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 Nature.

Projection of MOLA data of Martian north pole to equator. Black lines indicate position interpreted shorelines. Credit: MOLA Science Team

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 smoothness.

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 volcano.

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."