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Tilted: stable Earth, chaotic Mars.

Without the moon, life on Earth would likely face the same kinds of wild fluctuations in climate that Mars has apparently experienced through the eons. Its spin axis no longer maintained by the moon at an angle of 23.5 degrees, Earth could drastically change its tilt in just a few million years, sometimes dipping enough to bring more sunlight to polar regions than to equatorial zones.

These startling conclusions arise out of new calculations revealing that the chaotic wobbles of a planet's orbit around the sun can have a strong influence on the angle at which the planet's spin axis is tilted with respect to the plane of its orbit. In Earth's case, the moon has a stabilizing influence. In contrast, the tiny satellites of Mars are too small to keep the tilt, or obliquity, of the planet's spin axis from varying erratically over a wide range of angles.

These results follow from the recent discovery that the evolution of the solar system appears chaotic (SN: 2/22/92, p. 120). Because the details of its evolution over millions of years are sensitive to precisely where the planets are at any given moment, researchers cannot accurately calculate and predict the erratic variations possible in the past or future shapes and inclinations of planetary orbits.

"One of the most dramatic consequences of this chaotic evolution of the planets turns out to be that the [tilt of the spin axis] of Mars is wildly irregular over multimillion-year time scales:' says Jack Wisdom of the Massachusetts Institute of Technology The new results may alter considerably the kind of information that planetary scientists must put into computer models used for tracing and predicting the evolution of the climate and surface features of Mars.

Wisdom and Jihad Touma of the Massachusetts Institute of Technology report their findings on the chaotic obliquity of Mars in the Feb. 26 SCIENCE. Jacques Laskar and his colleagues at the Bureau des Longitudes in Paris describe in the Feb. 18 NATURE how the moon may act as Earth's climate regulator.

Astronomers have long known that the spin of Mars is strongly affected by variations in its orbit. However, previous calculations disagreed on how large this effect has been in the past and when changes in tilt might have occurred. Some researchers had also suggested that intense volcanic activity and other geologic processes could play an important role in these tilt fluctuations by altering the planet's mass distribution sufficient to change its spin and cause large shifts in its tilt angle.

"All sorts of things happen when you change the obliquity of Mars:' Wisdom says. "At high obliquity, the ice at the poles is probably no longer stable. At low obliquity, the atmosphere freezes out."

Wisdom and Touma's calculations show that orbital variations alone can cause such drastic changes. Their model suggests that the resulting tilt angles can range from about 11 degrees to 49 degrees (see diagram). Moreover, these irregular variations in the tilt of Mars over intervals longer than 10 million years appear inherently unpredictable.

However, despite this long-term uncertainty, orbital models can provide a glimpse of what has happened to Mars in the last 10 million years. Indeed, different orbital models now qualitatively agree that the average obliquity of Mars abruptly increased about 4 million years ago.

Curiously, Wisdom and Touma found no such tilt transition when they excluded the effects of general relativity from their equations of motion for the planets. These gravitational effects have a subtle, but apparently significant influence on the evolution of planetary orbits in the solar system.

"Perhaps the geology of Mars will ultimately provide another test of the validity of general relativity," the researchers note.

In Earth's case, the moons hefty mass forces Earth's spin axis to rotate, or process, rapidly enough to forestall wildly erratic variations in Earth's tilt. Although one complete rotation of the spin axis requires about 26,000 years, that's fast enough to keep Earth out of the range of disturbing resonances with other motions in the solar system.

[the moon] were not present, or lf it were smaller, for many values of [Earth's] primordial spin rate, the obliquity of the Earth would be chaotic with very large variations, reaching more than 50 degrees in a few million years and even, in the long term, more than 85 degrees;' Laskar and his colleagues conclude.

Thus, the moon may play a crucial role in regulating Earth's climate, stabilizing it enough to permit the evolution of life, the researchers speculate. Indeed, the presence of a moon-size satellite may be a necessary condition for finding Earthsize planets with Earth-like climates in orbits around neighboring stars.

In a separate paper in the Feb. 18 NATURE, Laskar and his colleagues suggest that the tilts of all the inner planets could have evolved chaotically at various times in the past. Earth itself may even enter such a chaotic zone when the distance between Earth and a slowly departing moon shifts in 2 billion years from its present-day distance of about 60 Earth radii to 68 Earth radii. Given that variations in tilt angle as small as 2 degrees may trigger ice ages, the forecast for Earth when its axis shifts to an angle of nearly 60 degrees would certainly be bleak.
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Title Annotation:changes in angle of axis affects climate on planets
Author:Peterson, Ivars
Publication:Science News
Date:Feb 27, 1993
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