91av

THE mystery of why the Moon’s orbit is tilted a few degrees away from the
Earth’s has been solved. Rings of debris thrown up by the impact that created
the Moon could have pulled it out of position.

Most astronomers agree that the Moon was created when an orbiting body about
the size of Mars smashed into Earth some 4.5 billion years ago
(91av supplement, 7 August 1999, p 3).
The impact splattered enough
dust, rock and vapour into orbit to form a broad ring centred about 12 000
kilometres above the planet.

Within a year, the Moon formed at the outer edge, while the inner part
eventually fell back to Earth. The new satellite then slowly retreated to its
present orbit, some 380 000 kilometres out.

This model explains most of what we know about the Moon—with one
puzzling exception. The lunar orbit is tilted 5 degrees from the orbital plane
of the Earth, and it must have been 10 degrees out when the Moon formed,
according to astronomers’ calculations. Yet the initial impact would have left
only a 1-degree tilt.

The missing ingredient, says Robin Canup of the Southwest Research Institute
in Boulder, could be gravitational interactions between the young Moon and the
inner ring. Saturn’s rings are similarly affected by its own satellites, she
says.

These interactions, known as gravitational resonances, occurred when
particles in the inner ring completed two or more orbits of the Earth in the
time the Moon completed one. That would have made them transfer their orbital
momentum to the new satellite before spiralling inwards.

Canup’s computer models show that particles orbiting three times as often as
the Moon would have survived the longest and had the biggest effect. “At this
resonance, the Moon launches a bending wave, which causes the disc to ripple,”
she says. With each pass of the Moon, this rippled distortion pulled its orbit
further out of kilter. And because the young Moon’s orbit only took a few hours,
it received thousands of tugs a year, tilting its orbit within a hundred
years.

“I think they’ve identified the right mechanism,” says Glen Stewart of the
University of Colorado. The Cassini spacecraft could test the model by observing
Saturn’s moons and rings in 2004, he notes.