Probing the subsurface of the two faces of Iapetus

¹ Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), UVSQ /CNRS/Paris VI, Guyancourt, France
² Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA), Observatoire de Paris-Meudon, Meudon, France
³ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, 75014 Paris, France
⁴ Univ. Grenoble Alpes, CNRS, IPAG, Grenoble, France
⁵ Institut de Radioastronomie Millimétrique, Granada, España

^* e-mail: lea.bonnefoy@latmos.ipsl.fr

Published online: 27 January 2020

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.