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 said.

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.

Images: NASA/LRO/LOLA/GSFC/MIT/Brown

See Also:

• Our Solar System: Now With 2 Million Years More Maturity
• Edge of Solar System Is Not What We Expected
• Pieces of Infant Solar System Found in Comet’s Wake
• Oldest Martian Meteorite Not as Old as Thought
• Moon Water Dreams Evaporate
• Asteroid Impact Craters on Earth as Seen From Space
• Millions of Tons of Water Ice Found at Moon’s North Pole

Authors: Jess McNally

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