'Ancient Mars had a huge ocean'
2007-06-14 10:20
Washington - Long, undulating features
on the northern plains of Mars probably are remnants of
shorelines of an ocean that covered a third of the planet's
surface at least 2 billion years ago, scientists said on
Wednesday.
The geological features, stretching thousands of kilometres, were first revealed in the 1980s in Viking
spacecraft images. But topographical data collected by Nasa's
Mars Global Surveyor in the 1990s cast doubt on whether the
features truly marked a long-gone sea coast.
The Global Surveyor found big, mountain-sized variations in
elevation along the suspected shorelines, whereas a shoreline
should be a constant elevation matching sea level.
Moving poles
But scientists writing in the journal Nature said the
movement of the Martian poles and also the planet's spin axis
by roughly 3 225 km in the past 2 billion to 3
billion years would have triggered deformation of surface
features just like that seen in the suspected coastlines.
"The pole moves and it warps the shorelines," planetary
scientist Taylor Perron of Harvard University, the study's lead
author, said in a telephone interview.
"We have don't have direct confirmation that there were
oceans because, of course, the water isn't there any more. But
what we've done is to eliminate one of the main reasons to
doubt that they were ever there."
Earth's poles also have moved in the past.
At some point, a big shift of mass on Mars caused its north
pole to shift 50 degrees toward its present location and the
planet's change in orientation changed the topography of the
shorelines, said physicist Jerry Mitrovica of the University of
Toronto, one of the researchers.
A Martian Pacific
The ocean may have covered a third of the Martian surface
during the first half of the planet's history before
disappearing at least 2 billion years ago for unknown reasons,
the researchers said.
"Relative to the size of the planet, this ocean would have
been about the same with respect to Mars as the Pacific Ocean
is with respect to Earth," Perron said.
Some water is retained as ice at the Martian poles and some
scientists believe much more is trapped underground.
Beyond adding to the understanding of Earth's next-door
neighbour in our solar system, evidence of water also is
critical to the issue of whether Mars has ever harboured life as
we know it.
'Issue of life linked to question of water'
"It is certainly true that the issue of life is
inextricably linked to the question of water. So, at least
indirectly, we have shown that there were once huge bodies of
water on Mars," Mitrovica said by e-mail.
Scientists have named the two features suspected of being
ancient shorelines Arabia and Deuteronilus.
The long lines on
the Martian surface rise and fall in a way resembling a sea
wave.
The elevation of the Arabia shoreline changes by 2.4 km, while the Deuteronilus shoreline varies by 0.7 km.
The researchers said another important Martian geological
feature - the volcano Tharsis, the biggest in the solar system
- lends support to their hypothesis on polar movement.
Tharsis is so massive it would always keep itself on the
spinning planet's equator, they said.
They calculated that the
suspected path taken by the moving poles would have preserved
the volcano's equatorial position.