Nucleosynthetic isotope anomalies provide some of the most informative sample-based constraints on the origin of the Solar System. An isotopic anomaly is a deviation in isotopic ratio relative to a standard that is made from natural terrestrial materials. Nucleosynthetic isotope anomalies are small (i.e., part-per-million scale) stable isotopic anomalies that are found in meteorites and some planetary bodies which are caused by the heterogeneous distribution of stardust in the protoplanetary disk. These subtle isotopic differences provide constraints on the combination of stellar precurors whose stardust comprises some of the matter in the protoplanetary disk. These anomalies also constrain how well stardust was mixed in the disk during accretion. Furthermore, discoveries of subtle nucleosynthetic isotope anomalies in samples from the Earth’s mantle have opened the door to the possibility of using nucleosynthetic isotope anomalies to trace Earth’s precursor material. New insights into the evolution of the nascent Solar System have come from interpreting nucleosynthetic isotope anomalies in the context of numerical stellar nucleosynthetic models and disk evolution models. This research is based on a thus far omnipresent isotopic dichotomy, termed the “NC–CC isotopic dichotomy”, that is recorded in the nucleosynthetic isotope composition of meteorites. This isotopic dichotomy has been interpreted to indicate that within the first few million years of Solar System history, the disk separated into two portions. This separation inhibited material in the presumptive inner Solar System (termed noncarbonaceous reservoir, NC group) from mixing with outer Solar System material (termed carbonaceous chondrite reservoir, CC group). This strict compositional division in the disk may have lasted until giant planet migration, which occurred at the tail end of the disk’s lifetime (<10 Ma of Solar System formation). Despite this application of the NC-CC isotopic dichotomy, fundamental questions remain about what part of Solar System history it preserves and if it can be used to reconstruct the architecture of the nascent Solar System.
The applications of nucleosynthetic isotope anomalies in cosmochemistry and the insights these data provide into the evolution of the Solar System and Earth are discussed. The nucleosynthetic isotope anomalies that are recorded in bulk cosmochemical and terrestrial materials are summarized. The likely stellar origins of the presolar grains responsible for the isotopic anomalies on the bulk sample scale are distilled, along with the constraints these data place on the distribution of presolar material in the disk. A review of stellar nucleosynthesis, the formation of the Solar System, the conceptual framework used to interpret nucleosynthetic isotope data, and reported bulk sample nucleosynthetic isotope anomalies is first provided. Following this, a discussion on how nucleosynthetic isotope anomalies are used to constrain the early architecture of the Solar System is presented. To conclude, possible future directions that the scientific community may pursue by applying nucleosynthetic isotope anomalies to questions about terrestrial accretion are presented.
Article
Nucleosynthetic Isotope Anomalies in Cosmochemistry and Geochemistry
Katherine Bermingham and Brad Meyer
Article
Planetary Spectroscopy
Alian Wang
Planetary spectroscopy uses physical methods to study the chemical properties of the geological materials on the planetary bodies in our solar system. This article will present twelve types of spectroscopy frequently used in planetary explorations. Their energy (or wavelength) varies from γ-ray (keV) to far-infrared (μm), which involves the transitions of nuclei, atoms, ions, and molecules in planetary materials. The article will cover the basic concept of the transition for each of the twelve types of spectroscopy, along with their legendary science discoveries made during the past planetary exploration missions by the international planetary science and engineering community.
The broad application of spectroscopy in planetary exploration is built upon the fact that only limited extraterrestrial materials were collected (meteorites, cosmic dust, and the returned samples by missions) that enabled the detailed investigations of their properties in laboratories, while spectroscopic measurements can be made on the objects of our solar system remotely and robotically, such as during the flyby, orbiting, lander, and rover missions. In this sense, the knowledge obtained by planetary spectroscopy has contributed to a major portion of planetary sciences.
In the coming era of space explorations, more powerful spacecraft will be sent out by mankind, go to deep space, and explore exotic places. Generations of new planetary science payloads, including planetary spectrometers, will be created and will fly. New sciences will be revealed.
Article
The Recognition of Meteorites and Ice Ages
Alan E. Rubin
Two important scientific questions that confronted 18th- and 19th-century naturalists were whether continental glaciation had occurred thousands of years earlier and whether extraterrestrial rocks occasionally fell to Earth. Eventual recognition of these hypotheses as real phenomena resulted from initial reports by nonprofessionals, subsequent investigation by skeptical scientists, and vigorous debate. Evidence that kilometer-thick glaciers had once covered Northern Europe and Canada included (a) the resemblance of scratched and polished rocks near mountain glaciers to those located in unglaciated U-shaped valleys; (b) the similarity of poorly sorted rocks and debris within “drift deposits” (moraines) to the sediment load of glaciers; and (c) the discovery of freezing meltwater at the base of glaciers, hypothesized to facilitate their movement. Three main difficulties naturalists had with accepting the notion that rocks fell from the sky were that (a) meteorite falls are localized events, generally unwitnessed by professional scientists; (b) mixed in with reports of falling rocks were fabulous accounts of falling masses of blood, flesh, milk, gelatin, and other substances; and (c) the phenomenon of falling rocks could neither be predicted nor verified by experiment. Five advances leading to the acceptance of meteorites were (a) Ernst Chladni’s 1794 treatise linking meteors, fireballs, and falling rocks; (b) meteor observations conducted in 1798 showing the high altitudes and enormous velocities of their meteoroid progenitors; (c) a spate of several widely witnessed meteorite falls between 1794 and 1807 in Europe, India, and America; (d) chemical analyses of several meteorites by Edward Charles Howard in 1802, showing all contained nickel (which is rare in the Earth’s crust); and (e) the discoveries of four asteroids between 1801 and 1807, providing a plausible extraterrestrial source for meteorites.
Article
Martian Paleoclimate
Robert M. Haberle
The climate of Mars has evolved over time. Early in its history, between 3.7 and 4.1 billion years ago, the climate was warmer and wetter and the atmosphere thicker than it is today. Erosion rates were higher than today, and liquid water flowed on the planet’s surface, carving valley networks, filling lakes, creating deltas, and weathering rocks. This implies runoff and suggests rainfall and/or snowmelt. Oceans may have existed. Over time, the atmosphere thinned, erosion rates declined, water activity ceased, and cooler and drier conditions prevailed. Ice became the dominate form of surface water. Yet the climate continued to evolve, driven now by large variations in Mars’ orbit parameters. Beating in rhythm with these variations, surface ice has been repeatedly mobilized and moved around the planet, glaciers have advanced and retreated, dust storms and polar caps have come and gone, and the atmosphere has collapsed and re-inflated many times. The layered terrains that now characterize both polar regions are telltale signatures of this cyclical behavior and owe their existence to modulations of the seasonal cycles of dust, water, and CO2. Contrary to the early images from the Mariner flybys of the 1960s, Mars is and has been a dynamically active planet whose surface has been partly shaped through its interaction with a changing atmosphere and climate system.
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The Atmosphere of Uranus
Leigh N. Fletcher
Uranus provides a unique laboratory to test current understanding of planetary atmospheres under extreme conditions. Multi-spectral observations from Voyager, ground-based observatories, and space telescopes have revealed a delicately banded atmosphere punctuated by storms, waves, and dark vortices, evolving slowly under the seasonal influence of Uranus’s extreme axial tilt. Condensables like methane and hydrogen sulphide play a crucial role in shaping circulation, clouds, and storm phenomena via latent heat release through condensation, strong equator-to-pole gradients suggestive of equatorial upwelling and polar subsidence, and the formation of stabilizing layers that may decouple different circulation and convective regimes as a function of depth. Phase transitions in the watery depths may also decouple Uranus’s atmosphere from motions within the interior. Weak vertical mixing and low atmospheric temperatures associated with Uranus’s negligible internal heat means that stratospheric methane photochemistry occurs in a unique high-pressure regime, decoupled from the influx of external oxygen. The low homopause also allows for the formation of an extensive ionosphere. Finally, the atmosphere provides a window on the bulk composition of Uranus—the ice-to-rock ratio, supersolar elemental and isotopic enrichments inferred from remote sensing, and future in situ measurements—providing key insights into its formation and subsequent migration.
As a cold, hydrogen-dominated, intermediate-sized, slowly rotating, and chemically enriched world, Uranus could be the closest and best example of atmospheric processes on a class of worlds that may dominate the census of planets beyond our own solar system. Future missions to the Uranian system must carry a suite of instrumentation capable of advancing knowledge of the time-variable circulation, composition, meteorology, chemistry, and clouds on this enigmatic “ice giant.”
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Lunar Exploration Missions and Environmental Discovery: Status and Progress
Kyeong J. Kim
Exploration of the Moon is currently one of the most important and interesting subjects. The Moon is considered not only a place to explore but also a place to live in preparation to explore planets beyond it. This opportunity has arisen due to a series of discoveries associated with water on the Moon during the past half century. Lunar exploration of the moon began with the flyby mission by the United States in 1959. Since then, scientific investigations of the Moon have increased understanding of the lunar geology and surface environment. Based on more than 70 lunar missions to date, a major goal is to explore how humans can live on the Moon for a long period of time to examine sustainability on the Moon. Consequently, the area of lunar science and technology is being employed to discover how in situ resources can be utilized for humans to live on the Moon and, eventually, Mars and beyond.
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Formation, Composition, and Evolution of the Earth’s Core
Francis Nimmo
The Earth’s core formed by multiple collisions with differentiated protoplanets. The Hf-W (hafnium-tungsten) isotopic system reveals that these collisions took place over a timescale of tens of megayears (Myr), in agreement with accretion simulations. The degree to which the iron and silicates re-equilibrated during each collision is uncertain and affects the apparent core age derived from tungsten isotopic measurements. Seismological data reveal that the core contains light elements in addition to Fe-Ni, and the outer core is more enriched in such elements than the inner core. Because O is excluded efficiently from solid iron, O is almost certainly an important constituent of the outer core. The identity of other elements is less certain, despite intensive measurements of their effects on seismic velocities, densities, and partitioning behavior at appropriate pressures and temperatures. Si and O are very likely present, with perhaps some S; C and H are less likely. Si and Mg may have exsolved over time, potentially helping to drive the geodynamo and producing a low-density layer at the top of the core. Radioactive elements (U, Th, K) are unlikely to be present in important concentrations. The cooling of the core is controlled by the mantle’s ability to extract heat. The geodynamo has existed for at least 3.5 gigayears (Gyr), placing a lower bound on the heat flow out of the core. Because the thermal conductivity of the core is uncertain by a factor of ~3, the lower bound on this heat flow is similarly uncertain. Once the inner core started to crystallize, additional sources of energy were available to power the geodynamo. Inner core crystallization likely started in the time range 0.5 to 2.0 Gyr Before Present (BP); paleomagnetic arguments have been advanced for inner core growth starting at several different epochs within this time range.
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Planetary Atmospheres: Chemistry and Composition
Channon Visscher
The observed composition of a planetary atmosphere is the product of planetary formation and evolution, including the chemical and physical processes shaping atmospheric abundances into the present day. In the solar system, the gas giant planets Jupiter, Saturn, Uranus, and Neptune possess massive molecular envelopes consisting mostly of H2 and He along with various minor amounts of heavy elements such as C, N, and O (present as CH4, NH3, and H2O, respectively) and numerous additional minor species. The terrestrial planets Venus, Earth, and Mars each possess a relatively thin atmospheric envelope surrounding a rocky surface. The atmospheres of Mars and Venus are characterized by abundant CO2 with a small amount of N2, whereas the atmosphere of the Earth is dominated by N2 and O2. Such differences provide clues to the divergent pathways of atmospheric evolution.
Numerous closely coupled physical and chemical processes give rise to the abundances observed in the planetary atmospheres of the solar system. These processes include the maintenance of thermochemical equilibrium, reaction kinetics, atmospheric transport, photochemistry, condensation (including cloud formation) and vaporization, deposition and sublimation, diurnal and seasonal effects, greenhouse effects, surface–atmosphere reactions, volcanic activity, and (in the case of Earth) biogenic and anthropogenic sources. The present understanding of the chemical composition of planetary atmospheres is the result of over a century of observations, including ground-based, space-based, and in situ measurements of the major, minor, trace, and isotopic species found on each planet. These observations have been accompanied by experimental studies of planetary materials and the development of theoretical models to identify the key processes shaping atmospheric abundances observed today.
Article
Planetary Systems Around White Dwarfs
Dimitri Veras
White dwarf planetary science is a rapidly growing field of research featuring a diverse set of observations and theoretical explorations. Giant planets, minor planets, and debris discs have all been detected orbiting white dwarfs. The innards of broken-up minor planets are measured on an element-by-element basis, providing a unique probe of exoplanetary chemistry. Numerical simulations and analytical investigations trace the violent physical and dynamical history of these systems from astronomical unit (au)-scale distances to the immediate vicinity of the white dwarf, where minor planets are broken down into dust and gas and accrete onto the white dwarf photosphere. Current and upcoming ground-based and space-based instruments are likely to further accelerate the pace of discoveries.
Article
Cosmogenic Nuclides
Rainer Wieler
Cosmogenic nuclides are produced by the interaction of energetic elementary particles of galactic cosmic radiation (GCR) and their secondaries with atomic nuclei in extraterrestrial or terrestrial material. In extraterrestrial samples cosmogenic nuclides produced by energetic particles emitted by the Sun (SCR) are also detectable. Cosmogenic nuclides usually are observable only for noble gas isotopes, whose natural abundances in the targets of interest are exceedingly low, with some radioactive isotopes having half-lives mostly in the million-year range, and a few stable nuclides of elements such as Gd and Sm whose abundance is appreciably modified by reactions with low-energy secondary cosmic-ray neutrons. In solid matter, the mean attenuation length of GCR protons is on the order of 50 cm. Therefore, cosmogenic nuclides are a major tool to study 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, that is, the time they spent as a small body in interplanetary space. In some cases, the previous history of the future meteorite in its parent-body regolith can also be constrained. Such information helps to understand delivery mechanisms of meteorites from their parent asteroids (mainly from the main belt) or parent planets, and to constrain the number of ejection events responsible for the meteorites in collections worldwide. 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 million 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 its parent-body surfaces to space. First measurements of cosmogenic noble gas isotopes on the surface of Mars demonstrate that the exposure and erosional history of planetary bodies can be obtained by in situ analyses. Exposure ages of presolar grains in meteorites provide at present the only quantitative constraint of 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 later in its history, as expected from observations of young stars. The increasing precision of modern 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 pre-atmospheric 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, although this is not a topic of this article. The foremost example is 14C produced in the atmosphere and incorporated into organic material, which is used for dating. Cosmogenic radionucuclides 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.
