Schumann resonance on Titan: A critical Re-assessment

Abstract

Extremely Low Frequency (ELF) electric field signals recorded by the Huygens probe have previously been interpreted as indicating a Schumann Resonance (SR) in the ionosphere-ocean cavity. This interpretation required accommodation of a number of anomalous features with respect to the terrestrial SR, namely a lack of observed lightning discharges with instead an appeal to excitation by interaction with the Saturnian magnetosphere, a horizontal field orientation, and the presence of only a single spectral peak speculated to be the second eigenmode. We suggest here that the data are dominated by artifacts due to mechanical oscillations of the booms or other elements of the probe. Although such an explanation was previously dismissed, we show in new analyses that the ELF time series correlates closely with the history of other mechanical vibration on the probe. In particular, the growth of the ‘ELF’ signals after the release of the main parachute, when the most extensive probe motions also began, and the disappearance of the signals within a few seconds after probe impact, point to a mechanical origin. We additionally review the capability of possible future SR measurements to constrain the thickness of an ice crust on Titan. Even if the magnetospheric excitation does not happen (it may or may not have occurred during the Huygens descent, we only argue that it is not required to explain the data), sporadic lightning during methane rainstorms could yet occur and excite occasional SR activity. However, the interpretation of the SR to retrieve ice thickness is challenged by ambiguities without simultaneous knowledge of the lower ionospheric structure.