What Happened to Mars?

Here's a sneak peek at a story that's likely to make a big splash — figuratively and literally — in a couple of months.

When spacecraft began orbiting Mars in the 1970s, planetary geologists realized that the Red Planet has a split personality. Most of its southern half is heavily cratered, much like the highlands of the Moon. But its northern half is dominated by vast, smooth plains, and these two very different terrains meet near the equator along an undulating boundary.

The mystery of this "crustal dichotomy" only deepened during the late 1990s, when Mars Global Surveyor found that the northern plains averaged 2 or 3 miles lower in elevation than the southern highlands, and that the planet's crust is distinctly thinner under the plains. Clearly, Mars's schizophrenic character was more than skin deep — but why?

Some scientists speculated that a giant impact could have leveled the top half of Mars early in its history. But when they tried to draw a circle along the dichotomy boundary, the fit just wasn't very good — particularly so in the area known as Tharsis, a continent-size bulge on the planet's equator that's capped by the largest volcanoes in the solar system.

Others then argued that a single, enormous blob of molten rock could have welled up from deep in the mantle and turned half of the Martian landscape to mush. It's geologically conceivable, perhaps, but rather far-fetched. (For example, why would it have happened only once?)

Now the "big splat" concept is suddenly back in vogue. At a conference about Mars last year, a trio of modelers led by Margarita Marinova (Caltech) argued that a single mega-impact could have given Mars the two-faced character we see today. They used what's called smooth-particle hydrodynamics to track the fate of 200,000 hypothetical bits of planet during computer simulations. Their results suggest that whacking Mars with a 500-mile-wide asteroid at about 4 miles per second would create an impact scar several thousand miles across and blanket the rest of the planet with a layer of crustal debris several miles thick.

But the problem of the poor fit of this putative mega-crater's rim to the current dichotomy boundary remained. And much of the boundary lies forever hidden under Tharsis.

A trio of geophysicists, led by Jeff Andrews-Hanna of MIT, has tackled the bad-fit problem anew. First, they "removed" Tharsis by carefully modeling how deeply this massive plateau affects the underlying crust. Then they reconstructed the dichotomy boundary and found a near-perfect fit to an ellipse measuring about 6,600 by 5,300 miles (10,650 by 8,520 km) centered at 66°N, 151°W.

Making his case yesterday to a roomful of planetary scientists, Andrews-Hanna explained that the mega-crater's rim doesn't have to be circular, particularly if the impactor struck Mars obliquely. For example, the Martian basin Hellas has an elliptical rim; so too does the South Pole-Aitken basin, an enormous largest impact on the Moon's far side.

More work needs to be done to refine the idea, but yesterday's audience was clearly impressed. Andrews-Hanna hopes to publish his conclusions within a couple months. Until then, you can peruse a summary of his team's analysis online.