SHARAD (Shallow Subsurface Radar) looks for liquid or frozen water in the first few hundreds of feet (up to 1 kilometer) of Mars’ crust.
SHARAD probes the subsurface using radar waves within a 15- to 25-megahertz frequency band to get the desired, high-depth resolution.
The radar wave return, which is captured by the SHARAD antenna, is sensitive to changes in the electrical reflection characteristics of rock, sand, and any water that may be present in the surface and subsurface. Water, like high-density rock, is highly conducting, and has a very strong radar return. Changes in the reflection characteristics of the subsurface, caused by layers deposited by geological processes in the ancient history of Mars, are also visible.
Written by: HIRISE Science Team (8 July 2009)
More info and image formats at http://hirise.lpl.arizona.edu/ESP_011878_1045
Image: NASA/JPL/University of Arizona