From the ArcaMax Publishing, Science & Technology Newsletter:
http://www.arcamax.com/news/technology/s-369368-156119
PASADENA, Calif. (UPI) -- U.S. scientists say data from two satellites
suggests Mars once suffered a gigantic impact, creating the planet's
distinctly different hemispheres.
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.