Hot light from a cold comet

By Phil Plait | April 7, 2009 7:00 am

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

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.

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (22)

  1. serenity

    I love posts like these! It feels as if (no offence intended!) many posts recently have been a bit lighter (heh) on the “hard” science, as if mentioning hydroxyl molecules and electrons would scare people off. By “hard” I mean not-everyone-knows-it-“hard”, not hard for anyone actually interested in this stuff.

    Heck, I don’t know about the *average* reader, but I’d love more advanced, sciencey posts (more so than this one).

  2. ccpetersen

    Phil, it wasn’t all that long ago that those of us who studied comets for a living still weren’t sure there would be x-ray emissions from the coma of a comet. In fact, I remember one conversation (or was it a lecture, I forget) about emissions we COULD detect in the coma as a result of interactions with the solar wind. But, we had a suspicion about it, which was eventually borne out with actual observations of a comet (Hyakutake) with x-ray emissions (ROSAT data). As you point out, it’s not so much the comet as it is in the interactions between the entrained magnetic fields in the plasma tail and that of the solar wind… and magnetic reconnection events can release HUGE amounts of energy.

    I remember the day we saw our first data on that (I was still at LASP) and the hallway conversations that day were quite interesting.

  3. Todd W.

    Didn’t Deep Impact already destroy the notion that comets were plasma balls? This X-ray bit is just icing (ahem) on the cake.

  4. Flying sardines

    To produce the amount of UV detected, it turns out that Lulin was shedding some 3000 liters of water every second.

    Whoa! How long can comet Lulin keep that up before its loses all its water? That must be a fair percentage of its mass just evapourating away surely?

  5. MadScientist

    Ooo. Pretty pictures. :) All the talk about xrays and a UV glow make me wish for some really violent sun activity to make some pretty aurorae.

    Oh, and call it a ‘hydroxyl radical’ – it’s fun to see the alternative health screwballs squeal whenever they see ‘radical’.

  6. Flying sardines

    @ Serenity :

    I love posts like these! It feels as if (no offence intended!) many posts recently have been a bit lighter (heh) on the “hard” science, as if mentioning hydroxyl molecules and electrons would scare people off. By “hard” I mean not-everyone-knows-it-”hard”, not hard for anyone actually interested in this stuff. Heck, I don’t know about the *average* reader, but I’d love more advanced, sciencey posts (more so than this one).

    Agreed. Jusdt as long as there’s no hard maths involved … *shudder*

    BTW. I don’t mind any of the BA’s posts. Variety is the spice of life to quote aphrase & one of the things I love about this blog – you’re never quite sure what you’re gonna find when you log on here. Sometimes I’m in the mood for lighter stuff sometimes for harder, sometimes I feel like hearing about detailed science, sometimes just a joke, sometimes I get caught up in the politics – but I *always* love those awesome, breathtaking astrophotos & esp. the BA’s yearly top 10.

    (Yeah, I like ‘Letterman’ too! ;-) )

    PS. Can I just take a moment to say “Thanks Phil, I love your work!?”

    No? Oh well too bad, I’ve said it now! ;-)

  7. Flying sardines

    @ MadScientist :

    Oh, and call it a ‘hydroxyl radical’ – it’s fun to see the alternative health screwballs squeal whenever they see ‘radical’.

    Radical? Takes one to know one I guess .. the health screwballs strike me as radical themsleves – & not in the good skateboard-y slang sense. ;-)

  8. Michael

    @Todd W:

    We had direct evidence about the nature of comets already in 1985, with the ISEE-3/ICE probe. Probably better known is the Giotto probe that flew past Halley in 1986. Deep Impact was much later in 2005.

  9. Todd W.

    @Michael

    Thanks for the info. I figured that there was already some idea, but the only thing I remembered (not being an astronomer) was Deep Impact. My guess was that DI solidified the evidence for the makeup of a comet.

  10. IVAN3MAN

    Todd W.:

    Didn’t Deep Impact already destroy the notion that comets were plasma balls? This X-ray bit is just icing (ahem) on the cake.

    I’m sorry to disappoint you Todd, but, like bloody creationist crackpots, those “Electric Universe/Plasma Cosmology” twats at Thunderbolts.info think otherwise:

    thunderbolts.info/tpod/2005/arch05/050705impression.htm

    thunderbolts.info/tpod/2005/arch05/050708smoking.htm

    thunderbolts.info/tpod/2006/arch06/060214comet.htm

    I have refrained from providing direct links to that pseudoscience web-site for obvious reasons.

  11. IVAN3MAN

    P.S. Nutters believe what they want to believe. :roll:

  12. Erasmussimo

    Putting on my prescriptionist finger-wagging schoolmarm hat (dress?) I’d like to rap your knuckles with a ruler for this statement:

    I think it’s incredible that comets emit X-rays,

    You don’t believe that comets emit X-rays. But previously you wrote that they DO emit X-rays. {rap!} (say ouch).

    Could we PLEASE stop using ‘incredible’ as an intensifier? We really need that word to describe things that are intrinsically devoid of credibility.

    Oh, and one more thing: {rap!} (say ouch)

  13. denadn03

    The loss rate struck me as pretty high. I’d be curious to know the stats on the loss rate throughout the orbit. Is it virtually nil at apogee? Simple questions, but I’m an economist who just happens to love the harder sciences from school days long since gone. Thanks.

  14. Gary Ansorge

    Dang. A casual scan of the interboob yields no mass estimates so I can’t answer Flying Sardines question about mass loss, though I expect the comet masses in the billions of tonnes, I have no hard data.

    Bummer. I am somewhat disappointed in the data available on the ‘net.

    GAry 7

  15. Brian

    Todd W.:

    My guess was that DI solidified the evidence for the makeup of a comet.

    Quite the opposite: Deep Impact sublimated the evidence!

    … I’ll get my coat.

  16. Retrogarde

    Ah! Good old Amiga DigiPaint.

  17. Chip

    Phil
    This blog from comic log highlights X-rays produced in small amounts from an incredibly non-violent event.
    http://www.msnbc.msn.com/id/27323869/
    X-rays from tape. Very cool (or hot or whatever)

  18. MadScientist

    @Erasmussimo:

    But ‘incredible’ only means ‘unbelievable’, not ‘I don’t believe’. So in english as in french, “c’est encroyable!” is just an expression like ‘wow!’. (Now back to watching The Incredibles)

  19. Torbjörn Larsson, OM

    The loss rate struck me as pretty high. I’d be curious to know the stats on the loss rate throughout the orbit. Is it virtually nil at apogee?

    I’m just speculating here (no astronomer), but Lulin seems to be on a parabolic trajectory (or orbit, as astronomers seems to say), so no apogee and a likely high loss rate for its presumably first and only visit.

    According to “Gary W. Kronk’s Cometography” it was at its closest to the sun in january, while the image was taken 28 feb. So lets’ say 2-4 months worth of ejecting ~ 3 ton/s mass would amount to ~ 2*10^7 m3 water, or ~ 0.01 km3 of loose volatiles.

    I can see papers of observed comet size distributions of 0.6-13 km radii, with 1-3 km most common. (And up to 100’s of km radii for parent Kuiper objects – and the typical size of Oort objects is anybody’s guess, perhaps.) So maybe Lulin is shedding about ~ 1 % of its mass, and ~ 10 % at the most? (But a larger comet, a larger surface area, is likelier with a relatively huge loss rate.) Not a big deal, it seems to me.

  20. Flying sardines

    @ Gary Ansorge :

    Dang. A casual scan of the interboob yields no mass estimates so I can’t answer Flying Sardines question about mass loss, though I expect the comet masses in the billions of tonnes, I have no hard data.

    & Torbjörn Larsson, OM

    Thanks. :-)

    3000 liters per sec still seems like an awful lot of water to me ..

  21. Torbjörn Larsson, OM

    Ah, sorry, I didn’t see Gary’s comment, otherwise I would have provided a link even if delayed for moderation. And now I can’t remember what key words I washed out the info from the intertubes with. :o

    And we still don’t know the evaporation rate for Kuiper objects when they still are “in place”. Maybe you can get something out of the new Pluto atmosphere observations?

    PS Anything over a shower seems like an awful lot of water to me. The fun thing is that space and space objects are so immense in scale!

  22. Lucas r.

    Hi friends. I am a m and have 32 years. I just wamna to say that the thing most beautifull i had seen i saw it in the sky on the 7th day of april about 10:00 pm in torreon mexico. I saw the biggest “thing” in the sky with numerous beautifull colors, and belive me, y have 2 children and no matter y have lived, but that night i saw the greatness of comets!! i will remember all my life. Thanks

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