The National Aeronautics and Space Administration's Mars Reconnaissance Orbiter and Mars Global Surveyor have transmitted elevation and gravity data that might resolve one of the solar systems biggest unanswered questions.

"The mystery of the two-faced nature of Mars has perplexed scientists since the first comprehensive images of the surface were beamed to Earth by spacecraft in the 1970s," NASA's Jet Propulsion Laboratory scientists said. "The main hypotheses have been an ancient impact or some internal process related to the planet's molten subsurface layers. The impact idea, proposed in 1984, fell into disfavor because the basin's shape didn't seem to fit the expected round shape for a crater."

The new data suggest the northern area -- the smoothest surface found so far in the galaxy -- is actually a massive elliptical impact crater approximately 5,300 miles in diameter. Scientists estimate it was caused about 3.9 billion years ago by an object that must have been about 1,200 miles across.

It took new gravity and elevation data to reveal the crater.