About the images
Two images are used in this activity. One is a "visible"
image: it's essentially what you would see if you were there.
It was taken by a camera onboard the Mars Global Surveyor orbiter
(MGS). The second is colored based on its elevation: blue is lower,
green or yellow is higher.
The elevation image was created using data from the Mars Orbiting
Laser Altimeter (MOLA) onboard MGS. MOLA bounces a laser off the
surface of the planet, then times how long it takes for the beam
to return to the spacecraft. From that it can calculate its distance
from the surface to an accuracy of a little over a foot. MOLA
does this over and over as it orbits, eventually building up enough
data for a global elevation map of the entire planet.
If you're looking for features that vary in elevation
(like flood channels), this kind of map can make
them much easier to see.