Properties of Icy Surfaces from their Thermal Emission: the Mimas case.

The icy surfaces of Saturn's synchronous satellites exhibit variations in thermal properties across their leading and trailing hemispheres. They are qualitatively attributed to changes in grain size properties and/or chemical composition which may originate from exogenous pollution, alteration by solar UV rays, or plasma and energetic charged particles trapped in Saturn magnetosphere, or from the coating or sandblasting by the particle of the E ring (Howett et al. 2018). It is difficult to understand which of these weathering processes dominates and what are the effects actually induced on the regolith.

On the surface of Mimas, the famous so-called “Pacman” asymmetry is very contrasted, with an asymmetry in thermal inertia of Γ_in= 66 ± 23 SI within a lens-shaped oval centered on the leading hemisphere, against Γ_out < 16 SI outside of it, for Bond albedos A_in ∼ A_out ∼ 0.6 (Howett et al. 2010, 2011). These estimates were obtained thanks to the high spatial resolution offered by the FP3 plane of the CIRS spectrometer which operates at wavelengths between 9 and 17 μm. They were recently revised to Γ_in = 98 ± 42 SI and Γ_out = 34 ± 32 SI for albedos A_in = 0.45 ± 0.08 and A_out = 0.41 ±0.07 after analysis of the whole set of FP3 observations acquired during the Cassini mission (Howett et al 2020). FP3 data did not cover the trailing hemisphere of Mimas. The thermal anomaly discovered on the leading hemisphere correlates well with the region where the energy accumulation due to high energy electrons bombardment occurs the fastest (Howett et al. 2020). The Bond albedo are systematically lower than those derived from near-infrared observations (Pitman et al. 2010).

This gain in high spatial resolution was achieved at the cost of the assumption of an emissivity equal to one. The emissivity of an icy regolith is expected to vary with the size of grains, the topography and the mixing of temperatures within the footprint of the observing instrument. The heat transfer within the regolith also depends on composition and size of grains, and on its porosity (Ferrari and Lucas 2016). On the other hand, CIRS focal plane FP1 plane (17-1000 μm) can be used to derive both the temperature and the emissivity at the cost of spatial resolution. We have therefore developed a thermal model that takes into account the size, composition of grains and the porosity of the regolith (Ferrari et al. 2021). It also includes the effect of topography and temperature mixing within the footprints of FP1 focal plane along an observation. Confronted with FP1 data, the model therefore makes it possible to interpret the regional variations in thermal emission with the variations in physical properties of the regolith.

Two FP1 observations of Mimas in which significant regional variations of emissivity e_F can be observed (figure 1), have been analyzed with this model. We have tested whether these regional variations observed on both the leading and trailing hemispheres around the predicted oval patterns of electrons energy deposition (Nordheim et al 2017) could be explained by variations in grain or regolith properties. Results obtained for both hemispheres will be presented and compared with constraints obtained thanks to near-infrared spectroscopy, in particular the variations in grain sizes and Bond albedo. The eventual sintering or amorphization of grains under MeV electrons bombardment on the leading side (Schaible et al 2016) or the competing processes of E ring deposition or sandblasting competing with eventual darkening by cold plasma on the trailing hemisphere will be discussed (Howett et al 2018).

Figure 1- Apparent Temperature T_F versus emissivity e_F within the footprints of one FP1 observation led in February 2005 on the trailing hemisphere of Mimas (Black full line). The red (dashed-dot) curve is the model best fit assuming uniform properties of the regolith. The green (dashed) curve yields a better fit assuming a lens-shaped pattern with maximum latitudinal extension of +/- 20 ° at the center of the hemisphere. Crystalline water ice and loose contacts between grains are considered here (Ferrari et al. 2021). Within the oval pattern, grains size may be R_in ∼ 100 μm with Bond albedo A_in ∼ 0.45, whereas grains outside of it, they may be much smaller, ∼ 15 μm with a brighter albedo A_out∼  0.61. In the uniform case, grains size may be ∼  20 μm in size with a Bond albedo A∼ 0.43.

 

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