November/December 2006 Table of Contents
courtesy of SOHO/ESA/NASA.
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.
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.
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.
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.
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.
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?
you enjoyed this excerpt from a feature article and would
like to receive our bi-monthly Mercury magazine, we invite you to
join the ASP and receive
6 issues a year.