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Article

Edik Dubinin, Janet G. Luhmann, and James A. Slavin

Knowledge about the solar wind interactions of Venus, Mars, and Mercury is rapidly expanding. While the Earth is also a terrestrial planet, it has been studied much more extensively and in far greater detail than its companions. As a result we direct the reader to specific references on that subject for obtaining an accurate comparative picture. Due to the strength of the Earth’s intrinsic dipole field, a relatively large volume is carved out in interplanetary space around the planet and its atmosphere. This “magnetosphere” is regarded as a shield from external effects, but in actuality much energy and momentum are channeled into it, especially at high latitudes, where the frequent interconnection between the Earth’s magnetic field and the interplanetary field allows some access by solar wind particles and electric fields to the upper atmosphere and ionosphere. Moreover, reconnection between oppositely directed magnetic fields occurs in Earth’s extended magnetotail—producing a host of other phenomena including injection of a ring current of energized internal plasma from the magnetotail into the inner magnetosphere—creating magnetic storms and enhancements in auroral activity and related ionospheric outflows. There are also permanent, though variable, trapped radiation belts that strengthen and decay with the rest of magnetospheric activity—depositing additional energy into the upper atmosphere over a wider latitude range. Virtually every aspect of the Earth’s solar wind interaction, highly tied to its strong intrinsic dipole field, has its own dedicated textbook chapters and review papers. Although Mercury, Venus, Earth, and Mars belong to the same class of rocky terrestrial planets, their interaction with solar wind is very different. Earth and Mercury have the intrinsic, mainly dipole magnetic field, which protects them from direct exposure by solar wind. In contrast, Venus and Mars have no such shield and solar wind directly impacts their atmospheres/ionospheres. In the first case, intrinsic magnetospheric cavities with a long tail are found. In the second case, magnetospheres are also formed but are generated by the electric currents induced in the conductive ionospheres. The interaction of solar wind with terrestrial planets also varies due to changes caused by different distances to the Sun and large variations in solar irradiance and solar wind parameters. Other important planetary differences like local strong crustal magnetization on Mars and almost total absence of the ionosphere on Mercury create new essential features to the interaction pattern. Solar wind might be also a feasible driver for planetary atmospheric losses of volatiles, which could historically affect the habitability of the terrestrial planets.

Article

Angel Abbud-Madrid

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.

Article

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.

Article

Rajeswari Pillai Rajagopalan

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.

Article

Fabio Tronchetti

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.

Article

Martha Mejía-Kaiser

International space law is a branch of public international law. Norms of treaty law and customary law provide a foundation for the behavior of the subjects of international law performing space activities. Five multilateral space treaties are in effect, which are complemented by important recommendations of international organizations such as United Nations (UN) General Assembly Resolutions and International Telecommunication Union (ITU) Regulations. The Inter-Agency Space Debris Mitigation Coordination Committee (IADC), a non-governmental body composed of several space agencies (for instance, the European Space Agency, the United States National Aeronautics and Space Administration, the Japanese Aerospace Exploration Agency, the Russian Federal Space Agency), issued its Space Debris Mitigation Guidelines in 2002. The IADC defines “space debris” as “all man-made space objects including fragments and elements thereof, in Earth orbit or re-entering the atmosphere, that are non-functional” (IADC, 2002, Revision 1, 2007, 3.1. Space Debris). Although the term “space debris” was not included in any space treaty, the drafters of the space treaties considered space objects as “hazardous” because “component parts of a space object as well as its launch vehicles and parts thereof” detach in course of normal launching operations, because space objects can fragment during an attempted launch, and because space objects that re-enter Earth’s atmosphere and survive friction have the potential to cause damage. In addition, radioactive and chemical substances on board space objects may represent a hazard to populations and the environment on the Earth. Besides the threats to aircraft in flight and to persons and property on the surface of the Earth, space debris in orbit is increasing alarmingly and poses a threat to manned space missions and non-manned space objects. While the Convention on International Liability for Damages Caused by Space Objects (Liability Convention, 1972) considers the threats of space objects during launch, in outer space, and when entering the Earth’s atmosphere, there have been efforts to minimize the generation of space debris in orbit, outside the framework of the space treaties. The IADC Space Debris Mitigation Guidelines are a comprehensive list of recommendations to launching states, owners, and operators of space objects. They are increasingly recognized by states through the creation of codes of conduct, national legislation, recommendations of international organizations, and state practice. Furthermore, non-governmental institutions, like the International Organization for Standardization, are providing more detailed technical instructions for the implementation of the Space Debris Mitigation Guidelines, which are a breakthrough for the application of the guidelines by states of different economic and technical standing. Even though states are reluctant to accept new obligations through treaties, recommendations and state practice are becoming powerful instruments to avert the dangers of hazardous space debris that may create damage on the Earth or in orbit. Space debris also is becoming one of the drivers for the initiatives of the United Nations on the long-term sustainability of outer space activities to promote the existing mitigation guidelines and to formulate new guidelines for clearing outer space of debris.

Article

Sa'id Mosteshar

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.”

Article

P.J. Blount

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.

Article

The subject of astronomy in folk tradition, or folk astronomy, requires some explication. It is, for instance, not the same as ethnoastronomy, which primarily studies the astronomical ideas of contemporary societies. However, the subject overlaps with archaeoastronomy when defined widely as the interdisciplinary study of prehistoric, ancient, and traditional astronomies worldwide within their cultural context that includes both written and archaeological records. The most useful definition of “astronomy in folk tradition” might be “astronomy of the people or of the common man,” or even “lay astronomy,” left to us through tradition, where the term “astronomy” may, for further clarity, be replaced by “ideas and observations of the sky.” In any case, it is worth keeping in mind that the content of folk astronomy of one society may overlap with the content of established astronomy of another society at another time and place. Scientific ideas or theories have their roots in the past, even before the advent of any “experts.” Folk astronomy of the past is often less accessible for historical studies than mainstream astronomy, especially in a society leaving few records or artifacts. Revealing sources may, however, be found by looking beyond the conventional. For instance, various sources on mythology and religion may give information on the astronomical and cosmological ideas of previous societies. Purportedly fictional literature, like the works of Dante and Chaucer, may also yield information of this kind, although they were not explicitly composed for that purpose. But there are also writers who have deliberately written on the astronomical ideas of their society at their time, although their works were outside of the best known corpus and sometimes intended for common people. Two Old Norse examples are the 13th-century Norwegian King’s Mirror and the Icelandic 12th- to 14th-century material edited in the volume of Alfræði íslenzk II. Among other things, these sources treat phenomena that are not observable outside the subarctic region. A third example is the 14th–15th century North European Seebuch with practical information for seamen, partly linked to astronomy. In any case, two types of folk astronomy can be distinguished: (a) practical astronomy that people use as a tool in daily life, for example, to determine the time of day or year, or for travel and navigation; (b) ideas related to cosmology or cosmogony, religion, or supernatural beliefs, which would neither imply practical uses nor consequences.

Article

M.A. Ivanov and J.W. Head

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.

Article

Paul K. Byrne

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.

Article

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.

Article

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.

Article

Susan Milbrath

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.

Article

The great diversity of extrasolar planetary systems has challenged our understanding of how planets form. During the formation process their orbits are modified while the protoplanetary disk is present. After its dispersal orbits may also be modified as a result of mutual gravitational interactions leading to their currently observed configurations in the longer term. A number of potentially significant phenomena have been identified. These include radial migration of solids in the protoplanetary disk, radial migration of protoplanetary cores produced by disk-planet interaction and how it can be halted by protoplanet traps, formation of resonant systems and subsystems, and gravitational interactions among planets or between a planet and an external stellar companion. These interactions may cause excitation of orbital inclinations and eccentricities which in the latter case may attain values close to unity. When the eccentricity approaches unity, tidal interaction with the central star could lead to orbital circularization and a close orbiting Hot Jupiter, providing a competitive process to direct migration through the disk or in-situ formation. Long-term dynamical instability may also account for the relatively small number of observed compact systems of super-Earths and Neptune class planets that have attained and subsequently maintained linked commensurabilities in the long term.

Article

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.

Article

Joachim Friedrich Quack

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.

Article

Susan Milbrath

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.

Article

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.

Article

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