Exam Review Notes

An object named Theia collided with Earth millions of years after its formation when Earth was still molten.
The collision ejected material into orbit around Earth, which coalesced over time to form the Moon.
The Earth's early atmosphere contained a lot of greenhouse gases to maintain liquid water on the surface, addressing the faint young Sun paradox.

Probability of Planets: Statistically, on average, each star has at least one planet.
The second variable in the Drake equation, $f_p$ , represents the fraction of stars with planets.
Stellar Luminosity: Stars increase in luminosity over their lifetime; therefore, the Sun was dimmer in the past.
Protoplanetary Disks: Primarily composed of hydrogen and helium (over 99%), with less than 1% consisting of rocks, metals, and other materials.

Early Life Records: The earliest evidence of life on Earth comes from carbon isotope data found in zircons, dating back approximately 4.2 billion years.
Mars Meteorite: Initial hopes that a meteorite from Mars contained evidence of past life have been debunked; subsequent investigations revealed false positives.
Europa's Oceans: Speculation exists regarding large amounts of liquid water oceans beneath Europa, one of Jupiter's moons.
Fermi Paradox: The Fermi Paradox questions why, given the high probability of extraterrestrial life, we haven't observed any evidence of it.
Rare Earth hypothesis: Suggests that the evolution of intelligent life is rare

The snow line is the boundary in a protoplanetary disc where hydrogen compounds can condense into ice.
In our solar system, the snow line is located between Mars and Jupiter.
Volatiles can only condense into liquid or solid form beyond this snow line where temperatures are cold enough.

Isotopes of an element differ in the number of neutrons in the nucleus.
Isotopes share the same number of protons and electrons, defining their chemical element.

The GOE occurred approximately 2.7 billion years ago, marking a significant increase in oxygen concentration in Earth's atmosphere.
Before this event, only trace amounts of oxygen were present.
- The sun will not experience the same evolution as early Earth to increase oxygen.

The Kepler mission was the first dedicated space telescope for exoplanet science.
The Keck Observatory in Hawaii houses twin 10-meter telescopes, among the largest in the world, used for astronomical observations.
Telescopes are built at high elevations to minimize atmospheric interference and humidity.

Upwelling: Returns nutrients to the surface ocean, supporting marine life.
Nutrient concentration in the ocean increases with depth due to biological activity at the surface.
Nuclear Fusion: Stars generate energy in their cores through nuclear fusion, combining lighter elements into heavier ones.
Respiration: Releases carbon dioxide ( $CO_2$ ), not oxygen, contrasting with photosynthesis.
Cell Types: Prokaryotes are simpler cell forms without a nucleus, while eukaryotes are more complex.

Variables in the Drake equation are not increasingly well-known from left to right; the opposite is true.
We have better knowledge of the earlier variables (star formation rate, fraction of stars with planets) compared to later variables (likelihood of life evolving intelligence).

Many researchers propose that life began on Earth in hydrothermal vents on the deep sea floor.

Stars maintain stability through a balance between outward pressure from nuclear fusion and inward gravity.
Animals do not form biofilms, but bacteria do today and the earliest records of life are NOT animals
Stromatolites are still forming on Earth, albeit slowly.

Expansion of Universe affects the concentration of elements.
Nuclear fusion converts hydrogen to helium and helium to heavier bio-essential elements.
The abundance of hydrogen decreases over time due to fusion.
Hypotheses for the origin of self-replicating molecules include both RNA world and metabolism-first scenarios.
Heavy Element Production: High-mass stars can produce elements up to iron in their cores; iron is the stopping point for fusion.

A star is defined as being on the main sequence when it is fusing hydrogen into helium in its core.

Infrared radiation is trapped by greenhouse gases in the atmosphere, warming the Earth.
Wave filter: Intestinal species may not progress due to the wave filter.
Prokaryotes are not animals, but related to the earliest animal life on Earth. Stromatolites are non-animal related as well.

The three essential ingredients for life that have been identified thus far are water, chemical building blocks, and an energy source are essential to life.

Potential moons, climate stability, and abundance of biochemical elements all affect habitability.
$CO_2$ is not the only greenhouse gas, water vapor also contributes

The location of the habitable zone around a star is defined by the star's properties (luminosity, temperature, etc.).
Earth is the next planet after Venus.
Oxygen was not abundant in Earth's earliest atmosphere; it was produced later.
Transiting exoplanet discovery through the dimming of the star from the planet orbiting.
Water is not absent on early Earth.