The Geology of Mercury: The View Prior to the MESSENGER Mission

Abstract

Mariner 10 and Earth-based observations have revealed Mercury, the innermost of the terrestrial planetary bodies, to be an exciting laboratory for the study of Solar System geological processes. Mercury is characterized by a lunar-like surface, a global magnetic field, and an interior dominated by an iron core having a radius at least three-quarters of the radius of the planet. The 45% of the surface imaged by Mariner 10 reveals some distinctive differences from the Moon, however, with major contractional fault scarps and huge expanses of moderate-albedo Cayley-like smooth plains of uncertain origin. Our current image coverage of Mercury is comparable to that of telescopic photographs of the Earth's Moon prior to the launch of Sputnik in 1957. We have no photographic images of one-half of the surface, the resolution of the images we do have is generally poor (∼1 km), and as with many lunar telescopic photographs, much of the available surface of Mercury is distorted by foreshortening due to viewing geometry, or poorly suited for geological analysis and impact-crater counting for age determinations because of high-Sun illumination conditions. Currently available topographic information is also very limited. Nonetheless, Mercury is a geological laboratory that represents (1) a planet where the presence of a huge iron core may be due to impact stripping of the crust and upper mantle, or alternatively, where formation of a huge core may have resulted in a residual mantle and crust of potentially unusual composition and structure; (2) a planet with an internal chemical and mechanical structure that provides new insights into planetary thermal history and the relative roles of conduction and convection in planetary heat loss; (3) a one-tectonic-plate planet where constraints on major interior processes can be deduced from the geology of the global tectonic system; (4) a planet where volcanic resurfacing may not have played a significant role in planetary history and internally generated volcanic resurfacing may have ceased at ∼3.8 Ga; (5) a planet where impact craters can be used to disentangle the fundamental roles of gravity and mean impactor velocity in determining impact crater morphology and morphometry; (6) an environment where global impact crater counts can test fundamental concepts of the distribution of impactor populations in space and time; (7) an extreme environment in which highly radar-reflective polar deposits, much more extensive than those on the Moon, can be better understood; (8) an extreme environment in which the basic processes of space weathering can be further deduced; and (9) a potential end-member in terrestrial planetary body geological evolution in which the relationships of internal and surface evolution can be clearly assessed from both a tectonic and volcanic point of view. In the half-century since the launch of Sputnik, more than 30 spacecraft have been sent to the Moon, yet only now is a second spacecraft en route to Mercury. The MESSENGER mission will address key questions ...

Support of long‐wavelength topography on Mercury inferred from MESSENGER measurements of gravity and topography

“…In conjunction with images from MESSENGER's Mercury Dual Imaging System [ Hawkins et al , ], MLA topography has helped to characterize and distinguish geological provinces [ Zuber et al , ]. Among the most distinctive physiographic features on Mercury's surface is the Caloris basin, the youngest and best preserved large impact basin on the planet [ Spudis and Guest , ; Head et al , ; Murchie et al , ]. The interior of the basin has been resurfaced with volcanic smooth plains, the Caloris interior plains (CIP), which were first observed in their entirety in images acquired during MESSENGER's flybys of Mercury prior to orbit insertion [ Murchie et al , ].…”

Global inventory and characterization of pyroclastic deposits on Mercury: New insights into pyroclastic activity from MESSENGER orbital data

“…Results of the survey of host crater morphology for the 46 pyroclastic deposits located within impact craters. Shown are the major periods in Mercury's global stratigraphy [Spudis and Guest, 1988] and an approximate geological timescale for Mercury, modified from Head et al [2007], on the left, and the number of pyroclastic deposits that are contained within host craters that have a degradation state comparable with craters from each geologic period as defined by Spudis and Guest [1988]. The nine deposits that fall at the Tolstojan-Calorian boundary are the nine deposits contained within the Caloris basin (which defines this stratigraphic boundary), and the deposit at the boundary between the Pre-Tolstojan and the Tolstojan is contained within the Tolstoj basin (which defines this stratigraphic boundary).…”

A search for Vulcanoids with the STEREO Heliospheric Imager

“…Based on the size-frequency distribution of craters on the Moon, Mars, Venus, and Mercury, it is thought that objects in the inner Solar System were resurfaced during the period of the Late Heavy Bombardment (LHB) 3.9 Gyr ago (Strom et al, 2005). Vulcanoids removed from stable orbits in the Vulcanoid zone by non-gravitational forces like the Yarkovsky effect (Vokrouhlický , 1999;Vokrouhlický et al, 2000) could have supplied a significant impactor population to Mercury after the LHB, making the surface appear older (Leake et al, 1987;Head et al, 2007).…”