41-60 of 139 Results

Article

The Interiors of Jupiter and Saturn  

Ravit Helled

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

International Geopolitics and Space Regulation  

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

International Liability for Commercial Space Activities and Related Issues of Debris  

Elina Morozova and Alena Laurenava

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

International Satellite Law  

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

International Space Law and Satellite Telecommunications  

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

State Responsibility and Commercial Space Activities  

Danielle Ireland-Piper, Makaela Fehlhaber, and Alana Bonenfant

Commercial activity in outer space has increased. However, space is a dual-use environment, with both military and civilian applications. This raises the important question as to the extent to which a nation-state is responsible for the actions of commercial activities undertaken by corporate entities. The international law principles of state responsibility are complex. However, in some circumstances, these principles do create that potential for states to be liable where, for example, a corporate entity is a de facto organ of the state, or where a corporation acts on the instructions of a state or is under its control. The United Nations Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies, or Outer Space Treaty, provides some guidance on this question. Notwithstanding that, this is an uncertain area of the law, not least because of the complexity of space as an operating environment and complications in determining corporate nationality.

Article

Interplanetary Dust Particles  

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: Composition, Age, and Origin  

Edward R. D. Scott

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

Isotopic Dating  

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

Jets in Planetary Atmospheres  

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.

Article

Landslides in the Solar System  

Maria Teresa Brunetti and Silvia Peruccacci

Landslides are mass movements of rock, earth, or debris. All of these surface processes occur under the influence of gravity, meaning that they globally move material from higher to lower places. On planets other than Earth, these structures were first observed in a lunar crater during the Apollo program, but mass movements have been found on many rocky worlds (solid bodies) in the Solar System, including icy satellites, asteroids, and comets. On Earth, landslides have the effect of shaping the landscape more or less rapidly, leaving a signature that is recognized through field surveys and visual analysis or automatic identification on ground-based, aerial, and satellite images. Landslides observed on Earth and on solid bodies of the Solar System can be classified into different types based on their movement and the material involved in the failure. Material is either rock or soil (or both), with a variable fraction of water or ice; a soil mainly composed of sand-sized or finer particles is referred as earth while debris is composed of coarser fragments. The landslide mass may be displaced in several types of movement, classified generically as falling, toppling, sliding, spreading, or flowing. Such diverse characteristics mean that the size of a landslide (e.g., area, volume, fall height, length) can vary widely. For example, on Earth, their area ranges up to 11 orders of magnitude, while their volume varies by 16 orders, from small rock fragments to huge submarine landslides. The classification of extraterrestrial landslides is based on terrestrial analogs having similarities and characteristics that resemble those found on planetary bodies, such as Mars. The morphological classification is made regardless of the geomorphological environment or processes that may have triggered the slope failure. Comparing landslide characteristics on various planetary bodies helps to understand the effect of surface gravity on landslide initiation and propagation—of tremendous importance when designing manned and unmanned missions with landings on extraterrestrial bodies. Regardless of the practical applications of such study, knowing the morphology and surface dynamics that shape solid bodies in the space surrounding the Earth is something that has fascinated the human imagination since the time of Galileo.

Article

Large Volcanic Channels of the Inner Solar System  

David W. Leverington

Many large volcanic channel systems are recognized at the surfaces of rocky bodies of the inner solar system. The more than 200 channels known for the Moon mainly have simple sinuous forms with widths of up to several kilometers and lengths of up to hundreds of kilometers, typically commencing at topographic depressions and extending downslope until they fade into associated volcanic units. The Rima Hadley system was a key target of the Apollo 15 mission and was confirmed as a product of volcanic processes related to the emplacement of lavas in the Palus Putredinis region of Mare Imbrium. The more than 200 channels known for Venus are in many cases morphologically similar to sinuous lunar rilles, but some systems are especially large and complex, with widths of up to tens of kilometers and lengths that can exceed 1,000 km. Such systems typically commence at structural features or in regions of disturbed terrain and possess anastomosing reaches associated with prominent streamlined uplands. In contrast, Venusian canali typically maintain sinuous forms with widths of only a few kilometers but remarkably can be characterized by lengths of thousands of kilometers. Some Venusian channels were involved in the emplacement of fluidized ejecta in the vicinities of impact craters whereas others may have formed in such environments as a result of later volcanic events. The 10 large volcanic channels that are recognized on Mercury have lengths no greater than ~161 km but can have widths of up to several tens of kilometers. These systems developed as conduits for voluminous lavas that extend across adjacent impact basins. Terrestrial komatiitic channels of Archean and Proterozoic ages can have sizes that are comparable to those of lunar rilles, and the formation of these systems is likely to have played an important role in the development of associated Ni-Cu-(PGE) ores. The outflow channels of Mars have widths of up to tens of kilometers and lengths of up to thousands of kilometers and are widely interpreted as aqueous systems formed by catastrophic discharges from aquifers, but the properties and geological associations of these features and numerous other large Martian channel systems are arguably well aligned with those expected of volcanic origins. Overall, large volcanic channels of the inner solar system are mainly ancient products of the emplacement of low-viscosity lava flows of mafic or ultramafic composition, involving eruptions that were characterized by extraordinarily high effusion rates and total lava volumes that in some cases are likely to have been as great as those that characterized some Large Igneous Provinces on Earth. The deeply rooted igneous plumbing systems most favorable to the development of large volcanic channels would have been especially common in the earlier history of the solar system, when the interior temperatures of rocky bodies were greater than today. The early development of large volcanic channel systems is likely typical of the geological histories of large rocky bodies in the universe.

Article

Legal Issues Related to Satellite Orbits  

P.J. Blount

Orbits are unique geophysical features that are best understood as natural resources that are exploitable by humans for a variety of space activities. As with any human activity, the exploitation of these resources results in a variety of legal questions that are driven by their physical features and their uniqueness and scarcity. The law of orbits, or orbital law, is the framework of governance mechanisms that regulate the use of orbits from the perspective of their exploitation as natural resources. This legal framework seeks to govern the allocation of these resources among potential users, the coordination mechanisms among users to avoid conflict, and the protection of orbital resources from detrimental activities.

Article

Liability and Patent Protection for Space Activities in China  

Guoyu Wang and Xiao Ma

Space tort and patent protection are becoming more and more urgent legal issues, in light of rapidly developing space technology and commercialization of space activities. China’s space industry and activities have witnessed rapid progress recently, yet the development of China’s national space law system lags far behind China’s other achievements in space. Existing space laws in China have not expressly stipulated space tort and patent protection. Thus, addressing these issues has to resort to the relevant rules in other national laws or legal documents to recognize and confirm the doctrines of space tort liability, mitigation or exemption of liability, assignment and protection of space patents, etc. Therefore, the two main tasks or topics for the Chinese space law community are defining the applicability of the general rules and establishing a systematic national space law regime. The national space law regime will need to address the uncertainties, loopholes, and insufficiencies in the existing legal system regarding space tort and space patent protection. National tribunals, researchers, lawmakers, and policymakers will require references and guidance for dealing with space tort and patent protection. Meanwhile, international academia and practitioners need to better understand Chinese laws related to space activities, in order to facilitate international cooperation and the settlement of disputes.

Article

Life in the Deep Subsurface and Its Implications for Astrobiology  

Lotta Purkamo

The thin layer known as the Earth’s crust contains an even thinner layer that is suitable for sustaining life. The boundary conditions that define where we can find life in the deep subsurface depend on the physical space and water availability, accessibility to suitable substrates for energy and growth, in addition to tolerable temperature and pressure. The prevailing rather extreme conditions in the deep subsurface dictate that it is mainly inhabited by prokaryotic microorganisms. In fact, most of Earth’s microbial biomass is dwelling in the deep subsurface. Understanding the microbial diversity, dynamics and ecology in the Earth’s crust provides knowledge on the role of microbes in global elemental cycling. These capacities are investigated with cultivation-based experiments or molecular biological methods. Using genetic material of the microbes to reveal insights into metabolic processes, community structure and evolutionary history of microorganisms is state-of the-art in the deep biosphere research. Upgraded cultivation methods, such as those mimicking the physical and chemical conditions of the deep subsurface, and especially the combination of different methodologies have proven to be a useful approach in unraveling the way of microbial life in the depths. Partly the same fundamentals that define the habitability in the deep subsurface, dictate the potential of finding life outside Earth. Thus, studying the deep biosphere may help us to understand and explore the potential of microbial life to survive and thrive in extreme environments and therefore be useful for future space missions trying to find life on other planetary objects. The study can also help us draw conclusions on our own planet’s history and how life originated on early Earth.

Article

Lunar and Planetary Geology  

Alexander T. Basilevsky

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

Article

Lunar Exploration Missions and Environmental Discovery: Status and Progress  

Kyeong J. Kim

Exploration of the Moon is currently one of the most important and interesting subjects. The Moon is considered not only a place to explore but also a place to live in preparation to explore planets beyond it. This opportunity has arisen due to a series of discoveries associated with water on the Moon during the past half century. Lunar exploration of the moon began with the flyby mission by the United States in 1959. Since then, scientific investigations of the Moon have increased understanding of the lunar geology and surface environment. Based on more than 70 lunar missions to date, a major goal is to explore how humans can live on the Moon for a long period of time to examine sustainability on the Moon. Consequently, the area of lunar science and technology is being employed to discover how in situ resources can be utilized for humans to live on the Moon and, eventually, Mars and beyond.

Article

Magnetosphere–Ionosphere Coupling  

N. Achilleos, L. C. Ray, and J. N. Yates

The process of magnetosphere-ionosphere coupling involves the transport of vast quantities of energy and momentum between a magnetized planet and its space environment, or magnetosphere. This transport involves extended, global sheets of electrical current, which flows along magnetic field lines. Some of the charged particles, which carry this current rain down onto the planet’s upper atmosphere and excite aurorae–beautiful displays of light close to the magnetic poles, which are an important signature of the physics of the coupling process. The Earth, Jupiter, and Saturn all have magnetospheres, but the detailed physical origin of their auroral emissions differs from planet to planet. The Earth’s aurora is principally driven by the interaction of its magnetosphere with the upstream solar wind—a flow of plasma continually emanating from the Sun. This interaction imposes a particular pattern of flow on the plasma within the magnetosphere, which in turn determines the morphology and intensity of the currents and aurorae. Jupiter, on the other hand, is a giant rapid rotator, whose main auroral oval is thought to arise from the transport of angular momentum between the upper atmosphere and the rotating, disc-like plasma in the magnetosphere. Saturn exhibits auroral behavior consistent with a solar wind–related mechanism, but there is also regular variability in Saturn’s auroral emissions, which is consistent with rotating current systems that transport energy between the magnetospheric plasma and localized vortices of flow in the upper atmosphere/ionosphere.

Article

The Magnetosphere of Saturn  

Sarah Badman

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. Saturn’s magnetosphere is the region of space surrounding Saturn that is controlled by the planetary magnetic field. Saturn’s magnetic field is aligned to within 1 degree of the rotation axis and rotates with a period of ~10.7 h. The magnetosphere is compressed on the dayside by the impinging solar wind, and stretched into a long magnetotail on the nightside. Its surface, the magnetopause, is located where the internal and external plasma and magnetic pressures balance. As a result of the pressure distributions, the magnetopause has a bimodal distribution of standoff distance at the sub-solar point and is flattened over the poles relative to the equator. Radiation belts composed of trapped energetic electrons and protons are present in the inner magnetosphere. Their intensity is limited by the moons and rings that can absorb the energetic particles. The icy moons and rings, particularly the cryovolcanic moon Enceladus, are the main sources of mass in the form of water. When the water molecules are ionized they are confined to the equatorial plane by the rapidly rotating magnetic field. This mass-loading acts to distend the magnetic field lines from a dipolar configuration into a radially stretched magnetodisk, with an associated eastward-directed current. In situ measurements of plasma velocity indicate it generally lags behind the planetary rotation, introducing an azimuthal component of the magnetic field. Despite the alignment of the magnetic and rotation axes, so-called planetary period oscillations are ubiquitous in field and plasma measurements in the magnetosphere. Radial transport of plasma involves the centrifugal interchange instability in the inner magnetosphere and magnetic reconnection in the middle and outer magnetosphere. This allows mass from the moons and rings to be lost from the system. The outermost regions of the magnetosphere are also influenced by the surrounding solar wind through magnetic reconnection and viscous interactions. Acceleration via reconnection or other processes, or scattering of plasma into the atmosphere leads to auroral emissions detected at radio, infrared, visible, and ultraviolet wavelengths.

Article

The Magnetosphere of Uranus  

Xin Cao and Carol Paty

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. A magnetosphere is formed by the interaction between the magnetic field of a planet and the high-speed solar wind. Those planets with a magnetosphere have an intrinsic magnetic field such as Earth, Jupiter, and Saturn. Mars, especially, has no global magnetosphere, but evidence shows that a paleo-magnetosphere existed billions of years ago and was dampened then due to some reasons such as the change of internal activity. A magnetosphere is very important for the habitable environment of a planet because it provides the foremost and only protection for the planet from the energetic solar wind radiation. The majority of planets with a magnetosphere in our solar system have been studied for decades except for Uranus and Neptune, which are known as ice giant planets. This is because they are too far away from us (about 19 AU from the Sun), which means they are very difficult to directly detect. Compared to many other space detections to other planets, for example, Mars, Jupiter, Saturn and some of their moons, the only single fly-by measurement was made by the Voyager 2 spacecraft in the 1980s. The data it sent back to us showed that Uranus has a very unusual magnetosphere, which indicated that Uranus has a very large obliquity, which means its rotational axis is about 97.9° away from the north direction, with a relative rapid (17.24 hours) daily rotation. Besides, the magnetic axis is tilted 59° away from its rotational axis, and the magnetic dipole of the planet is off center, shifting 1/3 radii of Uranus toward its geometric south pole. Due to these special geometric and magnetic structures, Uranus has an extremely dynamic and asymmetric magnetosphere. Some remote observations revealed that the aurora emission from the surface of Uranus distributed at low latitude locations, which has rarely happened on other planets. Meanwhile, it indicated that solar wind plays a significant impact on the surface of Uranus even if the distance from the Sun is much farther than that of many other planets. A recent study, using numerical simulation, showed that Uranus has a “Switch-like” magnetosphere that allows its global magnetosphere to open and close periodically with the planetary rotation. In this article, we will review the historic studies of Uranus’s magnetosphere and then summarize the current progress in this field. Specifically, we will discuss the Voyager 2 spacecraft measurement, the ground-based and space-based observations such as Hubble Space Telescope, and the cutting-edge numerical simulations on it. We believe that the current progress provides important scientific context to boost future ice giant detection.