Astrobiology Exam Notes

Unit 6, Assignments 1, 2, and 3
Topics:
- Stability of the Solar System
- Coevolution of Earth and Life
- Origin of the Universe
- Asteroid Orbits

Isotopes: Different versions of atoms of the same element, having the same number of protons but different numbers of neutrons.
Radioactive Decay: Spontaneous transformation of one element into another.
Half-life: The time taken for the radioactive decay of a specified isotope to fall to half its original value.
- Each isotope has a different half-life.
- Example: Half-life of Carbon-14 (C-14) is 5730 years.
  - Every 5730 years, approximately half of the C-14 atoms turn into Nitrogen-14.
  - Starting with 100% C-14:
    - After 1 half-life (5730 years): 50% of C-14
    - After 2 half-lives (11460 years): 25% of C-14
    - After 3 half-lives (17190 years): 12.5% of C-14
    - And so on, until one atom remains.
  - Carbon-14 changes into Nitrogen-14; Uranium-238 changes into Lead-206 because Carbon-14 and Uranium-238 are unstable.

Half-lives can be used to estimate the age of fossils and other Earth materials, a process called "dating."
Fossils and rocks from the same layers tend to have similar ages.
Fossils and rocks from deeper layers tend to be older.
Different isotopes are used depending on the age of the fossil and the materials.
- Examples:
  - Fossils are usually dated using C-14 because all once-living things were made of carbon.
  - Fossils that are too old cannot be estimated by C-14 because there is not much C-14 remaining in the fossil.

The estimated average age of the oldest minerals in our solar system is approximately 4.465 billion years.
- This suggests that our solar system is approximately 4.465 billion years old (or 4.6 billion years as scientists currently estimate).
The estimated average age of the impact melt rock samples is 3.84 billion years.
The age of impact melt rock samples is younger than the oldest minerals found on the planets of the solar system.
This suggests that during the early years of the solar system, there was a lot of large debris striking the surfaces of the young planets and moons.
Radioactive Dating is used to estimate the age of old rocks and fossils, and to understand what happened to our Earth and solar systems long before we were born!

During the Archean and early Proterozoic eons, the Earth's atmosphere had much lower oxygen levels (nearly 0%).
Around 2.4 billion years ago, cyanobacteria began to produce oxygen through photosynthesis, leading to oxygen accumulation in Earth’s atmosphere.

Archean Eon (beginning of Earth):
- Low oxygen levels allowed dissolved iron in the oceans to spread throughout the ocean and accumulate into layers.
Early Proterozoic Eon:
- When iron comes in contact with oxygen, it oxidizes the iron, preventing iron from accumulating.
Mid Proterozoic Eon:
- Around 1-2 billion years ago, atmospheric and oceanic $O_2$ levels were about 1% to 2% and remained steady for a billion years.
- This stable level of $O_2$ was due to photosynthesis and respiration being in balance during this billion-year period.
Late Proterozoic Eon:
- About 850–540 million years ago (0.85–0.54 billion years ago), atmospheric $O_2$ levels increased dramatically.
- This resulted in radical climate changes.
- With the increase in $O_2$ levels, multicelled (more complex) animals began to emerge.
Phanerozoic Eon:
- About 320–275 million years ago (0.320–0.275 billion years ago), the $O_2$ level reached its highest point (35% of the atmosphere).
- The dramatic increase in the $O_2$ levels was due to the evolution of large vascular land plants.
- The high level of $O_2$ in the atmosphere allowed larger organisms to evolve.
Modern day:
- Today, $O_2$ makes up about 21% of the atmosphere.
- Respiratory organisms use $O<em>2$ and turn it into $CO</em>2$ . The evolution of larger animals lowered the level of $O_2$ in the atmosphere.

Why do we use different isotopes for dating different materials?
- Each isotope has a different half-life.
- Example:
  - The half-life of U-235 is 713 million years.
  - The half-life of Rb-87 is 48.8 billion years.
  - U-235 can be used to estimate 2.5 billion-year-old rocks, but Rb-87 cannot because its half-life is too large on that scale.
  - Isotopes with shorter half-lives are useful when dating younger rocks/fossils.
  - Isotopes with longer half-lives are useful when dating older rocks/fossils.
What isotopes can we use to date human fossils?
- C-14 is useful when estimating human fossils that are 100 - 70,000 years old because organic materials have Carbon.
- K-40 is useful when estimating human fossils that are older (100,000 - 4.6 billion years old). This will not measure the human fossil but will allow us to measure rocks around the fossils.

Evidence for the Big Bang Theory:
- All distant stars are redshifting.
- The composition of the Universe has not changed much (mostly H and He).

As the galaxy gets further away from Earth, the absorption line wavelength increases.

Redshift: When the wavelength is increasing because the galaxy/star is getting further away from the observer.
Blueshift: When the wavelength is decreasing because the galaxy/star is getting closer to the observer.

Doppler Effect: The perceived change in frequency of sound/visible waves you perceive as the source of sound/light comes toward and away from the observer.
Redshift vs. Blueshift:
- When the galaxy is moving toward Earth:
  - Higher frequency (shorter wavelength) - blueshift
- When the galaxy is moving away from Earth:
  - Lower frequency (longer wavelength) - redshift

In the beginning, the universe was very, very small, smaller than a dot & very, very hot.
- All the galaxies were close together because the universe was so small.
Since the Big Bang, the universe has started to expand.
- The galaxies inside the Universe started to move apart from each other.
Since the Big Bang, the universe has started to cool down.

Cosmic Microwave Background Radiation (CMBR) is a remnant of the first light that could ever travel freely throughout the Universe.
We can still detect the CMBR if we direct the antenna in any direction. This is the third evidence of the Big Bang theory.

Eccentricity: The measure of how elliptical the path of orbit is.
- Higher eccentricity = more elliptical the orbital path of the planet.
- Eccentricity of 0 = circle.
- Eccentricity closer to 1 = ellipse.