December 2011

Water

The first detection of interstellar water was reported in 1969 by Cheung et al. via a single rotational line found in three sources, Sgr B2, Orion, and Westerhout 49 (W49). The observations were made at the Hat Creek Radio Observatory east of Redding, California. Buhl, Turner, Burke and their coworkers soon found water in additional sources. Extraterrestrial water is difficult to observe from the ground due to atmospheric water, so is fortuitous that the feature detected in the initial observations was accessible. A confirming detection of another line was reported in 1980 by Waters et al. using high altitude measurements with the Kuiper Airborne Observatory. The first extragalactic dectection of water was reported in 1977 by Churchwell et al. in the Triangulum galaxy (M33).

Although the notion of comets as "dirty snowballs" goes back at least to a 1950 model study by Fred Whipple, the first definite detection of water in a comet was not reported until 1986, when Mumma et al. found it in comet Halley. An earlier effort to find water in comet Bennett using the same line used to detect interstellar water failed.

UPDATE (July 2018): Water was detected in the protoplanetary disk TW Hydrae by Bergin et al. in 2011.

The source of earth's abundant water continues to be somewhat controversial, but the notion that it came from comets continues to find new support.


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