The polar regions of Mars contain layered ice deposits that are rich in detail of past periods of accumulation and erosion. These north and south polar layered deposits (NPLD and SPLD, respectively) contain primarily water–ice and ~5% and ~10% dust derived from the atmosphere, respectively. In addition, the SPLD has two known CO2 deposits—one thin unit at the surface and one buried, much thicker unit. Together, they comprise less than 1% of the SPLD volume. Mars also experiences seasonal deposits of CO2 that form in winter and sublimate in spring and early summer. These seasonal caps are visible from Earth and have been studied for centuries. Zooming in, exposed layers at the PLDs reveal histories of climate change that resulted when orbital parameters such as obliquity, eccentricity, and argument of perihelion changed over tens of thousands to millions of years. Simpler environmental conditions at the NPLD, especially related to seasonal and aeolian processes, make interpreting the history of that polar cap much easier than the SPLD. The history of Mars polar science is linked by numerous incremental advancements and unexpected discoveries related to the observed geology of both poles, the interpreted and modeled climatic conditions that gave rise to the PLDs, and the atmospheric conditions that modify the surface.
A Retrospective on Mars Polar Ice and Climate
Isaac B. Smith
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.
Alan E. Rubin and Chi Ma
Meteorites are rocks from outer space that reach the Earth; more than 60,000 have been collected. They are derived mainly from asteroids; a few hundred each are from the Moon and Mars; some micrometeorites derive from comets. By mid 2020, about 470 minerals had been identified in meteorites. In addition to having characteristic petrologic and geochemical properties, each meteorite group has a distinctive set of pre-terrestrial minerals that reflect the myriad processes that the meteorites and their components experienced. These processes include condensation in gaseous envelopes around evolved stars, crystallization in chondrule melts, crystallization in metallic cores, parent-body aqueous alteration, and shock metamorphism. Chondrites are the most abundant meteorites; the major components within them include chondrules, refractory inclusions, opaque assemblages, and fine-grained silicate-rich matrix material. The least-metamorphosed chondrites preserve minerals inherited from the solar nebula such as olivine, enstatite, metallic Fe-Ni, and refractory phases. Other minerals in chondrites formed on their parent asteroids during thermal metamorphism (such as chromite, plagioclase and phosphate), aqueous alteration (such as magnetite and phyllosilicates) and shock metamorphism (such as ringwoodite and majorite). Differentiated meteorites contain minerals formed by crystallization from magmas; these phases include olivine, orthopyroxene, Ca-plagioclase, Ca-pyroxene, metallic Fe-Ni and sulfide. Meteorites also contain minerals formed during passage through the Earth’s atmosphere and via terrestrial weathering after reaching the surface. Whereas some minerals form only by a single process (e.g., by high-pressure shock metamorphism or terrestrial weathering of a primary phase), other meteoritic minerals can form by several different processes, including condensation, crystallization from melts, thermal metamorphism, and aqueous alteration.
Migration of Low-Mass Planets
Frédéric S. Masset
Planet migration is the variation over time of a planet’s semimajor axis, leading to either a contraction or an expansion of the orbit. It results from the exchange of energy and angular momentum between the planet and the disk in which it is embedded during its formation and can cause the semimajor axis to change by as much as two orders of magnitude over the disk’s lifetime. The migration of forming protoplanets is an unavoidable process, and it is thought to be a key ingredient for understanding the variety of extrasolar planetary systems. Although migration occurs for protoplanets of all masses, its properties for low-mass planets (those having up to a few Earth masses) differ significantly from those for high-mass planets. The torque that is exerted by the disk on the planet is composed of different contributions. While migration was first thought to be invariably inward, physical processes that are able to halt or even reverse migration were later uncovered, leading to the realization that the migration path of a forming planet has a very sensitive dependence on the underlying disk parameters. There are other processes that go beyond the case of a single planet experiencing smooth migration under the disk’s tide. This is the case of planetary migration in low-viscosity disks, a fashionable research avenue because protoplanetary disks are thought to have very low viscosity, if any, over most of their planet-forming regions. Such a process is generally significantly chaotic and has to be tackled through high-resolution numerical simulations. The migration of several low-mass planets is also is a very fashionable topic, owing to the discovery by the Kepler mission of many multiple extrasolar planetary systems. The orbital properties of these systems suggest that at least some of them have experienced substantial migration. Although there have been many studies to account for the orbital properties of these systems, there is as yet no clear picture of the different processes that shaped them. Finally, some recently unveiled processes could be important for the migration of low-mass planets. One process is aero-resonant migration, in which a swarm of planetesimals subjected to aerodynamic drag push a planet inward when they reach a mean-motion resonance with the planet, while another process is based on so-called thermal torques, which arise when thermal diffusion in the disk is taken into account, or when the planet, heated by accretion, releases heat into the ambient gas.
Planet Formation Through Gravitational Instabilities
It is now widely accepted that planets form in discs around young stars, with the most widely accepted planet formation scenario being a bottom-up process typically referred to as “core accretion.” The basic process involves a core growing through the accumulation of solids and, if it gets massive enough while there is still gas present in the disc, undergoing a runaway gas accretion phase to form a Jupiter-like gas giant. However, early models of this process suggested that the formation timescale for a Jupiter-like gas giant exceeded the lifetime of the gas disc, suggesting that massive, gas giant planets form via some alternative process. One possibility is that they form via direct gravitational collapse. During the earliest stages of star formation, the disc around a young star can have a mass that is comparable to that of the central protostar and can be susceptible to the growth of a gravitational instability. One outcome of such an instability is that the disc fragments into bound objects that can then contract to become gas giant planets. This would happen very early in the star formation process and is very rapid, overcoming the timescale problem. Subsequent work has, however, both illustrated that core accretion may operate on timescales shorter than disc lifetimes and that disc fragmentation is very unlikely to operate in the inner parts of planet-forming discs. Hence, it is very unlikely that disc fragmentation plays a role in the direct formation of close-in exoplanets. However, disc fragmentation may operate at large orbital radii and is expected to preferentially form either massive gas giant planets or brown dwarfs. Therefore, it is intriguing that exactly such objects are starting to be directly imaged at orbital radii where disc fragmentation may operate. Additionally, even if a self-gravitating phase doesn’t play a direct role in the formation of gas giant planets, it may play an indirect role in the planet formation process. The spiral density waves that develop due to the gravitational instability can act to enhance the local density of solids, potentially accelerating their collisional growth or leading to the direct gravitational collapse of the solid component of the disc. This could then provide some of the building blocks for planets that later form via core accretion.
The Pluto−Charon System
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.
Records of Planetary Observations in Ancient Japan Before the 11th Century
Kiyotaka Tanikawa and Mitsuru Sôma
The records of planetary observations in Japan in the 7th century ad are treated separately from other records because they are written in the Nihongi. It is known that Japanese observational astronomy was recorded in the 7th century ad, but astronomy in Japan did not evolve straightforward in that century. There are thirty-one records that exist from that time, including four records on the Moon and planets. Correspondingly, a new interpretation of Japanese ancient history has been proposed. For the 8th, 9th, and 10th centuries, records have been compiled on the relative motion of the Moon and the planets, the motion of planets in the constellations, and stars seen in the daytime, as stated in Japanese recorded history. These records are written in Chinese, as in the case of the Nihongi, but have been translated into English. The orbits of the Moon and planets have been calculated using the NASA Jet Propulsion Laboratory (JPL) development ephemeris (DE) in order to confirm the validity of the records. The numbers of records and observations are not the same because one record may contain multiple observations. The accuracy of individual observations is discussed.
Larry W. Esposito
Saturn’s rings are not only a beautiful and enduring symbol of space, but astronomers’ best local laboratory for studying phenomena in thin cosmic disks like those where planets formed. All the giant planets have ring systems. Saturn’s are the biggest and brightest. Saturn’s rings are made of innumerable icy particles, ranging from the size of dust to that of football stadiums. Galileo discovered Saturn’s rings with his newly invented telescope, but they were not explained until Huygens described them as thin, flat disks surrounding the planet. In the space age, rings were found around the other giant planets in our solar system. Rings have been seen around asteroids and likely exist around exoplanets. Many of the ring structures seen are created by gravity from Saturn’s moons. Rings show both ongoing aggregation and disaggregation. After decades of study from space and by theoretical analysis, some puzzles still remain unexplained. There is evidence for youthful rings from Cassini results, but no good theory to explain their recent origin. A future Saturn Ring Observer mission would be able to determine the direct connections between the individual ring physical properties and the origin and evolution of larger structures.
Science and Exploration of the Moon: Overview
Bradley L. Jolliff
Earth’s moon, hereafter referred to as “the Moon,” has been an object of intense study since before the time of the Apollo and Luna missions to the lunar surface and associated sample returns. As a differentiated rocky body and as Earth’s companion in the solar system, much study has been given to aspects such as the Moon’s surface characteristics, composition, interior, geologic history, origin, and what it records about the early history of the Earth-Moon system and the evolution of differentiated rocky bodies in the solar system. Much of the Apollo and post-Apollo knowledge came from surface geologic exploration, remote sensing, and extensive studies of the lunar samples. After a hiatus of nearly two decades following the end of Apollo and Luna missions, a new era of lunar exploration began with a series of orbital missions, including missions designed to prepare the way for longer duration human use and further exploration of the Moon. Participation in these missions has become international. The more recent missions have provided global context and have investigated composition, mineralogy, topography, gravity, tectonics, thermal evolution of the interior, thermal and radiation environments at the surface, exosphere composition and phenomena, and characteristics of the poles with their permanently shaded cold-trap environments. New samples were recognized as a class of achondrite meteorites, shown through geochemical and mineralogical similarities to have originated on the Moon. New sample-based studies with ever-improving analytical techniques and approaches have also led to significant discoveries such as the determination of volatile contents, including intrinsic H contents of lunar minerals and glasses. The Moon preserves a record of the impact history of the solar system, and new developments in timing of events, sample based and model based, are leading to a new reckoning of planetary chronology and the events that occurred in the early solar system. The new data provide the grist to test models of formation of the Moon and its early differentiation, and its thermal and volcanic evolution. Thought to have been born of a giant impact into early Earth, new data are providing key constraints on timing and process. The new data are also being used to test hypotheses and work out details such as for the magma ocean concept, the possible existence of an early magnetic field generated by a core dynamo, the effects of intense asteroidal and cometary bombardment during the first 500 million–600 million years, sequestration of volatile compounds at the poles, volcanism through time, including new information about the youngest volcanism on the Moon, and the formation and degradation processes of impact craters, so well preserved on the Moon. The Moon is a natural laboratory and cornerstone for understanding many processes operating in the space environment of the Earth and Moon, now and in the past, and of the geologic processes that have affected the planets through time. The Moon is a destination for further human exploration and activity, including use of valuable resources in space. It behooves humanity to learn as much about Earth’s nearest neighbor in space as possible.
The Formation and Evolution of the Solar System
The formation and evolution of our solar system (and planetary systems around other stars) are among the most challenging and intriguing fields of modern science. As the product of a long history of cosmic matter evolution, this important branch of astrophysics is referred to as stellar-planetary cosmogony. Interdisciplinary by way of its content, it is based on fundamental theoretical concepts and available observational data on the processes of star formation. Modern observational data on stellar evolution, disc formation, and the discovery of extrasolar planets, as well as mechanical and cosmochemical properties of the solar system, place important constraints on the different scenarios developed, each supporting the basic cosmogony concept (as rooted in the Kant-Laplace hypothesis). Basically, the sequence of events includes fragmentation of an original interstellar molecular cloud, emergence of a primordial nebula, and accretion of a protoplanetary gas-dust disk around a parent star, followed by disk instability and break-up into primary solid bodies (planetesimals) and their collisional interactions, eventually forming a planet. Recent decades have seen major advances in the field, due to in-depth theoretical and experimental studies. Such advances have clarified a new scenario, which largely supports simultaneous stellar-planetary formation. Here, the collapse of a protosolar nebula’s inner core gives rise to fusion ignition and star birth with an accretion disc left behind: its continuing evolution resulting ultimately in protoplanets and planetary formation. Astronomical observations have allowed us to resolve in great detail the turbulent structure of gas-dust disks and their dynamics in regard to solar system origin. Indeed radio isotope dating of chondrite meteorite samples has charted the age and the chronology of key processes in the formation of the solar system. Significant progress also has been made in the theoretical study and computer modeling of protoplanetary accretion disk thermal regimes; evaporation/condensation of primordial particles depending on their radial distance, mechanisms of clustering, collisions, and dynamics. However, these breakthroughs are yet insufficient to resolve many problems intrinsically related to planetary cosmogony. Significant new questions also have been posed, which require answers. Of great importance are questions on how contemporary natural conditions appeared on solar system planets: specifically, why the three neighbor inner planets—Earth, Venus, and Mars—reveal different evolutionary paths.
Tidal Interactions Between Planets and Host Stars
Hundreds of planets are already known to have orbits only a few times wider than the stars that host them. The tidal interaction between a planet and its host star is one of the main agents shaping the observed distributions of properties of these systems. Tidal dissipation in the planet tends make the orbit circular, as well as synchronizing and aligning the planet’s spin with the orbit, and can significantly heat the planet, potentially affecting its size and structure. Dissipation in the star typically leads to inward orbital migration of the planet, accelerating the star’s rotation, and in some cases destroying the planet. Some essential features of tidal evolution can be understood from the basic principles that angular momentum and energy are exchanged between spin and orbit by means of a gravitational field and that energy is dissipated. For example, most short-period exoplanetary systems have too little angular momentum to reach a tidal equilibrium state. Theoretical studies aim to explain tidal dissipation quantitatively by solving the equations of fluid and solid mechanics in stars and planets undergoing periodic tidal forcing. The equilibrium tide is a nearly hydrostatic bulge that is carried around the body by a large-scale flow, which can be damped by convection or hydrodynamic instability, or by viscoelastic dissipation in solid regions of planets. The dynamical tide is an additional component that generally takes the form of internal waves restored by Coriolis and buoyancy forces in a rotating and stratified fluid body. It can lead to significant dissipation if the waves are amplified by resonance, are efficiently damped when they attain a very short wavelength, or break because they exceed a critical amplitude. Thermal tides are excited in a planetary atmosphere by the variable heating by the star’s radiation. They can oppose gravitational tides and prevent tidal locking, with consequences for the climate and habitability of the planet. Ongoing observations of transiting exoplanets provide information on the orbital periods and eccentricities as well as the obliquity (spin–orbit misalignment) of the star and the size of the planet. These data reveal several tidal processes at work and provide constraints on the efficiency of tidal dissipation in a variety of stars and planets.
Trans-Neptunian Dwarf Planets
Bryan J. Holler
The International Astronomical Union (IAU) officially recognizes five objects as dwarf planets: Ceres in the main asteroid belt between Mars and Jupiter, and Pluto, Eris, Haumea, and Makemake in the trans-Neptunian region beyond the orbit of Neptune. However, the definition used by the IAU applies to many other trans-Neptunian objects (TNOs) and can be summarized as follows: Any non-satellite large enough to be rounded by its own gravity. Practically speaking, this means any non-satellite with a diameter larger than 400 km. In the trans-Neptunian region, there are more than 150 objects that satisfy this definition, based on published results and diameter estimates. The dynamical structure of the trans-Neptunian region records the history of the migration of the giant planets in the early days of the solar system. The semi-major axes, eccentricities, and orbital inclinations of TNOs across various dynamical classes provide constraints on different aspects of planetary migration. For many TNOs, the orbital parameters are all that is known about them, due to their large distances, small sizes, and low albedos. The TNO dwarf planets are a different story. These objects are large enough to be studied in more detail from ground- and space-based observatories. Imaging observations can be used to detect satellites and measure surface colors, while spectroscopy can be used to constrain surface composition. In this way, TNO dwarf planets not only help provide context for the dynamical evolution of the outer solar system, but also reveal the composition of the primordial solar nebula as well as the physical and chemical processes at work at very cold temperatures. The largest TNO dwarf planets, those officially recognized by the IAU, plus others like Sedna, Quaoar, and Gonggong, are large enough to support volatile ices on their surfaces in the present day. These ices are able to exist as solids and gases on some TNOs, due to their sizes and surface temperatures (similar to water on Earth) and include N2 (nitrogen), CH4 (methane), and CO (carbon monoxide). A global atmosphere composed of these three species has been detected around Pluto, the largest TNO dwarf planet, with the possibility of local atmospheres or global atmospheres at perihelion for Eris and Makemake. The presence of non-volatile species, such as H2O (water), NH3 (ammonia), and complex hydrocarbons, provides valuable information on objects that may be too small to retain volatile ices over the age of the solar system. In particular, large quantities of H2O mixed with NH3 point to ancient cryovolcanism caused by internal differentiation of ice from rock. Complex hydrocarbons, formed through radiation processing of surface ices, such as CH4, record the radiation histories of these objects and provide clues to their primordial surface compositions. The dynamical, physical, and chemical diversity of the more than 150 TNO dwarf planets are key to understanding the formation of the solar system and its subsequent evolution to its current state. Most of our knowledge comes from a small handful of objects, but we are continually expanding our horizons as additional objects are studied in more detail.