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Images from the HIgh-Resolution Imager on NASA's EPOXI spacecraft.
Credit: NASA/JPL-Caltech UMD
More images can be found at http://epoxi.umd.edu/ |
In this amazing image released this week by NASA's EPOXI mission, two different dynamic processes are shown. The tiny (~0.93 mile length) peanut-shaped object is Comet Hartley 2. (It's actually being described as more like a drumstick with knobby ends than a peanut.) The images, in different infrared wavelengths, show the distribution of water vapor, dust, CO2, and ice crystals around the comet. The NASA press releases emphasize that the difference between the smooth and rough areas on this comet are associated with the a difference in the erupting gas: water vapor from the smooth central section, and CO2 from the jagged ends of the peanut. From press articles, it appears that Hartley 2 is producing about 200 tons per second of water vapor; the flux of CO2 has not been estimated as far as I can tell. The H2O vapor production rate is comparable to the rate estimated for production by the plumes at the south pole of Enceladus, an active satellite of Saturn.
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Artist's conception of degassing on Comet Hartley 2.
NASA/JPL-Caltech/UMD/McREL |
An analysis of the dynamics of the jets as geysers erupting into a vacuum can be found in Yelle, R.V., Soderblom, L.A., and Jokippi, J.R., Formation of jets in Comet 19P/Borrelly by subsurface geysers, Icarus, 167, 30-35, 2004. Their proposal is that there are cavities in the subsurface of the comet that have nozzle-like outlets to the vacuum of space. As gas flows from a cavity through a narrow nozzle, the flow becomes collimated, and focused jets such as those observed in these images are produced. Dust is entrained in the gas flow and is accelerated out into a fairly linear jet. When the gas leaves the cavity, it expands laterally because of the high overpressures compared to the vacuum of space, but the density drops so rapidly that the dust grains that it has been carrying continue on their collimated trajectories, decoupled from the gas. The result is a diffuse gas cloud with collimated beams of dust or ice. This appears to be the
first time that it's been shown that sublimation of subsurface carbon dioxide drives the outgassing of a comet's nucleus.
Other articles of interest: Emily Lakdawalla writes The Planetary Society Blog and has a nice
essay describing the features on the comet.
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