[meteorite-list] Five Giant Impact Basins Reveal the Ancient Equator of Mars

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Apr 18 16:43:34 2005
Message-ID: <200504182041.j3IKfQP10599_at_zagami.jpl.nasa.gov>

http://www.eurekalert.org/pub_releases/2005-04/agu-fgi041805.php

Public release date: 18-Apr-2005

Contact: Harvey Leifert
hleifert_at_agu.org <mailto:hleifert@agu.org>
1-202-777-7507
American Geophysical Union <http://www.agu.org>

Five giant impact basins reveal the ancient equator of Mars

WASHINGTON - Since the time billions of years ago when Mars was formed,
it has never been a spherically symmetric planet, nor is it composed of
similar materials throughout, say scientists who have studied the
planet. Since its formation, it has changed its shape, for example,
through the development of the Tharsis bulge, an eight kilometer [five
mile] high feature that covers one-sixth of the Martian surface, and
through volcanic activity. As a result of these and other factors, its
polar axis has not been stable relative to surface features and is known
to have wandered through the eons as Mars rotated around it and revolved
around the Sun.

Now, a Canadian researcher has calculated the location of Mars' ancient
poles, based upon the location of five giant impact basins on the
planet's surface. Jafar Arkani-Hamed of McGill University in Montreal,
Quebec, has determined that these five basins, named Argyre, Hellas,
Isidis, Thaumasia, and Utopia, all lie along the arc of a great circle.
This suggests that the projectiles that caused the basins originated
with a single source and that the impacts trace the Martian equator at
the time of impact, which was prior to the development of the Tharsis
bulge, he says.

Writing in the Journal of Geophysical Research (Planets), Arkani-Hamed
calculates that the source of the five projectiles was an asteroid that
had been circling the Sun in the same plane as Mars and most of the
other planets. At one point, it passed close to the planet, until the
force of Martian gravity surpassed the tensile strength of the asteroid,
at which point it fragmented. The five large fragments would have
remained in the same plane, that of Mars' then-equator. They hit in
different spots around the Martian globe, due to Mars' rotation on its
then-axis and the differing lengths of time the fragments took before
impacting on Mars.

Arkani-Hamed describes the locations of the resulting basins, only three
of which are well preserved. The two others have been detected by
analysis of Martian gravitational anomalies. The great circle they
describe on the Martian surface has its center at latitude -30 and
longitude 175. By realigning the map of Mars with that spot as the south
pole, the great circle marks the ancient equator.

Arkani-Hamed estimates that the mass of the asteroid captured by Mars
was about one percent of that of Earth's Moon. Its diameter was in the
range of 800 to 1,000 kilometers [500 to 600 miles], depending upon its
density, which cannot be determined.

The significance of Arkani-Hamed's findings, if borne out by further
research, is that the extent of presumed underground water on Mars would
have to be reassessed. "The region near the present equator was at the
pole when running water most likely existed," he said in a statement.
"As surface water diminished, the polar caps remained the main source of
water that most likely penetrated to deeper strata and has remained as
permafrost, underlain by a thick groundwater reservoir. This is
important for future manned missions to Mars."

###

The research was supported by the Natural Sciences and Engineering
Research Council (NSERC) of Canada.

 
Received on Mon 18 Apr 2005 04:41:26 PM PDT