EXTERNAL SUPPLY OF OXYGEN TO THE ATMOSPHERES OF THE GIANT PLANETS

The atmospheres of the giant planets are reducing, being mainly compos ed of hydrogen, helium and methane. But the rings and icy satellites t hat surround these planets, together with the flux of interplanetary d ust, could act as important sources of oxygen, which would be delivere d to the atmospheres mainly in the form of water ice or silicate dust( 1-7). Here we report the detection, by infrared spectroscopy, of gaseo us H2O in the upper atmospheres of Saturn, Uranus and Neptune. The imp lied H2O column densities are 1.5 x 10(15), 9 x 10(13) and 3 x 10(14) molecules cm(-2) respectively, CO2 in comparable amounts was also dete cted in the atmospheres of Saturn and Neptune, These observations can be accounted for by external fluxes of 10(5)-10(7) H2O molecules cm(-2 ) s(-1) and subsequent chemical processing in the atmospheres, The pre sence of gaseous water and infalling dust will affect the photochemist ry, energy budget and ionospheric properties of these atmospheres. Mor eover, our findings may help to constrain the injection rate and possi ble activity of distant icy objects in the Solar System.