Astronomers Explain Mars’s Lopsided Shape

[Dashken]

The lopsided shape of Mars may well be a result of a cataclysmic impact of a Pluto-size meteor billions of years ago, three teams of scientists are reporting. That would suggest that the lowlands of Mars’s northern hemisphere are a single gigantic impact crater, the largest crater in the solar system.

Artistic representation of the giant impact that formed the Martian dichotomy.

“The early solar system was a pretty exciting place,” said Francis Nimmo, an associate professor of earth and planetary sciences at the University of California, Santa Cruz, and the lead author of one of three scientific papers to appear in Thursday’s issue of Nature. “There were big collisions happening fairly frequently, and those collisions affected what the planets ultimately ended up looking like.”

About the same time, more than four billion years ago, Earth is believed to have been hit by a Mars-size object, which created the Moon, and signs of a giant impact have also been detected on Mercury.

NASA’s Viking orbiters observed in the 1970s that the bottom two-thirds of Mars was about two miles higher in altitude than its top third. Since then, planetary scientists have bandied about two hypotheses to explain the dichotomy: either some strangeness with the internal dynamics of Mars generated a thicker planetary crust in the south, or the northern surface was stripped away by a megameteor impact.

The impact idea, first proposed in the 1980s by Steven W. Squyres, now an astronomy professor at Cornell, and Don Wilhelms of the United States Geological Survey, ran into several objections. The boundary between lowlands and highlands does not have a simple round shape like most craters, there is no crater rim, and an impact that large should have, in theory, melted the entire surface. Then, when the rocks hardened again, the planet should have returned to the shape of a sphere, which is what occurred after the impact that created the Earth’s Moon.

The three Nature papers “have removed significant objections to the impact model,” said Walter S. Kiefer, a staff scientist at the Lunar and Planetary Institute in Houston, who wrote an accompanying commentary in Nature.

Dr. Squyres said the new findings did not prove that his idea was right, but “they’ve really gone and made some new observations, which make a strong case that the idea really makes sense.”

In the first paper, scientists at the Massachusetts Institute of Technology and NASA’s Jet Propulsion Laboratory took a closer look at the boundary, using topographical measurements by NASA’s Mars Global Surveyor orbiter and gravity measurements from the Mars Odyssey. About a third of the boundary is obscured by lava that flowed out of Tharsis, the largest volcano in the solar system. The topographic and gravity measurements enabled the scientists to peer at the geological structures below Tharsis.

“What we found, very surprisingly, is that the dichotomy boundary is very well matched by an ellipse on the surface of Mars,” said Jeffrey C. Andrews-Hanna, a postdoctoral researcher at M.I.T. and the lead author of the first Nature paper. “That was kind of the smoking gun for us.”

The ellipse, the scientists said, measures 6,500 miles by 5,300 miles.

Independently, two other teams of scientists performed computer simulations that indicate a meteor impact could plausibly create a crater of that size and shape. The two-dimensional simulations, performed by Dr. Nimmo and his collaborators at Santa Cruz and the University of London, show that “the crust basically gets stripped off half the planet,” Dr. Nimmo said. “You can watch the cavity it carved out wobble up and down and eventually come back to equilibrium.”

Simulations by scientists at Santa Cruz and the California Institute of Technology performed similar calculations, but in three-dimensions. Those calculations could only track the layer of crust coarsely, but could investigate impacts that struck at an angle; they found that a 30- to 60-degree angle produced elliptical-shaped craters and again showed that the impact would not have melted the entire Martian surface.

Oded Aharonson, an associate professor of geological and planetary sciences at Caltech and an author of the paper describing the three-dimensional model, said the impact would have released the energy of 75 trillion to 150 trillion megatons of TNT. To release that much energy, a meteor 1,250 miles wide — almost as large as Pluto — would have slammed into Mars at some 20,000 miles per hour.

None of the research disproves the alternate hypothesis that Mars’s internal dynamics produced the dichotomy, although it is unclear how such a process would generate an elliptical crater. Answering the question more conclusively will require more data from Mars.

Source: http://www.nytimes.com/2008/06/26/sc...ce/26mars.html

[Adrian Wong]

Wow.. That is very interesting!

[iModAMD]

Started to have an Head Ache at the 4th paragraph and started to nose bleeding at the 7th