Geology on the Moon

Abstract

The Mariner 10 mission to Mercury (1974–1975) revealed extensive smooth plains; unlike the lunar maria, however, they showed no distinctive albedo contrast with the surrounding cratered terrain, and no volcanic edifices or vents, placing their volcanic origin into question. Decades later, flyby data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission provided abundant evidence for the presence and style of volcanism on Mercury: detected were extrusive volcanic vents, pyroclastic deposits, lava flow margins, spectrally distinctive plains flow units of volcanic origin, flooded craters and embayment relationships (permitting lava deposit thickness estimates), and high-resolution crater counts on different units, supporting emplacement of multiple flows. Following the MESSENGER flybys, orbital data provided information on the global extent of volcanic plains, their associated features (flow fronts, etc.), style and mode of emplacement, temporal distribution, association with other geological features, and their mineralogical and elemental nature. Observed were: 1) The northern volcanic smooth plains, a huge contiguous expanse of smooth plains that occupies more than 6% of the surface. Unlike mare-filled lunar impact basins, no significant differences in the impact crater size-frequency distributions of superposed craters are seen within the northern volcanic plains, indicating that they were emplaced over a relatively short time geologically. 2) Although lunar-like lava flow channels and sinuous rilles are not common, a few very large outflow-like channels exists near the northern volcanic plains. Here an unusual assemblage of teardrop-shaped hills and rough plains are interpreted to represent lava sculpting of underlying terrain, and distal emplacement of extensive flow lobes, consistent with the rapid emplacement of high-temperature, low-viscosity flood lavas and the thermal erosion of subjacent terrain. 3) Many dozens of ghost craters are interpreted as volcanically flooded impact craters with rims now visible primarily as rings of deformational features; these contain volcanic plains material often in excess of ∼1.5 km in thickness. 4) A global census shows that ∼27% of the surface of Mercury is covered by smooth plains, the majority of which are interpreted to be volcanic in origin. Although smooth plains are found globally on Mercury, they are more heavily concentrated in the northern hemisphere and in the hemisphere surrounding Caloris. 5) More than 100 pit craters, rimless depressions ranging from 10 to ∼80 km in widest dimension, have been observed and most (77%) occur on impact crater floors. Two origins have been proposed, collapse calderas and explosion craters. 6) A total of 51 candidate pyroclastic deposits have been identified to date; these appear to be globally distributed with ∼90% found in impact craters. Deposits are commonly centered on rimless pits. Assuming Hawaiian-style eruptions on Mercury, the volatile contents required would be much greater than predicted by previous compositional models. The style of volcanism is indicated by the lack of large shield volcanoes and the paucity of low shield-like constructs and candidate calderas. No evidence has been discerned for extensive centers of volcanism as seen on Mars (e.g., Tharsis) or Venus (e.g., Beta Regio). Nor has evidence been seen for Venus-like corona or related annular deformational features displaying associated volcanism. In summary, magmatism on Mercury appears to be characterized predominantly by: 1) deeper magma sources of large volume, 2) minimal shallow crustal storage of magma, 3) vertically extensive and wide dikes penetrating completely through the lithosphere and crust, and 4) high-volume eruption rates of lava and correspondingly voluminous outpourings producing long/wide lava flows covering extensive areas.