Giant asteroid may explain origins of Mars landscape

An asteroid the size of Alaska slammed into Mars about 4.4 billion years ago, creating a 700-trillion kiloton blast that forever deformed the Red Planet, suggest three studies out Wednesday.

"It was a bad day for any Martians when this happened," says planetary scientist Francis Nimmo of the University of California, Santa Cruz, lead author of one of the studies in today's edition of the journal Nature.

Half the size of Earth, Mars has a curious split appearance, featuring a largely smooth northern plain and mottled southern highlands.

Since close-up mapping of Mars in the 1970s, scientists have debated the origins of the split, with massive volcanism seen as the most likely culprit for the planet's northern lava plains. A few others scientists, including Mars-rover expert Steve Squyres of Cornell University, suggested an asteroid impact instead.

Three papers in Thursday's journal Nature provide the most convincing evidence yet that an outside force was responsible.

According to the researchers, the asteroid or comet that hit Mars blasted away much of its northern crust and creating a giant hole over 40 percent of the surface.

New calculations reveal the crater known as the Borealis basin measures 5,300 miles across and 6,600 miles long — the size of Asia, Europe and Australia combined. It's believed to be four times bigger than the current titleholder, the South Pole-Aitken basin on Earth's moon.

Scientists who had no role in the studies said the latest research strengthens the case for a colossal Martian impact, but it does not rule out the other theory that hot rock from inside the planet could have welled up and formed the different crusts.

"The betting odds have gone up a lot in favor of the impact model," said Walter Kiefer, a staff scientist at the Lunar and Planetary Institute in Houston.