21-40 of 95 Results

Article

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.

Article

Since the launch of Sputnik on October 4, 1957, the development of space activities has provided a kind of evidence for the conduct of human affairs, to the point of neglecting to question these activities from an ethical point of view: only since the beginning of the 2000s has a real ethical interrogation within the space community (French Space Agency, International Space University, COPUOS) been developed, in parallel with international law. Taking advantage of a rich cultural background and a cooperative sustained effort, space ethics contributes, for example, to better management of debris orbiting the Earth, evaluation of the social impacts of observation satellite systems, and the arrival of new private entrepreneurs apparently less aware of the impacts of managing space as a common heritage of humanity. If space law provides a possible framework and a set of principles for the current and future management of space activities, ethical principles must be considered to accurately assess their reasons for being and their consequences. The following questions are pertinent today: Has space become a trash can? Is space “Big Brother’s” ally? Is space for sale? Should space be explored at any cost? These issues require special expertise of the situation (e.g., the distribution of debris around the Earth, the capabilities of observation satellites); consideration of the global, dual (civil, military) nature of space; and reference to ethical principles (responsibility, vigilance). Human space flight, space tourism, and the search for extraterrestrial life are also subject to ethical questioning. At the beginning of the 21st century, space ethics remained a goal for the space community.

Article

John C. Bridges

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

Article

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.

Article

V.V. Shevchenko

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

Article

Pascal Rosenblatt, Ryuki Hyodo, Francesco Pignatale, Antony Trinh, Sebastien Charnoz, Kevin Dunseath, Mariko Dunseath-Terao, and Hidenori Genda

The origin of the natural satellites or moons of the solar system is as challenging to unravel as the formation of the planets. Before the start of the space probe exploration era, this topic of planetary science was restricted to telescopic observations, which limited the possibility of testing different formation scenarios. This era has considerably boosted this topic of research, particularly after the Apollo missions returned samples from the Moon’s surface to Earth. Observations from subsequent deep space missions such as Viking 1 and 2 Orbiters, Voyager 1 and 2, Phobos-2, Galileo, Cassini-Huygens, and the most recent Mars orbiters such as Mars Express, as well as from the Hubble space telescope, have served to intensify research in this area. Each moon system has its own specificities, with different origins and histories. It is widely accepted that the Earth’s Moon formed after a giant collision between the proto-Earth and a body similar in size to Mars. The Galilean moons of Jupiter, on the other hand, appear to have formed by accretion in a circum-Jovian disk, while smaller, irregularly shaped satellites were probably captured by the giant planet. The small and medium-sized Saturnian moons may have formed from the rings encircling the planet. Among the terrestrial planets, Mercury and Venus have no moons, the Earth has a single large moon, and Mars has two very small satellites. This raises some challenging questions: What processes can lead to moon formation around terrestrial planets and what parameters determine the possible outcomes, such as the number and size of moons? The answer to such fundamental questions necessarily entails a thorough understanding of the formation of the Martian system and may have relevance to the possible existence of (exo)moons orbiting exoplanets. The formation of such exomoons is of great importance as they could influence conditions for habitability or for maintaining life over long periods of time on the surface of Earth-like exoplanets, for example by limiting the variations of the orientation of the planet’s rotation axis and thus preventing frequent changes of its climate. Our current knowledge concerning the origin of Phobos and Deimos has been acquired from observational data as well as theoretical work. Early observations led to the idea that the two satellites were captured asteroids but this created difficulties in reconciling the current orbits of Phobos and Deimos with those of captured bodies, hence suggesting the need for an alternative theory. A giant-impact scenario provides a description of how moons similar to Phobos and Deimos can be formed in orbits similar to those observed today. This scenario also restricts the range of possible composition of the two moons, providing a motivation for future missions that aim for the first time to bring material from the Martian system back to Earth.

Article

Long Xiao and James W. Head

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

Article

Valery I. Shematovich and Dmitry V. Bisikalo

The uppermost layers of a planetary atmosphere, where the density of neutral particles is vanishingly low, are commonly called exosphere or 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 thermal velocities can escape from the upper atmosphere into interplanetary space. This process is commonly called Jeans escape and 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 populate the coronas and escape from the atmospheres of terrestrial planets only through nonthermal processes such as photo- and electron-impact energizing, charge exchange, atmospheric sputtering, and ion pickup. The observations reveal that the planetary coronae 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 that expected for the exospheric temperature. These suprathermal (hot) atoms and molecules are the direct manifestation of the nonthermal processes taking place in the atmospheres. These hot particles populate the hot coronas, take a major part in the atmospheric escape, produce nonthermal emissions, and react with the ambient atmospheric gas, triggering the hot atom chemistry.

Article

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.

Article

Humans have always looked up at the stars and dreamed about outer space as the final frontier. The launch of the first artificial satellite—Sputnik—in 1957 by the Soviet Union and the first man on the Moon in 1969 represent significant missions in space exploration history. In 1972, Apollo 17 marked the last human program on the lunar surface. Nevertheless, several robotic spacecrafts have traveled to the Moon, such as the Soviet Luna 24 in 1976, and China’s Chang’e 4 in 2019, which was the first time a space vehicle touched down on the Moon’s far side. The international space community is currently assessing a return to the Moon in 2024 and even beyond, in the coming decades, toward the Red Planet, Mars. Robots and rovers (for example Curiosity, Philae, Rosetta, and Perseverance) will continue to play a major role in space exploration by paving the way for future long-duration missions on celestial bodies. It is still impossible to land humans on Mars or on other celestial bodies because there are significant challenges to overcome from technological and physiological perspectives. Therefore, the support of machines and artificial intelligence is essential for developing future deep space programs as well as to reach a sustainable space exploration. One can imagine a future scenario where robots and humans collaborate on the Moon’s surface or on celestial bodies to undertake scientific research, to extract and to analyze space resources for a possible in situ utilization, as well as to build sites for human habitation and work. The principles of free exploration and cooperation are core elements in the international space legal framework as mentioned in Article I of the 1967 Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies. In this context of new ‘robots–humans’ cooperation, it is also necessary to consider the provisions of the 1972 Convention on the International Liability for Damage Caused by Space Objects, the 1975 Convention on Registration of Objects Launched into Outer Space, the 1968 Agreement of the Rescue of Astronauts, the Return of Astronauts and the Return of Objects Launched into Outer Space, and the 1979 Agreement Governing the Activities of States on the Moon and Other Celestial Bodies, as well as some recent international agreements signed for future Moon missions given their significant importance for space exploration.

Article

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.

Article

Probing the interiors of the gaseous giant planets in our solar system is not an easy task. It requires a set of accurate measurements combined with theoretical models that are used to infer the planetary composition and its depth dependence. The masses of Jupiter and Saturn are 317.83 and 95.16 Earth masses (M ⊕ ), respectively, and since a few decades, it has been known that they mostly consist of hydrogen and helium. The mass of heavy elements (all elements heavier than helium) is not well determined, nor are their distribution within the planets. While the heavy elements are not the dominating materials inside Jupiter and Saturn, they are the key to understanding the planets’ formation and evolutionary histories. The planetary internal structure is inferred from theoretical models that fit the available observational constraints by using theoretical equations of states (EOSs) for hydrogen, helium, their mixtures, and heavier elements (typically rocks and/or ices). However, there is no unique solution for determining the planetary structure and the results depend on the used EOSs as well as the model assumptions imposed by the modeler. 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) of separation between the two envelopes. Alternative structure models assume a less distinct division between the layers and /or a non-homogenous distribution of the heavy elements. The fact that the behavior of hydrogen at high pressures and temperatures is not perfectly known and that helium may separate from hydrogen at the deep interior add sources of uncertainty to structure models. In the 21st century, 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 for planetary modelers.

Article

Gianfranco Gabriele Nucera

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.

Article

Space activities are technically sophisticated and challenging endeavors involving high risk. Notwithstanding precautionary measures that are taken by commercial operators, damage may be caused during space objects’ launching, passing through air space, in-orbit maneuvering and operating, and de-orbiting. The rules and procedures aimed at ensuring the prompt payment of a full and equitable compensation for such damage constitute the international liability regime, which is of crucial importance in space law. The first reference to international liability for damage caused by space objects and their component parts on Earth, in air space, or in outer space can be traced back to the very beginning of the space era. In 1963, just a few years after the first ever artificial satellite was launched, international liability was declared by the United Nations General Assembly as one of the legal principles governing the activities of states in the exploration and use of outer space. It was later made legally binding by inclusion in the 1967 Outer Space Treaty and received further development in the 1972 Liability Convention. The latter is generally referred to as lex specialis when the interrelation between the two international treaties is described and introduces several provisions that treat liability for damage caused in specific circumstances somewhat differently. International space law imputes liability on states that launch or procure launchings of space objects and states from whose territory or facility space objects are launched. This does not, however, exclude liability for damage caused by space objects that are operated by private entities. Still, international liability for accidents involving commercial operators stays with the so-called launching states, as this term is defined by the Liability Convention for the same states that are listed in the Outer Space Treaty as internationally liable. Insurance is well known to address damages and liability issues, including those arising from commercial launches; however, it is not always mandatory. Frequently, space-related accidents involve nonfunctional space objects and their component parts, which are usually referred to as “space debris.” This may include spent rocket stages and defunct satellites, as well as fragments from their disintegration. Since the nonfunctional state of a space object does not change its legal status, the relevant provisions of international space law that are applicable to space objects continue to apply to what is called space debris. This means, in particular, that launching states are internationally liable for damage caused by space debris, including cases where such debris was generated by private spacecraft. The probability of liability becomes even higher when it comes to active space debris removal. Such space activities, which are extensively developed by private companies, are inextricably linked to potential damage. Yet, practical problems arise with identification of space debris and, consequently, an efficient implementation of the liability regime.

Article

Frans von der Dunk

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.

Article

Elina Morozova and Yaroslav Vasyanin

International space law is a branch of international law that 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 (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 an integral part of modern human life and global economy. Among such applications, satellite telecommunications is 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 methods ensure instant delivery of huge amounts of data, relay of real-time voice and video, broadcasting of radio and television, and Internet access worldwide. By transmitting signals over any distance telecommunications satellites connect locations everywhere on 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 liability, registration, jurisdiction and control, debris mitigation, and touches upon ownership. Therefore, the first large group of international law rules applicable to satellite telecommunications includes provisions of three out of five UN space treaties, specifically, the 1967 Outer Space Treaty, 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 several UN General Assembly resolutions. To carry out a communication function, satellites need to be placed in a certain orbit and to use radio-frequency spectrum, both limited natural resources. Access to these highly demanded resources, 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 UN 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 technology advancement and market evolution, as well as with a growing number of states and non-state actors carrying on space activities. These tangible changes need to be addressed in the regulatory framework that cannot but serve as a driver for further development of satellite telecommunications.

Article

George J. Flynn

Scattered sunlight from interplanetary dust particles, mostly produced by comets and asteroids, orbiting the Sun are visible at dusk or dawn as the Zodiacal Cloud. Impacts onto the space-exposed surfaces of Earth-orbiting satellites indicate that, in the current era, thousands of tons of interplanetary dust enters the Earth’s atmosphere every year. Some particles vaporize forming meteors while others survive atmospheric deceleration and settle to the surface of the Earth. NASA has collected interplanetary dust particles from the Earth’s stratosphere using high-altitude aircraft since the mid-1970s. Detailed characterization of these particles shows that some are unique samples of Solar System and presolar material, never affected by the aqueous and thermal processing that overprints the record of formation from the Solar Protoplanetary Disk in the meteorites. These particles preserve the record of grain and dust formation from the disk. This record suggests that many of the crystalline minerals, dominated by crystalline silicates (olivine and pyroxene) and Fe-sulfides, condensed from gas in the inner Solar System and were then transported outward to the colder outer Solar System where carbon-bearing ices condensed on the surfaces of the grains. Irradiation by solar ultraviolet light and cosmic rays produced thin organic coatings on the grain surfaces that likely aided in grain sticking, forming the first dust particles of the Solar System. This continuous, planet-wide rain of interplanetary dust particles can be monitored by the accumulation of 3He, implanted into the interplanetary dust particles by the Solar Wind while they were in space, in oceanic sediments. The interplanetary dust, which is rich in organic carbon, may have contributed important pre-biotic organic matter important to the development of life to the surface of the early Earth.

Article

Iron meteorites are thought to be samples of metallic cores and pools that formed in diverse small planetary bodies. Their great diversity offers remarkable insights into the formation of asteroids and the early history of the solar system. The chemical compositions of iron meteorites generally match those predicted from experimental and theoretical considerations of melting in small bodies. These bodies, called planetesimals, were composed of mixtures of grains of silicates, metallic iron-nickel, and iron sulfide with compositions and proportions like those in chondrite meteorites. Melting in planetesimals caused dense metal to sink through silicate so that metallic cores formed. A typical iron meteorite contains 5–10% nickel, ~0.5% cobalt, 0.1–0.5% phosphorus, 0.1–1% sulfur and over 20 other elements in trace amounts. A few percent of iron meteorites also contain silicate inclusions, which should have readily separated from molten metal because of their buoyancy. They provide important evidence for impacts between molten or partly molten planetesimals. The major heat source for melting planetesimals was the radioactive isotope 26Al, which has a half-life of 0.7 million years. However, a few iron meteorites probably formed by impact melting of chondritic material. Impact processes were also important in the creation of many iron meteorites when planetesimals were molten. Chemical analysis show that most iron meteorites can be divided into 14 groups: about 15% appear to come from another 50 or more poorly sampled parent bodies. Chemical variations within all but three groups are consistent with fractional crystallization of molten cores of planetesimals. The other three groups are richer in silicates and probably come from pools of molten metal in chondritic bodies. Isotopic analysis provides formation ages for iron meteorites and clues to their provenance. Isotopic dating suggests that the parent bodies of iron meteorites formed before those of chondrites, and some irons appear to be the oldest known meteorites. Their unexpected antiquity is consistent with 26Al heating of planetesimals. Bodies that accreted more than ~2 million years after the oldest known solids (refractory inclusions in chondrites) should not have contained enough 26Al to melt. Isotopic analysis also shows that iron meteorites, like other meteorite types, display small anomalies due to pre-solar grains that were not homogenized in the solar nebula (or protoplanetary disk). Although iron meteorites are derived from asteroids, their isotopic anomalies provide the best clues that some come from planetesimals that did not form in the asteroid belt. Some may have formed beyond Jupiter; others show isotopic similarities to Earth and may have formed in the neighborhood of the terrestrial planets. Iron meteorites therefore contain important clues to the formation of planetesimals that melted and they also provide constraints on theories for the formation of planets and asteroids.

Article

Yuri Amelin

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

Article

Timothy E. Dowling

Jet streams, “jets” for short, are remarkably coherent streams of air found in every major atmosphere. They have a profound effect on a planet’s global circulation and have been an enigma since the belts and zones of Jupiter were discovered in the 1600s. Collaborations between observers, experimentalists, computer modelers, and applied mathematicians seek to understand what processes affect jet size, strength, direction, shear stability, and predictability. Key challenges include nonlinearity, nonintuitive wave physics, nonconstant-coefficient differential equations, and the many nondimensional numbers that arise from the competing physical processes that affect jets, including gravity, pressure gradients, Coriolis accelerations, and turbulence. Fortunately, the solar system provides many examples of jets, and both laboratory and computer simulations allow for carefully controlled experiments. Jet research is multidisciplinary but is united by a common language, the conservation of potential vorticity (PV), which is an all-in-one conservation law that combines the conservation laws of mass, momentum, and thermal energy into a single expression. The leading theories of how jets emerge out of turbulence, and why they are invariably zonal (east-west orientated), reveal the importance of vorticity waves that owe their existence to conservation of PV. Jets are observed to naturally group into equatorial, midlatitude, and polar types. Earth and Uranus have weakly retrograde equatorial jets, but most planets exhibit strongly prograde (superrotating) equatorial jets, which require eddies to transport momentum up-gradient in a manner that is not obvious but is beginning to be understood. Jupiter and Saturn exhibit multiple alternating jets spanning their midlatitudes, with deep roots that connect to their interior circulations. Polar jets universally exhibit an impressive inhibition of meridional (north-south) mixing, and the seasonal nature of the polar jets on Earth, Mars, and Titan contrasts with the permanence of those on the giant planets, including Saturn’s beautiful north-polar hexagon. Intriguingly, jets in atmospheres have strong analogies with jets in nonneutral plasmas, with practical benefits to both disciplines.