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Extraterrestrial Resources  

V.V. Shevchenko

Since the early 1990s, in analytical reviews, experts have increasingly been paying attention to the growing scarcity of rare and rare earth metals (REM) necessary for the development of advanced technologies in modern industry. The volume of the world market has increased over the past 50 years from 5,000 to 125,000 tons per year, which is explained by the extensive use of REM in the rapidly developing areas of industry associated with the advancement of high technology. Unique properties of REM are primarily used in the aerospace and other industrial sectors of the economy, and therefore are strategic materials. For example, platinum is an indispensable element that is used as a catalyst for chemical reactions. No battery can do without platinum. If all the millions of vehicles traveling along our roads installed hybrid batteries, all platinum reserves on Earth would end in the next 15 years! Consumers are interested in six elements known as the platinum group of metals (PGM): iridium (Ir), osmium (Os), palladium (palladium, Pd), rhodium (rhodium, Rh), ruthenium (ruthenium, Ru), and platinum itself. These elements, rare on the Earth, possess unique chemical and physical properties, which makes them vital industrial materials. To solve this problem, projects were proposed for the utilization of the substance of asteroids approaching the Earth. According to modern estimates, the number of known asteroids approaching the Earth reaches more than 9,000. Despite the difficulties of seizing, transporting, and further developing such an object in space, this way of solving the problem seemed technologically feasible and cost-effectively justified. A 10 m iron-nickel asteroid could contain up to 75 tons of rare metals and REM, primarily PGM, equivalent to a commercial price of about $2.8 billion in 2016 prices. However, the utilization of an asteroid substance entering the lunar surface can be technologically simpler and economically more cost-effective. Until now, it was believed that the lunar impact craters do not contain the rocks of the asteroids that formed them, since at high velocities the impactors evaporate during a collision with the lunar surface. According to the latest research, it turned out that at a fall rate of less than 12 km/s falling body (drummer) can partially survive in a mechanically fractured state. Consequently, the number of possible resources present on the lunar surface can be attributed to nickel, cobalt, platinum, and rare metals of asteroid origin. The calculations show that the total mass, for example, of platinum and platinoids on the lunar surface as a result of the fall of asteroids may amount more than 14 million tons. It should be noted that the world’s known reserves of platinum group metals on the Earth are about 80,000 tons.


Isotopic Dating  

Yuri Amelin

Isotopic dating is the measurement of time using the decay of radioactive isotopes and accumulation of decay products at a known rate. With isotopic chronometers, we determine the time of the processes that fractionate parent and daughter elements. Modern isotopic dating can resolve time intervals of ~1 million years over the entire lifespan of the Earth and the Solar System, and has even higher time resolution for the earliest and the most recent geological history. Using isotopic dates, we can build a unified scale of time for the evolution of Earth, the Moon, Mars, and asteroids, and expand it as samples from other planets become available for study. Modern geochronology and cosmochronology rely on isotopic dating methods that are based on decay of very long-lived radionuclides: 238U, 235U, 40K, 87Rb, 147Sm, etc. to stable radiogenic nuclides 206Pb, 207Pb, 40K, 40Ca, 87Sr, 143Nd, and moderately long-lived radionuclides: 26Al, 53Mn, 146Sm, 182Hf, to stable nuclides 26Mg, 53Cr, 142Nd, 182W. The diversity of physical and chemical properties of parent (radioactive) and daughter (radiogenic) nuclides, their geochemical and cosmochemical affinities, and the resulting diversity of processes that fractionate parent and daughter elements, allows direct isotopic dating of a vast range of earth and planetary processes. These processes include, but are not limited to evaporation and condensation, precipitation and dissolution, magmatism, metamorphism, metasomatism, sedimentation and diagenesis, ore formation, formation of planetary cores, crystallisation of magma oceans, and the timing of major impact events. Processes that cannot be dated directly, such as planetary accretion, can be bracketed between two datable events.


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.


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.


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.


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.”


The Formation and Evolution of the Solar System  

Mikhail Marov

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.