Ancient ice, wet and dry, from deep inside a comet

By Phil Plait | November 15, 2010 7:00 am

The comet Hartley 2 has come and gone, and the NASA mission EPOXI is also moving on after an exceedingly close flyby of the comet’s solid nucleus. The pictures we got were fantastic and beautiful… but their real power comes from coupling them with spectra.


In the picture above — an enhanced version of one of the images taken during the space probe’s flyby — you can see fan-like emission coming from the comet’s nucleus. These are jets; sprays of material coming out of the nucleus. Comets are made of rock and ice, and when the comet nears the Sun, the heat can turn that ice directly from a solid to a gas. This gas then shoots out from pockets on the nucleus, creating these jets. The EPOXI team (including my old boss, Don Lindler!) made a fantastic animation from a series of observations showing these jets in action.

But what are these ices made of? Lots of things we normally think of as liquids or gas (water, ammonia, carbon dioxide, and so on) exist in comets. In many comets, we see lots of water, and in fact the Swedish satellite Odin detected about 200 kg/sec (440 pounds per sec) of water coming off Hartley 2! So is water powering these jets?

hartley2_irplotWe can find out… using spectra. By breaking up the light from an object into its component colors, we can learn all manners of things including what it’s made of. EPOXI did just that with the jets streaming from Hartley 2, and while it did find water, amazingly, it found a lot more carbon dioxide!

The graph there shows an infrared spectrum of the comet’s jets, and the emission from water and CO2 are labeled. Apparently, it’s pockets of dry ice (frozen CO2) that are powering these jets, with water taking something of a back seat. That CO2 has probably been frozen deep inside the comet for billions of years, and is just now seeing sunlight for the first time. And when it does, it flashes into a gas, pouring forth from the comet. It takes other things with it, too, including dust grains, water, and even organic compounds like methanol. These gases then surround the nucleus, forming the fuzzy head of the comet, and stream away to form its tail.

And we couldn’t know this unless we went to the comet to see for ourselves. Observations like this are very difficult or impossible to do from the ground, where the Earth’s atmosphere absorbs the comet’s faint infrared signature (it’s the very presence of water and CO2 in our air that absorbs the light from those molecules in the comet!). Sometimes, there’s no substitute for being there.

And studying comets is important. For scientific reasons, they represent pristine example of the early solar system. Locked up inside them are chemical compounds and mixtures probably unchanged since they formed billions of years ago. Comets are the time capsules of the solar system.

They’re important for selfish reasons, too. Hartley 2 passed us at the relatively safe distance of 18 million km (11 million miles); close by in astronomical terms, but far on a human scale. Not all comets pass us safely, though, and we know they sometimes impact the Earth. A comet like Hartley 2 has a nucleus about 2 km across, small as comets go. Still, an impact by an object that size would cause a global catastrophe… and bigger comets exist. The more we know about these objects the more we can figure out what to do if we see one that decides to pay us a closer call than Hartley 2.

Related posts:

- 3D House of Comet Nucleus!
- Amazing close-ups of comet Hartley 2!
- Arecibo gives comes some radar love
- Three views to a comet

Comments (33)

  1. Messier Tidy Upper

    So dry ice (& wet ice too) drives a comet’s coma and fires its jets then, it seems. It looks like this* really is the recipe for making a comet :

    (Video via NASA & a comet expert)

    (written recipe & instructions.)

    By one of those strange co-incidences I was playing with dry ice ( ) myself earlier today – dunking it in a glass of water and watching the smoke-show and touching it without gloves – albiet briefly. Minus eighty degrees Celsius, pah! ;-)

    This “comet” is actually pretty good.

    Although perhaps not for eating! ;-)


    * Although I don’t know that EPOXI detected any traces ice cream or chocolate chips in Hartley 2′s spectrum. ;-)

  2. Messier Tidy Upper

    Superb image and marvellous science and awesome write-up. I love this blog! Thanks BA. :-)

    Thanks and congrats to the EXPOXI team and their Deep Impact spaceprobe too. :-)

    BTW. What – if anything – is planned for the DeepImpact craft itself now? Are there any further plans for it?

    It looks there like there are ribs or rings around a flat plain-like area on the middle right hand side of the nucleus there. Concentric cicles of (perhaps?) raised material in a set of three or so rings.

    Hartley-2′s surface there looks like there’s a sort of texture you can almost imagine running your hands over. (Not to be advised in reality I know esp. tactile contact without a spacesuit. ;-) )

    Wet ice, dry ice, old ice, new(~ly exposed) ice, all that’s left to find there is hot ice! ;-)

  3. Man, at that scale, I’m pretty sure I can see Bruce Willis on there!

  4. Chris

    Actually Phil you need to be careful here. This is the spectral intensity and you can’t directly make a comparison to the ratio of CO2 and Water. You need to account for the Einstein A coefficients. I took a quick look using the HITRAN program and CO2 is about 8 times more intense than water. So there is more water than CO2 coming off the comet.

    Put another way, this is like comparing apples to grapes. If you look at the weight, you may think you have more apples (CO2), but in reality you have more grapes (H2O).

  5. Methanol?

    Wake me up when it is ethanol. Nothing like a space martini.

  6. Gary Ansorge

    So much raw material going to waste. That single comet could supply hydrogen, oxygen and carbon for a hundred space colonies. Capture it now and relocate it to L5,,,ok, maybe not NOW, but soon.

    I wish I could live long enough(maybe as a download) to see space colonies strung across the solar system, housing ten trillion people and other critters. Note I say PEOPLE rather than just humans. Now we just need to define what we mean by people(my dogs might qualify, if I could just raise their effective IQs)(Oh,wait, maybe we should start that with current humans).

    Great pic.

    Gary 7

  7. Primordial CO2 can be characterized in the infrared by C-13/C-12 isotopic bands. There’s a lot of information in there. As has been metioned, water is the premier IR absorber. It’s worse than that! While dilute band absorbance is linear in concentration of absorber, saturated bands only increase as log(concentration). If there is a lot of water absorption, there is even more water then dilute spectral assumptions indicate.

    That is one of those little $trillion-levy things Envro-whiners conveniently overlook. Adding a few ppm CO2 more to the atmosphere is bupkis. Dumping intense absorbers into open spectral windows – hydrochlorofluorocarbons replacing Freons – is devastating. How do you know the Environment is being regulated unless it is being regulated badly?

  8. Jim

    Hmmm, water + carbon dioxide.

    OMG! Comets are made of club soda!!

  9. My crew of scientists have enhanced the distribution photos for a peek at what is not being announced. This discovery is huge! So why (if there is no conspiracy of silence) is there no word on the news? Isn’t anyone curious?

    Now you know the truth, pass it along (in your own special way) to save humanity.

  10. If comets are full of carbon dioxide couldn’t melting it be used, to alter it’s path? I guess it has a much lower warmth capacity than water and a lower boiling point.

    My physics are quite bad, but how much energy would be needed to alter the pass by doing this? Could a few kilo of radioactive substances do the trick? Like a burned out heating element out of an nuclear reactor? (Ok getting that one into space isn’t without a risk, but extinction is no option, too.)

  11. The Beer

    Uncle Al: I think you need to buy better spamming software….

  12. Joseph G (AG in SC)

    Fascinating stuff! I remember reading science books from not all that long ago (eh, several decades) that had very little insight as to the true nature of comets. They inferred that there was “dust” and “ice” involved, but that’s about it. It’s really cool to see science advance right before your eyes!

    Question: my understanding is that while comets vent those easily sublimated substances like CO2 and water, that they have a dark “crust” of complex organic molecules which are stable at much higher temperatures, and so never vaporize.
    Is this why comets often appear to have such weird shapes? As the volatiles vaporize, does this cause the crust to collapse inward, like a toothpaste tube being slowly emptied?
    Also, is it possible that some of these extremely carbonaceous low-density asteroids that we occasionally see are essentially old “empty” comets that have completely depleted themselves of gases?

  13. Joseph G (AG in SC)

    @#1 Messier: What do you do where you have the opportunity to play with dry ice regularly?
    Color me envious :D

  14. Gary Ansorge

    10. Darokthar

    That comet is WAY too massive to be significantly affected by a few hundred kg of Co2 and H2O boiling off the comet. Kinetic energy is directly proportional to the square of the exhaust plumes velocity. Comparing the kinetic energy of the boiling CO2 to the kinetic energy of the comet, it might affect the velocity of the comet by a few nanometers/sec, if that.(K.E.=1/2 MV^2).

    Gary 7

  15. Eddie Janssen

    This is the most impressive astronomical picture I have seen sofar. Even beats the Eagle Nebula pictures from 1994.

  16. Zetetic

    Uncle Al@ #7:
    I was expecting that some anti-AGW commenter would come on here to try and derail the thread at the mention of CO2.
    Thanks for the laugh….

    Just for the benefit of those still on the fence as to water versus CO2 in relation to Anthropogenic Global Warming.
    Water vapor is the most powerful greenhouse gas- Basic (The “Intermediate” tab option gives a more detailed explanation)
    CO2 is not the only driver of climate
    As to the issue of cost…
    Can we fix global warming?

    For any further anti-AGW commenters I would suggest reading the following list, to see if any any anti-AGW arguments you intend on using have already been debunked, before posting them. This will help to avoid looking like an anti-AGW spam bot.
    Skeptic Arguments and What the Science Says (List of already debunked arguments.)

    Thank you.

    Back to the subject of the thread… Very cool picture, in more ways than one!

    An example of the seeds of Earth’s early formation.
    Deflecting such an object is possible, but it would require a lot of time and energy to do so. A “Gravity Tow” is another option, but again requires a lot of time.

  17. But, but … the creationists have been saying that this comet is young!

  18. Pete Jackson

    @2Messier, there apparently isn’t enough maneuvering fuel to reach another comet or asteroid, but it’s excellent telescopes could do some observations of stars. Perhaps a program could be developed to obtain instant parallaxes of stars by observing portions of the sky simultaneously with EPOXI and from the Earth, or low Earth orbit (Hubble?, ISS?).

  19. Chris Winter

    Chris points out (#4) that the spectral data may be misinterpreted to give the impression that more CO2 than water is being released.

    However, that may also be a correct result. CO2 begins to volatilize at a temperature 80°F lower that water’s freezing point. I don’t know what the temperature was on the portions of Hartley 2 that were outgassing during the flyby, but logically the CO2 would go first.

  20. Jamey

    @15 Eddie: I have to say today’s APOD (11/15/2010) beats any picture I’ve seen so far. My first thought was “Who’s got a new SciFi novel coming out?” and then it hit me – no, that’s *REAL*. She’s in khakis and a pull-over, just relaxing, in orbit!

  21. MaDeR

    Crappy denialist bot is crappy and denialist.

    “My crew of scientists”
    Oh, multiple personality disorder. My favorite. ;)

    “have enhanced the distribution photos for a peek at what is not being announced.”
    Because it does not exists anywhere outside of your head.

    “This discovery is huge! So why (if there is no conspiracy of silence) is there no word on the news? Isn’t anyone curious?”
    Nah. Seen one crackpot, seen them all.

  22. Joseph G (AG in SC)

    @#8 Jim: Hmmm, water + carbon dioxide.
    OMG! Comets are made of club soda!!

    Well that explains the cleaning power:

  23. Joseph G (AG in SC)

    @#20 Jamey: Wow, that DOES look like a scene from a movie. You can even see the blue light on her face reflecting off the bluescreen.
    Oh, it’s the ocean? :P

  24. psuedonymous

    Maybe somebody more familiar with EPOXI can tell me: all the images released so far are from the MRI, but the coverage during the approach seemed to imply the HRI was also running. Why have no HRI images been released? Are they simply sat behind the MRI images in the data buffer?

  25. Regner Trampedach

    Thanks BA for yet another good post. EPOXI has done well. A great thanks goes to the team of what is formerly known as Deep Impact.
    Uncle Al @ 7: You have a very good point about the difference between optical thick or optical thin spectral features. Just for your info, however, CO2 in the Earth’s atmosphere only has 3 optical thick features: about 4.1-4.5micrometers, 4.8-4.9micrometers and 13-17micrometers,
    with 5 optical thin absorption bands in between (I defined optical thin as tau < 0.1). Ergo, there is plenty of linear absorption to wreck havoc with our climate! And they do occur in spectral regions with less H2O absorption. And even though the absorbed energy increases logarithmically with number of absorbers in the optical thick case, it still increases appreciatively. And it is of course included in climate models – so nothing changes in the conclusions. There are, after all, limits to how stupid climate scientists can be. That is still to be seen for AGW deniers… Please check your claims before you write them.
    Have a good day, Regner

  26. Messier Tidy Upper

    @18.Pete Jackson : Thanks. :-)

    @13. Joseph G (AG in SC) :

    @#1 Messier: What do you do where you have the opportunity to play with dry ice regularly? Color me envious

    No need to be envious, I don’t get to see dry ice regularly nor do I use it at work. In fact, yesterday was the first time that I’ve seen dry ice in years – a mate (science teacher) had some left over that he brought along to a writers group I attend. Which was just a nice co-incidence given this thread’s topic. :-)

    @ 8. Jim Says:

    Hmmm, water + carbon dioxide. OMG! Comets are made of club soda!!

    Reminds me of the old theory that Venus was covered in an ocean of carbonated fizzy soda water before we discovered thereality. It was one of many possibilities considered incl. an oil planet fuill of petroleumand the stereotypical prehistoric jungle planet. Wonder if there are world like any of those old Cytherean (alternate word for ‘Venusian’) theories out there somewhere?

  27. Messier Tidy Upper

    @12. Joseph G (AG in SC) :

    Also, is it possible that some of these extremely carbonaceous low-density asteroids that we occasionally see are essentially old “empty” comets that have completely depleted themselves of gases?

    It sure is and I think there are a number of examples known such as :

    an asteroid which was almost certainly a former comet and the source of the Geminid meteor shower :
    See also : for more. :-)

  28. Richard Woods

    @7 Uncle Al

    Before saying that others “conveniently overlook” something, it might be a good idea to proceed more humbly, to find out what you yourself have overlooked that they have not, or at least to _ask_ who’s doing the overlooking instead of making a flat-out accusation.

    In addition to what Zetetic @16 and Regner Trampedach @25 point out, see these explanations:

  29. Buzz Parsec

    Damn, went chasing down a URL and accidentally wiped my comet comment. :-(

    Trying to reconstruct…

    MTU – “hot ice”? Stolen diamonds? Wasn’t this the MacGuffin in an Arthur Clarke short story long ago?

    Using the jets to deflect a comet headed for the Earth… This has been seriously proposed. I first heard of it many years ago, possibly in an article in Sky and Telescope or Astronomy. (Someone may have recently written a book that mentions it.) The idea is to control the direction and strength of the jets by coating parts of the comet’s nucleus with either reflective material (e.g. aluminized mylar) to cool parts of it and shut down the jets and/or with absorptive material (e.g. powdered carbon or coal dust) to warm and enhance the jets. By depositing this stuff in appropriate places, you could basically turn the comet into a rocket that pushes itself out of a collision path with the Earth. It would probably take 5-10 years to build a spacecraft to do this and launch it to the comet, and it would probably take several months to a year to alter its trajectory sufficiently to make sure it doesn’t hit the earth, so warning time is an issue. (We usually only get a few months to a year of warning time of an approaching comet. Hale-Bopp at 2 years was exceptional.)

    I call this the “Paint It, Black” strategy.

    Comets have long been known to have fairly unpredictable orbits, which were explained to be due to nongravitational forces caused by the type of jets in these pictures. They are strong enough to significantly alter the course of a comet.

  30. Anchor

    @ Carey #3: “Man, at that scale, I’m pretty sure I can see Bruce Willis on there!”

    Well, if you looked at that hideouis movie carefully enough, you would have seen that every visual effect and the ridiculous set was absolutely nothing like the surface of a comet. You might also be amused to know that some gaffer’s equipment in the form of a ladder and other sundries are visible in some shots. Which just goes to show how incompetent and trustworthy the film production was in depicting ANYTHING.

  31. Messier Tidy Upper

    @ 28. Richard Woods & 25. Regner Trampedach & 16. Zetetic :

    Nice links & rebuttals there to which I’ll just add this one:

    The “Crock of the Week” take on that role of water vapour line. :-)

    @29. Buzz Parsec :

    MTU – “hot ice”? Stolen diamonds? Wasn’t this the MacGuffin in an Arthur Clarke short story long ago?

    Possibly although I’m not entirely sure.

    Might you be thinking of the diamonds from Jupiter’s core which provided a motive and problem / wonder or two in the third Space Odyssey novel 2061 perhaps?

    Can’t think of a particular Clarke short story involving hot ice tho’. Course that doesn’t mean there isn’t one I’ve missed or forgotten.

  32. Zetetic

    Messier Tidy Upper # @31:
    Thanks for the link to the video, yeah greeman3610 usually has some pretty good stuff.

  33. Messier Tidy Upper

    @ ^ Zetetic : No worries. That’s one of my fave sites & resources AGW~wise – as you may have noticed! ;-)


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