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Moon Crater Map Reveals Early Solar System History

The first complete topographic map of the moon and its craters has revealed details of billions of years of bombardment by asteroids, and the early history of our 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 present on Earth 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 Sept. 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, more than 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 [volcanic flow regions that 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 billion 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, whereas 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 that the oldest regions on the moon are the southern near side and the north-central far side. The moon was volcanic for 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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