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comets and coated ice creams

12 February 2015

I don't know what it is with NASA news releases that compare astronomical features with food and eating disorders but here we have another one at www.jpl.nasa.gov/news/news.php?features=4480&utm … where comets are being described as deep fried ice creams. Mind you, they are said to have a soft centre. Ostensibly, they are saying the hard surface of comets is caused by a coating – somewhat like the chocolate on an ice cream lolly (I'm doing it as well, must be catching).

The idea is that fluffy ice on the surface of a comet can crystalise and harden as the comet periodically approaches the Sun and warms up. As water crystals form they become more dense and the result is a crunchy comet crust sprinkled with organic dust. I suppose the peanuts in the chocolate are the rocks strewn about on the surface. The article is published in the Journal of Physical Chemistry so not to be sniffed at – but are they being too eager to hang on to the old idea comets are fluffy snowballs and this is one way round the problem when at least two missions have shown they aren't dirty snowballs but something quite different. NASAs Deep Impact mission as well as ESAs Rosetta mission have both found a hard surface. The black coating of comets was also common to both – but the exact composition of the comet remains unclear. Rosetta hopefully will fill in some of the blanks and the above explanation is simply filling in the holes.

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