[1] Orbital images from the MESSENGER spacecraft show that~27% of Mercury's surface is covered by smooth plains, the majority (>65%) of which are interpreted to be volcanic in origin. Most smooth plains share the spectral characteristics of Mercury's northern smooth plains, suggesting they also share their magnesian alkali-basalt-like composition. A smaller fraction of smooth plains interpreted to be volcanic in nature have a lower reflectance and shallower spectral slope, suggesting more ultramafic compositions, an inference that implies high temperatures and high degrees of partial melting in magma source regions persisted through most of the duration of smooth plains formation. The knobby and hummocky plains surrounding the Caloris basin, known as Odin-type plains, occupy an additional 2% of Mercury's surface. The morphology of these plains and their color and stratigraphic relationships suggest that they formed as Caloris ejecta, although such an origin is in conflict with a straightforward interpretation of crater size-frequency distributions. If some fraction is volcanic, this added area would substantially increase the abundance of relatively young effusive deposits inferred to have more mafic compositions. Smooth plains are widespread on Mercury, but they are more heavily concentrated in the north and in the hemisphere surrounding Caloris. No simple relationship between plains distribution and crustal thickness or radioactive element distribution is observed. A likely volcanic origin for some older terrain on Mercury suggests that the uneven distribution of smooth plains may indicate differences in the emplacement age of large-scale volcanic deposits rather than differences in crustal formational process.

This PDF file includes: SOM TextFigs. S1 to S3Background information is provided here on the major trends of wrinkle ridges in the northern smooth plains of Mercury (Fig. S1), on the sources and locations of images shown in Figs. 2 and 3, and on the crater size-frequency distributions shown in Fig. 4.

No abstract

Mercury, a planet with a lithosphere that forms a single tectonic plate, is replete with tectonic structures interpreted to be the result of planetary cooling and contraction. However, the amount of global contraction inferred from spacecraft images has been far lower than that predicted by models of the thermal evolution of the planet's interior. Here we present a synthesis of the global contraction of Mercury from orbital observations acquired by the MESSENGER spacecraft. We show that Mercury's global contraction has been accommodated by a substantially greater number and variety of structures than previously recognized, including long belts of ridges and scarps where the crust has been folded and faulted. The tectonic features on Mercury are consistent with models for large-scale deformation proposed for a globally contracting Earth-now obsolete-that pre-date plate tectonics theory. We find that Mercury has contracted radially by as much as 7 km, well in excess of the 0.8-3 km previously reported from photogeology and resolving the discrepancy with thermal models. Our findings provide a key constraint for studies of Mercury's thermal history, bulk silicate abundances of heat-producing elements, mantle convection and the structure of its large metallic core.G lobal contraction as a result of interior cooling was invoked as an explanation for mountain building and tectonic deformation on Earth in the nineteenth century 1,2 , but the idea was abandoned even before the recognition of the horizontal mobility of tectonic plates 3 , with the realization that contraction cannot account for the amount, style and distribution of deformation on the Earth's surface 4 . Large-scale deformational systems on Earth are localized along plate margins, unlike the quasihomogenous distribution of shortening structures predicted for a contracting planet 3 . However, other worlds in the Solar System do not exhibit plate tectonics today, so the intriguing possibility exists that some of the old concepts of contraction theory for global tectonics, long obsolete for Earth, may be valid for one-plate planets. Mercury, in particular, displays no evidence of plate boundaries that segment its globally continuous lithosphere. Yet observations made by the Mariner 10 and MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft have shown that the innermost planet displays myriad landforms-lobate scarps, wrinkle ridges and high-relief ridges-that have been interpreted as the tectonic result of horizontal shortening of the lithosphere as Mercury contracted in response to secular cooling of its interior 5-9 . Still, important details of Mercury's contraction, such as the timing, duration and spatial concentration of surface deformation, have remained elusive. Until the MESSENGER flybys of Mercury in [2008][2009], an entire hemisphere of Mercury had yet to be imaged, so inferences made on the basis of Mariner 10 data could not reliably be generalized globally. Furthermore, widespread topographic data for the planet we...

[1] We present the analysis of 205 spatially resolved measurements of the surface composition of Mercury from MESSENGER's X-Ray Spectrometer. The surface footprints of these measurements are categorized according to geological terrain. Northern smooth plains deposits and the plains interior to the Caloris basin differ compositionally from older terrain on Mercury. The older terrain generally has higher Mg/Si, S/Si, and Ca/Si ratios, and a lower Al/Si ratio than the smooth plains. Mercury's surface mineralogy is likely dominated by high-Mg mafic minerals (e.g., enstatite), plagioclase feldspar, and lesser amounts of Ca, Mg, and/or Fe sulfides (e.g., oldhamite). The compositional difference between the volcanic smooth plains and the older terrain reflects different abundances of these minerals and points to the crystallization of the smooth plains from a more chemically evolved magma source. High-degree partial melts of enstatite chondrite material provide a generally good compositional and mineralogical match for much of the surface of Mercury. An exception is Fe, for which the low surface abundance on Mercury is still higher than that of melts from enstatite chondrites and may indicate an exogenous contribution from meteoroid impacts.

Crater size–frequency analyses have shown that the largest volcanic plains deposits on Mercury were emplaced around 3.7 Ga, as determined with recent model production function chronologies for impact crater formation on that planet. To test the hypothesis that all major smooth plains on Mercury were emplaced by about that time, we determined crater size–frequency distributions for the nine next‐largest deposits, which we interpret also as volcanic. Our crater density measurements are consistent with those of the largest areas of smooth plains on the planet. Model ages based on recent crater production rate estimates for Mercury imply that the main phase of plains volcanism on Mercury had ended by ~3.5 Ga, with only small‐scale volcanism enduring beyond that time. Cessation of widespread effusive volcanism is attributable to interior cooling and contraction of the innermost planet.

International audienceGravitational deformation strongly infl uences the structure and eruptive behavior of large volcanoes. Using scaled analog models, we characterize a range of structural architectures produced by volcano sagging and volcano spreading. These arise from the interplay of variable basement rigidity and volcano-basement (de-)coupling. From comparison to volcanoes on Earth (La Réunion and Hawaii) and Mars (Elysium and Olympus Montes), the models highlight a structural continuum in which large volcanoes throughout the Solar System lie

Magnetized rocks can record the history of the magnetic field of a planet, a key constraint for understanding its evolution. From orbital vector magnetic field measurements of Mercury taken by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft at altitudes below 150 kilometers, we have detected remanent magnetization in Mercury's crust. We infer a lower bound on the average age of magnetization of 3.7 to 3.9 billion years. Our findings indicate that a global magnetic field driven by dynamo processes in the fluid outer core operated early in Mercury's history. Ancient field strengths that range from those similar to Mercury's present dipole field to Earth-like values are consistent with the magnetic field observations and with the low iron content of Mercury's crust inferred from MESSENGER elemental composition data.