Ancient Martian oceans warped by redistribution of planet's mass

William Atkins
Thursday, 14 June 2007 08:53

Science - Space

About one-third of the surface of Mars once was covered with water—that is, over two million years ago. It dried up and left flat shorelines—which is a sure sign that an ocean was once present. However, today, the water is gone and so are most of the flat shorelines.

Researchers are unsure where exactly the water went—hypothesizing that it was too much water to simply evaporate. Thus, some scientists contend that some of the water remains in underground reservoirs.

They also wondered where the flat shorelines went. However, that puzzle has been solved.

Researchers have now shown that warping of the Martian rock by a movement of the Martian poles over the past two million years or so altered the flat shorelines.

The results of the findings are published in the June 14, 2007 issue of the journal Nature. U.S. planetary scientists Mark Richards and Michael Manga, both from the University of California at Berkeley (UCB), Taylor Perron, previously from UCB but now from Harvard University’s Department of Earth and Planetary Sciences, and colleagues performed the research and co-authored the Nature article.

In the paper, the researchers talk about how two oceans—the older Arabia Ocean and the younger Deuteronilus Ocean—once had two shorelines that were thousands of miles in length. Now with the water gone, scientists have always contended that the dried out shorelines were too hilly to be edges of ancient oceans.

The researchers discovered, however, that several billion years ago, a shift occurred toward the equator, which forced the shores of the two oceans to warp. This shift is likely due to a massive re-distribution of the planet’s mass. More mass was moved toward the equator and away from the poles. This is the same reason why the Earth is shaped like an oblate spheroid, a sphere with a bulging middle.

In fact, Perron states: "On planets like Mars and Earth that have an outer shell, or lithosphere, that behaves elastically, the solid surface will deform differently than the sea surface, creating a non-uniform change in the topography.”

Near the equator the shores remained flat, however as the shoreline went north or south towards the poles they warped into hilly terrains in a long wave that rose and fell (what scientists called undulated) in height as much as several kilometers along their lengths.

This change in the shoreline agreed to what happened to the axis along which the planet spins. The poles of Mars have shifted by about 1,850 miles (3,000 kilometers) over the past two or three billion years. The tilt of the rotational axis of Mars did not actually move in relations to the position of the Sun—it remained the same. However, the mass of the planet shifted.

Scientists call this shifting process true polar wander. It is known to cause topographic changes to a planet’s surface. In the case of the two Martian oceans, it caused them to change from being flat to hilly; thus, rising and falling over long distances. Scientists say that the same thing happened on the Earth, however, in distances of centimeters and meters rather than kilometers.

What does this research result mean? It adds to the possibility that large amounts of water still exist on Mars and life once existed on Mars.

A enhanced image of Mars, taken from Viking Orbiter, and topographic data, taken from the Mars Orbiter Laser Altimeter onboard the Mars Global Surveyor spacecraft, shows Mars as how it might have looked two million years ago. See the image at the University of California at Berkeley’s website: http://www.berkeley.edu/news/media/releases/2007/06/13_mars.shtml. Additional information about the project is also found on this website.

The other collaborators include Jerry X. Mitrovica, from the University of Toronto (Ontario, Canada) and Isamu Matsuyama, from Carnegie Institution of Washington (Washington, D.C.).

The authors’ work is part of UCB’s BioMars project, which is funded by NASA’s Astrobiology Institute. It is supported by UCB’s Miller Institute for Basic Research in Science, the NASA Mars Data Analysis Program, and the Natural Sciences and Engineering Research Council of Canada.