QUESTION:
Why are the features on Mars (such as the Valles Marineras) huge in 
comparison to the features on Earth? Likewise why does Mars have so 
many craters and Earth has so few?

ANSWER from Ted Roush on February 14, 1997:
On Earth, the crust is relatively thin (tens of kilometers thick) 
whereas on Mars, it appears the crust is relatively thick (a couple of
hundred kilometers thick).  This means that throughout Mars' history the
crust has been less mobile than the Earth's crust.  This means the 
crust has remained in the same location on Mars and erosional processes
that have created land forms such as Valles Marineris could act over
longer periods of time and result in larger more extensive features.
Because of plate tectonics on the Earth, much of the crust is relatively
young and many of the craters created in the past have been erased.
Because the martian crust has been relatively immobile for most of Mars'
history, the impact craters would be better preserved.