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Reflections on a sunnier past.

Standard models of solar evolution assume that the sun shone 20 to 30 percent less brightly during its first billion years than now - a reduction big enough to chill Earth's surface and freeze water. That poses a paradox, however, because fossils and other evidence indicate that water remained liquid throughout nearly the entire history of our planet. In the past, researchers have attempted to circumvent the mismatch between a dimmer sun and the warmth of our planet's surface by suggesting that Earth's atmosphere once contained a much higher concentration of carbon dioxide. This greenhouse gas exerts a warming effect by trapping the sun's infrared rays within the atmosphere.

But the high carbon dioxide levels assumed in this theory do not fit with accepted climate models for Earth's atmosphere and its oceans. Moreover, the standard "weak-sun" model calls for far more lithium than observed in the solar spectra.

Astrophysicist I.-Juliana Sackmann of the California Institute of Technology in Pasadena and her colleagues now suggest an alternative theory to explain the surfs lithium depletion and temperatures warm enough for liquid water to exist in Eartus past. In the October GEOPHYSICAL RESEARCH LETTERS, they propose that the sun may once have had as much as 10 percent more mass - a feature that would make it more luminous during its early history than today.

The new model suggests that the sun shed its extra mass during the last few billion years - a notion that can readily account for the lower lithium abundance, Sackmann notes.

Observations of Milky Way stars younger than the sun but which have a similar mass, she says, hint that they have shed some of their material. If further observations confirm mass loss in such stars, this might lend more credence to the idea that the sun lost a significant amount of mass during its youth, Sackmann says. She notes, however, that a modification of the standard weak-sun model could also account for lithium depletion. In the modified scenario, lithium sinks from the sun's outer layer into the hot interior, where it burns.

To help decide between the theories, researchers should monitor young, sun-like stars more closely and theorists should calculate the effects of solar brightening and mass loss on Earth and the other planets, Sackmann says.
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Title Annotation:solar evolution
Publication:Science News
Date:Nov 2, 1991
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