Life in the Universe

“There are infinite worlds both like and unlike this world of ours...We must believe that in all worlds there are living creatures and plants and other things we see in this world.”--- Epicurus (c. 300 B.C)

"There are countless suns and countless earths all rotating around their suns in exactly the same way as the seven planets of our system. We see only the suns because they are the largest bodies and are luminous, but their planets remain invisible to us because they are smaller and non-luminous. The countless worlds in the universe are no worse and no less inhabited than our Earth”Giordano Bruno (1584) in De L'infinito Universo E Mondi

Fossil Evidence of Life

Because of impacts, volcanism, plate tectonics, and erosion, there is almost no record of the early Earth (4.5-4 billion yrs.)

Last Universal Common Ancestor

The analysis of genes indicates that all known life forms on Earth have descended from one organism known as the last universal common ancestor (LUCA), the root of the “tree of life”, which means that either• life arose only once on Earth, and so life arises rarelyor• multiple life forms arose, but the others became extinct due to competition or a catastrophe (large impact)

Fossil Evidence of Life

Microfossils of cyanobacteria are the earliest evidence of life (3.5 billion years). No information on the nature of life prior to that.

Was Life Born on Earth?

In the Miller-Urey experiment, a mixture of gases (NH3, CH4, CO2, H2O) similar to the primordial atmosphere of the Earth was exposed to electricity (lightning). After 1 week, amino acids and other complex molecules formed, which are the building blocks of life.

Or Did Life Arrive from Space?In 1969, a meteorite fell near the town of Murhison, Australia, that contained organic molecules and amino acids. Life could have formed on other bodies in the solar system (asteroids, comets, planets), which was then transferred to the Earth when those bodies experienced impacts.

Organic Molecules

Living organisms on Earth are based on molecules consisting of chains and rings of carbon atoms with H, O, and N. These are carbon-based molecules are called organic molecules.

Why Carbon?

Carbon is a versatile building block. It can form many complex molecules. C can bond to itself and many other atoms in long chains and rings. C has 4 bonds , while O has 2 and N has 3. Silicon has 4 bonds, but these tend to be either too strong or too weak for complex chemistry.

How Common are Organic Materials?

C and the 3 other major elements of organic molecules (H, N, O) are 4/5 most common elements in the universe.

Organic molecules are common in interstellar clouds, comets, asteroids, other planets, etc.

So organic elements and molecules are a promising basis for alien life.

The Role of Water

Life on Earth relies on the complex chemistry of carbon-based molecules reacting in a solvent of liquid water (H2O). A medium like water is crucial for the interaction and development of organic molecules.

The Role of Water

Water has other properties that are helpful for life.• Water’s solid form is lighter than its liquid form, so that oceans do not freeze solid.• In gaseous phase, it often forms ozone (O3), which shields the planet’s surface from destructive ultraviolet light.• Like organic molecules, water is relatively common in the universe.

Mars

Enceladus

Substitutes for Water in Alien Life?

• CH4 (methane) does not form a liquid under convenient pressures. • C2H4 (ethane) is the likely liquid recently found on the surface of Titan. It conceivably could be a solvent for life. • NH3 (ammonia) is a possible substitute but has many disadvantages (e.g. narrow range of temperatures in liquid phase) compared to H2O.

The Habitable ZoneThe range of distances from a star in which liquid water can survive on a planet’s surface is called the Habitable Zone. A planet in this zone has a better chance for producing life.

The Habitable ZoneThe distance of the habitable zone from a star depends on the star’s luminosity. For a brighter star, the temperature at a given location near it is higher, so the habitable zone is farther from the star.

The Carbonate-Silicate CycleBut the distance of a planet from a star is not the only factor in controlling surface temperature. CO2 in the atmosphere influences temperature through the greenhouse effect, and the amount of CO2 is regulated on Earth by the carbonate-silicate cycle, which is the transfer of CO2 from the air, to ocean water, to sea floors, and back into the air by volcanos.

The Carbonate-Silicate CycleWithout geologic activity and plate tectonics all of the CO2 in the atmosphere would eventually become locked up in the ocean floor, causing a planet to become very cold. This probably what happened to Mars, which has a low mass and therefore less internal heat and geologic activity.

Magnetic Fields Protect LifeIf a planet is massive enough, it will have a liquid metal core (like the Earth), and this generates a magnetic field. A magnetic field helps protect a planet’s surface from the destructive solar wind.

Not too big, not too smallFinally, a small planet has weaker gravity, making it harder to hold onto an atmosphere. But if the planet is too large, its gravity is strong enough to attract so much gas that it becomes a gas giant planet.

Favorable Conditions for Life• organic molecules• liquid water (in the Habitable Zone)• planet with large enough mass to

• retain an atmosphere• generate a magnetic field for protection against solar

wind• undergo plate tectonics

• planet with small enough mass so that it doesn’t attract too much gas and become a gas giant• stable conditions for a long time• source of energy (sunlight, geothermal)

Effects of the Moon on the EarthThis impact that created the Moon may have caused the Earth’s spin axis to become tilted. So we might not have seasons if it wasn’t for this collision. The Moon has several effects on the Earth that are probably beneficial for life. Without the Moon:

• There would be no lunar tides in the ocean. Only the much smaller tides from the Sun would remain. Tides may have helped wash minerals into the ocean that were needed for the formation of life.

• The Earth’s spin would not have slowed down, and the day would be as short as when the Earth was born (6 hours). With such fast rotation, the atmosphere would have much stronger winds, producing stronger ocean waves. Early organisms would have taken more of pounding.

• The tilt of the Earth’s axis would have been unstable, and could have changed drastically from time to time, which would have produced huge climate changes and earthquakes.

Promising Locations for Alien Life in our Solar System

Europa

TitanMars

CallistoGanymede

Enceladus

Detecting Life in other Solar SystemsThe presence of life on Earth has a distinct effect on the composition of its atmosphere. It might be possible to deduce the presence of life on planets around other stars by measuring their atmospheric compositions with spectroscopy.