AFTER a close look at Comet Hyakutake, astronomers are having to rethink their ideas about the make-up of comets. Hyakutake鈥檚 spectrum shows unexpected and 鈥渆normously bright鈥� lines of ethane, says Michael Mumma of NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland. 鈥淭his is the first clear evidence that there are at least two kinds of very distinct comets in the Solar System,鈥� he says.
Astronomers have long believed that comets are 鈥渄irty snowballs鈥濃�攈omogeneous chunks of ice and dust. Mumma and his colleagues say that Hyakutake and seven other recent comets suggest there are two types of comet鈥攐ne 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 ethane.
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One idea is that heat from the Sun could have produced ethane in comets that formed near Jupiter and Saturn, while methanol would have been more abundant in those condensing in cooler regions further out.