Probing the subsurface of the two faces of Iapetus

Léa E Bonnefoy,Alice Le Gall,Cédric Leyrat,Nicolas Ponthieu,Bilal Ladjelate,Emmanuel Lellouch,Jean-François Lestrade

doi:10.1051/epjconf/202022800006

Abstract

Saturn’s moon Iapetus, which is in synchronous rotation, is covered by an optically dark material mainly on its leading side, while its trailing side is significantly brighter. Because longer wavelengths probe deeper into the subsurface, observing both sides at a variety of wavelengths brings to light possible changes in thermal, compositional, and physical properties with depth. We have observed Iapetus’s leading and trailing hemispheres at 1.2 and 2.0 mm, using the NIKA2 camera mounted on the IRAM 30-m telescope, and compared our observations to others performed at mm to cm wavelengths. We calibrate our observations on Titan, which is simultaneously observed within the field of view. Due to the proximity of Saturn, it is sometimes difficult to separate Iapetus’s and Titan’s flux from that of Saturn, detected in the telescope’s side lobes. Preliminary results show that the trailing hemisphere brightness temperatures at the two wavelengths are equal within error bars, unlike the prediction made by Ries (2012)[1]. On the leading side, we report a steep spectral slope of increasing brightness temperature (by 10 K) from 1.2 to 2.0 mm, which may indicate rapidly varying emissivities within the top few centimeters of the surface. Comparison to a diffuse scattering model and a thermal model will be necessary to further constrain the thermophysical properties of the subsurface of Iapetus’s two faces.

Highlights

Saturn’s icy satellites, which are in synchronous rotation around Saturn, often display large differences between their leading and trailing sides, which interact differently with their orbital environment and, in particular, with Saturn’s dust rings
We have observed Iapetus’s leading and trailing hemispheres at 1.2 and 2.0 mm, using the NIKA2 camera mounted on the IRAM 30-m telescope, and compared our observations to others performed at mm to cm wavelengths
Preliminary results show that the trailing hemisphere brightness temperatures at the two wavelengths are equal within error bars, unlike the prediction made by Ries (2012)[1]

Summary

Introduction

Saturn’s icy satellites, which are in synchronous rotation around Saturn, often display large differences between their leading and trailing sides, which interact differently with their orbital environment and, in particular, with Saturn’s dust rings. Ries (2012) partially bridged the gap between CIRS and Cassini radiometry by observing Iapetus’s two faces at wavelengths varying from 3 to 10.8 mm using the Green Bank Telescope (GBT) [1] He observed that, while the trailing side is less emissive than the leading side, it shows a large absorption feature likely centered near 3 mm. While the trailing side is less emissive than the leading side, it shows a large absorption feature likely centered near 3 mm He attributed this feature to diffuse scattering by 1–2-mm ice particles, by comparison with the semi-empirical Microwave Emission Model for Layered Snowpacks (MEMLS) developed for and tested on snow on Earth [9]. He predicted that observations below 3 mm should show a progressive drop of the emissivity with increasing wavelength

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