Christian Klimczak

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.

Topography of the Northern Hemisphere of Mercury from MESSENGER Laser Altimetry

Mercury Inside and Out The MESSENGER spacecraft orbiting Mercury has been in a ∼12-hour eccentric, near-polar orbit since 18 March 2011 (see the Perspective by McKinnon). Smith et al. (p. 214, published online 21 March) present the most recent determination of Mercurys gravity field, based on radio tracking of the MESSENGER spacecraft between 18 March and 23 August 2011. The results point to an interior structure that differs from those of the other terrestrial planets: the density of the planets solid outer shell suggests the existence of a deep reservoir of high-density material, possibly an Fe-S layer. Zuber et al. (p. 217, published online 21 March) used data obtained by the MESSENGER laser altimeter through to 24 October 2011 to build a topographic map of Mercurys northern hemisphere. The map shows less variation in elevation, compared with Mars or the Moon, and its features add to the body of evidence that Mercury has sustained geophysical activity for much of its history. Mercury’s topography indicates sustained geophysical activity for most of the planet’s geological history. Laser altimetry by the MESSENGER spacecraft has yielded a topographic model of the northern hemisphere of Mercury. The dynamic range of elevations is considerably smaller than those of Mars or the Moon. The most prominent feature is an extensive lowland at high northern latitudes that hosts the volcanic northern plains. Within this lowland is a broad topographic rise that experienced uplift after plains emplacement. The interior of the 1500-km-diameter Caloris impact basin has been modified so that part of the basin floor now stands higher than the rim. The elevated portion of the floor of Caloris appears to be part of a quasi-linear rise that extends for approximately half the planetary circumference at mid-latitudes. Collectively, these features imply that long-wavelength changes to Mercury’s topography occurred after the earliest phases of the planet’s geological history.

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.

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.

Geomorphology of lunar grabens requires igneous dikes at depth

Dense coverage of high-resolution topographic measurements from the Lunar Orbiter Laser Altimeter (LOLA) now allows for a global survey of the long-wavelength morphology of lunar grabens, with important implications for the geologic processes that form them. Comparing model ground displacements for grabens that arise solely from faulting, and from faulting involving a dike at depth, with topographic measurements across some of the most prominent and best-preserved grabens shows that many of them are underlain by a dike. Matching ground displacement models to topographic observations is the only way to detect, and also provide specific geometric information about, dikes too small to be resolved in available gravity and magnetic measurements, and thus allows for quantitative assessments of magma source depths and required magma overpressures. Dike-forming magmas are likely mantle derived, originating from depths greater than 20 km, with dike widths exceeding 100–500 m. Such dike geometric properties are only plausible if a mechanically weak lunar lithosphere was under extension at the time of dike emplacement.

Evidence from MESSENGER for sulfur‐ and carbon‐driven explosive volcanism on Mercury

Targeted MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) X-Ray Spectrometer measurements of Mercury’s largest identified pyroclastic deposit are combined with neutron and reflectance spectroscopy data to constrain the composition of volatiles involved in the eruption that emplaced the pyroclastic material. The deposit, northeast of the Rachmaninoff basin, is depleted in S (relative to Ca and Si) and C, compared with the rest of Mercury’s surface. Spectral reflectance measurements of the deposit indicate relatively high overall reflectance and an oxygen-metal charge transfer (OMCT) absorption band at ultraviolet wavelengths. These results are consistent with oxidation of graphite and sulfides during magma ascent, via reaction with oxides in the magma or assimilated country rock, and the formation of Sand C-bearing volatile species. Consumption of graphite during oxidation could account for the elevated reflectance of the pyroclastic material, and the strength of the OMCT band is consistent with ~0.03–0.1wt% FeO in the deposit.

Limits on the brittle strength of planetary lithospheres undergoing global contraction

The degree and depth of fracturing of the lithospheres of Mars, Mercury, and the Moon remain poorly known. It is these two properties, however, that govern the mechanical behavior of a planetary lithosphere. Considering the lithosphere as a cohesive rock mass that consists of small and large blocky, interlocked rock fragments, as opposed to an intact body or a body entirely lacking cohesion, provides insight into the effect of lithospheric fracturing on tectonic processes on these bodies. Characterization of the near-surface lithospheric brittle strength that incorporates the degree of fracturing is necessary for a realistic assessment of the lithospheric response to global contraction resulting from interior secular cooling. Such an assessment shows that all of these bodies could have accommodated substantial amounts of global contraction prior to the formation of thrust fault-related landforms. In fact, their lithospheres were sufficiently strong so as to experience changes in radius of as much as 2.2 ± 0.4 km (Mars), 2.1 ± 0.4 km (Mercury), and 1.4 ± 0.3 km (the Moon) prior to the onset of widespread thrust faulting. These values imply that the process of global contraction begins before any evidence of it is established in the geologic record, requiring an earlier onset, and for Mars and Mercury a faster initial strain rate, of global contraction than previously thought. Such results add a heretofore unrecognized component of planetary radial decrease with implications for timing and strain rate to studies of global contraction on Mars, Mercury, and the Moon, in particular, and to planetary bodies, in general.