From: LARRY KLAES (firstname.lastname@example.org)
Date: Sat Dec 15 2001 - 09:00:01 PST
WAS MARS ALL WET?
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 1-1/4 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 Explorer 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 (11 parts
per billion), 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 3-1/2 billion years ago, testify to a time when the D:H
enrichment was only 1.9 roughly. Sometime earlier than that, water
vapor was so abundant around Mars, it could escape 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 abundance, Krasnopolsky has modeled the atmosphere's
evolution and deduces that the D:H enrichment rise from 1.9 to 5.5
represents a loss of Martian water equivalent to a planetwide ocean
about 30 meters deep. What little water remains today in the polar
caps and hidden elsewhere is probably enough for a 20-meter-deep
layer. Thus 3-1/2 billion years ago, the ocean was some 50 meters
deep. Working further back through time, he calculates that
hydrodynamic escape likely robbed the planet of all but 4 percent of
its original water inventory, yielding an original water table of
1-1/4 km. Krasnopolsky's model assumes 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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