[meteorite-list] Mars Express Images: 'Hourglass' Shaped Craters Filled with Traces of Glacier

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Mar 21 13:24:59 2005
Message-ID: <200503181938.j2IJcuR28407_at_zagami.jpl.nasa.gov>

http://www.esa.int/export/SPECIALS/Mars_Express/SEMN3IRMD6E_0.html

"Hourglass" shaped craters filled with traces of glacier
European Space Agency
Mars Express
18 March 2005

This image, taken by the High Resolution Stereo Camera (HRSC) on board
ESA's Mars Express spacecraft, shows flow features most likely formed by
glaciers or "block" glaciers.
 

Map showing location of craters in context

This unusual "hourglass"-shaped structure is located in Promethei Terra
at the eastern rim of the Hellas Basin, at about latitude 38? South and
longitude 104? East.

A so-called "block" glacier, an ice stream with a large amount of scree
(small rocks of assorted sizes), flowed from a flank of the massif into
a bowl-shaped impact crater (left), nine kilometres wide, which has been
filled nearly to the rim. The block glacier then flowed into a 17
kilometre wide crater, 500 metres below, taking advantage of downward
slope.

The Martian surface at mid latitudes and even near the equator was being
shaped by glaciers until a few million years ago. Today, water ice could
still exist at shallow depths as "fossil" remnants of these glaciers.

Numerous concentric ridges are visible and appear similar to "end
moraines" (hills of scree that form as an extending glacier pushes
material ahead and remain after its retreat). Furthermore, there are
parallel stripe-like structures that are interpreted as middle moraines,
displaying the flow direction of these glaciers.

 
 
Perspective view of "hourglass" shaped craters, looking south-east

In locations where glaciers creep over steep terrain, cracks are
visible. Similarly in terrestrial glaciers, cracks are formed when
tensile stress within the ice increases due to greater slope and uneven
terrain.

Further glacial features include elongated grooves, extending several
kilometres, and elongated hills observed on the surface of mountain
ridges some distance from potentially glaciated areas. These hills could
be analogous to so-called "drumlins", structures formed beneath ice by
glacial flow resulting in compression and accumulation of abraded
material. On Earth, drumlins appear in formerly glaciated regions such
as Germany's Bavarian alpine uplands. These glacial structures are seen
in a consistent spatial context, confirming the belief that scientists
are really seeing former glaciers on Mars.

Of particular interest is the age of these glacially shaped surfaces,
which seem to be fairly intact over a wide area of the formerly
glaciated terrain. Typical evidence for a significant loss of ice
volume, such as "kettle holes" present in ice-free regions of Iceland,
are almost entirely missing. The statistical analysis of the number of
craters formed by meteorite impacts used for age determination also
shows that part of the surface with its present-day glacial
characteristics was formed only a few million years ago. In planetology,
this age range is considered extremely young.

In these latitudes, ice on the surface of Mars is not stable over a long
period of time due to the extremely thin atmosphere. In theory it is
cold enough to allow for the existence of glaciers at the equator -
summer day temperatures rise to a maximum of 20? C while night and
winter time temperatures often drop below minus 50? C.

But under the prevailing atmospheric pressure, ice would sublimate
(transform directly from solid to gaseous state), and then escape from
the atmosphere into outer space.

 
 
3D anaglyph view of "hourglass" shaped craters

Therefore, glaciers must have formed until a few million years ago, in a
time that was warmer and possibly also had a thicker atmosphere, and
then became inactive or retreated due to the lack of continued supply of
ice. Since then, they have been protected from sublimation by a thin
dust layer. On Mars, dust is almost ubiquitous and would explain why
"fossil" ice present at depths of only a few metres could not be
detected by other instruments such as spectrometers.

If these conclusions prove to be true, the results would indicate a
climate change on Mars within the last million years. Such a dramatic
climate change has been discussed for some years by Mars researchers. It
could have been caused by a shift in the polar axis of the planet over
millions of years - a phenomenon long known to scientists. Martian
climate history is one of the main areas that ESA's Mars Express can
help to decipher.

The colour images were processed using the HRSC nadir (vertical view)
and three colour channels. The perspective views were calculated from
the digital terrain model derived from the stereo channels.

The 3D anaglyph image was created from the nadir channel and one of the
stereo channels. Stereoscopic glasses are needed to view the 3D image.
Image resolution has been decreased for use on the internet.
Received on Fri 18 Mar 2005 02:38:55 PM PST