DETECTION OF ICY SPECIES IN MERCURY’S PSRs: SPECTRAL SIMULATIONS FOR SIMBIO-SYS/VIHI ON BEPI COLOMBO

In this work, we apply a method previously discussed in [1] to assess the presence of water ice in Mercury’s Polar Shadowed Regions (PSR) mixed to S-rich volatile species like SO₂, H₂S, and volatile organics, intending to verify their detectability from Bepi Colombo’s orbit and to optimize SIMBIO-SYS/VIHI observations. PSR icy deposits located within the floor of the Kandinsky crater are simulated in terms of I/F spectra corresponding to mixtures of H₂O-H₂S and H₂O-SO₂ with grain sizes of 10, 100, and 1000 µm as resulting from indirect illumination by the scattered solar light coming from the crater’s rim. The spectral simulations, performed following the method described in [2], and including the ice-regolith mixing (areal or intimate) as modeled in [3], allow for exploring different volatile species abundances and grain size distribution.

The resulting ice detection threshold are evaluated by means of the computation of VIHI’s instrumental signal-to-noise ratio as given by the instrumental radiometric model [4]. In this way, a synergistic use of illumination models, spectral simulations and resulting SNR calculations will help us to optimize the VIHI’s observation strategy across Mercury’s polar regions with the aim to detect, identify and map volatile species. This task is one of the primary scientific goals of the 0.4-2.0 µm Visible and Infrared Hyperspectral Imager (VIHI) [5], one of the three optical channels of the SIMBIO-SYS experiment [6] on ESA’s BepiColombo mission.

Due to orbital characteristics and proximity to the Sun, Mercury’s polar regions undergo large variations in illumination conditions during the hermean year [1]. At poles, Permanent Shadowed Regions (PSRs) occur on deep craters and rough morphology terrains that are not directly illuminated by the Sun during the hermean day. Nevertheless, some of these areas could experience partial illumination caused by multiple scattered light coming from nearby illuminated areas. Despite the orbital vicinity to the Sun, Mercury’s PSRs can maintain cryogenic temperatures across geological timescales resulting in the condensation and accumulation of volatile species [7]. While water ice is the more certain species in Mercury’s PSR, it is not precluded the occurrence of other secondary species rich in sulfur or even organic matter.

In fact, the total surface area of PSRs between latitudes 80−90° south is not negligible, being estimated at about 25.000 km² [8], about two times larger than the same geographic area on the North Pole [9].

 

References: [1] Filacchione G. et al., MNRAS, 498, 1308-1318, 2020. [2] Raponi A. et al., Sci. Adv., 4, eaao3757, 2018. [3] Ciarniello. M. et al., Icarus, 214, 541, 2011. [4] Filacchione G. et al., Rev. Sci. Instrum., 88, 094502, 2017. [5] Capaccioni F. et al., IEEE Trans. Geosci. Remote Sens., 48, 3932, 2010. [6] Cremonese G. et al., Space Sci. Rev., 216, 75, 2020. [7] Paige D. A. et al., Science, 339, 300, 2013. [8] Chabot N. L. et al., J. Geophys. Res., 123, 666, 2018. [9] Deutsch A. N. et al., Icarus, 280, 158, 2016.

 

Acknowledgments: We gratefully acknowledge funding from the Italian Space Agency (ASI) under ASI-INAF agreement 2017- 47-H.0.