Compare and contrast the Dreadful Mars Face (1997) and the New Mars Face (2001) seen below. These are pictures of the same formation in Cydonia, but they have many differences, not the least of which is what appears to be additional erosion since 1997.

Erosion is alleged to occur at an extremely slow pace on Mars due to the absence of liquid water and the very thin atmosphere. Yet this formation -- like many others on Mars -- appears to have suffered considerable erosion in only 4 years. How could this have happened?

What if the formation were partially composed of ice? During the summer, this ice could sublime into the atmosphere, eroding the formation fairly quickly.