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One day scientists are fairly sure there's water on Mars. Then there's news of no water on Mars. Now there's evidence of an ancient body of water on Mars.

Signs of an ancient, and possibly habitable, lake were discovered by scientists analyzing images from the University of Arizona's High-Resolution Imaging Science Experiment, aboard NASA's Mars Reconnaissance Orbiter.

UA planetary scientists believe the orbiting camera returned pictures of an impact crater filled with sediment from what, apparently, was a lake "that filled Holden Crater on early Mars," according to a UA press release.

Megabreccia, a type of rock seen in the photos of the crater, is thought to have been formed when huge chunks of rock — possibly 150 feet or more across — were blasted from the Holden basin and fell back to the surface. The breccia is a conglomerate of large pieces of rock combined with smaller particles. The UA scientists say the exposed breccia outcroppings in the crater's walls may be some of the oldest exposed rock on Mars.

Analysis of the rock by another MRO instrument suggests that some of the material may be clay, which would have settled to the bottom through the calm waters of the lake.

"The origin of the clays is uncertain, but clays in the probable lake sediments implies quiescent conditions that may preserve signatures of a past habitable environment," HiRISE scientist John Grant said in a release issued by the university. He said it is the same type of environment scientists would look for on Earth to find preserved signs of earlier habitability.

Grant is a HiRISE co-investigator from the Smithsonian National Air and Space Museum, one of several institutions represented on the science team for the UA-designed camera.

The site may be a good place for a Mars rover or a sample- return mission to visit, according to the UA's Alfred McEwen, the principal investigator — lead scientist — for the orbiting camera.

Analysis of the Martian surface using HiRISE images led to a change in landing sites for the upcoming landing of the UA-led Phoenix Mars (lander) Mission.

The original site chosen for the May 25 Phoenix landing was found to be strewn with boulders that scientists feared could have overturned or blocked the lander. HiRISE images were used to choose another site with fewer obstacles.

At the top of the to-do list for Phoenix is investigating the presence and history of water on Mars' polar region.