BepiColombo's 5th Flyby: Early MERTIS Observations of Mercury's Surface Variations

Abstract

On December 1st, 2024, ESA/JAXA BepiColombo has become the 3rd spacecraft to observe the surface of Mercury, after Mariner 10’s flybys in 1974-75 and MESSENGER orbiting Mercury between 2011 and 2015. BepiColombo’s arrival is planned for November, 2026 - an 8 years journey which includes six flybys to guide the spacecraft into the orbit. The 5th flyby geometry has offered an exceptional and first-time opportunity for the MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS) to observe the planet’s surface through its Space Port. MERTIS is composed of a spectrometer (TIS) operating in the spectral range of 7-14 µm and a radiometer (TIR) with two channels at 8-14 µm and 7-40 µm [1]. The 5th flyby has marked the first time that Mercury’s surface has been observed spectrally resolved in the mid-IR range by a spacecraft [2]. This spectral range allows to better understand Mercury’s surface composition, as it is sensitive to elemental sulfur and sulfides [1], and iron-poor rock-forming minerals that are supposed to be common on Mercury [3].

Our understanding of the surface mineralogy and composition of Mercury is poorly constrained due to the lack of significant absorption features in the visible and near-infrared spectra acquired during the MESSENGER mission. This work focuses on (1) mapping the surface variations observed in the mid-IR range of the MERTIS instrument, within the coverage of the 5th flyby; (2) investigating the correlation with already reported surface features and geological units; (3) studying the global (within the 5th flyby coverage) effect of various surface- and temperature-dependent parameters in the MERTIS mid-IR signal variations.

The BepiColombo 5th flyby allowed observation of the surface for about 36 minutes from a distance of ~37268 km, resulting in a spatial resolution of 26-30 km/px (TIS). The TIS channel recorded 1,410,841 pixels on the surface of Mercury in full spatial resolution data acquisition mode with spectral 1x2 binning (no spatial binning). Deep space observations before and after intercepting with the planet were performed in order to provide a reference for a cold and “no emission” target. Here we see an increase in the measured signal shortly before intercepting with Mercury of less than one percent of the maximum radiance. This effect seems to be wavelength-dependent and reduces with increasing wavelength. This is a clear hint that our data are affected by straylight to a moderate extent. Observation through the space port is the main reason, which comes with some previously unknown parameters, since the space port was not initially planned for science observations. The team is currently investigating this effect.