Even the mission's chief scientist, Maria Zuber, was surprised when she saw the impact sites, which looked like dots.

NPR: NASA Moon Craft Spots Ebb And Flow Crash Sites

In a third study published in Nature, Jeffrey Andrews-Hanna and Maria Zuber of the Massachusetts Institute of Technology (MIT) in Cambridge, and Bruce Banerdt of Nasa's Jet Propulsion Laboratory (JPL) in California, provide impact evidence from gravitational and topographical signatures on Mars.

BBC: Mars' two-faced riddle 'solved'

"We have had very detailed mapping of that basin from images, from orbital spectral data, but we have not had high-resolution of its interior structure, " Grail's chief scientist, Prof Maria Zuber, who has been speaking at the Lunar and Planetary Science Conference (LPSC) in The Woodlands, Texas, told BBC News.

BBC: Nasa reveals new information about moon probes crash sites