EXPLORING THE DURATION OF EXPLOSIVE VOLCANISM ON MERCURY (Invited Presentation)

Mercury is a planet dominated by effusive volcanic and compressive tectonic morphologies. Despite often close and overlapping spatial proximity, these features tend to be relegated to distinct periods of mercurian history. Effusive volcanic plains appear to have been emplaced mostly prior to ~3.5 Ga, with little evidence for continued formation outside of young impact basins such as Rachmaninoff. In contrast, compressive tectonic morphologies, predominately lobate scarps (interpreted to be the surface manifestation of thrust faults), formed near the end of the emplacement of effusive volcanic plains, and sustained activity through recent mercurian history. These observations have been used to both inform and corroborate thermal models of Mercury’s evolution, with the transition in type of landform emplaced being tied to the cooling of Mercury and the imposition of a globally compressive stress state.

The MESSENGER mission also revealed new evidence of explosive volcanic morphologies on Mercury, including vent structures and associated pyroclastic deposits. Placing these features within the timeline of Mercury’s history provides important information about both the volcanic evolution of Mercury, and its thermal evolution. We have explored two methods for examining the ages of the explosive volcanic vents and their associated deposits. The first method uses the degradation state of craters hosting explosive volcanic vents to place an upper bound on the associated chronostratigraphic period of vent formation. Using this method, we observed vents associated with craters from all periods of Mercury’s history, including periods following the cessation of smooth plains emplacement. However, because this method only places an upper bound on formation period it is biased towards over-reporting vent ages; to address this, we have begun new investigations utilizing spectra from MESSENGER’s Visible and InfraRed Spectrograph instrument. Using the recognized spectral types previously identified for pyroclastic deposits, we are exploring how the spectra are influenced by space weathering. Our preliminary results suggest that the previous host-crater degradation method has indeed been too conservative, and that a larger number of pyroclastic vents may have been active through more recent mercurian history.