The use of impact crater densities to estimate the ages of planetary surfaces began in the 1960s. Some predictive successes have been confirmed with radiometric dating of sites on the Moon and Mars. The method is highly dependent on our understanding of the rate of crater formation on different worlds, and, more importantly, on the history of that rate, starting with intense cratering during planetary formation 4.5 Ga ago. The system is thus calibrated by obtaining radiometric dates from samples of relatively homogeneous geologic units on various worlds. Crater chronometry is still in its infancy. Future sample-returns and in situ measurements, obtained by missions from collaborating nations to various worlds, will provide ever-increasing improvements in the system in coming decades. Such data can lead to at least two-significant-figure measurements, not only of the ages of broad geologic provinces on solar system worlds, but of the characteristic survival times of various-sized smaller craters. Such data, in turn, clarify the rates of turnover of surface materials and the production rates of gravel-like regolith and megaregolith in the surface layers. Better measurements of the impact rate at various times, in turn, support better modeling of the accretion and fragmentation processes among early planetesimals as well as contemporary asteroids, in various parts of the solar system. Once the crater chronometry system is calibrated for various planetary bodies, important chronological information about those various planetary bodies can be obtained by orbital missions, without the need for expensive sample-return or lander missions on each individual surface.
William K. Hartmann
The asteroid impact near the Russian city of Chelyabinsk on February 15, 2013, was the largest airburst on Earth since the 1908 Tunguska event, causing a natural disaster in an area with a population exceeding 1 million. On clear morning at 9:20 a.m. local time, an asteroid about 19 m in size entered the Earth atmosphere near southern Ural Mountains (Russia) and, with its bright illumination, attracted the attention of hundreds of thousands of people. Dust trail in the atmosphere after the bolide was tens of kilometers long and was visible for several hours. Thousands of different size meteorites were found in the areas south-southwest of Chelyabinsk. A powerful airburst, which was formed due to meteoroid energy deposition, shattered thousands of windows and doors in Chelyabinsk and wide surroundings, with flying glass injuring many residents. The entrance and destruction of the 500-kt Chelyabinsk asteroid produced a number of observable effects, including light and thermal radiation; acoustic, infrasound, blast, and seismic waves; and release of interplanetary substance. This unexpected and unusual event is the most well-documented bolide airburst, and it attracted worldwide attention. The airburst was observed globally by multiple instruments. Analyses of the observational data allowed determination of the size of the body that caused the superbolide, its velocity, its trajectory, its behavior in the atmosphere, the strength of the blast wave, and other characteristics. The entry of the 19-m-diameter Chelyabinsk asteroid provides a unique opportunity to calibrate the different approaches used to model meteoroid entry and to calculate the damaging effects. The recovered meteorite material was characterized as brecciated LL5 ordinary chondrite, in which three different lithologies can be distinguished (light-colored, dark-colored, and impact-melt). The structure and properties of meteorites demonstrate that before encountering Earth, the Chelyabinsk asteroid had experienced a very complex history involving at least a few impacts with other bodies and thermal metamorphism. The Chelyabinsk airburst of February 15, 2013, was exceptional because of the large kinetic energy of the impacting body and the damaging airburst that was generated. Before the event, decameter-sized objects were considered to be safe. With the Chelyabinsk event, it is possible, for the first time, to link the damage from an impact event to a well-determined impact energy in order to assess the future hazards of asteroids to lives and property.
A magma ocean is a global layer of partially or fully molten rocks. Significant melting of terrestrial planets likely occurs due to heat release during planetary accretion, such as decay heat of short-lived radionuclides, impact energy released by continuous planetesimal accretion, and energetic impacts among planetary-sized bodies (giant impacts). Over a magma ocean, all water, which is released upon impact or degassed from the interior, exists as superheated vapor, forming a water-dominated, steam atmosphere. A magma ocean extending to the surface is expected to interact with the overlying steam atmosphere through material and heat exchange. Impact degassing of water starts when the size of a planetary body becomes larger than Earth’s moon or Mars. The degassed water could build up and form a steam atmosphere on protoplanets growing by planetesimal accretion. The atmosphere has a role in preventing accretion energy supplied by planetesimals from escaping, leading to the formation of a magma ocean. Once a magma ocean forms, part of the steam atmosphere would start to dissolve into the surface magma due to the high solubility of water into silicate melt. Theoretical studies indicated that as long as the magma ocean is present, a negative feedback loop can operate to regulate the amount of the steam atmosphere and to stabilize the surface temperature so that a radiative energy balance is achieved. Protoplanets can also accrete the surrounding H 2 -rich disk gas. Water could be produced by oxidation of H 2 by ferrous iron in the magma. The atmosphere and water on protoplanets could be a mixture of outgassed and disk-gas components. Planets formed by giant impact would experience a global melting on a short timescale. A steam atmosphere could grow by later outgassing from the interior. Its thermal blanketing and greenhouse effects are of great importance in controlling the cooling rate of the magma ocean. Due to the presence of a runaway greenhouse threshold, the crystallization timescale and water budget of terrestrial planets can depend on the orbital distance from the host star. The terrestrial planets in our solar system essentially have no direct record of their earliest history, whereas observations of young terrestrial exoplanets may provide us some insight into what early terrestrial planets and their atmosphere are like. Evolution of protoplanets in the framework of pebble accretion remains unexplored.
Impacts of small celestial bodies, in terms of energy density, occupy the range between ordinary chemical high explosives and nuclear explosions. The high initial energy density of impact gives them some features of an explosion (shock waves, melting and vaporization, mechanical disruption of target rocks). A near-surface burst creates an explosion crater, and an impact often results in the creation of an impact crater. The chain of processes connected to an impact crater’s formation is named “impact cratering” or simply “cratering.” The initial kinetic energy and momenta of the impacting body (“projectile”) generates shock waves (decaying with propagation to seismic waves), heats the material (at high impact velocities, to melt or to boil target rocks). A part of the kinetic energy is converted to target material motion, creating the crater cavity. The final crater geometry depends on the scale of event—while small craters are simple bowl-shaped cavities, large enough crater transient cavities collapse in the gravity field. If collapse takes place, the final crater has a complex geometry with central peaks and concentric inner rings. The boundary crater diameter, dividing simple and complex craters, varies with target body gravity and rock strength. Comparison of a crater’s morphology on remote planets and asteroids allows us to make some estimates about their mechanical parameters (e.g., strength and friction) even before future sample return missions. On many planets large impact craters can be seen, preserved much better than on the geologically active Earth. These observations help researchers to interpret the geological and geophysical data obtained for the relatively few and heavily modified large impact craters found on continents and (rarely) at the sea bottom.
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.
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.
The study of active asteroids is a relatively new field of study in Solar System science, focusing on objects with asteroid-like orbits but that exhibit comet-like activity. This field, which crosses traditionally drawn lines between research focused on inactive asteroids and active comets, has motivated reevaluations of classical assumptions about small Solar System objects and presents exciting new opportunities for learning more about the origin and evolution of the Solar System. Active asteroids whose activity appears to be driven by the sublimation of volatile ices could have significant implications for determining the origin of the Earth’s water—and therefore its ability to support life—and also challenge traditional assumptions about the survivability of ice in the warm inner Solar System. Meanwhile, active asteroids whose activity appears to be caused by disruptive processes such as impacts or rotational destabilization provide exciting opportunities to gain insights into fundamental processes operating in the asteroid belt and assessing their effects on the asteroid population seen in the 21st century.
Bradley L. Jolliff
Earth’s moon, hereafter referred to as “the Moon,” has been an object of intense study since before the time of the Apollo and Luna missions to the lunar surface and associated sample returns. As a differentiated rocky body and as Earth’s companion in the solar system, much study has been given to aspects such as the Moon’s surface characteristics, composition, interior, geologic history, origin, and what it records about the early history of the Earth-Moon system and the evolution of differentiated rocky bodies in the solar system. Much of the Apollo and post-Apollo knowledge came from surface geologic exploration, remote sensing, and extensive studies of the lunar samples. After a hiatus of nearly two decades following the end of Apollo and Luna missions, a new era of lunar exploration began with a series of orbital missions, including missions designed to prepare the way for longer duration human use and further exploration of the Moon. Participation in these missions has become international. The more recent missions have provided global context and have investigated composition, mineralogy, topography, gravity, tectonics, thermal evolution of the interior, thermal and radiation environments at the surface, exosphere composition and phenomena, and characteristics of the poles with their permanently shaded cold-trap environments. New samples were recognized as a class of achondrite meteorites, shown through geochemical and mineralogical similarities to have originated on the Moon. New sample-based studies with ever-improving analytical techniques and approaches have also led to significant discoveries such as the determination of volatile contents, including intrinsic H contents of lunar minerals and glasses. The Moon preserves a record of the impact history of the solar system, and new developments in timing of events, sample based and model based, are leading to a new reckoning of planetary chronology and the events that occurred in the early solar system. The new data provide the grist to test models of formation of the Moon and its early differentiation, and its thermal and volcanic evolution. Thought to have been born of a giant impact into early Earth, new data are providing key constraints on timing and process. The new data are also being used to test hypotheses and work out details such as for the magma ocean concept, the possible existence of an early magnetic field generated by a core dynamo, the effects of intense asteroidal and cometary bombardment during the first 500 million–600 million years, sequestration of volatile compounds at the poles, volcanism through time, including new information about the youngest volcanism on the Moon, and the formation and degradation processes of impact craters, so well preserved on the Moon. The Moon is a natural laboratory and cornerstone for understanding many processes operating in the space environment of the Earth and Moon, now and in the past, and of the geologic processes that have affected the planets through time. The Moon is a destination for further human exploration and activity, including use of valuable resources in space. It behooves humanity to learn as much about Earth’s nearest neighbor in space as possible.
Long Xiao and James W. Head
The geological characteristics of the Moon provide the fundamental data that permit the study of the geological processes that have formed and modified the crust, that record the state and evolution of the lunar interior, and that identify the external processes that have been important in lunar evolution. Careful documentation of the stratigraphic relationships among these features can then be used to reconstruct the sequence of events and the geological history of the Moon. These results can then be placed in the context of the geological evolution of the terrestrial planets, including Earth. The Moon’s global topography and internal structures include landforms and features that comprise the geological characteristics of its surface. The Moon is dominated by the ancient cratered highlands and the relatively younger flat and smooth volcanic maria. Unlike the current geological characteristics of Earth, the major geological features of the Moon (impact craters and basins, lava flows and related features, and tectonic scarps and ridges) all formed predominantly in the first half of the solar system’s history. In contrast to the plate-tectonic dominated Earth, the Moon is composed of a single global lithospheric plate (a one-plate planet) that has preserved the record of planetary geological features from the earliest phases of planetary evolution. Exciting fundamental outstanding questions form the basis for the future international robotic and human exploration of the Moon.