Potentially Once-Habitable Ancient Lake Discovered on Red Planet

Scientists studying images from The University of Arizona-led High Resolution Imaging Experiment camera on NASA's Reconnaissance Orbiter have discovered never-before-seen impact "megabreccia" and a possibly once-habitable ancient lake on at a place called Holden crater (NASA image).

The megabreccia, which may be some of the oldest deposits exposed on the surface of Mars. is topped by layers of fine sediments that formed in what apparently was a long-lived, calm lake that filled Holden crater on early Mars.

"Both contain minerals that formed in the presence of water and mark potentially habitable environments," said HiRISE's principal investigator, professor Alfred McEwen of the UA's Lunar and Planetary Laboratory. " This would be an excellent place to send a rover or sample-return mission to make major advances in understanding if Mars supported life."

At least 5 percent of the fine sediments in the layer on top of the megabreccia consists of clay.
l"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 co-investigator John Grant of the Smithsonian National Air and Space Museum said. "If we were looking on Earth for an environment that preserves signatures related to habitability, this is one of the kinds of environments we would look at."

And even the clay-containing layers aren't all that's icing the cake. Topping the clay layers that formed in the placid Holden crater lake are layers of great boulder-filled debris unleashed later, when water breached Holden crater rim, creating a torrential flood that eroded the older lake sediments.

The clay-rich layers would have remained buried from view, except for that great piece of luck, the fact that Holden crater rim could no longer withstand the force of an estimated 4,000 cubic kilometers of water dammed behind it. The body of water would have been larger than Lake Huron.

"The volume of water that poured through during this flood must have been spectacular," Grant said. "It ripped up finely bedded materials, including blocks 70 meters or 80 meters across -- blocks nearly the size of football fields."

The first, prolonged watery episode at Holden crater that settled out the fine-grain sediments probably lasted at least thousands of years. By contrast, the second lake, formed when the crater rim was breached, may have lasted only hundreds of years, not long at all, Grant said.

The megabreccia excavated when Holden crater formed is the first found on Mars, Grant said. "When large craters form, they produce very large blocks of material. We see them on Earth. Popigai Crater in Russia is one example. But we'd never seen them on Mars, and we knew they ought to be there. Now we've seen them with HiRISE."

Holden crater is one of six remaining landing site candidates for NASA's Science Laboratory, a mission scheduled for launch next year.

So far, most evidence for sustained wet conditions on is limited to the planet's earliest history, the HiRISE scientists say. While water certainly flowed over the planet later in its history, it may have flowed only in short-lived, or catastrophic events.

Posted by Casey Kazan.