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.
61-80 of 146 Results
Article
Lunar Exploration Missions and Environmental Discovery: Status and Progress
Kyeong J. Kim
Article
Magnetosphere–Ionosphere Coupling
N. Achilleos, L. C. Ray, and J. N. Yates
The process of magnetosphere-ionosphere coupling involves the transport of vast quantities of energy and momentum between a magnetized planet and its space environment, or magnetosphere. This transport involves extended, global sheets of electrical current, which flows along magnetic field lines. Some of the charged particles, which carry this current rain down onto the planet’s upper atmosphere and excite aurorae–beautiful displays of light close to the magnetic poles, which are an important signature of the physics of the coupling process. The Earth, Jupiter, and Saturn all have magnetospheres, but the detailed physical origin of their auroral emissions differs from planet to planet. The Earth’s aurora is principally driven by the interaction of its magnetosphere with the upstream solar wind—a flow of plasma continually emanating from the Sun. This interaction imposes a particular pattern of flow on the plasma within the magnetosphere, which in turn determines the morphology and intensity of the currents and aurorae. Jupiter, on the other hand, is a giant rapid rotator, whose main auroral oval is thought to arise from the transport of angular momentum between the upper atmosphere and the rotating, disc-like plasma in the magnetosphere. Saturn exhibits auroral behavior consistent with a solar wind–related mechanism, but there is also regular variability in Saturn’s auroral emissions, which is consistent with rotating current systems that transport energy between the magnetospheric plasma and localized vortices of flow in the upper atmosphere/ionosphere.
Article
Magnetosphere of Jupiter
Fran Bagenal
The magnetosphere of Jupiter is a vast region (50 million times the volume of the planet, 70,000 times the volume of the Sun) that is dominated by the magnetic field of Jupiter. Towards the Sun, upstream in the solar wind, the magnetosphere extends 60-100 jovian radii, compressing and expanding in response to changes in the solar wind pressure. Downstream in the solar wind, the magnetosphere extends past the orbit of Saturn. The vast volume of the magnetosphere encompasses the ring system and many of Jupiter’s 95 moons, including the four Galilean moons. The innermost Galilean moon, Io, ejects about one ton per second of gases, derived from its sulfur dioxide (SO2) atmosphere, that are dissociated and ionized to produce a toroidal region of plasma comprising electrons plus S and O ions of multiple charge states. Plasma from the Io plasma torus spreads out into the magnetosphere, getting heated (by poorly understood processes) to form a plasma sheet. Some of the magnetospheric particles are accelerated and precipitate into Jupiter’s atmosphere, where they excite intense auroral emissions, spanning the spectrum from X-rays to radio. The plasma sheet extends well beyond the orbits of the Galilean moons. The plasma interaction with Europa induces electrical currents in the ocean below the moon’s ice crust. Ganymede is the only moon known to have its own internal magnetic dynamo, producing a magnetosphere within a magnetosphere. Electrical currents generated in the plasma interactions with Io, Europa, and Ganymede travel along the magnetic field to Jupiter, where localized auroral emissions are excited at the satellites’ magnetic footprints. Large-scale electrical currents flow along the magnetic field between Jupiter and the plasma sheet, coupling the plasma sheet to Jupiter’s ionosphere that rotates with the planet’s ~10-hour spin period. Thus, Jupiter’s angular momentum drives the dynamics of the magnetosphere out to ~50 jovian radii. Farther out, the solar wind begins to influence the magnetospheric dynamics, coupled by processes at the magnetospheric boundary. Ultimately, the Iogenic material is ejected as plasma blobs down the magnetotail, eventually mixing with the solar wind, similar to the plasma tail of a comet.
Article
The Magnetosphere of Saturn
Sarah Badman
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.
Saturn’s magnetosphere is the region of space surrounding Saturn that is controlled by the planetary magnetic field. Saturn’s magnetic field is aligned to within 1 degree of the rotation axis and rotates with a period of ~10.7 h. The magnetosphere is compressed on the dayside by the impinging solar wind, and stretched into a long magnetotail on the nightside. Its surface, the magnetopause, is located where the internal and external plasma and magnetic pressures balance. As a result of the pressure distributions, the magnetopause has a bimodal distribution of standoff distance at the sub-solar point and is flattened over the poles relative to the equator.
Radiation belts composed of trapped energetic electrons and protons are present in the inner magnetosphere. Their intensity is limited by the moons and rings that can absorb the energetic particles. The icy moons and rings, particularly the cryovolcanic moon Enceladus, are the main sources of mass in the form of water. When the water molecules are ionized they are confined to the equatorial plane by the rapidly rotating magnetic field. This mass-loading acts to distend the magnetic field lines from a dipolar configuration into a radially stretched magnetodisk, with an associated eastward-directed current. In situ measurements of plasma velocity indicate it generally lags behind the planetary rotation, introducing an azimuthal component of the magnetic field. Despite the alignment of the magnetic and rotation axes, so-called planetary period oscillations are ubiquitous in field and plasma measurements in the magnetosphere.
Radial transport of plasma involves the centrifugal interchange instability in the inner magnetosphere and magnetic reconnection in the middle and outer magnetosphere. This allows mass from the moons and rings to be lost from the system. The outermost regions of the magnetosphere are also influenced by the surrounding solar wind through magnetic reconnection and viscous interactions. Acceleration via reconnection or other processes, or scattering of plasma into the atmosphere leads to auroral emissions detected at radio, infrared, visible, and ultraviolet wavelengths.
Article
The Magnetosphere of Uranus
Xin Cao and Carol Paty
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.
A magnetosphere is formed by the interaction between the magnetic field of a planet and the high-speed solar wind. Those planets with a magnetosphere have an intrinsic magnetic field such as Earth, Jupiter, and Saturn. Mars, especially, has no global magnetosphere, but evidence shows that a paleo-magnetosphere existed billions of years ago and was dampened then due to some reasons such as the change of internal activity. A magnetosphere is very important for the habitable environment of a planet because it provides the foremost and only protection for the planet from the energetic solar wind radiation.
The majority of planets with a magnetosphere in our solar system have been studied for decades except for Uranus and Neptune, which are known as ice giant planets. This is because they are too far away from us (about 19 AU from the Sun), which means they are very difficult to directly detect. Compared to many other space detections to other planets, for example, Mars, Jupiter, Saturn and some of their moons, the only single fly-by measurement was made by the Voyager 2 spacecraft in the 1980s. The data it sent back to us showed that Uranus has a very unusual magnetosphere, which indicated that Uranus has a very large obliquity, which means its rotational axis is about 97.9° away from the north direction, with a relative rapid (17.24 hours) daily rotation. Besides, the magnetic axis is tilted 59° away from its rotational axis, and the magnetic dipole of the planet is off center, shifting 1/3 radii of Uranus toward its geometric south pole. Due to these special geometric and magnetic structures, Uranus has an extremely dynamic and asymmetric magnetosphere.
Some remote observations revealed that the aurora emission from the surface of Uranus distributed at low latitude locations, which has rarely happened on other planets. Meanwhile, it indicated that solar wind plays a significant impact on the surface of Uranus even if the distance from the Sun is much farther than that of many other planets. A recent study, using numerical simulation, showed that Uranus has a “Switch-like” magnetosphere that allows its global magnetosphere to open and close periodically with the planetary rotation.
In this article, we will review the historic studies of Uranus’s magnetosphere and then summarize the current progress in this field. Specifically, we will discuss the Voyager 2 spacecraft measurement, the ground-based and space-based observations such as Hubble Space Telescope, and the cutting-edge numerical simulations on it. We believe that the current progress provides important scientific context to boost future ice giant detection.
Article
Mantle Convection in Terrestrial Planets
Elvira Mulyukova and David Bercovici
All the rocky planets in our solar system, including the Earth, initially formed much hotter than their surroundings and have since been cooling to space for billions of years. The resulting heat released from planetary interiors powers convective flow in the mantle. The mantle is often the most voluminous and/or stiffest part of a planet and therefore acts as the bottleneck for heat transport, thus dictating the rate at which a planet cools. Mantle flow drives geological activity that modifies planetary surfaces through processes such as volcanism, orogenesis, and rifting. On Earth, the major convective currents in the mantle are identified as hot upwellings such as mantle plumes, cold sinking slabs, and the motion of tectonic plates at the surface. On other terrestrial planets in our solar system, mantle flow is mostly concealed beneath a rocky surface that remains stagnant for relatively long periods. Even though such planetary surfaces do not participate in convective circulation, they deform in response to the underlying mantle currents, forming geological features such as coronae, volcanic lava flows, and wrinkle ridges. Moreover, the exchange of material between the interior and surface, for example through melting and volcanism, is a consequence of mantle circulation and continuously modifies the composition of the mantle and the overlying crust. Mantle convection governs the geological activity and the thermal and chemical evolution of terrestrial planets and understanding the physical processes of convection helps us reconstruct histories of planets over billions of years after their formation.
Article
Mars Atmospheric Entry, Descent, and Landing: An Atmospheric Perspective
Michael Mischna
Beginning in the very earliest years of the space age, a flotilla of robotic explorers have been sent to study Mars—first simply to fly by, then to orbit, and, later, to attempt landing on the surface. For these landers, separating the rapidly approaching spacecraft from the surface is little but a tenuous carbon dioxide atmosphere, too thin to be useful but too thick to ignore. The purpose of the entry, descent, and landing (EDL) process is to take these hypersonic spacecraft through the approximately 6 mb atmosphere and place them safely on the Martian surface. The sequence of steps required to progressively slow and control this descending spacecraft has been honed throughout the decades but follows the same basic approach. A period of frictional deceleration during the entry phase of EDL first slows the spacecraft to a point where a supersonic parachute can be deployed to further slow the spacecraft during its descent phase. Whether a spacecraft is following a ballistic or a guided entry determines the need to control the downrange motion of the spacecraft during the entry phase, providing more or less targeting accuracy, at the expense of EDL complexity. The third and terminal EDL phase, consisting of a powered or semi-powered landing, brings the spacecraft to the surface. Over the years, a range of different powered landing approaches have been employed, from basic retropropulsion, to airbags to the SkyCrane, as spacecraft size has grown and landing sites have become more challenging.
Despite this seemingly straightforward description, EDL at Mars is an exceptionally intricate process, with numerous failures over the decades; as of 2023, four space agencies have attempted, with varying degrees of success, to land on Mars. Environmental uncertainties during the EDL process typically remain a large mission concern. The process of characterizing the Martian atmosphere at the time, season, and location of touchdown has advanced incrementally from the earliest landings that relied on coarse orbital or flyby measurements of surface temperature and pressure to more modern efforts that incorporate sophisticated numerical models with high spatial and temporal resolution, pinpointing the most likely conditions that a spacecraft will experience during its traverse through the atmosphere and providing comprehensive uncertainty measurements to statistically bound the range of possible conditions.
As spacecraft become more complex, it has become possible to add in situ sensors to the descending spacecraft to directly measure the local environment. Combined with numerical modeling and information provided by other spacecraft, these data have helped increase knowledge of the local environment to a substantial degree, reducing environmental uncertainty from being a major risk to a manageable concern.
Article
Martian Dust
Steven W. Ruff
Dust makes the red planet red. Without dust, Mars would appear mostly as shades of gray. The reddish hue arises from a small amount of oxidized iron among its basaltic mineral constituents. In this sense, Mars is a rusty world. Martian dust is a ubiquitous material of remarkably uniform composition that spans the globe, filling the skies and covering the land in a temporally and spatially varying manner. It is routinely lifted into the atmosphere via convective vortices known as dust devils. Dust in the atmosphere waxes and wanes according to season. Every few Martian years, the planet is fully encircled in atmospheric dust of sufficient opacity that its surface markings and landforms are completely obscured from view of Earth-bound telescopes and Mars-orbiting satellites. Such global dust events last for weeks or months, long enough to jeopardize solar-powered spacecraft on the surface. Dust particles suspended in the thin Martian atmosphere ultimately fall to the surface, completing the cycle and contributing to a range of features that are still being discovered and investigated.
Article
Martian Ionospheric Observation and Modelling
Francisco González-Galindo
The Martian ionosphere is a plasma embedded within the neutral upper atmosphere of the planet. Its main source is the ionization of the CO2-dominated Martian mesosphere and thermosphere by energetic EUV solar radiation. The ionosphere of Mars is subject to an important variability induced by changes in its forcing mechanisms (e.g., the UV solar flux) and by variations in the neutral atmosphere (e.g., the presence of global dust storms, atmospheric waves and tides, changes in atmospheric composition, etc.). Its vertical structure is dominated by a maximum in electron concentration at altitude about 120–140 km, coincident with the peak of the ionization rate. Below, there is a secondary peak produced by solar X-rays and photoelectron-impact ionization. A sporadic third layer, possibly of meteoric origin, has been also detected below.
The most abundant ion in the Martian ionosphere is O2
+, although O+ can become more abundant in the upper ionospheric layers. While below about 180–200 km the Martian ionosphere is dominated by photochemical processes, above those altitudes the dynamics of the plasma becomes more important. The ionosphere is also an important source of escaping particles via processes such as dissociative recombination of ions or ion pickup. So, characterization of the ionosphere provides or can provide information about such disparate systems and processes as solar radiation reaching the planet, the neutral atmosphere, meteoric influx, atmospheric escape to space, or the interaction of the planet with the solar wind.
It is thus not surprising that the interest about this region dates from the beginning of the space era. From the first measurements provided by the Mariner 4 mission in the 1960s to observations by the Mars Express and MAVEN orbiters in the 2010s, our knowledge of this atmospheric region is the consequence of the accumulation of more than 50 years of discontinuous measurements by different space missions. Numerical simulations by computational models able to simulate the processes that shape the ionosphere have also been commonly employed to obtain information about this region, to provide an interpretation of the observations and to fill their gaps. As a result, at the end of the 2010s the Martian ionosphere was the best known one after that of the Earth. However, there are still areas for which our knowledge is far from being complete. Examples are the details and balance of the mechanisms populating the nightside ionosphere, the origin and variability of the lower ionospheric peak, and the precise mechanisms shaping the topside ionosphere.
Article
Martian Paleoclimate
Robert M. Haberle
The climate of Mars has evolved over time. Early in its history, between 3.7 and 4.1 billion years ago, the climate was warmer and wetter and the atmosphere thicker than it is today. Erosion rates were higher than today, and liquid water flowed on the planet’s surface, carving valley networks, filling lakes, creating deltas, and weathering rocks. This implies runoff and suggests rainfall and/or snowmelt. Oceans may have existed. Over time, the atmosphere thinned, erosion rates declined, water activity ceased, and cooler and drier conditions prevailed. Ice became the dominate form of surface water. Yet the climate continued to evolve, driven now by large variations in Mars’ orbit parameters. Beating in rhythm with these variations, surface ice has been repeatedly mobilized and moved around the planet, glaciers have advanced and retreated, dust storms and polar caps have come and gone, and the atmosphere has collapsed and re-inflated many times. The layered terrains that now characterize both polar regions are telltale signatures of this cyclical behavior and owe their existence to modulations of the seasonal cycles of dust, water, and CO2. Contrary to the early images from the Mariner flybys of the 1960s, Mars is and has been a dynamically active planet whose surface has been partly shaped through its interaction with a changing atmosphere and climate system.
Article
Mass Erosion and Transport on Cometary Nuclei, as Found on 67P/Churyumov-Gerasimenko
Wing-Huen Ip
The Rosetta spacecraft rendezvoused with comet 67P/Churyumov-Gerasimenko in 2014–2016 and observed its surface morphology and mass loss process. The large obliquity (52°) of the comet nucleus introduces many novel physical effects not known before. These include the ballistic transport of dust grains from the southern hemisphere to the northern hemisphere during the perihelion passage, thus shaping the dichotomy of two sides, with the northern hemisphere largely covered by dust layers from the recycled dusty materials (back fall) and the southern hemisphere consisting mostly of consolidated terrains. A significant amount of surface material up to 4–10 m in depth could be transferred across the nucleus surface in each orbit. New theories of the physical mechanisms driving the outgassing and dust ejection effects are being developed. There is a possible connection between the cometary dust grains and the fluffy aggregates and pebbles in the solar nebula in the framework of the streaming-instability scenario. The Rosetta mission thus succeeded in fulfilling one of its original scientific goals concerning the origin of comets and their relation to the formation of the solar system.
Article
Meteorite Mineralogy
Alan E. Rubin and Chi Ma
Meteorites are rocks from outer space that reach the Earth; more than 60,000 have been collected. They are derived mainly from asteroids; a few hundred each are from the Moon and Mars; some micrometeorites derive from comets. By mid 2020, about 470 minerals had been identified in meteorites. In addition to having characteristic petrologic and geochemical properties, each meteorite group has a distinctive set of pre-terrestrial minerals that reflect the myriad processes that the meteorites and their components experienced. These processes include condensation in gaseous envelopes around evolved stars, crystallization in chondrule melts, crystallization in metallic cores, parent-body aqueous alteration, and shock metamorphism. Chondrites are the most abundant meteorites; the major components within them include chondrules, refractory inclusions, opaque assemblages, and fine-grained silicate-rich matrix material. The least-metamorphosed chondrites preserve minerals inherited from the solar nebula such as olivine, enstatite, metallic Fe-Ni, and refractory phases. Other minerals in chondrites formed on their parent asteroids during thermal metamorphism (such as chromite, plagioclase and phosphate), aqueous alteration (such as magnetite and phyllosilicates) and shock metamorphism (such as ringwoodite and majorite). Differentiated meteorites contain minerals formed by crystallization from magmas; these phases include olivine, orthopyroxene, Ca-plagioclase, Ca-pyroxene, metallic Fe-Ni and sulfide. Meteorites also contain minerals formed during passage through the Earth’s atmosphere and via terrestrial weathering after reaching the surface. Whereas some minerals form only by a single process (e.g., by high-pressure shock metamorphism or terrestrial weathering of a primary phase), other meteoritic minerals can form by several different processes, including condensation, crystallization from melts, thermal metamorphism, and aqueous alteration.
Article
Meteorites
Kun Wang and Randy Korotev
For thousands of years, people living in Egypt, China, Greece, Rome, and other parts of the world have been fascinated by shooting stars, which are the light and sound phenomena commonly associated with meteorite impacts. The earliest written record of a meteorite fall is logged by Chinese chroniclers in 687 bce. However, centuries before that, Egyptians had been using “heavenly iron” to make their first iron tools, including a dagger found in King Tutankhamun’s tomb that dates back to the 14th century bce. Even though human beings have a long history of observing meteors and utilizing meteorites, we did not start to recognize their true celestial origin until the Age of Enlightenment. In 1794 German physicist and musician Ernst Chladni was the first to summarize the scientific evidence and to demonstrate that these unique objects are indeed from outside of the Earth. After more than two centuries of joint efforts by countless keen amateur, academic, institutional, and commercial collectors, more than 60,000 meteorites have been catalogued and classified in the Meteoritical Bulletin Database. This number is continually growing, and meteorites are found all over the world, especially in dry and sparsely populated regions such as Antarctica and the Sahara Desert. Although there are thousands of individual meteorites, they can be handily classified into three broad groups by simple examinations of the specimens. The most common type is stony meteorite, which is made of mostly silicate rocks. Iron meteorites are the easiest to be preserved for thousands (or even millions) of years on the Earth’s surface environments, and they are composed of iron and nickel metals. The stony-irons contain roughly the same amount of metals and silicates, and these spectacular meteorites are the favorites of many collectors and museums.
After 200 years, meteoritics (the science of meteorites) has grown out of its infancy and become a vibrant area of research today. The general directions of meteoritic studies are: (1) mineralogy, identifying new minerals or mineral phases that rarely or seldom found on the Earth; (2) petrology, studying the igneous and aqueous textures that give meteorites unique appearances, and providing information about geologic processes on the bodies upon which the meteorites originates; (3) geochemistry, characterizing their major, trace elemental, and isotopic compositions, and conducting interplanetary comparisons; and (4) chronology, dating the ages of the initial crystallization and later on impacting disturbances. Meteorites are the only extraterrestrial samples other than Apollo lunar rocks and Hayabusa asteroid samples that we can directly analyze in laboratories. Through the studies of meteorites, we have quested a vast amount of knowledge about the origin of the Solar System, the nature of the molecular cloud, the solar nebula, the nascent Sun and its planetary bodies including the Earth and its Moon, Mars, and many asteroids. In fact, the 4.6-billion-year age of the whole Solar System is solely defined by the oldest age dated in meteorites, which marked the beginning of everything we appreciate today.
Article
Migration of Low-Mass Planets
Frédéric S. Masset
Planet migration is the variation over time of a planet’s semimajor axis, leading to either a contraction or an expansion of the orbit. It results from the exchange of energy and angular momentum between the planet and the disk in which it is embedded during its formation and can cause the semimajor axis to change by as much as two orders of magnitude over the disk’s lifetime. The migration of forming protoplanets is an unavoidable process, and it is thought to be a key ingredient for understanding the variety of extrasolar planetary systems. Although migration occurs for protoplanets of all masses, its properties for low-mass planets (those having up to a few Earth masses) differ significantly from those for high-mass planets.
The torque that is exerted by the disk on the planet is composed of different contributions. While migration was first thought to be invariably inward, physical processes that are able to halt or even reverse migration were later uncovered, leading to the realization that the migration path of a forming planet has a very sensitive dependence on the underlying disk parameters.
There are other processes that go beyond the case of a single planet experiencing smooth migration under the disk’s tide. This is the case of planetary migration in low-viscosity disks, a fashionable research avenue because protoplanetary disks are thought to have very low viscosity, if any, over most of their planet-forming regions. Such a process is generally significantly chaotic and has to be tackled through high-resolution numerical simulations. The migration of several low-mass planets is also is a very fashionable topic, owing to the discovery by the Kepler mission of many multiple extrasolar planetary systems. The orbital properties of these systems suggest that at least some of them have experienced substantial migration. Although there have been many studies to account for the orbital properties of these systems, there is as yet no clear picture of the different processes that shaped them. Finally, some recently unveiled processes could be important for the migration of low-mass planets. One process is aero-resonant migration, in which a swarm of planetesimals subjected to aerodynamic drag push a planet inward when they reach a mean-motion resonance with the planet, while another process is based on so-called thermal torques, which arise when thermal diffusion in the disk is taken into account, or when the planet, heated by accretion, releases heat into the ambient gas.
Article
The Moon and Planets Among the Incas and Other Pre-Hispanic Andean Peoples
Mariusz Ziółkowski
Although the Inca state (ca. 1200–1572 ce) was called the Empire of the Sun, the Moon was, in some respects, an equally important divinity in the official state cult. The regulatory function of the phases of the synodic cycle of the Moon in different kinds of social activities, especially those framed in calendrical systems but also military campaigns, is well documented. As far as the orientation of architectural structures is concerned, the researchers focus their attention almost entirely on the position of the Sun. However, a more accurate analysis of two well-known sites—the caves of Intimachay and Inkaraqay—may provide evidence of their function as observatories of the lunar 18.6-year cycle. Those results may confirm the hypothesis, presented some years ago, that the Incas had elaborated a rudimentary method of predicting lunar eclipses.
The determination of the exact role of Venus and other planets in the Inca worldview encounters a serious limitation: in contrast to Mesoamerica, in Tahuantinsuyu and the Andes, there are no important “first-hand” sources such as the calendrical-astronomical data of the Maya or the Aztecs. Only Venus seems to have enjoyed a cult of Pan-American range. The morning appearance of Venus was apparently related to the puberty initiation rites of male adolescents, while its appearance as Evening Star seems to have been closely symbolically related to the Inca sovereign and his military activities. Putting aside the information available on Venus and its cult, there is an almost complete lack of data on the other planets.
Another problem must be considered: To what extent did the Incas inherit their knowledge from their predecessors, the Chimus, or even earlier cultures?
Article
The Moon and Planets in Ancient Mesopotamia
Mathieu Ossendrijver
In ancient Mesopotamia, all five planets visible to the naked eye were known and studied, along with the Moon, the Sun, the stars, and other celestial phenomena. In all Mesopotamian sources concerning the Moon and the planets, be they textual or iconographical, the astronomical, astrological, and religious aspects are intertwined. The term “astral science” covers all forms of Mesopotamian scholarly engagement with celestial entities, including celestial divination and astrology. Modern research on Mesopotamian astral science began in the 19th century. Much research remains to be done, because important sources remain unpublished and new questions have been posed to published sources.
From ca. 3000 bce onward, Mesopotamians used a calendar with months and years, which indicates that the Moon was studied at that early age. In cuneiform writing, the Sumerian and Akkadian names of the Moongod, Nanna/Sin, are attested since ca. 2500 bce. The most common Akkadian names of the five planets, Šiḫṭu (Mercury), Dilbat (Venus), Ṣalbatānu (Mars), White Star (Jupiter), and Kayyāmānu (Saturn), are attested first in 1800–1000 bce. The Moon, the Sun, and the planets were viewed as gods or manifestations of gods. From ca. 1800 bce onward, the phenomena of the Moon, the Sun, and the planets were studied as signs that were produced by the gods to communicate with humankind. Between ca. 600 bce and 100 ce, Babylonian scholars reported lunar and planetary phenomena in astronomical diaries and related texts. Their purpose was to enable predictions of the reported phenomena with period-based, so-called Goal-Year methods. After the end of the 5th century bce Babylonian astronomers introduced the zodiac and developed new methods for predicting lunar and planetary phenomena known as mathematical astronomy At about the same time they developed horoscopy and other forms of astrology that use the zodiac, the Moon, the Sun, and the planets to predict events on Earth.
Article
The Moon and Planets in Indigenous California
E.C. Krupp
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.
Article
Noble Gases
Mario Trieloff
Although the second most abundant element in the cosmos is helium, noble gases are also called rare gases. The reason is that they are not abundant on terrestrial planets like the Earth, which is characterized by orders of magnitude depletion of—particularly light—noble gases when compared to the cosmic element abundance pattern. Indeed, geochemical depletion and enrichment processes mean that noble gases are highly versatile tracers of planetary formation and evolution. When our solar system formed—or even before—small grains and first condensates incorporated small amounts of noble gases from the surrounding gas of solar composition, resulting in depletion of light He and Ne relative to heavy Ar, Kr, and Xe, leading to the “planetary type” abundance pattern. Further noble gas depletion occurred during flash heating of mm- to cm-sized objects (chondrules and calcium, aluminum-rich inclusions), and subsequently during heating—and occasionally differentiation—on small planetesimals, which were precursors of planets. Some of these objects are present today in the asteroid belt and are the source of many meteorites. Many primitive meteorites contain very small (micron to sub-micron size) rare grains that are older than our Solar System and condensed billions of years ago in in the atmospheres of different stars, for example, Red Giant stars. These grains are characterized by nucleosynthetic anomalies, in particular the noble gases, such as so-called s-process xenon.
While planetesimals acquired a depleted noble gas component strongly fractionated in favor of heavy noble gases, the Sun and also gas giants like Jupiter attracted a much larger amount of gas from the protosolar nebula by gravitational capture. This resulted in a cosmic or “solar type” abundance pattern, containing the full complement of light noble gases. In contrast, terrestrial planets accreted from planetesimals with only minor contributions from the gaseous component of the protosolar nebula, which accounts for their high degree of depletion and essentially “planetary” elemental abundance pattern. The strong depletion in noble gases facilitates their application as noble gas geo- and cosmochronometers; chronological applications are based on being able to determine noble gas isotopes formed by radioactive decay processes, for example, 40Ar by 40K decay, 129Xe by 129I decay, or fission Xe from 238U or 244Pu decay. Particularly ingrowth of radiogenic xenon is only possible due to the depletion of primordial nuclides, which allows insight into the chronology of fractionation of lithophile parent nuclides and atmophile noble gas daughters. Applied to large-scale planetary reservoirs, this helps to elucidate the timing of mantle degassing and evolution of planetary atmospheres. Applied to individual rocks and minerals, it allows radioisotope chronology using short-lived (e.g., 129I–129Xe) or long-lived (e.g., 40K–40Ar) systems.
The dominance of 40Ar in the terrestrial atmosphere allowed von Weizsäcker to conclude that most of the terrestrial atmosphere originated by degassing of the solid Earth, which is an ongoing process today at mid-ocean ridges, as indicated by outgassing of primordial helium from newly forming ocean crust. Mantle degassing was much more massive in the past, with most of the terrestrial atmosphere probably formed during the first few 100 million years of Earth’s history, in response to major evolutionary processes of accretion, terrestrial core formation, and the terminal accretion stage of a giant impact that formed our Moon. During accretion, solar noble gases were added to the mantle, presumably by solar wind irradiation of the small planetesimals and dust accreting to form the Earth. While the Moon-forming impact likely dissipated a major fraction of the primordial atmosphere, today’s atmosphere originated by addition of a late veneer of asteroidal and possibly cometary material combined with a decreasing rate of mantle degassing over time. As other atmophile elements behave similarly to noble gases, they also trace the origin of major volatiles on Earth, for example, water, nitrogen, and carbon.
Article
Nucleosynthetic Isotope Anomalies in Cosmochemistry and Geochemistry
Katherine Bermingham and Brad Meyer
Nucleosynthetic isotope anomalies provide some of the most informative sample-based constraints on the origin of the Solar System. An isotopic anomaly is a deviation in isotopic ratio relative to a standard that is made from natural terrestrial materials. Nucleosynthetic isotope anomalies are small (i.e., part-per-million scale) stable isotopic anomalies that are found in meteorites and some planetary bodies which are caused by the heterogeneous distribution of stardust in the protoplanetary disk. These subtle isotopic differences provide constraints on the combination of stellar precurors whose stardust comprises some of the matter in the protoplanetary disk. These anomalies also constrain how well stardust was mixed in the disk during accretion. Furthermore, discoveries of subtle nucleosynthetic isotope anomalies in samples from the Earth’s mantle have opened the door to the possibility of using nucleosynthetic isotope anomalies to trace Earth’s precursor material. New insights into the evolution of the nascent Solar System have come from interpreting nucleosynthetic isotope anomalies in the context of numerical stellar nucleosynthetic models and disk evolution models. This research is based on a thus far omnipresent isotopic dichotomy, termed the “NC–CC isotopic dichotomy”, that is recorded in the nucleosynthetic isotope composition of meteorites. This isotopic dichotomy has been interpreted to indicate that within the first few million years of Solar System history, the disk separated into two portions. This separation inhibited material in the presumptive inner Solar System (termed noncarbonaceous reservoir, NC group) from mixing with outer Solar System material (termed carbonaceous chondrite reservoir, CC group). This strict compositional division in the disk may have lasted until giant planet migration, which occurred at the tail end of the disk’s lifetime (<10 Ma of Solar System formation). Despite this application of the NC-CC isotopic dichotomy, fundamental questions remain about what part of Solar System history it preserves and if it can be used to reconstruct the architecture of the nascent Solar System.
The applications of nucleosynthetic isotope anomalies in cosmochemistry and the insights these data provide into the evolution of the Solar System and Earth are discussed. The nucleosynthetic isotope anomalies that are recorded in bulk cosmochemical and terrestrial materials are summarized. The likely stellar origins of the presolar grains responsible for the isotopic anomalies on the bulk sample scale are distilled, along with the constraints these data place on the distribution of presolar material in the disk. A review of stellar nucleosynthesis, the formation of the Solar System, the conceptual framework used to interpret nucleosynthetic isotope data, and reported bulk sample nucleosynthetic isotope anomalies is first provided. Following this, a discussion on how nucleosynthetic isotope anomalies are used to constrain the early architecture of the Solar System is presented. To conclude, possible future directions that the scientific community may pursue by applying nucleosynthetic isotope anomalies to questions about terrestrial accretion are presented.
Article
Origins of Life: Open Questions and Debates
André Brack
Stanley Miller demonstrated in 1953 that it was possible to form amino acids from methane, ammonia, and hydrogen in water, thus launching the ambitious hope that chemists would be able to shed light on the origins of life by recreating a simple life form in a test tube. However, it must be acknowledged that the dream has not yet been accomplished, despite the great volume of effort and innovation put forward by the scientific community. A minima, primitive life can be defined as an open chemical system, fed with matter and energy, capable of self-reproduction (i.e., making more of itself by itself), and also capable of evolving. The concept of evolution implies that chemical systems would transfer their information fairly faithfully but make some random errors.
If we compared the components of primitive life to parts of a chemical automaton, we could conceive that, by chance, some parts self-assembled to generate an automaton capable of assembling other parts to produce a true copy. Sometimes, minor errors in the building generated a more efficient automaton, which then became the dominant species. Quite different scenarios and routes have been followed and tested in the laboratory to explain the origin of life.
There are two schools of thought in proposing the prebiotic supply of organics. The proponents of a metabolism-first call for the spontaneous formation of simple molecules from carbon dioxide and water to rapidly generate life. In a second hypothesis, the primeval soup scenario, it is proposed that rather complex organic molecules accumulated in a warm little pond prior to the emergence of life. The proponents of the primeval soup or replication first approach are by far the more active. They succeeded in reconstructing small-scale versions of proteins, membranes, and RNA. Quite different scenarios have been proposed for the inception of life: the RNA world, an origin within droplets, self-organization counteracting entropy, or a stochastic approach merging chemistry and geology. Understanding the emergence of a critical feature of life, its one-handedness, is a shared preoccupation in all these approaches.
