Mystery behind 'Moon's bulging shape' revealed

Researchers have recently revealed that the shape of the moon could be attributed to the tidal effects acting early in the moon's history.

The researchers at UC Santa Cruz found that as the moon cooled and solidified more than 4 billion years ago, the sculpting effects of tidal and rotational forces became frozen in place. The idea of a frozen tidal-rotational bulge, known as the "fossil bulge" hypothesis, was first described in 1898.

But this fossil bulge process cannot fully account for the current shape of the moon. In the new paper, Ian Garrick- Bethelland his coauthors incorporated other tidal effects into their analysis. They also took into account the large impact basins that have shaped the moon's topography, and they considered the moon's gravity field together with its topography.

The results indicated that variations in the thickness of the moon's crust caused by tidal heating during its formation could account for most of the moon's large-scale topography, while the remainder was consistent with a frozen tidal-rotational bulge that formed later.

The researchers also found that the moon's overall gravity field was no longer aligned with the topography, as it would have been when the tidal bulges were frozen into the moon's shape. The principal axis of the moon's overall shape (the long axis of the lemon) was now separated from the gravity principal axis by about 34 degrees.

However, the details and timing of these processes were still uncertain but Garrick-Bethell said that the new analysis should help efforts to work out the details of the moon's early history.

The study is published in Nature.