Detailed assessment of morphologic characteristics indicates that these features are broad, low shield volcanoes. MES-SENGER images of the 1550-km-diameter Caloris basin reveal several irregular depressions surrounded by bright, relatively red deposits (compared to Mercury as a whole). Moderately volatile alkali metals are known to be important surface-derived species in Mercury's exosphere, and polar deposits postulated to consist of water ice have been documented by Earth-based radar on the floors of permanently shadowed impact craters near Mercury's poles these volatiles may be derived dominantly from meteoritic and cometary sources (e.g.,, however, and need not constrain interior volatile abundances.Īn important new constraint on interior volatile abundances on Mercury comes from imaging conducted during the first flyby of Mercury by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. Such heating should further deplete volatile species in Mercury's interior. Several of the scenarios proposed to account for Mercury's anomalously high bulk density (and inferred high ratio of metal to silicate) involve one or more episodes of further heating, either by the nebula itself or as a result of collision with another large object. Numerical simulations of planetary accretion indicate that Mercury is likely to be dominated by material formed in the inner solar nebula, where temperatures were comparatively high and volatile species remained in the gas phase throughout the time interval when nebular gas was present (e.g. The planet Mercury is generally thought to be deficient in interior volatiles compared with the other terrestrial planets (e.g.
Evidence for the presence of significant amounts of volatiles in partial melts derived from the interior of Mercury is an unexpected result and provides a new constraint on models for the planet's formation and early evolution. For comparison, measurements of the exsolution of volatiles (H 2 O, CO 2, S) from basaltic eruptive episodes at Kilauea volcano, Hawaii, indicate values of~1300-6500 ppm for the terrestrial mantle source. The minimum vent speed is~300 m/s, and the volatile content required to emplace the pyroclasts to this distance is hundreds to several thousands of parts per million (ppm) of the volatiles typically associated with pyroclastic eruptions on other bodies (e.g., CO, CO 2, H 2 O, SO 2, H 2 S). From the extent of the candidate pyroclastic deposit, we characterize the eruption parameters of the event that emplaced it, including vent speed and candidate volatile content. This candidate pyroclastic deposit has a mean radius of~24 km, greater in size than the third largest lunar pyroclastic deposit when scaled to lunar gravity conditions. Because of the importance of this inference for understanding the interior volatile inventory of Mercury, we focus on one of the best examples determined to date: a shield-volcano-like feature just inside the southwestern rim of the Caloris impact basin characterized by a near-central, irregularly shaped depression surrounded by a bright deposit interpreted to have a pyroclastic origin. Images obtained by the MESSENGER spacecraft have revealed evidence for pyroclastic volcanism on Mercury.