Santa Cruz
WHY do frozen balls of ice and dust from outer space emit bright X-rays? American physicists are suggesting that the clouds of gas surrounding comets, notably Comet Hyakutake on its flyby last year, send forth the rays because solar ions steal their electrons.
In March 1996, Michael Mumma of NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland, reported that images of Comet Hyakutake from the German-US satellite ROSAT revealed intense X-rays, far brighter than expected. Further satellite observations have now confirmed that this effect is common, having appeared on eight or nine comets.
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A host of theories to account for the X-rays appeared last year (快猫短视频, Science, 20 April 1996, p 18). But most relied on speculative processes rather than well-known physics, says Tamas Gombosi of the University of Michigan in Ann Arbor. In last week鈥檚 Science (vol 276, p 939), Gombosi and his colleagues report that they have explained the X-rays through well-understood physics.
Gombosi鈥檚 team followed up a suggestion by Thomas Cravens of the University of Kansas at Lawrence. Cravens proposed that the X-rays may appear because the Sun throws out a stream of charged particles, including heavy nuclei that have been stripped of their electrons by the Sun鈥檚 heat. Because they harbour a lust for electrons, these highly charged ions could steal electrons from neutral molecules such as water and carbon monoxide in the cloud of gas that forms around a comet near the Sun.
When the electrons were first captured, they would be in a highly excited state. They would then jump down to lower energy levels. As they did so, they would release their energy as photons of light. 鈥淪ome of these photons happen to be very energetic, and they鈥檙e observed as X-rays,鈥� says Gombosi.
To test the idea, Gombosi鈥檚 team calculated the speed of the ions coming out from the Sun. Using this, they could pin down the density of solar ions that surround the comet. They then calculated how the ions would interact with Hyakutake鈥檚 atmosphere, and the energies of photons the electrons would emit as they joined new atoms. Sure enough, the predicted X-ray spectrum almost exactly matched that observed for Hyakutake.
鈥淭he beauty of this model is that everything is known,鈥� says Gombosi. 鈥淭here is no magic here.鈥� Mumma agrees: 鈥淭he solar wind charge transfer mechanism seems to be the one that I would favour as providing an explanation for half to 80 per cent of the intensities.鈥�