Mars’s big splat

If Teddy Roosevelt had been able to visit Mars, he might have declared it a bully candidate for a national park. It boasts the highest known volcano in the solar system and the largest canyon. Now, three teams of scientists say Mars also has the largest impact basin in the solar system, encompassing some 42 percent of the planet’s surface.

The Borealis Basin is Mars’s broad northern bald spot. It has presented experts with a longstanding puzzle. On average, it’s about 2-1/2 miles lower than the southern highlands, which make up the rest of the planet. And the crust under the basin is nearly 16 miles thinner.

Over the years, scientists have offered dueling explanations for the difference. One involves a collision from a comet or asteroid. The other theory involves more mundane processes within the mantle beneath the crust.

One team, from the Massachusetts Institute of Technology and the US Jet Propulsion Laboratory, used geography and gravity data from two Mars orbiters to reconstruct the original elliptical shape of the basin – some 6,572 miles long and 5,270 miles wide – before later volcanic activity distorted its true outline. Voila! Signs of an impact.

The other two teams, from universities in California and England, backed up the “big splat” theory by modeling the kind of collision needed to sculpt the basin.

The results, which appear in today’s issue of the journal Nature, aren’t likely to settle the debate over the basin’s origins, other researchers note. But, they add, these studies certainly add weight to the idea.