DETECTION OF THE HYDROXYL RADICAL IN THE SATURN MAGNETOSPHERE

THE magnetosphere in the vicinity of the orbits of Saturn's icy satell ites consists of a low-density plasma, in which the electrons are an o rder of magnitude cooler than the accompanying heavy ions1. Most model s2-12 neglect this fact, even though radiative cooling and diffusive l oss rates are both too slow to account for the observed temperatures. Shemansky and Hall13 have recently proposed that the electrons could b e cooled by the presence of a large abundance of neutral gas, derived mainly from the breakdown products of H2O (mainly O and OH) from the i cy satellites. Hydrogen radicals have been reported in this region13, but these originate from the atmosphere of Saturn itself; no satellite -derived neutral species have been detected. Here we report the detect ion of neutral OH molecules near the orbit of Tethys, using the Faint Object Spectrograph on the Hubble Space Telescope. Our results suggest that neutral OH is one of the dominant species in Saturn's inner magn etosphere, implying a source rate for H2O twenty times greater than cu rrent theoretical estimates5,6. One possible explanation is that the m icrometeorite erosion rates of the inner satellites are significantly higher than expected.