Mars May Have Been Struck by Massive Object

ROBERT SIEGEL, host:

You're listening to All Things Considered from NPR News. The surface of Mars has a split personality. Even a casual observer will notice two distinct regions. In the south it appears craggy and pockmarked with craters. To the north, though, it looks smooth and flat. As NPR's Joe Palca reports, scientists now think they know why.

JOE PALCA: The first big telescopes revealed what scientists call the Mars dichotomy, a bumpy southern hemisphere and a flat northern hemisphere.

Dr. ODED AHARONSON (Associate Professor of Planetary Science, California Institute of Technology): Later on, when we measured Mars more accurately, we also recognized that there is a difference in elevation between the north and the south.

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PALCA: Oded Aharonson is a planetary scientist at Caltech in Pasadena. So not only is the south bumpier, the ground is much higher. There are two theories about how this could have happened. One is some internal mechanism that caused the thicker crust to move around the planet, sort of like the way the continents moved here on Earth.

Aharonson says the other theory is that something big smacked into the northern hemisphere on Mars. And by big, we are not talking big like a school bus or even big like the size of Texas. We are talking big!

Dr. AHARONSON: Imagine an object that is about 1,200 miles across, a little bit smaller than Australia, screaming through the sky at a speed roughly 20 times than a modern jetliner and colliding with Mars at roughly a 45-degree angle. And this would be quite an exciting day on Mars.

PALCA: Exciting anywhere, I'd say. Now, there have been objections to the impact theory.

Dr. AHARONSON: People expected that a large object hitting Mars would melt a significant faction of the planet and distribute the melt sufficiently globally that it would erase the record of the event.

PALCA: In other words, molten rock would engulf the entire planet, and the surface would be uniform, which it clearly isn't. But a paper published today in the journal Nature removes that objection. Aharonson used mathematical models to show an Australia-size rock would only cause molten rock to cover the northern half of the planet. That fits with what scientists see. But there is another objection.

Dr. AHARONSON: Smaller impact craters generally produce circular holes in the ground, circular craters.

PALCA: A large object should also produce a circular hole or possibly an elliptical one if it came in at an angle. But just looking at the site where the object would have hit Mars, you don't see the smooth edge of a crater. The boundary meanders around quite a bit.

That is where the work of Jeffrey Andrews-Hanna comes in. He is a planetary scientist at MIT. He used satellites orbiting Mars to study the boundary.

Dr. JEFFREY ANDREWS-HANNA (Planetary Scientist, MIT): The key was to use not only the topography, which only shows you the shape of the surface, but to use the gravity field of Mars, which provides this information about the structure of the planet beneath the surface.

PALCA: Much of the structure is hidden by lava from more recent volcanoes. But as he reports in this week's Nature, when you look below the newer material, you see something very different.

Dr. ANDREWS-HANNA: This dichotomy boundary didn't seem to follow an irregular path around the planet at all. It actually followed a very smooth and regular path.

PALCA: In fact, Andrews-Hanna says the boundary had an elliptical shape, just as you'd expect from a large object striking a glancing blow. So is the impact theory proven? No. Francis Nimmo is planetary scientist at the University of California Santa Cruz. He also has a paper in Nature suggesting the impact theory is the right one. But he says proving it probably means drilling into the surface of Mars to find chemical changes in the rock that would occur following an impact.

Dr. FRANCIS NIMMO (Earth and Planetary Sciences, University of California Santa Cruz): And so if you have good enough samples from Mars, you might be able to detect the chemical signature.

PALCA: Unfortunately, none of the landers currently on Mars are in a position to do that. Joe Palca, NPR News, Washington. Transcript provided by NPR, Copyright NPR.