A comet seen from close up - the surface looks like rock, but is a mixture of water ice, carbonaceous particles and interesting compounds. Image credit: ESA/Rosetta/Philae/ROLIS/DLR

Nov 28, 2014 Water. But not as we know it.

Sen—My first Sen blog! I should write about some up to the minute exciting astrophysics, but instead I can't stop thinking about little Philae, still stuck in a hole on the surface of comet Churyumov–Gerasimenko. I'm not the only one, either—the relieved scientists who were already toasting in champagne the acquisition of their data while the rest of us wiped away a tear or two have been hard at work. So what do we know now that we didn't know a fortnight ago?

Quite a lot, as it turns out. We know that the Ptolemy team have detected complex organic molecules, though we don't know the details. This is pleasing in many ways, not least because they seemed to have come off worst in the fight to make use of what little power Philae had. We knew from the ground that comets were sites for complex chemistry, but the ability to get an in situ sample is incredibly exciting; astronomers normally only get access to comet molecules once they've had the ride of the lives and are hundred of kilometers up in the coma, which makes things more complicated.

Speaking of molecules, it was the comet's water that had been attracting most of the attention. The logic was that a careful study of its isotropic signature—of the ratio of normal water to 'heavy' water with deuterium instead of regular hydrogen—would reveal whether comets like good old Churyumov–Gerasimenko provided Earth's water. The early Earth was parched for water, and so it must have come from somewhere.

You can argue that it remerged after being buried deep under the scorching surface and you can—and people will—look to asteroids, but you have to admit there's a certain elegance to the comet theory. Here our poor planet is, in need of water, and there these icy balls of water, conveniently contaminated with lovely organic molecule to give life a kick start. Studies of other comets had given a mixed picture; as far as we could tell from the ground the critical ratio didn't always match but going to the source, without that pesky processing by sunlight, was a critical test.

It seems Churyumov–Gerasimenko failed that test. Initial results from Philae suggest a lack of a match between Earth and comet. The case isn't closed yet—there are, always, other comets, and the kind of chemistry that happens on the surface of dust grains in the early Solar System tends to wreak havoc with deuterium ratios. But let's assume it holds up.

Asteroids are plainly less water-rich than their cometary counterparts and that means that if they're the source the required impact rate is much higher. It also means that the excitement of finding those organic molecules is dampened a little. They're still fascinating if you care, as I do, about comets, but perhaps they're less relevant for life's story. We should get our first look at the real science from Philae at a meeting next month, and in the meantime, let's hope against hope it wakes up. It's got more work to do, if only it can catch the Sun for a day or two.