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Giant Asteroid Flattened Half of Mars, Studies Suggest

Hemisphere-size crater documents solar system's violent origins

The Phoenix Lander may have dominated Mars news in recent weeks, but a new study performed here on Earth has turned up a whopper of a finding: The Red Planet seems to have been the victim of a massive hit and run more than four billion years ago.

That is the conclusion of researchers who have finally mapped the edges of something known as the Martian hemispheric dichotomy.

That feature—in which the crust thickness drops from 30 to about 10 miles (50 to 20 kilometers) over a large area that is the most visible feature on Mars—has been known to astronomers for more than 30 years and was long suspected to be due to an asteroid impact that flung most of the crust out the area.

Scientists could not say for sure, however, because the dichotomy's exact shape was unclear: As much as a third of its edge was obscured beneath a 20-mile- (30-kilometer-) high pile of volcanic rock (the second largest feature on Mars) known as the Tharsis Rise.

To uncover the dichotomy's true edge, researchers from the Massachusetts Institute of Technology (M.I.T.) and NASA's Jet Propulsion Laboratory in Pasadena, Calif., used geologic data to probe the structure of the crust underneath Tharsis.

They combined data on the surface height, or topography, with variations in mass revealed by disparities in the surface's gravitational force, looking for telltale changes in mass under Tharsis.

The analysis revealed an elongated round shape measuring about 6,600 by 5,300 miles (10,600 by 8,500 kilometers) and covering 42 percent of the planet. The team calls it the Borealis basin.

"There's basically only one process we know of that will produce a giant elliptical depression...and that's a giant impact," says M.I.T. planetary scientist Jeffrey Andrews-Hanna, lead author of the report, published in Nature.

One longstanding puzzler that did not quite fit with the asteroid idea was that although the asteroid would have left a circular crater or impact basin, the dichotomy seemed at best elliptical in shape. But according to a second Nature study, an asteroid impact still could have caused that shape.

A separate team from the California Institute of Technology in Pasadena and the University of California, Santa Cruz, simulated the effect of impacts made at different energies, speeds and angles on the Martian crust.

They found that the dichotomy's newly revealed shape matched an impact by an asteroid measuring 1,000 to 1,700 miles (1,600 to 2,700 kilometers) wide, moving at about four to six miles (six to 10 kilometers) per second and striking at an angle of 30 to 60 degrees with the ground, with the most likely angle being 45 degrees.

The study also throws cold water on a second objection to the impact hypothesis: Scientists have thought that rock melted by the powerful strike would have simply filled in the basin and erased any record of its effects. According to the new simulation, the impactor would have thrown clear enough of the crust to leave a marked depression.

In yet another consistency check, a second simulation paper in Nature reports that reverberations through the crust caused by the collision could account for a known decline in magnetism in Martian rock on the opposite side of the planet from the dichotomy.

That a large asteroid would have hit Mars is not unexpected: Craters that pepper the inner planets and moons of the solar system indicate that space rocks crashed together like pool balls when the planets were young. For example, researchers believe that our moon was formed when a Mars-size object crashed into a young Earth.

The new studies have "upped the odds a lot" that an impact caused the dichotomy, says planetary scientist Walter Kiefer of the Lunar and Planetary Institute in Houston. The next line of evidence, says Kiefer, would be looking for chemical differences in Martian rocks indicating that crust in the dichotomy formed from material deep inside the mantle that welled up after the impact.

The kind of asteroid needed to form the Martian dichotomy would fall in between that size and those of the rocks that formed other large craters, such as the South Pole—Aitken impact basin on the moon and the Hellas Basin in Mars's southern hemisphere, both more than 1,30 miles (2,000 kilometers) wide.

Although Earth's inhabitants still face the threat of deadly asteroid strikes, researchers say that impacts of the magnitude seen in the Martian dichotomy have long since died off, leaving only scars to tell their story.

"The early solar system was a very dangerous place to be," Andrews-Hanna says, "but if we didn't have the impacts, we wouldn't have the planets as we see them today."

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