AFTER a close look at Comet Hyakutake, astronomers are having to rethink their ideas about the make-up of comets. Hyakutake's spectrum shows unexpected and "enormously bright" lines of ethane, says Michael Mumma of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "This is the first clear evidence that there are at least two kinds of very distinct comets in the Solar System," he says.
Astronomers have long believed that comets are "dirty snowballs"homogeneous chunks of ice and dust. Mumma and his colleagues say that Hyakutake and seven other recent comets suggest there are two types of cometone rich in methanol and poor in ethane, the other low in methanol and rich in ethane (Science vol 272, p 1310).
Mumma thinks the crucial variable was the temperature at which the comets formed. Below about 20 kelvin, hydrogen combines with carbon monoxide to give methanol ice; at higher temperatures the reaction yields ...
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