THE AUTHOR OF the new analysis of gentle, parallel ridges in the
blandest, flattest northern plains of enigmatic Mars argues instead that the
features are large landscape bumps that resulted simply to relieve surface
pressure from massive volcanoes, such as Tharsis, and other structures on
the planet's surface.
"We don't see the paleo (ancient) shorelines that some people see,"
said Paul Withers of the Lunar and Planetary Laboratory at the University of
Arizona. "We see an network of tectonic ridges that helps us explain how the
Tharsis volcano formed. I wouldn't make too many sweeping conclusions about
whether there was or was not an ocean based on this. The northern plains are
still flat, and nobody has a clue why."
TECTONIC FORCES
Unlike plate tectonics, which collide, disrupt and mold huge land
forms on Earth, the tectonic forces at Mars result from the stresses
required to hold up mountains and volcanoes - some of which reach as high as
10 miles.
Those stresses build up over time and cause crustal compression that
eventually squeezes large slabs of land and forms the ridges that initially
look like shorelines, Withers said. The features likely formed about 3
billion years ago.
Withers and his colleague Gregory Neumann of the Massachusetts
Institute of Technology and NASA's Goddard Space Flight Center in Maryland
published their brief analysis in Thursday's issue of the journal Nature.
The tip-off to a non-oceanic origin for the ridges included raised
lips adjacent to ridges on what is the most featureless landscape in the
solar system, Withers said.
Withers and Neumann analyzed topographic data on the ridges measured
in the past few years by NASA's Mars Global Surveyor, currently orbiting the
Red Planet.
"Specifically what we saw on one group was a nice flat terrace you'd
associate with a shorelines, and then a raised lip on what would be the
ocean-ward side of this terrace," Withers said. "It's difficult to imagine
how you'd form this by an ocean or sea washing in and then receding away to
leave this lip."
HARBORED LIFE
The new research, arguing against a 1999 paper published by Brown
University's James Head and colleagues, relies on more data than the
previous team had.
Head's team argued for an ocean that covered the northern third
of the planet - adding to mounting evidence that Mars once harbored life. It
was a finding compatible with others coming earlier and later, pointing to
what looked like enormous dry river channels once flooded with water or
carved by ice. Those channels feed into the northern flatlands basin.
But the new data show a network of ridges crisscrossing the plains,
as well as many smaller ridges there, Withers said - features you wouldn't
expect on an ocean floor.
"All the attempts to find shorelines or not - they don't seem to be
succeeding too well," Withers said, referring to similar work by Mike Malin
and colleagues, where they searched for evidence of shorelines and failed to
find them.
Withers remained open to the idea that the flatlands once harbored an
ocean, but said that, thus far, the evidence is not apparent.
"No one has a clear (idea) what a billion-year-old shoreline on Mars
is going to look like, but it's unlikely to look like the candidates that we
've tested," he said. "This featureless area on Mars is covered with this
network of tectonic features will give geophysicists something to think
about."
James Head is thought to be working on a longer analysis of the ridge
features that could be published later this year. He was out of the country
Wednesday and could not be reached for comment.
THE EARTH MODEL
Victor Baker, a planetary geologist at the University of Arizona who
also has studied Mars' northern flatland ridges, was an early advocate of
the northern ocean idea.
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Competing teams of researchers have been studying somewhat
different sets of ridge features in this area on Mars, Baker said.
"The ones that Head and others had interpreted had always been
suspiciously possible tectonic forms because of their scale," he said, "so
that's not an interpretation that would surprise me."
And Withers' and Neumann's finding actually could still point to the
idea of a northern ocean, Baker said.
"It's not inconsistent with this having been a basin which could be a
possible receptacle for water because obviously the Earth's ocean basins are
tectonic features. Tectonic processes resulted in those depressions. So the
fact that these are tectonic features that line up along the margins doesn't
say anything about whether there was ponded water there.
"It's very difficult to prove the positive and it's difficult to say
there never was any water there. We have a lot of other indications that
inundation of the northern plains to some degree or other, by some processes
related to water, possibly mud flows, has occurred during Martian history.
But it's difficult to establish that in an absolute way that will get the
whole community to believe it."
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