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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 6 Mar 2008 08:52:15 -0800 (PST)
Message-ID: <200803061652.IAA28206_at_zagami.jpl.nasa.gov>

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
Received on Thu 06 Mar 2008 11:52:15 AM PST