There are three features I want to point out in this map. First of all, most of the small-scale variation visible in the free-air gravity map has gone away. That means that most of the small-scale signal in the gravity field really is due to uncompensated topography. It also means that the assumption of the crust being pretty much the same density everywhere works pretty well. There are local variations visible especially in the highlands, though it's tough to tell how large the variation is without contour lines.

Now, the Moon should have local, small-scale variations in crustal density due to local geology; the fact that the variations are small and smooth means (Zuber argued while presenting these results) that impact cratering has completely battered, bashed, fragmented, mixed, and remixed the upper couple of kilometers of the crust, totally homogenizing it. Previous maps of lunar gravity have not had high enough detail to see this beautiful match between topography and gravity; so this is a new GRAIL result.

A second notable feature: generally, the crust is thick (low Bouguer gravity, blue color) on the farside and thin (high Bouguer gravity, red color) on the nearside. This is a known feature of the Moon; it produces what's known as its center-of-figure/center-of-mass offset. The Moon's geometric center is offset from its rotational center by several kilometers. One hypothesis to explain this oddity is that most of the farside is covered by very thick deposits of ejecta from the giant south pole-Aitken basin.

Finally, many of the lunar basins have great honking huge positive gravity anomalies. They're noticeable in the free-air gravity map but even more so in the Bouguer map. These, too, have been recognized for quite some time. They're called "mascons," mass concentrations, places where there is much more mass than it seems there should be. The Moon's mascons are a real pain for orbital navigation; their gravitational effect destabilizes orbits, and it takes a lot of work to keep lunar orbiters from crashing. In fact, Zuber said at last week's press briefing that in their new, low-elevation extended-mission orbit, GRAIL has to perform trajectory correction maneuvers three times a week in order to keep from crashing. And once they quit doing those maneuvers, they're going to crash very quickly. The mission will end on December 17 at 14:28 PT / December 18 00:28 UT. (There's a press briefing planned for Thursday to announce where the crash will be.)

The crust must be ultra-thin under the mascons. In some places it would appear that the crustal thickness is effectively zero -- the impacts dug holes all the way through the low-density crust to reach the high-density mantle. In many mascons, even this isn't enough to account for all the Bouguer anomaly. You also need an uncompensated pile of extra mass. Many of the basins are, in fact, filled with basaltic lavas, which (if not compensated) could account for some of the positive gravity anomaly. But not all of them are. The puzzle of why some lunar basins that aren't filled with volcanic flows do have mascons is an area of active research and debate.

All of that was to explain the first of the three Science papers published this week, the one introducing the GRAIL data. But there were two more related papers, in which GRAIL team members performed computational tricks to wring more information out of the data set.

The first of these was a paper about the lunar crust presented at the briefing by Mark Wieczorek. He took the GRAIL data set and essentially performed a high-pass filter on it, removing long-wavelength features (large basins and other regional features). What's left behind, the short-wavelength features, should all be uncompensated, too small for isostasy to be able to make mantle flow to lift up crater floors. Therefore, the Bouguer gravity anomaly in such a map should be zero.

A few paragraphs up, I told you that in order to calculate the Bouguer anomaly from the free-air anomaly, you have to assume a crustal density. Well, Wieczorek and coworkers approached the problem a different way. They took the high-pass-filtered gravity map, forced the Bouguer anomaly to be zero, and solved for what the crustal density would have to be in order to make that happen. They avoided places where dense mare basalts have covered up the primordial lunar crust. Here is their map of lunar crustal density in all the places not covered by mare basalt.