AST103 – Exam #2 Review Sheet
Early Earth
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Compare/contrast terrestrial planets with the giant planets.
- Terrestrial planets are smaller, less massive, denser, mostly
solid, possess a weaker (if any) magnetic field, fewer (if any)
moons, a thinner (if any) atmosphere, while typically orbiting
closer to their parent star.
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Giant planets are larger, more massive, less dense, mostly
gaseous, possess a stronger magnetic field, many moons, a
thicker atmosphere, while typically orbiting further from their
parent star.
What is the cause of the Heavy Bombardment Period? What effect
did it have for the development of life within the solar system?
- The Heavy Bombardment Period came to be as a result of the solar
nebula’s “condensing” and forming increasingly-larger bodies.
For 500-million years the protoplanets of the Solar System were
regularly impacted by orbiting planetesimals massive enough
to remain despite the constant solar wind.
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The impacts would have devastating effects on any forming life, so
the end of the HBP is considered to be the absolute earliest
that life could begin to form on Earth. Any life that formed
between impacts would likely not be traceable to our own “tree”.
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Explain the process by which the Moon was formed.
- The Moon is thought to be the result of a catastrophic impact
with the Earth by a Mars-sized planet called Theia. The ejecta of
this impact was put into orbit around the Earth, where it
eventually coalesced into our moon.
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What is differentiation and what was its effect on the structure of the
Earth?
- Differentiation is the process by which materials of varying
density, over time, naturally order themselves around the center
of gravity. The layers of the Earth thus decrease in density from
the inner core to the lithosphere, and continue decreasing through
the layers of our atmosphere as well.
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Explain the process by which the Earth’s magnetic field is generated.
- The Earth’s magnetic field is generated as a result of interaction
between the Earth’s core layers. The Coriolis force drives the
molten nickel-iron outer core, which is electrically conductive
and thermally convective, to revolve around the solid nickeliron inner core. The combination of these three factors results in
a magnetic field that envelopes the Earth itself. This process is not
unique to the Earth.
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Describe the evolution of Earth’s atmosphere. Pay particular
attention to the types of chemicals that made up the atmosphere at
different points in the planet’s history, as well as the processes that
fed them into the atmosphere.
- As the Earth cooled, the outer crust solidified first, trapping
various gases within the mantle. These gases, over time, were
released from within the Earth through volcanic outgassing. The
earliest permanent atmosphere consisted of relatively-heavy
molecules, including water vapor, carbon dioxide, nitrogen,
hydrochloric acid, and sulfuric acid. Most importantly, the early
atmosphere was anaerobic (containing no free oxygen).
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Over hundreds of millions of years, the atmosphere’s composition
changed as a result of various physical and biological processes.
Atmospheric water vapor, exposed to solar ultraviolet radiation,
underwent photochemical dissociation and formed the
protective ozone layer in our stratosphere. Early plant life evolved
to exploit the abundance of sunlight and carbon dioxide in their
energy-producing process, photosynthesis. A by-product of
photosynthesis is free oxygen, which is then released by the plant
life. Over time, this transformed our atmosphere into an
aerobic state (containing free oxygen).
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Eventually, the composition of our atmosphere reached that which
we observe today: 78% nitrogen, 21% oxygen, and less than 1%
argon, carbon dioxide, neon, helium, methane, krypton, and
hydrogen (% by volume).
Properties of Life
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What are the two properties that do the best job of distinguishing
life from non-living things on Earth? Can you give examples of living
things that do not have either of the two properties?
- The ability to reproduce and the ability to evolve are considered
two best distinguishing criteria for life
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One example of life that does not meet these criteria is the mule,
which cannot reproduce. Evolution is an iterative process by which
advantageous traits outlast the disadvantageous in a species, but
a mule cannot contribute any advantageous traits beyond their
own lifetime because they cannot reproduce.
What properties are necessary for life but aren’t solely properties of
living things? Can you can give examples of non-living things that
have these properties?
- Growth and development, energy utilization, environmental
response, and an ordered nature are properties that living
things exhibit, but are not themselves exclusive to living things.
o Order: gradient of stone size along a shoreline (smaller
particles are carried further inland by the tides)
o Growth and development: the pile of dirty laundry in the
hamper.
o Energy utilization: windmill/watermill.
o Response to environment: thermostat switch.
Why is it important to have a way of defining the properties of living
organisms?
- If we are to search for the existence of life outside of the Earth, we
need to be able to compare the properties and behavior of an
observed phenomenon to a well-defined set in order to
determine whether or not we have found life. Extraterrestrial
life will probably not be obvious (like a bipedal alien) by our
subjective standards.
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What are the properties of carbon that made it beneficial to the
development of life on Earth? What other element(s) could serve a
similar purpose as carbon to the development of life?
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Carbon is very abundant (4th most abundant by mass in universe),
joins readily with other elements in sturdy double-bonded forms,
dissolves easily in water, and is capable of forming various
compounds in conditions like those found on Earth (temperature,
pressure, etc.).
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Silicon has properties similar to carbon, though it is heavier than
carbon and less abundant.
What are the unique properties of water that assisted in the
development of life on Earth? Why are other substances not
considered to be as good in facilitating the development of life on
another world?
- Water is a very capable solvent, conducive to biochemical
reactions along boundaries/interfaces with other chemicals, it
does not destroy/dissolve cellular structure, protects from
UV/environment by freezing “top-down” (water ice is less dense
than liquid water), and remains liquid in a wide range of
acceptable temperatures.
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These properties are all conducive to the development of life on
Earth, which is thought to have originated underwater.
Life on Earth
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What are the three (3) main factors any world (planet or moon)
needs to have in order for life to form?
- For life to form, a world must have liquid water, a sufficient
energy source, and organic compound ingredients.
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What is meant by the top-down and bottom-up approach to
determining the time when life started on Earth?
- We study how organic chemicals can form in an energetic-butlifeless environment (the bottom-up part) and work backwards
through time to trace how life evolved on Earth (the top-down
part). From there, the goal becomes connecting these two efforts
and forming a unified timeline.
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When do scientists think that life started on Earth and what evidence
do scientists point to that supports that time frame? What makes it
difficult to accurately narrow down the time period when life began
on Earth?
- Life is thought to have developed at least 3.85 ByA (rocks dated at
this age have 13C/12C ratios indirectly suggesting presence of life.
- The Earth’s active geology does a good job of erasing many of the
clues left behind (e.g. fossils are relatively rare, compared to the
number of organisms who have ever lived and died on Earth).
Describe the current model of how life formed from chemical
reactions to the earliest organisms.
- Through millions of years of chemical reactions, “simple” RNA
strands are formed. Those which are able to catalyze their own
reproduction do so. Amino acids form protective shells (precells), isolating them and allowing them to continue their
mutations, eventually producing the more complex DNA, creating
the genetic framework for life as we know it today.
Why do scientists think that life started in the oceans instead of on
land? What benefits do the oceans offer in the development of life?
- Scientists think life started in the oceans instead of on land because
of its protective characteristics. Organisms can benefit from solar
UV radiation protection, abundance of energy (near deep-sea
vents), and an abundant source of inorganic “food”.
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What were the conditions of Earth’s early permanent atmosphere
like? What role did the early atmosphere play in the development of
life? How did the Earth’s water cycle alter the atmosphere over the
course of billions of years?
- The earliest permanent atmosphere on Earth was anaerobic, thus
the earliest surface life did not require oxygen to live.
- The Earth’s water cycle removed much of the atmospheric SO4,
CO2, and HCl with precipitation and surface runoff into the oceans.
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What are the key differences between a prokaryotic cell and a
eukaryotic cell? Which type of cell was the foundation of the first
lifeforms on Earth? What led to the development of eukaryotic cells?
- Prokaryotic cells do not feature a nucleus, nor a permeable
membrane, and are smaller.
- Eukaryotic cells feature a cellular nucleus to house its genetic
material, have a semi-permeable membrane, and are larger (an
order of magnitude larger than prokaryotic cells).
- Prokaryotic cells eventually developed into Eukaryotic cells,
able to interact with and function within their environment.
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How did the development of photosynthesis alter the atmosphere
and, by extension, the evolution of life on Earth?
- Photosynthesis takes sunlight and carbon dioxide and turns it into
chemical energy and free oxygen. The vast amount of plant life,
collectively photosynthesizing over millions of years, drastically
increased the proportion of free oxygen present in the Earth’s
atmosphere. Aerobic life then develops and evolves on the surface.
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What are some factors for why there was a rapid diversification of
life during the Cambrian Period?
- Breakup of the supercontinent. Rodina, “stirred the pot” by
mixing together environments that were previously separated.
- Disruption and realignment of Earth’s magnetic polar axis
rearranged the global climate.
- Multicellular life began specializing/optimizing for these newly
combined/formed local environments, creating niches of
different species and dramatically increasing biodiversity.
Key Concepts & Definitions
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Stromatolites
- Mound-shaped masses of layered sediment and microbial growth
- Modern stromatolite microbes are largely similar to those found
in the earliest microbe fossils, (dated 3.5 ByA), providing an idea
of what life looked at that time.
Murchison Meteorite
- Meteorite impact observed 1969, carrying 20 different types of
common and rare amino acids.
- Supports the idea of Panspermia, in which extraterrestrial object
impacts seed the Earth with organic compounds.
Evolution & Natural Selection
- The process by which a species’ nature is collectively shaped by
its environment, leading to a new sub-species.
Natural Selection
- The process by which organisms with useful traits survive their
local environment and are able to reproduce, furthering the
presence of a beneficial trait for that species.
- Responsible for Earth’s biodiversity, given the vast assortment
of local environments around the world.
Heavy Bombardment Period/Hadean Era
- The period (4.6 ByA to 4.1 ByA) of Earth’s formation during which
there were periodic impacts by planetesimals and other orbital
debris in the solar system, keeping it in a generally hellish state.
Outgassing
- The escape of gasses trapped within the Earth’s mantle,
typically through volcanic activity and plate divergence along the
ocean floor.
Impact Sterilization
- The consequence of periodic orbital bombardment of the Earth by
Hadean-era planetesimals, keeping the conditions of the young
Earth far too hostile for life to form and survive.
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Panspermia
- The theory that life, or at least its ingredients, came to Earth from
an extraterrestrial source, such as a comet.
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Miller-Urey Experiment
- Attempts to demonstrate that abiogenesis is possible, given the
correct ingredients and conditions.
- Recreated ancient atmospheric conditions and chemical presences,
added a strong energy source, resulting in formation of organic
molecules.
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Cambrian Explosion
- A period (530 MyA to 515 Mya) of notable acceleration in the
development of complex, diverse life on Earth.
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Proteins
- Complex nitrogenous organic molecules, essential to all life.
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Enzymes
- Organic molecules, typically proteins, which catalyze a certain
biochemical reaction (or group thereof).
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Lipids
- Organic molecules capable of storing chemical energy and
forming membrane structures.
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Carbohydrates
- Organic molecules capable of utilizing stored energy and
forming cellular structure.
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DNA/RNA
- Organic macromolecules containing the genetic information
instructing cellular functions.
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Amino Acids
- Simple organic molecules, of which all proteins are constructed.