Water among the outer planets.

Often dubbed the European Space Agency's answer to the Hubble Space Telescope, the Infrared Space Observatory (ISO) spends much of its time studying dying stars in the Milky Way and emissions from galaxies lying far beyond it (SN: 3/16/96, p. 168). The Earth-orbiting observatory also casts its eye closer to home. Recently, ISO measured the concentration of water vapor in the atmospheres of Saturn, Uranus, and Neptune.

Water vapor there is about one-thousandth the concentration in Earth's atmosphere, report H. Feuchtgruber of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and his colleagues in the Sept. 11 Nature.

Donald M. Hunten of the University of Arizona in Tucson notes that the measured concentrations do not exceed 20 parts per billion. Hunten, who was not involved in the study, adds that the measurements appear to be at odds with an 11-year-old theory, recently bolstered by new data, that thousands of small, house-size comets pelt Earth each day. These icy bodies, as yet unseen, could have delivered enough water to account for Earths-oceans (SN: 8/23/97, p. 117).

All comets are thought to originate in the deep freeze of the outer solar system. Thus, the number of house-size bodies per unit area bombarding the outer planets should far exceed the number reaching Earth. The amount of water delivered to the outer planets should be correspondingly greater. However, the water vapor recorded by ISO is only one ten-thousandth the concentration that would be expected if the proposed population of these comets were delivering the molecule, Hunten says.