Assess your responses on the basis of 3 points total

1. Compare and contrast the surface appearance (i.e., at GROUND LEVEL) of the planets and moons in the inner part of the solar system. Qualitatively explain why these surfaces appear as they do today.
a. All surfaces show some signs of impact. Surfaces with relatively low crater counts are those of Earth, Venus, and Mars, which all are believed to have obliterated many craters with volcanic activity and/or weathering processes. Mercury probably also obliterated some very early craters with volcanic activity.
b. Earth's surface shows outlines of tectonic plates as well as movement of those plates, which is unique in the inner solar system. The smaller planets may have cooled too much to retain the energy to support plate tectonics, but we don't really know why Venus shows no sign of plate tectonics.
c. Earth's Moon has craters that are filled with lava on the Moon's near side - because the crust is thin enough for lava to have welled up into early craters before the molten interior cooled off. The crust on the Moon's far side and that of the other inner planets was too thick for volcanism to result in filling in most impact craters on those surfaces.
d. The surfaces of Mars and Earth have numerous canyon-like features that result from flood erosion. Liquid water probably never formed on the other inner system objects, with the possible exception of ancient Venus.
e. All objects show signs of volcanism to some extent - Earth, Venus and Mars have large volcanoes, and on Earth (and probably Venus) that activity is still alive. Mercury and Earth's moon lack volcanoes but show signs of flood lava mentioned earlier.
f. Mercury has a unique feature in the inner planets - the scarps. Scarps are cliff-like features that result from shrinkage (when the inner part cooled and shrank, the crust settled and formed the scarps).

2. Compare and contrast the atmospheres of the four inner solar system planets, and summarize the likely evolution of the atmosphere of each of those planets, beginning with planet formation and ending with the present.
Mercury has no atmosphere, Venus has a thick atmosphere (about 100 times that of Earth) of carbon dioxide, Earth has a thin atmosphere of nitrogen and oxygen, and Mars has a very thin (about 100 times less than that of Earth) carbon dioxide atmosphere. Any atmosphere that Mercury had at formation would have been lost due to very high temperature (close to the Sun) and low escape velocity for the planet. For the same reasons, any vented gases from the interior would be lost as well. Venus' and Earth's and Mars' original atmospheres probably contained light gases including molecules made of the elements hydrogen, helium, oxygen, carbon, and nitrogen - especially molecular nitrogen, carbon dioxide, and water vapor. On these three planets, solar ultraviolet radiation (U.V.) would break down most of these molecules and the planets would lose much of their hydrogen and all their helium. Venus is close enough to the Sun so that water would not condense and remove carbon dioxide from the atmosphere, and a runaway greenhouse effect resulted in high enough temperature that water could never condense. As a result, solar U.V. radiation would break down water vapor molecules and all hydrogen would escape, leaving what we now see. Earth went through the most complex evolution, which differs from Venus in that water vapor could condense and over long time periods remove carbon dioxide from the atmosphere (via the carbonate-silicate cycle) and sequester most of Earth's carbon in limestone rock. The major natural source of carbon dioxide is from volcanism. Earth may well have received water and other volatiles like carbon dioxide from space (icy comets) over long time periods, possibly enough to account for all the water on Earth now. The carbonate-silicate cycle we observe now keeps our atmosphere almost free of carbon dioxide, but leaves enough for our moderate greenhouse effect. When life on Earth began in the oceans, it gradually spread and ultimately was able to generate free oxygen (i.e., oxygen that did not combine chemically with something), and the presence of oxygen led to formation of our ozone layer. The appearance of a high-altitude ozone layer made life on land possible because ozone shields * land-based life from life-killing ultraviolet radiation, and * lower atmosphere molecules (especially water vapor) from destructive ultraviolet radiation. On Mars, the planet has a low escape velocity that makes atmosphere retention tough, and it is far enough from the Sun that any water bombarding the planet freezes out. The consensus is that Mars at one point about 3-4 billion years ago had water which weathered its surface and could have supported life, but as the planet cooled and gradually lost its atmosphere the water froze and disappeared into the sub-surface and froze near the poles. Future Mars missions will test this hypothesis within the next 10 years.  Results from the Mars Rover & Opportunity vehicles seem to support strongly the hypothesis of plenty of water on the earlier Martian surface.

3. Explain why we believe life on the surface of Earth could not evolve until the atmosphere contained free oxygen. What present-day practices threaten to negate the unique feature of our atmosphere that permits land-based life (if you answer with global warming, may a storm cloud immediately descend on your home)?
Just joking about the storm part - it's frustrating that so many students (and the general population) confuse these two issues. The main absorber of the Sun's ultraviolet (UV) radiation is ozone in our upper atmosphere. That layer could not form until the appearance of free oxygen, which in turn was produced by life that evolved under water (water protects life under the surface from UV). Various life forms used carbon dioxide in sea-water (obtained from carbon dioxide in the atmosphere when rain dissolved it and fell into the ocean) for photosynthesis and emitted oxygen as a waste product. Thus, the explosion of life under water ultimately led to surface life-forms that depend on the waste products of the under-water life. In very recent times, if we had continued to deplete our ozone layer by using CFCs for refrigeration and propellants, we would have assured our own extinction via UV radiation, which no surface life-forms can tolerate. UV radiation is energetic enough that such photons cause cell damage that leads to skin cancer (cell damage in plants is also fatal). We hope and believe that production and use of these chemicals stopped (will stop) in time to salvage the situation. Although ozone holes still appear every spring at the respective poles, even ozone at mid-latitudes continues to exhibit up to 10% depletion (that got the attention of powerful world governments - even the rich and powerful depend on the ozone layer).