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In planetary science, accretion is the process in which solids agglomerate to form larger and larger objects, and eventually planets are produced. The initial conditions are a disc of gas and microscopic solid particles, with a total mass of about 1% of the gas mass. These discs are routinely detected around young stars and are now imaged with the new generation of instruments. Accretion has to be effective and fast. Effective, because the original total mass in solids in the solar protoplanetary disk was probably of the order of ~300 Earth masses, and the mass incorporated into the planets is ~100 Earth masses. Fast, because the cores of the giant planets had to grow to tens of Earth masses to capture massive doses of hydrogen and helium from the disc before the dispersal of the latter, in a few millions of years.
The surveys for extrasolar planets have shown that most stars have planets around them. Accretion is therefore not an oddity of the solar system. However, the final planetary systems are very different from each other, and typically very different from the solar system. Observations have shown that more than 50% of the stars have planets that don’t have analogues in the solar system. Therefore the solar system is not the typical specimen. Models of planet accretion have to explain not only how planets form, but also why the outcomes of the accretion history can be so diverse.
There is probably not one accretion process but several, depending on the scale at which accretion operates. A first process is the sticking of microscopic dust into larger grains and pebbles. A second process is the formation of an intermediate class of objects called planetesimals. There are still planetesimals left in the solar system. They are the asteroids orbiting between the orbits of Mars and Jupiter, the trans-Neptunian objects in the distant system, and other objects trapped along the orbits of the planets (Trojans) or around the giant planets themselves (irregular satellites). The Oort cloud, source of the long period comets, is also made of planetesimals ejected from the region of formation of the giant planets. A third accretion process has to lead from planetesimals to planets. Actually, several processes can be involved in this step, from collisional coagulation among planetesimals to the accretion of small particles under the effect of gas drag, to giant impacts between protoplanets. Adopting a historical perspective of all these processes provides details of the classic processes investigated in the past decades to those unveiled in the last years.
The quest for planet formation is ongoing. Open issues remain, and exciting future developments are expected.
The study of active asteroids is a relatively new field of study in Solar System science, focusing on objects with asteroid-like orbits but that exhibit comet-like activity. This field, which crosses traditionally drawn lines between research focused on inactive asteroids and active comets, has motivated reevaluations of classical assumptions about small Solar System objects and presents exciting new opportunities for learning more about the origin and evolution of the Solar System. Active asteroids whose activity appears to be driven by the sublimation of volatile ices could have significant implications for determining the origin of the Earth’s water—and therefore its ability to support life—and also challenge traditional assumptions about the survivability of ice in the warm inner Solar System. Meanwhile, active asteroids whose activity appears to be caused by disruptive processes such as impacts or rotational destabilization provide exciting opportunities to gain insights into fundamental processes operating in the asteroid belt and assessing their effects on the asteroid population seen in the 21st century.
The Agreement on the Rescue and Return of Astronauts and the Return of Objects Launched into Outer Space (ARRA) of 1968 deals with the obligation of states toward astronauts in distress or in emergency situations and with the obligation to return space objects. It is the second of the five United Nations space treaties, after the Outer Space Treaty (OST) of 1967 and before the Liability Convention (LIAB) of 1972. The historical development of ARRA and how this agreement reflects the needs and interests of the two important space-faring nations at the time of its entry into force, the United States and the Soviet Union, are important factors for understanding the space race. ARRA is related to the OST and regards the various obligations of states concerning rescue and assistance as well as the return of astronauts, which stand in the middle between a general humanitarian duty and political and national security considerations. The return of space objects and the question of costs of rescue and return operations are important concerns and can be compared to the situation with the law of the sea, the United Nations Convention on the Law of the Sea (UNCLOS) of 1982 and the Convention for the Unification of Certain Rules of Law Respecting Assistance and Salvage at Sea (Salvage Convention) of 1989.
ARRA has never been applied with respect to accidents or distress of astronauts or cosmonauts but several times with respect to the recovering and returning of space objects. Finally, current challenges, such as the commercialization and privatization of outer space activities need to be addressed. This includes the increased interests of private individuals to enter outer space (so-called space tourism) and the question of the application of the ARRA to suborbital flights. Many legal challenges created by technological progress can be resolved via an evolving interpretation and application of the ARRA. Yet, some issues might warrant a new legal framework.
Astrology was a central feature of Greek and Roman culture. A knowledge of astrology’s claims, practices, and world view is essential for a full understanding of religion, politics, and science in the Greek and Roman worlds. Astrology is the name given to a series of diverse practices based in the idea that the stars, planets, and other celestial phenomena possess significance and meaning for events on Earth. It assumes a link between Earth and sky in which all existence, spiritual, psychological, and physical, is interconnected. Most premodern cultures practice a form of astrology. A particularly complex variety of it evolved in Mesopotamia in the first and second millennia
Venus is a slowly rotating planet with a thick atmosphere (~9.2 MPa at the surface). Ground- and satellite-based observations have shown atmospheric superrotation (atmospheric rotation much faster than solid surface rotation), global-scale cloud patterns (e.g., Y-shaped and bow-shaped structures), and polar vortices (polar hot dipole and fine structures). The Venusian atmospheric circulation, controlled by the planet’s radiative forcing and astronomical parameters, is quite different from the earth’s. As the meteorological data have been stored, understanding of the atmospheric circulation has been gradually enriched with the help of theories of geophysical fluid dynamics and meteorology.
In the cloud layer far from the surface (49–70 km altitude), superrotational flows (east-to-west zonal winds) exceeding 100 m/s and meridional (equator-to-pole) flows have been observed along with planetary-scale brightness variations unique to Venus. The fully developed superrotation, which is ~60 times faster than the planetary rotation, is maintained by meridional circulation and waves. For the planetary-scale variations, slow-traveling waves with stationary and solar-locked structures and fast-traveling waves with phase velocities of around the superroational wind speeds are dominant in the cloud layer. Thermal tides, Rossby waves, Kelvin waves, and gravity waves play important roles in mechanisms for maintaining fast atmospheric rotation. In the lower atmosphere below the cloud layer, the atmospheric circulation is still unknown because of the lack of global observations. In addition to the limited observations, the atmospheric modeling contributes to deep understanding of the atmospheric circulation system. Recent general circulation models have well simulated the dynamical and thermal structures of Venus’s atmosphere, though there remain outstanding issues.
Karen Aplin and Georg Fischer
Electricity occurs in atmospheres across the Solar System planets and beyond, spanning spectacular lightning displays in clouds of water or dust, to more subtle effects of charge and electric fields. On Earth, lightning is likely to have existed for a long time, on the basis of evidence from fossilized lightning strikes in ancient rocks, but observations of planetary lightning are necessarily much more recent. The generation and observations of lightning and other atmospheric electrical processes, both from within-atmosphere measurements, and spacecraft remote sensing, can be readily studied using a comparative planetology approach, with the Earth as a model.
All atmospheres contain charged molecules, electrons, and/or molecular clusters created by ionization from cosmic rays and other processes, which may affect an atmosphere’s energy balance both through aerosol and cloud formation and direct absorption of radiation. Several planets are anticipated to host a “global electric circuit” by analogy with the circuit occurring on the Earth, where thunderstorms drive the current of ions or electrons through weakly conductive parts of the atmosphere. This current flow may further modulate an atmosphere’s radiative properties through cloud and aerosol effects.
Lightning could potentially have implications for life through its effects on atmospheric chemistry and particle transport. It has been observed on many of the Solar System planets (Earth, Jupiter, Saturn, Uranus, and Neptune), and it may also be present on Venus and Mars. On Earth, Jupiter, and Saturn, lightning is thought to be generated in deep water and ice clouds, but discharges can be generated in dust, as for terrestrial volcanic lightning, and on Mars. Other, less well-understood mechanisms causing discharges in non-water clouds also seem likely. The discovery of thousands of exoplanets has recently led to a range of further exotic possibilities for atmospheric electricity, though lightning detection beyond our Solar System remains a technical challenge to be solved.
This is an advance summary of a forthcoming article in the Oxford Encyclopedia of Planetary Science. Please check back later for the full article.
The Austrian Outer Space Act, which entered into force in December 2011; and the Austrian Outer Space Regulation, which has been in force since February 2015, form the legal framework for Austrian national space activities. The elaboration of national space legislation became necessary to ensure compliance with Austria’s obligations as State Party to the five United Nations Space Treaties when the first two Austrian satellites were launched in 2012 and Austria became a launching state on its own. The legislation comprehensively regulates legal aspects related to space activities, such as authorization, supervision, and termination of space activities; registration and transfer of space objects; recourse of the government against the operator; as well as implementation of the law and sanctions for its infringement. One of the main purposes of the law is to ensure the authorization of national space activities. The Outer Space Act sets forth the main conditions for authorization, which inter alia refer to the expertise of the operator; requirements for orbital positions and frequency assignments; space debris mitigation, insurance requirements, and the safeguard of public order; public health; national security as well as Austrian foreign policy interests; and international law obligations. The Austrian Outer Space Regulation complements these provisions by specifying the documents the operator must submit as evidence of the fulfillment of the authorization conditions, which include the results of safety tests, emergency plans, and information on the collection and use of Earth observation data. Particular importance is attached to the mitigation of space debris. Operators are required to take measures in accordance with international space debris mitigation guidelines for the avoidance of operational debris, the prevention of on-orbit break-ups and collisions, and the removal of space objects from Earth orbit after the end of the mission. Another specificity of the Austrian space legislation is the possibility of an exemption from the insurance requirement or a reduction of the insurance sum, if the space activity is in the public interest. This allows support to space activities that serve science, research, and education. Moreover, the law also provides for the establishment of a national registry for objects launched into outer space by the competent Austrian Ministry. The first two Austrian satellites have been entered into this registry after their launch in 2012. The third Austrian satellite, launched in June 2017, will be the first satellite authorized under the Austrian space legislation.
Mark J. Sundahl
Throughout the history of human activity in outer space, the role of private companies has steadily grown, and, in some cases, companies have even replaced government agencies as the primary actors in space. As private space activity has grown and diversified, the laws and regulations that govern private actors have been forced to evolve in reaction to the new realities of the industry. On the international level, the treaties concluded in the 1960s and 1970s continue to be in force today. However, these treaties only govern state activity in space. The rules regulating private industry are necessarily domestic in nature, and it is in these domestic laws that the evolution of space law can be most clearly seen. That said, new industries, such as asteroid mining, are testing the limits of international law and have forced the international community to examine whether changes to long-standing laws are needed.
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.
Anni Määttänen and Franck Montmessin
This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Planetary Science. Please check back later for the full article.
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 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 of 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 are a fairly recent discovery and have put our understanding of the Martian atmosphere to a test.
On Mars, cloud crystals form on ice nuclei, mostly provided by the omnipresent 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.
Leonid V. Ksanfomality
Cometary nuclei are small, despite the cosmic scale of the comet tails that they produce. The nuclei have the ability to create rarefied atmospheres, extending as a tail to giant distances comparable to the orbital distances of the planets. Giant tails of comets are sometimes observed for several years and cover a significant part of the sky. The cometary nucleus is capable of continuously renewing tails and supporting the material that is constantly dissipating in space. Large comets do not appear so often that they have become trivial celestial phenomena, but they appear often enough to allow astronomers to complete detailed studies. Many remarkable discoveries, such as the discovery of solar wind, were made during the study of comets. Comets are characterized by great diversity, and their appearance often becomes an ornament of the night sky. Comets have become remote laboratories, where experiments are performed in physical conditions that are not achievable on Earth.
Jay M. Pasachoff and Roberta J.M. Olson
Since the landmark lunar landing of Apollo 11 on July 20, 1969, NASA’s Lunar Reconnaissance Orbiter (launched in 2009), and the Japanese Aerospace Exploration Agency’s Kaguya spacecraft (2007–2009), among other efforts, have now mapped the Moon’s surface. Before those technological advances and since the beginning of recorded time, people and civilizations have been entranced by Earth’s only natural satellite, which is the second-brightest celestial object visible in the sky from the surface of the planet. Selected examples, among thousands, show how the history of the Moon has been regarded, illustrated, and mapped in visual culture in the Western world. Early examples include representations of a formulaic crescent Moon in Babylonian times; later this dominant stylized depiction of the Moon gave way to more naturalistic images based on observation, culminating in Leonardo da Vinci’s manuscript drawings, which study the lunar structure and cratered surface, and Galileo Galilei’s first telescopic images of the Moon recorded in wash drawings and woodcuts for his book Sidereus Nuncius. Both the artistic and scientific visual acuity that made this evolution possible belonged to the burgeoning empiricism of the 14th through the 17th centuries, which eventually yielded modern observational astronomy and impacted lunar iconography. The subsequent dramatic mapping of the Moon’s surface and the naming of its features became a preoccupation of many astronomers and some artists, who assisted scientists in illustrating their work. With the seeming physical mapping of the Earth-facing side of the Moon well underway in the late 18th and early 19th centuries, the function of Earth’s satellite as a Romantic symbol gained force in the all the arts but most dramatically in the works of landscape painters in Germany (e.g., Caspar David Friedrich and Carl Gustav Carus) and in England (e.g., Samuel Palmer). At the same time, William Blake, who was obsessed with astronomical imagery, used the Moon for expressive purposes, which reached a fever pitch later in the century in the work of Vincent Van Gogh. Along with the increasing accuracy of the Moon’s portrayal through both artists’ and scientists’ representations, the dramatic history of its mapping from Earth crescendoed with the development of photography and William Cranch Bond’s first successful daguerreotype of the Moon in 1851. Further exploration of the Moon, including its far side, has gravitated to aerospace engineers in cooperation with physicists, astronomers, mathematicians, and Apollo astronauts. Nevertheless, the Moon has remained an enduring object of fascination for artists—among the many, Surrealist Joan Miró, Veja Celmins, and Andy Warhol.
Matthew R. Balme
Dust devils are rotating columns or cones of air, loaded with dust and other fine particles, that are most often found in arid or desert areas. They are common on both Mars and Earth, despite Mars’ very thin atmosphere. The smallest and least intense dust devils might last only a few 10s of seconds and be just a meters or two across. The largest dust devils can persist for hours and are intensely swirling columns of dust with “skirts” of sand at their base, 10s or more meters in diameter and hundreds of meters high; even larger examples have been seen on Mars. Dust devils on Earth have been documented for thousands of years, but scientific observations really began in the early 20th century, culminating in a period of intense research in the 1960s. The discovery of dust devils on Mars was made using data from the NASA Viking lander and orbiter missions in the late 1970s and early 1980s and stimulated a renewed scientific interest in dust devils. Observations from subsequent lander, rover, and orbital missions show that Martian dust devils are common but heterogeneously distributed in space and time and have a significant effect on surface albedo (often leaving “tracks” on the surface) but do not appear to be triggers of global or major dust storms. An aspiration of future research is to synthesize observations and detailed models of dust devils to estimate more accurately their role in dust lifting at both local and global scales, both on Earth and on Mars.
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.
Dmitry V. Bisikalo, Pavel V. Kaygorodov, and Valery I. Shematovich
The history of exoplanetary atmospheres studies is strongly based on the observations and investigations of the gaseous envelopes of hot Jupiters—exoplanet gas giants that have masses comparable to the mass of Jupiter and orbital semi-major axes shorter than 0.1 AU. The first exoplanet around a solar-type star was a hot Jupiter discovered in 1995. Researchers found an object that had completely atypical parameters compared to planets known in the solar system. According to their estimates, the object might have a mass about a half of the Jovian mass and a very short orbital period (four days), which means that it has an orbit roughly corresponding to the orbit of Mercury. Later, many similar objects were discovered near different stars, and they acquired a common name—hot Jupiters. It is still unclear what the mechanism is for their origin, because generally accepted theories of planetary evolution predict the formation of giant planets only at large orbital distances, where they can accrete enough matter before the protoplanetary disc disappears. If this is true, before arriving at such low orbits, hot Jupiters might have a long migration path, caused by interactions with other massive planets and/or with the gaseous disc. In favor of this model is the discovery of many hot Jupiters in elliptical and highly inclined orbits, but on the other hand several observed hot Jupiters have circular orbits with low inclination. An alternative hypothesis is that the cores of future hot Jupiters are super-Earths that may later intercept matter from the protoplanetary disk falling on the star.
The scientific interest in hot Jupiters has two aspects. The first is the peculiarity of these objects: they have no analogues in the solar system. The second is that, until recently, only for hot Jupiters was it possible to obtain observational characteristics of their atmospheres. Many of the known hot Jupiters are eclipsing their host stars, so, from their light curve and spectral data obtained during an eclipse, it became possible to obtain information about their shape and their atmospheric composition.
Thus it is possible to conclude that hot Jupiters are a common type of exoplanet, having no analogues in the solar system. Many aspects of their evolution and internal structure remain unclear. Being very close to their host stars, hot Jupiters must interact with the stellar wind and stellar magnetic field, as well as with stellar flares and coronal mass ejections, allowing researchers to gather information about them. According to UV observations, at least a fraction of hot Jupiters have extended gaseous envelopes, extending far beyond of their upper atmospheres. The envelopes are observable with current astronomical instruments, so it is possible to develop their astrophysical models. The history of hot Jupiter atmosphere studies during the past 20 years and the current status of modern theories describing the extended envelopes of hot Jupiters are excellent examples of the progress in understanding planetary atmospheres formation and evolution both in the solar system and in the extrasolar planetary systems.
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.
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.
Valery I. Shematovich and Dmitry V. Bisikalo
This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Planetary Science. Please check back later for the full article.
The uppermost layers of a planetary atmosphere, where the density of neutral particles is vanishingly low, are commonly called the exosphere or the planetary corona. Since the atmosphere is not completely bound to the planet by the planetary gravitational field, light atoms, such as hydrogen and helium with sufficiently large velocities, can escape from the upper atmosphere into interplanetary space. This process is commonly called Jeans escape, and it depends on the temperature of the ambient atmospheric gas at an altitude where the atmospheric gas is virtually collisionless. The heavier carbon, nitrogen, and oxygen atoms can escape from the atmospheres of the terrestrial planets only through non-thermal processes such as photo- and electron-impact dissociation, charge exchange, atmospheric sputtering, and ion pick-up. Theories of planetary exospheres have been based on ground-based and space observations of emission features such as the 121.6 nm Ly-α and 102.6 nm Ly-β hydrogen lines, the 58.4 nm helium line, and the 130.4 and 135.6 nm atomic oxygen lines. Such observations, together with in situ mass-spectrometer measurements, as at Titan, allow the density and temperature height profiles of the exospheric components to be constructed. The measurements reveal that planetary coronas contain both a fraction of thermal neutral particles with a mean kinetic energy corresponding to the exospheric temperature and a fraction of hot neutral particles with mean kinetic energy much higher than the exospheric temperature. These suprathermal (hot) atoms and molecules are a direct manifestation of the non-thermal processes taking place in the atmospheres. These hot particles lead to the atmospheric escape, determine the coronal structure, produce non-thermal emissions, and react with the ambient atmospheric gas triggering hot atom chemistry.
One of the brightest manifestations of these processes is a formation of hot oxygen corona around terrestrial planets. Oxygen atom is one of the lightest among heavy atmospheric species, so it is a best species to form corona, and another important aspect is that it produces a lot of observational evidence. The transport of suprathermal oxygen atoms to exospheric heights leads to the formation of hot oxygen coronas around Venus, Earth, and Mars. It has been well established by both observations and theoretical calculations that hot oxygen is an important constituent in the transition region between upper thermosphere and exosphere at terrestrial planets.
The study of the planetary coronas is based on direct observations and numerical simulations. It is a rarefied gas, therefore, production and transport of suprathermal particles into the corona requires solving a Boltzmann equation or a DSMC simulation. The stochastic simulation method had been widely used to investigate the formation, kinetics, and transport of suprathermal particles in the hot planetary coronas. This approach was first used to study the formation of the hot oxygen geocorona, taking into account the exothermic chemistry and the precipitation of magnetospheric protons and high-energy O+ ions from the ring current. It was found that only atmospheric sputtering results in the formation of the escape flux of energetic oxygen atoms, providing an important source of heavy atoms for the magnetosphere and geospace. A stochastic modeling approach was also applied to study the escape of hot oxygen atoms from the upper atmosphere of Mars and Venus; the kinetics and transport of suprathermal atoms and molecules in the hot oxygen corona at Jovian satellite Europa, which is an example of a highly non-equilibrium near-surface atmosphere; and the hot extended corona at Saturnian satellite Titan, which was directly measured by the spacecraft Cassini.
Emergence of ballistic missile technology after World War II enabled human flight into the Earth’s orbit, fueling the imagination of those fascinated with science, technology, exploration, and adventure. The performance of astronauts in the early flights assuaged concerns about the functioning of “the human system” in the absence of the Earth’s gravity. However, researchers in space medicine have observed degradation of crews after longer exposure to the space environment and have developed countermeasures for most of them, although significant challenges remain. With the dawn of the 21st century, well-financed and technically competent commercial entities have begun to provide more affordable alternatives to historically expensive and risk-averse government-funded programs. The growing accessibility to space has encouraged entrepreneurs to pursue plans for potentially autarkic communities beyond the Earth, exploiting natural resources on other worlds. Should such dreams prove to be technically and economically feasible, a new era will open for humanity with concomitant societal issues of a revolutionary nature.
This article consists of three sections. The first discusses how we determine satellite internal structures and what we know about them. The primary probes of internal structure are measurements of magnetic induction, gravity, and topography, as well as rotation state and orientation. Enceladus, Europa, Ganymede, Callisto, Titan, and (perhaps) Pluto all have subsurface oceans; Callisto and Titan may be only incompletely differentiated. The second section describes dynamical processes that affect satellite interiors and surfaces: tidal and radioactive heating, flexure and relaxation, convection, cryovolcanism, true polar wander, non-synchronous rotation, orbital evolution, and impacts. The final section discusses how the satellites formed and evolved. Ancient tidal heating episodes and subsequent refreezing of a subsurface ocean are the likeliest explanation for the deformation observed at Ganymede, Tethys, Dione, Rhea, Miranda, Ariel, and Titania. The high heat output of Enceladus is a consequence of Saturn’s highly dissipative interior, but the dissipation rate is strongly frequency-dependent and does not necessarily imply that Saturn’s moons are young. Major remaining questions include the origins of Titan’s atmosphere and high eccentricity, the regular density progression in the Galilean satellites, and the orbital evolution of the Saturnian and Uranian moons.