Paul K. Byrne

Flood Volcanism in the Northern High Latitudes of Mercury Revealed by MESSENGER

MESSENGER observations of Mercury’s high northern latitudes reveal a contiguous area of volcanic smooth plains covering more than ~6% of the surface that were emplaced in a flood lava mode, consistent with average crustal compositions broadly similar to terrestrial komatiites. MESSENGER observations from Mercury orbit reveal that a large contiguous expanse of smooth plains covers much of Mercury’s high northern latitudes and occupies more than 6% of the planet’s surface area. These plains are smooth, embay other landforms, are distinct in color, show several flow features, and partially or completely bury impact craters, the sizes of which indicate plains thicknesses of more than 1 kilometer and multiple phases of emplacement. These characteristics, as well as associated features, interpreted to have formed by thermal erosion, indicate emplacement in a flood-basalt style, consistent with x-ray spectrometric data indicating surface compositions intermediate between those of basalts and komatiites. The plains formed after the Caloris impact basin, confirming that volcanism was a globally extensive process in Mercury’s post–heavy bombardment era.

Chemical heterogeneity on Mercury's surface revealed by the MESSENGER X-Ray Spectrometer

[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.

A sagging-spreading continuum of large volcano structure

Gravitational 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 Reunion and Hawaii) and Mars (Elysium and Olympus Montes), the models highlight a structural continuum in which large volcanoes throughout the Solar System lie.

Widespread effusive volcanism on Mercury likely ended by about 3.5 Ga: End of effusive volcanism on Mercury

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.

Extension and contraction within volcanically buried impact craters and basins on Mercury

Orbital images of Mercury obtained by the MESSENGER spacecraft have revealed families of troughs, interpreted to be graben, on volcanic plains material that largely or completely buried preexisting craters and basins. The graben are partially to fully encircled by rings of contractional wrinkle ridges localized over the rims of the buried impact features to form systems of associated contractional and extensional landforms. Most of the buried craters and basins with graben identified to date are located in the extensive volcanic plains that cover much of Mercury’s northern high latitudes. The distinctive relationship between wrinkle ridges and graben in buried craters and basins on Mercury is interpreted to be the result of a combination of extensional stresses from cooling and thermal contraction of thick lava flow units and compressional stresses from cooling and contraction of the planet’s interior.

Low-altitude magnetic field measurements by MESSENGER reveal Mercury’s ancient crustal field

Old minerals expose an ancient field Mercury is the only terrestrial planet other than Earth with an active, internally generated magnetic field. Results from the MESSENGER spacecraft indicate that the field is almost as old as the planet. Johnson et al. took advantage of close flybys to extract evidence of an ancient magnetic field. Certain minerals are able to “lock in” the signature of a field at the time they crystallize. This remnant magnetization was found in a region on Mercury believed to be 3.8 billion years old. Science, this issue p. 892 Magnetic field measurements of Mercury’s crust uncover a global magnetic field operating at least 3.8 billion years ago. 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.

Investigating the origin of candidate lava channels on Mercury with MESSENGER data: Theory and observations

Received 12 April 2012; revised 31 August 2012; accepted 1 November 2012; published 31 March 2013. [1] Volcanic plains identified on Mercury are morphologically similar to lunar mare plains but lack constructional and erosional features that are prevalent on other terrestrial planetary bodies. We analyzed images acquired by the MESSENGER spacecraft to identify features on Mercury that may have formed by lava erosion. We used analytical models to estimate eruption flux, erosion rate, and eruption duration to characterize the formation of candidate erosional features, and we compared results with analyses of similar features observed on Earth, the Moon, and Mars. Results suggest that lava erupting at high effusion rates similar to those required to form the Teepee Butte Member of the Columbia River flood basalts (0.1–1.2 � 10 6 m 3 s –1 ) would have been necessary to form wide valleys (>15km wide) observed in Mercury’s northern hemisphere, first by mechanical erosion to remove an upper regolith layer, then by thermal erosion once a lower rigid layer was encountered. Alternatively, results suggest that lava erupting at lower effusion rates similar to those predicted to have formed Rima Prinz on the Moon (4400 m 3 s –1 ) would have been required to form, via thermal erosion, narrower channels (<7km wide) observed on Mercury. Although these results indicate how erosion might have occurred on Mercury, the observed features may have formed by other processes, including lava flooding terrain sculpted during the formation of the Caloris basin in the case of the wide valleys, or impact melt carving channels into impact ejecta in the case of the narrower channels. Citation: Hurwitz, D. M., J. W. Head, P. K. Byrne, Z. Xiao, S. C. Solomon, M. T. Zuber, D. E. Smith, and G. A. Neumann (2013), Investigating the origin of candidate lava channels on Mercury with MESSENGER data: Theory and observations, J. Geophys. Res. Planets, 118, 471–486, doi:10.1029/2012JE004103.

On the Origin of Graben and Ridges within and near Volcanically Buried Craters and Basins in Mercury's Northern Plains

[1] Images of Mercury’s northern volcanic plains taken by the MESSENGER spacecraft reveal a large number of buried impact craters and basins discernible by wrinkle-ridge rings that overlie their rims. Many of these “ghost” craters and basins contain interior graben of diverse widths and orientations. Here we use finite element models to test a variety of mechanisms for the formation of these graben and ridges. Results show that graben are best explained by cooling of large thicknesses of flood lavas within the craters and basins; conservation of surface area during cooling induces the required extensional stress state. In contrast, the development of wrinkle-ridge rings is best explained as the result of cooling and contraction of Mercury’s interior, during which a reduction in Mercury’s surface area led to a compressional state of stress. The critical factor in determining where large graben form is the thickness of the youngest cooling unit, the topmost sequence of lavas that cooled coevally. A thicker cooling unit leads to a deeper initiation of normal faulting (wider graben floors). Consistent with observations, the widest graben are predicted to occur where pooled lavas were thickest, and no graben are predicted within generally thinner plains outside of major craters. Observed concentrically oriented graben can be explained by variations in the thickness of the youngest cooling unit. In contrast, none of the basin uplift mechanisms considered, including isostatic response to crater topography, inward flow of the lower crust, or exterior loading by volcanic plains, can account for concentrically oriented graben.

Duration of activity on lobate-scarp thrust faults on Mercury: Thrust Fault Activity on Mercury

Lobate scarps, landforms interpreted as the surface manifestation of thrust faults, are widely distributed across Mercury and preserve a record of its history of crustal deformation. Their formation is primarily attributed to the accommodation of horizontal shortening of Mercurys lithosphere in response to cooling and contraction of the planets interior. Analyses of images acquired by the Mariner 10 and MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft during flybys of Mercury showed that thrust faults were active at least as far back in time as near the end of emplacement of the largest expanses of smooth plains. However, the full temporal extent of thrust fault activity on Mercury, particularly the duration of this activity following smooth plains emplacement, remained poorly constrained. Orbital images from the MESSENGER spacecraft reveal previously unrecognized stratigraphic relations between lobate scarps and impact craters of differing ages and degradation states. Analysis of these stratigraphic relations indicates that contraction has been a widespread and long-lived process on the surface of Mercury. Thrust fault activity had initiated by a time near the end of the late heavy bombardment of the inner solar system and continued through much or all of Mercurys subsequent history. Such deformation likely resulted from the continuing secular cooling of Mercurys interior.

The origin of graben and ridges in Rachmaninoff, Raditladi, and Mozart basins, Mercury

the basin floor, and (3) subsidence following volcanic loading. Our results suggest that only thermal contraction can account for the observed pattern of graben, whereas some combination of subsidence and global contraction is the most likely explanation for the central ridges in Rachmaninoff and Mozart. Thermal contraction models, however, predict the formation of graben in the centermost region of each basin, where no graben are observed. We hypothesize that graben in this region were buried by a thin, late-stage flow of plains material, and images of partially filled graben provide evidence of such late-stage plains emplacement. These results suggest that the smooth plains units in these three basins are volcanic in origin. The thermal contraction models also imply a cooling unit ~1km thick near the basin center, further supporting the view that plains-forming lavas on Mercury were often of sufficiently high volume and low viscosity to pool to substantial thicknesses within basins and craters.