This week, a mechanical geologist the size of a golf cart and
nearly 156 million miles away galvanized the world with news that
Mars bears unequivocal evidence of once-watery conditions capable of
supporting life as we know it.
Yet for all the excitement surrounding the discovery, the value
of the Mars exploration program may lie as much in what it suggests
about the early history of Earth and about the prospects for
habitable planets around other stars as it does about Mars.
Thus, the results announced Tuesday are whetting astronomers'
appetites for coming missions to Mercury, and perhaps later to
Venus. These missions are expected to help fill in the outlines of a
planetary story whose plot - at least in general terms - is being
repeated throughout the universe. And they are highlighting a
renewed focus on the inner planets of our solar system.
Scientists are trying "to understand the diversity of outcomes
from a common set of physical and chemical building blocks," says
Sean Solomon, director of the department of terrestrial magnetism at
the Carnegie Institution of Washington and lead scientist on the
upcoming Messenger mission to Mercury, scheduled for launch in May.
Where Mars represents a potential reference point for
environmental and possibly biological comparisons with Earth,
Mercury represents "one of the most extreme outcomes of planetary
formation," Dr. Solomon says. "It has much to tell us about how the
inner planets assembled themselves" from the disk of dust and gas
that surrounded the young sun some 4.6 billion years ago. Venus, by
contrast, "is the most Earth-like planet" when looking at mass,
density, and distance from the sun. And its searing, noxious
atmosphere is an example of the greenhouse effect run amok.
This broader view represents what a top panel of astronomers two
years ago called "a new paradigm for solar system exploration." The
flyby views of other worlds in our cosmic neighborhood are yielding
to a deeper search for answers to "fundamental questions about our
place in the universe," the panel said. "Exploration of the inner
solar system is vital to understanding how Earth-like planets form
and evolve and how habitable planets may arise throughout the
Little doubt remains that Mars once hosted habitats suitable for
at least simple life forms. Tuesday's announcement that the rover
Opportunity scored, drilled, and analyzed rocks containing minerals
that form only in very wet environments is adding fresh impetus to a
reexamination of the Mars exploration program. …