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Mercury, November/December 2006 Table of Contents

Image courtesy of SOHO/ESA/NASA.

by Jennifer Birriel

Imagine a young Sun—some four billion years in the past—that is much fainter and cooler than today's mature Sun. So much less luminous is this young Sun that Earth and Mars are frigid. It remains far too cold on either planet for liquid water to exist. Both planets are locked in a so-called icehouse state for nearly the first half of their history.

Such is the picture suggested by standard models of stellar evolution: as a star ages on the main sequence, its luminosity increases. This increase results from changes in the chemical composition of the star's core due to thermonuclear reactions that turn hydrogen into helium and liberate energy. In fact, the standard model predicts that the Sun's luminosity has increased by some thirty per cent over the last 4.5 billion years.

As a result of a fainter Sun, the temperature on ancient Earth should have been some 25 °C lower than today. Such a low temperature should have kept large parts of Earth frozen until about one to two billion years ago. The case for Mars is even more extreme due to its greater distance from the Sun. Yet there is compelling geologic evidence suggesting that liquid water was abundant on both planets three to four billion years ago.

Earth's oldest rocks, which are found in northern Canada and in the southwestern part of Greenland, date back nearly four billion years to the early Archean eon. Within these ancient rock samples are rounded "pebbles" that appear to be sedimentary—laid down in a liquid-water environment. Rocks as old as 3.2 billion years exhibit mud cracks, ripple marks, and microfossil algae. All of these pieces of evidence indicate that early Earth must have had an abundant supply of liquid water in the form of lakes or oceans.

Mars also exhibits abundant evidence for liquid water in its ancient past. It has networks of dry channels and valleys. Its massive canyon systems appear to have been modified by the flow of liquid water. Sediments deposited within the canyons suggest the presence of standing lakes in the past. And some features appear to have resulted from the former action of large glaciers that have since disappeared.

This apparent contradiction—between the icehouse that one would expect based upon stellar evolution models and the geologic evidence for copious amounts of liquid water—has become known as the "faint young sun paradox." It was first pointed out more than three decades ago by Carl Sagan and George Mullen. Does the paradox indicate a problem with our stellar evolution models? Or is there another way around this conundrum?

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