Hidden beneath the dark, solidified lava plains lie highly dense material that distort the Moon’s gravity from spherical consistency. Seen here is a gravity map of the Moon.
The fact that the Moon’s gravitational field is lumpy became apparent when the Soviet Luna 10 spacecraft deviated from its lunar orbit significantly. Likewise, America’s Lunar Orbiter missions faced 100 times more deviation at certain points than expected whilst in low lunar orbits. Apollo missions thus had to take into account the gravitational lumpiness of the Moon for a successful endeavor.
During the Apollo 15 and 16 missions, two small satellites were put in low lunar orbit. Despite taking into account lumpy behavior affecting past missions, both the small satellites crashed into the lunar surface! Something was definitely fishy. Practically any spacecraft left in lunar orbit would be doomed to crash because of these stronger gravity areas that seem to be unevenly distributed on the Moon.
It is only after NASA’s Lunar Prospector mission (1998) mapped these gravitational anomalies that scientists fully understood what’s going on. You’d expect typically that a mountain would represent a higher gravity region and a crater a lower one. And that’s true in some cases. But surprisingly, many vast basins on the Moon, on which the dark lava plains are laid, showed higher gravitational pull than even the rocky highland regions!
It was suggested that the lava flows in these basins are dense and can go as deep as 6 km. But this accumulated, dense lava material could only account for some of the concentrated mass in such regions. What about the rest of the mass?
Using data from the twin GRAIL spacecraft, scientists think they figured out the missing mass mystery. All of the large lava plain areas on the Moon are actually part of deep basins; the basins being massive impact craters formed during the time of Late Heavy Bombardment period in the solar system. These impacts excavated deep into the lunar mantle, melted the material and concentrated it to make it denser. This in turn must have pulled in material from nearby and made the region a mass concentration i.e a mascon. Simplest way of naming things. 🙂
Mascons have also been found at Mercury and Mars. Surprisingly, Earth is not known to have any mascons! Go look up why.
The wonderful guys over at NASA’s Lunar Reconnaissance Orbiter have made an interactive lunar map with the GRAIL gravity data overlaid on it! ❤