Researchers Trace Earth’s Stability to Moon’s Influence

Of all the profound influences on the Earth’s delicate climate, from volcanic eruptions to asteroid impacts, scientists now suggest that the most important may be the most unexpected: the moon.

Two French scientists report in today’s edition of the journal Nature that the moon apparently acts as a sort of gravitational gyroscope to stabilize the 23-degree tilt of Earth’s axis, the slight skew that gives the planet its seasons.

Jacques Laskar and Philippe Robutel of the Bureau des Longitudes in Paris assert that without the moon, Earth--like the other planets of the inner solar system, Mercury, Venus and Mars--would tilt as much as 85 degrees off vertical. (Vertical is defined as perpendicular to the plane of Earth’s orbit around the sun.)

A radical obliquity, or tilt, of as much as 85 degrees would be catastrophic because--as other scientists have suggested--a mere 1.3-degree shift in Earth’s tilt may have resulted in ice ages.

Carl A. Murray, an astronomer at the University of London, noted elsewhere in Nature that a tilt greater than 54 degrees would give the Equator less sunshine than the poles.

“Given that,” he concluded, “the forecast for a moon-less Earth would have been bleak.”

The distant future may be just that, because the moon is slowly spinning away from Earth at the rate of a little more than an inch a year. The farther it creeps away, the less influence it exerts on Earth. In about 2 billion years--when the moon will be 269,000 miles from Earth, compared to 238,000 miles today--scientists suspect Earth could tilt as much as 60 degrees.

Laskar and Robutel calculated the history of Earth’s tilt by studying how the gravitational pull of each planet in the solar system affects other planets. This process, called long-term perturbation, both influences a planet’s orbit around the sun and applies a slight torque, or twisting power, that can shift the axis on which each planet spins.

By applying their equations to the other planets in the solar system, Laskar and Robutel calculate that Mercury and Venus wobbled with violently chaotic tilts for millions of years until they became stabilized in their current attitudes.

The two scientists suggest this may explain one of the solar system’s vexing mysteries: Why does Venus, which in many ways is similar to Earth, rotate in the opposite direction to Earth and the other planets? Laskar and Robutel suggest that Venus at one point rotated in the same direction as the other planets and has simply tilted 180 degrees.

Mars, being farther from the sun and having only two very small moons, still has a chaotic tilt. Over the last 45 million years, its tilt has ranged from 0 to 60 degrees. It is now 25 degrees.

The outer planets--Jupiter, Saturn, Uranus, Neptune and Pluto--all have relatively even tilts, Laskar and Robutel report, although they may have experienced similar instability soon after the solar system’s formation 4.5 billion years ago.

Since the moon evidently played a vital role in the development of an Earth atmosphere that is amenable to life, the scientists suggest that moons may be a key to finding life on planets elsewhere in the universe--if astronomers can just find those planets in the first place.

No one has confirmed the discovery of planets beyond our solar system, but theorists think they are relatively common and astronomers are beginning to find circumstantial evidence that supports their theories.