H O and CO in the Upper Atmospheres of the Giant Planets
E. Lellouch (Obs. Meudon), H. Feuchtgruber (MPI Garching), Th. de Graauw (SRON Groningen), B. Bézard, Th. Encrenaz (Obs. Meudon), M. Griffin (QMWC London)
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 m in the spectrum of Saturn, Uranus and Neptune. These emissions originate from the sub-millibar region and the implied H O column densities are about 1.5 10 , 9 10 and 3 10 molec. cm , respectively. The CO band at 14.98 m was also detected in emission on Saturn and Neptune but not on Uranus. The CO column density is about 8 10 molec. cm for both planets (assuming uniform mixing at p 10 mbar in Saturn's case).
Because of the tropopause cold trap, the detection of H O at stratospheric levels implies an external origin. Similarly, CO must be deposited or formed locally in Neptune's stratosphere. On Saturn, another possibility if updraft of CO 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 bar range by meteoritic ablation and the main loss is assumed to be vertical transport, external fluxes in the range 10 to 10 H O molec. cm s are required to explain the observed H O amounts. The presence of CO in Saturn and Neptune may result from similar ablation of CO 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.