Is the Solar System unique, or are planets ubiquitous in the universe? The answer to this long-standing question implies the understanding of planet formation, but perhaps more relevant, the observational assessment of the existence of other worlds and their frequency in the galaxy.
The detection of planets orbiting other suns has always been a challenging task. Fortunately, technological progress together with significant development in data reduction and analysis processes allowed astronomers to finally succeed. The methods used so far are mostly based on indirect approaches, able to detect the influence of the planets on the stellar motion (dynamical methods) or the planet’s shadow as it crosses the stellar disk (transit method). For a growing number of favorable cases, direct imaging has also been successful. The combination of different methods also allowed probing planet interiors, composition, temperature, atmospheres, and orbital architecture. Overall, one can confidently state that planets are common around solar-type stars, low mass planets being the most frequent among them.
Despite all the progress, the discovery and characterization of temperate Earth-like worlds, similar to the Earth in both mass and composition and thus potential islands of life in the universe, is still a challenging task. Their low amplitude signals are difficult to detect and are often submerged by the noise produced by different instrumentation sources and astrophysical processes. However, the dawn of a new generation of ground and space-based instruments and missions is promising a new era in this domain.
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Nuno C. Santos, Susana C.C. Barros, Olivier D.S. Demangeon, and João P. Faria
Article
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.
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William K. Hartmann
The use of impact crater densities to estimate the ages of planetary surfaces began in the 1960s. Some predictive successes have been confirmed with radiometric dating of sites on the Moon and Mars. The method is highly dependent on our understanding of the rate of crater formation on different worlds, and, more importantly, on the history of that rate, starting with intense cratering during planetary formation 4.5 Ga ago. The system is thus calibrated by obtaining radiometric dates from samples of relatively homogeneous geologic units on various worlds.
Crater chronometry is still in its infancy. Future sample-returns and in situ measurements, obtained by missions from collaborating nations to various worlds, will provide ever-increasing improvements in the system in coming decades. Such data can lead to at least two-significant-figure measurements, not only of the ages of broad geologic provinces on solar system worlds, but of the characteristic survival times of various-sized smaller craters. Such data, in turn, clarify the rates of turnover of surface materials and the production rates of gravel-like regolith and megaregolith in the surface layers. Better measurements of the impact rate at various times, in turn, support better modeling of the accretion and fragmentation processes among early planetesimals as well as contemporary asteroids, in various parts of the solar system. Once the crater chronometry system is calibrated for various planetary bodies, important chronological information about those various planetary bodies can be obtained by orbital missions, without the need for expensive sample-return or lander missions on each individual surface.
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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.
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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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Francisco González-Galindo
The Martian ionosphere is a plasma embedded within the neutral upper atmosphere of the planet. Its main source is the ionization of the CO2-dominated Martian mesosphere and thermosphere by energetic EUV solar radiation. The ionosphere of Mars is subject to an important variability induced by changes in its forcing mechanisms (e.g., the UV solar flux) and by variations in the neutral atmosphere (e.g., the presence of global dust storms, atmospheric waves and tides, changes in atmospheric composition, etc.). Its vertical structure is dominated by a maximum in electron concentration at altitude about 120–140 km, coincident with the peak of the ionization rate. Below, there is a secondary peak produced by solar X-rays and photoelectron-impact ionization. A sporadic third layer, possibly of meteoric origin, has been also detected below.
The most abundant ion in the Martian ionosphere is O2
+, although O+ can become more abundant in the upper ionospheric layers. While below about 180–200 km the Martian ionosphere is dominated by photochemical processes, above those altitudes the dynamics of the plasma becomes more important. The ionosphere is also an important source of escaping particles via processes such as dissociative recombination of ions or ion pickup. So, characterization of the ionosphere provides or can provide information about such disparate systems and processes as solar radiation reaching the planet, the neutral atmosphere, meteoric influx, atmospheric escape to space, or the interaction of the planet with the solar wind.
It is thus not surprising that the interest about this region dates from the beginning of the space era. From the first measurements provided by the Mariner 4 mission in the 1960s to observations by the Mars Express and MAVEN orbiters in the 2010s, our knowledge of this atmospheric region is the consequence of the accumulation of more than 50 years of discontinuous measurements by different space missions. Numerical simulations by computational models able to simulate the processes that shape the ionosphere have also been commonly employed to obtain information about this region, to provide an interpretation of the observations and to fill their gaps. As a result, at the end of the 2010s the Martian ionosphere was the best known one after that of the Earth. However, there are still areas for which our knowledge is far from being complete. Examples are the details and balance of the mechanisms populating the nightside ionosphere, the origin and variability of the lower ionospheric peak, and the precise mechanisms shaping the topside ionosphere.
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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.
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Peter J. Mouginis-Mark and Lionel Wilson
More than 50 years of solar system exploration have revealed the great diversity of volcanic landscapes beyond Earth, be they formed by molten rock, liquid water, or other volatile species. Classic examples of giant shield volcanoes, solidified lava flows, extensive ash deposits, and volcanic vents can all be identified, but except for eruptions seen on the Jovian moon Io, no planetary volcanoes have been observed in eruption. Consequently, the details of the processes that created these landscapes must be inferred from the available spacecraft data. Despite the increasing improvement in the spatial, temporal, compositional, and topographic characteristics of the data for planetary volcanoes, details of the way they formed are not clear. However, terrestrial eruptions can provide numerous insights into planetary eruptions, whether they are effusive eruptions resulting in the emplacement of lava flows or explosive eruptions due to either volatiles in the magma or the interaction between hot lava and water or ice. In recent decades, growing attention has been placed on the use of terrestrial analogs to help interpret volcanic landforms and processes on the rocky planets (Mercury, Venus, the Moon, and Mars) and in the outer solar system (the moons of Jupiter and Saturn, and the larger asteroids). In addition, terrestrial analogs not only provide insights into the geologic processes associated with volcanism but also can serve as test sites for the development of instrumentation to be sent to other worlds, as well as provide a training ground for crewed and uncrewed missions seeking to better understand volcanism throughout the solar system.
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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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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.
