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Article

Alexander V. Zakharov

The Moon was the first extraterrestrial body to attract the attention of space pioneers. It has been about half a century since an active lunar exploration campaign was carried out. At that time, a series of Russian and American automatic landing vehicles and the American manned Apollo Program carried out an unprecedented program of lunar exploration in terms of its saturation and volume. Unique breakthrough data on the lunar regolith and plasma environment were obtained, a large number of experiments were carried out using automated and manned expeditions, and more than 300 kg of lunar regolith and rock samples were delivered to Earth for laboratory research. A wealth of experience has been accumulated by performing direct human activities on the lunar surface. At the same time, the most unexpected result of the studies was the detection of a glow above the surface, recorded by television cameras installed on several lunar landers. The interpretation of this phenomenon led to the conclusion that sunlight is scattered by dust particles levitating above the surface of the Moon. When the Apollo manned lunar exploration program was being prepared, this fact was already known, and it was taken into account when developing a program for astronauts’ extravehicular activities on the lunar surface, conducting scientific research, and ground tests. However, despite preparations for possible problems associated with lunar dust, according to American astronauts working on the lunar surface, the lunar dust factor turned out to be the most unpleasant in terms of the degree of impact on the lander and its systems, on the activities of astronauts on the surface, and on their health. Over the past decades, theoretical and experimental model studies have been carried out aimed at understanding the nature of the lunar horizon glow. It turned out that this phenomenon is associated with the complex effect of external factors on the lunar regolith, as a result of which there are a constant processing and grinding of the lunar regolith to particles of micron and even submicron sizes. Particles of lunar regolith that are less than a millimeter in size are commonly called lunar dust. As a result of the influence of external factors, the upper surface of the regolith acquires an electric charge, and a cloud of photoelectrons and a double layer are formed above the illuminated surface. Coulomb forces in the electric field of this layer, acting on microparticles of lunar dust, under certain conditions are capable of tearing microparticles from the surface of the regolith. These dust particles, near-surface plasma, and electrostatic fields form the near-surface dusty plasma exosphere of the Moon. The processes leading to the formation of regolith and microparticles on the Moon, their separation from the surface, and further dynamics above the surface include many external factors affecting the Moon and physical processes on the surface and near-surface dusty plasma exosphere. As a result of the research carried out, a lot has been understood, but many unsolved problems remain. Recently, since the space agencies of the leading space powers have been turning their attention to intensive research and subsequent exploration of the Moon, interest in the processes associated with the dynamics of lunar dust and its influence on landing vehicles and their engineering systems is increasing, and significant attention is being paid to reducing and mitigating the negative impact of lunar dust on the activities of astronauts and their health.

Article

Matthew R. Balme

Dust devils are rotating columns or cones of air, loaded with dust and other fine particles, that are most often found in arid or desert areas. They are common on both Mars and Earth, despite Mars’ very thin atmosphere. The smallest and least intense dust devils might last only a few 10s of seconds and be just a meters or two across. The largest dust devils can persist for hours and are intensely swirling columns of dust with “skirts” of sand at their base, 10s or more meters in diameter and hundreds of meters high; even larger examples have been seen on Mars. Dust devils on Earth have been documented for thousands of years, but scientific observations really began in the early 20th century, culminating in a period of intense research in the 1960s. The discovery of dust devils on Mars was made using data from the NASA Viking lander and orbiter missions in the late 1970s and early 1980s and stimulated a renewed scientific interest in dust devils. Observations from subsequent lander, rover, and orbital missions show that Martian dust devils are common but heterogeneously distributed in space and time and have a significant effect on surface albedo (often leaving “tracks” on the surface) but do not appear to be triggers of global or major dust storms. An aspiration of future research is to synthesize observations and detailed models of dust devils to estimate more accurately their role in dust lifting at both local and global scales, both on Earth and on Mars.

Article

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

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

Alessandro Morbidelli

In planetary science, accretion is the process in which solids agglomerate to form larger and larger objects, and eventually planets are produced. The initial conditions are a disc of gas and microscopic solid particles, with a total mass of about 1% of the gas mass. These discs are routinely detected around young stars and are now imaged with the new generation of instruments. Accretion has to be effective and fast. Effective, because the original total mass in solids in the solar protoplanetary disk was probably of the order of ~300 Earth masses, and the mass incorporated into the planets is ~100 Earth masses. Fast, because the cores of the giant planets had to grow to tens of Earth masses to capture massive doses of hydrogen and helium from the disc before the dispersal of the latter, in a few millions of years. The surveys for extrasolar planets have shown that most stars have planets around them. Accretion is therefore not an oddity of the solar system. However, the final planetary systems are very different from each other, and typically very different from the solar system. Observations have shown that more than 50% of the stars have planets that don’t have analogues in the solar system. Therefore the solar system is not the typical specimen. Models of planet accretion have to explain not only how planets form, but also why the outcomes of the accretion history can be so diverse. There is probably not one accretion process but several, depending on the scale at which accretion operates. A first process is the sticking of microscopic dust into larger grains and pebbles. A second process is the formation of an intermediate class of objects called planetesimals. There are still planetesimals left in the solar system. They are the asteroids orbiting between the orbits of Mars and Jupiter, the trans-Neptunian objects in the distant system, and other objects trapped along the orbits of the planets (Trojans) or around the giant planets themselves (irregular satellites). The Oort cloud, source of the long period comets, is also made of planetesimals ejected from the region of formation of the giant planets. A third accretion process has to lead from planetesimals to planets. Actually, several processes can be involved in this step, from collisional coagulation among planetesimals to the accretion of small particles under the effect of gas drag, to giant impacts between protoplanets. Adopting a historical perspective of all these processes provides details of the classic processes investigated in the past decades to those unveiled in the last years. The quest for planet formation is ongoing. Open issues remain, and exciting future developments are expected.

Article

George J. Flynn

Scattered sunlight from interplanetary dust particles, mostly produced by comets and asteroids, orbiting the Sun are visible at dusk or dawn as the Zodiacal Cloud. Impacts onto the space-exposed surfaces of Earth-orbiting satellites indicate that, in the current era, thousands of tons of interplanetary dust enters the Earth’s atmosphere every year. Some particles vaporize forming meteors while others survive atmospheric deceleration and settle to the surface of the Earth. NASA has collected interplanetary dust particles from the Earth’s stratosphere using high-altitude aircraft since the mid-1970s. Detailed characterization of these particles shows that some are unique samples of Solar System and presolar material, never affected by the aqueous and thermal processing that overprints the record of formation from the Solar Protoplanetary Disk in the meteorites. These particles preserve the record of grain and dust formation from the disk. This record suggests that many of the crystalline minerals, dominated by crystalline silicates (olivine and pyroxene) and Fe-sulfides, condensed from gas in the inner Solar System and were then transported outward to the colder outer Solar System where carbon-bearing ices condensed on the surfaces of the grains. Irradiation by solar ultraviolet light and cosmic rays produced thin organic coatings on the grain surfaces that likely aided in grain sticking, forming the first dust particles of the Solar System. This continuous, planet-wide rain of interplanetary dust particles can be monitored by the accumulation of 3He, implanted into the interplanetary dust particles by the Solar Wind while they were in space, in oceanic sediments. The interplanetary dust, which is rich in organic carbon, may have contributed important pre-biotic organic matter important to the development of life to the surface of the early Earth.

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

The study of planetary ionospheres helps us to understand the composition, losses, and electrical properties of the atmosphere. The structure of the ionosphere depends on the neutral gas composition as well. Models based on fundamental equations have been able to simulate the neutral and ion structure of the Martian atmosphere. These models couple chemical, physical, radiative, and dynamical processes at various levels of complexities. The lower ionosphere (below 80 km) and its composition have not been observed and studied as comprehensively as the upper ionosphere. Most of our current understanding of the plasma environment in the lower atmosphere is based on theoretical models. Models indicate that Mars contains a D region, similar to that in the Earth’s ionosphere, produced primarily due to high-energy galactic cosmic rays that can penetrate to the lower altitudes. The D layer has been simulated to lie in the altitude range of ~25 to 35 km on the dayside ionosphere of Mars. A one-dimensional model, used to calculate the densities of 35 positive and negative ions, predicts hydrated ions to be dominant in the troposphere of Mars. Due to the variability of water vapor, these cluster ions show seasonal variability and can be measured by future experiments on Mars landers. Dust is an important component of the climate of Mars, wherein dust storms are known to affect the temperatures and winds of the lower atmosphere. The inclusion of ion–dust interactions in the model for the Martian ionosphere has yielded important effects of dust storms on the ionosphere. It has been found that during dust storms, the ion densities can significantly diminish, reducing the total ion conductivity in the troposphere by an order of magnitude. Also, large electric fields could be generated due to the charging of dust in the ionosphere, leading to electric discharges and, possibly, lightning.

Article

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.