The first issue relates to the end of Class 18 - why are Venus and Earth so different? Part of that issue is what happened to carbon dioxide on the Earth - it got turned into limestone. This is a process that requires WATER to go from a vapor to liquid state. Clearly, Venus is too hot for this to happen.
So, what about on Mars? Is Mars just too cold? Well, sometimes. Below is a more complicated carbon dioxide - silicate cycle that allows for temporal variation ( from the Nature articles).
Discussion of this issue is beyond the topic of the class (take Planetary Atmospheres next semester!) - but the point is that atmosphere-surface interactions play a CRITICAL role in planetary science - particularly in the case of the Earth, and most especially considering the issue of anthropogenic climate change.
So, what happened to the water on Mars?
Sometime around 3.7 billion years ago the volcanism stopped sending gases into the atmosphere, the magnetic dynamo turned off (no longer protected the atmosphere from being stripped away by the solar wind) - the greenhouse effect turned off and Mars began to freeze.
Now, let's back to the topic at hand.
First, let's dispel a myth - erosion is not the primary source of geological change on Earth - plate tectonics is. Erosion is just the small scale gology that wears down the plates and make pretty features like rivers and canyons. We notice it most because it happens on closer to human time scales (while the cm per year motion of plates is not noticable unless you use GPS locators).
For examples of erosion on Mars - see Chapter 13 of the text and the Ancient Life on Mars as well as the Red Planet slide sets from the Lunar and Planetary Institute.
Finally, we did the following excercise in class:
For each of the following questions use the geological concept maps to explain your answer, tracing your logic back to the formation properties ( size/mass, distance from Sun, composition, spin).
Take turn to roll die to decide which of Qs 1-6 to answer
(1) In 2006 the Messenger mission to Mercury photographs part of the surface never seen before and detects vast fields of sand dunes - is this "discovery" likely/reasonable/surprising?
(2) A radar mapper in orbit around Venus detects a new volcano larger than any other on Venus but not seen by the Magellan spacecraft - is this "discovery" likely/reasonable/surprising?
(3) A Venus rover discovers extensive regions of layered sedimentary rocks - is this "discovery" likely/reasonable/surprising?
(4) Clear-cutting in the Amazon rain forest exposes vast regions of ancient terrain that is as heavily cratered as the lunar highlands - is this "discovery" likely/reasonable/surprising?
(5) Why is the Moon heavily cratered but not the Earth?
(6) Why is there little erosion on Venus?
Each person roll die - person with highest score picks Q7 vs. Q8.
(7) Suppose Mars had turned out to be significantly smaller than its current size - say, the size of Moon. How would this have affected the number of geological features due to each of the four major geological processes - cratering, volcanism, tectonics, erosion? Do you think such a small Mars would still be a good candidate for haboring life?
(8) Consider scenarios for the geological evolution of the Earth: if you change - increase or decrease - each of the formation properties one at a time what would you expect to be the change the Earth's geological history and likelihood of developing life in each case?