Homework #4 has been posted, due Tuesday, Oct. 13, 11 pm

Building the Planets. ICOLLAPSE OF PROTOSTELLAR CLOUD INTO A ROTATING DISK

Composition of disk:

98% hydrogen and helium 2% heavier elements (carbon, nitrogen, oxygen, silicon, iron, etc.).

Most of this was in gaseous form!

Building the Planets. II

There was a range of temperatures in the proto-solar disk, decreasing outwards

Condensation: the formation of solid or liquid particles from a cloud of gas (from gas to solid or liquid phase)

Different kinds of planets and satellites were formed out of different condensates

Building the Planets. III Accretion

Accretion is growing by colliding and stickingThe growing objects formed by accretion – planetesimals (“pieces of planets”)

Small planetesimals came in a variety of shapes, reflected in many small asteroids

Large planetesimals (>100 km across) became spherical due to the force of gravity

In the inner solar system (interior to the frost line), planetesimals grew by accretion into the Terrestrial planets.

In the outer solar system (exterior to the frost line), accretion was not the final mechanism for planet building – nebular capture followed once accretion of planetesimals built a sufficiently massive protoplanet.

Building the Planets. IV. Nebular Capture

Nebular capture – growth of icy planetesimals by capturing larger amounts of hydrogen and helium. Led to the formation of the Jovian planets

Numerous moons were formed by the same processes that formed the proto-planetary disk

Condensation and accretion created “mini-solar systems” around each Jovian planet

Building the Planets. V.

Expulsion of remaining gas

The Solar wind is a flow of charged particles ejected by the Sun in all directions. It was stronger when the Sun was young. The wind swept out a lot of the remaining gas

Building the Planets. VI.

Period of Massive Bombardment

Planetesimals remaining after the clearing of the solar nebula became comets and asteroids

Rocky leftovers became asteroids

Icy leftovers became comets

Many of them impacted on objects within the solar system during first few 100 million years (period of massive bombardment - creation of ubiquitous craters).

BriefSummary

To aid in our search for life and suitable environments, we will be examining various timescales of importance, e.g.,

How old is the Earth

How long did it take for the Earth to develop an oxygen atmosphere

How long did it take for life to form on Earth

Many others…

How do we determine ages like these?

Radioactive dating

Recall that some isotopes of an element are unstable and will decay into another element.

Parent: the atoms of the original unstable isotope

Daughter: atoms of the element that results from the decay of the parent

Parent decays into daughter

Half-Life: the Time it Takes for Half of the Original (Parent) Atoms in a Sample to Decay to a "Daughter"

Product

Examples of radioactive isotopes useful in dating

Parent Daughter Half Change in...

Carbon-14 Nitrogen-14 5730 years

Uranium-235 Lead-207 704 million years

Uranium-238 Lead-206 4,470 million years

Potassium-40 Argon-40 1,280 million years

Thorium-232 Lead-208 14,010 million years

Rubidium-87 Strontium-87 48,800 million years

Life depends critically on environment. We will examine how life-friendly

environments can form in the universe.

TemperatureLiquids (particularly H2O) Sources of EnergyChemical environmentRadiation environment

Fundamentals:

What determines the environments of terrestrial-like planets? A look at:

(much of what follows also has applications to Jovian moons).

interiors surfaces

atmospheres

What accounts for interior, surface, and atmospheric structures?

Terrestrial planets are mostly made of rocky materials (with some metals) that can deform and flow.

Likewise, the larger moons of the Jovian planets are made largely of icy materials (with some rocks and metals) that can deform and flow.

The ability to deform and flow leads every object exceeding approximately 500 km in diameter to become spherical under the influence of gravity.

Early in their existence, the Terrestrial planets and the large moons had an extended period when they were mostly molten.

The heating that led to this condition was caused by impacts, where the kinetic energy of the impacting material was converted to thermal energy.

Today, the interiors of planets are heated mainly by radioactive decay.

Differentiation – the process by which gravity separates materials according to their densities

Denser materials sink, less dense material “float” towards top

Layering of interiors by density due to differentiation

Terrestrial planets and many large moon had an extended period where their interiors were “molten”.

During this time, denser material sank towards center of planet while less dense material “floated” towards top

Terrestrial planets have metallic cores (which may or may not be molten) & rocky mantles

Earth (solid inner, molten outer core)

Mercury (solid core)Earth’s interior structure

Differentiated Jovian moons have rocky cores & icy

mantles

Europa Io

CallistoGanymeade

Layering by strength

(mantle)

The Lithosphere…

Layer of rigid rock (crust plus upper mantle) that floats on softer (mantle) rock below

While interior rock is mostly solid, at high pressures stresses can cause rock to deform and flow (think of silly putty)

This is why we have spherical planets/moons

The interiors of the terrestrial planets slowly cool as their heat escapes.

Interior cooling gradually makes the lithosphere thicker and moves molten rocks deeper.

Larger planets take longer to cool, and thus:

1) retain molten cores longer

2) have thinner (weaker) lithospheres

The stronger (thicker) the lithosphere, the less geological activity the planet exhibits.

Planets with cooler interiors have thicker lithospheres.

lithospheres of the Terrestrial planets:

Geological activity is driven by the thermal energy of the interior of the planet/moon

Earth has lots of geological activity today, as does Venus. Mars, Mercury and the Moon have little to no geological activity (today)

This has important repercussions for life:1) Outgassing produces atmosphere2) Magnetic fields (need molten cores)

protect planet surface from high energy particles from a stellar wind.

Larger planets stay hot longer.

Earth and Venus (larger) have continued to cool over the lifetime of the solar system thin lithosphere, lots of geological activity

Mercury, Mars and Moon (smaller) have cooled earlier thicker lithospheres, little to no geological activity

Initially, accretion provided the dominant source of heating.

Very early in a terrestrial planet’s life, it is largely molten (differentiation takes place).

Today, the high temperatures inside the planets are due to residual heat of formation and radioactive decay heating.

Stresses in the lithosphere lead to “geological activity” (e.g., volcanoes, mountains, earthquakes, rifts, …) and, through outgassing, leads to the formation and maintenance of atmospheres.

Cooling of planetary interiors (energy transported from the planetary interior to the surface) creates these stresses

Convection is the main cooling process for planets with warm interiors.

Convection - the transfer of thermal energy in which hot material expands and rises while cooler material contracts and falls (e.g., boiling water).

Convection is the main cooling process for planets with warm interiors.

Side effect of hot interiors - global planetary magnetic fields

Requirements:

• Interior region of electrically conducting fluid (e.g., molten iron, salty water)

• Convection in this fluid layer

• “rapid” rotation of planet/moon

Earth fits requirements

Venus rotates too slowly

Mercury, Mars & the Moon lack molten metallic cores

Sun has strong field

Planetary Surfaces4 major processes affect planetary surfaces:

Impact cratering – from collisions with asteroids and comets

Volcanism – eruption of molten rocks

Tectonics – disruption of a planet's surface by internal stresses

Erosion – wearing down or building up geological feature by wind, water, ice, etc.

Impact Cratering: The most common geological process shaping the surfaces of rigid objects in the solar system (Terrestrial

planets, moon, asteroids)

Volcanism

Volcanoes help erase impact craters

Volcanic outgassing: source of atmospheres and water

Erosion: the breakdown and transport of rocks and soil by an atmosphere.

Wind, rain, rivers, glaciers contribute to erosion.

Erosion can build new formations: sand dunes, river deltas, deep valleys).

Erosion is significant only on planets with substantial atmospheres.

Tectonics: refers to the action of internal forces and stresses on the lithosphere to create surface features.

Tectonics can only occur on planets or moons with convection in the mantle

Earth & Venus Jupiter’s moons Europa & Ganymede?

http://www.astro.indiana.edu/~classweb/a100s1275/Chapter10/Applets/tectonics_convect_of_mantle.htm

Tectonics…

•raises mountains

•creates huge valleys (rifts) and cliffs

•creates new crust

•moves large segments of the lithosphere (plate tectonics)

Portion of Valles Marineris on Mars – created by tectonic

stresses

Tectonic plates

divergent plate boundary (plates move away from each other).

Atlantic Ocean

Great Rift Valley in Africa

Valles Marineris (Mars)

Portion of Valles Marineris on Mars – created by tectonic stresses

convergent plate boundary with subduction : plates move towards each other & one slides beneath the other.

Nazca plate being subducted under the South American plate to form the Andes Mountain Chain.

Island arc system

convergent plate boundary without subduction : plates move towards each other and compress.

Formation of Himalayas.

Plates sliding past each other: earthquakes, valleys, mountain building

Tectonic plates

Half of the world’s volcanoes surround the Pacific plate