PHOTOS: Blue Moon of 2015 Thrills Skywatchers
Computer models suggest the blitz was triggered by Jupiter and the giant planets shifting orbits, causing a barrage of rocky bodies and comets to be gravitationally slingshot inward. But impact basins uncovered by GRAIL don't fit neatly into those models, astronomer and geophysicist Gregory Neumann, with NASA's Goddard Space Flight Center in Greenbelt, Maryland, told Discovery News.
"We have more of the medium-size impactors that formed the ‘face' of the moon, Imbrium (a vast lava-filled crater), and so forth, and we don't find evidence for larger impactors other than Aitken," Neumann said. "The implications are enormous."
"We have been writing this paper since 1996, trying to get an inventory of lunar impact basins from gravity. We haven't had the gravity field that we've needed to do the job until now," added GRAIL lead scientist Maria Zuber, with the Massachusetts Institute of Technology.
Overall, the number of impact basins uncovered by GRAIL was in the ballpark of what most researchers expected, Zuber told Discovery News.