Second moon may have collided with our moon, say scientists

A collision with a smaller moon may explain why the terrains on the far and near sides of the moon are so different
Artist's impression of a collision between our moon and a smaller moon
Artist's impression of a collision between the moon and a companion moon 4% the lunar mass. Such a collision could explain the highlands on the moon's far side. Photograph: Martin Jutzi/Erik Asphaug/Nature

The remnants of a second moon that orbited the Earth billions of years ago may be splattered across the far side of our moon, scientists claim.

The two moons are believed to have been created at the same time and followed a similar path to the moon we're familiar with today, but after tens of millions of years of peaceful co-existence, the two appear to have crunched together in a gentle collision that left the smaller, just a third of the size, spread across the larger like a cosmic pancake.

Researchers put forward the idea after computer simulations found that a collision with a second, sibling moon in Earth's early history might solve the longstanding puzzle of why the two faces of the moon differ so dramatically.

While the near side, which always faces the Earth, is low-lying and relatively flat, the far side is high and mountainous, with a crust tens of kilometres thicker.

The idea builds on what planetary scientists call the "big impact" model of the moon, in which a planet the size of Mars slammed into the Earth in the early days of the solar system and knocked out a vast shower of rocky debris, which later coalesced as the moon.

"The impact produced a disc of debris around the Earth and from this disc we got the moon, but there is no reason why only one moon would be formed," Martin Jutzi at the University of Bern in Switzerland told the Guardian.

Jutzi and his colleague, Erik Asphaug at the University of California in Santa Cruz, decided to simulate what might happen if a second moon was created from the rock and dust that fell into orbit around the Earth.

Graphic: how a second moon may have crashed into the first
Graphic: how a second moon may have crashed into the first Photograph: Graphic

Computer models showed that a sister moon roughly 1,200km in diameter could have accompanied the larger moon around the Earth for tens of millions of years. But as the two moons' orbit moved further away from Earth, the balance of forces became unstable and they collided.

A high-speed impact would have punched a giant crater into the moon and kicked a shower of rock into space, but if the two bodies met at less than three kilometres a second, the smaller moon would have splatted onto the surface of the larger and stayed there. The study appears in the journal, Nature.

"A slower collision doesn't produce such intense shockwaves and causes much less damage than a high-velocity collision," Jutzi said. "It's kind of a gentle collision that doesn't form a big crater. The smaller moon gets more or less pancaked onto the larger moon."

If Jutzi is right, the impact thickened the moon's crust on the far side, creating the highlands and forcing subsurface magma to the opposite side. "It wouldn't matter where the impact happens, because after the collision, the moon would reorient itself so that the material left from the impact was on the far side," Jutzi said.

While speculative, scientists hope to find ways of testing the idea. The smaller moon would have formed before the moon we see today, so rock samples from the far side of our moon should be older than rocks collected from the near side.

Another approach under consideration is to compare Jutzi and Asphaug's simulations with details of the moon's internal structure, gleaned from lunar maps drawn up by Nasa's Lunar Reconnaissance Orbiter, and high-resolution gravity maps of the moon, which will be obtained next year by Nasa's Gravity Recovery and Interior Laboratory (GRAIL) mission.

In an accompanying article, Maria Zuber, a geophysicist at Massachusetts Institute of Technology, said the study raised "the legitimate possibility that, after the giant impact, our Earth perhaps fleetingly possessed more than one moon. Furthermore, significant remnants of this long-departed member of the Earth-moon collisional family may be preserved today on the lunar far side."

The moon shows only one face to the Earth because its centre of mass is slightly off-centre – around 2km closer to our planet than the geometric centre. There is no dark side of the moon, though much of the surface spends 14 days in daylight and 14 days in darkness.

Last year, Ian Garrick-Bethell and Francis Nimmo at the University of California, Santa Cruz, published an alternative explanation for the different thicknesses in the moon's crust that suggested tidal forces rather than an impact were responsible.

"The fact that the near side of the moon looks so different to the far side has been a puzzle since the dawn of the space age, perhaps second only to the origin of the moon itself," said Nimmo. "One of the elegant aspects of [this] study is that it links these two puzzles together: perhaps the giant collision that formed the moon also spalled off some smaller bodies, one of which later fell back to the moon to cause the dichotomy that we see today."