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.
Alan E. Rubin and Chi Ma
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.
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.