Anthropologists distinguish the U.S. State of California as a primary zone of prehistoric and tribal North America—it was one of the most linguistically and cultural diverse regions on earth. The original population of Native California and traditional cultures were decimated by the Spanish, the Mexicans, and the Anglos, who successively settled California and transformed it. For that reason, knowledge of the character and function of astronomy in what is now California prior to European contact in the 16th century is incomplete and fragmented. Traditional astronomical lore is preserved in a few ethnohistoric commentaries, in some archaeological remains, and in ethnographic research conducted primarily in the early 20th century, when elements of indigenous knowledge still survived. Throughout Native California, the moon’s conspicuous brightness, movement, and systematically changing appearance prompted its affiliation with seasonal change, the passage of time, and cyclical renewal, and most California tribes monitored and counted lunations in one way or another, but not necessarily throughout the entire year. In some cases, individual lunations were affiliated with and named for seasonal circumstances. There is little evidence, however, for even minimal interest in or recognition of the planets visible to the unaided eye, with the exception of Venus as the “Morning Star” or “Evening Star.” Venus, like the moon and other celestial objects, was personified and regarded as a fundamental and active agent of the cosmos. There is no evidence, however, for detailed monitoring of Venus and quantitative knowledge of its synodic behavior.
The Moon and Planets in Indigenous California
The Planets in Pawnee Culture
Von Del Chamberlain
We can be certain that all cultures wondered about the Sun, Moon, planets, and stars, and that they found ways of incorporating what they observed into comprehension of themselves existing within their perceptible surroundings, both on earth and in the sky. Thanks to the gleanings of anthropologists in the late 1800s and early 1900s, we have a treasure trove revealing that the Native American Skidi Band of the Pawnee Nation possessed a unique creation tradition rich in astronomical symbolism. This includes the belief that the two bright planets encompassing within their orbits the orbit of planet Earth were considered by the Skidi to be the cosmic parents of the very first human child, a girl; the Sun and Moon were considered parents of the first male child. This story of human origin includes the legendary journey of the male Great Red Warrior from the east to court the Beautiful Bright White female star of the west, followed by the birth of their daughter transported to earth. This is a striking allegory of the apparent migrations of Mars and Venus, continually changing in brightness, undergoing retrograde motions and sometimes seeming to unite in close conjunctions. Watching these interrelations, repeated over and over with intriguing variations, likely led to and continually reinforced this tradition. Likewise, the apparent monthly relationships of Sun and Moon, with occasional eclipses, visually reinforced the account of the initial male human birth. Thus, the Skidi Pawnee tradition of human origins is an interesting, indeed beautiful, example of human interpretation of natural phenomena.
Planetary aurorae are some of the most iconic and brilliant (in all senses of that word) indicators not only of the interconnections on Planet Earth, but that these interconnections pertain throughout the entire Solar System as well. They are testimony to the centrality of the Sun, not just in providing the essential sunlight that drives weather systems and makes habitability possible, but also in generating a high velocity wind of electrically charged particles—known as the Solar Wind—that buffets each of the planets in turn as it streams outward through interplanetary space. Aurorae are created when electrically charged particles—predominantly negatively charged electrons or positive ions such as protons, the nuclei of hydrogen—crash into the atoms and molecules of a planetary or lunar atmosphere. Such particles can excite the electrons in atoms and molecules from their ground state to higher levels. The atoms and molecules that have been excited by these high-energy collisions can then relax; the emitted radiation is at certain well-defined wavelengths, giving characteristic colors to the aurorae. Just how many particles, how much atmosphere, and what strength of magnetic field are required to create aurorae is an open question. But giant planets like Jupiter and Saturn have aurorae, as does Earth. Some moons also show these emissions. Overall, the aurorae of the Solar System are very varied, variable, and exciting.
Water Ice Permafrost on Mars and on the Moon
Maxim Litvak and Anton Sanin
The Moon and Mars are the most explored planetary bodies in the solar system. For the more than 60 years of the space era, dozens of science robotic missions have explored the Moon and Mars. The primary scientific goal for many of these missions was declared to be a search for surface or ground water/water ice and gaining an understanding of its distribution and origin. Today, for the Moon, the focus of scientific exploration has moved to the lunar polar regions and permanently shadowed regions (PSRs). PSRs do not receive any direct sunlight and are frozen at very low temperatures (< 120 K), acting as cold traps. They are considered to be a storehouse that preserves records of the solar system’s evolution by trapping water ice and potentially other volatile deposits brought by comets and asteroids over billions of years. For Mars, the water/water ice search was part of an attempt to find traces of ancient extraterrestrial life and possibly to understand how life appeared on Earth. Current Mars is cold and dry, but its high latitudes and some equatorial regions are enriched with surface and subsurface water ice. Scientists argue that oceans could have existed on ancient Mars if it was warm and wet and that different life forms could have originated similar to Earth’s. If this is the case, then biomarkers could be preserved in the Martian ground ice depositions. Another popular idea that ties water ice permafrost on the Moon and Mars is related to the expected future human expansion to deep space. The Moon and Mars are widely considered to be the first destinations for future manned space-colony missions or even space-colony missions. In this scenario, the long-term presence and survival of astronauts on the lunar or Martian surface strongly depend on in situ resource utilization (ISRU). Water ice is at the top of the ISRU list because it could be used as water for astronauts’ needs. Its constituents, oxygen and hydrogen, could be used for breathing and for rocket fuel production, respectively. The Moon is the closest body to Earth and discussion about presence of water ice on the Moon has both scientific and practical interest, especially for planning manned space missions. The focus further in space is on how subsurface water ice is distributed on Mars. A related topic is the debates about whether ancient Mars was wet and warm or if, for most of its history, the Martian surface was covered with glaciers. Finally, there are fundamental questions that should be answered by upcoming Mars and Moon missions.
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.
The Surface Composition of Terrestrial Planets
Nicolas Mangold, Jessica Flahaut, and Véronique Ansan
Planetary surface compositions are fundamental to an understanding of both the interior activity through differentiation processes and volcanic activity and the external evolution through alteration processes and accumulations of volatiles. While the Moon has been studied since early on using ground-based instruments and returned samples, observing the surface composition of the terrestrial planets did not become practical until after the development of orbital and in situ missions with instruments tracking mineralogical and elemental variations. The poorly evolved, atmosphere-free bodies like the Moon and Mercury enable the study of the formation of the most primitive crusts, through processes such as the crystallization of a magma ocean, and their volcanic evolution. Nevertheless, recent studies have shown more diversity than initially expected, including the presence of ice in high latitude regions. Because of its heavy atmosphere, Venus remains the most difficult planetary body to study and the most poorly known in regards to its composition, triggering some interest for future missions. In contrast, Mars exploration has generated a huge amount of data in the last two decades, revealing a planet with a mineralogical diversity close to that of the Earth. While Mars crust is dominated by basaltic material, recent studies concluded for significant contributions of more felsic and alkali-rich igneous material, especially in the ancient highlands. These ancient terrains also display widespread outcrops of hydrous minerals, especially phyllosilicates, which are key in the understanding of past climate conditions and suggest a volatile-rich early evolution with implications for exobiology. Recent terrains exhibit a cryosphere with ice-rich landforms at, or close to the surface, of mid- and high latitudes, generating a strong interest for recent climatic variability and resources for future manned missions. While Mars is certainly the planetary body the most similar to Earth, the observation of specific processes such as those linked to interactions with solar wind on atmosphere-free bodies, or with a thick acidic atmosphere on Venus, improve our understanding of the differences in evolution of terrestrial bodies. Future exploration is still necessary to increase humankind’s knowledge and further build a global picture of the formation and evolution of planetary surfaces.
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.
The Moon and the Planets in Classical Greece and Rome
While the moon naturally featured in Mediterranean cultures from time immemorial, principally noted in the earliest literature as a marker of time, time-dependent constructs such as the calendar, and time-related activities, awareness and recognition of the five visible planets came relatively late to the Greeks and thence to the Romans. The moon underlies the local calendars of the Greeks, with documentary and literary evidence from the Late Bronze Age through the Imperial Roman period, and there are signs that the earliest Roman calendar also paid homage to the moon in its divisions of the month. However, although Homer in the 8th century BCE knows of a Morning and an Evening Star, he shows no indication of realizing that these are one and the same, the planet Venus. That particular identification may have come in the 6th century BCE, and it appears to have been not until the 4th century BCE that the Greeks recognized the other four planets visible to the naked eye—Saturn, Jupiter, Mars, and Mercury. This awareness probably came via contact with Babylonian astronomy and astrology, where identification and observations of the planets had figured from the 2nd millennium BCE and served as a basis for astrological prognostications. But it is time, not astrology, that lies at the heart of Greek and Roman concerns with the moon and the planets. Indeed, the need to tell time accurately has been regarded as the fundamental motivation of Greek astronomy. A major cultural issue that long engaged the Greeks was how to synchronize the incommensurate cycles of the moon and the sun for calendrical purposes. Given the apparent irregularities of their cycles, the planets might seem to offer no obvious help with regard to time measurement. Nonetheless they were included by Plato in the 4th century BCE in his cosmology, along with the sun and moon, as heavenly bodies created specifically to compute time. Astrology then provided a useful framework in which the sun, moon, planets, and stars all combined to enable the interpretation and forecasting of life events. It became necessary for the Greeks, and their successors the Romans, to be able to calculate as accurately as possible the positions of the heavenly bodies in order to determine readings of the past, present, and future. Greek astronomy had always had a speculative aspect, as philosophers strove to make sense of the visible cosmos. A deep-seated assumption held by Greek astronomers, that the heavenly bodies moved in uniform, circular orbits, lead to a desire over the centuries to account for or explain away the observed irregularities of planetary motions with their stations and retrogradations. This intention “to save the phenomena,”— that is, to preserve the fundamental circularity—was said to have originated with Plato. While arithmetical schemes had sufficed in Babylonia for such calculation, it was a Greek innovation to devise increasingly complex geometric theories of circular motions (eccentrics and epicycles) in an effort to understand how the sun, moon, and planets moved, so as to place them more precisely in time and space.
The Moon in Meso-America
What is known about the Moon among the ancient Maya of southern Mexico and Guatemala and the Nahuatl-speaking people of central Mexico, especially the Aztecs who lived in the Valley of Mexico and their neighbors in Puebla-Tlaxcala Valley, has been obtained from records related to astronomy and lunar cycles inscribed on Classic Maya monuments dating between ad 250 and 850/900. Modern scholarship focusing on the mathematical units and glyphic writing has helped in deciphering the records. Postclassic Maya codices dating from 1300 to 1500, sent to Europe shortly after the Spanish conquest, also have lunar tables that have been decoded by study of the lunar cycles and glyphs. Painted books dating prior to the conquest in 1521 are also known from central Mexico, but these can only be understood with the help of books that were painted by native artists later in the 16th century and annotated with texts written in Spanish and Nahuatl. These glosses provide information about lunar deities and beliefs about the Moon. Furthermore, knowledge of the Moon in Meso-America is greatly enhanced by ethnographic studies and study of iconographic representations of deities representing different lunar roles and phases.
The Formation of the Martian Moons
Pascal Rosenblatt, Ryuki Hyodo, Francesco Pignatale, Antony Trinh, Sebastien Charnoz, Kevin Dunseath, Mariko Dunseath-Terao, and Hidenori Genda
The origin of the natural satellites or moons of the solar system is as challenging to unravel as the formation of the planets. Before the start of the space probe exploration era, this topic of planetary science was restricted to telescopic observations, which limited the possibility of testing different formation scenarios. This era has considerably boosted this topic of research, particularly after the Apollo missions returned samples from the Moon’s surface to Earth. Observations from subsequent deep space missions such as Viking 1 and 2 Orbiters, Voyager 1 and 2, Phobos-2, Galileo, Cassini-Huygens, and the most recent Mars orbiters such as Mars Express, as well as from the Hubble space telescope, have served to intensify research in this area. Each moon system has its own specificities, with different origins and histories. It is widely accepted that the Earth’s Moon formed after a giant collision between the proto-Earth and a body similar in size to Mars. The Galilean moons of Jupiter, on the other hand, appear to have formed by accretion in a circum-Jovian disk, while smaller, irregularly shaped satellites were probably captured by the giant planet. The small and medium-sized Saturnian moons may have formed from the rings encircling the planet. Among the terrestrial planets, Mercury and Venus have no moons, the Earth has a single large moon, and Mars has two very small satellites. This raises some challenging questions: What processes can lead to moon formation around terrestrial planets and what parameters determine the possible outcomes, such as the number and size of moons? The answer to such fundamental questions necessarily entails a thorough understanding of the formation of the Martian system and may have relevance to the possible existence of (exo)moons orbiting exoplanets. The formation of such exomoons is of great importance as they could influence conditions for habitability or for maintaining life over long periods of time on the surface of Earth-like exoplanets, for example by limiting the variations of the orientation of the planet’s rotation axis and thus preventing frequent changes of its climate. Our current knowledge concerning the origin of Phobos and Deimos has been acquired from observational data as well as theoretical work. Early observations led to the idea that the two satellites were captured asteroids but this created difficulties in reconciling the current orbits of Phobos and Deimos with those of captured bodies, hence suggesting the need for an alternative theory. A giant-impact scenario provides a description of how moons similar to Phobos and Deimos can be formed in orbits similar to those observed today. This scenario also restricts the range of possible composition of the two moons, providing a motivation for future missions that aim for the first time to bring material from the Martian system back to Earth.
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.
Science and Exploration of the Moon: Overview
Bradley L. Jolliff
Earth’s moon, hereafter referred to as “the Moon,” has been an object of intense study since before the time of the Apollo and Luna missions to the lunar surface and associated sample returns. As a differentiated rocky body and as Earth’s companion in the solar system, much study has been given to aspects such as the Moon’s surface characteristics, composition, interior, geologic history, origin, and what it records about the early history of the Earth-Moon system and the evolution of differentiated rocky bodies in the solar system. Much of the Apollo and post-Apollo knowledge came from surface geologic exploration, remote sensing, and extensive studies of the lunar samples. After a hiatus of nearly two decades following the end of Apollo and Luna missions, a new era of lunar exploration began with a series of orbital missions, including missions designed to prepare the way for longer duration human use and further exploration of the Moon. Participation in these missions has become international. The more recent missions have provided global context and have investigated composition, mineralogy, topography, gravity, tectonics, thermal evolution of the interior, thermal and radiation environments at the surface, exosphere composition and phenomena, and characteristics of the poles with their permanently shaded cold-trap environments. New samples were recognized as a class of achondrite meteorites, shown through geochemical and mineralogical similarities to have originated on the Moon. New sample-based studies with ever-improving analytical techniques and approaches have also led to significant discoveries such as the determination of volatile contents, including intrinsic H contents of lunar minerals and glasses. The Moon preserves a record of the impact history of the solar system, and new developments in timing of events, sample based and model based, are leading to a new reckoning of planetary chronology and the events that occurred in the early solar system. The new data provide the grist to test models of formation of the Moon and its early differentiation, and its thermal and volcanic evolution. Thought to have been born of a giant impact into early Earth, new data are providing key constraints on timing and process. The new data are also being used to test hypotheses and work out details such as for the magma ocean concept, the possible existence of an early magnetic field generated by a core dynamo, the effects of intense asteroidal and cometary bombardment during the first 500 million–600 million years, sequestration of volatile compounds at the poles, volcanism through time, including new information about the youngest volcanism on the Moon, and the formation and degradation processes of impact craters, so well preserved on the Moon. The Moon is a natural laboratory and cornerstone for understanding many processes operating in the space environment of the Earth and Moon, now and in the past, and of the geologic processes that have affected the planets through time. The Moon is a destination for further human exploration and activity, including use of valuable resources in space. It behooves humanity to learn as much about Earth’s nearest neighbor in space as possible.
Geological Characteristics of the Moon
Long Xiao and James W. Head
The geological characteristics of the Moon provide the fundamental data that permit the study of the geological processes that have formed and modified the crust, that record the state and evolution of the lunar interior, and that identify the external processes that have been important in lunar evolution. Careful documentation of the stratigraphic relationships among these features can then be used to reconstruct the sequence of events and the geological history of the Moon. These results can then be placed in the context of the geological evolution of the terrestrial planets, including Earth. The Moon’s global topography and internal structures include landforms and features that comprise the geological characteristics of its surface. The Moon is dominated by the ancient cratered highlands and the relatively younger flat and smooth volcanic maria. Unlike the current geological characteristics of Earth, the major geological features of the Moon (impact craters and basins, lava flows and related features, and tectonic scarps and ridges) all formed predominantly in the first half of the solar system’s history. In contrast to the plate-tectonic dominated Earth, the Moon is composed of a single global lithospheric plate (a one-plate planet) that has preserved the record of planetary geological features from the earliest phases of planetary evolution. Exciting fundamental outstanding questions form the basis for the future international robotic and human exploration of the Moon.
Depictions of the Moon in Western Visual Culture
Jay M. Pasachoff and Roberta J.M. Olson
Since the landmark lunar landing of Apollo 11 on July 20, 1969, NASA’s Lunar Reconnaissance Orbiter (launched in 2009), and the Japanese Aerospace Exploration Agency’s Kaguya spacecraft (2007–2009), among other efforts, have now mapped the Moon’s surface. Before those technological advances and since the beginning of recorded time, people and civilizations have been entranced by Earth’s only natural satellite, which is the second-brightest celestial object visible in the sky from the surface of the planet. Selected examples, among thousands, show how the history of the Moon has been regarded, illustrated, and mapped in visual culture in the Western world. Early examples include representations of a formulaic crescent Moon in Babylonian times; later this dominant stylized depiction of the Moon gave way to more naturalistic images based on observation, culminating in Leonardo da Vinci’s manuscript drawings, which study the lunar structure and cratered surface, and Galileo Galilei’s first telescopic images of the Moon recorded in wash drawings and woodcuts for his book Sidereus Nuncius. Both the artistic and scientific visual acuity that made this evolution possible belonged to the burgeoning empiricism of the 14th through the 17th centuries, which eventually yielded modern observational astronomy and impacted lunar iconography. The subsequent dramatic mapping of the Moon’s surface and the naming of its features became a preoccupation of many astronomers and some artists, who assisted scientists in illustrating their work. With the seeming physical mapping of the Earth-facing side of the Moon well underway in the late 18th and early 19th centuries, the function of Earth’s satellite as a Romantic symbol gained force in the all the arts but most dramatically in the works of landscape painters in Germany (e.g., Caspar David Friedrich and Carl Gustav Carus) and in England (e.g., Samuel Palmer). At the same time, William Blake, who was obsessed with astronomical imagery, used the Moon for expressive purposes, which reached a fever pitch later in the century in the work of Vincent Van Gogh. Along with the increasing accuracy of the Moon’s portrayal through both artists’ and scientists’ representations, the dramatic history of its mapping from Earth crescendoed with the development of photography and William Cranch Bond’s first successful daguerreotype of the Moon in 1851. Further exploration of the Moon, including its far side, has gravitated to aerospace engineers in cooperation with physicists, astronomers, mathematicians, and Apollo astronauts. Nevertheless, the Moon has remained an enduring object of fascination for artists—among the many, Surrealist Joan Miró, Veja Celmins, and Andy Warhol.
Records of Planetary Observations in Ancient Japan Before the 11th Century
Kiyotaka Tanikawa and Mitsuru Sôma
The records of planetary observations in Japan in the 7th century ad are treated separately from other records because they are written in the Nihongi. It is known that Japanese observational astronomy was recorded in the 7th century ad, but astronomy in Japan did not evolve straightforward in that century. There are thirty-one records that exist from that time, including four records on the Moon and planets. Correspondingly, a new interpretation of Japanese ancient history has been proposed. For the 8th, 9th, and 10th centuries, records have been compiled on the relative motion of the Moon and the planets, the motion of planets in the constellations, and stars seen in the daytime, as stated in Japanese recorded history. These records are written in Chinese, as in the case of the Nihongi, but have been translated into English. The orbits of the Moon and planets have been calculated using the NASA Jet Propulsion Laboratory (JPL) development ephemeris (DE) in order to confirm the validity of the records. The numbers of records and observations are not the same because one record may contain multiple observations. The accuracy of individual observations is discussed.
Landslides in the Solar System
Maria Teresa Brunetti and Silvia Peruccacci
Landslides are mass movements of rock, earth, or debris. All of these surface processes occur under the influence of gravity, meaning that they globally move material from higher to lower places. On planets other than Earth, these structures were first observed in a lunar crater during the Apollo program, but mass movements have been found on many rocky worlds (solid bodies) in the Solar System, including icy satellites, asteroids, and comets. On Earth, landslides have the effect of shaping the landscape more or less rapidly, leaving a signature that is recognized through field surveys and visual analysis or automatic identification on ground-based, aerial, and satellite images. Landslides observed on Earth and on solid bodies of the Solar System can be classified into different types based on their movement and the material involved in the failure. Material is either rock or soil (or both), with a variable fraction of water or ice; a soil mainly composed of sand-sized or finer particles is referred as earth while debris is composed of coarser fragments. The landslide mass may be displaced in several types of movement, classified generically as falling, toppling, sliding, spreading, or flowing. Such diverse characteristics mean that the size of a landslide (e.g., area, volume, fall height, length) can vary widely. For example, on Earth, their area ranges up to 11 orders of magnitude, while their volume varies by 16 orders, from small rock fragments to huge submarine landslides. The classification of extraterrestrial landslides is based on terrestrial analogs having similarities and characteristics that resemble those found on planetary bodies, such as Mars. The morphological classification is made regardless of the geomorphological environment or processes that may have triggered the slope failure. Comparing landslide characteristics on various planetary bodies helps to understand the effect of surface gravity on landslide initiation and propagation—of tremendous importance when designing manned and unmanned missions with landings on extraterrestrial bodies. Regardless of the practical applications of such study, knowing the morphology and surface dynamics that shape solid bodies in the space surrounding the Earth is something that has fascinated the human imagination since the time of Galileo.
Use of Outer Space for Peaceful Purposes
The great rise and diversification of the use of outer space raises the question of the limitations to space activities. The ultimate restriction posed by space law is the use of outer space “for peaceful purposes.” Regardless of the semantic approach one adopts with respect to the definition of the term “peaceful purposes” in the text of the Outer Space Treaty, it is the underlying substantive legal normativity which constitutes the determining factor. The applicable international legal rules confirm that the ultimate limit is the prohibition of the use of force laid down in Article 2 (4) of the UN Charter, which applies to outer space along with the exceptions stipulated in the UN Charter and general international law. In addition, the Outer Space Treaty establishes a particular legal regime on celestial bodies, declaring them a demilitarized zone, and bans the stationing of weapons of mass destruction in outer space. Space law, as any other branch of public international law, is of evolutive nature, so future adjustments and developments of its legal normativity in light of the abrupt growth and multiplication of the exploration and uses in the space arena remain open.
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.
Impact Crater Densities as a Tool for Dating Planetary Surfaces
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.