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 reported in today's issue 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.
Without the moon, Jacques Laskar and Philippe Robutel of the Bureau des Longitudes in Paris assert, the Earth -- like the 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 tilt of as much as 85 degrees off vertical 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 with 238,000 miles today -- scientists suspect Earth could tilt as much as 60 degrees.
Mr. Laskar and Mr. 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, which can shift the axis on which each planet spins.
By applying their equations to the other planets in the solar system, Mr. Laskar and Mr. 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 of Earth and the other planets? Mr. Laskar and Mr. Robutel suggest that Venus at one point ro
tated 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.
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.