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.
Composition of Earth
Large Volcanic Channels of the Inner Solar System
David W. Leverington
Many large volcanic channel systems are recognized at the surfaces of rocky bodies of the inner solar system. The more than 200 channels known for the Moon mainly have simple sinuous forms with widths of up to several kilometers and lengths of up to hundreds of kilometers, typically commencing at topographic depressions and extending downslope until they fade into associated volcanic units. The Rima Hadley system was a key target of the Apollo 15 mission and was confirmed as a product of volcanic processes related to the emplacement of lavas in the Palus Putredinis region of Mare Imbrium. The more than 200 channels known for Venus are in many cases morphologically similar to sinuous lunar rilles, but some systems are especially large and complex, with widths of up to tens of kilometers and lengths that can exceed 1,000 km. Such systems typically commence at structural features or in regions of disturbed terrain and possess anastomosing reaches associated with prominent streamlined uplands. In contrast, Venusian canali typically maintain sinuous forms with widths of only a few kilometers but remarkably can be characterized by lengths of thousands of kilometers. Some Venusian channels were involved in the emplacement of fluidized ejecta in the vicinities of impact craters whereas others may have formed in such environments as a result of later volcanic events. The 10 large volcanic channels that are recognized on Mercury have lengths no greater than ~161 km but can have widths of up to several tens of kilometers. These systems developed as conduits for voluminous lavas that extend across adjacent impact basins. Terrestrial komatiitic channels of Archean and Proterozoic ages can have sizes that are comparable to those of lunar rilles, and the formation of these systems is likely to have played an important role in the development of associated Ni-Cu-(PGE) ores. The outflow channels of Mars have widths of up to tens of kilometers and lengths of up to thousands of kilometers and are widely interpreted as aqueous systems formed by catastrophic discharges from aquifers, but the properties and geological associations of these features and numerous other large Martian channel systems are arguably well aligned with those expected of volcanic origins. Overall, large volcanic channels of the inner solar system are mainly ancient products of the emplacement of low-viscosity lava flows of mafic or ultramafic composition, involving eruptions that were characterized by extraordinarily high effusion rates and total lava volumes that in some cases are likely to have been as great as those that characterized some Large Igneous Provinces on Earth. The deeply rooted igneous plumbing systems most favorable to the development of large volcanic channels would have been especially common in the earlier history of the solar system, when the interior temperatures of rocky bodies were greater than today. The early development of large volcanic channel systems is likely typical of the geological histories of large rocky bodies in the universe.
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.
Terrestrial Analogs to Planetary Volcanic Phenomena
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.
Kun Wang and Randy Korotev
For thousands of years, people living in Egypt, China, Greece, Rome, and other parts of the world have been fascinated by shooting stars, which are the light and sound phenomena commonly associated with meteorite impacts. The earliest written record of a meteorite fall is logged by Chinese chroniclers in 687 bce. However, centuries before that, Egyptians had been using “heavenly iron” to make their first iron tools, including a dagger found in King Tutankhamun’s tomb that dates back to the 14th century bce. Even though human beings have a long history of observing meteors and utilizing meteorites, we did not start to recognize their true celestial origin until the Age of Enlightenment. In 1794 German physicist and musician Ernst Chladni was the first to summarize the scientific evidence and to demonstrate that these unique objects are indeed from outside of the Earth. After more than two centuries of joint efforts by countless keen amateur, academic, institutional, and commercial collectors, more than 60,000 meteorites have been catalogued and classified in the Meteoritical Bulletin Database. This number is continually growing, and meteorites are found all over the world, especially in dry and sparsely populated regions such as Antarctica and the Sahara Desert. Although there are thousands of individual meteorites, they can be handily classified into three broad groups by simple examinations of the specimens. The most common type is stony meteorite, which is made of mostly silicate rocks. Iron meteorites are the easiest to be preserved for thousands (or even millions) of years on the Earth’s surface environments, and they are composed of iron and nickel metals. The stony-irons contain roughly the same amount of metals and silicates, and these spectacular meteorites are the favorites of many collectors and museums. After 200 years, meteoritics (the science of meteorites) has grown out of its infancy and become a vibrant area of research today. The general directions of meteoritic studies are: (1) mineralogy, identifying new minerals or mineral phases that rarely or seldom found on the Earth; (2) petrology, studying the igneous and aqueous textures that give meteorites unique appearances, and providing information about geologic processes on the bodies upon which the meteorites originates; (3) geochemistry, characterizing their major, trace elemental, and isotopic compositions, and conducting interplanetary comparisons; and (4) chronology, dating the ages of the initial crystallization and later on impacting disturbances. Meteorites are the only extraterrestrial samples other than Apollo lunar rocks and Hayabusa asteroid samples that we can directly analyze in laboratories. Through the studies of meteorites, we have quested a vast amount of knowledge about the origin of the Solar System, the nature of the molecular cloud, the solar nebula, the nascent Sun and its planetary bodies including the Earth and its Moon, Mars, and many asteroids. In fact, the 4.6-billion-year age of the whole Solar System is solely defined by the oldest age dated in meteorites, which marked the beginning of everything we appreciate today.
Space Resource Utilization
Throughout human history, resources have been the driving force behind the exploration and settling of our planet and also the means to do so. Similarly, resources beyond Earth will make space the next destination in the quest for further exploration and economic expansion of our species. The multitude of celestial bodies surrounding Earth and the space between them hold a vast wealth of resources for a variety of applications. The unlimited solar energy, vacuum, radiation, and low gravity in space, as well as the minerals, metals, water, atmospheric gases, and volatile elements on the Moon, asteroids, comets, and the inner and outer planets of the Solar System and their moons, constitute potential valuable resources for robotic and human space missions and for future use on our own planet. In the short term, these resources could be transformed into useful materials at the site where they are found to extend mission duration and to reduce the costly dependence on materials sent from Earth. Making propellants and human consumables from local resources can significantly reduce mission mass, cost, and risk, enabling longer stays and fueling transportation systems for use within and beyond the planetary surface. Use of finely grained surficial dust and rocks can serve for habitat and infrastructure construction, radiation protection, manufacturing parts, and growing crops. In the long term, material resources and solar energy could also be brought to Earth if obtaining these resources and meeting energy demands locally prove to be no longer economically or environmentally acceptable. However, just like on Earth, not all challenges to identify, extract, and utilize space resources are scientific and technological. As nations and private companies start working toward extracting extraterrestrial resources, an international legal framework and sound socioeconomic policies need to be put in place to ensure that these resources are used for the benefit of all humanity. Space resources promise to unleash an unprecedented wave of exploration and of economic prosperity by utilizing the full potential and value of space. As we embark on this new activity, it will be up to us, humans on planet Earth, to find the best alternatives to use resources beyond our planet effectively, responsibly, and sustainably to make this promise a reality.