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  • Planetary Science x

Article

Alessandro Morbidelli

In planetary science, accretion is the process in which solids agglomerate to form larger and larger objects, and eventually planets are produced. The initial conditions are a disc of gas and microscopic solid particles, with a total mass of about 1% of the gas mass. These discs are routinely detected around young stars and are now imaged with the new generation of instruments. Accretion has to be effective and fast. Effective, because the original total mass in solids in the solar protoplanetary disk was probably of the order of ~300 Earth masses, and the mass incorporated into the planets is ~100 Earth masses. Fast, because the cores of the giant planets had to grow to tens of Earth masses to capture massive doses of hydrogen and helium from the disc before the dispersal of the latter, in a few millions of years. The surveys for extrasolar planets have shown that most stars have planets around them. Accretion is therefore not an oddity of the solar system. However, the final planetary systems are very different from each other, and typically very different from the solar system. Observations have shown that more than 50% of the stars have planets that don’t have analogues in the solar system. Therefore the solar system is not the typical specimen. Models of planet accretion have to explain not only how planets form, but also why the outcomes of the accretion history can be so diverse. There is probably not one accretion process but several, depending on the scale at which accretion operates. A first process is the sticking of microscopic dust into larger grains and pebbles. A second process is the formation of an intermediate class of objects called planetesimals. There are still planetesimals left in the solar system. They are the asteroids orbiting between the orbits of Mars and Jupiter, the trans-Neptunian objects in the distant system, and other objects trapped along the orbits of the planets (Trojans) or around the giant planets themselves (irregular satellites). The Oort cloud, source of the long period comets, is also made of planetesimals ejected from the region of formation of the giant planets. A third accretion process has to lead from planetesimals to planets. Actually, several processes can be involved in this step, from collisional coagulation among planetesimals to the accretion of small particles under the effect of gas drag, to giant impacts between protoplanets. Adopting a historical perspective of all these processes provides details of the classic processes investigated in the past decades to those unveiled in the last years. The quest for planet formation is ongoing. Open issues remain, and exciting future developments are expected.

Article

Henry Hsieh

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.

Article

The Agreement on the Rescue and Return of Astronauts and the Return of Objects Launched into Outer Space (ARRA) of 1968 deals with the obligation of states toward astronauts in distress or in emergency situations and with the obligation to return space objects. It is the second of the five United Nations space treaties, after the Outer Space Treaty (OST) of 1967 and before the Liability Convention (LIAB) of 1972. The historical development of ARRA and how this agreement reflects the needs and interests of the two important space-faring nations at the time of its entry into force, the United States and the Soviet Union, are important factors for understanding the space race. ARRA is related to the OST and regards the various obligations of states concerning rescue and assistance as well as the return of astronauts, which stand in the middle between a general humanitarian duty and political and national security considerations. The return of space objects and the question of costs of rescue and return operations are important concerns and can be compared to the situation with the law of the sea, the United Nations Convention on the Law of the Sea (UNCLOS) of 1982 and the Convention for the Unification of Certain Rules of Law Respecting Assistance and Salvage at Sea (Salvage Convention) of 1989. ARRA has never been applied with respect to accidents or distress of astronauts or cosmonauts but several times with respect to the recovering and returning of space objects. Finally, current challenges, such as the commercialization and privatization of outer space activities need to be addressed. This includes the increased interests of private individuals to enter outer space (so-called space tourism) and the question of the application of the ARRA to suborbital flights. Many legal challenges created by technological progress can be resolved via an evolving interpretation and application of the ARRA. Yet, some issues might warrant a new legal framework.

Article

James D. Burke and Erik M. Conway

The Jet Propulsion Laboratory (JPL) of the California Institute of Technology had its origins in a student project to develop rocket propulsion in the late 1930s. It attracted funding from the U.S. Army just prior to U.S. entry into World War II and became an Army missile research facility in 1943. Because of its origins as a contractor-operated Army research facility, JPL is the National Aeronautics and Space Administration’s (NASA) only contractor-operated field center. It remains a unit of the California Institute of Technology. In the decades since its founding, the laboratory, first under U.S. Army direction and then as a NASA field center, has grown and evolved into an internationally recognized institution generally seen as a leader in solar system exploration but whose portfolio includes substantial Earth remote sensing. JPL’s history includes episodes where the course of the laboratory’s development took turning points into new directions. After developing short-range ballistic missiles for the Army, the laboratory embarked on a new career in lunar and planetary exploration through the early 1970s and abandoned its original purpose as a propulsion technology laboratory. It developed the telecommunications infrastructure for planetary exploration too. It diversified into Earth science and astrophysics in the late 1970s and, due to a downturn in funding for planetary exploration, returned to significant amounts of defense work in the 1980s. The end of the Cold War between 1989 and 1991 resulted in a declining NASA budget, but support for planetary exploration actually improved within NASA management—as long as that exploration could be done more cheaply. This resulted in what is known as the “Faster Better Cheaper” period in NASA history. For JPL, this ended in 2000, succeeded by a return to more rigorous technical standards and increased costs.

Article

Sean McMahon

Astrobiology seeks to understand the origin, evolution, distribution, and future of life in the universe and thus to integrate biology with planetary science, astronomy, cosmology, and the other physical sciences. The discipline emerged in the late 20th century, partly in response to the development of space exploration programs in the United States, Russia, and elsewhere. Many astrobiologists are now involved in the search for life on Mars, Europa, Enceladus, and beyond. However, research in astrobiology does not presume the existence of extraterrestrial life, for which there is no compelling evidence; indeed, it includes the study of life on Earth in its astronomical and cosmic context. Moreover, the absence of observed life from all other planetary bodies requires a scientific explanation, and suggests several hypotheses amenable to further observational, theoretical, and experimental investigation under the aegis of astrobiology. Despite the apparent uniqueness of Earth’s biosphere— the “n = 1 problem”—astrobiology is increasingly driven by large quantities of data. Such data have been provided by the robotic exploration of the Solar System, the first observations of extrasolar planets, laboratory experiments into prebiotic chemistry, spectroscopic measurements of organic molecules in extraterrestrial environments, analytical advances in the biogeochemistry and paleobiology of very ancient rocks, surveys of Earth’s microbial diversity and ecology, and experiments to delimit the capacity of organisms to survive and thrive in extreme conditions.

Article

Astrology was a central feature of Greek and Roman culture. A knowledge of astrology’s claims, practices, and world view is essential for a full understanding of religion, politics, and science in the Greek and Roman worlds. Astrology is the name given to a series of diverse practices based in the idea that the stars, planets, and other celestial phenomena possess significance and meaning for events on Earth. It assumes a link between Earth and sky in which all existence, spiritual, psychological, and physical, is interconnected. Most premodern cultures practice a form of astrology. A particularly complex variety of it evolved in Mesopotamia in the first and second millennia bce from where it was imported into the Hellenistic world from the early 4th century bce onward. There it became attached to three philosophical schools, those pioneered by Plato, Aristotle and the Stoics, all of which shared the assumption that the cosmos is a single, living, integrated whole. Hellenistic astrology also drew on Egyptian temple culture, especially the belief that the soul could ascend to the stars. By the 1st century ce, the belief in the close link between humanity and the stars had become democratized and diversified into a series of practices and schools of thought which ranged across Greek and Roman culture. It was practiced at the imperial court and in the street. It could be used to predict individual destiny, avert undesirable events, and arrange auspicious moments to launch new enterprises. It could advise on financial fortunes or the condition of one’s soul. It was conceived of as natural science and justified by physical influences or considered to be divination, concerned with communication with the gods and goddesses. In some versions, the planets were neither influences nor causes of events on Earth, but timing devices, which indicated the ebb and flow of human affairs, like the hands on a modern clock. Astrology had a radical view of time in which the future already existed, at least in potential, and the astrologer’s task was to intercede in time, altering the future to human advantage. In this sense astrology was a form of “participation mystique” in which time and space were conceived of as a single entity and individual and social benefits were to be derived from engaging with it. There was no one single version of astrology and there were disputes about what it was and what it could do, for example, whether it could make precise predictions about individual affairs or merely general statements. From the early 4th century it went into a progressive decline, facing challenges from Christianity and the fragmentation of classical culture, especially in Western Europe. It survived in Persia, exerted a powerful influence on Indian astrology, and was transmitted to the Islamic world, from where it was reimported into the Latin West in the 12th century.

Article

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. Venus is a slowly rotating planet with a thick atmosphere (~9.2 MPa at the surface). The ground- and satellite-based observations have shown atmospheric superrotation (atmospheric rotation much faster than the solid surface rotation), cloud patterns (e.g., Y-shaped clouds), and polar vortices (polar dipole, cold collar, and fine structures). The Venusian atmospheric circulation, controlled by the planet’s radiative forcing and astronomical parameters, is quite different from the Earth’s one. Since the meteorological data have been accumulated, understanding of the atmospheric circulation has been gradually enriched with the help of theories of geophysical fluid dynamics and meteorology. Observations and fundamental dynamics of the atmospheric circulation are overviewed in this article. In the cloud layer (49-70 km altitude) far from the surface, planetary-scale brightness variations unique to Venus and thermally induced meridional winds have been observed, along with superrotational flows of >100 m/s. The fully developed superrotation ~60-times faster than the planetary rotation is maintained by meridional circulation and waves. Thermal tides, Rossby wave, Kelvin wave, and gravity wave play important roles in some promising mechanisms for maintaining the fast atmospheric rotation. In the lower atmosphere below the cloud layer, unlike the Earth, the general circulation is still unknown, because there is a lack of the global simultaneous observation. Thus, in addition to the limited observations, development of the atmospheric modeling and understanding of the fluid dynamics help to elucidate the atmospheric circulation system. Recently, general circulation models have simulated the dynamical and thermal structures of the Venus atmosphere. The recent advances and future perspectives are marshaled, along with the outstanding issues.

Article

Karen Aplin and Georg Fischer

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. Electricity occurs in atmospheres across the Solar System planets and beyond, spanning spectacular lightning displays in clouds of water or dust, to more subtle effects of charge and electric fields. On Earth, lightning is likely to have existed for a long time, based on evidence from fossilized lightning strikes in ancient rocks, but observations of planetary lightning are necessarily much more recent. The generation and observations of lightning and other atmospheric electrical processes, both from within-atmosphere measurements, and spacecraft remote sensing, can be readily studied using a comparative planetology approach, with Earth as a model. All atmospheres contain charged molecules, electrons, and/or molecular clusters created by ionization from cosmic rays and other processes, which may affect an atmosphere’s energy balance both through aerosol and cloud formation, and direct absorption of radiation. Several planets are anticipated to host a “global electric circuit” by analogy with the circuit occurring on Earth, where thunderstorms drive current of ions or electrons through weakly conductive parts of the atmosphere. This current flow may further modulate an atmosphere’s radiative properties through cloud and aerosol effects. Lightning could potentially have implications for life through its effects on atmospheric chemistry and particle transport. It has been observed on many of the Solar System planets (Earth, Jupiter, Saturn, Uranus, and Neptune) and it may also be present on Venus and Mars. On Earth, Jupiter, and Saturn, lightning is thought to be generated in deep water and ice clouds, but discharges can be generated in dust, as for terrestrial volcanic lightning, and on Mars. Other, less well-understood mechanisms causing discharges in non-water clouds also seem likely. The discovery of thousands of exoplanets has recently led to a range of further exotic possibilities for atmospheric electricity, though lightning detection beyond our Solar System remains a technical challenge to be solved.

Article

Cordula Steinkogler

The Austrian Outer Space Act, which entered into force in December 2011, and the Austrian Outer Space Regulation, which has been in force since February 2015, form the legal framework for Austrian national space activities. The elaboration of this national space legislation became necessary when the first two Austrian satellites were developed, to ensure compliance with Austria’s obligations as State Party to the five United Nations space treaties. The legislation comprehensively regulates legal aspects related to space activities, including the authorization, supervision, and termination of space activities; the registration of space objects; insurance requirements; and possibilities for recourse of the government against the operator. One of the main purposes of the law is to ensure the authorization of national space activities. The Outer Space Act sets forth the conditions for authorization, which, inter alia, refer to the expertise of the operator, requirements for orbital positions and frequency assignments, space debris mitigation, insurance requirements, and the safeguard of public order, public health, and national security, as well as of Austrian foreign policy interests and international law obligations. The Austrian Outer Space Regulation complements these provisions by specifying the documents the operator must submit as evidence of the fulfillment of the authorization conditions, which include the results of safety tests, emergency plans, and information on the collection and use of Earth observation data. Particular importance is attached to the mitigation of space debris. Operators are required to take measures in accordance with international space debris mitigation guidelines for the avoidance of operational debris, the prevention of on-orbit breakups and collisions, and the removal of space objects from Earth orbit after the end of the mission. Another specificity of the Austrian space legislation is the possibility of an exemption from the insurance requirement or a reduction of the insurance sum if the space activity is in the public interest. This allows the support of space activities that serve science, research, and education. Moreover, the law also provides for the establishment of a national registry for objects launched into outer space by the competent Austrian ministry.

Article

Olavo de O. Bittencourt Neto and Daniel Freire e Almeida

The article provides an overview of the Brazilian legal framework for space-related activities, highlighting the main legal instruments and their most relevant provisions. Domestic regulatory initiatives are appraised and contextualized through the review of specific provisions and legal instruments. The Brazilian space program’s normative structure is acknowledged, considering national space policy and applicable legislation. Brazil regulates national space activities through a myriad of regulations and edicts, forming a broad—although fragmented—body of rules. Considered an emerging space power, Brazil has a long-standing and ambitious space program, involving artificial satellites, launch centers, and the eventual development of a national launch vehicle. However, a domestic, general space law, as required by the Federal Constitution of 1988, still awaits to be enacted. The latest developments at the Brazilian Space Agency indicate that it might not be too long for such a federal law to materialize. The importance of a national space law for the implementation of international obligations as well as to ensure legal certainty for governmental and non-governmental national space activities is increasingly realized by space-faring nations. The Brazilian space legal framework represents a relevant case study toward the identification of appropriate legal mechanisms for the regulation of national space activities, taking into account international principles and local perspectives.

Article

Throughout the history of human activity in outer space, the role of private companies has steadily grown, and, in some cases, companies have even replaced government agencies as the primary actors in space. As private space activity has grown and diversified, the laws and regulations that govern private actors have been forced to evolve in reaction to the new realities of the industry. On the international level, the treaties concluded in the 1960s and 1970s continue to be in force today. However, these treaties only govern state activity in space. The rules regulating private industry are necessarily domestic in nature, and it is in these domestic laws that the evolution of space law can be most clearly seen. That said, new industries, such as asteroid mining, are testing the limits of international law and have forced the international community to examine whether changes to long-standing laws are needed.

Article

Olga Popova

The asteroid impact near the Russian city of Chelyabinsk on February 15, 2013, was the largest airburst on Earth since the 1908 Tunguska event, causing a natural disaster in an area with a population exceeding 1 million. On clear morning at 9:20 a.m. local time, an asteroid about 19 m in size entered the Earth atmosphere near southern Ural Mountains (Russia) and, with its bright illumination, attracted the attention of hundreds of thousands of people. Dust trail in the atmosphere after the bolide was tens of kilometers long and was visible for several hours. Thousands of different size meteorites were found in the areas south-southwest of Chelyabinsk. A powerful airburst, which was formed due to meteoroid energy deposition, shattered thousands of windows and doors in Chelyabinsk and wide surroundings, with flying glass injuring many residents. The entrance and destruction of the 500-kt Chelyabinsk asteroid produced a number of observable effects, including light and thermal radiation; acoustic, infrasound, blast, and seismic waves; and release of interplanetary substance. This unexpected and unusual event is the most well-documented bolide airburst, and it attracted worldwide attention. The airburst was observed globally by multiple instruments. Analyses of the observational data allowed determination of the size of the body that caused the superbolide, its velocity, its trajectory, its behavior in the atmosphere, the strength of the blast wave, and other characteristics. The entry of the 19-m-diameter Chelyabinsk asteroid provides a unique opportunity to calibrate the different approaches used to model meteoroid entry and to calculate the damaging effects. The recovered meteorite material was characterized as brecciated LL5 ordinary chondrite, in which three different lithologies can be distinguished (light-colored, dark-colored, and impact-melt). The structure and properties of meteorites demonstrate that before encountering Earth, the Chelyabinsk asteroid had experienced a very complex history involving at least a few impacts with other bodies and thermal metamorphism. The Chelyabinsk airburst of February 15, 2013, was exceptional because of the large kinetic energy of the impacting body and the damaging airburst that was generated. Before the event, decameter-sized objects were considered to be safe. With the Chelyabinsk event, it is possible, for the first time, to link the damage from an impact event to a well-determined impact energy in order to assess the future hazards of asteroids to lives and property.

Article

Mikhail Zolotov

Chemical and phase compositions of the surface of Venus could reflect a history of gas–rock and fluid–rock interactions, recent and past climate changes, and a loss of water from the Earth’s sister planet. The concept of chemical weathering on Venus through gas–solid type reactions was established in the early 1960s after the discovery of the hot and dense CO2-rich atmosphere of the planet, inferred from Earth-based and Mariner 2 radio emission data. Initial models suggested carbonation, hydration, and oxidation of exposed igneous rocks and a control (buffering) of atmospheric gases by solid–gas type chemical equilibria in the near-surface rocks. Carbonates, phyllosilicates and Fe oxides were considered likely secondary minerals. From the late 1970s onward, measurements of trace gases in the sub-cloud atmosphere by the Pioneer Venus and Venera entry probes and by Earth-based infrared spectroscopy challenged the likelihood of hydration and carbonation. The atmospheric H2O gas content appeared to be low enough to allow the stable existence of H2O-bearing and a majority of OH-bearing minerals. The concentration of SO2 gas was too high to allow the stability of Ca-rich carbonates and silicates with respect to sulfatization to CaSO4. In the 1980s, the detection of an elevated bulk S content at the Venera and Vega landing sites suggested ongoing consumption of atmospheric SO2 to surface sulfates. The supposed composition of the near-surface atmosphere implied oxidation of ferrous minerals to Fe oxides, magnetite and hematite, consistent with the infrared reflectance of surface materials. The likelihood of sulfatization and oxidation has been illustrated in modeling experiments in simulated Venus’ conditions. The morphology of Venus’ surface suggests contact of atmospheric gases with hot surface materials of mainly basaltic composition during the several hundreds of millions years since a global volcanic/tectonic resurfacing. Some exposed materials could have reacted at higher and lower temperatures in a presence of diverse gases at different altitudinal, volcanic, impact, and atmospheric settings. On highly deformed tessera terrains, more ancient rocks of unknown composition may reflect interactions with putative water-rich atmospheres and even aqueous solutions. Geological formations rich in salt, carbonate, Fe oxide, or silica will indicate past aqueous processes. The apparent diversity of affected solids, surface temperatures, pressures, and gas/fluid compositions throughout Venus’ history implies multiple signs of chemical alterations that remain to be investigated. The current understanding of chemical weathering is limited by the uncertain composition of the deep atmosphere, by the lack of direct data on the phase and chemical composition of surface materials, and by the uncertain data on thermodynamics of minerals and their solid solutions. In preparation for further atmospheric entry probe and lander missions, rock alteration could be investigated through chemical kinetic experiments and calculations of solid-gas/fluid equilibria to constrain past and present processes.

Article

A. Määttänen and F. Montmessin

Although resembling an extremely dry desert, planet Mars hosts clouds in its atmosphere. Every day somewhere on the planet a part of the tiny amount of water vapor held by the atmosphere can condense as ice crystals to form mainly cirrus-type clouds. The existence of water ice clouds has been known for a long time and they have been studied for decades, leading to the establishment of a well-known climatology and understanding on their formation and properties. Despite their thinness, they have a clear impact on the atmospheric temperatures, thus affecting the Martian climate. Another, more exotic type of clouds forms as well on Mars. The atmospheric temperatures can plunge to such frigid values that the major gaseous component of the atmosphere, CO2, condenses as ice crystals. These clouds form in the cold polar night where they also contribute to the formation of the CO2 ice polar cap, and also in the mesosphere at very high altitudes, near the edge of space, analogously to the noctilucent clouds on Earth. The mesospheric clouds, discovered in the early 2000s, have put our understanding of the Martian atmosphere to a test. On Mars, cloud crystals form on ice nuclei, mostly provided by the omnipresent mineral dust. Thus, the clouds link the three major climatic cycles: those of the two major volatiles, H2O and CO2, and that of dust, which is a major climatic agent itself.

Article

Comets  

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.

Article

The atmosphere of Venus is quite different from that of Earth: it is much hotter and denser. The temperature and pressure at the surface are 740 K and 92 atmospheres respectively. Its atmosphere is primarily composed of carbon dioxide (96.5%) and nitrogen (3.5%), the rest being trace gases such as carbon monoxide (CO), water vapor (H2O), halides (HF, HCl), sulfur-bearing species (SO2, SO, OCS, H2S), and noble gases. Sulfur compounds are extremely important in understanding the formation of the Venusian clouds which are believed to be composed of sulfuric acid (H2SO4) droplets. These clouds completely enshroud the planet in a series of layers, extending from 50 to 70 km altitude, and are composed of particles of different sizes and different H2SO4/H2O compositions. These act as a very effective separator between the atmospheres below and above the clouds, which show very distinctive characteristics.

Article

H. Palme

Early models of the composition of the Earth relied heavily on meteorites. In all these models Earth had different layers, each layer corresponded to a different type of meteorite or meteorite component. Later, more realistic models based on analyses of samples from Earth began with Ringwood’s pyrolite composition in the 1960s. Further improvement came with the analyses of rare MgO rich peridotites from a variety of occurrences all over the Earth, as xenoliths enclosed in melts from the upper mantle or as ultramafic massifs, tectonically emplaced on the Earth’s surface. Chemical systematics of these rocks allow the determination of the major element composition of the primitive upper mantle (PUM), the upper mantle after core formation and before extraction of basalts ultimately leading to the formation of the crust. Trace element analyses of upper mantle rocks confirmed their primitive nature. Geochemical and geophysical evidence argue for a bulk Earth mantle of uniform composition, identical to the PUM, also designated as “bulk silicate Earth” (BSE). The formation of a metal core was accompanied by the removal of siderophile and chalcophile elements into the core. Detailed modeling suggests that core formation was an ongoing process parallel to the accretion of Earth. The composition of the core is model dependent and thus uncertain and makes reliable estimates for siderophile and chalcophile element concentrations of bulk Earth difficult. Improved stable isotope analyses show isotopic similarities with noncarbonaceous chondrites (NCC), while the chemical composition of the mantle of the Earth indicates similarities with carbonaceous chondrites (CC). In detail, however, it can be shown that no single known meteorite group, nor any mixture of meteorite groups can match the chemical and isotopic composition of Earth. This conclusion is extremely important for any formation model of the Earth.

Article

The Sun’s chemical and isotopic composition records the composition of the solar nebula from which the planets formed. If a piece of the Sun is cooled to 1,000 K at 1 mbar total pressure, a mineral assemblage is produced that is consistent with the minerals found in the least equilibrated (most chemically heterogeneous), oldest, and compositionally Sunlike (chondritic), hence most “primitive,” meteorites. This is an equilibrium or fractional condensation experiment. The result can be simulated by calculations using equations of state for hundreds of gaseous molecules, condensed mineral solids, and silicate liquids, the products of a century of experimental measurements and recent theoretical studies. Such calculations have revolutionized our understanding of the chemistry of the cosmos. The mid-20th century realization that meteorites are fossil records of the early solar system made chemistry central to understanding the origin of the Earth, Moon, and other bodies. Thus “condensation,” more generally the distribution of elements and isotopes between vapor and condensed solids and/or liquids at or approaching chemical equilibrium, came to deeply inform discussion of how meteoritic and cometary compositions bear on the origins of atmospheres and oceans and the differences in composition among the planets. This expansion of thinking has had profound effects upon our thinking about the origin and evolution of Earth and the other worlds of our solar system. Condensation calculations have also been more broadly applied to protoplanetary disks around young stars, to the mineral “rain” of mineral grains expected to form in cool dwarf star atmospheres, to the expanding circumstellar envelopes of giant stars, to the vapor plumes expected to form in giant planetary impacts, and to the chemically and isotopically distinct “shells” computed and observed to exist in supernovae. The beauty of equilibrium condensation calculations is that the distribution of elements between gaseous molecules, solids, and liquids is fixed by temperature, total pressure, and the overall elemental composition of the system. As with all sophisticated calculations, there are inherent caveats, subtleties, and computational difficulties. In particular, local equilibrium chemistry has yet to be consistently integrated into gridded, dynamical astrophysical simulations so that effects like the blocking of light and heat by grains (opacity), absorption and re-emission of light by grains (radiative transfer), and buffering of heat by grain evaporation/condensation are fed back into the physics at each node or instance of a gridded calculation over time. A deeper integration of thermochemical computations of chemistry with physical models makes the prospect of a general protoplanetary disk model as hopeful in the 2020s as a general circulation model for global climate may have been in the early 1970s.

Article

Rainer Wieler

Cosmogenic nuclides are produced by the interaction of energetic elementary particles of galactic (or solar) cosmic radiation and their secondaries with atomic nuclei in extraterrestrial or terrestrial material. Cosmogenic nuclides usually are observable only for some noble gas isotopes, whose natural abundances in the targets of interest are exceedingly low; some radioactive isotopes with half-lives mostly in the million-year range; and a few stable nuclides of elements, such as Gd and Sm, whose abundance is sizably modified by reactions with low energy secondary cosmic ray neutrons. In solid matter, the mean attenuation length of galactic cosmic ray protons is on the order of 50 cm. Therefore, cosmogenic nuclides are a major tool in studying the history of small objects in space and of matter near the surfaces of larger parent bodies. A classical application is to measure “exposure ages” of meteorites, namely the time they spent as a small body in interplanetary space. In some cases, also the previous history of the future meteorite in its parent-body regolith can be constrained. Such information helps to understand delivery mechanisms of meteorites from their parent asteroids or parent planets and to constrain the number of ejection events responsible for the collected meteorites. Cosmogenic nuclides in lunar samples from known depths of up to ~2 m serve to study the deposition and mixing history of the lunar regolith over hundreds of millions of years, as well as to calibrate nuclide production models. Present and future sample return missions rely on cosmogenic nuclide measurements as important tools to constrain the sample’s exposure history or loss rates of their parent body surfaces to space. The first data from cosmogenic noble gas isotopes measured on the surface of Mars demonstrate that the exposure and erosional history of planetary bodies can be obtained by in-situ analyses. For the foreseeable future, exposure ages of presolar grains in meteorites are presumably the only means to quantitatively constrain their presolar history. In some cases, irradiation effects of energetic particles from the early sun can be detected in early solar system condensates, confirming that the early sun was likely much more active than today, as expected from observations of young stars. The ever-increasing precision of isotope analyses also reveals tiny isotopic anomalies induced by cosmic-ray effects in several elements of interest in cosmochemistry, which need to be recognized and corrected for. Cosmogenic nuclide studies rely on the knowledge of their production rates, which depend on the elemental composition of a sample and its “shielding” during irradiation, that is, its position within an irradiated object and for meteorites their preatmospheric size. The physics of cosmogenic nuclide production is basically well understood and has led to sophisticated production models. They are most successful if a sample’s shielding can be constrained by the analyses of several cosmogenic nuclides with different depth dependencies of their production rates. Cosmogenic nuclides are also an important tool in Earth Sciences. The foremost example is 14C produced in the atmosphere and incorporated into organic material, which is used for dating. Cosmogenic radionuclides and noble gases produced in-situ in near surface samples, mostly by secondary cosmic-ray neutrons, are an important tool in quantitative geomorphology and related fields.

Article

Boris Ivanov

Impacts of small celestial bodies, in terms of energy density, occupy the range between ordinary chemical high explosives and nuclear explosions. The high initial energy density of impact gives them some features of an explosion (shock waves, melting and vaporization, mechanical disruption of target rocks). A near-surface burst creates an explosion crater, and an impact often results in the creation of an impact crater. The chain of processes connected to an impact crater’s formation is named “impact cratering” or simply “cratering.” The initial kinetic energy and momenta of the impacting body (“projectile”) generates shock waves (decaying with propagation to seismic waves), heats the material (at high impact velocities, to melt or to boil target rocks). A part of the kinetic energy is converted to target material motion, creating the crater cavity. The final crater geometry depends on the scale of event—while small craters are simple bowl-shaped cavities, large enough crater transient cavities collapse in the gravity field. If collapse takes place, the final crater has a complex geometry with central peaks and concentric inner rings. The boundary crater diameter, dividing simple and complex craters, varies with target body gravity and rock strength. Comparison of a crater’s morphology on remote planets and asteroids allows us to make some estimates about their mechanical parameters (e.g., strength and friction) even before future sample return missions. On many planets large impact craters can be seen, preserved much better than on the geologically active Earth. These observations help researchers to interpret the geological and geophysical data obtained for the relatively few and heavily modified large impact craters found on continents and (rarely) at the sea bottom.