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Wandering Poles Could Explain Vagaries of Ancient Martian Shoreline
Cambridge, Mass. - June 13, 2007 - Writing this week in the journal Nature, geophysicists demolish one of the key arguments against the past presence of large oceans on Mars. The finding, from scientists at Harvard University, the University of California, Berkeley, the University of Toronto, and the Carnegie Institution of Washington, suggests that there was likely once an ocean on Mars.
The scientists report that mysterious undulating shorelines along a large, sediment-filled plain surrounding Mars' north pole -- a plain that even from Earth looks like an ocean basin -- can be explained by the movement of Mars' spin axis, and thus its poles, by nearly 3,000 kilometers along the surface sometime within the past 2 or 3 billion years. Because spinning objects bulge at their equator, this so-called "true polar wander" could have caused shoreline elevation shifts.
"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," says lead author Taylor Perron, a postdoctoral fellow in Harvard's Department of Earth and Planetary Sciences. Perron conducted the research as a graduate student at UC Berkeley.
"When the spin axis moves relative to the surface, the surface deforms, and that is recorded in the shoreline," adds coauthor Michael Manga, professor of earth and planetary science at UC Berkeley.
In the 1980s, Viking spacecraft images revealed two possible ancient shorelines near the Martian north pole, each thousands of kilometers long with features like those found in Earth's coastal regions. The apparent shorelines -- in Martian regions dubbed Arabia and the younger Deuteronilus -- date from between 2 and 4 billion years ago.
But in the 1990s, NASA's Mars Global Surveyor mapped the Martian topography to a resolution of 300 meters, and found, curiously, that the purported shoreline varies in elevation by several kilometers (more than a mile), rising and falling like a wave with several thousand kilometers from one peak to the next. Typical shoreline elevations on Earth, measured relative to sea level, are constant, leading many experts to argue against the putative Martian shorelines' connection to past oceans.
Perron's calculations show that the resistance of Mars' elastic crust could create several-kilometer elevation differences for topographic features like a shoreline, in accord with topographic measurements. The Arabia shoreline varies in elevation by about 2.5 kilometers, while the Deuteronilus shoreline varies by about 0.7 kilometers.
"This is a beautiful result that Taylor got," says co-author Mark Richards, professor of earth and planetary science and dean of mathematical and physical sciences at UC Berkeley. "The mere fact that you can explain a good fraction of the information about the shorelines with such a simple model is just amazing. It's something I never would have guessed at the outset."
Richards adds: "This really confirms that there was an ocean on Mars."
The tilt of the rotation axis of a planet remains fixed relative to the sun, but the crust can move relative to this axis. The question remains: What caused Mars' rotation axis to move relative to the crust?
Any major shift of mass on a planet -- within the mantle, or between the mantle and the crust to form a volcano, or even via impact from outer space -- could cause a shift of the rotation axis because a spinning planet is most stable with its mass farthest from its spin axis. Richards has modeled true polar wander in Earth's past that was generated by the upwelling of hot mantle in the interior of the planet, which some scientists claim shifted our planet's rotation axis 90 degrees some 800 million years ago, essentially tipping the planet on its side.
Perron, Manga, Richards, and colleagues calculate that on Mars, an initial shift of 50 degrees from today's pole, equal to about 3,000 kilometers on the surface, would be sufficient to disrupt the Arabia shoreline, while a subsequent shift of 20 degrees from today's pole, or 700 kilometers, could have altered the Deuteronilus shoreline.
Interestingly, today's Martian pole and the two ancient poles lie in a straight line equidistant from the planet's biggest feature, the Tharsis rise, a bulge just north of the equator that contains Mars' most recent volcanic vent, Olympus Mons. Tharsis is the largest volcano in the solar system, and formed about 4 billion years ago, not long after Mars solidified. Dynamically, the relative positions of Tharsis and the pole path is exactly what would be expected for any mass shift on Mars that is smaller than the Tharsis rise, since the planet would reorient in a way that keeps Tharsis on the equator.
"This alignment is unlikely to occur by coincidence," the team writes in Nature.
Manga has a hunch about the mass shift that precipitated the tilt of Mars' rotation axis. If a flood of water had filled the Arabia ocean about 3 billion years ago, to a depth some have calculated at 700 meters, that mass at the pole might have been enough to shift the pole 50 degrees to the south. Once the water disappeared, the pole could have shifted back, then shifted again by 20 degrees during the deluge that created the Deuteronilus shoreline.
Because it's unclear whether the two shorelines represent separate inundations or whether one is the receded shoreline of a larger sea, an alternative scenario features the Arabia ocean receding to the Deuteronilus shoreline, shifting the pole from 50 to 20 degrees. Then, once the Arabia ocean disappears entirely, the pole returns to its current position.
Richards is skeptical of this, however, pointing out that thermal convection within the hot interior of Mars could also have caused the poles to wander.
"There must certainly be thermal convection in Mars now because Olympus Mons had new lava flows very recently, within the last 100 million years," he says. "But the jury's still out."
Manga says, too, that the source of the water, while unknown, must have produced a deluge greater than any observed on Earth, since huge canyons are cut in the flanks of the Tharsis rise. The water may have evaporated, but it may also have sunk back into underground dikes, frozen near the surface but possibly liquid below.
Perron, Manga, and Richards' coauthors are Jerry X. Mitrovica in the Department of Physics at the University of Toronto and Isamu Matsuyama of the Department of Terrestrial Magnetism at the Carnegie Institution of Washington, who have developed models for the effect of polar wander and internal dynamic processes on the surface deformation of Mars.
The work is funded by NASA's Astrobiology Institute, the Reginald A. Daly Postdoctoral Fellowship at Harvard, UC Berkeley's Miller Institute for Basic Research in Science, the Natural Sciences and Engineering Research Council of Canada and the NASA Mars Data Analysis Program.
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NOTE: Computer-generated images of how Mars' oceans may have looked are available.