As the first spacecraft to orbit Mercury, NASA’s MESSENGER mission revolutionized our understanding of the Solar System’s innermost planet. One the six major science question that motivated the MESSENGER mission was “What are the radar-reflective materials at Mercury’s poles?” Earth-based radar observations in the 1990s had discovered high reflectivity signals near both Mercury’s north and south polar regions that were suggestive of being water ice, postulated to be located in permanently shadowed regions near the poles. But with only three flybys by the Mariner 10 mission from 1974-1975, more than half of the planet had never even been mapped, limiting evaluation of the hypothesis that water ice was present on the planet closest to the Sun.

For over four Earth years, in 2011-2015, the MESSENGER spacecraft orbited Mercury, returning a wealth of scientific data. These datasets provided compelling evidence that the radar-reflective materials near Mercury’s poles are indeed composed of water ice, including results based on measurements from the mission’s neutron spectrometer, laser altimeter, and imager. MESSENGER’s results provided new insights into Mercury’s polar deposits, in particular revealing that there are volatiles in addition to water ice present in the deposits and that the deposits are a few tens of meters in thickness, with distinct surface reflectance values and sharp boundaries.

Building on MESSENGER’s discoveries, key open questions drive new investigations of Mercury’s polar deposits. In particular, MESSENGER’s measurements were largely limited to Mercury’s north polar region, given the highly eccentric orbit of the spacecraft that passed far from the south pole. A fundamental open question is how similar are the polar deposits at both of Mercury’s poles? Additionally, located at Mercury’s south pole is the 180-km crater Chao Meng-Fu, the largest crater to host a significant polar deposit on the entire planet, and MESSENGER’s observations were highly limited for this intriguing ice-rich location. Additionally, while MESSENGER data show evidence for the presence of low-reflectance volatile compounds stable to higher temperatures than water ice, consistent with being due to the presence of complex organic compounds, spectral or compositional measurements of these additional volatile materials have not been obtained. The joint ESA-JAXA BepiColombo mission is well positioned to advance our understanding of these open questions and more when the mission enters Mercury orbit in late 2026.