Detection of a strongly negative surface potential at Saturn’s moon Hyperion

 

Illustration showing how a magnetic connection between the spacecraft and Hyperion can lead to the reduction in 18–100 keV electron counts observed by LEMMS. Not to scale

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. Observations by the Voyager and Cassini spacecraft found that Hyperion has a chaotic spin state, rotating nearly about its long axis by 72–75°/d. Its orbital semimajor axis is 24.55 Saturn radii (1  Rs  =  60,268 km), which takes it outside Saturn’s magnetosphere for parts of its orbit. Hence, Hyperion is exposed to plasma conditions representative of the outer magnetosphere, magnetosheath, or solar wind, depending on its orbital position and the overall magnetospheric configuration at the time.

On 26 September 2005, Cassini conducted its only close targeted flyby of Saturn’s small 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. 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.