Mars Odyssey is a tougher
sell because it has no true camera
on board.
Instead, it has a radiation
counter (not too different
from a Geiger Counter);
a thermal imaging camera;
and a gamma-ray spectrometer.
What can we learn from these instruments?
Well, we can find out the compositions of
some of the rocks in those great MGS
images. Here,
dust and sand are red and orange.
Basaltic lava (an igneous rock) is
blue, and purple shows a high concentration
of the igneous mineral olivine, shown
in an eroded layer.
Because olivine is easily destroyed be liquid
water, their interpretation is that
this area has been dry for a very long
time (otherwise the olivine would not
have survived).