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Earth may once have had three moons
Friday, 16 May 2008
by Bruce Dorminey
Cosmos Online
Earth may once have had three moons
Missing moons: The Earth and Moon as seen by one of the Voyager space probes. But did the Moon once have two miniature celestial companions?
Image: NASA


In its youth our Moon may have once been bracketed by two asteroidal companions, says a new study. From Earth, these tiny moons – up to 100 km wide – would have appeared as two extremely bright stars.

The mathematical modelling study, detailed in the journal Icarus, predicts that the moonlets could have existed for hundreds of millions of years in gravitationally stable zones of equilibrium know as Lagrange points. These are where the gravitational pull of the Earth and Moon is cancelled out, allowing objects (such as research probes) to remain in a long-term stationary orbit.

Lone wolf

Until now, the accepted view had been that our anomalously large Moon had always been a lone wolf. It is believed to have formed some 4.4 billion years ago in the aftermath of a chance encounter between a Mars-sized planetary embryo and our early Earth.

But the paper's co-authors, planetary scientists Jack Lissauer, at NASA Ames Research Centre in Mountain View, California, USA and John Chambers at the Carnegie Institution in Washington DC, believe the moonlets (named 'Trojans' after similar captured satellites of Jupiter) may have formed simultaneously with the larger impact.

To get an idea of the orbital location of the missing moons: imagine the face of a clock, with Earth at the centre and the Moon at 12 o'clock. The two orbiting moonlets would appear to be leading or behind the Moon at positions of 10 and 2 o'clock and would orbit Earth in the same time period as the Moon.

Previously, it was assumed that any leftover debris lurking in odd orbits around the Earth or Moon would have eventually left the pair's gravitational clutches or else slammed into the surface of either body.

"A Trojan satellite [today] … couldn't last more than a few million years," said Lissauer. "But when the Moon was closer to the Earth early on, Trojan positions would have been more stable and the Trojan satellites presumably could have lasted a lot longer."

The researchers suggest that the moonlets could have remained stable at the spatial equilibrium points first espoused more than 200 years ago by the Italian-French mathematician Joseph-Louis Lagrange.

If they hit Lagrange's equilibrium sweet spots, they may have circled Earth for up to one billion years. However, as the Moon moved away from the Earth over time, the orbits of the relatively puny bodies would have become less stable; perturbed by the gravitational influence of other planets in the Solar System and the Sun itself.

Asteroid bombardment

Matija Cuk, an expert in celestial mechanics at the University of British Columbia in Vancouver, Canada, said the study is "very solid".

He was hoping that the data from this study might answer a long-standing question about an intense period of asteroid bombardment that occurred a few hundred million years after the Moon's formation. That is: were these moonlets the source of the material that fuelled the 'late heavy bombardment'?

The scars of the bombardment still pockmark the surface of the Moon today.

"We were hopeful that these Trojans might solve the problem of where the asteroids came from," said Cuk, whose own study is also slated for publication in Icarus. But his calculations require much larger Trojans, some 500 km in diameter, large enough to break up into smaller bodies.

So, if the original moonlets didn't slam into the Earth or Moon, or burn up in the Sun, they may have eventually been ejected from the Solar System altogether. It's even conceivable that they are still drifting out their somewhere, said the experts.