HOT on the heels of last week’s announcement that water has recently flowed
on Mars comes further encouraging news. The Red Planet has kept two to three
times as much of its water as we thought. But there is a catch—since
estimates of what it started with vary wildly, we still have very little idea of
exactly how much there is.
“It would be more exciting if I could tell you how much water is in the
Martian crust,” says Laurie Leshin of the University of Arizona in Tucson.
Nevertheless, her results do give us more reason to be optimistic when it comes
to finding evidence of life on Mars, she says.
Leshin’s calculation is based on her study of deuterium, a heavy isotope of
hydrogen, on Mars. The concentration of deuterium increases over time because
lighter hydrogen in the atmosphere gradually drifts off into space. The more of
its hydrogen—and thus ultimately water—a planet loses, the greater
the change in the deuterium ratio. On Mars, the atmospheric deuterium
concentration is now five times that on Earth.
Advertisement
So if the water on Mars originally had the same isotopic composition as the
young Earth’s, as scientists have assumed, the Red Planet must have lost about
90 per cent of its water. But Leshin’s study of the water trapped within an
ancient Martian meteorite suggests this assumption is wrong.
The tiny 12-gram rock, found in Antarctica in 1994, was born around 4.5
billion years ago in magma flowing deep within Mars. Any “primal” water trapped
within it should have the same isotopic composition as the first water on Mars.
But instead of being the same as Earth’s, Leshin found that the
deuterium-to-hydrogen ratio of the water was twice as high. That would mean that
only 70 to 80 per cent of Mars’s water has left the planet, she says in a paper
that will appear in Geophysical Research Letters.
Leshin’s results could upset theories about the formation of Mars. It is
possible, for example, that Mars did not get its water from the same source as
Earth, but that some of it came from comets instead, whose water has a different
isotopic composition. “I find it a little disturbing to accept that the material
that made up Mars was so different,” says Thomas Donahue, a planetary scientist
at the University of Michigan.