Moon’s craters can help improve Solar System surface-dating methods

LONDON - Images of the Moon’s surface, sent by NASA’s Lunar Reconnaissance Orbiter (LRO), are shedding new light into the mechanics of asteroid and comet impacts and how frequently they occur-information that could improve estimates of the age of geological formations on other planets.

The work, said planetary geologist Peter Schultz of Brown University in Providence, Rhode Island, “gives us another foothold into dating the Solar System”, reports Nature.

Craters on Earth are quickly eroded, so there are few well preserved impact sites here for researchers to study. However, the Moon offers a natural laboratory for understanding how impacts excavate craters and generate pools of molten rock. Brett Denevi, a planetary scientist at Arizona State University in Tempe, and her colleagues have found that craters have a wide range of melt volumes and they are working to determine the factors, such as the speed, composition and approach angle of the impactor, that might account for this variability.

Other scientists are using the data to find newly formed craters. By comparing the LRO pictures with images collected by Apollo missions in the 1970s, they have found five craters that have appeared in the past four decades. That is helping the team to determine how frequently objects strike the Moon, according to planetary geologist Alfred McEwen of the University of Arizona in Tucson. They have only surveyed a small sliver of the Moon, and expect to find more craters in the course of several more years of study.

The data could fill a gap in scientists’ knowledge of contemporary collision rates for Earth as well as for the Moon, because the pair should have impact rates proportional to their size. Large asteroids that might threaten Earth can be observed in space, but smaller objects can fall undetected or disintegrate in the atmosphere, whereas they would leave a mark on the Moon.

The crater count could also lead to a re�calibration of methods for estimating the age of surfaces elsewhere in the Solar System. Right now, the Moon acts as a sort of fundamental clock. Scientists have dated lunar samples returned to Earth by Apollo and linked those dates to the crater density of the sample’s original terrain. So when a surface with a certain crater density is found on Mars, for example, researchers compare it with surfaces on the Moon to determine its age.

However, corrections must be applied, owing to differences in impact rates between the Moon and Mars. These are estimated from asteroid orbit calculations, Mars’s location in the Solar System and models that account for its greater size and gravity.

By combining LRO observations with those from other spacecraft, scientists may be able to determine relative impact rates throughout the Solar System more directly. McEwen and his team have been finding new craters on Mars for the last four years, using data from NASA’s Mars Reconnaissance Orbiter. And a current impact rate for Mercury may emerge when NASA’s MESSENGER mission begins to orbit the planet next year, although McEwen believes new craters would have to be very large to be detected.

Schultz said this is an opportunity to improve the dating of surfaces on other planets with measurements rather than models. (ANI)