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Spacecraft data reveal that volcanism was active on Mercury. Evidence of large-volume effusive and smaller-scale explosive eruptions has been detected. However, only large (>similar to 15km) volcanic features or vents have been found so far, despite abundant high-resolution imagery. On other volcanic planets, the size of volcanoes is anticorrelated with their frequency; small volcanoes are much more numerous than large ones. Here we present results of a numerical model that predicts the shapes of ballistically emplaced volcanic edifices and hence can explain the lack of kilometer-sized constructional explosive volcanoes on the surface of Mercury. We find that due to the absence of the atmosphere, particles are spread on this planet over a larger area than is typical for Earth or Mars. Erupted volumes are likely insufficient to build edifices with slope angles that enable their easy recognition with currently available data or that could survive destruction by subsequent impact bombardment.

Plain Language Summary Volcanic eruptions have occurred on planetary bodies throughout the solar system, including Mercury. Eruptions have different styles, which affect the volcanoes they build. On Earth, small-volume explosive eruptions, which occur because expanding gas bubbles in the magma fragment the erupting molten rock, can form piles of material called scoria cones. Features resembling scoria cones have been observed on the Moon and Mars but not yet on Mercury. We used computer simulations to calculate where rock chunks would accumulate during explosive eruptions with different eruption volumes, speeds, and angles, under Mercury gravity. We found that, under most plausible scenarios, explosive eruptions on Mercury ejected material over too great an area to build a cone but instead built gentle slopes that would be undetectable in data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging mission. This is because Mercury has no atmosphere to reduce the maximum range of ejected rock and cause it to build up close to the vent. We suggest that BepiColombo, the next spacecraft to visit Mercury, should concentrate on searching for compositional, rather than topographical, evidence for explosive volcanism. We suggest that volcanic cones on the Moon may have formed differently to scoria cones on Earth, since the Moon also has no atmosphere.