ACIS/comet 1999S4
Credit: NASA/SAO/CXC/STScI/Lisse et al.

A Snowball's X-rays Solved

One of the biggest surprises to be uncovered by the ROSAT X-ray observatory was the discovery that comets, mountain-sized snowballs travelling around the sun, produced X-rays. X-rays are generally produced by very high energy processes, and very few people expected such processes to be associated with cold objects like comets. Thanks to a new observation by the ACIS X-ray camera on the Chandra X-ray observatory, we now know the mechanism by which comets produced X-ray emission. The ACIS observation of comet C/1999 S4 (LINEAR) shown above shows the intense X-ray halo surrounding the sunward side of the snowball. ACIS also has the capability to split the observed X-ray emission into its component wavelengths; by using ACIS to look at the X-ray spectrum, scientists have shown that the X-rays are produced as ions from the solar wind capture electrons from the comet's halo; the X-rays are generated as the captured electron cascades down to the atomic nucleus.

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Page Author: Dr. Michael F. Corcoran
Last modified August 18, 2000