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Volcanism on Mercury  

David A. Rothery

The history of volcanism on Mercury is almost the entire history of the formation of its crust. There are no recognized tracts of intact primary crust analogous to the Moon’s highland crust, probably because the density of Mercury’s iron-poor magma ocean was insufficient to enable crystalized silicate phases to float. Mercury’s surface consists of multiple generations of lavas. These were emplaced, rather like terrestrial “large igneous provinces” or LIPs, in their greatest volumes prior to about 3.5 Ga. Subsequently, erupted volumes decreased, and sites of effusive eruption became largely confined to crater floors. Plains lava surfaces younger than about 3.7 Ga have become scarred by sufficiently few impact craters that they are mapped as “smooth plains.” The older equivalents, which experienced the inner solar system’s “late heavy bombardment,” are mapped as intercrater plains. There is no consensus over whether plains with superimposed-crater characteristics that are intermediate between the smooth plains and intercrater plains end members can be consistently mapped as “intermediate plains.” However, any subdivision of the volcanic plains for mapping purposes arbitrarily splits apart a continuum. The volcanic nature of Mercury’s smooth plains was ambiguous on the basis of the imagery returned by the first mission to Mercury, Mariner 10, which made three fly-bys in 1974–1975. Better and more complete imaging by MESSENGER (in orbit 2011–2015) removed any doubt by documenting innumerable ghost craters and wrinkle ridges. No source vents for the plains are apparent, but this is normal in LIPs where effusion rate and style characteristically flood the vent beneath its own products. However, there are good examples of broad, flat-bottomed valleys containing streamlined islands suggesting passage of fast-flowing low viscosity lava. Although the causes of the mantle partial melting events supplying surface eruptions on Mercury are unclear, secular cooling of a small, one-plate planet such as Mercury would be expected to lead to the sort of temporal decrease in volcanic activity that is observed. Factors include loss of primordial heat and declining rate of radiogenic heat production (both of which would make mantle partial melting progressively harder), and thermal contraction of the planet (closing off ascent pathways). Lava compositions, so far as can be judged from the limited X-ray spectroscopic and other geochemical measurements, appear to be akin to terrestrial komatiites but with very low iron content. Variations within this general theme may reflect heterogeneities in the mantle, or different degrees of partial melting. The cessation of flood volcanism on Mercury is hard to date, because the sizes of the youngest flows, most of which are inside <200-km craters, are too small for reliable statistics to be derived from the density of superposed craters. However, it probably continued until approximately 1 Ga ago. That was not the end of volcanism. MESSENGER images have enabled the identification of over a hundred “pits,” which are noncircular holes up to tens of km in size and up to about 4 km deep. Many pits are surrounded by spectrally red deposits, with faint outer edges tens of km from the pit, interpreted as ejecta from explosive eruptions within the pit. Many pits have complex floors, suggesting vent migration over time. Pits usually occur within impact craters, and it has been suggested that crustal fractures below these craters facilitated the ascent of magma despite the compressive regime imposed by the secular thermal contraction. These explosive eruptions must have been driven by the violent expansion of a gas. This could be either a magmatic volatile expanding near the top of a magma conduit, or result from heating of a near-surface volatile by rising magma. MESSENGER showed that Mercury’s crust is surprisingly rich in volatiles (S, Cl, Na, K, C), of which the one likely to be of most importance in driving the explosive eruptions is S. We do not know when explosive volcanism began on Mercury. Cross-cutting relationships suggest that some explosion pits are considerably less than 1 Ga old, though most could easily be more than 3 Ga. They characteristically occur on top of smooth plains (or less extensive smooth fill of impact craters), and while some pits have no discernible “red spot” around them (perhaps because over time, it has faded into the background), there is no known example of part of a red spot peeping out from beneath the edge of a smooth plains unit. There seems to have been a change in eruptive style over time, with (small volume) explosions supplanting (large volume) effusive events.


Petrology and Geochemistry of Mercury  

Shoshana Z. Weider

Having knowledge of a terrestrial planet’s chemistry is fundamental to understanding the origin and composition of its rocks. Until recently, however, the geochemistry of Mercury—the Solar System’s innermost planet—was largely unconstrained. Without the availability of geological specimens from Mercury, studying the planet’s surface and bulk composition relies on remote sensing techniques. Moreover, Mercury’s proximity to the Sun makes it difficult to study with Earth/space-based telescopes, or with planetary probes. Indeed, to date, only NASA’s Mariner 10 and MESSENGER missions have visited Mercury. The former made three “flyby” encounters of Mercury between 1974 and 1975, but did not carry any instrument to make geochemical or mineralogical measurements of the surface. Until the MESSENGER flyby and orbital campaigns (2008–2015), therefore, knowledge of Mercury’s chemical composition was severely limited and consisted of only a few facts. For example, it has long been known that Mercury has the highest uncompressed density (i.e., density with the effect of gravity removed) of all the terrestrial planets, and thus a disproportionately large Fe core. In addition, Earth-based spectral reflectance observations indicated a dark surface, largely devoid of Fe within silicate minerals. To improve understanding of Mercury’s geochemistry, the MESSENGER scientific payload included a suite of geochemical sensing instruments: in particular, an X-Ray spectrometer and a gamma-ray and neutron spectrometer. The datasets obtained from these instruments (as well as from other complementary instruments) during MESSENGER’s 3.5-year orbital mission allow a much more complete picture of Mercury’s geochemistry to be drawn, and quantitative abundance estimates for several major rock-forming elements in Mercury’s crust are now available. Overall, the MESSENGER data reveal a surface that is rich in Mg, but poor in Al and Ca, compared with typical terrestrial and lunar crustal materials. Mercury’s surface also contains high concentrations of the volatile elements Na, S, K, and Cl. Furthermore, the total surface Fe abundance is now known to be <2 wt%, and the planet’s low-reflectance is thought to be primarily caused by the presence of C (in the form of graphite) at a level of >1 wt%. Such data are key to constraining models of Mercury’s formation and early evolution. Large-scale spatial variations in the MESSENGER geochemical datasets have also led to the designation of several geochemical “terranes,” which do not always align with otherwise mapped geological regions. Based on the MESSENGER geochemical results, petrological experiments and calculations have been, and continue to be, performed to study Mercury’s surface mineralogy and petrology. The results show that there are likely to be substantial differences in the precise mineral compositions and abundances amongst the different terranes, but Mercury’s surface appears to be dominated by Mg-rich olivine and pyroxene, as well as plagioclase and sulfide phases. Depending on the classification scheme used, Mercury’s ultramafic surface rocks can thus be described as similar in nature to terrestrial boninites, andesites, norites, or gabbros.


Tectonism of Mercury  

Paul K. Byrne

Mercury, like its inner Solar System planetary neighbors Venus, Mars, and the Moon, shows no evidence of having ever undergone plate tectonics. Nonetheless, the innermost planet boasts a long record of tectonic deformation. The most prominent manifestation of this history is a population of large scarps that occurs throughout the planet’s cratered terrains; some of these scarps rise kilometers above the surrounding landscape. Mercury’s smooth plains, the majority of which are volcanic and occupy over a quarter of the planet, abound with low-relief ridges. The scarps and ridges are underlain by thrust faults and point to a tectonic history dominated by crustal shortening. At least some of the shortening strain recorded by the ridges may reflect subsidence of the lavas in which they formed, but the widespread distribution of scarps attests to a planetwide process of global contraction, wherein Mercury experienced a reduction in volume as its interior cooled through time. The onset of this phenomenon placed the lithosphere into a net state of horizontal compression, and accounts for why Mercury hosts only a few instances of extensional structures. These landforms, shallow troughs that form complex networks, occur almost wholly in volcanically flooded impact craters and basins and developed as those lavas cooled and thermally contracted. Tellingly, widespread volcanism on Mercury ended at around the same time the population of scarps began to form. Explosive volcanism endured beyond this point, but almost exclusively at sites of lithospheric weakness, where large faults penetrate deep into the interior. These observations are consistent with decades-old predictions that global contraction would shut off major volcanic activity, and illustrate how closely Mercury’s tectonic and volcanic histories are intertwined. The tectonic character of Mercury is thus one of sustained crustal shortening with only localized extension, which started almost four billion years ago and extends into the geologically recent past. This character somewhat resembles that of the Moon, but differs substantially from those of Earth, Venus, or Mars. Mercury may represent how small rocky planets tectonically evolve and could provide a basis for understanding the geological properties of similarly small worlds in orbit around other stars.