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.
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Article
The Lunar Dust Puzzle
Alexander V. Zakharov
Article
The Recognition of Meteorites and Ice Ages
Alan E. Rubin
Two important scientific questions that confronted 18th- and 19th-century naturalists were whether continental glaciation had occurred thousands of years earlier and whether extraterrestrial rocks occasionally fell to Earth. Eventual recognition of these hypotheses as real phenomena resulted from initial reports by nonprofessionals, subsequent investigation by skeptical scientists, and vigorous debate. Evidence that kilometer-thick glaciers had once covered Northern Europe and Canada included (a) the resemblance of scratched and polished rocks near mountain glaciers to those located in unglaciated U-shaped valleys; (b) the similarity of poorly sorted rocks and debris within “drift deposits” (moraines) to the sediment load of glaciers; and (c) the discovery of freezing meltwater at the base of glaciers, hypothesized to facilitate their movement. Three main difficulties naturalists had with accepting the notion that rocks fell from the sky were that (a) meteorite falls are localized events, generally unwitnessed by professional scientists; (b) mixed in with reports of falling rocks were fabulous accounts of falling masses of blood, flesh, milk, gelatin, and other substances; and (c) the phenomenon of falling rocks could neither be predicted nor verified by experiment. Five advances leading to the acceptance of meteorites were (a) Ernst Chladni’s 1794 treatise linking meteors, fireballs, and falling rocks; (b) meteor observations conducted in 1798 showing the high altitudes and enormous velocities of their meteoroid progenitors; (c) a spate of several widely witnessed meteorite falls between 1794 and 1807 in Europe, India, and America; (d) chemical analyses of several meteorites by Edward Charles Howard in 1802, showing all contained nickel (which is rare in the Earth’s crust); and (e) the discoveries of four asteroids between 1801 and 1807, providing a plausible extraterrestrial source for meteorites.
Article
Thermal Physics of Cometary Nuclei
Dina Prialnik
Cometary nuclei, as small, spinning, ice-rich objects revolving around the sun in eccentric orbits, are powered and activated by solar radiation. Far from the sun, most of the solar energy is reradiated as thermal emission, whereas close to the sun, it is absorbed by sublimation of ice. Only a small fraction of the solar energy is conducted into the nucleus interior. The rate of heat conduction determines how deep and how fast this energy is dissipated. The conductivity of cometary nuclei, which depends on their composition and porosity, is estimated based on vastly different models ranging from very simple to extremely complex. The characteristic response to heating is determined by the skin depth, the thermal inertia, and the thermal diffusion timescale, which depend on the comet’s structure and dynamics. Internal heat sources include the temperature-dependent crystallization of amorphous water ice, which becomes important at temperatures above about 130 K; occurs in spurts; and releases volatiles trapped in the ice. These, in turn, contribute to heat transfer by advection and by phase transitions. Radiogenic heating resulting from the decay of short-lived unstable nuclei such as 26Al heats the nucleus shortly after formation and may lead to compositional alterations. The thermal evolution of the nucleus is described by thermo-physical models that solve mass and energy conservation equations in various geometries, sometimes very complicated, taking into account self-heating. Solutions are compared with actual measurements from spacecraft, mainly during the Rosetta mission, to deduce the thermal properties of the nucleus and decipher its activity pattern.
Article
Trans-Neptunian Dwarf Planets
Bryan J. Holler
The International Astronomical Union (IAU) officially recognizes five objects as dwarf planets: Ceres in the main asteroid belt between Mars and Jupiter, and Pluto, Eris, Haumea, and Makemake in the trans-Neptunian region beyond the orbit of Neptune. However, the definition used by the IAU applies to many other trans-Neptunian objects (TNOs) and can be summarized as follows: Any non-satellite large enough to be rounded by its own gravity. Practically speaking, this means any non-satellite with a diameter larger than 400 km. In the trans-Neptunian region, there are more than 150 objects that satisfy this definition, based on published results and diameter estimates.
The dynamical structure of the trans-Neptunian region records the history of the migration of the giant planets in the early days of the solar system. The semi-major axes, eccentricities, and orbital inclinations of TNOs across various dynamical classes provide constraints on different aspects of planetary migration. For many TNOs, the orbital parameters are all that is known about them, due to their large distances, small sizes, and low albedos. The TNO dwarf planets are a different story. These objects are large enough to be studied in more detail from ground- and space-based observatories. Imaging observations can be used to detect satellites and measure surface colors, while spectroscopy can be used to constrain surface composition. In this way, TNO dwarf planets not only help provide context for the dynamical evolution of the outer solar system, but also reveal the composition of the primordial solar nebula as well as the physical and chemical processes at work at very cold temperatures.
The largest TNO dwarf planets, those officially recognized by the IAU, plus others like Sedna, Quaoar, and Gonggong, are large enough to support volatile ices on their surfaces in the present day. These ices are able to exist as solids and gases on some TNOs, due to their sizes and surface temperatures (similar to water on Earth) and include N2 (nitrogen), CH4 (methane), and CO (carbon monoxide). A global atmosphere composed of these three species has been detected around Pluto, the largest TNO dwarf planet, with the possibility of local atmospheres or global atmospheres at perihelion for Eris and Makemake. The presence of non-volatile species, such as H2O (water), NH3 (ammonia), and complex hydrocarbons, provides valuable information on objects that may be too small to retain volatile ices over the age of the solar system. In particular, large quantities of H2O mixed with NH3 point to ancient cryovolcanism caused by internal differentiation of ice from rock. Complex hydrocarbons, formed through radiation processing of surface ices, such as CH4, record the radiation histories of these objects and provide clues to their primordial surface compositions.
The dynamical, physical, and chemical diversity of the more than 150 TNO dwarf planets are key to understanding the formation of the solar system and its subsequent evolution to its current state. Most of our knowledge comes from a small handful of objects, but we are continually expanding our horizons as additional objects are studied in more detail.
Article
Vesta and Ceres
Kevin Righter
Asteroids 1 Ceres and 4 Vesta are the two most massive asteroids in the asteroid belt, with mean diameters of 946 km and 525 km, respectively. Ceres was reclassified as a dwarf planet by the International Astronomical Union as a result of its new dwarf planet definition which is a body that (a) orbits the sun, (b) has enough mass to assume a nearly round shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a moon. Scientists’ understanding of these two bodies has been revolutionized in the past decade by the success of the Dawn mission that visited both bodies. Vesta is an example of a small body that has been heated substantially and differentiated into a metallic core, silicate mantle, and basaltic crust. Ceres is a volatile-rich rocky body that experienced less heating than Vesta and has differentiated into rock and ice. These two contrasting bodies have been instrumental in learning how inner solar system material formed and evolved.
Article
Water Ice Permafrost on Mars and on the Moon
Maxim Litvak and Anton Sanin
The Moon and Mars are the most explored planetary bodies in the solar system. For the more than 60 years of the space era, dozens of science robotic missions have explored the Moon and Mars. The primary scientific goal for many of these missions was declared to be a search for surface or ground water/water ice and gaining an understanding of its distribution and origin.
Today, for the Moon, the focus of scientific exploration has moved to the lunar polar regions and permanently shadowed regions (PSRs). PSRs do not receive any direct sunlight and are frozen at very low temperatures (< 120 K), acting as cold traps. They are considered to be a storehouse that preserves records of the solar system’s evolution by trapping water ice and potentially other volatile deposits brought by comets and asteroids over billions of years.
For Mars, the water/water ice search was part of an attempt to find traces of ancient extraterrestrial life and possibly to understand how life appeared on Earth. Current Mars is cold and dry, but its high latitudes and some equatorial regions are enriched with surface and subsurface water ice. Scientists argue that oceans could have existed on ancient Mars if it was warm and wet and that different life forms could have originated similar to Earth’s. If this is the case, then biomarkers could be preserved in the Martian ground ice depositions.
Another popular idea that ties water ice permafrost on the Moon and Mars is related to the expected future human expansion to deep space. The Moon and Mars are widely considered to be the first destinations for future manned space-colony missions or even space-colony missions. In this scenario, the long-term presence and survival of astronauts on the lunar or Martian surface strongly depend on in situ resource utilization (ISRU). Water ice is at the top of the ISRU list because it could be used as water for astronauts’ needs. Its constituents, oxygen and hydrogen, could be used for breathing and for rocket fuel production, respectively.
The Moon is the closest body to Earth and discussion about presence of water ice on the Moon has both scientific and practical interest, especially for planning manned space missions. The focus further in space is on how subsurface water ice is distributed on Mars. A related topic is the debates about whether ancient Mars was wet and warm or if, for most of its history, the Martian surface was covered with glaciers. Finally, there are fundamental questions that should be answered by upcoming Mars and Moon missions.
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