Detection of the hydroxyl radical in the Saturn magnetosphere

Abstract

THE magnetosphere in the vicinity of the orbits of Saturn's icy satellites consists of a low-density plasma, in which the electrons are an order of magnitude cooler than the accompanying heavy ions¹. Most models^2-12 neglect this fact, even though radiative cooling and diffusive loss rates are both too slow to account for the observed temperatures. Shemansky and Hall¹³ have recently proposed that the electrons could be 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 icy 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 detection 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 magnetosphere, implying a source rate for H₂O twenty times greater than current theoretical estimates^5,6. One possible explanation is that the micrometeorite erosion rates of the inner satellites are significantly higher than expected.