Cassini microwave radiometry observations of Enceladus' South Pole: Detection of a warm subsurface?

Abstract : At the beginning of the Cassini mission, the ISS (Imaging Science Subsystem) and CIRS (Composite Infra-Red Spectrometer) instruments discovered a geologically active region at the south pole of Saturn's moon Enceladus (e.g. Porco et al., 2005). Plumes venting material emanate from this region. Six years later, on November 6, 2011, the first-ever Synthetic Aperture Radar (SAR) image of Enceladus was acquired during the E16 flyby of the moon at the wavelength of 2-cm (Mitchell et al., AGU 2011). The SAR swath is located within the seemingly young South Pole Terrains, not far from the active sulci also known as the "tiger stripes" identified as the sources of the plumes. Concurrently to the SAR image, radiometry data were collected in the passive mode of the instrument with a ground footprint of 25-40 km across the track and ~5 km along. The Cassini radiometer records the thermal emission from the surface in the microwave domain, at 2-cm. More specifically, it measures the brightness temperature of the surface that varies both with the emissivity and the vertical temperature profile below the surface down to a depth, which depends on the electrical properties of the subsurface. Typically, radio instruments sense 10 to 100 wavelengths into an icy crust and can thus provide unique insight into the compositional, thermal and physical (porosity, roughness) state of planetary regoliths at depths much greater than the ones sampled by thermal IR spectrometers. In particular, microwave radiometer can be used to detect possible endogenic activity beneath the surface. The measured calibrated brightness temperatures during E16 cover a range from 33 to 60 K. In order to analyze these dataset, we have modeled the expected thermal emission from Enceladus' surface. In absence of endogenic emission, the temperature structure of any airless satellite results from a balance between solar insolation, heat transport within the subsurface and reradiation outward. The developed thermal model accounts not only for the diurnal but also for the seasonal variations of the temperature that cannot be ignored at the depths sensed by the radiometer. Outputs are coupled with a radiative transfer model to infer the blackbody brightness temperature sensed by the instrument. The surface emissivity is estimated from SAR measurements using Kirchhoff's law of thermal radiation and is found to lie between 0.2 and 0.8 along the scan, consistent with Ostro et al., (2006). Comparison between the E16 observations and the model outputs reveal that the measured brightness temperatures are much higher than expected when assuming no endogenic emission. Even for thermal inertias as high as 1000 Jm-2K-1s-1/2 (the CIRS-derived value is <30, Howett et al., 2010), the model predicts temperatures that are almost 20 K lower than the ones recorded. Only a locally much lower bolometric Bond albedo than assumed (0.8 based on Howett et al., 2010) could explain such results unless they point to a geothermal anomaly in the subsurface. A warmer near-surface could easily be masked to CIRS by a thermally insulating layer. Though maximum at the center of the scan i.e. closer to the South pole, the discrepancy seems to be present everywhere, which would argue in favor of a laterally extensive buried heat source and provide crucial new constrains for the moon heating mechanisms.
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Submitted on : Wednesday, December 26, 2012 - 10:16:57 AM
Last modification on : Saturday, June 15, 2019 - 3:20:05 PM

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  • HAL Id : hal-00768858, version 1

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Alice Le Gall, Cedric Leyrat, Michael A. Janssen, Aurélien Stolzenbach, Lauren C. Wye, et al.. Cassini microwave radiometry observations of Enceladus' South Pole: Detection of a warm subsurface?. AGU Fall Meeting, Dec 2012, San Francisco, United States. ⟨hal-00768858⟩

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