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Article

James D. Burke and Erik M. Conway

The Jet Propulsion Laboratory (JPL) of the California Institute of Technology had its origins in a student project to develop rocket propulsion in the late 1930s. It attracted funding from the U.S. Army just prior to U.S. entry into World War II and became an Army missile research facility in 1943. Because of its origins as a contractor-operated Army research facility, JPL is the National Aeronautics and Space Administration’s (NASA) only contractor-operated field center. It remains a unit of the California Institute of Technology. In the decades since its founding, the laboratory, first under U.S. Army direction and then as a NASA field center, has grown and evolved into an internationally recognized institution generally seen as a leader in solar system exploration but whose portfolio includes substantial Earth remote sensing. JPL’s history includes episodes where the course of the laboratory’s development took turning points into new directions. After developing short-range ballistic missiles for the Army, the laboratory embarked on a new career in lunar and planetary exploration through the early 1970s and abandoned its original purpose as a propulsion technology laboratory. It developed the telecommunications infrastructure for planetary exploration too. It diversified into Earth science and astrophysics in the late 1970s and, due to a downturn in funding for planetary exploration, returned to significant amounts of defense work in the 1980s. The end of the Cold War between 1989 and 1991 resulted in a declining NASA budget, but support for planetary exploration actually improved within NASA management—as long as that exploration could be done more cheaply. This resulted in what is known as the “Faster Better Cheaper” period in NASA history. For JPL, this ended in 2000, succeeded by a return to more rigorous technical standards and increased costs.

Article

Mikhail Zolotov

Chemical and phase compositions of the surface of Venus could reflect a history of gas–rock and fluid–rock interactions, recent and past climate changes, and a loss of water from the Earth’s sister planet. The concept of chemical weathering on Venus through gas–solid type reactions was established in the early 1960s after the discovery of the hot and dense CO2-rich atmosphere of the planet, inferred from Earth-based and Mariner 2 radio emission data. Initial models suggested carbonation, hydration, and oxidation of exposed igneous rocks and a control (buffering) of atmospheric gases by solid–gas type chemical equilibria in the near-surface rocks. Carbonates, phyllosilicates and Fe oxides were considered likely secondary minerals. From the late 1970s onward, measurements of trace gases in the sub-cloud atmosphere by the Pioneer Venus and Venera entry probes and by Earth-based infrared spectroscopy challenged the likelihood of hydration and carbonation. The atmospheric H2O gas content appeared to be low enough to allow the stable existence of H2O-bearing and a majority of OH-bearing minerals. The concentration of SO2 gas was too high to allow the stability of Ca-rich carbonates and silicates with respect to sulfatization to CaSO4. In the 1980s, the detection of an elevated bulk S content at the Venera and Vega landing sites suggested ongoing consumption of atmospheric SO2 to surface sulfates. The supposed composition of the near-surface atmosphere implied oxidation of ferrous minerals to Fe oxides, magnetite and hematite, consistent with the infrared reflectance of surface materials. The likelihood of sulfatization and oxidation has been illustrated in modeling experiments in simulated Venus’ conditions. The morphology of Venus’ surface suggests contact of atmospheric gases with hot surface materials of mainly basaltic composition during the several hundreds of millions years since a global volcanic/tectonic resurfacing. Some exposed materials could have reacted at higher and lower temperatures in a presence of diverse gases at different altitudinal, volcanic, impact, and atmospheric settings. On highly deformed tessera terrains, more ancient rocks of unknown composition may reflect interactions with putative water-rich atmospheres and even aqueous solutions. Geological formations rich in salt, carbonate, Fe oxide, or silica will indicate past aqueous processes. The apparent diversity of affected solids, surface temperatures, pressures, and gas/fluid compositions throughout Venus’ history implies multiple signs of chemical alterations that remain to be investigated. The current understanding of chemical weathering is limited by the uncertain composition of the deep atmosphere, by the lack of direct data on the phase and chemical composition of surface materials, and by the uncertain data on thermodynamics of minerals and their solid solutions. In preparation for further atmospheric entry probe and lander missions, rock alteration could be investigated through chemical kinetic experiments and calculations of solid-gas/fluid equilibria to constrain past and present processes.

Article

Boris Ivanov

Impacts of small celestial bodies, in terms of energy density, occupy the range between ordinary chemical high explosives and nuclear explosions. The high initial energy density of impact gives them some features of an explosion (shock waves, melting and vaporization, mechanical disruption of target rocks). A near-surface burst creates an explosion crater, and an impact often results in the creation of an impact crater. The chain of processes connected to an impact crater’s formation is named “impact cratering” or simply “cratering.” The initial kinetic energy and momenta of the impacting body (“projectile”) generates shock waves (decaying with propagation to seismic waves), heats the material (at high impact velocities, to melt or to boil target rocks). A part of the kinetic energy is converted to target material motion, creating the crater cavity. The final crater geometry depends on the scale of event—while small craters are simple bowl-shaped cavities, large enough crater transient cavities collapse in the gravity field. If collapse takes place, the final crater has a complex geometry with central peaks and concentric inner rings. The boundary crater diameter, dividing simple and complex craters, varies with target body gravity and rock strength. Comparison of a crater’s morphology on remote planets and asteroids allows us to make some estimates about their mechanical parameters (e.g., strength and friction) even before future sample return missions. On many planets large impact craters can be seen, preserved much better than on the geologically active Earth. These observations help researchers to interpret the geological and geophysical data obtained for the relatively few and heavily modified large impact craters found on continents and (rarely) at the sea bottom.

Article

John C. Bridges

This is an advance summary of a forthcoming article in the Oxford Encyclopedia of Planetary Science. Please check back later for the full article. Mars, which has a tenth of the mass of Earth, has cooled as a single lithospheric plate. Current topography gravity maps and magnetic maps do not show signs of the plate tectonics processes that have shaped the Earth’s surface. Instead, Mars has been shaped by the effects of meteorite bombardment, igneous activity, and sedimentary—including aqueous—processes. Mars also contains enormous igneous centers—Tharsis and Elysium, with other shield volcanoes in the ancient highlands. In fact, the planet has been volcanically active for nearly all of its 4.5 Gyr history, and crater counts in the Northern Lowlands suggest that may have extended to within the last tens of millions of years. Our knowledge of the composition of the igneous rocks on Mars is informed by over 100 Martian meteorites and the results from landers and orbiters. These show dominantly tholeiitic basaltic compositions derived by melting of a relatively K, Fe-rich mantle compared to that of the Earth. However, recent meteorite and lander results reveal considerable diversity, including more silica-rich and alkaline igneous activity. These show the importance of a range of processes including crystal fractionation, partial melting, and possibly mantle metasomatism and crustal contamination of magmas. The figures and plots of compositional data from meteorites and landers show the range of compositions with comparisons to other planetary basalts (Earth, Moon, Venus). A notable feature of Martian igneous rocks is the apparent absence of amphibole. This is one of the clues that the Martian mantle had a very low water content when compared to that of Earth. The Martian crust, however, has undergone hydrothermal alteration, with impact as an important heat source. This is shown by SNC analyses of secondary minerals and Near Infra-Red analyses from orbit. The associated water may be endogenous. Our view of the Martian crust has changed since Viking landers touched down on the planet in 1976: from one almost entirely dominated by basaltic flows to one where much of the ancient highlands, particularly in ancient craters, is covered by km deep sedimentary deposits that record changing environmental conditions from ancient to recent Mars. The composition of these sediments—including, notably, the MSL Curiosity Rover results—reveal an ancient Mars where physical weathering of basaltic and fractionated igneous source material has dominated over extensive chemical weathering.

Article

V.V. Shevchenko

Since the early 1990s, in analytical reviews, experts have increasingly been paying attention to the growing scarcity of rare and rare earth metals (REM) necessary for the development of advanced technologies in modern industry. The volume of the world market has increased over the past 50 years from 5,000 to 125,000 tons per year, which is explained by the extensive use of REM in the rapidly developing areas of industry associated with the advancement of high technology. Unique properties of REM are primarily used in the aerospace and other industrial sectors of the economy, and therefore are strategic materials. For example, platinum is an indispensable element that is used as a catalyst for chemical reactions. No battery can do without platinum. If all the millions of vehicles traveling along our roads installed hybrid batteries, all platinum reserves on Earth would end in the next 15 years! Consumers are interested in six elements known as the platinum group of metals (PGM): iridium (Ir), osmium (Os), palladium (palladium, Pd), rhodium (rhodium, Rh), ruthenium (ruthenium, Ru), and platinum itself. These elements, rare on the Earth, possess unique chemical and physical properties, which makes them vital industrial materials. To solve this problem, projects were proposed for the utilization of the substance of asteroids approaching the Earth. According to modern estimates, the number of known asteroids approaching the Earth reaches more than 9,000. Despite the difficulties of seizing, transporting, and further developing such an object in space, this way of solving the problem seemed technologically feasible and cost-effectively justified. A 10 m iron-nickel asteroid could contain up to 75 tons of rare metals and REM, primarily PGM, equivalent to a commercial price of about $2.8 billion in 2016 prices. However, the utilization of an asteroid substance entering the lunar surface can be technologically simpler and economically more cost-effective. Until now, it was believed that the lunar impact craters do not contain the rocks of the asteroids that formed them, since at high velocities the impactors evaporate during a collision with the lunar surface. According to the latest research, it turned out that at a fall rate of less than 12 km/s falling body (drummer) can partially survive in a mechanically fractured state. Consequently, the number of possible resources present on the lunar surface can be attributed to nickel, cobalt, platinum, and rare metals of asteroid origin. The calculations show that the total mass, for example, of platinum and platinoids on the lunar surface as a result of the fall of asteroids may amount more than 14 million tons. It should be noted that the world’s known reserves of platinum group metals on the Earth are about 80,000 tons.

Article

Long Xiao and James W. Head

The geological characteristics of the Moon provide the fundamental data that permit the study of the geological processes that have formed and modified the crust, that record the state and evolution of the lunar interior, and that identify the external processes that have been important in lunar evolution. Careful documentation of the stratigraphic relationships among these features can then be used to reconstruct the sequence of events and the geological history of the Moon. These results can then be placed in the context of the geological evolution of the terrestrial planets, including Earth. The Moon’s global topography and internal structures include landforms and features that comprise the geological characteristics of its surface. The Moon is dominated by the ancient cratered highlands and the relatively younger flat and smooth volcanic maria. Unlike the current geological characteristics of Earth, the major geological features of the Moon (impact craters and basins, lava flows and related features, and tectonic scarps and ridges) all formed predominantly in the first half of the solar system’s history. In contrast to the plate-tectonic dominated Earth, the Moon is composed of a single global lithospheric plate (a one-plate planet) that has preserved the record of planetary geological features from the earliest phases of planetary evolution. Exciting fundamental outstanding questions form the basis for the future international robotic and human exploration of the Moon.

Article

Yuri Amelin

Isotopic dating is the measurement of time using the decay of radioactive isotopes and accumulation of decay products at a known rate. With isotopic chronometers, we determine the time of the processes that fractionate parent and daughter elements. Modern isotopic dating can resolve time intervals of ~1 million years over the entire lifespan of the Earth and the Solar System, and has even higher time resolution for the earliest and the most recent geological history. Using isotopic dates, we can build a unified scale of time for the evolution of Earth, the Moon, Mars, and asteroids, and expand it as samples from other planets become available for study. Modern geochronology and cosmochronology rely on isotopic dating methods that are based on decay of very long-lived radionuclides: 238U, 235U, 40K, 87Rb, 147Sm, etc. to stable radiogenic nuclides 206Pb, 207Pb, 40K, 40Ca, 87Sr, 143Nd, and moderately long-lived radionuclides: 26Al, 53Mn, 146Sm, 182Hf, to stable nuclides 26Mg, 53Cr, 142Nd, 182W. The diversity of physical and chemical properties of parent (radioactive) and daughter (radiogenic) nuclides, their geochemical and cosmochemical affinities, and the resulting diversity of processes that fractionate parent and daughter elements, allows direct isotopic dating of a vast range of earth and planetary processes. These processes include, but are not limited to evaporation and condensation, precipitation and dissolution, magmatism, metamorphism, metasomatism, sedimentation and diagenesis, ore formation, formation of planetary cores, crystallisation of magma oceans, and the timing of major impact events. Processes that cannot be dated directly, such as planetary accretion, can be bracketed between two datable events.

Article

Alexander T. Basilevsky

Lunar and planetary geology can be described using examples such as the geology of Earth (as the reference case) and geologies of the Earth’s satellite the Moon; the planets Mercury, Mars and Venus; the satellite of Saturn Enceladus; the small stony asteroid Eros; and the nucleus of the comet 67P Churyumov-Gerasimenko. Each body considered is illustrated by its global view, with information given as to its position in the solar system, size, surface, environment including gravity acceleration and properties of its atmosphere if it is present, typical landforms and processes forming them, materials composing these landforms, information on internal structure of the body, stages of its geologic evolution in the form of stratigraphic scale, and estimates of the absolute ages of the stratigraphic units. Information about one body may be applied to another body and this, in particular, has led to the discovery of the existence of heavy “meteoritic” bombardment in the early history of the solar system, which should also significantly affect Earth. It has been shown that volcanism and large-scale tectonics may have not only been an internal source of energy in the form of radiogenic decay of potassium, uranium and thorium, but also an external source in the form of gravity tugging caused by attractions of the neighboring bodies. The knowledge gained by lunar and planetary geology is important for planning and managing space missions and for the practical exploration of other bodies of the solar system and establishing manned outposts on them.

Article

Elvira Mulyukova and David Bercovici

All the rocky planets in our solar system, including the Earth, initially formed much hotter than their surroundings and have since been cooling to space for billions of years. The resulting heat released from planetary interiors powers convective flow in the mantle. The mantle is often the most voluminous and/or stiffest part of a planet and therefore acts as the bottleneck for heat transport, thus dictating the rate at which a planet cools. Mantle flow drives geological activity that modifies planetary surfaces through processes such as volcanism, orogenesis, and rifting. On Earth, the major convective currents in the mantle are identified as hot upwellings such as mantle plumes, cold sinking slabs, and the motion of tectonic plates at the surface. On other terrestrial planets in our solar system, mantle flow is mostly concealed beneath a rocky surface that remains stagnant for relatively long periods. Even though such planetary surfaces do not participate in convective circulation, they deform in response to the underlying mantle currents, forming geological features such as coronae, volcanic lava flows, and wrinkle ridges. Moreover, the exchange of material between the interior and surface, for example through melting and volcanism, is a consequence of mantle circulation and continuously modifies the composition of the mantle and the overlying crust. Mantle convection governs the geological activity and the thermal and chemical evolution of terrestrial planets and understanding the physical processes of convection helps us reconstruct histories of planets over billions of years after their formation.

Article

M.A. Ivanov and J.W. Head

This chapter reviews the conditions under which the basic landforms of Venus formed, interprets their nature, and analyzes their local, regional, and global age relationships. The strong greenhouse effect on Venus causes hyper-dry, almost stagnant near-surface environments. These conditions preclude water-driven, and suppress wind-related, geological processes; thus, the common Earth-like water-generated geological record of sedimentary materials does not currently form on Venus. Three geological processes are important on the planet: volcanism, tectonics, and impact cratering. The small number of impact craters on Venus (~1,000) indicates that their contribution to resurfacing is minor. Volcanism and tectonics are the principal geological processes operating on Venus during its observable geologic history. Landforms of the volcanic and tectonic nature have specific morphologies, which indicate different modes of formation, and their relationships permit one to establish their relative ages. Analysis of these relationships at the global scale reveals that three distinct regimes of resurfacing comprise the observable geologic history of Venus: (1) the global tectonic regime, (2) the global volcanic regime, and (3) the network rifting-volcanism regime. During the earlier global tectonic regime, tectonic resurfacing dominated. Tectonic deformation at this time caused formation of strongly tectonized terrains such as tessera, and deformational belts. Exposures of these units comprise ~20% of the surface of Venus. The apparent beginning of the global tectonic regime is related to the formation of tessera, which is among the oldest units on Venus. The age relationships among the tessera structures indicate that this terrain is the result of crustal shortening. During the global volcanic regime, volcanism overwhelmed tectonic activity and caused formation of vast volcanic plains that compose ~60% of the surface of Venus. The plains show a clear stratigraphic sequence from older shield plains to younger regional plains. The distinctly different morphologies of the plains indicate different volcanic formation styles ranging from eruption through broadly distributed local sources of shield plains to the volcanic flooding of regional plains. The density of impact craters on units of the tectonic and volcanic regimes suggests that these regimes characterized about the first one-third of the visible geologic history of Venus. During this time, ~80%–85% of the surface of the planet was renovated. The network rifting-volcanism regime characterized the last two-thirds of the visible geologic history of Venus. The major components of the regime include broadly synchronous lobate plains and rift zones. Although the network rifting-volcanism regime characterized ~2/3 of the visible geologic history of Venus, only 15%–20% of the surface was resurfaced during this time. This means that the level of endogenous activity during this time has dropped by about an order of magnitude compared with the earlier regimes.

Article

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.

Article

The planetary boundary layer of Mars is a crucial component of the Martian climate and meteorology, as well as a key driver of the surface-atmosphere exchanges on Mars. As such, it is explored by several landers and orbiters; high-resolution atmospheric modeling is used to interpret the measurements by those spacecrafts. The planetary boundary layer of Mars is particularly influenced by the strong radiative control of the Martian surface and, as a result, features a more extreme version of planetary boundary layer phenomena occurring on Earth. In daytime, the Martian planetary boundary layer is highly turbulent, mixing heat and momentum in the atmosphere up to about 10 kilometers from the surface. Daytime convective turbulence is organized as convective cells and vortices, the latter giving rise to numerous dust devils when dust is lifted and transported in the vortex. The nighttime planetary boundary layer is dominated by stable-layer turbulence, which is much less intense than in the daytime, and slope winds in regions characterized by uneven topography. Clouds and fogs are associated with the planetary boundary layer activity on Mars.

Article

Cometary nuclei, as small, spinning, ice-rich objects revolving around the sun in eccentric orbits, are powered and activated by solar radiation. Far from the sun, most of the solar energy is reradiated as thermal emission, whereas close to the sun, it is absorbed by sublimation of ice. Only a small fraction of the solar energy is conducted into the nucleus interior. The rate of heat conduction determines how deep and how fast this energy is dissipated. The conductivity of cometary nuclei, which depends on their composition and porosity, is estimated based on vastly different models ranging from very simple to extremely complex. The characteristic response to heating is determined by the skin depth, the thermal inertia, and the thermal diffusion timescale, which depend on the comet’s structure and dynamics. Internal heat sources include the temperature-dependent crystallization of amorphous water ice, which becomes important at temperatures above about 130 K; occurs in spurts; and releases volatiles trapped in the ice. These, in turn, contribute to heat transfer by advection and by phase transitions. Radiogenic heating resulting from the decay of short-lived unstable nuclei such as 26Al heats the nucleus shortly after formation and may lead to compositional alterations. The thermal evolution of the nucleus is described by thermo-physical models that solve mass and energy conservation equations in various geometries, sometimes very complicated, taking into account self-heating. Solutions are compared with actual measurements from spacecraft, mainly during the Rosetta mission, to deduce the thermal properties of the nucleus and decipher its activity pattern.

Article

Nicolas Mangold, Jessica Flahaut, and Véronique Ansan

Planetary surface compositions are fundamental to an understanding of both the interior activity through differentiation processes and volcanic activity and the external evolution through alteration processes and accumulations of volatiles. While the Moon has been studied since early on using ground-based instruments and returned samples, observing the surface composition of the terrestrial planets did not become practical until after the development of orbital and in situ missions with instruments tracking mineralogical and elemental variations. The poorly evolved, atmosphere-free bodies like the Moon and Mercury enable the study of the formation of the most primitive crusts, through processes such as the crystallization of a magma ocean, and their volcanic evolution. Nevertheless, recent studies have shown more diversity than initially expected, including the presence of ice in high latitude regions. Because of its heavy atmosphere, Venus remains the most difficult planetary body to study and the most poorly known in regards to its composition, triggering some interest for future missions. In contrast, Mars exploration has generated a huge amount of data in the last two decades, revealing a planet with a mineralogical diversity close to that of the Earth. While Mars crust is dominated by basaltic material, recent studies concluded for significant contributions of more felsic and alkali-rich igneous material, especially in the ancient highlands. These ancient terrains also display widespread outcrops of hydrous minerals, especially phyllosilicates, which are key in the understanding of past climate conditions and suggest a volatile-rich early evolution with implications for exobiology. Recent terrains exhibit a cryosphere with ice-rich landforms at, or close to the surface, of mid- and high latitudes, generating a strong interest for recent climatic variability and resources for future manned missions. While Mars is certainly the planetary body the most similar to Earth, the observation of specific processes such as those linked to interactions with solar wind on atmosphere-free bodies, or with a thick acidic atmosphere on Venus, improve our understanding of the differences in evolution of terrestrial bodies. Future exploration is still necessary to increase humankind’s knowledge and further build a global picture of the formation and evolution of planetary surfaces.

Article

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.