How we got to know the Moon’s gravity is lumpy

Gravitational anomalies on the Moon have been crashing spacecraft since the 1960s.

When you look up to the Moon, you see several dark regions. Most of 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 when large asteroids and comets were bombarding planets and moons in the solar system. Over the next several hundred million years, the Moon’s volcanoes filled these craters with lava, which is why they don’t look like craters anymore.

Some of these lava-filled craters are denser than the rest of the Moon which means that force of lunar gravity isn’t the same everywhere, as shown in this gravity map.

Gravity map of the Moon (right), red areas indicate dense regions possessing higher gravitational pull than the rest. Credit: Lick Observatory (left) and NASA Lunar Prospector (right)

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, NASA’s “Lunar Orbiter” missions deviated from their paths by up to 100 times than expected in their low lunar orbits. NASA took these gravitational anomalies into account for successfully steering and landing humans on the Moon with the Apollo missions.

Both Apollo 15 and 16 put a small satellite in low lunar orbit. Despite mission planners having accounted for Moon’s lumpiness, both the satellites crashed into the Moon’s surface. The Apollo spacecraft, which took different routes than the satellites, were unaffected. Nevertheless, it was clear that the Moon’s gravity variations needed to be better understood.

In 1998, NASA launched the Lunar Prospector mission which 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 flat or even cratered areas in the vast lava plains have higher gravitational pull than several bright, rocky regions!

Scientists thought that this could be due to dense lava deposits in these large craters filled as deep as six kilometers. But that could only account for some of the concentrated mass in such regions. Where is the rest of the mass?

Highest resolution gravity map of the Moon, measured by NASA’s Gravity Recovery and Interior Laboratory (GRAIL) twin spacecraft in 2012. Credit: NASA

Using the highest resolution mapping data from the purpose-built twin GRAIL spacecraft launched in 2011, scientists think they figured out the missing mass mystery. The asteroid and cometary impacts that formed these deep craters, where the anomalies lie, excavated denser mantle material sitting below the Moon’s crust. This in turn must have pulled in more nearby material, making the region a mass concentration i.e a mascon at the expense of surrounding regions losing mass.

Mascons have also been found on other bodies like Mercury and Mars. Given that they have been extensively cratered much like the Moon, it makes sense. There’s even a mascon on Earth near 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.

This post was made possible thanks to Justin Alva and Hemant S supporting me on Patreon.

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