Abstract
On 26 September 2005, Cassini conducted its only close targeted flyby of Saturn's small, irregularly shaped moon Hyperion. Approximately 6 min before the closest approach, the electron spectrometer (ELS), part of the Cassini Plasma Spectrometer (CAPS) detected a field-aligned electron population originating from the direction of the moon's surface. Plasma wave activity detected by the Radio and Plasma Wave instrument suggests electron beam activity. A dropout in energetic electrons was observed by both CAPS-ELS and the Magnetospheric Imaging Instrument Low-Energy Magnetospheric Measurement System, indicating that the moon and the spacecraft were magnetically connected when the field-aligned electron population was observed. We show that this constitutes a remote detection of a strongly negative (∼ −200 V) surface potential on Hyperion, consistent with the predicted surface potential in regions near the solar terminator.
Highlights
Hyperion is a highly irregular outer moon of Saturn, with dimensions of 180 × 133 × 103 km and a mean radius of 133 ± 8 km
A dropout in energetic electrons was observed by both Cassini Plasma Spectrometer (CAPS)-electron spectrometer (ELS) and the Magnetospheric Imaging Instrument Low-Energy Magnetospheric Measurement System, indicating that the moon and the spacecraft were magnetically connected when the field-aligned electron population was observed
We report on the discovery of a strongly negative surface potential at Hyperion by the Cassini low-energy Electron Spectrometer (ELS), part of the Cassini Plasma Spectrometer (CAPS) [Young et al, 2004]
Summary
Hyperion is a highly irregular outer moon of Saturn, with dimensions of 180 × 133 × 103 km and a mean radius of 133 ± 8 km. It has a low mean density, indicating that it may consist primarily of water ice with an unusually high degree of porosity (>40%), resulting in its characteristic “sponge-like” appearance [Thomas et al, 2007]. Due to interaction with solar photons and magnetospheric plasma, it is expected that surfaces in Saturn’s magnetosphere may acquire a net electric charge. We report on the discovery of a strongly negative surface potential at Hyperion by the Cassini low-energy Electron Spectrometer (ELS), part of the Cassini Plasma Spectrometer (CAPS) [Young et al, 2004]. Presented are measurements made by the Magnetospheric Imaging Instrument Low-Energy Magnetospheric Measurement System (MIMI-LEMMS) [Krimigis et al, 2004], the Radio and Plasma Wave (RPWS) instrument [Gurnett et al, 2004], and magnetometer (MAG) [Dougherty et al, 2004], which provide crucial context for the CAPS-ELS observations
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