Spectral Detectability of Carbon dioxide Ice in the Permanently Shadowed Regions of Mercury

Introduction: Radar observations of Mercury’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’s North Pole may actually exhibit temperatures potentially conducive to the presence of CO₂ ice. The SIMBIO-SYS instrument onboard the BepiColombo’s mission (ESA/JAXA) will observe Mercury’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 µm in all panels, while CO₂ ice grain size decreases from 100 mm (A) to 1 mm (B) to 100 µm (C).

Method: VIHI is a visible and near-infrared spectrometer ranging from 0.4 µm to 2.0 µm. 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 µm. 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 µm and 1.96 µm. These bands could serve as primary indicators of CO₂ ice presence in a mixture with water ice. The 1.96 µm is stronger than the 1.43 µm (Fig. 1C), however, due to the spectral range of VIHI that terminates at 2 µm, distinguishing the 1.96 µm 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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