H_2O and CO_2 in the Upper Atmospheres of the Giant Planets

Details H_2O and CO_2 in the Upper Atmospheres of the Giant Planets H_2O and CO_2 in the Upper Atmospheres of the Giant Planets

Jul 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997dps....29.1504l&link_type=abstract

American Astronomical Society, DPS meeting #29, #15.04; Bulletin of the American Astronomical Society, Vol. 29, p.992

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Observations with the Short Wavelength Spectrometer (SWS) of the Infrared Space Observatory (ISO) have allowed the detection of a series of emissions due to the rotational lines of water vapor between 28 and 45 mu m in the spectrum of Saturn, Uranus and Neptune. These emissions originate from the sub-millibar region and the implied H_2O column densities are about 1.5x10(15) , 9x10(13) and 3x10(14) molec. cm(-2) , respectively. The CO_2 nu_2 band at 14.98 mu m was also detected in emission on Saturn and Neptune but not on Uranus. The CO_2 column density is about 8x10(14) molec. cm(-2) for both planets (assuming uniform mixing at p<=10 mbar in Saturn's case). Because of the tropopause cold trap, the detection of H_2O at stratospheric levels implies an external origin. Similarly, CO_2 must be deposited or formed locally in Neptune's stratosphere. On Saturn, another possibility if updraft of CO_2 from the deep interior. Sources of water include interplanetary dust, infall of asteroidal/cometary objects and material sputtered from rings and satellites. Using a simple model, in which water is deposited in the 0.1-3 mu bar range by meteoritic ablation and the main loss is assumed to be vertical transport, external fluxes in the range 10(5) to 10(7) H_2O molec. cm(-2) s(-1) are required to explain the observed H_2O amounts. The presence of CO_2 in Saturn and Neptune may result from similar ablation of CO_2 ice contained in infalling meteorites, or, perhaps more likely given its non-detection at Uranus, secondary production from CO + OH. Preliminary calculations suggest that this mechanism is quantitatively viable at least for Neptune. The existence of external sources of water in Giant Planets has also implications for the energy budget, photochemistry and ionospheric properties of these atmospheres. Elucidating the interplanetary vs. ring/satellite origin of water may also provide constraints on cometary activity at large heliocentric distances.

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