The study of active asteroids is a relatively new field of study in Solar System science, focusing on objects with asteroid-like orbits but that exhibit comet-like activity. This field, which crosses traditionally drawn lines between research focused on inactive asteroids and active comets, has motivated reevaluations of classical assumptions about small Solar System objects and presents exciting new opportunities for learning more about the origin and evolution of the Solar System. Active asteroids whose activity appears to be driven by the sublimation of volatile ices could have significant implications for determining the origin of the Earth’s water—and therefore its ability to support life—and also challenge traditional assumptions about the survivability of ice in the warm inner Solar System. Meanwhile, active asteroids whose activity appears to be caused by disruptive processes such as impacts or rotational destabilization provide exciting opportunities to gain insights into fundamental processes operating in the asteroid belt and assessing their effects on the asteroid population seen in the 21st century.
Leonid V. Ksanfomality
Cometary nuclei are small, despite the cosmic scale of the comet tails that they produce. The nuclei have the ability to create rarefied atmospheres, extending as a tail to giant distances comparable to the orbital distances of the planets. Giant tails of comets are sometimes observed for several years and cover a significant part of the sky. The cometary nucleus is capable of continuously renewing tails and supporting the material that is constantly dissipating in space. Large comets do not appear so often that they have become trivial celestial phenomena, but they appear often enough to allow astronomers to complete detailed studies. Many remarkable discoveries, such as the discovery of solar wind, were made during the study of comets. Comets are characterized by great diversity, and their appearance often becomes an ornament of the night sky. Comets have become remote laboratories, where experiments are performed in physical conditions that are not achievable on Earth.
Dmitry V. Bisikalo, Pavel V. Kaygorodov, and Valery I. Shematovich
The history of exoplanetary atmospheres studies is strongly based on the observations and investigations of the gaseous envelopes of hot Jupiters—exoplanet gas giants that have masses comparable to the mass of Jupiter and orbital semi-major axes shorter than 0.1 AU. The first exoplanet around a solar-type star was a hot Jupiter discovered in 1995. Researchers found an object that had completely atypical parameters compared to planets known in the solar system. According to their estimates, the object might have a mass about a half of the Jovian mass and a very short orbital period (four days), which means that it has an orbit roughly corresponding to the orbit of Mercury. Later, many similar objects were discovered near different stars, and they acquired a common name—hot Jupiters. It is still unclear what the mechanism is for their origin, because generally accepted theories of planetary evolution predict the formation of giant planets only at large orbital distances, where they can accrete enough matter before the protoplanetary disc disappears. If this is true, before arriving at such low orbits, hot Jupiters might have a long migration path, caused by interactions with other massive planets and/or with the gaseous disc. In favor of this model is the discovery of many hot Jupiters in elliptical and highly inclined orbits, but on the other hand several observed hot Jupiters have circular orbits with low inclination. An alternative hypothesis is that the cores of future hot Jupiters are super-Earths that may later intercept matter from the protoplanetary disk falling on the star.
The scientific interest in hot Jupiters has two aspects. The first is the peculiarity of these objects: they have no analogues in the solar system. The second is that, until recently, only for hot Jupiters was it possible to obtain observational characteristics of their atmospheres. Many of the known hot Jupiters are eclipsing their host stars, so, from their light curve and spectral data obtained during an eclipse, it became possible to obtain information about their shape and their atmospheric composition.
Thus it is possible to conclude that hot Jupiters are a common type of exoplanet, having no analogues in the solar system. Many aspects of their evolution and internal structure remain unclear. Being very close to their host stars, hot Jupiters must interact with the stellar wind and stellar magnetic field, as well as with stellar flares and coronal mass ejections, allowing researchers to gather information about them. According to UV observations, at least a fraction of hot Jupiters have extended gaseous envelopes, extending far beyond of their upper atmospheres. The envelopes are observable with current astronomical instruments, so it is possible to develop their astrophysical models. The history of hot Jupiter atmosphere studies during the past 20 years and the current status of modern theories describing the extended envelopes of hot Jupiters are excellent examples of the progress in understanding planetary atmospheres formation and evolution both in the solar system and in the extrasolar planetary systems.
Since the early 1990s, in analytical reviews, experts have increasingly been paying attention to the growing scarcity of rare and rare earth metals (REM) necessary for the development of advanced technologies in modern industry. The volume of the world market has increased over the past 50 years from 5,000 to 125,000 tons per year, which is explained by the extensive use of REM in the rapidly developing areas of industry associated with the advancement of high technology. Unique properties of REM are primarily used in the aerospace and other industrial sectors of the economy, and therefore are strategic materials. For example, platinum is an indispensable element that is used as a catalyst for chemical reactions. No battery can do without platinum. If all the millions of vehicles traveling along our roads installed hybrid batteries, all platinum reserves on Earth would end in the next 15 years! Consumers are interested in six elements known as the platinum group of metals (PGM): iridium (Ir), osmium (Os), palladium (palladium, Pd), rhodium (rhodium, Rh), ruthenium (ruthenium, Ru), and platinum itself. These elements, rare on the Earth, possess unique chemical and physical properties, which makes them vital industrial materials. To solve this problem, projects were proposed for the utilization of the substance of asteroids approaching the Earth. According to modern estimates, the number of known asteroids approaching the Earth reaches more than 9,000. Despite the difficulties of seizing, transporting, and further developing such an object in space, this way of solving the problem seemed technologically feasible and cost-effectively justified. A 10 m iron-nickel asteroid could contain up to 75 tons of rare metals and REM, primarily PGM, equivalent to a commercial price of about $2.8 billion in 2016 prices.
However, the utilization of an asteroid substance entering the lunar surface can be technologically simpler and economically more cost-effective. Until now, it was believed that the lunar impact craters do not contain the rocks of the asteroids that formed them, since at high velocities the impactors evaporate during a collision with the lunar surface. According to the latest research, it turned out that at a fall rate of less than 12 km/s falling body (drummer) can partially survive in a mechanically fractured state. Consequently, the number of possible resources present on the lunar surface can be attributed to nickel, cobalt, platinum, and rare metals of asteroid origin. The calculations show that the total mass, for example, of platinum and platinoids on the lunar surface as a result of the fall of asteroids may amount more than 14 million tons. It should be noted that the world’s known reserves of platinum group metals on the Earth are about 80,000 tons.
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.
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.
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.
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.
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.
Pluto orbits the Sun at a mean distance of 39.5 AU (astronomical units; 1 AU is the mean distance between the Earth and the Sun), with an orbital period of 248 Earth years. Its orbit is just eccentric enough to cross that of Neptune. They never collide thanks to a 2:3 mean-motion resonance: Pluto completes two orbits of the Sun for every three by Neptune. The Pluto system consists of Pluto and its large satellite Charon, plus four small satellites: Styx, Nix, Kerberos, and Hydra. Pluto and Charon are spherical bodies, with diameters of 2,377 and 1,212 km, respectively. They are tidally locked to one another such that each spins about its axis with the same 6.39-day period as their mutual orbit about their common barycenter. Pluto’s surface is dominated by frozen volatiles nitrogen, methane, and carbon monoxide. Their vapor pressure supports an atmosphere with multiple layers of photochemical hazes. Pluto’s equator is marked by a belt of dark red maculae, where the photochemical haze has accumulated over time. Some regions are ancient and cratered, while others are geologically active via processes including sublimation and condensation, glaciation, and eruption of material from the subsurface. The surfaces of the satellites are dominated by water ice. Charon has dark red polar stains produced from chemistry fed by Pluto’s escaping atmosphere.
The existence of a planet beyond Neptune had been postulated by Percival Lowell and William Pickering in the early 20th century to account for supposed clustering in comet aphelia and perturbations of the orbit of Uranus. Both lines of evidence turned out to be spurious, but they motivated a series of searches that culminated in Clyde Tombaugh’s discovery of Pluto in 1930 at the observatory Lowell had founded in Arizona. Over subsequent decades, basic facts about Pluto were hard-won through application of technological advances in astronomical instrumentation. During the progression from photographic plates through photoelectric photometers to digital array detectors, space-based telescopes, and ultimately, direct exploration by robotic spacecraft, each revealed more about Pluto. A key breakthrough came in 1978 with the discovery of Charon by Christy and Harrington. Charon’s orbit revealed the mass of the system. Observations of stellar occultations constrained the sizes of Pluto and Charon and enabled the detection of Pluto’s atmosphere in 1988. Spectroscopic instruments revealed Pluto’s volatile ices. In a series of mutual events from 1985 through 1990, Pluto and Charon alternated in passing in front of the other as seen from Earth. Observations of these events provided additional constraints on their sizes and albedo patterns and revealed their distinct compositions. The Hubble Space Telescope’s vantage above Earth’s atmosphere enabled further mapping of Pluto’s albedo patterns and the discovery of the small satellites. NASA’s New Horizons spacecraft flew through the system in 2015. Its instruments mapped the diversity and compositions of geological features on Pluto and Charon and provided detailed information on Pluto’s atmosphere and its interaction with the solar wind.
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.
Asteroids 1 Ceres and 4 Vesta are the two largest asteroids in the asteroid belt, with mean diameters of 946 km and 525 km, respectively. Ceres was reclassified as a dwarf planet by the IAU (International Astronomical Union) as a result of their new dwarf planet definition, which is a body that (a) orbits the sun, (b) has enough mass to assume a nearly round shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a moon. Our understanding of these two bodies has been revolutionized in the last decade by the success of the Dawn mission that visited both bodies. Vesta is an example of a small body that has been heated substantially, and differentiated into a metallic core, silicate mantle, and basaltic crust. Ceres is a volatile-rich rocky body that did not experience significant heating and therefore has only partially differentiated. These two contrasting bodies have been instrumental in learning how inner solar system material formed and evolved.