NASA artist Greg Shirah depicts Mars as it might have looked some 4 billion years ago, with much of its northern hemisphere submerged under water.

Courtesy NASA/Goddard Space Flight Center.

Future astronauts roaming the surface of Mars will be
hard-pressed to find sources of water, but the red planet was not
always so arid a place. From minuscule gullies to giant flood plains,
the face of Mars bears mute witness to eras when water must have gushed
forth onto the surface — at least temporarily.

In fact, a recent study shows indirectly, but convincingly,
that Mars may have formed with enough water to cover its entire globe
to a depth of at least 1¼ kilometers (about 4,000 feet). The
implication is that this ruddy, arid world actually started out with
more water, relative to its overall mass, than we did. This provocative
evidence comes not from some robotic sentinel on Mars itself, but
from the Far Ultraviolet Spectroscopic
orbiting 760 km above Earth.

In the November 30th issue of Science, Vladimir
A. Krasnopolsky (Catholic University of America) and Paul D. Feldman
(Johns Hopkins University) describe how they used FUSE to make the
first-ever detection of hydrogen molecules (H2)
in the upper Martian atmosphere. Present at just 15 parts per million,
the hydrogen represents water molecules that have been broken down
by sunlight. Four years ago Krasnopolsky used the Hubble Space Telescope
to determine the extent of deuterium ("heavy" hydrogen) in the Martian
atmosphere, and these two isotopic abundances provide important clues
to unraveling water's history there.

Today Mars's atmosphere has a deuterium-to-hydrogen
(D:H) ratio 5.5 times higher than Earth's. Yet Martian meteorites,
ejected from Mars's surface 3½ billion years ago, testify to
a time when the D:H enrichment was only 1.9. Water reacted with iron
on early Mars and released huge quantities of hydrogen, which escaped
wholesale into space. When this so-called hydrodynamic escape shut
off, water continued to leak away, albeit gradually. The molecules
first broke down into their component atoms, followed by the H and
D atoms flying off into space. The process continues even today, and
since the lighter hydrogen escapes more readily than deuterium, the
deuterium becomes enriched over time.

Knowing the H2 and D abundances,
Krasnopolsky has modeled the atmosphere's evolution and deduces that
the rise of D:H enrichment from 1.9 to 5.5 represents a loss of Martian
water equivalent to a planetwide ocean about 30 meters deep. What's
left today, sequestered in the polar caps and in the ground, would
make a 20-meter-deep layer. Thus 3½ billion years ago, Mars
must have had enough water for a 50-meter layer. Working further back
through time, Krasnopolsky calculates that hydrodynamic escape likely
robbed the planet of all but 4 percent of its original water inventory,
yielding an original water table of 1¼ km or more. His model
assumes that Mars and Earth acquired their water the same way and
thus had equal D:H ratios to begin with. However these assumed conditions
could easily have been upset by varying the proportion of incoming
water-bearing comets (known to have high D:H ratios).


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