Thursday, September 16, 2010

Moon Crater Map Reveals Early Solar System History


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The first complete topographic map of the moon and its craters has revealed
details of billions of years of bombardment by asteroids, and the history of our
early solar system. Among other things, the map confirms theories of an
onslaught of massive asteroids around 3.9 billion years ago that likely
evaporated any water on Earth present at the time.

“Ever since the surface of the moon could be photographed, scientists have
counted craters on the moon and tried to decipher the projectile bombardment
rate and the geological history of the moon,” said geologist James Head of Brown
University, lead author of the study in Science September 16. “But
until now we’ve had uneven or low resolution coverage.”

The map was created using data from NASA’s Lunar Reconnaissance Orbiter
that has been circling the moon since June 2009. The orbiter measured the
height of the surface by sending billions of laser pulses towards the surface
and measuring the time it took for the pulses to return. The method is precise
enough it would have been able to detect a small house if there were one, Head

Once the map was made, Head and his team cataloged all the craters bigger
than 12.5 miles across, over 5,000 craters in total.

With the catalog of craters, the scientists were able to confirm there have
been two different eras of asteroid pummeling in the solar system. In the most
ancient regions of the moon the craters are all different sizes, large and
small. But in the younger regions of the moon that were resurfaced by volcanic
activity, the craters sizes are much smaller.

“The evidence we have is that the shift happened
before the dominant mare
[these volcanic flow regions appear as dark spots on the moon] were created 3.6
billion years ago, and probably before that,” said geologist Caleb Fassett of
Brown University, co-author of the study.

Planetary surface geologist Robert Strom, who first proposed the theory of
the shift in asteroid types in 2005, argues the shift was a result of of a
repositioning of Saturn and Jupiter around 3.9 billion years ago. The
gravitational pull of the planets causes there to be regions of the meteor belt
where anything that enters gets ejected. A shift in the positioning of the
planets would have changed the location of these regions and caused a relatively
sudden expulsion of meteors of all sizes that happened to be in those, now
vacant, areas.

“The intense bombardment that happened around 3.9 million years ago, the
earth didn’t escape that,” said Strom. “Earth was impacted so much and by such
big objects that it’s likely that if there was any water on Earth at that time
it would have been evaporated. And any life would have been terminated.”

Since this recalibration of the asteroid belt, only relatively small
asteroids have drifted into these vacant regions and been booted out, Strom
said. Small asteroids drift over time because they are affected by solar energy,
where as large ones have so much mass they are relatively motionless.

Mapping the topography of Mars and Mercury in the future will also help to
confirm this theory, Strom said.

The accurate map of the moon was also used to confirm the oldest regions on
the moon are the southern near side and the north-central far side. The moon was
volcanic about half of its history, until it cooled to the point where the
volcanism shut down around 2 billion years ago.


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