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Space Science

Evidence For 200-Year-Old Comet Impact On Neptune 83

Posted by kdawson
from the cold-case-file dept.
astroengine writes "Astronomers using ESA's Herschel space observatory have spotted evidence of a cometary impact in Neptune's upper atmosphere (publication, PDF). Whereas impact craters on rocky planetary bodies can remain for billions of years, an impact in the dynamic atmospheres of gas giants aren't obvious, especially if long periods of time have elapsed. This ultimate 'cold case' tracked the unusual distribution of carbon monoxide in Neptune's stratosphere, a sure sign it was deposited there by an external source. Once they realized they were looking at a comet impact, researchers were able to deduce when the impact occurred: 200 years ago."
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Evidence For 200-Year-Old Comet Impact On Neptune

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  • Impact probability (Score:5, Interesting)

    by JoshuaZ (1134087) on Tuesday July 20, 2010 @09:34PM (#32973324) Homepage

    Note that this hypothesis is more plausible than it might seem at first glance since we've seen comets impact gas giants before. Most famously, in 1994 Shoemaker-Levy 9 was observed directly impacting on Jupiter http://en.wikipedia.org/wiki/Comet_Shoemaker-Levy_9 [wikipedia.org]. This also isn't the first time this sort of technique has been used to detect historic comet impacts. As TFA notes, this technique was previously used to show that a similar event likely occurred around 230 years ago on Saturn.

    Although comets hit the outer planets frequently, this is due to a variety of issues including the large size of the planets and the exact orbit of Jupiter (which makes Jupiter very effective at clearing interplanetary debris). Thus, this sort of situation doesn't pose much of a risk for Earth. However, even a single such comet colliding with Earth would be an extinction level event. The asteroid that caused the Chicxulub crater in the Yucatan is generally estimated to be around 10 km diameter and most comets are generally larger than that (Halley's Comet has a mean diameter of 11 km, and many have larger mean diameters). Comets are also much easier to spot generally than asteroids and so we have a better idea about their orbits and are more likely to have a lot of warning before a potential impact event on Earth. Asteroids are much harder to see and pose much more of a threat even though they are smaller objects (with the exception of a handful such as Ceres).

    • Re: (Score:1, Troll)

      by BitZtream (692029)

      You and I have different definitions of impact, but I suppose you're correct about SL9. I tend to think exploding in the atmosphere isn't an impact, but on a gaseous planet you aren't going to get much other than an airburst.

      What I want to know is how we know what a comet that impacted a gas giant 200 years ago would look like in the atmosphere 200 years after the fact.

      Maybe its just me, but it seems like an aweful lot of 'science' recently has been based on pure speculation. I mean, I know there are some

      • On earth we measure the intensity of UV light in Antarctica and conclude that the wrong sort of gas is being used in air conditioners in the tropics. Of course you have to follow a few steps from the observation to the conclusion, and uncertainty accumulates along the way. But thats science for you.

        While we are at it, how can Toyota assemble an engine in a factory, sell it, and have it operate flawlessly for 20 years without even testing it once? I don't know either. Thats engineering for you.

      • by MidnightBrewer (97195) on Wednesday July 21, 2010 @12:57AM (#32974246)

        RTFA.

        We're talking about the distribution of gases between layers of the atmosphere. This same technique could be used on earth to extrapolate when the industrial revolution started to have an impact on the upper atmosphere, and is based on a similar principle as analyzing ice cores to determine the composition of the atmosphere of the earth a thousand years ago, and as a result, make inferences regarding the general climate at the time. This isn't that far-fetched.

      • Re: (Score:2, Flamebait)

        by DerekLyons (302214)

        What I want to know is how we know what a comet that impacted a gas giant 200 years ago would look like in the atmosphere 200 years after the fact.

        If you read the abstract or the article linked in the summary, you'd know.

        The balance of your reply makes it clear why you didn't however.

        And someone else believes this bullshit?

        Yes, I believe you're full of bullshit. Willfully and knowingly so. And you revel in it.

      • Re: (Score:2, Insightful)

        by euphemistic (1850880)
        Science is educated guessing. People get the facts and attempt to make the most plausible theory which fits said facts. When more facts are discovered, the theory is altered to fit said facts.

        All science should be therefore taken with a grain of salt, it's kind of the point. There is now a theory for a spot on Neptune, you aren't obligated to take it as some sort of absolute truth.

        And then to apply the 'logic' of "well we suck at this aspect of science so how could we be right about this other compl
      • by Bemopolis (698691)

        Maybe its just me, but it seems like an aweful lot of 'science' recently has been based on pure speculation.

        I wrote a long response to this. Then I deleted it, because it can be easily summarized...

        It is just you. Just because you are a fucking "aweful" idiot doesn't mean that everybody is.

      • Re: (Score:3, Insightful)

        by radtea (464814)

        Maybe its just me, but it seems like an aweful lot of 'science' recently has been based on pure speculation.

        It is, indeed, just you.

        "Pure" speculation would be speculation without reference to facts or well established abstract principles.

        In the present case, there is a distinct feature in the upper atmosphere of Neptune. That is a fact. We also have a whole bunch of facts about the details of the feature. Furthermore, we have a bunch of facts about the properties of comets and the odds of a comet of a given size hitting Neptune in the past few hundred years. And finally, we have a bunch of facts about how th

    • by gstoddart (321705)

      Note that this hypothesis is more plausible than it might seem at first glance since we've seen comets impact gas giants before. Most famously, in 1994 Shoemaker-Levy 9 was observed directly impacting on Jupiter. This also isn't the first time this sort of technique has been used to detect historic comet impacts. As TFA notes, this technique was previously used to show that a similar event likely occurred around 230 years ago on Saturn.

      You know, what's actually kind of scary about what you've written is ju

      • My guess is that a comet orbits the sun dozens or hundreds of times before it hits anything, and many of them never come as close to the sun as the Earth anyway. When it does hit something the probability of an impact with a comet may scale with the mass of the planet. Jupiter has 317.8 time the mass of earth and 10 times the radius. Another guess is that the probability scales with cross sectional area (for the actual impact) and mass (for the effective range of the gravitational field). Multiply the two (

        • by Marcx77 (1193559)
          How do you figure the mass has anything to do with the probability of impact? The cross section, yes, that's obvious, but the mass? The trajectories of the colliding objects are overwhelmingly determined by the gravitational interaction between those objects and the sun, not between those objects.
          • Well for a start the gravitational field of Jupiter (for example) can change the path of a comet to collide with the planet. Additionally if the comet goes within the roche limit of a planet it can fall to bits. This causes momentum to be split between parts of the comet, resulting in much of the object entering the atmosphere. This was why we say shoemaker levy 9 on its last orbit. Looking further out the gravitational field of a planet can alter the orbits of comets so they have a resonant relationship wi

            • by Marcx77 (1193559)
              I understand all that, but I don't see how these arguments warrant a *linear* relationship between the relative masses and the probability of impact. This is why I used the word "overwhelmingly" in my post, which I admit isn't backed up by calculations.
              • I don't see how these arguments warrant a *linear* relationship between the relative masses and the probability of impact

                Oh okay. My argument was really intended to be back of the envelope but I looked up Gravitational acceleration [wikipedia.org] (its been a while since I had to use it). Acceleration due to gravity is proportional to mass so if you pass Earth at 100000km and Jupiter at the same distance you will get 317 times the acceleration from Jupiter at that distance. So if you think about a target 100000 km in radius the gravity alone should make it 317 times more likely you will hit the planet, if the planet is Jupiter.

                • by Marcx77 (1193559)
                  See, and that's where I think you're wrong. The chance an object is going to hit a target isn't determined by its acceleration like you describe it is. You could say that its trajectory will be more affected by Jupiter than by Earth, which will mean that it'll probably pass by Jupiter closer (only a little, mind you) than by Earth, and will move away from Jupiter at a greater angle with the original trajectory than from Earth, all other circumstances being equal. But an object would have to be moving very s
                  • by Marcx77 (1193559)
                    RE:myself: .. than the 1:300,000 times the probability of a Jupiter impact, of course.
      • by clong83 (1468431)
        According to Carl Sagan in Cosmos, some astronomers think that Jupiter may have a small solid core at it's center made up of precisely all the asteroids and bits of rock from comets it's eaten up over the last few eons. But I don't think there's really any consensus on the issue.

        I am not an astronomer, but I did watch Cosmos the other night...
        • by gstoddart (321705)

          I am not an astronomer, but I did watch Cosmos the other night...

          Which is more than I knew.

          Thanks for the info.

      • by Ana10g (966013)
        I believe the current consensus is that Jupiter and Saturn (and I'm not sure about the other Gas Giants) has a compressed "liquid metal Hydrogen" core, (where Hydrogen at sufficient pressure acts as a metal): http://en.wikipedia.org/wiki/Metallic_hydrogen#Astrophysics [wikipedia.org]. IANA Physicist, so take that wiki article at face value. I'm still with you on not knowing what would happen, but I suspect that pressures significant to turn Hydrogen into a metal, I'd lean towards something akin to your diamond explanatio
        • by gstoddart (321705)

          Metallic Hydrogen? Man, from the article you linked, I see the words "alkali metal" and "superconductor, up to room temperature".

          Man, is science ever cool. :-P

          Cheers

    • Re: (Score:3, Informative)

      by khallow (566160)

      Note that this hypothesis is more plausible than it might seem at first glance since we've seen comets impact gas giants before. Most famously, in 1994 Shoemaker-Levy 9 was observed directly impacting on Jupiter http://en.wikipedia.org/wiki/Comet_Shoemaker-Levy_9 [wikipedia.org]. This also isn't the first time this sort of technique has been used to detect historic comet impacts. As TFA notes, this technique was previously used to show that a similar event likely occurred around 230 years ago on Saturn.

      So why do you think this technique shows evidence of a comet impact at a particular date? At best, it shows evidence of comet impact. Going from that to make a particular claim about the number of large impacts that could generate the observed atmospheric details, seems hasty. We may be seeing the results of many impacts over thousands of years rather than single large impacts a couple centuries ago.

      • Re: (Score:3, Informative)

        by DerekLyons (302214)

        We may be seeing the results of many impacts over thousands of years rather than single large impacts a couple centuries ago.

        If your hypothesis was true - they wouldn't be a sharp gradient of CO concentrations between atmospheric layers. However, such a gradient was observed, showing the impacts occurred in a relatively short time frame a relatively short time ago.

        • by khallow (566160)

          If your hypothesis was true - they wouldn't be a sharp gradient of CO concentrations between atmospheric layers. However, such a gradient was observed, showing the impacts occurred in a relatively short time frame a relatively short time ago.

          Why is that assertion true (that a sharp gradient implies what they claim it implies)? From what I understand, there appears to be observations of perhaps two or three impacts that they're basing this assertion on, Shoemaker-Levy and single addition impacts on Jupiter and Saturn since. That seems very sparse evidence on which to base such claims.

          • Why is that assertion true (that a sharp gradient implies what they claim it implies)?

            Why shouldn't it be true? (With the caveat that science doesn't claim to produce truth - only to produce explanations that match observations.) The theory matches all available evidence, so it'll do until someone comes up with a better explanation.

            From what I understand, there appears to be observations of perhaps two or three impacts that they're basing this assertion on, Shoemaker-Levy and single addition impac

            • by khallow (566160)

              Why shouldn't it be true? (With the caveat that science doesn't claim to produce truth - only to produce explanations that match observations.) The theory matches all available evidence, so it'll do until someone comes up with a better explanation.

              I didn't come up with a "better" explanation, but I did come up with a different one that apparently is just as compatible with the evidence.

              That's the way science works - they examine the available facts and produce a theory that explains it. Then other people seek to find if the theory holds up over time, as more facts are discovered.

              Save the lecture. My point was that there was other hypotheses that could explain the existing evidence. Why should we get into a discussion of the scientific method when it isn't an issue? Isn't that a bit unscientific to introduce extraneous information?

              • I didn't come up with a "better" explanation, but I did come up with a different one that apparently is just as compatible with the evidence.

                If it's the one I replied to, it's not compatible with the evidence.

                Save the lecture. My point was that there was other hypotheses that could explain the existing evidence. Why should we get into a discussion of the scientific method when it isn't an issue? Isn't that a bit unscientific to introduce extraneous information?

                You asked a question, and I answered it

                • by khallow (566160)

                  If it's the one I replied to, it's not compatible with the evidence.

                  This is what I'm talking about. There isn't enough evidence for the claim you make. Recall that you claimed a sharp gradient in carbon monoxide implies a single impact sequence some point about two centuries ago. That may well be true and I do not disagree that the hypothesis is compatible with the evidence. But again, that differential can also be explained through a combination of a steady rain of comets combined with some enhanced mechanism for destroying/sequestering carbon monoxide in the lower layer o

    • by johno.ie (102073)

      Ceres isn't an asteroid.

      • And Pluto isn't a planet. Not any more. Sniff!

      • by osu-neko (2604)

        Ceres isn't an asteroid.

        Depends on who you ask. It arguably is. And before someone leaps forward with links about Ceres being classified as a "dwarf planet", let me note that saying what Ceres is doesn't prove what it isn't, as things can answer to more than one description (you don't contradict someone claiming that a man is a father by noting that he's a brother -- he can be both). "Planet" has now received a much less ambiguous definition than it once had, and "dwarf planet" has be coined, but, as far as I know, "asteroid" h

  • We are rapidly learning more about the cometary impact rate on Jupiter, and now Neptune. It should be possible to extrapolate from this to calculate the impact rate on Earth.

    Clearly, a dangerous object spends some time orbiting across the orbits of the planets before it hits something, and the probability of an impact on Jupiter is much greater than an impact on Earth.

    We seem to be getting a handle on the risk from asteroids, but a comet can come our way without warning.

    • by JoshuaZ (1134087) on Tuesday July 20, 2010 @09:49PM (#32973402) Homepage

      We are rapidly learning more about the cometary impact rate on Jupiter, and now Neptune. It should be possible to extrapolate from this to calculate the impact rate on Earth.

      Better extrapolation method: look at historic impacts on Earth. See Chapman's 1994 paper in Nature "Impacts on the Earth by asteroids and comets: assessing the hazard" v. 367, Issue 6458, pg. 33-40. This paper gives a good summary of the literature at the time (my impression is that this hasn't changed much since then but this is far from my area of expertise).

      We seem to be getting a handle on the risk from asteroids, but a comet can come our way without warning.

      Not exactly. Comets that are anywhere near the inner system become visible very quickly due to their outgassing. In contrast asteroids are much harder to spot. On the other hand, asteroids stay where they are supposed to and don't have wildly elliptic orbits so they are much easier to track in the long run and tag. So there's a mix here, but overall asteroids are more likely to strike without warning. Comets will likely give us at least a few days to have a giant orgy.

      • by aliquis (678370)

        Comets will likely give us at least a few days to have a giant orgy.

        Big deal, I won't be invited that time either :(

        Oh well, atleast that time I won't have to wish for everyone else to die... They will anyway! >:]

      • by MichaelSmith (789609) on Tuesday July 20, 2010 @10:28PM (#32973606) Homepage Journal

        So there's a mix here, but overall asteroids are more likely to strike without warning.

        ...until we catalog them. We can do that from low earth orbit with infrared telescopes. The wise mission [berkeley.edu] has massively increased the rate of discovery, which is why I think the uncertainty about impacts will come from comets in the future.

      • by GIL_Dude (850471)
        I seem to recall reading that the out-gassing is assumed to fade over time (after several close passes to the sun) after which the bright tail and nimbus dissipate leaving a fairly low albedo object that, while still a comet, is hard to find. If that's really true, then "burned out comets" - still in their easily perturbed, long elliptical orbits would present a lot of danger.
        • Another issue with the highly eccentric orbits is that a comet can do a close pass past the sun, and hit us almost directly from the direction of the sun. A large object on that trajectory could easily be missed entirely.

  • Did they find water? After all, we're talking about Neptune.
  • Perhaps this is why we have never seen any Neptunians?
  • by damn_registrars (1103043) <damn.registrars@gmail.com> on Tuesday July 20, 2010 @11:26PM (#32973898) Homepage Journal
    If that was 200 earth years ago that the comet hit, then Neptune has made less than 1.5 orbits around the sun since then [wikipedia.org].
  • Voyager 2? (Score:4, Informative)

    by dpille (547949) on Wednesday July 21, 2010 @12:43AM (#32974196)
    When the comet Shoemaker-Levy 9 hit Jupiter sixteen years ago, scientists all over the world were prepared: instruments on board the space probes Voyager 2, Galileo and Ulysses documented every detail of this rare incident.

    That's funny, because my back-of-the-napkin estimate is that at the time, Voyager 2 was 3 billion miles further away from Jupiter than the Earth is. Wonder what they thought they were gonna see with 15-year-old technology that they weren't going to see with, say, the Hubble telescope, new ground-based instruments, or hell, even the naked eye that was 3 billion miles closer to the event.
    • by Vulch (221502)

      The Shoemaker-Levy impacts were happening on the side of Jupiter not visible from Earth. Voyager, Galileo and Ulysses all had a different view of the planet, so the view from 3 billion miles away is better than "none at all".

    • Re:Voyager 2? (Score:5, Informative)

      by Sockatume (732728) on Wednesday July 21, 2010 @05:45AM (#32975316)

      PUBLIC INFORMATION OFFICE
      JET PROPULSION LABORATORY
      CALIFORNIA INSTITUTE OF TECHNOLOGY
      NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
      PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011

                                                    VOYAGER MISSION STATUS
                                                            August 1, 1994

                Both the Voyager 1 and 2 spacecraft are healthy and they are
      continuing to take data on fields and particles in interplanetary
      space.

                The Voyager 2 spacecraft used two of its scientific
      instruments to look at the impacts of Comet Shoemaker-Levy 9
      fragments as they impacted Jupiter July 16-22. Both the
      ultraviolet spectrometer and the planetary radio astronomy
      experiments were used in the observations. Neither instrument
      detected any UV emission or radio signals during the impacts.
      The spacecraft began its observations of Jupiter on July 8 and
      will continue to observe the planet until August 17. At the
      time of the comet impacts, Voyager 2 was 6.1 billion kilometers
      (3.7 billion miles) from Jupiter.

                Voyager 1 is currently 8.4 billion kilometers (5.2 billion
      miles) from Earth. Voyager 2 is 6.4 billion kilometers (4
      billion miles) from Earth.

      My question would be, why not try? It's not like it took time away from mission-critical operations.

  • Evidence For 200-Year-Old Comet Impact On Neptune

    I thought most comets were much older than 200 years. How does a comet form, and crash, in a mere 200 years?

  • If the moon kingdom had a damned militia this wouldn't have happened....

    Instead they put all their funding for palaces built in the inhospitable airless vacuum that is the surface of the moon. Instead of building some sorta real defense force...

I tell them to turn to the study of mathematics, for it is only there that they might escape the lusts of the flesh. -- Thomas Mann, "The Magic Mountain"

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