Scientists controlling instruments on the orbiter found bright ice exposed at five Martian sites, with new craters that range in depth from approximately 1.5 feet to 8 feet. The craters did not exist in earlier images of the same sites. Some of the craters show a thin layer of bright ice atop darker underlying material. The bright patches darkened in the weeks following initial observations, as the freshly exposed ice vaporized into the thin Martian atmosphere. One of the new craters had a bright patch of material large enough for one of the orbiter's instruments to confirm it is water ice.
(Photo below: This series of images spanning a period of 15 weeks shows a pair of fresh, middle-latitude craters on Mars in which some bright, bluish material apparent in the earliest images disappears by the later ones. Each panel is 75 meters (246 feet) across. The two craters are each about 4 meters (13 feet) in diameter and half a meter (1.5 feet) deep.
The bright material is water ice that was uncovered by the meteorite impact that excavated these small craters less than 15 weeks before the initial image of this series. Sublimation of the ice during the Martian summer leaves behind a dust layer that gradually thickens to the point where it obscures the ice.
The High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter took these images of this site at 46.33 degrees north latitude, 176.90 degrees east longitude. The HiRISE camera's targeting of the site was prompted by two earlier images from the Context Camera on the Mars Reconnaissance Orbiter, which showed that the impact responsible for these craters had not yet occurred by June 4, 2008, but had occurred by Aug. 10, 2008.
The dates when these six HiRISE images were taken were (left to right, top row; then left to right, bottom row): Sept. 12, 2008; Sept. 28, 2008; Oct. 9, 2008; Oct. 14, 2008; Nov. 22, 2008; and Dec. 25, 2008. The span of time corresponded to a period from mid to late summer in Mars' northern hemisphere. The images are subframes of the observations made on those dates.)
The finds indicate water ice occurs beneath Mars' surface halfway between the north pole and the equator, a lower latitude than expected in the Martian climate. "We now know we can use new impact sites as probes to look for ice in the shallow subsurface," said Megan Kennedy of Malin Space Science Systems in San Diego, a co-author of the paper and member of the team operating the orbiter's Context Camera.
During a typical week, the Context Camera returns more than 200 images of Mars that cover a total area greater than California. The camera team examines each image, sometimes finding dark spots that fresh, small craters make in terrain covered with dust. Checking earlier photos of the same areas can confirm a feature is new. The team has found more than 100 fresh impact sites, mostly closer to the equator than the ones that revealed ice. The Mars orbiter is designed to facilitate coordination and quick response by the science teams, making it possible to detect and understand rapidly changing features. The ice exposed by fresh impacts suggests that NASA's Viking 2 lander, digging into mid-latitude Mars in 1976, might have struck ice if it had dug four inches deeper.