Evidence For 200-Year-Old Comet Impact On Neptune 83
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."
Impact probability (Score:5, Interesting)
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).
If that is 200 earth years (Score:5, Interesting)