One of the reasons the ESA launched the Rosetta spacecraft, which sent the Philae lander down to the surface of Comet 67P/Churyumov-Gerasimenko last month, was to study the composition of the comet. They are the oldest celestial bodies, so they contain chemical clues to how the Solar System formed, and perhaps how life arrived on our planet. Like asteroids, comets are also known to contain water, and scientists have theorized that just as life may have hitched a ride to Earth aboard one, perhaps water did too. But some of Rosetta’s early findings challenge that idea.
As you know, water contains two hydrogen atoms and oxygen atom, H20. But according to data recently published in Science, water from Comet 67P has three times more deuterium than normal, or more accurately, than terrestrial water molecules. Deuterium is a heavy isotope of hydrogen, and is a common element on Mars (and not so much on Earth). On our planet, roughly .0003% of water molecules contain deuterium.