Acidalia Planitia is a large, flat region north of where the Mars Pathfinder spacecraft landed in July 1997. This nighttime infrared image of part of Acidalia little resembles a daytime view. But it can tell scientists about the physical properties of the martian surface, helping them map the proportion of rocks and dust.

Detecting Warmth
Unlike visual images, infrared images show surface temperatures. At night, dust cools rapidly, while rocks give up heat much more slowly. Seen by infrared late at night, rocky areas appear bright because they are still radiating daytime heat, while dusty patches have cooled and appear dark.

Aprons of ejected rocks surround many impact craters. Most appear bright (as above) because of the rocks' warmth. Other craters (such as at right) are ringed by patterns of dark ejecta, suggesting the debris is smaller in size, dustier, and therefore cooler.

Streamlined Islands
Liquid water is not stable on the surface of Mars today, but much geologic evidence suggests that large quantities of water flowed across the surface in the distant past.

Streamlined islands are one piece of evidence for these ancient floods. The tremendous force of surging water from a catastrophic flood carved these teardrop-shaped islands within a much larger channel called Ares Valles. The orientation of the islands indicates the direction the water flowed. Here the narrow end of the island points downstream.