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This is an advance summary of a forthcoming article in the Oxford Encyclopedia of Planetary Science. Please check back later for the full article.

Space objects are subject to registration in order to allocate “jurisdiction and control” over those objects in the sovereign-free environment of outer space. This approach is similar to the registration of ships in view of the high sea and for aircrafts with respect to the international airspace. Registration is one of the basic principles of space law, starting with UN General Assembly Resolution 1721 B (XVI) of December 20, 1961, followed by Resolution 1962 (XVIII) of December 13, 1963, then formulated in Article VIII of the Outer Space Treaty of 1967 and as specified in the Registration Convention of 1975. Registration of space objects can be seen today as a principle of customary international law, relevant for each spacefaring state. Registration is divided into a national and an international level. The State Party establishes a national registry for its space objects, and those registrations have to be communicated via diplomatic channel to the UN Register of space objects. This UN Register is handled by the UN Office for Outer Space Affairs (UNOOSA) and is an open source of information for space objects worldwide. Registration is linked to the so-called launching state of the relevant space object. There might be more than one launching state for the specific launch event, but only one state actor can register a specific space object. The state of registry gains “jurisdiction and control” over the space object and therefore no double registration is permissible.

Based on the established UN Space Law, registration practice was subject to some adaptions due to technical developments and legal challenges. After the privatization of the major international satellite organizations, a number of non-registrations had to be faced. The state actors reacted with the UN Registration Practice Resolution of 2007 as elaborated in the Legal Subcommittee of UNCOPUOS, the Committee for the Peaceful Use of Outer Space. In this context an UNOOSA Registration Information Submission Form had been developed. Today the complexity of launch activities and the concepts of mega-constellations lead to new challenges to the registration system. The Registration Practice Resolution already recommends that in cases of joint launches, each space object should be registered separately. Registration of space objects is a legal instrument in the context of state responsibility; it is not an instrument of traffic management. The orbit information of the registration system is indicative for identification purposes but not real-time positioning information. Such traffic management information follows different rules.

This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Planetary Science. Please check back later for the full article.

The space and multitude of celestial bodies surrounding Earth hold a vast wealth of resources for a variety of space and terrestrial applications. The unlimited solar energy, vacuum, 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 in 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 from materials sent from Earth. Making propellants and human consumables from local resources can significantly reduce mission mass and cost, enabling longer stays and fueling transportation systems for use within and beyond the planetary surface. Use of finely grained soils and rocks can serve for habitat construction, radiation protection, solar cell fabrication, and food growth. The same material could also be used to develop repair and replacement capabilities using advanced manufacturing technologies. Following similar mining practices utilized for centuries on Earth, identifying, extracting, and utilizing extraterrestrial resources will enable further space exploration, while increasing commercial activities beyond our planet. In the long term, planetary resources and solar energy could also be brought to Earth if obtaining these resources locally prove to be no longer economically or environmentally acceptable.

Throughout human history, resources have been the driving force for the exploration and settling of our planet. Similarly, extraterrestrial resources will make space the next destination in the quest for further exploration and expansion of our species. However, just like on Earth, not all challenges are scientific and technological. As private companies start working toward exploiting the resources from asteroids, the Moon, and Mars, an international legal framework is also needed to regulate commercial exploration and the use of space and planetary resources for the benefit of all humanity. These resources hold the secret to unleash an unprecedented wave of exploration and of economic prosperity by utilizing the full potential and value of space. It is up to us humans here on planet Earth to find the best way to use these extraterrestrial resources effectively and responsibly to make this promise a reality.

Shortly after the launch of the first manmade satellite in 1957, the United Nations (UN) took the lead in formulating international rules governing space activities. The five international conventions (the 1967 Outer Space Treaty, the 1968 Rescue Agreement, the 1972 Liability Convention, the 1975 Registration Convention, and the 1979 Moon Agreement) within the UN framework constitute the nucleus of space law, which laid a solid legal foundation securing the smooth development of space activities in the next few decades. Outer space was soon found to be a place with abundant opportunities for commercialization. Telecommunications services proved to be the first successful space commercial application, to be followed by remote sensing and global navigation services. In the last decade, the rapid development of space technologies has brought space tourism and space mining to the forefront of space commercialization. With more and more commercial activities taking place on a daily basis from the 1980s, the existing space law faces severe challenges. The five conventions, enacted in a time when space was monopolized by two superpowers, failed to take into account the commercial aspect of space activities. While there is an urgent need for new rules to deal with the ongoing trend of space commercialization, international society faces difficulties in adopting new rules due to diversified concerns over national interests and adjusts the legislative strategies by enacting soft laws. In view of the difficulty in adopting legally binding rules at the international level, states are encouraged to enact their own national space legislation providing sufficient guidance for their domestic space commercial activities. In the foreseeable future, it is expected that the development of soft laws and national space legislation will be the mainstream regulatory activities in the space field, especially for commercial space activities.

Outer space is once again facing renewed competition. Unlike in the earlier decades of space exploration when there were two or three spacefaring powers, by the turn of the 21st century, there are more than 60 players making the outer space environment crowded and congested. Space is no more a domain restricted to state players. Even though it is mostly a western phenomenon, the reality of commercial players as a major actor is creating new dynamics. The changing power transitions are making outer space contested and competitive. Meanwhile, safe and secure access to outer space is being challenged by a number of old and new threats including space debris, militarization of space, radio frequency interference, and potential arms race in space. While a few foundational treaties and legal instruments exist in order to regulate outer space activities, they have become far too expansive to be useful in restricting the current trend that could make outer space inaccessible in the longer term. The need for new rules of the road in the form of norms of responsible behavior, transparency and confidence building measures (TCBMs) such as a code of conduct, a group of governmental experts (GGE), and legal mechanisms, is absolutely essential to have safe, secure, and uninterrupted access to outer space. Current efforts to develop these measures have been fraught with challenges, ranging from agreement on identifying the problems to ideating possible solutions. This is a reflection of the shifting balance of power equations on the one hand, and the proliferation of technology to a large number of players on the other, which makes the decision-making process a lot problematic. In fact, it is the crisis in decision making and the lack of consensus among major space powers that is impeding the process of developing an effective outer space regime.

China has made remarkable achievements in the space sector and has become one of the most relevant players in the outer space domain. Highlights of this process have been the deployment in orbit of the first Chinese space station, Tiangong-1, on September 29, 2011, and the landing of the Yutu rover on the lunar surface on December 14, 2013.

While technological developments have occurred at such a rapid pace, the same cannot be said of the regulatory framework governing Chinese space activities, which still lays at its infant stage. Indeed, unlike other major spacefaring countries, China lacks a comprehensive and uniform national space legislation; as of now, China has enacted two low-level administrative regulations addressing the issues of launching and registration of space objects.

With the growth of the Chinese space program, such a lack of structured national space law is beginning to show its limits and to raise concerns about its negative impact on business opportunities and the ability of China to fully comply with international obligations. One should keep in mind that international space treaties (China is part to four international space law treaties) are not self-executing, thus requiring states to adopt domestic measures to ensure their effective implementation.

Importantly, Chinese authorities appear to be aware of these issues; as stated by the Secretary-General of the Chinese National Space Administration (CNSA) in 2014, national space law has been listed in the national legislation plan, and the CNSA is directly engaged in such a process. However, questions remain as to how this drafting process will be conducted and what legal form and content the law will have. For example, China could either decide to proceed with a gradual approach, consisting in the adoption of laws addressing selected issues to be eventually assembled into one single law or to directly move to the adoption of one comprehensive law.

In any case, if enacted, a Chinese national space law would represent an important step in the advancement of the Chinese space program and in the progress of international space law as such.

Although legal principles to govern space were discussed as early as the mid-1950s, they were not formalized until the Outer Space Treaty (OST) 1967 was adopted and came into force. The OST establishes a number of principles affecting the placement of weapons in outer space. In particular it provides that “the Moon and other celestial bodies shall be used exclusively for peaceful purposes” and prohibits the testing of any types of weapons on such bodies. More generally the OST forbids the placement of nuclear weapons or other weapons of mass destruction in outer space. In addition there are a number of disarmament treaties and agreements emanating from the United Nations Office for Disarmament Affairs and the Conference on Disarmament that are relevant to weapons in space.

Although the disarmament provisions and international humanitarian laws place some restrictions on the use or manner of use of space weapons, none prohibit space weaponization. The absence of such prohibition is not due to many attempts over the years to prevent an arms race in space. Notable among these are Prevention of an Arms Race in Space Draft Treaty and the Prevention of the Placement of Weapons in Space Draft Treaty.

In considering the laws affecting space weapons a fundamental question that arises is what constitutes a weapon and does its placement in space breach the requirement that outer space be used exclusively for peaceful purposes? As an example, does a satellite used to control and direct an armed drone breach the peaceful use provision of the OST? There may be risks that without international norms governments and substate groups may acquire and use armed drones in ways that threaten regional stability, laws of war, and the role of domestic rule of law in decisions to use force.

Given their orbital velocity, any object in space could be a weapon with capability to destroy a satellite or other space object. There is also a growing population of dual-use satellites with military as well as civilian applications. These present great difficulty in arriving at a workable definition of a space weapon in the formulation of a generally acceptable treaty. In addition, there are divergent views of the meaning of peaceful use. Some, in particular the United States, consider the meaning to be “nonaggressive” rather than “nonmilitary.”

The use and exploration of space by humans is historically implicated with international and national security. Space exploration itself was sparked, in part, by the race to develop intercontinental ballistic missiles (ICBM), and the strategic uses of space enable the global projection of force by major military powers. The recognition of space as a strategic domain spurred states to develop the initial laws and policies that govern space activities to reduce the likelihood of conflict. Space security, therefore, is a foundational concept to space law.

Since the beginning of the Space Age, the concept of security has morphed into a multivariate term, and contemporary space security concerns more than just securing states from the dangers of ICBMs. The prevalence of space technologies across society means that security issues connected to the space domain touch on a range of legal regimes. Specifically, space security law involves components of international peace and security, national security, human security, and the security of the space environment itself.

This chapter reviews the conditions under which the basic landforms of Venus formed, interprets their nature, and analyzes their local, regional, and global age relationships. The strong greenhouse effect on Venus causes hyper-dry, almost stagnant near-surface environments. These conditions preclude water-driven, and suppress wind-related, geological processes; thus, the common Earth-like water-generated geological record of sedimentary materials does not currently form on Venus. Three geological processes are important on the planet: volcanism, tectonics, and impact cratering. The small number of impact craters on Venus (~1,000) indicates that their contribution to resurfacing is minor. Volcanism and tectonics are the principal geological processes operating on Venus during its observable geologic history.

Landforms of the volcanic and tectonic nature have specific morphologies, which indicate different modes of formation, and their relationships permit one to establish their relative ages. Analysis of these relationships at the global scale reveals that three distinct regimes of resurfacing comprise the observable geologic history of Venus: (1) the global tectonic regime, (2) the global volcanic regime, and (3) the network rifting-volcanism regime. During the earlier global tectonic regime, tectonic resurfacing dominated. Tectonic deformation at this time caused formation of strongly tectonized terrains such as tessera, and deformational belts. Exposures of these units comprise ~20% of the surface of Venus. The apparent beginning of the global tectonic regime is related to the formation of tessera, which is among the oldest units on Venus. The age relationships among the tessera structures indicate that this terrain is the result of crustal shortening. During the global volcanic regime, volcanism overwhelmed tectonic activity and caused formation of vast volcanic plains that compose ~60% of the surface of Venus. The plains show a clear stratigraphic sequence from older shield plains to younger regional plains. The distinctly different morphologies of the plains indicate different volcanic formation styles ranging from eruption through broadly distributed local sources of shield plains to the volcanic flooding of regional plains. The density of impact craters on units of the tectonic and volcanic regimes suggests that these regimes characterized about the first one-third of the visible geologic history of Venus. During this time, ~80%–85% of the surface of the planet was renovated. The network rifting-volcanism regime characterized the last two-thirds of the visible geologic history of Venus. The major components of the regime include broadly synchronous lobate plains and rift zones. Although the network rifting-volcanism regime characterized ~2/3 of the visible geologic history of Venus, only 15%–20% of the surface was resurfaced during this time. This means that the level of endogenous activity during this time has dropped by about an order of magnitude compared with the earlier regimes.

Mercury, like its inner Solar System planetary neighbors Venus, Mars, and the Moon, shows no evidence of having ever undergone plate tectonics. Nonetheless, the innermost planet boasts a long record of tectonic deformation. The most prominent manifestation of this history is a population of large scarps that occurs throughout the planet’s cratered terrains; some of these scarps rise kilometers above the surrounding landscape. Mercury’s smooth plains, the majority of which are volcanic and occupy over a quarter of the planet, abound with low-relief ridges. The scarps and ridges are underlain by thrust faults and point to a tectonic history dominated by crustal shortening. At least some of the shortening strain recorded by the ridges may reflect subsidence of the lavas in which they formed, but the widespread distribution of scarps attests to a planetwide process of global contraction, wherein Mercury experienced a reduction in volume as its interior cooled through time.

The onset of this phenomenon placed the lithosphere into a net state of horizontal compression, and accounts for why Mercury hosts only a few instances of extensional structures. These landforms, shallow troughs that form complex networks, occur almost wholly in volcanically flooded impact craters and basins and developed as those lavas cooled and thermally contracted. Tellingly, widespread volcanism on Mercury ended at around the same time the population of scarps began to form. Explosive volcanism endured beyond this point, but almost exclusively at sites of lithospheric weakness, where large faults penetrate deep into the interior. These observations are consistent with decades-old predictions that global contraction would shut off major volcanic activity, and illustrate how closely Mercury’s tectonic and volcanic histories are intertwined.

The tectonic character of Mercury is thus one of sustained crustal shortening with only localized extension, which started almost four billion years ago and extends into the geologically recent past. This character somewhat resembles that of the Moon, but differs substantially from those of Earth, Venus, or Mars. Mercury may represent how small rocky planets tectonically evolve and could provide a basis for understanding the geological properties of similarly small worlds in orbit around other stars.

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.

The planetary boundary layer of Mars is a crucial component of the Martian climate and meteorology, as well as a key driver of the surface-atmosphere exchanges on Mars. As such, it is explored by several landers and orbiters; high-resolution atmospheric modeling is used to interpret the measurements by those spacecrafts. The planetary boundary layer of Mars is particularly influenced by the strong radiative control of the Martian surface and, as a result, features a more extreme version of planetary boundary layer phenomena occurring on Earth. In daytime, the Martian planetary boundary layer is highly turbulent, mixing heat and momentum in the atmosphere up to about 10 kilometers from the surface. Daytime convective turbulence is organized as convective cells and vortices, the latter giving rise to numerous dust devils when dust is lifted and transported in the vortex. The nighttime planetary boundary layer is dominated by stable-layer turbulence, which is much less intense than in the daytime, and slope winds in regions characterized by uneven topography. Clouds and fogs are associated with the planetary boundary layer activity on Mars.

The five visible planets are certainly attested to in Egyptian sources from about 2000 bce. The three outer ones are religiously connected with the falcon-headed god Horus, Venus with his father Osiris, and Mercury with Seth, the brother and murderer of Osiris. Clear attestations of the planets are largely limited to decoration programs covering the whole night sky. There are a number of passages in religious texts where planets may be mentioned, but many of them are uncertain because the names given to the planets are for most of them not specific enough to exclude other interpretations. There may have been a few treatises giving a more detailed religious interpretation of the planets and their behavior, but they are badly preserved and hardly understandable in the details.

In the Late Period, probably under Mesopotamian influence, the sequence of the planets as well as their religious associations could change; at least one source links Saturn with the Sun god, Mars with Miysis, Mercury with Thot, Venus with Horus, son of Isis, and Jupiter with Amun, arranging the planets with those considered negative in astrology first, separated from the positive ones by the vacillating Mercury. Late monuments depicting the zodiac place the planets in positions which are considered important in astrology, especially the houses or the place of maximum power (hypsoma; i.e., “exaltation”).

Probably under Babylonian influence, in the Greco-Roman Period mathematical models for calculating the positions and phases of the planets arose. These were used for calculating horoscopes, of which a number in demotic Egyptian are attested. There are also astrological treatises (most still unpublished) in the Egyptian language which indicate the relevance of planets for forecasts, especially for the fate of individuals born under a certain constellation, but also for events important for the king and the country in general; they could be relevant also for enterprises begun at a certain date.

There is some reception of supposedly or actually specific Egyptian planet sequences, names and religious associations in Greek sources.

The Spanish chronicles do not mention planets other than Venus, although they compare certain Aztec gods with classical gods such as Jupiter and Mars. Creation myths recorded by the Spanish chroniclers frequently name Venus gods, most notably Ehecatl-Quetzalcoatl and Tlahuizcalpantecuhtli. The focus on Venus seen in these texts is also mirrored in colonial period Aztec codices, which feature several Venus gods as rulers of calendar periods associated with the 260-day calendar. The famous Aztec Calendar Stone represents Venus symbols prominently in an image showing the predicted demise of the Sun in an eternal solar eclipse, to be accompanied by earthquakes. Venus is apparently seen as the cause of a total solar eclipse in the Codex Borgia, a pre-conquest codex from Tlaxcala, a community neighboring the Aztecs in central Mexico. Although no pre-conquest Aztec codices survive, the painted screenfold books attributed to neighboring communities in central Mexico provide evidence of the kinds of almanacs that were probably also found in Preconquest Aztec screenfold books. The Codex Borgia has two Venus almanacs associated with heliacal rise events and another focusing on dates that coordinate with events involving Venus and possibly other planets. A unique narrative in the Codex Borgia traces Venus over the course of a year, representing different aspects of the synodical cycle. The transformation of Venus in the narrative is evidenced by subtle changes in the Venus god, Quetzalcoatl, who represents the planet Venus throughout the synodical cycle. Another god, Tlahuizcalpantecuhtli (“lord of dawn”), appears in the narrative associated with Venus as the morning star and also is represented in a death aspect during superior conjunction. This is in keeping with Aztec legends that tell how the Sun killed Tlahuizcalpantecuhtli with his solar rays. The Borgia narrative also helps identify Xolotl as the planet Mercury and provides hints about other planets that may be linked with different aspects of Tezcatlipoca, an Aztec god who ruled the night sky.

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.

In addition to ground-based observations beginning in the 1970s, NASA’s Voyager 2 spacecraft flew by Triton in 1989, and NASA’s New Horizons spacecraft flew by Pluto in 2015. Prior to the flyby of New Horizons, Pluto and Triton were termed “sister worlds” due to what appeared to be a high degree of similarity in solid-body density, surface ices, diameter, and surface pressures. Despite being small, cold, icy bodies, both Pluto and Triton have been found to have atmospheres that behave as a continuous fluid up to 300 km altitude above the surface and thereby have a defined temperature, surface pressure, and global general circulation (wind). The primary constituent of these atmospheres is molecular nitrogen, with methane and carbon monoxide comprising the largest abundances of trace gases. The surface pressure as measured in the 2010s on both worlds is of the order of 10 microbars (1 Pa = 10 µbar), for these exotic atmospheres exchange mass between sublimation of surface ice and deposition of nitrogen over the course of each body’s year. Ground-based stellar occultation measurements observed a dramatic change in surface pressure, which one study found was as much as a factor of two increase between 1988 and 2003 on Pluto, presumably due to Pluto’s seasonal volatile cycle. Voyager 2 observed plumes and surface “streaks” on Triton, while New Horizons observed dunes (indicating wind speeds of 1–10 m s−1) as well as streaks, evidently indicating the presence of surface and near-surface winds.

While wind velocity aloft has not been directly measured on Pluto or Triton, 3-D general circulation modeling studies of both worlds have shown zonal (east–west) wind speeds of the order of 10 m/s, meridional (north–south) wind speeds of the order of 1 m/s, and extremely weak vertical wind speeds.

In 2015, New Horizons showed that Pluto and Triton were much more different than previously thought. New Horizons uncovered many spectacular views of Pluto’s atmosphere. First, while hydrocarbon haze was observed on Triton, Pluto had multiple, very distinct stratified haze layers bearing a similar appearance to the layers of an onion. Second, Pluto’s surface elevation was found to be largely inhomogeneous (in contrast to Triton) in the form of a large depression (Sputnik Planitia). Third, the characteristics of the surface markings on Pluto were found to be different than the streaks observed on Triton, which has implications for surface wind patterns.

Further major discoveries made by New Horizons included evidence for many hydrocarbon species in trace concentrations, a lower than expected surface pressure, which could previously only be indirectly ascertained from ground-based observations, and a higher mixing ratio of methane at higher altitudes than at lower due to gravitational diffusive separation. Using radio occultation experiments (not conducted by Voyager 2 at Triton), New Horizons confirmed the existence of a stratosphere (temperature increasing with height) extending to 25 km altitude at both the entry and exit locations. The entry location had a shallow troposphere (temperature decreasing with height) extending to 3.5 km altitude above the surface, while the exit location did not.

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.

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.

During the last three millennia before the Spanish Conquest, the peoples living in the central and southern parts of modern Mexico and the northern part of Central America evolved into complex societies with a number of common characteristics that define the cultural area known as Mesoamerica and are expressed in technology, forms of subsistence, government, architecture, religion, and intellectual achievements, including sophisticated astronomical concepts. For the Aztecs, the Maya, and many other Mesoamerican societies, Venus was one of the most important celestial bodies. Not only were they aware that the brightest “star” appearing in certain periods in the pre-dawn sky was identical to the one that at other times was visible in the evening after sunset; they also acquired quite accurate knowledge about the regularities of the planet’s apparent motion. While Venus was assiduously observed and studied, it also inspired various beliefs, in which its morning and evening manifestations had different attributes. Relevant information is provided by archaeological data, prehispanic manuscripts, early Spanish reports, and ethnographically recorded myths that survive among modern communities as remnants of pre-Conquest tradition.

The best-known is the malevolent aspect of the morning star, whose first appearances after inferior conjunction were believed to inflict harm on nature and humanity in a number of ways. However, the results of recent studies suggest that the prevalent significance of the morning star was of relatively late and foreign origin. The most important aspect of the symbolism of Venus was its conceptual association with rain and maize, in which the evening star had a prominent role. It has also been shown that these beliefs must have been motivated by some observational facts, particularly by the seasonality of evening star extremes, which approximately delimit the rainy season and the agricultural cycle in Mesoamerica. As revealed by different kinds of evidence, including architectural alignments to these phenomena, Venus was one of the celestial agents responsible for the timely arrival of rains, which conditioned a successful agricultural season. The planet also had an important place in the concepts concerning warfare and sacrifice, but this symbolism seems to have been derived from other ideas that characterize Mesoamerican religion. Human sacrifices were believed necessary for securing rain, agricultural fertility, and a proper functioning of the universe in general. Since the captives obtained in battles were the most common sacrificial victims, the military campaigns were religiously sanctioned, and the Venus-rain-maize associations became involved in sacrificial symbolism and warfare ritual. These ideas became a significant component of political ideology, fostered by rulers who exploited them to satisfy their personal ambitions and secular goals. In sum, the whole conceptual complex surrounding the planet Venus in Mesoamerica can be understood in the light of both observational facts and the specific socio-political context.

This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Planetary Science. Please check back later for the full article.

Asteroids 1 Ceres and 4 Vesta are the two largest asteroids in the asteroid belt, with mean diameters of 946 km and 525 km, respectively. Ceres was reclassified as a dwarf planet by the IAU (International Astronomical Union) as a result of their new dwarf planet definition, which is a body that (a) orbits the sun, (b) has enough mass to assume a nearly round shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a moon. Our understanding of these two bodies has been revolutionized in the last decade by the success of the Dawn mission that visited both bodies. Vesta is an example of a small body that has been heated substantially, and differentiated into a metallic core, silicate mantle, and basaltic crust. Ceres is a volatile-rich rocky body that did not experience significant heating and therefore has only partially differentiated. These two contrasting bodies have been instrumental in learning how inner solar system material formed and evolved.

The history of volcanism on Mercury is almost the entire history of the formation of its crust. There are no recognized tracts of intact primary crust analogous to the Moon’s highland crust, probably because the density of Mercury’s iron-poor magma ocean was insufficient to enable crystalized silicate phases to float. Mercury’s surface consists of multiple generations of lavas. These were emplaced, rather like terrestrial “large igneous provinces” or LIPs, in their greatest volumes prior to about 3.5 Ga. Subsequently, erupted volumes decreased, and sites of effusive eruption became largely confined to crater floors. Plains lava surfaces younger than about 3.7 Ga have become scarred by sufficiently few impact craters that they are mapped as “smooth plains.” The older equivalents, which experienced the inner solar system’s “late heavy bombardment,” are mapped as intercrater plains. There is no consensus over whether plains with superimposed-crater characteristics that are intermediate between the smooth plains and intercrater plains end members can be consistently mapped as “intermediate plains.” However, any subdivision of the volcanic plains for mapping purposes arbitrarily splits apart a continuum.

The volcanic nature of Mercury’s smooth plains was ambiguous on the basis of the imagery returned by the first mission to Mercury, Mariner 10, which made three fly-bys in 1974–1975. Better and more complete imaging by MESSENGER (in orbit 2011–2015) removed any doubt by documenting innumerable ghost craters and wrinkle ridges. No source vents for the plains are apparent, but this is normal in LIPs where effusion rate and style characteristically flood the vent beneath its own products. However, there are good examples of broad, flat-bottomed valleys containing streamlined islands suggesting passage of fast-flowing low viscosity lava.

Although the causes of the mantle partial melting events supplying surface eruptions on Mercury are unclear, secular cooling of a small, one-plate planet such as Mercury would be expected to lead to the sort of temporal decrease in volcanic activity that is observed. Factors include loss of primordial heat and declining rate of radiogenic heat production (both of which would make mantle partial melting progressively harder), and thermal contraction of the planet (closing off ascent pathways).

Lava compositions, so far as can be judged from the limited X-ray spectroscopic and other geochemical measurements, appear to be akin to terrestrial komatiites but with very low iron content. Variations within this general theme may reflect heterogeneities in the mantle, or different degrees of partial melting.

The cessation of flood volcanism on Mercury is hard to date, because the sizes of the youngest flows, most of which are inside <200-km craters, are too small for reliable statistics to be derived from the density of superposed craters. However, it probably continued until approximately 1 Ga ago.

That was not the end of volcanism. MESSENGER images have enabled the identification of over a hundred “pits,” which are noncircular holes up to tens of km in size and up to about 4 km deep. Many pits are surrounded by spectrally red deposits, with faint outer edges tens of km from the pit, interpreted as ejecta from explosive eruptions within the pit. Many pits have complex floors, suggesting vent migration over time. Pits usually occur within impact craters, and it has been suggested that crustal fractures below these craters facilitated the ascent of magma despite the compressive regime imposed by the secular thermal contraction. These explosive eruptions must have been driven by the violent expansion of a gas. This could be either a magmatic volatile expanding near the top of a magma conduit, or result from heating of a near-surface volatile by rising magma. MESSENGER showed that Mercury’s crust is surprisingly rich in volatiles (S, Cl, Na, K, C), of which the one likely to be of most importance in driving the explosive eruptions is S.

We do not know when explosive volcanism began on Mercury. Cross-cutting relationships suggest that some explosion pits are considerably less than 1 Ga old, though most could easily be more than 3 Ga. They characteristically occur on top of smooth plains (or less extensive smooth fill of impact craters), and while some pits have no discernible “red spot” around them (perhaps because over time, it has faded into the background), there is no known example of part of a red spot peeping out from beneath the edge of a smooth plains unit. There seems to have been a change in eruptive style over time, with (small volume) explosions supplanting (large volume) effusive events.