[meteorite-list] Did An Impact Basin Make Water 'Reservoir' on Mars?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:32:52 2004
Message-ID: <200403221754.JAA05354_at_zagami.jpl.nasa.gov>

http://www.astronomy.com/Content/Dynamic/Articles/000/000/001/685ysvia.asp

Did an impact basin make a water "reservoir" on Mars?

More evidence surfaces that the Red Planet has long been rich in water.

by Robert Burnham
astronomy.com
March 19, 2004

According to an analysis by a team of planetary scientists headed by James
Dohm (University of Arizona), a large, underground aquifer lies within the
remains of an ancient impact basin in Arabia Terra on Mars.

Specifically, the team proposes that the Arabia basin spanned 1,800 miles
(3,000 km) in diameter, and that it formed sometime before about 3.5 billion
years ago. As essentially a big hole in the ground, such a place would have
attracted and trapped a lot of water and sediments during much of Mars's
history. The team has combined data from spacecraft in Mars orbit with
geophysical arguments to build what is admittedly a circumstantial case.

Waters of Arabia

This is not the first time Arabia Terra and water have been in the news
together. In 2002-3, the Mars Odyssey spacecraft's Gamma-Ray and Neutron
Spectrometers (GRS, NS) detected hydrogen in the upper yard (meter) of
martian soil. In fact, the Opportunity rover's landing site in Meridiani
Planum lies just outside the southwest margin of the putative basin.

Planetary scientists think the near-surface water detected at Arabia Terra
is likely a relic of the last martian ice age. This was a period ending
about 350,000 years ago during which the planet's rotation axis had a
greater tilt - 30 to 35 - than it does today (25). The greater tilt
caused snow to migrate from the polar caps down into the middle and even low
latitudes.

However, the subsurface water proposed by the Arizona team is older, buried
deeper, and may have only an incidental connection with the ice-age water
found near the surface.

Building a case

First things first. Does an ancient basin lie beneath Arabia Terra?
According to Dohm and the team, several lines of indirect evidence point in
this direction.

Cratering. Ejecta blankets surround an unusually high proportion of Arabia's
craters. The shape of the ejecta indicates that the ground contained ice or
water when the meteorites struck. "In addition," says Barlow, "many craters
also have central pits, which indicate the ground contained water at the
time of impact." The central pit craters also point to a long-term presence
of water. Says Barlow, "We find central pit craters in a wide range of ages.
This tells us that volatiles - water, ice - were in the ground during much
of Arabia's history."

Topography. Although no obvious impact basin appears in elevation data of
the region, it's possible an extremely old, degraded impact basin could be
there. "This area is ancient," says Nadine Barlow (Northern Arizona
University), a member of the team. "Thus the basin's appearance has had lots
of time to be overprinted by more recent geological activity."

Geophysics. "There's no strong positive or negative gravity anomaly in
Arabia," notes Dohm. "This contrasts with the two big known impact basins,
Hellas and Argyre." While this observation might seem like evidence against
the impact basin, Dohm says it isn't. He notes the proposed event occurred
so long ago the that the martian crust has had ample time to erase its
geophysical traces. "Arabia lies approximately antipodal to the volcanic
region of Tharsis," he says. "As that area grew over Mars's history, areas
on the opposite side of the planet would have risen in isostatic
compensation."

Another geophysical observation concerns the magnetic field impressed on
rocks as old as the suspected basin. These became magnetized when they were
molten, during the early period in which Mars had a geomagnetic dynamo in
its core (as Earth still does today). Any basin impact would heat rocks to
high temperatures and destroy the magnetization, as happened in the case of
ancient rocks within both Argyre and Hellas.

The rocks in Arabia Terra carry a weak magnetization still, which may appear
at first to argue against the basin. But the scientists think that if the
impact occurred while Mars still had a dynamo operating, then as the rocks
cooled, they would take on traces of the magnetic field then present.
"Arabia's rocks have a weak magnetic field, which indicates the basin's
ancient age," says Dohm.

Chemical elements. "The GRS data adds another layer of evidence," Dohm
notes. "Besides the maps of hydrogen (which indicate water), similar maps of
chlorine and silicon abundance show Arabia holds a concentration of these
elements not seen elsewhere on the planet." This fits the "catchment basin"
model of a big impact scar.

Geomorphology. "The northeastern edge of the proposed Arabia basin contains
the largest extent of 'fretted terrain' on Mars," says Dohm. This portion of
what would have been the basin's rim also lies toward another, smaller
impact basin, Isidis. "It's no surprise to find such a region of broken,
dissected terrain here," Dohm explains.

The northern side of Arabia's rim shows similar evidence of disturbance in a
network of water-carved channels that flow northward into the lowlands. On
the northwest side lies a major region of central pit craters, which Barlow
notes prefer to form on rims of big impact structures. In the southwest, the
scientists mapped drainage networks that issue into a basin not far from the
Opportunity landing area.

"The idea of an aquifer in Arabia . . . makes sense," Barlow says.

Digging deeper

When asked by Astronomy what would help clinch the case for the Arabia
basin, Dohm replied, "Ground-penetrating radar." That would let the
scientists look below the surface into the deep structure of the region. The
radar waves could also help determine to what extent water remains present
in the basin.

The MARSIS instrument on Mars Express, currently in martian orbit, will soon
begin operations. Its radar was designed to look 2 or 3 miles (3 or 4 km)
below the surface. Similarly, the SHARAD radar on the Mars Reconnaissance
Orbiter, due for launch in 2005, will look specifically for water, but it
will also map the subsurface - and perhaps reveal the outlines of what may
be one of Mars's oldest and largest impact basins.
Received on Mon 22 Mar 2004 12:54:35 PM PST


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