When you look up to the Moon, you see several dark regions. These are vast, solidified lava plains sitting on top of huge, deep, ancient craters. The craters formed ~4.1 to 3.8 billion years ago during a time when large asteroids and comets were colliding with all planets and moons of the solar system. Later on, volcanic activity on the Moon filled those craters and hence they don’t look like such.
Some of these lava-filled craters are denser than the rest of the Moon which means that the Moon’s gravity isn’t really the same everywhere, as shown in this gravity map.
The fact that the Moon’s gravitational field is lumpy first became apparent when the Soviets noticed that their Moon orbiter Luna 10 was deviating from its orbit significantly. Likewise, America’s Lunar Orbiter missions faced up to 100 times more deviation than expected in their low lunar orbits. NASA thus had to take into account the Moon’s gravitational lumpiness for successfully steering and landing humans on the Moon with the Apollo missions.
Both the Apollo 15 and 16 missions put two small satellites in low lunar orbit respectively. Despite taking into account lumpy behavior affecting past missions, both the satellites crashed into the Moon’s surface! The Apollo spacecraft were unaffected but nevertheless it was clear that local variations in lunar gravity needed to be mapped in greater detail.
NASA’s Lunar Prospector mission, launched in 1998, mapped the Moon’s gravitational anomalies in great detail and revealed surprises. Typically, you’d expect a region with a mountain to have higher gravity than a region with a crater. But Lunar Prospector’s data revealed that many plain or even cratered areas in the vast lava plains have higher gravitational pull than even the bright, rocky regions!
Scientists thought that this could be due to dense lava flows in these large craters going as deep as six kilometers. But even the multiple layers of accumulated, dense lava material could only account for some of the concentrated mass in such regions. Where is the rest of the mass coming from?
Using the highest resolution mapping data from the said purpose-built twin GRAIL spacecraft launched in 2011, scientists think they finally figured out the missing mass mystery. The asteroid and cometary impacts that formed these deep craters, where the anomalies lie, must’ve excavated denser material from the Moon’s mantle i.e. below the crust. This in turn must’ve pulled in material from nearby and made the region a mass concentration i.e a mascon and the nearby areas lack said mass.
Mascons have also been found at Mercury and Mars. Given their extensive cratering record, it makes sense. There’s even a mascon on Earth around Hawaii.
NASA has made an interactive Moon map with the GRAIL gravity data overlaid on it, click on the image below to explore it yourself!
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