By Lori Stiles

What scientists suspect might be ancient ocean shorelines on the northern
plains of Mars is actually a network of tectonic ridges related to
dramatic martian volcanism, a University of Arizona planetary sciences
graduate student and a collaborating post-doctoral researcher at the
Massachusetts Institute of Technology report in the April 5 issue of

Their new findings don’t rule out the possibility that an ancient ocean
once did cover the northern half of Mars. However, what previously has
been reported to be ancient shorelines apparently are not. The discovery
of the network of ridges “opens a new tectonic window into Mars,” the
authors say.

Paul Withers of the UA and Gregory A. Neumann of MIT analyzed dazzlingly
precise new views of Mars’ topography from the Mars Orbiter Laser
Altimeter (MOLA). The instrument continues an extended mission in orbit
around Mars on the Mars Global Surveyor spacecraft. MOLA transmits
infrared laser pulses towards the surface of Mars, and the measurements
are used to create topographic maps accurate to within a meter of
elevation. Viking era topographic maps of Mars were accurate only to
about a kilometer.

Withers worked last summer through a graduate student program with
members of the MOLA science team at the NASA Goddard Space Flight Center.
He and Neumann analyzed ridges that cover the enigmatic northern plains
of Mars. The region is the flattest known surface in the solar system,
and a leading theory is that an ocean created such extraordinary

Authors of a December 1999 article in Science identified candidate
shorelines of the possible ancient ocean based on the new MOLA maps. The
topographical profile shows a succession of flat terraces along a linear
slope in one case, and in another case a series of slopes in the right
relation to be shorelines.

Withers and Neuman specifically re-examined two leading candidate
paleoshoreline groups, one group near the Utopia impact basin and the
other on the opposite side of the proposed ocean near the Alba Patera

The details of the ridges near the Utopia basin don’t look like
paleocoastline, Withers said in an interview. “The morphologies are
inconsistent with formation by shoreline processes. There are the flat
terraces, but the ridges are on what would be the oceanward side. That’s
difficult to explain if you have an ocean coming in, flattening things
smooth over the terrace and then receding again.

He and Neumann conclude that the ridges record a history of enormous
tectonic stress and strain that forced the martian crust to form
10-mile-high volcanoes.

“Most ridges appear to be related to obvious stress centres, such as
the volcanic Tharsis Rise, the Utopia impact basin and the Alba Patera
volcano,” they report in Nature. The direction and shapes of these
ridges indicate that they have a tectonic origin.

The network of ridges is the only tectonic feature in the region.

“In future work, we hope that studying these ridges will reveal how the
huge martian volcanoes formed, what the martian crust and lithosphere
were like at the time, and what the northern plains of Mars are like
today beneath their blanketing surface layer of martian dust.”

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