A comet is essentially a bunch of rocks and dust mixed in with ice; the ice is what we normally think of as gases or liquids on Earth like carbon dioxide, carbon monoxide, ammonia, and water. These are frozen when in the depths of space, but as the comet approaches the Sun, the warmth penetrates the comet’s surface to where there are big repositories of ice. They turn directly from a solid into a gas (a process called sublimation), and can burst out. The material expands, reflects sunlight, and the comet gets much brighter. But there’s more. Dust mixed in with the gas blows off the comet, forming the fuzzy coma around it, and then follows behind in the comet’s orbit, forming the long, wispy tail. In general, that dust tail looks yellowish or red, since it is reflecting sunlight. As the comet nears the Sun, though, the solar wind slams into the gas emitted by the comet, exciting the electrons in the gas atoms, or even ionizing the gas – stripping it of an electron or two. As the electrons dance with their parent atoms, they emit light, generally in the green or blue part of the spectrum. Not only that, but the solar wind is traveling very rapidly compared to the comet’s own motion, so it blows the ionized gas particles straight back, away from the Sun. So we can get more than one tail: the yellow dust tail, and a vibrant green/blue ion tail, which is generally much straighter and narrower. The ion tail can be beautifully structured depending on how steady or how turbulent the solar wind is. A strong burst of wind from the Sun can even detach the ion tail, causing it to literally separate from the comet and drop away. Generally, once the wind calms down again, another ion tail (or tails) will grow Phil Plait/Slate
Posted on: Fri, 22 Nov 2013 05:12:44 +0000
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