Potential for Water Ice Detection at Mercury’s Poles: Contribution of Night-Side Observations by BepiColombo/PHEBUS Near the Terminator.

Radar-bright features near Mercury’s poles were first identified through ground-based radar observations, which suggested the presence of water ice at or near the surface in these regions [1,2]. Subsequent data from MESSENGER revealed a strong correlation between these radar-bright deposits and deep impact craters that remain in persistent shadow – areas where sunlight never reaches the crater floors and where surface temperatures are low enough for water ice to be thermally stable [3,4]. This stability is especially likely if the ice is insulated by a layer of organic-rich volatile compounds, as suggested by topographic, thermal, and spectral measurements from MESSENGER [5-8].

In these permanently shaded regions (Figure 1), the main source of illumination is solar Lyman-α radiation at 121.6 nm, produced by the scattering of solar photons by interplanetary hydrogen. This process generates a diffuse ultraviolet glow that uniformly bathes Mercury’s surface. If water ice is present, it is expected to induce detectable variations in surface albedo at this wavelength, as the Lyman-α reflectance from icy areas should be lower than that of the surrounding regolith.

Figure 1: North Polar Stereographic view of Mercury. The pink circles represent the Permanent Shadowed Regions (PSRs). 

The EUV (Extreme Ultraviolet) detector of the PHEBUS (Probing the Hermean Exosphere By Ultraviolet Spectroscopy) instrument on board BepiColombo [9], operating in the 55–155 nm wavelength range, is well-suited to investigate this phenomenon. PHEBUS/EUV will enable the detection of variations of surface reflectance variations at Lyman-α, offering a unique opportunity to confirm the presence of surface or near-surface water ice in Mercury’s polar regions. In this study, we identify potential observation opportunities for these PSRs with PHEBUS/EUV during the nominal science phase of the BepiColombo mission, starting in April 2027. Observations of Mercury’s polar regions on the night side, near the terminator, with PHEBUS are scheduled during the so-called “terminator seasons” (Figure 2). However, these observations come with a risk that needs to be carefully considered: because the observations involve pointing close to the illuminated surface of Mercury, even a small misalignment could allow direct sunlight to enter the baffle and irreversibly damage the detector. Scheduling the observations toward the end of the mission would ensure that most exospheric data will have already been acquired, minimizing the risk of detector degradation.

Figure 2: The diagram shows the orbit of Mercury as seen from the Sun's north pole and the polar orbits of BepiColombo (green). It also indicates the boundaries of the four seasonal phases during Mercury's 88-day orbital period: in blue, the perihelion season, in orange the aphelion season and in magenta, the terminator seasons.

 

References:

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