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Wednesday, 16 February, 2000, 22:05 GMT
Moon's orbit betrays its violent birth

The Earth was hit by a Mars-sized body The Earth was hit by a Mars-sized body

By BBC News Online science editor Dr David Whitehouse

The mysterious tilt of the Moon's orbit around the Earth is probably due to the satellite's violent origin, say scientists writing in the current issue of the journal Nature.

A team of researchers from the Southwest Research Institute (SwRI), Colorado, US, have used a computer model to trace the Moon's orbit back in time.

The Moon, scarred by impacts itself The Moon, scarred by impacts itself
The study suggests that the gravitational interaction between the forming Moon and the disk of debris from which it emerged was responsible for putting the body in its present orbit.

Popular theory has it that the Moon formed violently and quickly. Approximately 4,500 million years ago, when the Solar System was young, a so-called proto-planet, probably about the size of the present-day Mars, smashed into the young Earth with incredible violence.

Within minutes, a jet of vaporised rock was blasted into space and begun to settle around the Earth forming a disk. Within hours, the entire Earth's surface, already hot, had melted. Computer simulations of the event suggest that the lunar orbit should have been nearly aligned with the Earth's equator, with only about a one degree tilt.

Inclination problem

The problem is that the Moon's orbit is much more tilted - about five degrees.

This is unusual because most other planetary satellites in our Solar System have orbital inclinations smaller than one or two degrees. The cause of the Moon's large orbital tilt has long been a mystery.

"The inclination problem had been one of the last remaining obstacles for the impact hypothesis of Moon formation," says SwRI Institute scientist Dr William Ward.

His team believes the Moon acquired its large tilt soon after it formed because of a gravitational interaction with the other debris left over from the impact event.

Computer simulations of the giant impact suggest that about two lunar masses of material were put into an Earth-orbiting disk. In the model, debris particles in the inner regions of such a disk are prevented from coalescing by Earth's gravity, which tends to pull objects apart.

Gravitational interaction

The Moon forms in about a year at the outer edge of the debris disk, at a distance of about 22,500 kilometres (14,000 miles) from the Earth.

The key point is that after the Moon coalesced, its gravity would generate waves in the inner disk. The gravitational interaction of the Moon with these waves would then modify the young Moon's orbit.

The computer model simulated the interaction of the Moon and the inner debris disk, assuming that the Moon formed in an orbit with only a one degree tilt.

They found that the interaction of the young Moon with the disk can amplify the lunar inclination to as much as 15 degrees.

"This theory explains the Moon's anomalous orbital tilt as a natural consequence of its formation from a giant impact event," says Dr Ward.

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See also:
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