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Long Xiao and James W. Head
The geological characteristics of the Moon provide the fundamental data that permit the study of the geological processes that have formed and modified the crust, that record the state and evolution of the lunar interior, and that identify the external processes that have been important in lunar evolution. Careful documentation of the stratigraphic relationships among these features can then be used to reconstruct the sequence of events and the geological history of the Moon. These results can then be placed in the context of the geological evolution of the terrestrial planets, including Earth. The Moon’s global topography and internal structures include landforms and features that comprise the geological characteristics of its surface. The Moon is dominated by the ancient cratered highlands and the relatively younger flat and smooth volcanic maria. Unlike the current geological characteristics of Earth, the major geological features of the Moon (impact craters and basins, lava flows and related features, and tectonic scarps and ridges) all formed predominantly in the first half of the solar system’s history. In contrast to the plate-tectonic dominated Earth, the Moon is composed of a single global lithospheric plate (a one-plate planet) that has preserved the record of planetary geological features from the earliest phases of planetary evolution. Exciting fundamental outstanding questions form the basis for the future international robotic and human exploration of the Moon.
Valery I. Shematovich and Dmitry V. Bisikalo
This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Planetary Science. Please check back later for the full article.
The uppermost layers of a planetary atmosphere, where the density of neutral particles is vanishingly low, are commonly called the exosphere or the planetary corona. Since the atmosphere is not completely bound to the planet by the planetary gravitational field, light atoms, such as hydrogen and helium with sufficiently large velocities, can escape from the upper atmosphere into interplanetary space. This process is commonly called Jeans escape, and it depends on the temperature of the ambient atmospheric gas at an altitude where the atmospheric gas is virtually collisionless. The heavier carbon, nitrogen, and oxygen atoms can escape from the atmospheres of the terrestrial planets only through non-thermal processes such as photo- and electron-impact dissociation, charge exchange, atmospheric sputtering, and ion pick-up. Theories of planetary exospheres have been based on ground-based and space observations of emission features such as the 121.6 nm Ly-α and 102.6 nm Ly-β hydrogen lines, the 58.4 nm helium line, and the 130.4 and 135.6 nm atomic oxygen lines. Such observations, together with in situ mass-spectrometer measurements, as at Titan, allow the density and temperature height profiles of the exospheric components to be constructed. The measurements reveal that planetary coronas contain both a fraction of thermal neutral particles with a mean kinetic energy corresponding to the exospheric temperature and a fraction of hot neutral particles with mean kinetic energy much higher than the exospheric temperature. These suprathermal (hot) atoms and molecules are a direct manifestation of the non-thermal processes taking place in the atmospheres. These hot particles lead to the atmospheric escape, determine the coronal structure, produce non-thermal emissions, and react with the ambient atmospheric gas triggering hot atom chemistry.
One of the brightest manifestations of these processes is a formation of hot oxygen corona around terrestrial planets. Oxygen atom is one of the lightest among heavy atmospheric species, so it is a best species to form corona, and another important aspect is that it produces a lot of observational evidence. The transport of suprathermal oxygen atoms to exospheric heights leads to the formation of hot oxygen coronas around Venus, Earth, and Mars. It has been well established by both observations and theoretical calculations that hot oxygen is an important constituent in the transition region between upper thermosphere and exosphere at terrestrial planets.
The study of the planetary coronas is based on direct observations and numerical simulations. It is a rarefied gas, therefore, production and transport of suprathermal particles into the corona requires solving a Boltzmann equation or a DSMC simulation. The stochastic simulation method had been widely used to investigate the formation, kinetics, and transport of suprathermal particles in the hot planetary coronas. This approach was first used to study the formation of the hot oxygen geocorona, taking into account the exothermic chemistry and the precipitation of magnetospheric protons and high-energy O+ ions from the ring current. It was found that only atmospheric sputtering results in the formation of the escape flux of energetic oxygen atoms, providing an important source of heavy atoms for the magnetosphere and geospace. A stochastic modeling approach was also applied to study the escape of hot oxygen atoms from the upper atmosphere of Mars and Venus; the kinetics and transport of suprathermal atoms and molecules in the hot oxygen corona at Jovian satellite Europa, which is an example of a highly non-equilibrium near-surface atmosphere; and the hot extended corona at Saturnian satellite Titan, which was directly measured by the spacecraft Cassini.
Emergence of ballistic missile technology after World War II enabled human flight into the Earth’s orbit, fueling the imagination of those fascinated with science, technology, exploration, and adventure. The performance of astronauts in the early flights assuaged concerns about the functioning of “the human system” in the absence of the Earth’s gravity. However, researchers in space medicine have observed degradation of crews after longer exposure to the space environment and have developed countermeasures for most of them, although significant challenges remain. With the dawn of the 21st century, well-financed and technically competent commercial entities have begun to provide more affordable alternatives to historically expensive and risk-averse government-funded programs. The growing accessibility to space has encouraged entrepreneurs to pursue plans for potentially autarkic communities beyond the Earth, exploiting natural resources on other worlds. Should such dreams prove to be technically and economically feasible, a new era will open for humanity with concomitant societal issues of a revolutionary nature.
This article consists of three sections. The first discusses how we determine satellite internal structures and what we know about them. The primary probes of internal structure are measurements of magnetic induction, gravity, and topography, as well as rotation state and orientation. Enceladus, Europa, Ganymede, Callisto, Titan, and (perhaps) Pluto all have subsurface oceans; Callisto and Titan may be only incompletely differentiated. The second section describes dynamical processes that affect satellite interiors and surfaces: tidal and radioactive heating, flexure and relaxation, convection, cryovolcanism, true polar wander, non-synchronous rotation, orbital evolution, and impacts. The final section discusses how the satellites formed and evolved. Ancient tidal heating episodes and subsequent refreezing of a subsurface ocean are the likeliest explanation for the deformation observed at Ganymede, Tethys, Dione, Rhea, Miranda, Ariel, and Titania. The high heat output of Enceladus is a consequence of Saturn’s highly dissipative interior, but the dissipation rate is strongly frequency-dependent and does not necessarily imply that Saturn’s moons are young. Major remaining questions include the origins of Titan’s atmosphere and high eccentricity, the regular density progression in the Galilean satellites, and the orbital evolution of the Saturnian and Uranian moons.
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.
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.
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.
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.
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.
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.
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.
This is an advance summary of a forthcoming article in the Oxford Encyclopedia of Planetary Science. Please check back later for the full article.
Space debris has grown to be a significant problem for outer space activities. The remnants of human activities in space are very diverse; they can be tiny paint flakes, all sorts of fragments, or entirely intact—but otherwise nonfunctional spacecraft and rocket bodies. The amount of debris is increasing at a growing pace, thus raising the risk of collision with operational satellites. Due to the relative high velocities involved in on-orbit collisions, their consequences are severe; collisions lead to significant damage or the complete destruction of the affected spacecraft. Protective measures and collision avoidance have thus become a major concern for spacecraft operators. The pollution of space with debris must, however, not only be seen as an unfavorable circumstance that accompanies space activities and increases the costs and complexity of outer space activities. Beyond this rather technical perspective, the presence of man-made, nonfunctional objects in space represents a global environmental concern. Similar to the patterns of other environmental problems on Earth, debris generation appears to have surpassed the absorption capacity of the space environment. Studies indicate that the evolution of the space object environment has crossed the tipping point to a runaway situation in which an increasing number of collisions―mostly among debris―leads to an uncontrolled population growth. It is thus in the interest of all mankind to address the debris problem in order to preserve the space environment for future generations.
International space law protects the space environment. Article IX of the Outer Space Treaty obligates States to avoid the harmful contamination of outer space. The provision corresponds to the obligation to protect the environment in areas beyond national jurisdiction under the customary “no harm” rule of general environmental law. These norms are applicable to space debris and establish the duty not to pollute outer space by limiting the generation of debris. They become all the more effective when the principles of sustainable development are taken into account, which infuse considerations of intra- as well as inter-generational justice into international law. In view of the growing debris pollution and its related detrimental effects, it is obvious that questions of liability and responsibility will become increasingly relevant. The Liability Convention offers a remedy for victims having suffered damage caused by space debris. The launching State liability that it establishes is even absolute for damage occurring on the surface of the Earth. The secondary rules of international responsibility law go beyond mere compensation: States can also be held accountable for the environmental pollution event itself, entailing a number of consequential obligations, among them―under certain circumstances―a duty to active debris removal. While international law is, therefore, generally effective in addressing the debris problem, growing use and growing risks necessitate the establishment of a comprehensive traffic management regime for outer space. It would strengthen the rule of law in outer space and ensure the sustainability of space utilization.
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.
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.
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.
Elvira Mulyukova and David Bercovici
All the rocky planets in our solar system, including the Earth, initially formed much hotter than their surroundings and have since been cooling to space for billions of years. The resulting heat released from planetary interiors powers convective flow in the mantle. The mantle is often the most voluminous and/or stiffest part of a planet and therefore acts as the bottleneck for heat transport, thus dictating the rate at which a planet cools. Mantle flow drives geological activity that modifies planetary surfaces through processes such as volcanism, orogenesis, and rifting. On Earth, the major convective currents in the mantle are identified as hot upwellings such as mantle plumes, cold sinking slabs, and the motion of tectonic plates at the surface. On other terrestrial planets in our solar system, mantle flow is mostly concealed beneath a rocky surface that remains stagnant for relatively long periods. Even though such planetary surfaces do not participate in convective circulation, they deform in response to the underlying mantle currents, forming geological features such as coronae, volcanic lava flows, and wrinkle ridges. Moreover, the exchange of material between the interior and surface, for example through melting and volcanism, is a consequence of mantle circulation and continuously modifies the composition of the mantle and the overlying crust. Mantle convection governs the geological activity and the thermal and chemical evolution of terrestrial planets and understanding the physical processes of convection helps us reconstruct histories of planets over billions of years after their formation.
The Martian ionosphere is a plasma embedded within the neutral upper atmosphere of the planet. Its main source is the ionization of the CO2-dominated Martian mesosphere and thermosphere by energetic EUV solar radiation. The ionosphere of Mars is subject to an important variability induced by changes in its forcing mechanisms (e.g., the UV solar flux) and by variations in the neutral atmosphere (e.g., the presence of global dust storms, atmospheric waves and tides, changes in atmospheric composition, etc.). Its vertical structure is dominated by a maximum in electron concentration at altitude about 120–140 km, coincident with the peak of the ionization rate. Below, there is a secondary peak produced by solar X-rays and photoelectron-impact ionization. A sporadic third layer, possibly of meteoric origin, has been also detected below.
The most abundant ion in the Martian ionosphere is O2 +, although O+ can become more abundant in the upper ionospheric layers. While below about 180–200 km the Martian ionosphere is dominated by photochemical processes, above those altitudes the dynamics of the plasma becomes more important. The ionosphere is also an important source of escaping particles via processes such as dissociative recombination of ions or ion pickup. So, characterization of the ionosphere provides or can provide information about such disparate systems and processes as solar radiation reaching the planet, the neutral atmosphere, meteoric influx, atmospheric escape to space, or the interaction of the planet with the solar wind.
It is thus not surprising that the interest about this region dates from the beginning of the space era. From the first measurements provided by the Mariner 4 mission in the 1960s to observations by the Mars Express and MAVEN orbiters in the 2010s, our knowledge of this atmospheric region is the consequence of the accumulation of more than 50 years of discontinuous measurements by different space missions. Numerical simulations by computational models able to simulate the processes that shape the ionosphere have also been commonly employed to obtain information about this region, to provide an interpretation of the observations and to fill their gaps. As a result, at the end of the 2010s the Martian ionosphere was the best known one after that of the Earth. However, there are still areas for which our knowledge is far from being complete. Examples are the details and balance of the mechanisms populating the nightside ionosphere, the origin and variability of the lower ionospheric peak, and the precise mechanisms shaping the topside ionosphere.
Kun Wang and Randy Korotev
For thousands of years, people living in Egypt, China, Greece, Rome, and other parts of the world have been fascinated by shooting stars, which are the light and sound phenomena commonly associated with meteorite impacts. The earliest written record of a meteorite fall is logged by Chinese chroniclers in 687
After 200 years, meteoritics (the science of meteorites) has grown out of its infancy and become a vibrant area of research today. The general directions of meteoritic studies are: (1) mineralogy, identifying new minerals or mineral phases that rarely or seldom found on the Earth; (2) petrology, studying the igneous and aqueous textures that give meteorites unique appearances, and providing information about geologic processes on the bodies upon which the meteorites originates; (3) geochemistry, characterizing their major, trace elemental, and isotopic compositions, and conducting interplanetary comparisons; and (4) chronology, dating the ages of the initial crystallization and later on impacting disturbances. Meteorites are the only extraterrestrial samples other than Apollo lunar rocks and Hayabusa asteroid samples that we can directly analyze in laboratories. Through the studies of meteorites, we have quested a vast amount of knowledge about the origin of the Solar System, the nature of the molecular cloud, the solar nebula, the nascent Sun and its planetary bodies including the Earth and its Moon, Mars, and many asteroids. In fact, the 4.6-billion-year age of the whole Solar System is solely defined by the oldest age dated in meteorites, which marked the beginning of everything we appreciate today.
Although the Inca state (ca. 1200–1572
The determination of the exact role of Venus and other planets in the Inca worldview encounters a serious limitation: in contrast to Mesoamerica, in Tahuantinsuyu and the Andes, there are no important “first-hand” sources such as the calendrical-astronomical data of the Maya or the Aztecs. Only Venus seems to have enjoyed a cult of Pan-American range. The morning appearance of Venus was apparently related to the puberty initiation rites of male adolescents, while its appearance as Evening Star seems to have been closely symbolically related to the Inca sovereign and his military activities. Putting aside the information available on Venus and its cult, there is an almost complete lack of data on the other planets.
Another problem must be considered: To what extent did the Incas inherit their knowledge from their predecessors, the Chimus, or even earlier cultures?
In ancient Mesopotamia, all five planets visible to the naked eye were known and studied, along with the Moon, the Sun, the stars, and other celestial phenomena. In all Mesopotamian sources concerning the Moon and the planets, be they textual or iconographical, the astronomical, astrological, and religious aspects are intertwined. The term “astral science” covers all forms of Mesopotamian scholarly engagement with celestial entities, including celestial divination and astrology. Modern research on Mesopotamian astral science began in the 19th century. Much research remains to be done, because important sources remain unpublished and new questions have been posed to published sources.
From ca. 3000