[meteorite-list] Mars Express Radar Finds Possible Ocean Sediments

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 6 Feb 2012 16:28:59 -0800 (PST)
Message-ID: <201202070028.q170Sxl1017843_at_zagami.jpl.nasa.gov>

http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=49949

Mars radar finds possible ocean sediments
European Space Agency
06 Feb 2012

The MARSIS radar instrument on board ESA's Mars Express orbiter has
discovered a subsurface blanket of low density material around the north
polar cap, supporting theories that the northern lowlands of Mars were
once covered by a large body of water.

Although Mars is currently a frozen desert where liquid water cannot
exist on the surface for more than a few seconds, there is plenty of
evidence that the planet was once much wetter. Particularly striking are
the enormous channels ??? now dry - which were carved by raging torrents
of sediment-laden water. These relatively short-lived flash floods seem
to have occurred episodically from the late Noachian period until the
early Amazonian period, between 4 billion and 2 billion years ago.

The majority of these outflow channels disgorged onto the vast lowlands
which surround the ice cap at the north pole, leading many scientists to
favour the idea that they fed a shallow Martian ocean that once covered
more than 30 per cent of the planet's surface.

Indications have also been found of possible ancient shorelines that may
have been eroded by waves several billion years ago. Within these
shorelines, a possible sedimentary remnant of a former ocean, known as
the Vastitas Borealis Formation, was recognised on the basis of
geomorphologic and topographic observations. Now analysis of MARSIS
radar data has provided further geophysical evidence in favour of a
northern ocean.

The radar probes the nature of the planet's subsurface by revealing
contrasts in dielectric constant. In the case of Mars, these variations
are caused by two main parameters: the density of the materials and the
amount of ice in the ground. A high dielectric value indicates the
presence of dense material, whereas a low value shows a fairly low
density or high content of ice.

Unlike previous observations, which have only been able to study the
surface of Mars, the MARSIS low-frequency sounding radar is able to
probe the subsurface regions to depths of 60-80 m. Its observations of
the planet's hidden layers provide new insights into what Mars was like
in the distant past.

In a paper published in Geophysical Research Letters, an international
team of researchers has compared the physical properties of the
subsurface materials around the south and north poles by analysing two
years of MARSIS data. Their maps reveal some fundamental differences
between the two hemispheres.

In the south, the area corresponding to the lowest dielectric values
defines a roughly circular region which extends from the pole to 60??S
latitude. The radar observations probably indicate a distribution of
shallow ground ice in equilibrium with present surface temperatures and
the water vapour content of the atmosphere. The data are consistent with
the presence of ice-rich permafrost.

In the northern hemisphere, the MARSIS dielectric map shows a very
different pattern. The region with low dielectric value extends towards
the equator, far beyond the latitudes where shallow subsurface ice is
expected to be stable. This huge area covers the northern plains and
extends toward lower latitudes in the regions of Amazonis Planitia

"The measured value can only be explained by the presence of low
density sedimentary deposits, massive deposits of ground ice, or a
combination of the two," explained J??r??mie Mouginot, lead author of the
paper, who undertook the research as a PhD student under the leadership
of Wlodek Kofman at the Institut de Plan??tologie et d'Astrophysique in
Grenoble, France. Mouginot is currently a researcher at the University
of California, Irvine.

"It is not possible to distinguish between these different
compositions. However, we can definitely say that the Vastitas Borealis
Formation is not made of hardened volcanic flows, as some have suggested."

Of particular interest to the team is the apparent correspondence
between the region of low dielectric values and the postulated northern
ocean.

"The presence of low density material, potentially mixed with ice,
suggests that we are observing sedimentary material, tens of metres
thick, that was eroded by fast-flowing water and carried downstream
before being deposited in an ocean," said Wlodek Kofman, co-author of
the paper and a co-investigator on the MARSIS instrument team.

"The pattern of low dielectric values is consistent with the extent of
the Vastitas Borealis Formation, which extends toward low latitudes and
encompasses the three main catchment areas of the large outflow channels
that emanate from the southern highlands. The evidence clearly points
toward an oceanic origin for the shallow subsurface."

"The MARSIS subsurface radar gives us new and complementary information
about the history and distribution of water on Mars, compared to
previous results," said Olivier Witasse, ESA's Mars Express Project
Scientist. "This adds new pieces to the puzzle. The question still
remains: where did all the water go?"

Reference publication

"Dielectric map of the Martian northern hemisphere and the nature of
plain filling materials" by Jeremie Mouginot, Antoine Pommerol, Pierre
Beck, Wlodek Kofman, and Stephen M. Clifford, Geophysical Research
Letters, Vol. 39, L02202, 2012. DOI: 10.1029/2011GL050286

Contacts

Jeremie Mouginot
Department of Earth System Science
University of California, Irvine, USA
Phone: (mobile) +1 949 202 7132, (office) +1 949 824 3931
Email: jmouginouci.edu

Wlodek Kofman
Conseiller scientifique aupr??s du DGAR, Ecole Polytechnique
Institut de Planetologie et d'Astrophysique de Grenoble, France
Phone: +33 476514147
Email: wlodek.kofmanobs.ujf-grenoble.fr

Olivier Witasse
Mars Express Project Scientist
Research and Scientific Support Department
ESA Directorate of Science & Robotic Exploration
Noordwijk, The Netherlands
Phone: +31 71 5658015
Email: Olivier.Witasseesa.int
Received on Mon 06 Feb 2012 07:28:59 PM PST