Mars probe snaps dramatic new images of gullies

 作者:隆姒鸷     |      日期:2017-12-21 05:01:22
By David Shiga (Image: NASA/JPL/University of Arizona) (Image: NASA/JPL/University of Arizona) Dramatic new images of gullies and dried-up streambeds bolster evidence that water once gushed across the surface of Mars. The images are the latest from NASA’s Mars Reconnaissance Orbiter (MRO), which boasts the most powerful camera ever sent to Mars. MRO arrived at the Red Planet in March 2006 and began its main observing phase in November of that year, training its High Resolution Imaging Science Experiment (HiRISE) camera on the planet’s surface. With its sharp vision, it has already spotted several spacecraft sitting on the on the planet’s surface, as well as interesting features such as a patch of water ice lying inside a crater. Now, the spacecraft has returned images showing more signs of past water flow on Mars. One image shows a set of gullies cutting into a crater wall. Liquid water probably carved the gullies, says HiRISE team member Kelly Kolb of the University of Arizona in Tucson, US. But researchers are not sure which of three different scenarios explains where the water came from. One possibility is that it came from an underground reservoir of liquid water somewhere beneath the surface. A second idea is that frozen water – deposited as frost in the winter – melted in the summertime to produce the gullies. Indeed, bright patches in the upper half of the image are areas where frost – made either of frozen carbon dioxide or water – covers the surface, Kolb says. A third possibility is that snow accumulated on these slopes long ago as a result of climate swings caused by periodic changes in the tilt of Mars’s axis. This snow layer later melted from underneath, releasing water to carve the gullies. Patches of very smooth terrain visible in this and other images could be the dust-covered remnants of this snow layer. The flows that carved the gullies appear to have occurred repeatedly here, since sediment from some gullies has partially buried previously existing ones. “The source of water was either replenished or available for an extended period of time,” Kolb told New Scientist. The age of the gullies is unclear, but the lack of overlying craters suggests they formed within the past few million years, she says. Another new image shows branching ridges that appear to be the remains of water-carrying streams. Oddly, the streambeds are now higher up than the surrounding terrain. But Colin Dundas, another HiRISE team member at the University of Arizona, says this can happen when a stream cements material beneath it or covers it with rocks. In either case, the streambed could end up rising above the surrounding terrain because it is stronger and more resistant to erosion, he says. These dried-up streambeds are ancient, though their exact age is not known,