Astronomical evidence shows that cometary water may have variable deuterium to hydrogen
(D to H) ratios, and that the average ratio differs from that of water on Earth.
But unless Earth has lost a lot of hydrogen to space, which he says doesn't seem likely, the overall ratio of deuterium to hydrogen
on the planet shouldn't change much.
As Kathrin Altwegg (University of Bern, Switzerland) and colleagues report December 10th in Science, the ratio of deuterium to hydrogen
in Comet 67P/ChuryumovGerasimenko's water is 3V2 times that in Earth's oceans.
Caption: Different flavors The ratio of deuterium to hydrogen
, the D/H ratio, varies widely among solar system bodies.
But the water vapor is natural to Mars and shows five times as much deuterium to hydrogen
as Earth's seawater.
If two planetary bodies have similar ratios of deuterium to hydrogen
, then their water probably came from the same place.
The ratio of this primordial deuterium to hydrogen
can be used to determine whether the universe is "open" or "closed." That ratio, as measured toward the two stars Alexander targeted, is about 1.65x[10.sup.-5], which, according to current Big Bang models, suggests that the universe is open and could expand forever.
The ratio of deuterium to hydrogen
in the formic acid, a key precursor to the chemicals that make up cell membranes, indicates that the acid had an extraterrestrial origin.
It is harder for the heavy form to escape gravity, so if lots of hydrogen from water left Venus, the ratio of deuterium to hydrogen
left behind would rise.
In the May 18 SCIENCE, scientists reported that C/1999 S4 is the first comet whose ratio of deuterium to hydrogen
is similar to that found in water on Earth.
Assuming that both Earth and Mars began with the same ratio of deuterium to hydrogen
, researchers had calculated that about 90 percent of the water in the Martian atmosphere and upper crust had been lost since the planet formed.
However, the ratio of deuterium to hydrogen
in cometary water is much higher than in water on our planet.