Effusive and Explosive Volcanism on Mercury from MESSENGER Orbital Observations

Abstract

Observations from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, after one Earth year in orbit about Mercury, have provided new insights into the nature of volcanic activity on the innermost planet and shown that volcanic characteristics on Mercury differ from those on other terrestrial planetary bodies. MESSENGER observations reveal no evidence for large shield volcanoes like those on Earth, Mars, and Venus, and small numbers of low shield-like constructs, pit craters, and candidate calderas. No evidence has been discerned for extensive centers of volcanism as seen on Mars (e.g., Tharsis, Elysium) or Venus (e.g., Beta and Atla Regiones), or less well-developed ones as seen on the Moon (e.g., Marius and Rumker Hills). Nor has evidence been seen for any Venus-like coronae or related annular deformational features displaying associated volcanism. Only one radial graben structure (Pantheon Fossae), centrally located in the Caloris basin, has been documented. Observations of Mercury to date also reveal no evidence for several types of volcanic features (cones, leveed flows, or widespread sinuous rilles). Instead, we see evidence on Mercury for extensive flooding of the surface to form regional smooth plains that appear to be very extensive lava sheet flows, and intercrater plains (found between large, old impact craters) that may also have been formed by volcanic eruptions. Smooth volcanic plains filling the interior of the Caloris basin show generally uniform ages and spectral characteristics and are up to several kilometers thick. Exterior plains of volcanic origin have similar to slightly younger ages. Contiguous plains at northern high latitudes cover ~6% of the surface of Mercury, have surface ages and spectral properties that show no resolvable variation, and have no locatable source regions. The general characteristics of the plains deposits and features on Mercury strongly suggest that they were emplaced by flood-lava-style eruptions rather than collections of narrow, leveed flows typical of small dike-emplacement events and more limited-volume surface eruptions. In summary, effusive volcanic deposits on Mercury appear to be characterized predominantly by: (1) deep magma sources of large volume, (2) little very shallow crustal storage of magma, and (3) high-volume eruption rates of lava and correspondingly voluminous outpourings that produced long and wide lava flows that covered extensive areas. These observations are consistent with theoretical predictions of vertically extensive and wide dikes penetrating through the lithosphere and crust. Deposits of pyroclastic origin and associated source depressions are globally distributed. The dimensions of individual pyroclastic deposits around vents signal the involvement of substantial amounts of magmatic volatiles in the eruptions. Observations by the suite of instruments on the ESA/JAXA BepiColombo spacecraft will markedly improve our knowledge of volcanism on Mercury.