Spectral Detectability of Carbon dioxide Ice in the Permanently Shadowed Regions of Mercury
Authors: Leclef Apolline , Vincendon Mathieu , Lantz Cateline , Filacchione Gianrico , Fornassier Sonia , Capaccioni Fabrizio 
Published: July 2024

Introduction: Radar observations of Mercury&#8217;s poles revealed bright features within its Permanently Shadowed Regions (PSRs), interpreted as possible water ice [1]. Subsequent neutron measurements by MESSENGER supported this finding, as they are consistent with a water-like-hydrogen saturated soil, potentially indicating layers of water ice about a few meters thick [2]. The probable detection of water ice at the poles of Mercury raises the question of its origin. Different scenarios relying on either endogenic or exogenic sources have been suggested, such as release through volcanism, hydrated asteroids or comets impacts, or solar wind implantation. These two latter mechanisms could have taken place during recent timescales. While the impact mechanism implies bodies that are already H2O rich (estimated average of 50% inside comets, up to 10-20% in asteroids[3]), the solar wind mechanism on the other hand will only bring H+, which is then implanted in the soil to form OH or H2O [4]. The possible presence of other species brought alongside water within small bodies was considered to distinguish between these two mechanisms [5]. The impact mechanism could actually introduce organic material and other types of volatiles, notably CO2 [3, 6]. CO2 ice could then be present within Mercury PSR [7], as suggested for the Moon too [8]. Based on temperature maps derived from MESSENGER's data and CO2 sublimation rates [9, 7], some PSRs on Mercury&#8217;s North Pole may actually exhibit temperatures potentially conducive to the presence of CO2 ice. The SIMBIO-SYS instrument onboard the BepiColombo&#8217;s mission (ESA/JAXA) will observe Mercury&#8217;s PSRs in 2026. In particular, the Visible Infrared Hyperspectral Imager Channel (VIHI) [10] will produce spectra that are expected to provide spectral evidence for water ice within the PSRs [11], as tentatively obtained on the Moon [12]. Our goal is to simulate PSR spectra of various water and CO2 ice mixtures to evaluate the detectability of putative CO2 ice with SIMBIO-SYS.

Figure 1: Simulations of areal mix between H2O ice and CO2 ice, for a ratio of 50% each. In yellow and blue we show the high-resolution CO2 and H2O ice reference spectrum (obtained from [13]) respectively, measured at 179 K completed with 28 K data for CO2, and 140-145 K for H2O. In green we mix these two spectra downgraded to the spectral sampling of VIHI. Water ice grain size is fixed at 100 &#181;m in all panels, while CO2 ice grain size decreases from 100 mm (A) to 1 mm (B) to 100 &#181;m (C).
Method: VIHI is a visible and near-infrared spectrometer ranging from 0.4 &#181;m to 2.0 &#181;m. With a spatial resolution down to 100 meters for a spacecraft altitude of 400 km, this instrument will provide data with a spectral resolution of 6.25 nm. The VIHI wavelength range includes several absorption features diagnostic of CO2 ice, in particular near 1.4 and 2.0 &#181;m. Fig. 1 shows an initial simplified simulation of spectra corresponding to a 50% mixture of water and CO2 ices. Three combinations of optical path lengths within each ice are shown (this allows e.g. different grain size configurations to be represented). Panel A corresponds to a much larger path length within CO2 than in H2O: we can see that most CO2 ice features are clearly distinguishable. Panels B and C explore configurations that may be more plausible, with lower to no relative differences between CO2 and H2O path lengths. In such configurations, we only observe the presence of two remaining spectral bands diagnostic of CO2 ice, at 1.43 &#181;m and 1.96 &#181;m. These bands could serve as primary indicators of CO2 ice presence in a mixture with water ice. The 1.96 &#181;m is stronger than the 1.43 &#181;m (Fig. 1C), however, due to the spectral range of VIHI that terminates at 2 &#181;m, distinguishing the 1.96 &#181;m band may be more challenging.
Next steps: We intend to enhance our simulations by incorporating the SNR conditions specific to VIHI observations of Mercury's poles. This will involve integrating the performances of VIHI [14, 15] and considering the illumination conditions of the PSR coldest areas possibly compatible with CO2 ice presence.
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