[meteorite-list] HiRISE Discovers a Possibly Once-Habitable Ancient Mars Lake

[Ron Baalke]

FROM: Lori Stiles (520-626-4402; lstiles at u.arizona.edu)

HiRISE Discovers a Possibly Once-Habitable Ancient Mars Lake

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

The megabreccia 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, HiRISE scientists say.

The Holden Crater image is on the HiRISE Website at  
http://hirise.lpl.arizona.edu/PSP_003077_1530

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

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 percent, 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.

"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."

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, Grant said. "These 
blocks could be derived from the earlier Holden basin that were 
excavated on impact, then later re-eroded, with the sediments settling 
to the bottom of the long-lived lake. It's intriguing to think the clays 
we see in Holden crater now might actually have been recycled."


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.

Grant is first author on a research paper about Holden crater, published 
in the journal Geology last week.

The mission is managed by NASA's Jet Propulsion Laboratory, a division 
of the California Institute of Technology, for the NASA Science Mission 
Directorate. Lockheed Martin Space Systems, based in Denver, is the 
prime contractor and built the spacecraft. Ball Aerospace and 
Technologies Corp., of Boulder, Colo., built the HiRISE camera, which is 
operated by the UA Lunar and Planetary Laboratory.

CONTACTS:
John Grant (202-633-2474; grantj at si.edu)
Alfred McEwen (520-621-4573)

LINKS:
HiRISE - http://hirise.lpl.arizona.edu
CRISM - http://crism.jhuapl.edu/
MRO - http://www.nasa.gov/mro