The same side of the Moon always faces us. This means that we never get a chance to see the other side of it unless a spacecraft flies there. If we could see it in the night sky, though, it would be like a whole different world. Where the lunar face we’re used to, the nearside, has deep craters known as mare, the farside has comparatively few large craters. Instead, it’s covered in a series of small craters, which makes it look a bit more like a golf ball.
Why are the two sides different?
The reason for this vast hemispheric differences could be the result of a literal imbalance in the Moon. According to research published in Nature, there’s about a 2 to 3 percent mass asymmetry between the sides, mostly in the mantle layer. The driver seems to be heat from deep within the Moon, which also causes there to be moonquakes — more in some areas than others.
The discovery was made through data from the Gravity Recovery and Interior Laboratory (GRAIL) spacecraft, a NASA mission consisting of two different orbiters spaced a bit apart. The spacecraft, which were otherwise identical, used their distance to map the gravity of the Moon. Mapping gravity is actually a good way to measure the internal structure of an object, because when gravity is just a little bit different, it indicates a sort of aberration from a homogenous internal structure. Thus, we might know where, say, an ancient collision took place by measuring differences in the structure below the surface.
The driver of these structural differences is tidal forces. Yes, the same ones that drive our oceans. “Similar things happen at the Moon, except there’s no oceans, but the surface actually goes through a tidal deformation,” Ryan S. Park, a NASA-JPL planetary scientist and lead author of the paper, says. Park says there’s more tidal deformation at the nearside than the farside.
As the Moon orbits Earth, this tidal deformation follows along, distorting the general shape of the Moon. These tidal stresses, in turn, may lead to more moonquakes on the nearside (where temperatures also appear to be hotter.)
What made this happen?
But Park says, so far, they don’t entirely understand what initially caused the anomaly. “This could be due to some stochastic process over time, or this could be some interaction, such as a large impact, causing the Moon to change its internal structure,” Park says. Upcoming lunar missions, including seismometers planned as part of Artemis III, could shed more insight.
Nicholas Schmerr, a University of Maryland College Park geologist, says that more work may be needed to compare the team’s higher asymmetries than past models against more archival data. “That said, the increase of the shear modulus of 2 to 3 percent on the near vs. far side of the Moon is an intriguing, if not entirely surprising, finding,” Schmerr says. “We’ve known since the Apollo era that there seem to be fundamental differences between the near and far side of the Moon, and there have been many models proposing that a deep set difference in the interior could be the source. This seems like another important, previously unrecognized, piece of evidence for that.” He also said that while temperature differences internally could explain this asymmetry, it could also be a combination of temperature and other forces mixing to create the effect.
In addition to potentially cracking the mystery of the strange differences between lunar hemispheres, the methods used in the paper could also be applied to other areas of the solar system with potentially intriguing interiors, such as Mars, Saturn’s moon Enceladus, or Jupiter’s satellite Ganymede. Given the potential past or present habitable conditions on those worlds, it could provide clues to how the internal structures and tidal forces shape possibly biotic conditions.
