There are also signs that water ice exists below the surface of Mars in underground deposits. These signs include rampart craters, also called moat craters. Rampart craters are impact holes surrounded by ejecta material. They have a strong ridge around the far edge of the ejecta, suggesting that the material in the crater flowed out instead of being shot out. This means, when the impacting meteorite hit the surface, it may have liquefied underground ice. The ejecta would then be a water/dirt mixture flowing out from the center of the crater. The result is craters that tend to look like mud pies!

Only the larger, deeply-penetrating craters have this moat-like ridge around them; small shallow craters do not. This implies that there is a certain depth one must pass before any ice can be found. Scientists' best guess is that the depth to ice is about 100 meters [at mid-latitudes], and about 400 meters near the hotter equator. Any ice above this depth has sublimated away.

Gullies (Figure 2) are found at cliff edges and crater walls, where the long channels come out of a cut in the cliff. The channels look like they were formed by water, being that they are very long with aprons and fingers of material extending far onto the flatlands.

Again, this suggests that the ground contains water or an ice-rich layer that periodically melts. It is believed that if the soil did not contain any ice-like material, it would not behave this way.

Chaotic terrain (Figure 3) is a jumble of cracked and collapsed blocks. It is often associated with outflow channels. These broken up regions are thought to be created by the melting of underground ice. The new water flows downhill, which causes the ground directly above it, at the surface, to crack and slump in a disordered fashion.

[The common presence of rampart craters, gullies and channels, and chaotic terrain suggests that there has been ice present near the surface of Mars throughout its history, and probably even today.] Knowing where critical resources like water can be found is essential for human exploration to Mars someday.

Figure 3. Images like the one above suggest to scientists that water once flowed across the surface of Mars.