The moon may look like a perfectly round ball in the sky, but its shape is actually surprisingly distorted. Scientists have long wondered what caused our satellite to adopt a slight lemon shape and while several theories have been proposed in the past, none could fully explain its morphology. Now, building on previous work, researchers have published work in Nature that suggests the majority of the moon’s bulging shape is attributable to tidal effects that took place early in the moon’s history.
Investigating the shape of the moon has been tricky business in the past because it’s peppered with scars from impacts that deformed its crust long ago. Large craters and basins therefore left gaps in the data which scientists previously struggled to fill. “We did a lot of work to estimate the uncertainties in the analysis that result from those gaps,” lead researcher Ian Garrick-Bethell said in a news-release.
A few years ago, Garrick-Bethell and University of California Santa Cruz colleagues proposed that part of the moon’s odd shape was attributable to tidal heating. Some 4.4 billion years ago, an ocean of magma decoupled the crust from the underlying mantle, causing huge tidal forces. This intense heating was greatest at the poles, causing the moon’s crust to become thinner here whilst thickening at the equators. While this can account for some of the moon’s odd bulge, it couldn’t solely explain the morphology we see today.
However, in this new study, the researchers took into account another effect- rotational forces- which helps to fill in the blanks. When the moon started to cool and solidify more than 4 billion years ago, the sculpting effects of both tidal and rotational forces became stuck in place, giving the moon a slight lemon shape with one bulge facing us and another on the opposite side. This frozen tidal-rotational bulge, called a “fossil bulge,” was first hypothesized in 1898.
“If you imagine spinning a water balloon, it will start to flatten at the poles and bulge at the equator,” said Garrick-Bethall. “On top of that you have tides due to the gravitational pull of the Earth, and that creates sort of a lemon shape with the long axis of the lemon pointing at the Earth.”
The researchers therefore concluded that while the majority of the topography of the moon can be explained by variations in crust thickness caused by tidal heating, the rest was likely caused by this frozen tidal-rotational bulge that occurred later on. However, they are still unable to explain why there are topographical differences between the near and far side.
Another interesting finding was that the orientation of the moon has been shifted, meaning that its overall gravity field is no longer aligned with the topography as it would have been when the forces were frozen in place. According to Garrick-Bethell, this is likely due to changes in mass distribution due to a combination of internal changes and impacts that removed mass from the surface. When these events occurred, however, remains a mystery.
[Header image "moon - waxing gibbous," by Dino Abatzidis, used in accordance with CC BY-NC-SA 2.0]