IT鈥橲 hard to think of two more wildly different possibilities: did life on Earth get its start amid the gentle bubbles of a 鈥渨arm little pond鈥�, as Darwin put it, or was it born amid the colossal blows of space rocks slamming into our young planet?
The Rosetta spacecraft has now found water in the coma of comet 67P 鈥� the tenuous envelope of gas forming around the comet as it approaches the sun. That鈥檚 not unexpected: water has already been detected in other comets.
What was unexpected is that it鈥檚 not the same kind of water that we find on Earth: it is much richer in heavy hydrogen (see 鈥�#RosettaWatch: Comet water is not like Earth鈥檚鈥�). That鈥檚 a complication for the idea that comets delivered vast quantities of water to early Earth. Other comets do have Earth-like water, though, and an analysis of lunar rocks suggests the moon鈥檚 water may have come from meteorites (see 鈥�Lunar water delivered by soggy meteorites鈥�). If the Rosetta result holds up, we鈥檒l have to explain why we haven鈥檛 seen more comets with heavy water.
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But life here may have begun differently. The four bases that make up RNA have been created by mimicking meteorite impacts on early Earth 鈥� an environment where water was scarce (see 鈥�Formation of life鈥檚 building blocks recreated in lab鈥�). It鈥檚 a long way from life, but our own kind of big bang may have sparked it.
This article appeared in print under the headline 鈥淭he wrong kind of water鈥�
