When a meteorite strikes at high speed, the impact's energy blasts a crater in the ground and ejects debris in all directions.

Despite the violence, however, an impact can tell scientists much about the surface where the meteorite struck by the pattern it leaves. For example, Bacolor Crater, 20 kilometers (12 miles) wide, shows many signs that at the time of impact, the subsurface held a lot of water, presumably as ice.

Bacolor lies in Utopia Plantitia, a vast, flat region in Mars' northern lowlands and home to distinctive-looking craters. In 2001, the Gamma-Ray Spectrometer on NASA's Mars Odyssey orbiter mapped abundant hydrogen in the subsurface of Utopia. The data indicate that near the surface, ice makes up 50 percent or more of the volume even now.

Another instrument on Mars Odyssey, the Thermal Emission Imaging System (THEMIS), imaged Bacolor Crater in visible and infrared wavelengths. Combined into a false-color image, THEMIS data can help scientists decipher how much rock, hardened sediments, and dust make up the surface at Bacolor Crater. The data also help them draw a scenario for the crater's formation.

http://themis.asu.edu/featuresimg/acherongraben.jpg

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Images: NASA/JPL/Arizona State University.
Research: Christensen, P.R., B.M. Jakosky, H.H. Kieffer, M.C. Malin, H.Y. McSween, Jr., K. Nealson, G.L. Mehall, S.H. Silverman, S. Ferry, M. Caplinger, and M. Ravine, The Thermal Emission Imaging System (THEMIS) for the Mars 2001 Odyssey Mission, Space Science Reviews, 110, 85-130, 2004.