This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Friday, November 19, 2010

Degassing of Comet Hartley 2--Geysers causing a cosmic snow storm!

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.

Artist's conception of degassing on Comet Hartley 2.
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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