A NASA spacecraft charting the topography of the moon in exceptional detail has produced a catalogue of lunar craters that traces billions of years of impact history on the moon. The cratering record on the moon provides a proxy for similar impacts by interplanetary debris such as comets and asteroids on Earth, the effects of which have largely been erased by billions of years of erosion and geologic activity.

The Lunar Orbiter Laser Altimeter (LOLA), one of seven instruments that have circled the moon since June 2009 on board the Lunar Reconnaissance Orbiter (LRO), bounces a laser beam off the lunar surface and times its return to gauge the elevation of the terrain below. Fifty kilometers above the moon, LOLA can pinpoint the elevation of lunar landforms to within 10 centimeters.

A new study based on more than two billion laser readings taken as LRO cruised through its orbit has provided what may be the most complete accounting of lunar craters yet. The research appears in the September 17 issue of Science.

The researchers used LOLA's topography to identify more than 5,000 craters on the lunar surface, restricting the search to craters 20 kilometers in diameter and larger. The crater catalogue roughly doubles the size of that produced in 1978 by Don Wilhelms of the U.S. Geological Survey and his colleagues using photographs from NASA's Lunar Orbiter missions in the mid-1960s. The 20-kilometer-size cutoff was primarily set for the sake of expediency, as each crater must be manually counted. "At some point you would be doing nothing but counting craters for the rest of your life," says James Head, a planetary geoscientist at Brown University and the lead author of the study.

Previous crater counts had relied on less uniform data, including collections of photographs taken at different lighting angles that changed the craters' shadow profiles. But the new study's use of a single laser range finder means the results are not dependent on lighting or on differences between instruments. "It's a very useful contribution in the sense that it's the first time a homogeneous data set of the entire moon, front and back, has been examined from the point of view of crater distribution," says Jay Melosh, a planetary scientist at Purdue University who did not contribute to the new research.

With that data set, Head and his colleagues were able to determine which areas are truly ancient and which have been resurfaced relatively recently in lunar history. Lava flows three billion or four billion years ago, for instance, flooded lunar plains and filled craters there, whereas large impacts excavated vast amounts of lunar material that fell to the surface, burying or obscuring nearby craters.

The researchers found that large impactors, in addition to excavating huge basins, sprayed ejecta over vast swaths of the lunar surface. The impact that formed a large basin known as Orientale in the western area of the moon's near side, which the group investigated in detail, obliterated all prior craters within the basin itself, an area of nearly 700,000 square kilometers. But it also overwrote parts of the cratering record up to 500 kilometers away from the basin, reducing crater counts over an area of roughly three million square kilometers, or about 8 percent of the lunar surface. And, Head notes, Orientale is not even the biggest lunar basin—a larger impact such as the one that formed the Imbrium Basin would affect an even larger fraction of the terrain.

The crater map derived from LOLA data also confirms an idea put forth in 2005 by Robert Strom of the University of Arizona and his colleagues: that a shift in the impactor population took place around 3.8 billion years ago, around the time that Orientale Basin was excavated. The proportion of large (50- to 100-kilometer) craters is greater in the relatively ancient lunar highlands than in the younger lava plains, implying that the moon was pelted by two different populations of impactors in its history, the latter of which had fewer large bodies. One explanation for such a transition would be an orbital migration of the giant planets that disrupted the asteroid belt, sending a slew of rocky bodies flying toward Mars, Mercury, Venus, Earth and its moon early on.

Studying the craters on the moon offers a window into that violent history of the young solar system that is not nearly as accessible on Earth. "This all has implications for the Earth," Head says. "The Earth's record is abysmal back then because it's such a dynamic planet."