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A VIEW OF

EARTH AND SPACE Click to edit Master subtitle style

Produced by Docents for Docents and Guides

3Earth and Space

TABLE OF CONTENTS

INTRODUCTION 5

EARTH

A Brief History of Earth and Life Through Time 7

Geologic Timeline 11

Plate Tectonics 12

Fossils - What They Are and How They’re Formed 16

Minerals 18

Rocks 19

The Theory of Evolution 22

VISTA 26

Evolving Story and Human Journey 26

SPACE

The Geology of the Solar System 27 The Sun 27 The Planets and Their Moons 28 Kuiper Belt Objects 36 Planets Beyond the Solar System 36

A Tour of the Universe Beyond the Solar System 37 Stars 38 Brown Dwarfs 38 Red Dwarfs 38 White Dwarfs 39 Black Dwarfs 39 Red Giants 39 Blue Giants 39 Supernovas 40 Pulsars and Neutron Stars 40 Black Holes 40 Quasars 41

Suggested Websites 42

FOREWORD

The “Earth and Space” topics such as rocks, fossils, plate tectonics, astronomy and cosmology are major interests among docents, guides and the general public. These interests will be addressed at a number of locations in the new Academy, especially the new state-of-the art Morrison Planetarium. This guidebook is designed to assist docents and guides with their public interactions about these topics.

A number of docents contributed written material to this View of Earth and Space:

Jean Johnson – Rocks and Minerals

Henri Lese – VISTA, A Brief History of Earth and Life Through Time, A Tour of the Universe, Updates to The Geology of the Solar System

Sandy Linder – Plate Tectonics, Theory of Evolution

Bing Quock – The Geology of the Solar System

Jim Scheihing – Rocks and Minerals

Some of this material was written for earlier guidebooks and carts. For example, much of the geology and fossil content was taken from written material for the geology cart and Life Through Time cart, respectively.

Special thanks go to Bing Quock for allowing us to again use his Geology of the Solar System, first published in the Astrobiology section of the Howard Street Exhibit Handbook and for checking the few updates necessary.

Our thanks also go to the scientific staff who provided important information and helped review the content: Drs. Carol Tang, Jean DeMouthe, Peter Roopnarine and Bing Quock. Support from VIP staff was essential and very much appreciated: Dr. Carol Tang, Dan Weinstein, Docent Manager Kathleen Lilienthal, Velma Schnoll, Jim Richard and Linda Huang. Thanks, too, to Lois Kelley, Roberta Borgonovo, and Marilyn Swartz, who reviewed this material and made valuable suggestions for improving the content and presentation, and special thanks to Sandy Linder, formatter.

Henri Lese, Editor

4 California Academy of Sciences

5Earth and Space

INTRODUCTION

The California Academy of Sciences is the only institution in the world that houses an Aquarium, Planetarium, and Natural History Museum under one roof—a statement that today, with our signature Living Roof, is not just figuratively, but literally true as well. This fact also means that Earth and Space Sciences are not subjects found in a single corner, but ones that permeate the exhibits of the New Academy.

The Universe and our Earth within it—from their earliest beginnings to the present—have changed over time. On Earth, atmosphere, oceans, land masses have been and remain in flux, providing the physical template against which life has emerged and evolved. From the appearance of single-celled organisms to the modern biodiversity, both beautiful and bizarre, that now fills almost every conceivable niche, the physical Earth has molded its inhabitants. The Academy—from rainforest to coral reef, from Africa to Galapagos, California to Madagascar—tells this story of how Earth and life have changed over time. The Planetarium, our state-of-the-art digital dome, will carry the story from Earth to Space, exploring the Universe in ways we can only begin to imagine.

So given the reach of these two subjects—Earth and Space—this View presents a brief introduction to some topics relevant to understanding broad, underlying concepts and themes that can be applied to many areas of the exhibit floor. We begin with a brief look at how Earth and life has changed over time—a topic presented in a dramatic mural on the Exhibit Hall’s southeast wall. An introduction to the workings of plate tectonics gives some insight into the geological process that has been a prime driver in effecting the evolution of life by bringing change to Earth’s climates and habitats, transporting once polar regions to the equator, opening new oceans and changing their circulation patterns, building mountains where plains or empty ocean once existed, producing earthquakes and volcanoes, sending once continental land masses to an island existence, or creating land where once there was only water.

THE TERRESTRIAL PLANETS - MERCURY, VENUS, EARTH, AND MARS

And how do we know what we know about life’s change over time? The fossil record tells much of the story, so an overview of types of fossils and how they are formed, complemented with a look at the characteristics of minerals and a description of rock types, rounds out the picture.

When Earth’s physical environments or the relationships of living things change, a new set of conditions exists and life responds. Through the processes of evolution, those organisms with features best suited to meet new conditions are favored and more likely to reproduce, passing their successful traits on to future generations, at least until the evolutionary playing field changes again, bringing new responses. Humans are subject to the same selective forces, as highlighted in Evolving Story on the northeast Exhibit Hall wall. And so we review the theory of evolution and the mechanisms that explain life’s “descent with modification” over the broad sweep of time.

Human intelligence, curiosity, and technology have now sent astronauts and probes beyond Earth, to the Solar System and farther still. The Geology of the Solar System and A Tour of the Universe Beyond the Solar System take us on an outward journey from our home planet to places where perhaps we or our children may one day travel—or, in the near future, go on wondrous digital voyages in the Morrison Planetarium. Enjoy this trip through time and space.


7Earth and Space

EARTH

A BRIEF HISTORY OF EARTH AND LIFE THROUGH TIME

On the southeast wall of the Kimball Natural History Exhibit Area a striking collage of images, created by photographer Franz Lanting, creates a timeline of life’s evolution over Earth’s 4.6 billion year history. The images are beautiful and compelling, using modern life forms to provide windows on the past. The specific times and pictures were chosen to mark important evolutionary milestones, and to suggest the change evolutionary processes have produced over an immense period of time.

The universe evolves. Stars are born, evolve and die. Earth itself evolves. And life on Earth evolves.

4.6 Billion Years Ago After the universe was born in the Big Bang some 9.1 billion years earlier, our Sun condenses out of a cloud of hydrogen, helium and lesser amounts of heavier elements. Once conditions at the sun’s core are extreme enough, hydrogen begins to fuse into helium, releasing vast amounts of energy in a process that continues today.

At the same time as the birth of the Sun, planets begin to condense out of a dusty ring surrounding the Sun. The primeval solar system is a hellish place, with collisions occurring constantly. The surface of early Earth, absorbing the heat of these collisions, is molten.

Earth’s Moon is created some 50 million years after the birth of the solar system, when a Mars-sized object strikes Earth, hurling debris into Earth orbit that eventually coalesce to form the Moon.

4.0 Billion Years Ago Earth’s crust cools and water vapor condenses to form oceans. The cooling Earth establishes an atmosphere, one that would be toxic to life today, but it makes possible the formation of organic molecules necessary for life’s origin.


Around this time, an intense bombardment probably remelts Earth’s surface and creates huge impact structures on the Moon that lead to the Moon’s “maria.” When this bombardment ends the Earth’s surface solidifies. The oldest rocks found today date to about 3.8 billion years ago.

3.5 Billion Years Ago Single-celled organisms—the ancestors of all life today—evolve. For over two billion years, more than two thirds of the history of life on Earth, single-celled organisms are the only forms of life on Earth.

The oldest direct evidence for life dates to 3.5 billion years ago as microfossils of bacteria, single-celled organisms that lack a nucleus. Cyanobacteria, capable of using energy from the sun to manufacture carbohydrates and in the process releasing oxygen into the atmosphere, are among these early forms.

Fossil stromatolites, layered mounds formed of cyanobacteria, other single-celled organisms, and sediments, appear about 3.2 billion years ago, and are abundant by 2.5 billion years ago. Banded iron formations, which date mostly from about 2.5 to 1.8 billion years ago, provide indirect indications of increasing photosynthesis that produces more and more oxygen. The oxygen reacts with iron dissolved in sea water, causing the iron to precipitate out into brilliant red layers of iron oxide. Once the oceans are depleted of iron, oxygen in the atmosphere begins to increase significantly by 2.0 billion years ago.

Eukaryotic cells, characterized by a nucleus and other complex structures enclosed within membranes, evolve somewhat later. Indirect evidence suggests eukaryotes may have appeared as long as 2.7 billion years ago.

850 Million Years Ago At this time, a series of massive ice ages occur that may have covered the entire planet in ice – the so-called “Snowball Earth.” Eukaryotic cells, new single-celled life forms with more complex cells, are now found in the layered mats of stromatolites. These complex cells eventually evolve into the first multicellular organisms, presumably able to reproduce sexually.

650 Million Years Ago Simple, soft-bodied marine organisms evolve, during what is known as the Ediacaran Period. Some of these simple life forms may have evolved into the rich variety of more modern marine animals that begin to appear about 540 million years ago.

450 Million Years Ago At this time, land is invaded by both plants and animals. Land plants come first, with a waxy exterior to keep them from drying out and pores that allow them to take in carbon dioxide and give off oxygen. Ancestors of modern scorpions, spiders, and insects follow, adapted by outer coverings that protect from water loss and predators, and give support to move against gravity.

440 Million Years Ago Primitive fishes and jawed relatives of sharks evolve in marine waters. Many marine invertebrates die off during a mass extinction. Early forms of reef-building corals appear.

9Earth and Space

415 Million Years Ago More complex plants appear though they still lack true leaves and roots and reproduce by spores.

310 Million Years Ago Forests of tree ferns, giant club mosses and horsetails grow on land. Amphibians are widespread, but, like today, dependent on water for reproduction. Vertebrates with scaly, waterproof skin and shelled eggs that protect the embryo within fluid-filled membranes evolve. Free from dependence on standing water, they are able to invade dry land. Their descendents include reptiles, birds, and mammals.

250 Million Years Ago The end-Permian extinction occurs, the largest mass extinction in Earth’s history. The cause or causes are still widely debated, but the effect is to wipe out 95 percent of all species on Earth. Eventually, survivors colonize the land and radiate opportunistically into habitats with few competitors. Within a few million years, reptiles occupy land (dinosaurs), air (pterosaurs), and ocean (ichthyosaurs and plesiosaurs); cycads and conifers replace giant club moss forests; new species of fishes, squid, and ammonites reclaim the seas.

Extinction events are a major force for natural selection: Earth changes, life responds. Five major extinctions and many more minor events have occurred in the past. Evidence suggests we are in midst of the Sixth Great Extinction, this one caused not by geological events, but by humans.

225 Million Years Ago Another successful group evolves about the same time as dinosaurs, but occupies a different niche. Mammals—able to regulate their body temperature independent of the external environment—make their living as small, nocturnal insect eaters.

125 Million Years Ago Dominant for 160 million years, dinosaurs continue to roam a land that is beginning to look very different. Flowering plants are replacing conifers and other gymnosperms in many areas, and modern groups of insects, such as ants, butterflies, aphids, grasshoppers, and termites evolve, many coevolving with flowering plants as pollinators.

50 Million Years Ago 15 million years earlier, a major extinction wipes out the non-avian dinosaurs. This event is tied to the famous Chicxulub impact, although many scientists believe that flood basalts also contributed to the extinction. Whatever the reason, the door opens for the radiation of mammals. Most modern mammal groups appear, including primates and early grazing mammals, such as horses and camels that coevolve with the first grasses.

30 Million Years Ago Grasslands are extensive. Mammals occupy nearly every conceivable niche. Monkeys and the ancestors of apes appear, and whales ply the oceans. Modern birds fly overhead, and marine groups, especially fishes and invertebrates, diversify.

5.3 Million Years Ago Life on Earth is looking distinctly modern. Cattle, sheep, and other grazing mammals roam across prairie and savannas, hunted by predators such as saber-toothed cats, bears, hyenas, and dire wolves.


5.0 Million Years Ago The line of primates leading to humans has diverged from the one leading to chimpanzees, our closest living relatives with whom we share over 98% of the same DNA.

2.0 Million Years Ago Our genus Homo evolves. At least 10 species are known from the ensuing 2 million years. Today, we modern humans, Homo sapiens, are the only member of our genus still living.

STROMATOLITES BANDED IRON FORMATION

11Earth and Space

Paleocene

Eocene

Oligocene

Miocene

First horsesFirst w

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First hominids

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Hadean

Archean

Proterozoic

Paleozoic

Mesozoic

Cenozoic

Phanerozoic

Cam

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4500

3800

2500

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First evidence of life (microfossils)

First stromatolites

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360

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Mass Extinction

Years in Millions

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Moon

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Mass Extinction

Mass Extinction

Mass Extinction

First primates

Pleistocene


PLATE TECTONICS

An Historical Overview

In 1915, a young German scientist, Alfred Wegener, published evidence to support his theory that Earth’s continents had once been joined in a single landmass. According the Wegener, this supercontinent, which he called Pangaea, drifted apart over millions of years to form the continents familiar to us today. His evidence included, among other points, the lock-and-key shape of the South American and African coastlines and the correlation between geologic structures, such as South African and Argentinian mountain ranges that line up with each other. In addition, fossils of particular land plants and reptiles that lived some 250 million years ago had been found in South America, Africa, India, Australia, and Antarctica. Such distribution patterns suggested to Wegener that these land masses, though now separated by oceans, were once attached. He noted, too, that areas such as South Africa, now in latitudes with mild climates, showed signs of ancient glaciation. Other areas in polar regions contained fossils of tropical plants. These indications of extreme climate change could be explained by what Wegener called the theory of continental drift, the movement of land masses over time.

For many years, Wegener’s ideas were rejected, mainly because he could offer no explanation of the mechanism that drove these movements. However, in the 1950s, radar mapping of the sea floor revealed parts of the Mid-Ocean Ridge, the now well-known 50,000-mile mountain range that girdles Earth. Later exploration by deepsea submersibles discovered the ridges were strewn with pillow basalts, rocks formed by the sudden cooling of magma underwater. In addition, radiometric dating of cores showed that the farther rocks were from ridge areas the older they were. Given this new evidence, the scientific community soon embraced the idea of seafloor spreading: at mid-ocean ridges material from the mantle rises, erupting at the surface to form new oceanic crust. This new material forces older crust away from the ridge on either side.

245 MILLION YEARS AGO 200 MILLION YEARS AGO

65 MILLION YEARS AGO PRESENT DAY

13Earth and Space

The Theory of Plate Tectonics

These discoveries, coupled with a revival of Wegener’s ideas and a greater understanding of heat circulation within the Earth’s mantle, produced a revolution in earth science known today as the theory of plate tectonics. This theory seeks to provide an overarching explanation for movements of the Earth’s surface and the geological features and events they produce, such as mid-ocean ridges, volcanoes, and earthquakes.

“Plates” refers to discrete sections of the Earth’s crust that float on the mantle, colliding, pulling apart, or grinding past each other. The crust is split into seven major plates and a number of smaller ones. Heat from the Earth’s hot metallic core, left over from the energy of impacts early in the planet’s history and from the decay of radioactive materials, rises to the upper, viscous levels of the mantle and crust, generating convection currents. Like pudding that bubbles on the stove, the hot material rises toward the surface, cools, and becomes denser as molecules lose energy and pack more closely together. The material then begins to sink to depth where it once again picks up heat, expands, and completes the circular currents that most scientists believe drive plate movements on the surface in complex interactions.

Divergent Boundaries Temperatures are especially high at divergent boundaries, such as seafloor spreading zones or the African Rift Zone Here magma pushes up from the mantle, creating new crust. At the Mid-Atlantic Ridge, which spreads east-west, this process creates new crust at about one inch (2.5 cm) per year, and pushes the North American Plate, carrying both the North American continent and the west Atlantic seafloor, westward against the north-northwestward moving Pacific Plate. This movement seems inconsequential in yearly terms, but over a million years translates to 10 miles (25 km)! In 200 million years, seafloor spreading could easily account for the splitting of Pangaea into today’s far-flung continents.

Convergent Boundaries As the theory of seafloor spreading gained popularity, scientists puzzled about why, if new crust is constantly being produced, Earth’s size has not increased significantly. The explanation—the recycling of oceanic plates in deep-ocean trenches through the process of subduction—not only solved this problem, but explained other natural phenomena as well. At convergent boundaries, three possible scenarios play out.

In the first, oceanic crust meets oceanic crust. As they converge, the plate that carries crustal material that is cooler and denser, usually because it is farther away from the spreading zone, sinks by gravity below the other plate, more buoyant with heat. Subduction of one oceanic plate under another often results in magma rising close to the surface, which in turn triggers the formation of volcanic island arcs, such as Japan and the Aleutians, and the strong earthquakes associated with them.

OCEANIC-OCEANIC COMVERGENCE


When oceanic crust meets continental crust, the lighter continental material, made mostly of granite, rides over the more dense oceanic crust, composed largely of basalt. Pick up an equal- sized piece of each rock; the density difference is obvious. Crustal material from the descending plate is usually scraped off, creating complex melanges such as the Franciscan Formation along our coast. The subducting plate also forces the overriding continental crust upward; for example, the Andes are being pushed upward by the subduction of the Nazca Plate along the western flank of South America. As noted above, subducting plates often result in magma formation that sustains volcanic activity near the edge of continental margins, as in the Andes or Cascades.

Most impressive, though a process invisible on a human scale, is the collision of two plates carrying continental crust. Here, subduction does not take place because the materials are of similar density. Instead, a slow-motion crash compresses the crust into sometimes spectacular mountain ranges. The impact 50 million years ago of India into the Eurasian Plate continues to force the Tibetan Plateau and the Himalayan Range upwards.

Transform Boundaries Where plates slide past one another, crust is neither created nor destroyed. Most transform boundaries are found on the ocean floor, but a few, such as the San Andreas Fault, occur on land. Transform boundaries typically lack the spectacular features found at divergent or convergent zones. Linear valleys sometimes mark the line where plates slip past each other, crumbling and compacting rock. In some instances stream beds or landforms are offset as plates move in opposite directions.

The west side of the San Andreas Fault moves northwestward relative to the rest of California, carrying Los Angeles closer to San Francisco at about 2 inches (5 cm) per year. If this rate remains constant, the two cities will meet in 10 million years! When plates lock together, constraining their movement, stress builds up in the rock. This stress, a form of energy, grows until the rock suddenly breaks or changes shape; in other words, an earthquake occurs.

OCEANIC-CONTINENTAL CONVERGENCE

CONTINENTAL-CONTINENTAL CONVERGENCE

15Earth and Space

Astrobiology and Plate Tectonics

Astrobiology is the study of the origin, evolution, adaptation and distribution of past and present life in the universe. Plate tectonics and the forces that drive them have created extreme environments where life has formed and thrived on Earth, including hydrothermal vents and hot springs. These habitats and others may provide models for equivalent niches on other extraterrestrial bodies where we might find life.

Evidence suggests that tectonics were active on some planets in the distant past. For example, certain areas on Mars’ surface show evidence of magnetic striping, a pattern that reflects reversals of a planet’s magnetic poles. Earth’s seafloor displays this same pattern. As new-formed basalt solidifies from magma, its iron-rich particles line up with Earth’s poles. In the 1960s, the theory of seafloor spreading was validated with the discovery that the pattern of reversals was mirrored on either side of mid-ocean ridges. Did a similar tectonic activity take place on early Mars? Was Mars far more similar to Earth in its early history, and potentially more hospitable to life?

Earth’s active surface movements result from dissipating heat from its core. The Jovian moons Io and Europa also appear to have active surface movements, though their cause is probably quite different from Earth’s. Surface movement on Io is driven by tidal heating, caused by friction from the gravitational pull of their immense parent planet and other nearby moons. As the surface rises and falls, somewhat like tides on earth, frictional energy is released in the form of heat. Some scientists speculate that Europa’s heat may be generated by a core still cooling from its earliest history. Perhaps this internal energy drives hydrothermal vents that maintain temperatures warm enough to sustain an ocean of salt water beneath Europa’s thick crust of ice. Such scenarios prompt intriguing speculation about the existence of life beyond Earth.

EARTH’S MAJOR TECTONIC PLATES


FOSSILS

What They Are, What They Tell Us, and How They’re Formed

Fossils are the remains or traces of organisms that lived in past geologic times. Fossils give clues about the form of ancient plants and animals, the communities in which they lived, and the age of the strata where they were found. Carefully linked together by paleontologists, fossils form a record of past life and its evolution over 3.8 billion years.

Only a minute fraction of all the organisms that lived over the span of geologic time have been fossilized, and only a fraction of those have been found. Certain conditions favor fossilization:

Hard Parts Animals or plants that have hard parts such as teeth, bone, shell, or wood are well-represented in the fossil record. Structures with cellulose (plant tissue) and chitin (exoskeletons of insects and other arthropods) are also decay resistant and relatively good candidates for fossilization. Soft tissues soon deteriorate, but hard parts persist long enough to allow preservation.

Rapid Burial Usually, when an organism dies, it is destroyed by predators, scavengers, bacteria, acids, oxidation, or water movement. Because of these factors, fossilization is a rare occurrence. Most fossils result from the rapid accumulation of sediments on top of a newly dead plant or animal that lived on or sinks to the bottom of a lake, sea, or river.

Wide, Plentiful, and Long-term Distribution Obviously, the laws of probability favor fossilization of organisms that had hard parts, were widespread, numerous, and persisted for a long period of time.

Types of Fossil Preservation

Original Material Hard parts, such as teeth and shells, may be preserved with little or no change. Occasionally, whole carcasses of animals have been preserved in ice or frozen soils. Tar pits and peat bogs are sources for unchanged bones or plant material. Whole organisms, from insects to reptiles, have been preserved in amber.

Mineralization Circulating ground water carries minerals that fill in open spaces of the original fossil material. The process may be partial, with much of the original material remaining, or complete if the organic matter is dissolved, usually by acidic ground water and totally replaced by minerals. Common replacement minerals are silica, calcium carbonate, and iron pyrite.

Impression The two-dimensional imprint of an organism. Leaves, feathers, insects, and soft-bodied animals as well as footprints and trails of animals are most often preserved as impressions on fine-grained sediments, such as clay or slit that hardens to stone.

17Earth and Space

Compression If the fossil retains some organic material, it is called a compression fossil.

Carbonization A kind of compression fossil. Complex hydrocarbons make up a significant portion of living cells. During the compression process, volatile compounds are driven off by heat. Sometimes, especially in the case of plants, only a thin film of carbon remains, leaving a characteristic black or dark outline of the original organism, often in fine detail.

Molds and Casts The shell or exoskeleton of an organism may act as a mold. For example, a shell may be filled with sediment that hardens to rock with deep burial. If the shell material is subsequently lost, a cast of the original may remain.

Sometimes bones, wood, or shells dissolve, leaving hollows, or molds, that preserve the shape of the original organism. If such a mold is later filled in with sediment, this new material may harden to form a cast; in this case it’s called a cast of a mold rather than a cast of the original.

Trace Fossils Leave a record of an organism’s activities.

Burrows and Borings of animals traveling through or living in sediment are common trace fossils.

Footprints, Tracks, and Trails animals make as they move about are important lifestyle clues.

Coprolites, or fossilized dung, may give clues to the diet of ancient animals.

Gastroliths are smooth stones found within the rib cage of certain dinosaurs, including sauropods. Swallowed and passed to a muscular gizzard, they are thought to have aided in the breakdown of fibrous plant material.

Chemofossils are chemical compounds or isotopes that indicate biological activity from past geologic times. For example, lipids leave a distinct chemical signature; their detection in ancient rocks is adding to our understanding of when and how life evolved.

TRILOBITEAMMONITE COPROLITE

FOSSIL FISH INSECT IN AMBER


MINERALS AND ROCKS

MINERALS

Composition

A mineral is an inorganic solid that occurs naturally in the Earth. “Inorganic” means that minerals do not contain organic substances, which are compounds of carbon, oxygen, and hydrogen. Though man has learned to reproduce some minerals, such as diamonds and quartz, these synthetic substances are not classified as true minerals because they do not form naturally.

Each mineral species has a definite chemical composition of elements. A few minerals are made up of only one element, such gold (Au), silver (Ag), copper (Cu), or sulfur (S). Most minerals, however, are compounds of more than one element. For example, quartz always consists of one silicon atom (an element, Si) and two oxygen atoms (another element, O). Its chemical composition is therefore expressed as SiO2.

Crystal Structure

Each mineral species has a characteristic internal structure because the atoms that make up a mineral are bound together in orderly, repeating patterns. There are six crystal systems, each defined by a set of internal axes (imaginary lines). Each system always has the same crystal symmetry, no matter how big or small the crystals are. For example, salt (NaCl) is always cubed shaped with the sodium and chlorine atoms stacked together in a box-like pattern. Quartz (SiO2) crystals form in a distinctive six-sided shape that terminates pyramidally.

Large crystals visible to the naked eye form only under optimal conditions when they have time and space to grow. Such crystals most often form (1) from atoms and molecules that precipitate from hot fluids within the Earth’s crust, or (2) from magma that cools slowly underground. Most crystals, however, are not large enough to see without a microscope.

Other physical properties

Color

Luster How a mineral reflects light

Cleavage The tendency to break along definite planes

Fracture The tendency to break in ways other than along a regular plane

Hardness Resistance to abrasion

Streak Color of mark left when a mineral is rubbed across an unglazed white porcelain surface

While more than 3000 minerals have been identified, only about 100 are common. Eight elements account for 98% of the Earth’s crust, with silicon and oxygen accounting for 74%.

19Earth and Space

ROCKS

A rock is a natural solid made up of one or more minerals, bound together either by interlocking grains or by natural cement. Rocks typically are defined by their mineral content, texture, and/or how they were formed.

There are three basic types of rock: igneous, metamorphic, and sedimentary. Their origins are all different. Any one of these rock types can be derived from rocks of the other types. This transformation over long periods of time from one rock type to another is part of the Rock Cycle.

Igneous Rocks

Igneous rocks form by cooling and hardening of magma, a partially molten mixture of minerals, gases, and water that originates in the Earth’s lower crust and upper mantle. There are two types of igneous rocks—plutonic, which cool and solidify slowly beneath the surface of the earth, and volcanic, which cool relatively quickly on the surface of the earth. Slow cooling allows larger crystal growth, as in coarse-grained rocks like granite, while rapid cooling promotes fine-grained rocks like basalt.

As the percent of silica increases in volcanic rocks so does the melting point and viscosity, or resistance to flow. High viscosity hinders the release of gases, which can build up and produce violent eruptions when the pressure on them is reduced. Low silica lavas, like those on Hawaii, are low viscosity and flow easily for miles. High silica – high viscosity rhyolitic lavas, like those of Yellowstone and of Mono Craters just east of Yosemite, often produce huge explosions.

Some common igneous rocks are listed below. Note each volcanic rock has a compositionally equivalent plutonic rock (e.g., basalt/gabbro), because each pair cooled from similar magma melts.

Plutonic Silica Content Volcanic Silica Content Color

Granite 72% Rhyolite 72% Light

Granodiorite 65% Dacite 65%

Diorite 57% Andesite 57%

Gabbro 48% Basalt 48% Dark

LAVA PUMICE VESICULAR BASALT


Sedimentary Rocks

Sedimentary rocks are deposited in stratified fashion, layer upon layer, in both marine and terrestrial environments. Though igneous and metamorphic rocks make up most of the crust by volume, sedimentary rocks are by far the most commonly exposed rock on the Earth’s surface. Sedimentary rocks are produced in three different ways:

Mechanical sediments are accumulations of mineral and rock fragments. Most mechanically deposited sediments, such as mud, sand, and gravel, are products of surface weathering and erosion. They consist of the disintegrated and decomposed debris of older rocks (igneous, metamorphic, or other sedimentary rocks), transported and deposited by water, air, or ice. They are categorized according to the size of particles that compose them.

Chemical sediments originate by precipitation of material in solution, usually in oceans or lakes. Precipitation takes place when the concentration of dissolved material exceeds its solubility and can be caused by evaporation or by inorganic chemical reactions.

Organic sediments result from the activities of living things, such as clams, mollusks, and microscopic plants and animals that concentrate dissolved minerals to make their shells or skeletons. When these organisms die, the mineral material accumulates, sometimes to great depth, on the ocean floor.

Most sedimentary rock is formed by the compaction and cementation of previously unconsoli-dated sediments. As thick layers of sediment build up, the pressure compacts the underlying lay-ers, squeezing particles tightly together. Minerals dissolved in water, most often quartz or calcium carbonate, can seep into spaces between particles. When these minerals crystallize, they cement the particles together, forming rock.

Sedimentary rocks are extremely important to geologists and paleontologists because they can tell about the conditions at the time of their deposition. For example, evaporites, such as salt, suggest the climate must have been dry and hot. Limestone suggests formation in a marine environment, even though it may now be found in mountains high above sea level. Also, sediments regularly bury and preserve the remains of dead plants and animals. If these sediments turn to rock at low temperature, these organisms may become fossilized, rather than being destroyed by high pressure and temperature. Fossils can reveal the environment of deposition, help date the rock, and ultimately tell the story of how life evolved on Earth.

SANDSTONE BEDS OF LIMESTONE AND SHALE

21Earth and Space

Metamorphic Rocks

Metamorphic rocks (meta - “change,” morpho - “shape”) result from changes in the mineral composition and/or texture of preexisiting rocks that take place beneath the Earth’s surface. Unlike igneous rocks, metamorphic rocks are formed in the solid rather than molten state. The agents of metamorphism are pressure, temperature, and fluid activity. Pressure, produced by the weight of overlying rocks, increases with depth. Temperature also increases with depth and/or from proximity to magma. Hot fluids, which can circulate in the pore spaces between mineral grains and accelerate chemical change, come from water trapped in the rocks themselves or from fluids within nearby magmas. Common types of metamorphism:

Contact metamorphism occurs when magma is injected into surrounding solid rock (country rock). The changes are greatest wherever the magma comes into contact with the rock, because the temperatures are highest at this boundary and decrease with distance from it.

Regional metamorphism is the name given to changes in great masses of rock over a wide area. Rocks can be metamorphosed simply by being at great depths below the Earth’s surface, subjected to high temperatures and the great pressure caused by the immense weight of the rock layers above. Much of the lower continental crust is metamorphic. Horizontal tectonic movements such as the collision of continents often cause high temperatures, pressures and deformation in the rocks. If the metamorphosed rocks are later uplifted and exposed by erosion, they may occur in long belts or other large areas at the surface.

SERPENTINITESCHIST

GNEISS WITH GARNET CRYSTALS


THE THEORY OF EVOLUTION

FROM DARWIN TO THE 21ST CENTURY

Nothing in biology makes sense except in the light of evolution. Theodosius Dobshansky, American Biology Teacher, March 1973

Evolution, in the simplest terms, is descent with modification. Evolution informs our understanding of the history of life on Earth. Over the some 3.8 billion years that life has continuously inhabited Earth, many millions of species have evolved. Most are extinct, yet all known species, past and present, are related, descended with modification from a single common ancestor that gave rise to the astounding diversity that surrounds us today.

Our Debt to Darwin

Our modern understanding of evolution has been built on the work of many, Charles Darwin still the most influential among them. The idea of evolution was not new in Darwin’s day, as the fossil record and the history of changing life forms it records were becoming more widely known. However, Darwin’s extraordinary contribution was his ability to marshal his broad knowledge of geology, paleontology, biogeography, artificial selection, and his intimate familiarity with various plant and animal groups to provide overwhelming evidence that all living organisms had evolved from a common ancestor over a vast period of time. Even more important, he was the first to clearly suggest a viable way these changes occurred over time, a mechanism he outlined in 1859 in The Origin of Species.

Darwin was acquainted with Thomas Malthus’ Essay on the Principles of Population (1798), which states that human populations, though capable of increasing geometrically, do not actually expand at this rate because natural causes, such as disease or famine, limit growth. Darwin made the brilliant step from Malthus’ observation of humankind to a more general view of the natural world. First, Darwin noted that living things produce many more offspring than actually survive. Second, individuals within groups

23Earth and Space

exhibit variation; some are larger or smaller, some have variations in color or shape, some are faster or stronger, and so on. Third, individuals are able to pass these various traits to their offspring.

Given these facts, Darwin made what seems today a remarkably simple conclusion: Those individuals that possess qualities which improve their chances for survival in competition with others of their species will be more likely to survive long enough to reproduce and pass their successful traits on to their descendants. Over time, if the environment remains fairly stable, these traits will continue to be selected and become more common within a species. In Darwin’s view a variety within a species may eventually build up many such traits, making the subset different enough to be defined as a new species. This process Darwin called “natural selection” because it is the natural world, the environment in which an organism lives, that selects a given trait as an adaptive advantage. Natural selection was Darwin’s engine of evolution.

The Rise of Gene Theory

By the 1870s most serious scientists accepted the theory of evolution, the idea that life forms have changed over time. Even so, exactly how it worked remained a subject of intense debate. One problem, of course, involved inheritance; exactly how are traits carried from parent to offspring, a mystery that Darwin admitted he could not solve.

Gregor Mendel, an obscure Moravian monk who lived and worked during Darwin’s lifetime, pro-vided an answer, clarified in famous experiments well known to high school students today. Mendel first carefully developed strains of peas that would breed true for different traits. He then crossbred the strain and discovered that, in the second generation, all individuals were similar in selected traits. For example, if tall and short plants were crossbred, the second-generation plants were all tall. However, when second-generation plants were crossbred, they produced on average a 3:1 ratio of tall to short plants.

In these seminal experiments, Mendel proved that some traits that characterize an individual are inherited from its parents as discrete units, some of which are dominant, such as tallness in peas, or recessive, such as shortness. His work showed patterns of inheritance to be real and predictable. Unfortunately, though Mendel published his findings in 1865, his work remained virtually unknown until early in the 20th century.

The Theory of Evolution Today

During the last nearly 150 years, many brilliant scientists have stood on Darwin’s shoulders, interweaving Darwinian ideas with new discoveries and ideas that inform our current understanding of the process of evolution. Fisher, Haldane, Dobshansky, Mayr, Watson, Crick, Gould, and Wilson are a few of the most notable.

During the past century, the discovery of DNA and the genetic code, as well as a more sophisticated understanding of how genes carry the messages of inheritance have refined evolutionary theory. Today, we know that variation occurs at the cellular level, where the genetic code is carried in an organism’s DNA, and that the information encoded changes over time, through


the mixing of parental DNA during sexual reproduction, mutation, and other random events. It is this rearrangement of genes that provides variation and its expression at the phenotypical level, which refers to anything that is part of the observable structure, function, or behavior of an organism. Genetic variation is the raw material of evolution.

Today we understand that biological evolution is descent with modification through genetic inheritance. Evolution takes place not on the level of the individual, as Darwin thought, but rather at the population level. Populations are groups of individuals that interbreed with each other and so share a gene pool. Gene frequencies change in a population, often from one generation to the next. Over even short periods of time, these changes can produce relatively small-scale evolution, such as the growing resistance of many bacterial diseases to antibiotics in just the last 20 years. Changes in gene frequencies also produce the large-scale evolution visible over the 3.8 billion years of the fossil record, including the creation of new species, the origins of clades such as mammals, and the radiation of dinosaurs.

Natural selection, while a potent evolutionary force, is not the only player that acts to promote change over time. Other causes for change include mutation, gene flow, and genetic drift. Mutation is an obvious factor in changing the genetic makeup of a population and its gene frequency, though significant mutations are relatively rare. Even so, because of the expanse of time involved, random mutations have created considerable variation for natural selection and so have had a powerful effect on the evolution of life.

Gene flow—also called migration—is the movement of genes from one population to another. Migration takes many forms, from the movement of an animal from one place to another, to the drifting of planktonic forms to distant locations, to the pollen of flowering plants carried randomly on the wind. The end result, though, is the same. The gene pool of a population changes over time, setting up new equations and novel variations.

Genetic drift is a phenomenon that can cause drastic change, especially in small populations, because of random events. For example, some individuals may produce more young than others, not through the working of natural selection, but simply by chance. This reproductive anomaly might increase or remove significant genetic variation in the population as a whole. Occasionally populations undergo bottlenecks, when large numbers of a population are lost. Northern elephant seal and African cheetah populations, for example, were severely reduced in size because of human activity. Though the populations have rebounded significantly, they lack genetic diversity, making the whole group less likely to adapt to environmental change or disease. Another cause of genetic drift is the founder effect, which occurs when a few individuals leave the original population and start a new colony, again potentially reducing the gene pool.

Mutation, gene flow, genetic drift, and natural selection all produce changes in gene frequencies. The first three processes work at the level of the genotype. Natural selection, the most potent causative factor, acts on the phenotype: the physical appearance, physiology, or behavior of an organism. If an organism has traits adaptive to its environment, it will more likely prosper and reproduce in greater numbers than organisms that lack the trait.

25Earth and Space

Selective pressures may be extremely subtle, such as small temperature changes in the environment, or the introduction of a new and mildly competitive organism into an area. They may also be catastrophic, such as meteor or asteroid impacts or the global shifting of tectonic plates, which may create new climatic and vegetation regimes over large areas and cause mass extinctions. Five great mass extinctions are recognized in the fossil record, the most dramatic being the Permian extinction 245 million years ago, which wiped out well over 90% of marine species and cleared the way on land for the domination of reptiles, including dinosaurs, during the Mesozoic Era. The extinction event at the end of the Mesozoic, some 65 million years ago, brought the demise of the dinosaurs and set the stage for the radiation of mammals. Today, most scientists agree that the Earth is experiencing a sixth major mass extinction: the cause—human activity and overpopulation that have led to serious loss of habitat and widespread pollution.

The Evidence for Evolution

Evidence for large-scale change comes from many sources. Even casual observation supports the fact that most organisms exhibit adaptive advantages, making them often exquisitely fit for the niches they occupy. Other lines of evidence are also well known. The fossil record provides indisputable evidence of progressive evolutionary change toward increasing complexity. Molecular studies, which can show relatedness of species and how long ago their evolutionary lines diverged, have for the most part confirmed the findings of the fossil record. The prevalence of shared structures subsequently modified in different groups, such as the configuration of limb bones in vertebrates, also illustrates descent with modification. Often developmental patterns of organisms show the evolutionary history and relationships among groups, with new features being added to more basic patterns over time. The fact that all vertebrates possess gills at some stage in their embryonic development is but one example.

Scientists do not always agree on the patterns and rates of evolution. In the 1970s, Stephen Jay Gould and Niles Eldridge proposed the idea of punctuated equilibrium, proposing that the fossil re-cord does not show Darwin’s gradual change over time, but rather long periods of stasis punctuated by sudden, dramatic change when many new species appear. However, though details are debated, no serious biologist doubts the theory of evolution. And, even though our understanding of how evolution works and what it has produced over time has matured during the last 150 years, Charles Darwin still stands as a giant, both in the clarity of his insight, and the depth of his argument in sup-port of evolution. The ending of The Origin of Species is as eloquent as its arguments are illuminating:

There is a grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and wonderful have been, and are being, evolved.


VISTA: MAJOR CONCEPTS OF THE EVOLUTION OF LIFE

There are five requirements for the evolution of species summarized by the acronym VISTA

Variation No two individuals of a species are exactly the same. Variation occurs at the genetic level, and can occur by sexual reproduction or mutation. Variation makes selection possible.

Inheritance Advantageous traits can be passed from generation to generation.

Selection Advantageous traits will increase in frequency from generation to generation, if they confer an enhanced ability to survive and reproduce. Severely disadvantageous traits will eventually disappear. The theory of evolution by natural selection was independently developed by Charles Darwin and Alfred Russel Wallace. “Survival of the fittest” is a misleading statement about natural selection, because it is not necessary to be “the fittest;” it is only necessary to be fit enough.

Time The divergence of species takes many generations to occur.

Adaptation An adaptation is a trait that helps an individual to survive better, the result of the interplay between variation, inheritance, selection and time.

EVOLVING STORY appears on the northeast wall of the Kimball Natural History Museum Exhibit Hall and emphasizes the importance of variation as the fuel for natural selection.

The idea of variation is shown through a collection of ladybird beetle specimens, each a member of the same species (Hippodamia convergens), yet each visually different, emphasizing the concept that no two individuals of a species are exactly alike. Many other important variations are not visible, but nonetheless may have adaptive advantage. Variation is the fuel of evolution; a particular difference may provide an advantage to an individual or group within a population, allowing survival, preferential reproduction, and the increase of the trait within the population. These differences in traits over time may produce new species.

Humans have manipulated variation in the selective breeding of plants and animals, as seen in a series of dog skulls, all obviously different from one another. As dog breeders and growers of monogenetic crops know, however, there is a downside to selecting for a few desirable traits; variation is reduced and the species may be subject to disease, deformity, or other pressures and not have the genetic plasticity to respond successfully to change.

Human variation is also obvious, in our faces, eyes, skin color, and bodies. Each of us has inherited certain traits from our parents, traits—some visible, some not—passed down in our DNA through generations. No two of us are alike, yet we are all one species.

THE HUMAN JOURNEY world map traces the evolution of modern humans in Africa more that 100,000 years ago, and our migration throughout the world. Distinctive traits or markers we display today—skin color, disease resistance, and many other genetic traits—evolved over time, and are recorded in our DNA, a record that reveals the times and places of our journey. This map is an extension in time of the story of human evolution presented in Africa Hall.

27Earth and Space

SPACE

THE GEOLOGY OF THE SOLAR SYSTEM

THE SUN

Our solar system consists of one star, eight planets, at least three dwarf planets, many satellites (planet moons), countless small solar system bodies such as asteroids, comets, and meteoroids, and dust. More than 99% of the solar system’s mass is contained in the Sun, which is a sphere mostly of hydrogen and helium measuring 865,000 miles (1.39 x 106 km) in diameter, or 109 times the diameter of Earth.

The difference between a star and a planet is that stars are so massive that the pressure at their cores generates a temperature high enough to make hydrogen atoms slam together and fuse, forming helium. This process, called “thermonuclear fusion,” gives off energy, and is where the heat and light of stars comes from. Planets aren’t massive enough for thermonuclear fusion to occur in their cores.

The sun converts about 700 million tons of hydrogen into helium per second! At this rate the sun will last about another 5 billion years. Obviously it is very, very large. The literature often refers to the sun as an ordinary star. This is misleading. The sun is much larger than most other stars. A small number of stars are much larger than the sun, but many stars are smaller.

ORION NEBULA

SUN WITH JOVIAN PLANETS FOR SIZE COMPARISON


THE PLANETS

The four nearest planets to the Sun are small, rocky objects known collectively as the “terrestrial (or Earthlike) planets,” and the four after that are very large, gaseous bodies called the “jovian (or

Jupiterlike) planets.” By examining the characteristics of each planet, we can learn what they have in common with Earth and ultimately better understand our own planet and what makes life possible on its surface.

EARTH

The complex life we find on Earth is not possible without abundant water, and 3/4ths of the planet is covered by it. This is made possible by Earth’s distance from the Sun (making Earth neither too hot nor too cold for liquid water to exist) as well as by the density of its atmosphere (providing enough pressure so that the water doesn’t all evaporate). The oxygen and ozone in the atmosphere protect Earth’s surface from solar ultraviolet radiation. Earth’s magnetic field also traps energetic particles released by solar flares, collecting them into the Van Allen radiation belts around the planet. Without the protection of the last two factors, Earth’s surface would experience much higher levels of radiation than it does today.

THE MOON

Earth’s only moon is a small, dry, rocky object. At only a quarter the diameter of Earth, it has insufficient gravity (only 1/6th that of Earth) to hold onto an appreciable atmosphere. The lack of an atmosphere means that very little erosion has taken place on the Moon’s surface, preserving a relatively permanent record of the heavy bombardment that took place in the solar system’s past. Without the moderating effects of an atmosphere, the Moon is exposed to extremes in temperature, ranging from 250 °F (121 °C) in sunlight to -250 °F (-157 °C) in shadow.

The Moon’s craters can be seen easily from Earth, and are most prominent during partial phases—particularly the quarter phases, when the Moon is lit from either the right or left sides. This is when the low Sun-angle casts long shadows, making the surface relief stand out along the edge of the sunlit regions (this edge is known as the “terminator”). The youngest craters can be easily identified by the “splash” pattern of rays radiating from them, well-defined edges, and a central mountain peak, formed by a “rebound” of the crater floor after the impact. Older craters may have more worn-down edges, and subsequent craters may have obliterated their rays and central peaks. Some may even have been flooded by lava, giving them very dark, flat floors.

TERRESTRIAL PLANETS JOVIAN PLANETS

EARTH FROM ORBITING SATELLITE

29Earth and Space

The maria (“MAH-ree-uh”) are dark, flat lowlands of hardened lava that welled up from underground (the brighter, cratered regions are called the highlands). Some of the maria are thought to have formed when an asteroid crashed into the Moon and blasted out the giant, circular basin known today as Mare (“MAH-ray”) Imbrium (the “Sea of Rains”). The Moon’s nearside has more maria than the farside, and it’s thought that this is because the Earth’s gravity pulled the core of the Moon toward it, causing the lunar crust to be thinner on the Earthward side and thus more subject to upwelling of lava.

The maria were formed during the “Late Heavy Bombardment” (LHB) about 3.9 billion years ago. A new theory is that the LHB impacts originated when the planet Neptune invaded the Kuiper Belt of small solar system bodies, displacing some of these bodies and sending them into the inner solar system.

Another large impact feature on the Moon is a multi-ringed basin just beyond the western horizon called Mare Orientalis (or the “Eastern Sea”). The South Pole–Aitken Basin, 1400 miles (2250 km) in diameter and 7.5 miles (12 km) deep, may be the largest impact basin in the solar system. In fact, the Moon itself is believed to be the result of an impact. The current theory for the formation of the Moon is that a stray, Mars-sized planetoid struck the Earth shortly after its formation, splashing out a large blob of material which then went into orbit.

Some geological structures hint at past volcanic activity on the Moon, such as the maria, rilles, or collapsed lava tubes. One of these, Hadley Rille, was visited by the astronauts of Apollo 15 in 1971. Puzzling activity is also observed by amateur astronomers, called Transient Lunar Phenomena, consisting of brief glows in shadowed areas. Some have suggested that these may be evidence of current volcanism, but this theory remains unconfirmed.

The Moon was first visited by the Soviet spacecraft Luna 2, in 1959. From 1969 to 1972, the Apollo astronauts explored parts of the Moon and left laser-ranging reflectors that allowed astronomers to precisely measure the distance from Earth to the Moon, revealing that the Moon is slowly moving away from Earth at about an inch (2.5 cm) per year. They also collected solar wind samples and about 800 pounds (363 kg) of lunar surface material, one piece of which is in the Academy’s possession—a 3.3 ounce (102.58 gm) specimen from Apollo 17 which was chipped off from a larger rock near the landing site.

The moon rock on display was taken from the 243.6 pounds (110.5 kg) of rocks collected during the Apollo 17 mission. The landing site was on the southeastern rim of Mare Serenitatis, in the Taurus-Littrow highlands and valley area. Our specimen is kept under nitrogen gas to avoid degradation by oxygen and water.

Another curious lunar mystery is whether water ice may exist at the lunar poles. In 1964, scientists reported that two years earlier, radar reflections from the lunar pole displayed characteristics similar

MOON’S MARIA

HADLEY RILLE


to radar that had been bounced off an icy surface. They suggested that ice—transported from space by comets—might be located, mixed with the lunar soil, at the bottom of deep craters at the north and south poles of the Moon, where direct sunlight never shines. Recently, however, radar studies seem to indicate that less water than originally thought may exist, in the form of ice crystals mixed with the lunar soil, rather than as large bodies of ice. This does not, however, disprove the theory that comets may have brought water to the planets, since comets are known to be composed largely of frozen water, and the hundreds of comets observed throughout human history suggest that it’s likely that many have hit the planets. This was actually witnessed in 1994 when fragments of Comet Shoemaker-Levy 9 struck Jupiter.

MERCURY

Mercury is the nearest planet to the Sun, at a distance of 36 million miles (5.8 x 107 km). It’s typically washed from view by the Sun’s glare as seen from Earth, but it can be seen for about a week when it’s at its greatest angular separation from the Sun. It can also be seen when it transits the Sun’s disk—that is, moves in line between Earth and the Sun so that its silhouette can be seen against the Sun’s disk. Transits of Mercury happen only about a dozen times per century.

Mercury is about 1.5 times the diameter of Earth’s Moon, and is very moonlike, having no appreciable atmosphere and a Sun-baked surface with temperatures close to 800 °F (427 °C). These conditions make life unlikely.

Like the Moon, Mercury is pocked by countless impact craters, though its surface has no maria. The largest impact feature on the planet, photographed in 1974 by the Mariner 10 spacecraft, is the Caloris Basin, a multi-ringed feature measuring about 800 miles (1287 km) in diameter. On the exact opposite side of the planet

from (“antipodal to”) Caloris, the surface is jumbled and chaotic, as if the shock waves from the Caloris impact were somehow focused on that spot, causing severe seismic upheavals. A similar phenomenon is seen on the surface of Mars.

Mercury has no known moons.

VENUS

Venus is the brightest planet visible in the sky. It can be seen to undergo phases, which were one proof offered by Galileo in the early 1600s that the Sun—and not the Earth—is at the center of the solar system. Venus rotates on its axis in the opposite direction from all the other planets, leading some to conclude that its axis of rotation is tilted nearly 180° from the vertical, making Venus an “upside-down” planet. Venus is so bright because its atmosphere is filled with

MERCURY

31Earth and Space

clouds that reflect 65% of the light striking it—the most of any planet (the next most reflective is Jupiter, reflecting 52%, while Earth reflects 37% and the Moon, believe it or not, reflects only 12%). However, these clouds are composed partly of sulfuric acid, and the atmosphere itself is 97% carbon dioxide—a heavy gas which causes the surface atmospheric pressure on Venus to be about 90 times that of Earth; i.e., more than 1,300 pounds per square inch as opposed to about 14.7 pounds per square inch.

The high concentration of carbon dioxide in Venus’ atmosphere also causes a “runaway greenhouse effect,” allowing solar heat through to warm the surface of the planet. However, the heat given off by the warmed surface can’t escape back into space, and so it remains trapped within the atmosphere, like in an automobile on a sunny day. As a result, the surface temperature climbs to nearly 900 °F (482 °C)—hot enough to melt lead, tin, and other soft metals. Here, too, the conditions are too inhospitable to expect any life to have arisen.

Scientists have estimated that the surface temperatures of Earth and Venus should not be so different—all things being equal, the difference should be only about 55 °F (31 °C). The cause of the temperature difference is the runaway greenhouse effect, resulting from all the carbon dioxide in Venus’ atmo-sphere, but why is there so much CO2 in Venus’ atmosphere and not in Earth’s?

The difference is Earth’s water. In the presence of liquid water, carbon dioxide bonds with calcium to form carbonates, such as sea-shells, corals, and limestone. On Venus, the absence of water means that the CO2 stayed in the atmosphere, where it caused the greenhouse effect.

Radar scans have penetrated the clouds of Venus and revealed several continent-sized plateaus rising from an enormous lowland plain, extinct volcanoes, and other very odd geological landforms.

Venus has no known moons.

MARS

In our solar system, Venus, Earth, and Mars are within the range of distances from the Sun where liquid water can exist. This is known as the “habitable zone.” Mars receives major media attention every 26 months, when it comes in to opposition, meaning that it is opposite the Sun in the sky. This is when it comes closest to Earth, and in 2003, it reached its closest opposition in 60,000 years. This is also when spacecraft headed for Mars can take advantage of the shortest route, so that’s why NASA and other space agencies send landers or orbiters to Mars about every 2 years.

VENUS


Mars is half the diameter of Earth, with about a third the gravity, which is enough to hold onto a substantial atmosphere. The surface temperatures range from -220 °F to 68 °F, ( -140 °C to 20 °C) with an average of -81 °F (-62 °C). Even through small telescopes, dark permanent markings can be seen, along with seasonal ice caps at the poles. Spacecraft have given better views of the surface, revealing features such as a huge, 3000-mile (4830 km) long chasm (Valles Marineris, or “the Mariner Valley”), and giant volcanoes (including Olympus Mons, or “Mount Olympus”) larger than any on Earth. Olympus Mons is the largest mountain in the solar system at 15 miles (24 km) high, and a base more than 310 miles (500 km) in diameter. The largest volcanoes are found on an enormous rise known as Tharsis, the most volcanic region on Mars. The second most volcanic region is called Elysium.

The volcanoes of Mars are as large as they are because Mars has no tectonic plate motion. This allows upwelling lava to accumulate and form shield volcanoes over the same spot. On Earth, tectonic plates slowly pass over such “hot spots,” forming chains of volcanoes such as the Hawaiian Islands. Volcanoes are the method by which planets outgas from their interiors and replace original methane atmospheres with new atmospheres of carbon dioxide.

Mars has enough of an atmosphere for certain types of meteorological activity to take place, such as cloud formation, cyclonic airflow, condensation and frost, seasonal dust storms, and tornadoes (or “dust devils”). Wind-formed features such as sand dunes are also abundant, although Martian dunes appear to be more permanent than their Earthly counterparts, which gradually move as a result of the wind.

Orbital photos of Mars also show evidence of liquid flow across the surface. Whether this is caused by water is still under debate. These include twisting, branching channels resembling riverbeds; flood features such as teardrop-shaped islands downstream of plateaus and craters, and gullies running down the inside walls of large craters. One very curious feature on Mars was not discovered until topographic data from the most recent satellites showed that the northern hemisphere is generally lower than the southern hemisphere. If enough water had once existed on Mars, it would have naturally occupied this lowland basin, forming an ocean surrounding the northern ice cap. However, alternate theories suggest that some of these features may have been formed by some process involving carbon dioxide, rather than water. However, indications of carbonates and hematite were found by the two Mars Exploration Rovers, Spirit and Endeavor, in early 2004, and NASA went so far as to announce that this, along with other data from the rovers, constituted conclusive evidence of past water on Mars.

OLYMPUS MONS

GULLY CHANNELS ON MARS

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Other features reminiscent of Mercury’s Caloris Basin and antipodal jumbled terrain have been found on Mars, relating to two large impact basins, Hellas and Argyre. Exactly opposite each one of these basins are Tharsis and Elysium, the two most volcanic areas on Mars, as if the impacts also resulted in volcanic bulges on the opposite side of the planet.

Impacts of another kind resulted in the transport of Martian material to Earth. Twenty-eight Martian meteorites are known, identified by the similarity of gas bubbles embedded within them to the composition of Mars’ atmosphere as analyzed by the Viking landers in 1976. These are thought to have been chipped off by asteroids that struck the planet at shallow angles, chipping pieces from the crust with enough force to send them into space. Odd mineral structures found inside some of them have scientists debating whether they have discovered fossilized evidence of ancient microorganisms on Mars. This remains an ongoing controversy, and few scientists accept this interpretation

Mars has two tiny moons, Phobos and Deimos. Phobos circles Mars so rapidly that it would be seen to rise in the west and set in the east. Their orbits are unstable, and they will some day crash into the Mars surface.

ASTEROIDS

The asteroid belt includes tens of thousands of objects with orbits mostly between Mars and Jupiter. In general, the asteroids are too small to have atmospheres or conditions that would in any way be hospitable to life. Ceres, the largest, is only about 500 miles (805 km) in diameter, or roughly a quarter the diameter of Earth’s Moon. Ceres is large enough to be classified as a dwarf planet.

JUPITER

Moving farther out from the Sun, Jupiter is the first and largest of the jovian planets. The jovians are far enough away from the Sun so that its heat doesn’t prevent methane and ammonia from forming, and large enough so that their gravity can hang onto large amounts of hydrogen. Jupiter is 11 times the Earth’s diameter and more than 1000 times its volume, but because it’s composed mostly of the light elements hydrogen (75%) and helium (25%), it has only 318 times the Earth’s mass. Being gaseous in nature, Jupiter—like the other jovian planets—has no “geology” to speak of, but it does have a huge family of fascinating moons, several of which are larger than Earth’s. At last

SPIRIT’S WINTER ROOST 2006

STORM ON JUPITER


count, Jupiter has 63 satellites, named after mythological characters associated in one way or another with the Roman god Jupiter. Some of these are only tentatively identified, and their verification requires further observation by astronomers. Some orbit in the opposite direction as the others, indicating that they are captured asteroids, like the moons of Mars.

The largest of Jupiter’s moons are easily visible through small telescopes and were first seen by Galileo in 1610. Hence, they’re collectively known as the Galilean Moons. Ganymede is the largest moon in the entire solar system, being slightly larger than even the planet Mercury. Callisto, the outermost of the Galilean Moons, is one of the most heavily-cratered bodies in the entire solar system. Europa is believed to be covered by a relatively recent shell of ice, and high-resolution photos taken by the Galileo spacecraft show features that look very similar to ice-rafts that are seen in Arctic ice packs on Earth. Scientists think that a mantle of liquid water lies beneath the crust, amounting to more water than there is on Earth, and some speculate that internal stresses caused by the gravity of Jupiter and its other moons may warm the water to the point where conditions may be hospitable to life. Similar gravitational stresses explain the internal heat that drives the volcanoes of Io, the innermost of the Galilean Moons. Io is the most volcanically-active body in the solar system, and at least eight volcanoes were photographed erupting when the Voyager 1 spacecraft passed Io in 1979. The eruption plumes are 185 miles (300 km) high. Io’s surface is covered with calderas, long “lava” flows and volcanic vents.

Jupiter has rings like Saturn’s but they are much smaller and fainter than Saturn’s. The jovian rings reflect only about 5% of the sunlight that falls on them. In July, 1994 pieces of a comet struck Jupiter, with spectacular results. Each collision released a fireball of enormous energy, and the collision spots on the planet remained dark for weeks.

SATURN

Saturn is the sixth planet from the sun and the second largest. Like Jupiter it is about 75% hydrogen and 25% helium by weight. It is the least dense of the planets at 0.7 grams per cubic centimeter, less dense than water.

Saturn is justly renowned for its magnificent rings. These were first observed by Galileo in 1610, but his telescope was not powerful enough to fully resolve them. In 1659 Christiaan Huygens first established the geometry of the rings.

THE GALILEAN MOONS: GANYMEDE, CALLISTO, IO, EUROPA

35Earth and Space

Three major rings are observable from Earth, and spacecraft have observed many others. The rings are composed of small particles, mostly water ice, and are less than 0.6 mile (1 km) thick.

Saturn has at least 60 moons. These are mostly icy objects, and many show evidence of catastrophic or near-catastrophic impacts that in one case fragmented the moon into small pieces; others came very close, showing huge craters and fractures that run all the way around their circumferences. One moon has strange, dark material covering one half of its surface. The largest of Saturn’s moons, Titan, is the second largest moon in the solar system and is larger than Mercury. Titan’s atmosphere is thick and hazy, and blocks our view of the surface. The Cassini spacecraft parachuted a lander called Huygens into Titan’s atmosphere and onto the surface in 2005.

URANUS

Uranus is unique among the planets in that its axis of rotation is tilted 97°, so it seemingly rotates on its side. This causes very unusual seasons on the planet and its moons during its 84-year long orbit: each pole receives continuous sunlight for 42 years! Uranus has 27 known moons, named after characters from the writings of William Shakespeare and Alexander Pope. Like the moons of Saturn, these are mostly icy bodies and show signs of heavy bombardment. One, called Miranda, may have been shattered to pieces and reassembled by gravity, resulting in a patchwork of surface textures. Another theory is that its terrain may have been caused by the upwelling of partially melted ices.

Uranus, like Jupiter and Neptune, has dark rings of unknown composition.

NEPTUNE

Neptune’s discovery was a triumph of Newtonian physics. Astronomers had noticed that the orbit of Uranus did not fully obey Newton’s laws. John Couch Adams and Urbain Le Verrier independently predicted the existence and location of another planet, and Neptune was discovered on September 23, 1846, very close to the predicted position. Ironically, Galileo twice observed Neptune during 1613 but failed to identify it as a planet.

Neptune is the most distant planet that we have seen up-close, thanks to the Voyager 2 spacecraft in 1989. Nearly identical in size

and mass to Uranus, Neptune is the last of the giant gas planets, and has 13 known moons. The largest of these is Triton, which orbits in the opposite direction as the other moons, indicating that it didn’t form with the planet. It has some very unusual textures on its surface, one type called “cantaloupe terrain,” and it has an atmosphere dense enough for wind to exist. Triton also has “ice volcanoes,” giving rise to

HUBBLE CAPTURES RINGS OF URANUS

NEPTUNE ON TRITON’S HORIZON


plumes 5 miles (8 km) above the surface. These may be made of liquid nitrogen or methane compounds. Neptune, like Jupiter and Uranus, has dark rings of unknown composition.

KUIPER BELT OBJECTS

Recent discoveries have revealed that Pluto is actually a member of a cometary reservoir just beyond the orbit of Neptune called the Kuiper Belt. Kuiper Belt Objects (KBOs) are mostly made up of frozen volatiles such as methane, ammonia and water.

Pluto and several recently discovered KBOs are large enough to qualify as dwarf planets. Eris is a dwarf planet that is larger than Pluto. Other large KBOs are Quaoar, Sedna, Varuna and Orcus. Pluto has three moons, and Eris has at least one. Pluto’s moon Charon is itself a large KBO.

The number of known KBOs is greater than 1,000. More than 70,000 KBOs with diameters greater than 60 miles (100 km) are believed to exist, and there are huge numbers of smaller objects. The Kuiper Belt is where short-period comets are believed to originate. Long-period comets are thought to reside in a hypothetical halo of comets surrounding the Sun called the Oort Cloud, that may stretch as much as 2 light years out from the Sun. The Oort Cloud is believed to be the source of long-period comets. Short-period comets originate in the Kuiper Belt.

PLANETS BEYOND THE SOLAR SYSTEM

Since 1995 the number of known planets has included objects located outside our solar system, or “extrasolar planets.” By studying the light of nearby stars that are similar to the Sun, astronomers have compiled data showing that some of those stars wobble rhythmically. This wobble, they have deduced, is caused by the gravitational pull of a massive planetary body (or bodies) orbiting the stars. As of January 2008, 271 such planets have been identified—most in the mass-range Jupiter and above—and these are only the largest (and easiest) to find. If those planets are like the jovians in our solar system and are within their stars’ “habitable zones,” any moons they might have may have conditions hospitable to life.

Small, Earth-sized planets or moons of the large planets are beyond current technology’s ability to detect directly. The Kepler spacecraft may change this. Kepler is designed to spend 4 years observing about 100,000 stars and trying to observe the slight drop in the stars’ light as planets move in front of them, rather like a transit of Venus or Mercury as seen from Earth. However, Kepler’s sensitivity should allow it to detect light-intensity changes caused by planets as small as Earth—this is comparable to detecting the change in light level as a gnat flies in front of a car headlight.

From the foregoing, we can see that the “world of nature” consists of far more than just one world. If you add the number of known extrasolar planets to the number of planets and moons in our solar system, the total as of May 2008 comes to 620. That’s 620 worlds of varying sizes, masses, and composition—so far. This diversity of planets and moons shows us what’s possible in this immense Universe and gives us a greater range of environments to consider as potential habitats for life . . . or not, and by learning more about what’s possible, we can ultimately come to understand more about our own planet and the conditions that make life possible on its surface.

37Earth and Space

A TOUR OF THE UNIVERSE BEYOND THE SOLAR SYSTEM

There is nothing new to be discovered in physics now. All that remains is more and more precise measurements. Lord Kelvin (William Thomson), 1900 Not only is the universe stranger than we imagine, it is stranger than we can imagine.

Sir Arthur Eddington

Look at a clear night sky, with no Earth-bound pollution of any kind and no moon to create light pollution. Our eyes can see 5,000–6,000 stars of various brightnesses, a few planets, occasional meteors, and the Milky Way. Rarely, a comet may appear, and even more rarely, a bright star may appear seemingly out of nowhere.

In the 1800s Newtonian physics was hugely successful at describing the motions of the planets and comets. Still, there were some unresolved problems. In the late 1800s the velocity of light was found to be constant regardless of the motion of the source or the observer. The motion of the planet Mercury couldn’t be predicted precisely by Newtonian physics. But perhaps these observations were due to imprecise measurements. Many physicists at that time held the same opinion as that expressed by Lord Kelvin in 1900.

Compare this to some of the known weird wonders of today: particles so tiny that trillions of them pass through our bodies every second; enormous celestial bodies that cannot be seen; compact objects brighter than most galaxies; objects more massive than the sun that spin hundreds of times per second; and—73% of the universe is made up of something called “dark energy” the physical nature of which is still unknown.

SOMBRERO GALAXY


STARS

Most stars are large objects consisting mainly of hydrogen, helium and small amounts of heavier substances. The energy created by stars results from the sustained “fusion” of four atoms of

hydrogen to create one helium atom. One helium atom, however, has only 99.28% the mass of four hydrogen atoms. The missing mass is converted to huge amounts of energy according to E = mc2.

Most stars are similar to planets like Jupiter, which is also made up mostly of hydrogen and helium. But Jupiter is too small to fuse hydrogen and thus does not produce energy like a star. It would have to be 80 times more massive to fuse hydrogen.

The temperature at the sun’s core is 28,000,000 °F (15,600,000 °C) and the pressure is 250 billion times greater than our atmosphere at sea level! Only conditions this extreme can get hydrogen atoms to collide fast enough to fuse together. Among the byproducts of the fusion reactions are tiny particles called neutrinos. Trillions of these pass through our bodies every second! Since neutrinos travel at nearly the speed of light and interact very little with matter, they are very difficult to detect, but physicists believe that they may lead to a better understanding of dark matter.

There are many different kinds of stars, depending upon their size and age:

BROWN DWARFS

Brown dwarfs are at least 13 times the mass of Jupiter (13 Mj) but smaller than 80 Mj. They are too small to fuse hydrogen, but are large enough to fuse deuterium (heavy hydrogen) into helium. There is only a small amount of deuterium to begin with, so this type of fusion quickly sputters out.

Astronomers suspect that there are large numbers of brown dwarfs in our Milky Way galaxy. Because they are small and dim they are very hard to detect.

RED DWARFS

Red dwarfs are small stars that are large enough to fuse hydrogen. They range from 0.075 the mass of the Sun (solar mass) to about 0.4 solar mass.

Red dwarfs are—by far—the most common stars. But if you go outside on a very clear night with no smog or light pollution, you won’t see any red dwarfs. They are too faint! Because of their small size they fuse hydrogen very, very slowly, and thus don’t produce much energy. The slow fusion rate guarantees that red dwarfs will live for a very long time—perhaps trillions of years.

STAR-FORMING REGION IN THE LARGE MAGELLANIC CLOUD

PLANET IN ORBIT AROUND A RED DWARF(ARTIST’S CONCEPTION)

39Earth and Space

WHITE DWARFS

Stars end their lives as white dwarfs if the star is smaller than 8 to 12 solar masses. When fusion reactions stop, the star shrinks under its own gravity to a very small size because it no longer produces energy to counterbalance gravity. White dwarfs shine only by stored heat, not by fusion reactions. About 9% of all stars are white dwarfs.

Sirius, the brightest star in the night sky, has a white dwarf companion. “Sirius B” has slightly more mass than our Sun, compressed into a volume smaller than Earth’s! One tablespoon of this stuff would weigh over 60 tons!

BLACK DWARFS

White dwarfs cool continuously. As they cool, their color will change from white to yellow to red, and eventually become too dark to see. This end product is called a black dwarf. The cooling process is believed to take a very long time—much longer than the current age of the universe. So black dwarfs do not yet exist!

RED GIANTS

When stars near their end-of-life they swell into the red giant phase. Red giants are enormous in size. Our Sun will reach this phase in about 5 billion years. It will swell to approximately the size of Earth’s orbit! Mercury, Venus and possibly Earth will be consumed. Even if Earth is not consumed, life will have been extinguished by the ferocious energy output of the swollen Sun.

A famous example of a red Giant is Betelgeuse, in the constellation Orion.

BLUE GIANTS

Blue giants are extremely hot, large stars that “burn” their nuclear fuel at a prodigious rate. An example is Rigel, the hot blue star in the constellation Orion. Rigel produces 41,000 times as much energy as the Sun. Even though it is 17 times the mass of the Sun it will burn through its nuclear fuel very rapidly and has a short life expectancy. Blue Giants live in the fast lane and die young, in a spectacular supernova explosion. They are quite rare—about one star in 10,000 is a blue giant.

It is interesting to compare Rigel with Proxima Centauri, a red dwarf that is the closest star to Earth, other than the Sun. Rigel is 780 light-years from the Earth; Proxima Centauri is only 4.2 light years away. (One light year is the distance that light travels in one year, almost six trillion miles.) Yet Rigel is one of the brightest stars in the sky; Proxima Centauri is so faint that it can only be seen by large telescopes. The difference in energy production is enormous; Rigel produces 730 million times as much energy as Proxima Centauri.

PORTRAIT OF BETELGEUSE BY HUBBLE


SUPERNOVAS

In the year 1054 Chinese observers noticed a “Guest Star” in what is today the constellation Taurus. This star was so bright that it was easily visible in the daytime sky for about a month. It eventually faded from view but remained visible in the night sky for approximately two years.

Supernovas are titanic end-of-life explosions of very large stars like blue giants. The energy released by these explosions can, for a short time, equal that produced by an entire galaxy of billions of stars. The ferocious nuclear reactions that occur before and during a supernova explosion produce heavy elements that are essential to life. Other than hydrogen, all the elements (like carbon) that make up living things were forged by stars. Supernova explosions scatter these into space, where they can be incorporated into newer stars and their associated planets. Supernova explosions often leave a core remnant with strange, exotic properties.

PULSARS AND NEUTRON STARS

In 1967 Jocelyn Bell, a graduate student at Cambridge University, was observing the sky with a radio telescope. She noticed something that was emitting pulses of radio waves once every 3.4 seconds. The time interval between pulses was so precise that it could be used as a clock.

Today we call these objects pulsars, and more than 1,500 are known. Only a very small, spinning object could produce the precise pulses. All pulsars are neutron stars.

Neutron stars make white dwarfs seem like puffballs. White dwarfs pack a stellar-sized mass into a volume like the Earth’s. By comparison a neutron star packs the same mass into a sphere the width of New York City! A thimbleful of its material would weigh 100 million tons. Its gravitational pull would be so strong that a human weighing 150 pounds on Earth would weigh 10 billion tons on the surface of a neutron star. Unfortunately the gravity at the surface is so intense that the human would be squashed into a layer thinner than a piece of paper.

The pulsar discovered by Jocelyn Bell spins at a very leisurely rate—a little more than once per second. Some pulsars, more massive then the Sun, spin almost 1,000 times per second! At that rate, a point on such a star’s equator is moving at more than 20 percent the speed of light.

BLACK HOLES

A black hole is a region of space where gravity is so powerful that nothing, not even light, can escape. The boundary is called the event horizon.

Black holes cannot be observed directly. Their existence is shown by their gravitational impact on

SUPERNOVA REMMANT

41Earth and Space

the motion of objects near to them. For example, the center of our Milky Way Galaxy is estimated to have a mass of 3.7 million Suns in a region smaller than the orbit of the planet Neptune. This object can only be a black hole.

QUASARS

In the early 1960s astronomers noted a number of radio sources for which—at first—no visible counterpart could be found. Astronomers eventually found two visible counterparts, and realized that they were very remote objects moving away from us at enormous speeds. Moreover they were incredibly bright—generating the energy of thousands of galaxies. More than 30,000 quasars are now known.

Astronomers believe that quasars are the active cores of very distant—and therefore very young—galaxies. They give off enormous amounts of energy; they can be a trillion times brighter than the Sun! Quasars are believed to produce their energy from massive black holes in the center of the galaxies in which they are located.

CRAB NEBULA

MASSIVE BLACK HOLE AT THE CENTER OF A GALAXY(ARTIST’S CONCEPTION)


A FEW SUGGESTED WEBSITES

History of Life

http://www.ucmp.berkeley.edu/exhibits/geologictime.php (A tour of geologic time with the University of California Museum of Paleontology-UCMP)

http://www.scotese.com/earth.htm (Paleomap Project: a fascinating look at how Earth has changed over time due to the shifting of tectonic plates, and excellent narrative—click on the top left “More Info” on pages for each time period on left)

Fossil Formation

http://www.ucmp.berkeley.edu/paleo/fossils/ (More information from UCMP about how fossils are formed)

Plate Tectonics

http://pubs.usgs.gov/gip/dynamic/dynamic.html (Excellent explanation of the what and why of plate tectonics)

http://visionlearning.com/library/module_viewer2.php?mid=65&l=&let1=Ear

http://visionlearning.com/library/module_viewer.php?mid=66&l=11

Evolution

http://evolution.berkeley.edu/evolibrary/article/0_0_0/evo_toc_01 (Excellent overview of mechanics and issues of evolution – UC Museum of Paleontology)

http://www.agiweb.org/news/evolution.pdf (An overview of the mechanics and evidence for evolution - American Geophysical Institute)

Space

http://www.nasa.gov/home/index.html (NASA’s website – explore!)

http://www.nineplanets.org/ (Some good, basic information about the solar system)

43Earth and Space

PICTURE CREDITS

Cover: (left) NASA, (right) National Aeronautic and Space Agency (NASA)-Hubble Heritage Team; p 3: NASA; p 5:

NASA; p 8: (left) NASA, (right) NASA; p 10: United States Geological Survey (USGS); p 11: USGS; p 12: USGS; p 13 USGS; p

15 all public domain; p 17: Sandy Linder; p 18: (left) Sandy Linder, (right) public domain; p 19 (top) Sandy Linder, (bottom)

public domain; p 20: public domain; p 25: (top) NASA, (left) NASA; p 26 NASA; p 27 NASA; p 28: NASA, p 29: (top right)

NASA Venus Pioneer Orbiter 1979, (bottom left) NASA Hubble Heritage Team; p 30: (top) NASA/JPL, (bottom right) NASA/

JPL/University of Arizona; p 31: (top right) NASA/JPL/Cornell, (bottom left) NASA, ESA, I. de Pater and M. Wong; p 32:

(top) NASA/JPL, (bottom) NASA, ESA, J. Clarke (Boston University), and Z. Levay (STScI), p 33 (top right) NASA and Erich

Karkoschka, University of Arizona, (bottom left) NASA; p 35 NASA; p 36 (top left) NASA, ESA, (bottom right) NASA; p 37:

ndrea Dupree (Harvard-Smithsonian CfA), Ronald Gilliland (STScI), NASA and ESA; p 38: NASA, ESA, HEIC, and the Hubble

Heritage Team STScI-AURA; p 39 (top left) NASA, ESA, HEIC, and the Hubble Heritage Team STScI-AURA, (bottom right)

NASA, ESA, and J. Hester.

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