Astrobiology Study Guide for Exam I – Spring 2025

I. THE UNIVERSE

• What is Deep Time?

Geologic/Cosmic time, measured in billions of years.

• What is Deep Space?

Space beyond the Earth and Moon.

• What did Copernicus accomplish?

He proposed the heliocentric model for the first time.

• What did Galileo do?

He proved the heliocentric model, showed that the Milky Way was made of stars.

• Distance to the nearest star—about 4 Lt Years

• Light year = 6 trillion miles = 6,000,000,000,000 mile

• Milky Way Galaxy contains about 400 billion stars

• Universe composed of many galaxies (hundreds of billions)

• Distance to the nearest galaxy — the Andromeda Galaxy—2.2 million light years

• Most distant galaxies observed—about 13 Billion light years away

• The observable Universe—What is it, and what are its limits?

100 billion galaxies, more than 10^22 stars, most distant galaxy 13.3 billion light years away

2. ORIGIN OF THE UNIVERSE

• Age of the Universe –13.8 Billion years

• Origin of the Universe in the hot Big Bang

• Evidence for the Big Bang

• Expanding universe

• CMBR

• Redshift of distant galaxies

  • Light wave gets longer as object moves away (red long/blue short): amount tells velocity

• Hubble’s Law

The farther away a galaxy is, the faster it moves. Velocity = a*distance

• The Primordial Fireball

Early phase of the big bang: universe was hot and dense (radiation era)

• Cosmic Microwave Background Radiation

Low level of radiation that fills the universe.

• Temperature of space = +3 degrees above absolute zero

• CMB Detected by Penzias and Wilson (Bell Labs)

• Is Earth at the Center of the expanding Universe?

No, everything’s getting further apart (like a loaf of bread).

• Creation of primordial hydrogen and helium in the Big Bang

Came out of the Big Bang

• What percentages of H and He were created?

¾ H, ¼ He

• Will the Universe stop expanding and start contracting? Why or why not?

Will keep expanding, dark energy is a repulsive force

3. STARS AND THE CREATION OF THE ELEMENTS

• What is the structure of the atom?

Electron, proton, neutron. Atomic nucleus contains protons and neutrons

• The basic forces—gravitational, electromagnetic, nuclear

• How many naturally occurring elements are there in the Universe?

98

• Formation of stars by gravitational self-contraction of large gas clouds

Stellar birthplaces, e.g., gas clouds like the Orion nebula. Gas clouds contract and temperatures rise in the cores of the clouds

• Magic temperature = 10 million degrees C

• Nuclear fusion reactions can take place in stars

Protons are fused into Helium nuclei. A small amount of mass is converted to energy—that is the source of the light and heat given off by a star.

• Evolution and life history of small stars

Hydrogen "burning" (fusion of hydrogen nuclei (protons) into helium)

• 10 million degrees C

Helium "burning" (fusion of helium nuclei into carbon)

• 100 million degrees C

• Red Giant stars, White dwarf stars

Older stars become red giants, turn helium into carbon and become white dwarfs

• Evolution and life history of large stars

Large stars burn the carbon too and make up to iron, which means they will turn into supernovas and not white dwarfs.

• Creation of the Elements in Stars by nuclear fusion

Protons start to collide and fuse, turning the nuclei into helium nuclei with two protons.

• Manufacture of heavy elements in large stars by nuclear fusion reactions up to IRON (element #26). Why only up to iron? 

Iron absorbs the nuclear energy, the star explodes

• Why are the light elements much more abundant than the heavy elements? 

Heavy elements are only made in the supernova explosion itself, as the star is dying.

• Supernova explosions: Dispersal of heavy elements to space

• The Sun is a second generation star. How do we know this?

It doesn’t have heavy elements like iron.

• Why does a star’s life history depend on its size and mass?

Dictates the rate of nuclear fusion, the rate at which it burns.

• What are neutron stars (pulsars)? What are stellar black holes?

Pulsars are the rotating cores left behind by supernovas. Stellar black holes are very large stars that collapse at the end of their life and contract into a dense core with a very strong gravitational pull.

4. THE SOLAR SYSTEM

• The eight planets—distances from Sun in AU

• Terrestrial planets versus Giant planets—What are the differences?

Terrestrial planets are made of rock/iron while giant planets are made of gas, and they are much smaller.

• Age of the solar system – 4.6 Billion years. 

• Origin of the solar system from a rotating accretion disk

Evidence of this origin from current properties of the planets

Evidence from meteorites relating to the age and origin of the planets

• How did the giant planets form? 

Smaller knots of condensed matter appeared in the solar nebula’s outer regions.

• How did the terrestrial planets form?

They form from rocky material in the warm inner regions of the solar nebula.

• How can we explain the differences in planetary sizes and composition?

Solar flare-up drove gases out of the inner solar system

• The icy moons of the giant planets:

• Europa and Titan: Prospects for life?

  • Titan has an atmosphere, methane/ethane, Europa has ice

• Asteroids: Originally larger solid bodies that have been ground down to smaller size by repeated impacts with each other

• Comets: snowball-like mixtures of ice and rock that originated during the formation of  the Solar System

• Abundance of planets in the Universe based on origin from accretion discs?

Dust grains collide, will grow and lead to planets. Gas planets form if they are massive enough to capture hydrogen and helium from the disk.

5. THE MOON

• The origin of the Moon: Collision between proto-Earth and a Mars-sized body

• Evidence of origin by collision? 

• Composition of the Moon: lack of volatile elements

• Radiometric dating: 

• Parent and daughter isotopes: 

• U-Pb dating: 

• Potassium-40/Argon-40 method: 

• Lunar highlands

original crust of the moon is heavily cratered 

• Origin of lunar craters by impact 

Early heavy meteorite bombardment of an earthlike planet

• Lunar maria - large, flat, dark plains on the moon that are made of solidified lava flooded from the Moon’s interior

• How does a planet heat up?

Heat comes from radioactive decay of unstable elements (e.g. Uranium)

Heat builds up over hundreds of millions of years, and reaches the melting point of rock

• Ages of lunar rocks

Highland rocks — 4.4 billion years old—original crust of the Moon

Maria rocks — 3.8 billion years to 3.0 billion years old—lava flows, evidence of volcanic activity from the Moon’s hot interior

• Lack of lunar atmosphere, dryness, and lack of life — why? 

Size of the Moon relative to Earth

• Relationship of the size of a planet with its geologic activity When and why did the moon become inactive?

It became geologically inactive 3 billion years ago, which is when the last lunar maria rocks were formed, meaning volcanic activity created new rocks before that point only.

• Where do the gases in a planet’s atmosphere come from?

Volcanic activity.

• Why doesn’t the Moon have any atmosphere? 

Too small to have volcanic activity

6. MARS

• Size relative to Earth and Moon; Distance from the Sun—1.5 AU

• Evidence for Martian volcanism; when did volcanism die out? 

Craters, about 500 million years ago

• Evidence for past liquid water on Mars

• Winding valleys

• Branching channels

• River deltas? 

• Mars had a “Golden Age” with a thicker atmosphere and liquid water. Where is the water now?

Polar ice caps and buried ice 

• The Viking spacecraft--labeled release experiment--Results

Determined that there was no conclusive evidence of life on Mars, but they did find indications of metabolic activity.

• Why would the discovery of even primitive life on Mars be very important?

Would give us a frame of reference, something to compare Earth too.

7. VENUS

• Size relative to Earth/Distance from Sun ~0.7 AU

• Surface conditions on Venus—temperature, pressure, atmospheric composition

Very hot, toxic

• Major atmospheric constituents compared with the Earth

More CO2 & sulfur, less O2 & water, higher pressure.

• The Greenhouse effect—Explain how it works to heat a planet’s atmosphere

The atmosphere acts as an insulating blanket, trapping heat and warming the planet.

• The Carbon Cycle

Removal of carbon dioxide from a planet’s atmosphere through rock weathering

Transport of dissolved CO2 and calcium to the oceans

Precipitation of solid calcium carbonate in the oceans

Origin of limestone

• The Runaway Greenhouse Effect on Venus—Why did this happen on Venus but not on the Earth?

Earth’s rocks can absorb CO2 through silicates. The reactions need water, which Venus doesn’t have.

• What is a habitable planet?

Needs to be large and volcanically active.

• Habitable zone around the Sun: What controls the boundaries of the Habitable Zone?

Around Earth. Determined by distance from the Sun, size, and atmosphere.

8. DETECTION OF PLANETS AROUND NEARBY STARS

• Method of discovery of nearby solar systems: Spectral shifts in starlight

  • Doppler effect, red shift

• Show that planets are present and making the stars wobble

• Means a planet is nearby

• Blink method—starlight dims as planet passes by