Hospitable Lake Identified On Mars

The HiRISE camera on the Mars Reconnaissance Orbiter took this image of the largest fan in Holden crater when the orbiter was flying about 162 miles over the surface in March 2007. Geologists discovered a complex geologic history for the site, including two wet episodes that may have been amenable to life.

Photo: NASA, JPL, University of Arizona

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In the exploration of the solar system and the most extreme environments on Earth, scientists pursue the same thing: Evidence of life, and how it manages in hostile conditions.

Astrobiologists study microbes living locked in polar ice, and those living in sulphurous steam vents in the deep ocean. And one day, they may focus on the "megabreccia" in the Holden Crater on Mars.

There, scientists studying images from the Mars Reconnaissance Orbiter have seen evidence of an ancient, long-lived calm lake that is among the most likely have witnessed the spark of life.

"Holden crater has some of the best-exposed lake deposits and ancient megabreccia known on 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 University of Arizona's Lunar and Planetary Laboratory. (It is the University's High Resolution Imaging Experiment camera – HiRISE – that captured the images.) "This would be an excellent place to send a rover or sample-return mission to make major advances in understanding if Mars supported life."

Here's how the University described the crater:

Holden crater is an impact crater that formed within an older, multi-ringed impact basin called Holden basin. Before an impact created Holden crater, large channels crossed and deposited sediments in Holden basin.

Blocks as big as 50 meters across were blasted from Holden basin when Holden crater formed, then fell chaotically back to the surface and eventually formed "megabreccia," a conglomeration of large, broken boulders mixed with smaller particles. HiRISE images show megabreccia outcrops in Holden crater walls. This megabreccia may be some of the oldest deposits exposed on the surface of Mars.

At least 5%, by weight, of the fine sediments in the layer on top of the megabreccia consists of clay, according to another instrument on the Mars Reconnaissance Orbiter, the Compact Reconnaissance Imaging Spectrometer for Mars, or CRISM.

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 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. The megabreccia excavated when Holden crater formed is the first found on Mars.

The observations suggest that the clays originally could have formed before the impact created Holden crater in the older Holden basin. Many of the blocks in the megabreccia appear to erode more easily than the surrounding crater wall material. These blocks could be chunks of Holden basin sediments that predate the impact crater.

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

So far, most evidence for sustained wet conditions on Mars 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.

Take it away, Mr. Bowie:

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