Hot light from a cold comet

Hot light from a cold comet

Hot light from a cold comet

Bad Astronomy
The entire universe in blog form
April 7 2009 7:00 AM

Hot light from a cold comet

A couple of months ago, the bright comet Lulin passed about 60 million kilometers from the Earth, relatively close as such things go. It was observed by a fleet of telescopes, including a few in orbit. One such observatory, my old friend Swift, took this very cool image of the comet:

Swift sees UV and X-rays from comet Lulin

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


Swift has three telescopes on board; one detects gamma rays, another high-energy X-rays, and third is sensitive to optical (the kind we see with our eyes) and ultraviolet light. This false-color image is a composite of X-rays (red) and UV and optical (blue/green), superposed on a deeper ground-based optical image to show the background stars.

There's a lot of cool stuff going on in this image. The optical light is centered on the comet itself. Really, a comet is a giant chunk of water ice and rock, together with other things we normally think of as gases like ammonia. When it gets close enough to the Sun, those ices turn into gas and surround the solid part (called the nucleus).

So the bulk of the optical and UV emission is coming from the denser gas surrounding the nucleus. In fact, the UV glow tells us how much water the comet is shedding: the water molecule breaks down into a hydrogen atom and a hydroxyl (OH-) molecule. Hydroxyl gives off a specific color of UV light, and how much UV we see tells us how much water is surrounding the comet. To produce the amount of UV detected, it turns out that Lulin was shedding some 3000 liters of water every second.

That's a heckuva fire hose.


But now take a look at the red part: that's from X-rays. But why aren't they centered on the nucleus?

The gaseous part of the comet, called the coma, gets blown around by both the solar wind and sunlight, producing the tail. The solar wind is really just a stream of subatomic particles (electrons, protons, neutrons, helium nuclei, and other wee bits) streaming out from the Sun at several hundred km/sec. When these hit the molecules in the comet tail, a series of processes occur which generate X-rays.

It's a bit funny: we usually associate X-rays with incredibly violent events like exploding stars, black holes gobbling down matter, and über-dense neutron stars with terrifyingly strong magnetic fields. Yet comets are frozen snowballs. You might not think they can emit such high-energy light.

But I guess that's a failing of our language. When a subatomic particle from the solar wind hits the comet's coma, the impact speeds are huge, and the event really is pretty extreme. It's just really really tiny. It's not enough to significantly heat up the comet, but it's certainly enough to make it glow in X-rays.


I'll note that a few years ago, an antiscientist was claiming that comets were not frozen chunks of ice, but were energetic balls of plasma, and had lots of silly ideas he invented to try to back this claim up. He was incredibly wrong in pretty much everything he said, including the idea that ice cubes can't emit X-rays.

I wish I had a dollar for every crank who had some elaborate theory that could fill a dozen notebooks, but fell flat on its face due to some basic physics.

As I like to point out, the real Universe is cooler and more surprising than any goofy antiscience theory about it. I think it's incredible that comets emit X-rays, that we can understand why, and that with remarkable observatories like Swift we can learn more about them. But of course, that's what real science does: it learns.

Credit: NASA/Swift/Univ. of Leicester/Bodewits et al.