Back in 2010 NASA discovered one of the sources of trajectory errors: bumps in the gravity field of the Moon. From 2010, here’s a map:
Source: NASA
NewScientist (11 March 2017) is now reporting that those mascons are old impact craters:
Jay Melosh at Purdue University in Indiana and his colleagues were searching data from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission for traces of underground lava tubes when they came across two large buried craters.
These had been hinted at last year, when Alex Evans at the University of Arizona and colleagues used GRAIL maps to find evidence of more than 100 craters buried beneath seas of basalt formed by ancient volcanic eruptions.
One of the new craters, called Earhart, is about 200 kilometres across and is almost completely masked by a later impact and subsequent lava flooding. Another discovery is a buried crater 160 kilometres in diameter, which has been called the Ashoka Anomaly (Icarus, doi.org/b2j9).
From the GRAIL mission page:
On a map of the moon’s gravity field, a mascon appears in a target pattern. The bulls-eye has a gravity surplus. It is surrounded by a ring with a gravity deficit. A ring with a gravity surplus surrounds the bulls-eye and the inner ring. This pattern arises as a natural consequence of crater excavation, collapse and cooling following an impact. The increase in density and gravitational pull at a mascon’s bulls-eye is caused by lunar material melted from the heat of a long-ago asteroid impact.
“Knowing about mascons means we finally are beginning to understand the geologic consequences of large impacts,” Melosh said. “Our planet suffered similar impacts in its distant past, and understanding mascons may teach us more about the ancient Earth, perhaps about how plate tectonics got started and what created the first ore deposits.”
And knowing what created the ore deposits might help guide us when searching for new deposits – here or on other planets.
