The second finding is that there is a high correlation between variations in the Moon’s gravitational field and its topography. “This, to me, is one of the most surprising things we found,” says Zuber. The result suggests that the gravitational field of the Moon is dominated by its deep craters rather than the structure of its interior. This relationship between topography and gravity was closer for the Moon than for other terrestrial bodies such as Mars, Venus and Earth, Zuber says. “It actually makes you wonder how well we understand the other terrestrial planets.”
Bradley Jolliff, a planetary scientist at Washington University in St Louis, Missouri, says that this high correlation ”will spark a good bit of debate”. He adds that he also is confident that the full GRAIL data set will include some regions where the correlation is weaker — revealing important density variations in the Moon's subsurface.
Third, Zuber reported that GRAIL has not been able to confirm some of the larger and older impact basins hypothesized to exist on the Moon’s surface. Lunar scientists examining the Moon’s surface have tried to unravel the history of asteroid impacts — which would illuminate the evolution of not only the Moon but also other bodies of the inner Solar System — by defining areas where large impact basins might underlie more visible and obvious craters. But GRAIL hasn’t found many of the hypothetical basins, Zuber says.
Zuber did not discuss one key goal of GRAIL: understanding the Moon’s deep interior structure and core. Clive Neal, a lunar geologist at the University of Notre Dame in Indiana, says that it will take more time to make these more complicated interpretations from the data. Zuber says the data from GRAIL’s primary mission have already been processed and analyzed, which puts the mission a year ahead of schedule.