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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.

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 bce from where it was imported into the Hellenistic world from the early 4th century bce onward. There it became attached to three philosophical schools, those pioneered by Plato, Aristotle and the Stoics, all of which shared the assumption that the cosmos is a single, living, integrated whole. Hellenistic astrology also drew on Egyptian temple culture, especially the belief that the soul could ascend to the stars. By the 1st century ce, the belief in the close link between humanity and the stars had become democratized and diversified into a series of practices and schools of thought which ranged across Greek and Roman culture. It was practiced at the imperial court and in the street. It could be used to predict individual destiny, avert undesirable events, and arrange auspicious moments to launch new enterprises. It could advise on financial fortunes or the condition of one’s soul. It was conceived of as natural science and justified by physical influences or considered to be divination, concerned with communication with the gods and goddesses. In some versions, the planets were neither influences nor causes of events on Earth, but timing devices, which indicated the ebb and flow of human affairs, like the hands on a modern clock. Astrology had a radical view of time in which the future already existed, at least in potential, and the astrologer’s task was to intercede in time, altering the future to human advantage. In this sense astrology was a form of “participation mystique” in which time and space were conceived of as a single entity and individual and social benefits were to be derived from engaging with it. There was no one single version of astrology and there were disputes about what it was and what it could do, for example, whether it could make precise predictions about individual affairs or merely general statements. From the early 4th century it went into a progressive decline, facing challenges from Christianity and the fragmentation of classical culture, especially in Western Europe. It survived in Persia, exerted a powerful influence on Indian astrology, and was transmitted to the Islamic world, from where it was reimported into the Latin West in the 12th century.

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.

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.

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.

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.

Chemical and phase compositions of Venus’s surface could reflect history of gas- 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 has been established in 1960s after the discovery of hot and dense CO₂-rich atmosphere inferred from Earth-based and Mariner 2 radio emission data. Initial works 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 lithosphere. Calcite, quartz, wollastonite, amphiboles, and Fe oxides were considered likely secondary minerals. Since the late 1970s, measurements of trace gases in the sub-cloud atmosphere by Pioneer Venus and Venera entry probes and Earth-based infrared spectroscopy doubted the likelihood of hydration and carbonation. The H₂O gas content appeared to be low to allow a stable existence of hydrated and a majority of OH-bearing minerals. The concentration of SO₂ was too high to allow the stability of calcite and Ca-rich silicates with respect to sulfatization to CaSO₄. In 1980s, the supposed ongoing consumption of atmospheric SO₂ to sulfates gained support by the detection of an elevated bulk S content at Venera and Vega landing sites. The induced composition of the near-surface atmosphere implied oxidation of ferrous minerals to magnetite and hematite, consistent with the infrared reflectance of surface materials. The likelihood of sulfatization and oxidation has been illustrated in modeling experiments at simulated Venus conditions.

Venus’s surface morphology suggests that hot surface rocks and fines of mainly mafic composition contacted atmospheric gases during several hundreds of millions years since a global volcanic 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 could reflect interactions with putative water-rich atmospheres and even aqueous solutions. Salt-, Fe oxide, or silica-rich formations would indicate past aqueous processes. The apparent diversity of affected solids, surface temperatures, pressures, and gas/fluid compositions throughout Venus’s history implies multiple signs of chemical alteration, which 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 composition of surface materials, and by the uncertain data on thermodynamics of minerals and their solid solutions. In the preparation for further entry probe and lander missions, rock alteration needs to be investigated through chemical kinetic experiments and calculations of solid-gas(fluid) equilibria to constrain past and present processes.

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.

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.

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.

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.

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.

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.

In recent years, in analytical reviews, experts are increasingly 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 rare earth metals find their application primarily in the aerospace and other industrial sectors of the economy, and therefore they 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 millions of vehicles traveling along our roads install hybrid batteries, all platinum reserves on Earth will 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 (Pd), rhodium (Rh), ruthenium (Ru), and actually platinum (Pt). These elements, rare on Earth, possess unique chemical and physical properties, which makes them vital industrial materials. To solve this problem, in the future, projects were proposed for the utilization of a substance of asteroids approaching the Earth. According to modern estimates, the number of known asteroids approaching the Earth reaches 9,000 objects. 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. Really, a 10-m iron-nickel asteroid could contain up to 75 tons of rare and rare earth metals, primarily platinum group metals, which is 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 more simple 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, the 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 to more than 14 million tons. It should be noted that the world’s known reserves of platinum group metals on Earth are about 80,000 tons.

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 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, ttherefore, 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.

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.

Probing the interiors of the giant planets in our Solar System is not an easy task. This requires a set of observations combined with theoretical models that are used to infer the planetary composition and its depth dependence. The masses of Jupiter and Saturn are 318 and 96 Earth masses, respectively, and since a few decades, we know that they mostly consist of hydrogen and helium.

It is the mass of heavy elements (all elements heavier than helium) that is not well determined, as well as its distribution within the planets. While the heavy elements are not the dominating materials in Jupiter and Saturn, they are the key for our understanding of their formation and evolution histories.

The planetary internal structure is inferred to fit the available observational constraints including the planetary masses, radii, 1-bar temperatures, rotation rates, and gravitational fields. Then, using theoretical equations of states (EOSs) for hydrogen, helium, their mixtures, and heavier elements (typically rocks and/or ices), a structure model is developed. However, there is no unique solution for the planetary structure, and the results depend on the used EOSs and the model assumptions imposed by the modeler.

Standard interior models of Jupiter and Saturn include three main regions: (1) the central region (core) that consists of heavy elements, (2) an inner metallic hydrogen envelope that is helium rich, and (3) an outer molecular hydrogen envelope depleted with helium. The distribution of heavy elements can be either homogenous or discontinuous between the two envelopes.

Major model assumptions that can affect the derived internal structure include the number of layers, the heat transport mechanism within the planet (and its entropy), the nature of the core (compact vs. diluted), and the location/pressure where the envelopes are divided. Alternative structure models assume a less distinct division between the layers and/or a less non-homogenous distribution of the heavy elements. The fact that the behavior of hydrogen at high pressures and temperatures in not perfectly known, and that helium separates from hydrogen at the deep interior add sources of uncertainties to the interior model. Today, with accurate measurements of the gravitational fields of Jupiter and Saturn from the Juno and Cassini missions, structure models can be further constrained. At the same time, these measurements introduce new challenges and open question for planetary modelers.

Outer space has always assumed a relevant geopolitical value due to strategic and economic reasons. Since the beginning of the so-called space age, national space policies have pursued both political and economic objectives, taking into account fundamental security and military considerations. After the Second World War, the international relations were based on the dichotomy between the United States and the Soviet Union. The foundation of activities in outer space finds its roots in the Cold War and reproduces the distinctive geopolitical dynamics of that historical moment. The diverging interests between the two states were reflected in the political tensions that characterized the competition to reach outer space.

The classical geopolitics deals with how states should act in outer space to increase their influence in the international arena. According to the theories developed during the space race, whoever controls outer space controls the world. In this sense, security on Earth depends on the security in space, ensured by national control over the strategic assets. Space applications had indeed a central role in the context of deterrence. In addition, conducting activities in outer space represented an important tool of foreign policy and for the enhancement of international cooperation, mainly within the blocs.

International geopolitical dynamics were reflected on space regulations developed during the Cold War era. The 1967 Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space (OST) is the main legal instrument, which codifies the general principles in international law of space activities.

Over the past few decades, space activities have changed due to the growing participation of non-state actors to the so-called space economy. The end of the Cold War era produced a structural change of the international relations in the space sector. The traditional scheme of cooperation within the Western, or Eastern, bloc was overcome by a stronger multilateral cooperation, such in the case of the International Space Station. Furthermore, the end of the Cold War busted the regionalization of space cooperation.

Furthermore, space activities are relevant for the well-being of humankind. Many services provided by public and private companies, such as satellite broadcasting, weather forecasts, or satellite navigation, have a strong socioeconomic impact. In addition, the protection of the environment in outer space has become a central theme in the international debate, with a focus on mitigation and removal of space debris. These issues are reflected in increasing legislation, adopted to regulate space activities on a national level.

This evolution, along with technological changes, poses political challenges to the actors involved in the space arena and creates a competitive geopolitical situation in which states aim at protecting their national interests in outer space. In this context, the international space governance plays a fundamental role in bringing together national interests toward a collective interest in protecting and promoting space activities for the benefit of humankind and with due regard to the corresponding interests of all states.

International satellite law can best be described as that subset of international space law that addresses the operations of satellites in orbit around the Earth. Excluding, therefore, topics such as manned space flight, suborbital space operations, and any activities beyond Earth orbits, this means addressing the use of satellites for telecommunications purposes, for Earth observation and remote sensing, and for positioning, timing, and navigation.

These three major sectors of space activities are, in addition to jointly being subject to the body of international space law, each subject to their specific dedicated legal regime—international satellite communications law, international satellite remote sensing law, and international satellite navigation law.

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.

International space law is a branch of international law, which regulates the conduct of space activities. Its core instruments include five space-specific international treaties, which were adopted under the auspices of the United Nations. The first and the underlying one—the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies (the 1967 Outer Space Treaty)—establishes that outer space is free for exploration and use by all states. Such fundamental freedom is exercised by a number of space applications that have become integral parts of modern human life, of which satellite telecommunications remains among the most widespread, essential, and advanced.

Indeed, since 1957 when the Soviet Union launched Sputnik 1, the first artificial satellite merely capable of continuous beeping during its 21-day trip around the globe, space technologies have progressed in leaps and bounds. Cutting-edge satellite telecommunications ensures instant delivery of huge amounts of data, relay of real-time voice and video, broadcasting of radio and television, and Internet access worldwide. It is telecommunications satellites that, by transmitting signals over a distance, connect locations on the surface of the Earth.

A telecommunications satellite’s lifetime, starting from the launch and ending at de-orbiting, is governed by international space law. The latter considers satellites as “space objects” and regulates ownership, jurisdiction and control, registration, liability, debris mitigation, and more. Therefore, four out of five United Nations space treaties, specifically, the 1967 Outer Space Treaty; the 1968 Agreement on the Rescue of Astronauts, the Return of Astronauts and the Return of Objects Launched into Outer Space; the 1972 Convention on International Liability for Damage Caused by Space Objects; and the 1976 Convention on Registration of Objects Launched into Outer Space, as well as the United Nations General Assembly’s resolutions, compose a large group of international law rules that apply to satellite telecommunications.

To carry out a communication function, satellites need to be placed in a certain orbit and use the radio-frequency spectrum, both being limited natural resources. Access to such increasingly sought-for parts of outer space, which are not subject to national appropriation and require rational, efficient, and economical uses in an interference-free environment, is managed by the International Telecommunication Union (ITU)—the United Nations’ specialized agency for information and communication technologies. The ITU’s core regulatory documents are its constitution, convention, and the radio regulations, which collectively make up another group of international law rules relevant to satellite telecommunications.

Both groups of international law rules constitute the international legal regime of satellite telecommunications and face the challenge of keeping pace with the times, triggered by a growing number of states and nonstate actors, which place priority on satellite telecommunications and a continuous application of the latest technical advances. These tangible changes need to be addressed in the regulatory framework that cannot but serve as a driver for further development of satellite telecommunications.