Why the Moon is lopsided: a new explanation

N. Gopal Raj

A whack from a smaller sibling could, it seems, account for one of the abiding mysteries about the Moon — why the side seen from the Earth is so different from its other hemisphere.

The nearside is low and flat, with lava-filled basins that are readily visible as dark patches which early astronomers mistakenly labelled as ‘maria' (or seas). In 1959, the Soviet Union's Luna 3 became the first spacecraft to go behind the Moon and the photographs it sent back showed the far-side to be mountainous.

Later studies also showed that the Moon's crust was much thicker on the far-side. The nearside, on the other hand, has rocks rich in potassium, rare earth elements and phosphorous, which the other side lacked.

Strange asymmetry

A new explanation for this strange asymmetry has been put forward in a paper in the latest issue of the journal Nature. The Moon, it suggests, may once have had a companion that was one-third its diameter and just four per cent of its weight. Both Moons formed from the huge cloud of debris that was flung into space when a Mars-sized object slammed into the Earth some 4.5 billion years ago.

For tens of millions of years, the smaller moon was held in place by the competing gravitational pull from the Earth and the Moon. During that period, the Moon cooled sufficiently for a crust to form, with an ocean of molten rock (known as magma) beneath it. Meanwhile, the companion Moon, being smaller, solidified, according to the scenario envisaged by Martin Jutzi and Erik Asphaug of the University of California at Santa Cruz in the U.S.

Then, about 70 million years after their formation, the companion moon's orbit became unstable and it ended up crashing into the Moon. The collision occurred by cosmic standards at a ‘low-impact velocity.' As a result, the smaller companion got splattered over the Moon, “pasting on a thickened crust and forming a mountainous region comparable in extent to the far-side highlands,” the two scientists noted in their paper.

The crash also pushed the magma ocean, which had higher levels of potassium, rare earth elements and phosphorous, into the other hemisphere.

Plausibility

Many processes in nature could produce the observed pattern of lunar asymmetry and the current study demonstrated “plausibility rather than proof,” observed Maria Zuber of the Massachusetts Institute of Technology in a commentary published in the same issue of the journal.

Such proof could come from the America's Gravity Recovery and Interior Laboratory spacecraft that will head to the Moon next month. Dr. Zuber is the principal investigator for the mission. The mission will provide “very detailed information about the crustal thickness,” observed Dr. Jutzi in an e-mail.

The paper had made “an interesting suggestion that appears viable conceptually but may not be the real story,” cautioned J.N. Goswami, director of the Physical Research Laboratory at Ahmedabad and chairman of the Science Advisory Board for India's Chandrayaan-2 lunar mission.

Bringing back samples from more than one location on the far-side on the Moon would provide useful evidence, he pointed out. If the scenario given in the paper was correct, there should be rocks there that were far older than those found on the nearside.