The polar regions of Mars contain layered ice deposits that are rich in detail of past periods of accumulation and erosion. These north and south polar layered deposits (NPLD and SPLD, respectively) contain primarily water–ice and ~5% and ~10% dust derived from the atmosphere, respectively. In addition, the SPLD has two known CO2 deposits—one thin unit at the surface and one buried, much thicker unit. Together, they comprise less than 1% of the SPLD volume. Mars also experiences seasonal deposits of CO2 that form in winter and sublimate in spring and early summer. These seasonal caps are visible from Earth and have been studied for centuries. Zooming in, exposed layers at the PLDs reveal histories of climate change that resulted when orbital parameters such as obliquity, eccentricity, and argument of perihelion changed over tens of thousands to millions of years. Simpler environmental conditions at the NPLD, especially related to seasonal and aeolian processes, make interpreting the history of that polar cap much easier than the SPLD. The history of Mars polar science is linked by numerous incremental advancements and unexpected discoveries related to the observed geology of both poles, the interpreted and modeled climatic conditions that gave rise to the PLDs, and the atmospheric conditions that modify the surface.
A Retrospective on Mars Polar Ice and Climate
Isaac B. Smith
Astrobiology seeks to understand the origin, evolution, distribution, and future of life in the universe and thus to integrate biology with planetary science, astronomy, cosmology, and the other physical sciences. The discipline emerged in the late 20th century, partly in response to the development of space exploration programs in the United States, Russia, and elsewhere. Many astrobiologists are now involved in the search for life on Mars, Europa, Enceladus, and beyond. However, research in astrobiology does not presume the existence of extraterrestrial life, for which there is no compelling evidence; indeed, it includes the study of life on Earth in its astronomical and cosmic context. Moreover, the absence of observed life from all other planetary bodies requires a scientific explanation, and suggests several hypotheses amenable to further observational, theoretical, and experimental investigation under the aegis of astrobiology. Despite the apparent uniqueness of Earth’s biosphere— the “n = 1 problem”—astrobiology is increasingly driven by large quantities of data. Such data have been provided by the robotic exploration of the Solar System, the first observations of extrasolar planets, laboratory experiments into prebiotic chemistry, spectroscopic measurements of organic molecules in extraterrestrial environments, analytical advances in the biogeochemistry and paleobiology of very ancient rocks, surveys of Earth’s microbial diversity and ecology, and experiments to delimit the capacity of organisms to survive and thrive in extreme conditions.
Chemical Weathering on Venus
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.
Clouds in the Martian Atmosphere
A. Määttänen and F. Montmessin
Although resembling an extremely dry desert, planet Mars hosts clouds in its atmosphere. Every day somewhere on the planet a part of the tiny amount of water vapor held by the atmosphere can condense as ice crystals to form mainly cirrus-type clouds. The existence of water ice clouds has been known for a long time and they have been studied for decades, leading to the establishment of a well-known climatology and understanding on their formation and properties. Despite their thinness, they have a clear impact on the atmospheric temperatures, thus affecting the Martian climate. Another, more exotic type of clouds forms as well on Mars. The atmospheric temperatures can plunge to such frigid values that the major gaseous component of the atmosphere, CO2, condenses as ice crystals. These clouds form in the cold polar night where they also contribute to the formation of the CO2 ice polar cap, and also in the mesosphere at very high altitudes, near the edge of space, analogously to the noctilucent clouds on Earth. The mesospheric clouds, discovered in the early 2000s, have put our understanding of the Martian atmosphere to a test. On Mars, cloud crystals form on ice nuclei, mostly provided by the omnipresent mineral dust. Thus, the clouds link the three major climatic cycles: those of the two major volatiles, H2O and CO2, and that of dust, which is a major climatic agent itself.
Composition of Earth
Early models of the composition of the Earth relied heavily on meteorites. In all these models Earth had different layers, each layer corresponded to a different type of meteorite or meteorite component. Later, more realistic models based on analyses of samples from Earth began with Ringwood’s pyrolite composition in the 1960s. Further improvement came with the analyses of rare MgO rich peridotites from a variety of occurrences all over the Earth, as xenoliths enclosed in melts from the upper mantle or as ultramafic massifs, tectonically emplaced on the Earth’s surface. Chemical systematics of these rocks allow the determination of the major element composition of the primitive upper mantle (PUM), the upper mantle after core formation and before extraction of basalts ultimately leading to the formation of the crust. Trace element analyses of upper mantle rocks confirmed their primitive nature. Geochemical and geophysical evidence argue for a bulk Earth mantle of uniform composition, identical to the PUM, also designated as “bulk silicate Earth” (BSE). The formation of a metal core was accompanied by the removal of siderophile and chalcophile elements into the core. Detailed modeling suggests that core formation was an ongoing process parallel to the accretion of Earth. The composition of the core is model dependent and thus uncertain and makes reliable estimates for siderophile and chalcophile element concentrations of bulk Earth difficult. Improved stable isotope analyses show isotopic similarities with noncarbonaceous chondrites (NCC), while the chemical composition of the mantle of the Earth indicates similarities with carbonaceous chondrites (CC). In detail, however, it can be shown that no single known meteorite group, nor any mixture of meteorite groups can match the chemical and isotopic composition of Earth. This conclusion is extremely important for any formation model of the Earth.
Cosmogenic nuclides are produced by the interaction of energetic elementary particles of galactic cosmic radiation (GCR) and their secondaries with atomic nuclei in extraterrestrial or terrestrial material. In extraterrestrial samples cosmogenic nuclides produced by energetic particles emitted by the Sun (SCR) are also detectable. Cosmogenic nuclides usually are observable only for noble gas isotopes, whose natural abundances in the targets of interest are exceedingly low, with some radioactive isotopes having half-lives mostly in the million-year range, and a few stable nuclides of elements such as Gd and Sm whose abundance is appreciably modified by reactions with low-energy secondary cosmic-ray neutrons. In solid matter, the mean attenuation length of GCR protons is on the order of 50 cm. Therefore, cosmogenic nuclides are a major tool to study the history of small objects in space and of matter near the surfaces of larger parent bodies. A classical application is to measure “exposure ages” of meteorites, that is, the time they spent as a small body in interplanetary space. In some cases, the previous history of the future meteorite in its parent-body regolith can also be constrained. Such information helps to understand delivery mechanisms of meteorites from their parent asteroids (mainly from the main belt) or parent planets, and to constrain the number of ejection events responsible for the meteorites in collections worldwide. Cosmogenic nuclides in lunar samples from known depths of up to ~2 m serve to study the deposition and mixing history of the lunar regolith over hundreds of million years, as well as to calibrate nuclide production models. Present and future sample return missions rely on cosmogenic nuclide measurements as important tools to constrain the sample’s exposure history or loss rates of its parent-body surfaces to space. First measurements of cosmogenic noble gas isotopes on the surface of Mars demonstrate that the exposure and erosional history of planetary bodies can be obtained by in situ analyses. Exposure ages of presolar grains in meteorites provide at present the only quantitative constraint of their presolar history. In some cases, irradiation effects of energetic particles from the early Sun can be detected in early solar system condensates, confirming that the early Sun was likely much more active than later in its history, as expected from observations of young stars. The increasing precision of modern isotope analyses also reveals tiny isotopic anomalies induced by cosmic-ray effects in several elements of interest in cosmochemistry, which need to be recognized and corrected for. Cosmogenic nuclide studies rely on the knowledge of their production rates, which depend on the elemental composition of a sample and its “shielding” during irradiation, that is, its position within an irradiated object, and for meteorites their pre-atmospheric size. The physics of cosmogenic nuclide production is basically well understood and has led to sophisticated production models. They are most successful if a sample’s shielding can be constrained by the analyses of several cosmogenic nuclides with different depth dependencies of their production rates. Cosmogenic nuclides are also an important tool in Earth sciences, although this is not a topic of this article. The foremost example is 14C produced in the atmosphere and incorporated into organic material, which is used for dating. Cosmogenic radionucuclides and noble gases produced in situ in near-surface samples, mostly by secondary cosmic-ray neutrons, are an important tool in quantitative geomorphology and related fields.
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.
Element Partitioning (Mineral-Melt, Metal-/Sulfide-Silicate) in Planetary Sciences
Element partitioning—at its most basic—is the distribution of an element of interest between two constituent phases as a function of some process. Major constituent elements generally affect the thermodynamic environment (chemical equilibrium) and therefore trace element partitioning is often considered, as trace elements are present in minute quantities and their equilibrium exchange reactions do not impart significant changes to the larger system. Trace elements are responsive to thermodynamic conditions, and thus they act as passive tracers of chemical reactions without appreciably influencing the bulk reactions themselves. In planetary sciences, the phase pairs typically considered are mineral-melt, metal-silicate, and sulfide-silicate, owing largely to the ubiquity of their coexistence in planetary materials across scales and context, from the micrometer-sized components of meteorites up to the size of planets (thousands of kilometers). It is common to speak of trace elements in terms of their tendency toward forming metallic, sulfidic, or oxide phases, and the terms “siderophile,” “chalcophile,” and “lithophile” (respectively) are used to define these tendencies under what is known as the Goldschmidt Classification scheme. The metric of an element’s tendency to concentrate into one phase relative to another is expressed as the ratio of its concentration (as a weight or molar fraction) in one phase over another, where convention dictates the reference frame as solid over liquid, and metal or sulfide over silicate; this mathematical term is the element’s partition coefficient, or distribution coefficient, between the two respective phases, D M Phase B Phase A (where M is the element of interest, most often reported as molar fraction), or simply D M . In general, trace elements obey Henry’s Law, where the element’s activity and concentration are linearly proportional. Practically speaking, this means that the element is sufficiently dilute in the system such that its atoms interact negligibly with one another compared to their interactions with major element phases, and thus the trace element’s partition coefficient in most settings is not appreciably affected by its concentration. The radius and charge of an element’s ionized species (its ionic radius and valence state)—in relation to either the major element ion for which it is substituting or the lattice site vacancy or interstitial space it is filling—generally determine the likelihood of trace element substitution or vacancy/interstitial fill (along with the net charge of the lattice space). The key energy consideration that underlies an element’s partitioning is the Gibbs free energy of reaction between the phases involved. Gibbs free energy is the change in internal energy associated with a chemical reaction (at a given temperature and pressure) that can be used to do work, and is denoted as Δ G rxn . Reactions with negative Δ G rxn values are spontaneous, and the magnitude of this negative value for a given phase, for example, a metal oxide, denotes the relative affinity of the metal toward forming oxides. That is to say, an element with a highly negative Δ G rxn for its oxide species at relevant pressure-temperature conditions will tend to be found in oxide and silicate minerals, that is, it will be lithophile (and vice versa for siderophile elements). Trace element partitioning systematics in mineral-melt and metal-/sulfide-silicate systems have boundless applications in planetary science. A growing collective understanding of the partition coefficients of elements has been built on decades of physical chemistry, deterministic theory, petrology, experimental petrology, and natural observations. Leveraging this immense intellectual, technical, and methodological foundation, modern trace element partitioning research is particularly aimed at constraining the evolution of plate tectonics on Earth (conditions and timing of onset), understanding the formation history of planetary materials such as chondrite meteorites and their constituents (e.g., chondrules), and de-convolving the multiply operating processes at play during the accretion and differentiation of Earth and other terrestrial planets.
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.
Geological Characteristics of the Moon
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.
Impact Crater Densities as a Tool for Dating Planetary Surfaces
William K. Hartmann
The use of impact crater densities to estimate the ages of planetary surfaces began in the 1960s. Some predictive successes have been confirmed with radiometric dating of sites on the Moon and Mars. The method is highly dependent on our understanding of the rate of crater formation on different worlds, and, more importantly, on the history of that rate, starting with intense cratering during planetary formation 4.5 Ga ago. The system is thus calibrated by obtaining radiometric dates from samples of relatively homogeneous geologic units on various worlds. Crater chronometry is still in its infancy. Future sample-returns and in situ measurements, obtained by missions from collaborating nations to various worlds, will provide ever-increasing improvements in the system in coming decades. Such data can lead to at least two-significant-figure measurements, not only of the ages of broad geologic provinces on solar system worlds, but of the characteristic survival times of various-sized smaller craters. Such data, in turn, clarify the rates of turnover of surface materials and the production rates of gravel-like regolith and megaregolith in the surface layers. Better measurements of the impact rate at various times, in turn, support better modeling of the accretion and fragmentation processes among early planetesimals as well as contemporary asteroids, in various parts of the solar system. Once the crater chronometry system is calibrated for various planetary bodies, important chronological information about those various planetary bodies can be obtained by orbital missions, without the need for expensive sample-return or lander missions on each individual surface.
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.
Landslides in the Solar System
Maria Teresa Brunetti and Silvia Peruccacci
Landslides are mass movements of rock, earth, or debris. All of these surface processes occur under the influence of gravity, meaning that they globally move material from higher to lower places. On planets other than Earth, these structures were first observed in a lunar crater during the Apollo program, but mass movements have been found on many rocky worlds (solid bodies) in the Solar System, including icy satellites, asteroids, and comets. On Earth, landslides have the effect of shaping the landscape more or less rapidly, leaving a signature that is recognized through field surveys and visual analysis or automatic identification on ground-based, aerial, and satellite images. Landslides observed on Earth and on solid bodies of the Solar System can be classified into different types based on their movement and the material involved in the failure. Material is either rock or soil (or both), with a variable fraction of water or ice; a soil mainly composed of sand-sized or finer particles is referred as earth while debris is composed of coarser fragments. The landslide mass may be displaced in several types of movement, classified generically as falling, toppling, sliding, spreading, or flowing. Such diverse characteristics mean that the size of a landslide (e.g., area, volume, fall height, length) can vary widely. For example, on Earth, their area ranges up to 11 orders of magnitude, while their volume varies by 16 orders, from small rock fragments to huge submarine landslides. The classification of extraterrestrial landslides is based on terrestrial analogs having similarities and characteristics that resemble those found on planetary bodies, such as Mars. The morphological classification is made regardless of the geomorphological environment or processes that may have triggered the slope failure. Comparing landslide characteristics on various planetary bodies helps to understand the effect of surface gravity on landslide initiation and propagation—of tremendous importance when designing manned and unmanned missions with landings on extraterrestrial bodies. Regardless of the practical applications of such study, knowing the morphology and surface dynamics that shape solid bodies in the space surrounding the Earth is something that has fascinated the human imagination since the time of Galileo.
Large Volcanic Channels of the Inner Solar System
David W. Leverington
Many large volcanic channel systems are recognized at the surfaces of rocky bodies of the inner solar system. The more than 200 channels known for the Moon mainly have simple sinuous forms with widths of up to several kilometers and lengths of up to hundreds of kilometers, typically commencing at topographic depressions and extending downslope until they fade into associated volcanic units. The Rima Hadley system was a key target of the Apollo 15 mission and was confirmed as a product of volcanic processes related to the emplacement of lavas in the Palus Putredinis region of Mare Imbrium. The more than 200 channels known for Venus are in many cases morphologically similar to sinuous lunar rilles, but some systems are especially large and complex, with widths of up to tens of kilometers and lengths that can exceed 1,000 km. Such systems typically commence at structural features or in regions of disturbed terrain and possess anastomosing reaches associated with prominent streamlined uplands. In contrast, Venusian canali typically maintain sinuous forms with widths of only a few kilometers but remarkably can be characterized by lengths of thousands of kilometers. Some Venusian channels were involved in the emplacement of fluidized ejecta in the vicinities of impact craters whereas others may have formed in such environments as a result of later volcanic events. The 10 large volcanic channels that are recognized on Mercury have lengths no greater than ~161 km but can have widths of up to several tens of kilometers. These systems developed as conduits for voluminous lavas that extend across adjacent impact basins. Terrestrial komatiitic channels of Archean and Proterozoic ages can have sizes that are comparable to those of lunar rilles, and the formation of these systems is likely to have played an important role in the development of associated Ni-Cu-(PGE) ores. The outflow channels of Mars have widths of up to tens of kilometers and lengths of up to thousands of kilometers and are widely interpreted as aqueous systems formed by catastrophic discharges from aquifers, but the properties and geological associations of these features and numerous other large Martian channel systems are arguably well aligned with those expected of volcanic origins. Overall, large volcanic channels of the inner solar system are mainly ancient products of the emplacement of low-viscosity lava flows of mafic or ultramafic composition, involving eruptions that were characterized by extraordinarily high effusion rates and total lava volumes that in some cases are likely to have been as great as those that characterized some Large Igneous Provinces on Earth. The deeply rooted igneous plumbing systems most favorable to the development of large volcanic channels would have been especially common in the earlier history of the solar system, when the interior temperatures of rocky bodies were greater than today. The early development of large volcanic channel systems is likely typical of the geological histories of large rocky bodies in the universe.
Lunar and Planetary Geology
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.
Lunar Exploration Missions and Environmental Discovery: Status and Progress
Kyeong J. Kim
Exploration of the Moon is currently one of the most important and interesting subjects. The Moon is considered not only a place to explore but also a place to live in preparation to explore planets beyond it. This opportunity has arisen due to a series of discoveries associated with water on the Moon during the past half century. Lunar exploration of the moon began with the flyby mission by the United States in 1959. Since then, scientific investigations of the Moon have increased understanding of the lunar geology and surface environment. Based on more than 70 lunar missions to date, a major goal is to explore how humans can live on the Moon for a long period of time to examine sustainability on the Moon. Consequently, the area of lunar science and technology is being employed to discover how in situ resources can be utilized for humans to live on the Moon and, eventually, Mars and beyond.
Mantle Convection in Terrestrial Planets
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.
Steven W. Ruff
Dust makes the red planet red. Without dust, Mars would appear mostly as shades of gray. The reddish hue arises from a small amount of oxidized iron among its basaltic mineral constituents. In this sense, Mars is a rusty world. Martian dust is a ubiquitous material of remarkably uniform composition that spans the globe, filling the skies and covering the land in a temporally and spatially varying manner. It is routinely lifted into the atmosphere via convective vortices known as dust devils. Dust in the atmosphere waxes and wanes according to season. Every few Martian years, the planet is fully encircled in atmospheric dust of sufficient opacity that its surface markings and landforms are completely obscured from view of Earth-bound telescopes and Mars-orbiting satellites. Such global dust events last for weeks or months, long enough to jeopardize solar-powered spacecraft on the surface. Dust particles suspended in the thin Martian atmosphere ultimately fall to the surface, completing the cycle and contributing to a range of features that are still being discovered and investigated.
Robert M. Haberle
The climate of Mars has evolved over time. Early in its history, between 3.7 and 4.1 billion years ago, the climate was warmer and wetter and the atmosphere thicker than it is today. Erosion rates were higher than today, and liquid water flowed on the planet’s surface, carving valley networks, filling lakes, creating deltas, and weathering rocks. This implies runoff and suggests rainfall and/or snowmelt. Oceans may have existed. Over time, the atmosphere thinned, erosion rates declined, water activity ceased, and cooler and drier conditions prevailed. Ice became the dominate form of surface water. Yet the climate continued to evolve, driven now by large variations in Mars’ orbit parameters. Beating in rhythm with these variations, surface ice has been repeatedly mobilized and moved around the planet, glaciers have advanced and retreated, dust storms and polar caps have come and gone, and the atmosphere has collapsed and re-inflated many times. The layered terrains that now characterize both polar regions are telltale signatures of this cyclical behavior and owe their existence to modulations of the seasonal cycles of dust, water, and CO2. Contrary to the early images from the Mariner flybys of the 1960s, Mars is and has been a dynamically active planet whose surface has been partly shaped through its interaction with a changing atmosphere and climate system.
Mass Erosion and Transport on Cometary Nuclei, as Found on 67P/Churyumov-Gerasimenko
The Rosetta spacecraft rendezvoused with comet 67P/Churyumov-Gerasimenko in 2014–2016 and observed its surface morphology and mass loss process. The large obliquity (52°) of the comet nucleus introduces many novel physical effects not known before. These include the ballistic transport of dust grains from the southern hemisphere to the northern hemisphere during the perihelion passage, thus shaping the dichotomy of two sides, with the northern hemisphere largely covered by dust layers from the recycled dusty materials (back fall) and the southern hemisphere consisting mostly of consolidated terrains. A significant amount of surface material up to 4–10 m in depth could be transferred across the nucleus surface in each orbit. New theories of the physical mechanisms driving the outgassing and dust ejection effects are being developed. There is a possible connection between the cometary dust grains and the fluffy aggregates and pebbles in the solar nebula in the framework of the streaming-instability scenario. The Rosetta mission thus succeeded in fulfilling one of its original scientific goals concerning the origin of comets and their relation to the formation of the solar system.