Mystery of cometary X-rays solved

Comets, which resemble “dirty snow balls” a few miles in diameter, until recently were thought to be too cold to emit X-rays. So the detection of X-rays from comet Hyakutake in 1996 was a surprise. Several explanations were suggested, but the source of cometary X-ray emission remained a puzzle. NASA’s Chandra X-ray Observatory in July 2000 captured images of Comet C/1999 S4 (LINEAR) and detected X-rays from oxygen and nitrogen ions. The observation showed that they are produced by collisions of ions racing away from the sun with gas in the comet. “This observation solves one mystery. It proves how comets produce X-rays,” said Carey Lisse, leader of the scientific team. “With an instrument like Chandra, we can now study the chemistry of the solar wind, and observe the X-ray glow from the atmospheres of comets as well as planets such as Venus. It may even be possible to observe other, nearby solar systems.”