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Probing the subsurface of the two faces of Iapetus


  • Ms. Lea BONNEFOY

Auteurs principaux



Saturn’s icy satellites, which are in synchronous rotation around Saturn, often present a different albedo on their leading and trailing sides, which interact differently with Saturn’s dusty rings. Because longer wavelengths probe deeper into the subsurface, observing both sides at a variety of wavelengths indicates possible changes in thermal and physical properties with depth. The Cassini mission has observed the near-surface thermal emission from Saturn’s moons in the far-infrared (with Cassini’s Composite Infrared Spectrometer CIRS) [1] and at 2.2 cm wavelength (with the Cassini Radar/Radiometer) [2,3]. Ongoing analysis of the radiometry data by comparison with a thermal model of the surface suggests several of these satellites also show a leading/trailing dichotomy at 2.2 cm. The contrast in brightness temperature is especially strong for Iapetus; this dichotomy should also be visible at millimeter wavelengths i.e. at IRAM NIKA2 operating wavelengths.

We are currently conducting an observing campaign to bridge the gap between the CIRS and radar datasets by observing different longitudes of Iapetus at mm and cm wavelengths with, respectively, IRAM NIKA2 and the JVLA (Jansky Very Large Array, New Mexico, USA). In May 2018, we acquired 1- and 2-mm observations of Iapetus’ anti-Saturn side from IRAM NIKA2, and in February-March 2019 we collected data on Iapetus’s leading and trailing sides. These observations will allow us to detect and quantify the brightness temperature variation with longitude. Combined with data from CIRS, the Cassini Radar/radiometer and JVLA, NIKA2 will provide insights into the thermal, physical, and compositional properties of Iapetus’s near-surface as well as into their variations with both depth and longitude. We will report on the analysis of the NIKA2 data to produce an image of the satellites Titan and Iapetus in the proximity of Saturn, which is 1,000 to 10,000 times brighter, respectively.


[1] Howett et al., 2010, Icarus 206, 573–593.

[2] Ostro et al., 2006, Icarus 183 (2), 479–490.

[3] Le Gall et al., 2014, Icarus 241, 221–238.