[meteorite-list] Geologic Features in Martian Craters Suggest Deposition and Flow of Water and/or Ice

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 9 Feb 2009 16:09:36 -0800 (PST)
Message-ID: <200902100009.QAA08260_at_zagami.jpl.nasa.gov>

http://www.psi.edu/press/

Geologic Features in Martian Craters Suggest Deposition and Flow
of Water and/or Ice

Planetary Science Institute

Feb. 5, 2009 - Scientists at the Tucson-based Planetary Science
Institute (PSI) have found further evidence for the large role that
water has likely played in shaping the Martian landscape.

Their results, which will be published in Icarus and are now available
online at Science Direct <http://www.sciencedirect.com> (search All
fields:10.1016/j.icarus.2008.10.026), provide strong evidence that
multiple wet and/or icy climate cycles have shaped the topography of the
planet's large craters. Icarus is the journal of the American
Astronomical Society's Division of Planetary Sciences.

"Studying crater degradation in potentially ice-rich environments is
vital to understanding the geology of craters and their surroundings, as
well as for determining whether the ice comes from the atmosphere or
from below the ground," said Daniel Berman, a PSI associate research
scientist and lead author of the paper.

Berman, along with PSI Senior Scientist David Crown and PSI Research
Scientist Leslie Bleamaster III, surveyed the geologic features in two
sets of mid-latitude craters. Each set included about 100 craters, with
the first set in the Arabia Terra region of the northern hemisphere and
the second set in an area east of Hellas basin in the southern hemisphere.

The researchers selected craters that are greater than 20 km (about 12.5
miles) in diameter that have been completely or nearly completely
photographed by cameras on various spacecraft, including the Mars
Odyssey THEMIS VIS camera, the Mars Global Surveyor Mars Orbiter Camera,
and the Viking Orbiter cameras.

They looked specifically for the following erosional or depositional
features, the number and sizes of those features, and how the features
are oriented (i.e., whether they face the equator or the planet's pole
in their hemisphere):

-- Lobate flows - Lobe-shaped flow features that have pitted surfaces
and raised ridges on their lateral margins are observed on the walls of
some craters. These lobes resemble rock glaciers on Earth.

-- Channels - Narrow channels often breach crater walls and extend
outside the craters, as well as across crater floors, These channels may
have been formed by flowing water.

-- Crater-wall valleys - Trough-like crater-wall valleys, wider than the
above-mentioned channels, typically start at the top of the crater rim
and terminate where the wall meets the floor. These valleys are
sometimes filled with rough-textured deposits, which may be glacial.

-- Gullies and alcoves - Gullies are typically composed of three parts:
alcoves at the head of a channel, channels, and debris fans, and are
thought to have been formed by flowing water.

-- Arcuate ridges - These are small, arc-shaped ridges that enclose
depressions at the base of crater walls, often below gullies. Berman
interprets these to be glacial moraines, remnants of glacial deposits
that have since evaporated.

-- Debris aprons - These aprons are pitted and lineated deposits on
crater floors. They are similar to debris-covered glaciers or ice-rich
landslides seen on Earth.

All of these features suggest a landscape shaped by liquid water and/or
ice, Berman said. He found that lobate flows, gullies, and arcuate
ridges on the crater walls between latitudes of 30 to 45 degrees face
the pole in their hemisphere, whereas equator-facing orientations are
more common than pole-facing ones at latitudes between 45 and 60
degrees. In the southern study area, narrow channels generally had
pole-facing orientations, whereas wider valleys generally have
equator-facing orientations.

The features' pole-facing or equator-facing orientations could result
from uneven heating of the crater walls. Ice on walls that get more
sunlight would melt faster, causing more water to flow and form the
gullies and other features.

Unlike Earth, whose axis only oscillates through an arc of about four
degrees over millions of years, Mars appears to have an axis that tilts
between vertical and as much as 60 degrees, according to recent studies.

Such tilting could enhance ice deposition, Berman said. When one pole
begins leaning toward the sun, ice evaporates and then falls as snow at
the other pole, which is getting little sunlight. Such tilting could
have caused ice sheets to form in areas that are now ice-free, he added.

Further evidence for flowing ice is found on the crater floors, Berman
observed. He found that the floors of small craters slope away from the
walls that exhibit erosional/depositional features toward the more
pristine ones. These slopes have inclines of about 0.5 to 3 degrees.
This suggests that ice-rich materials flowed from one crater wall to the
other. Tilting floors are less evident in larger craters, although some
have gradually sloping floors where debris apron material is evident.

The PSI team's crater study has led them to make the following conclusions:

-- The orientation of erosional/depositional features (whether they face
the equator or the pole) suggests a direct relationship to total solar
heating along the crater walls.

-- Differences in the shape and size of various erosional/depositional
features can be explained by differences in crater-wall slopes, local
topography and orientation.

-- The geologic features and suites of features found in the craters may
have been created by multiple cycles of ice-sheet formation in response
to changes in the tilt of Mars' rotation axis.

This work was funded by a grant to PSI from NASA's Mars Data Analysis
Program (PI David Crown).

Figure: THEMIS VIS image V07798008, centered at -40.32? N, 132.5? E,
showing a 16 km diameter crater with gullies and arcuate ridges on its
north, pole-facing interior wall in the center of a larger (60 km
diameter) crater with lobate flows on its north, interior wall. Image
width is 17.4 km.
Received on Mon 09 Feb 2009 07:09:36 PM PST