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Evidence For 200-Year-Old Comet Impact On Neptune
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."
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.
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 has massively increased the rate of discovery, which is why I think the uncertainty about impacts will come from comets in the future.
http://michaelsmith.id.au
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. 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.
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.
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.
"Give a man fire, and he'll be warm for a day; set a man on fire, and he'll be warm for the rest of his life
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.