1. Max-Planck Institut für Extraterrestrische Physik, 85748 Garching, Germany
2. ISO Science Operation Center, ESA, 28080 Villafranca, Spain
3. DESPA, Observatoire de Paris, 92195 Meudon, France
4. SRON, 9700 AV Groningen, The Netherlands
5. QMW College, London E1 4NS, UK

Correspondence to: E. Lellouch³ Correspondence should be addressed to E.L. (e-mail: Email: lellouch:obspm.fr).

The atmospheres of the giant planets are reducing, being mainly composed of hydrogen, helium and methane. But the rings and icy satellites that surround these planets, together with the flux of interplanetary dust, could act as important sources of oxygen, which would be delivered to the atmospheres mainly in the form of water ice or silicate dust^{1, 2, 3, 4, 5, 6, 7}. Here we report the detection, by infrared spectroscopy, of gaseous H₂O in the upper atmospheres of Saturn, Uranus and Neptune. The implied H₂O column densities are 1.5 10¹⁵, 9 10¹³ and 3 10¹⁴ molecules cm^-2 respectively. CO₂ in comparable amounts was also detected in the atmospheres of Saturn and Neptune. These observations can be accounted for by external fluxes of 10⁵–10⁷ H₂O molecules cm^-2 s^-1 and subsequent chemical processing in the atmospheres. The presence of gaseous water and infalling dust will affect the photochemistry, energy budget and ionospheric properties of these atmospheres. Moreover, our findings may help to constrain the injection rate and possible activity of distant icy objects in the Solar System.