[Mars-discuss] New data suggest Mars' northern hemisphere is an impact crater

[Thomas Coffee]

Given the role of impact events in Earth's biological history, this
could have serious implications for theories of early life formation
and propagation on Mars ...


---------- Forwarded message ----------
From: NASA Jet Propulsion Laboratory <info at jpl.nasa.gov>
Date: Wed, Jun 25, 2008 at 3:24 PM
Subject: NASA Spacecraft Reveal Largest Crater in Solar System
To: Thomas Coffee <tcoffee at mit.edu>


NASA Spacecraft Reveal Largest Crater in Solar System

PASADENA, Calif. -- New analysis of Mars' terrain using NASA
spacecraft observations reveals what appears to be by far the largest
impact crater ever found in the solar system.

NASA's Mars Reconnaissance Orbiter and Mars Global Surveyor have
provided detailed information about the elevations and gravity of the
Red Planet's northern and southern hemispheres. A new study using this
information may solve one of the biggest remaining mysteries in the
solar system: Why does Mars have two strikingly different kinds of
terrain in its northern and southern hemispheres? The huge crater is
creating intense scientific interest.

The mystery of the two-faced nature of Mars has perplexed scientists
since the first comprehensive images of the surface were beamed home
by NASA spacecraft in the 1970s. The main hypotheses have been an
ancient impact or some internal process related to the planet's molten
subsurface layers. The impact idea, proposed in 1984, fell into
disfavor because the basin's shape didn't seem to fit the expected
round shape for a crater. The newer data is convincing some experts
who doubted the impact scenario.

"We haven't proved the giant-impact hypothesis, but I think we've
shifted the tide," said Jeffrey Andrews-Hanna, a postdoctoral
researcher at the Massachusetts Institute of Technology in Cambridge.

Andrews-Hanna and co-authors Maria Zuber of the Massachusetts
Institute of Technology, and Bruce Banerdt of NASA's Jet Propulsion
Laboratory in Pasadena, Calif., report the new findings in the journal
Nature this week.

A giant northern basin that covers about 40 percent of Mars' surface,
sometimes called the Borealis basin, is the remains of a colossal
impact early in the solar system's formation, the new analysis
suggests. At 8,500 kilometers (5,300 miles) across, it is about four
times wider than the next-biggest impact basin known, the Hellas basin
on southern Mars. An accompanying report calculates that the impacting
object that produced the Borealis basin must have been about 2,000
kiolometers (1,200 miles) across. That's larger than Pluto.

"This is an impressive result that has implications not only for the
evolution of early Mars, but also for early Earth's formation," said
Michael Meyer, the Mars chief scientist at NASA Headquarters in
Washington.

This northern-hemisphere basin on Mars is one of the smoothest
surfaces found in the solar system. The southern hemisphere is high,
rough, heavily cratered terrain, which ranges from 4 to 8 kilometers
(2.5 to 5 miles) higher in elevation than the basin floor.

Other giant impact basins have been discovered that are elliptical
rather than circular. But it took a complex analysis of the Martian
surface from NASA's two Mars orbiters to reveal the clear elliptical
shape of Borealis basin, which is consistent with being an impact
crater.

One complicating factor in revealing the elliptical shape of the basin
was that after the time of the impact, which must have been at least
3.9 billion years ago, giant volcanoes formed along one part of the
basin rim and created a huge region of high, rough terrain that
obscures the basin's outlines. It took a combination of gravity data,
which tend to reveal underlying structure, with data on current
surface elevations to reconstruct a map of Mars elevations as they
existed before the volcanoes erupted.

"In addition to the elliptical boundary of the basin, there are signs
of a possible second, outer ring -- a typical characteristic of large
impact basins," Banerdt said.

JPL manages the Mars Reconnaissance Orbiter for NASA's Science Mission
Directorate, Washington. For more information about the mission,
visit: http://www.nasa.gov/mro .



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Thomas Coffee
Massachusetts Institute of Technology
Department of Aeronautics & Astronautics
Space Systems Laboratory

NASA Jet Propulsion Laboratory
Mission Systems Concepts
Systems Analysis & Modeling Group
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