MARS
WAS ONCE ALL WET Although
covered by frozen deserts today, Mars could have been born with more water in
proportion to its mass than the Earth, according to new observations from NASA's
Far Ultraviolet Spectroscopic Explorer (FUSE) spacecraft. The
new research is the first detection of molecular hydrogen (H2) in the upper atmosphere
of Mars. Molecular hydrogen, which consists of two hydrogen atoms, can be formed
from the breakup of water, which is comprised of two hydrogen atoms bound to an
oxygen atom (H2O). With
the result, Dr. Vladimir Krasnopolsky of the Catholic University of America, Washington,
D.C., and Dr. Paul Feldman of the Johns Hopkins University, Baltimore, MD, were
able to derive the quantity of Martian water lost to space and estimate the amount
of water Mars had shortly after its formation. "Our
result is an important clue to reconstruct the history of Martian water, because
with it and other results, we can estimate the volume of primordial Martian oceans,"
said Krasnopolsky, who is lead author of a paper on the research to be published
in the journal Science November 30. "We calculate that if the initial quantity
of water on Mars could have been evenly distributed across the planet somehow,
it would have been equivalent to a global Martian ocean at least three-quarters
of a mile (1.25 kilometers) deep. This is 1.3 times more water per mass than the
Earth." An ancient ocean could have covered most of the northern hemisphere
of Mars, which is a vast basin according to a topographic survey by NASA's Mars
Global Surveyor (MGS) spacecraft. Mars
is a mystery because it has features, such as those resembling dry riverbeds,
that imply it was quite wet in its past. Since Mars is apparently so dry now,
a primary goal of NASA's Mars exploration program is to determine what happened
to all that water and discover how much remains. Understanding the history of
Martian water is of interest because liquid water is required to support known
forms of life. With such a history, scientists will learn if Mars was once capable
of supporting life. Ultraviolet
radiation from the Sun energizes H2 molecules in the Martian upper atmosphere,
causing them to glow with ultraviolet light as well. Krasnopolsky and Feldman
determined the quantity of H2 molecules present (only about 15 parts per million)
from the intensity of their faint ultraviolet emission as recorded by the advanced
detectors on FUSE. They
compared the amount of H2 to the amount of deuterium in the Martian atmosphere,
obtained from a 1997 observation by Krasnopolsky using the Hubble Space Telescope.
Deuterium is a form of hydrogen made heavier due to the presence of a neutron
in its nucleus. Like hydrogen, deuterium can link to an oxygen atom and another
hydrogen atom to form water, which in this case is called "heavy water"
due to the inclusion of the more massive deuterium atom (HDO). Both
forms of water are broken down by solar ultraviolet radiation and form some quantities
of H2 and HD, respectively. H2 and HD rise high in the Martian atmosphere where
they may be broken down to their component atoms by chemical reactions. Due to
their random thermal (temperature-related) motion, collisions with energetic particles,
and chemical reactions, a certain percentage of H and D atoms, and H2 and HD molecules,
will have enough velocity to escape the pull of Mars's gravity, so Mars gradually
loses its hydrogen and deuterium to space. Hydrogen loss (or deuterium loss) equates
to water loss because the atoms are no longer available to recombine and form
water in the Martian atmosphere. Since
deuterium is heavier than hydrogen, less deuterium will escape because it takes
more energy to get it moving at the necessary speed. By measuring the amounts
of deuterium and molecular hydrogen in the Martian atmosphere, the team discovered
the degree to which deuterium is preferentially left behind, called the fractionation
factor. Because
deuterium is left behind more often, the portion of Martian water that is heavy
water rises over time. In fact, earlier measurements revealed that Martian water
is 5.5 times richer in heavy water than the water on Earth. Scientists
assume that the Earth and Mars were created with the same initial proportions
of heavy water and normal water, called the D to H ratio. If this is correct,
once the rate at which deuterium builds up determined using the fractionation
factor - is known, they can work backwards to determine how much additional water
would be required to dilute the current Martian water so that its D to H ratio
is the same as Earth's. However,
this requires that the current amount of water on Mars be known. Mars is a frigid
world, so most of its water is ice. The team used measurements of the volume of
the Martian polar caps by the MGS spacecraft for an estimate of the water remaining
on Mars today. Additional water may remain frozen in the Martian soil, but this
quantity is unknown. However, any water found there only increases the current
amount of deuterium-enriched Martian water, which will require an even larger
primordial supply to dilute it to an Earth-like level. The
team could calculate backward for as long as the fractionation factor can be applied,
which extends to a period about 3.6 billion years ago. Prior to that, the Martian
surface was a lot warmer due to heat left over from Mars's formation, and much
of the water was in vapor form. This permitted a much greater quantity of water
to escape Mars via a different process called hydrodynamic escape. Other
researchers previously determined the D to H ratio for Martian water at the end
of the hydrodynamic escape period some 3.6 billion years ago by deriving it from
the analysis of the D to H ratio in Martian meteorites. Using the derived ratio
for the end of hydrodynamic escape, Krasnopolsky and Feldman calculate that the
amount of water required to dilute the D to H ratio in the current Martian water
supply so that it matches the D to H ratio of that earlier era is equivalent to
a global ocean 100 feet (about 30 meters) deep. Adding
a 30-meter global Martian ocean to the current supply gives the estimated water
remaining on Mars after the hydrodynamic escape period. Since the Martian D to
H ratio at the end of the hydrodynamic escape era is still higher than the terrestrial
ratio, the team calculated that a much larger volume of water would have been
required to dilute the Martian water supply at the end of hydrodynamic escape
so that it matched the D to H ratio in Earth's water. It is this final calculation
that yields such an abundant estimate of the water present on Mars shortly after
its formation about 4.5 billion years ago. Back
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