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.
Leonid V. Ksanfomality
Cometary nuclei are small, despite the cosmic scale of the comet tails that they produce. The nuclei have the ability to create rarefied atmospheres, extending as a tail to giant distances comparable to the orbital distances of the planets. Giant tails of comets are sometimes observed for several years and cover a significant part of the sky. The cometary nucleus is capable of continuously renewing tails and supporting the material that is constantly dissipating in space. Large comets do not appear so often that they have become trivial celestial phenomena, but they appear often enough to allow astronomers to complete detailed studies. Many remarkable discoveries, such as the discovery of solar wind, were made during the study of comets. Comets are characterized by great diversity, and their appearance often becomes an ornament of the night sky. Comets have become remote laboratories, where experiments are performed in physical conditions that are not achievable on Earth.
Astrobiology seeks to understand the origin, evolution, distribution, and future of life in the universe and thus to integrate biology with planetary science, astronomy, cosmology, and the other physical sciences. The discipline emerged in the late 20th century, partly in response to the development of space exploration programs in the United States, Russia, and elsewhere. Many astrobiologists are now involved in the search for life on Mars, Europa, Enceladus, and beyond. However, research in astrobiology does not presume the existence of extraterrestrial life, for which there is no compelling evidence; indeed, it includes the study of life on Earth in its astronomical and cosmic context. Moreover, the absence of observed life from all other planetary bodies requires a scientific explanation, and suggests several hypotheses amenable to further observational, theoretical, and experimental investigation under the aegis of astrobiology. Despite the apparent uniqueness of Earth’s biosphere— the “n = 1 problem”—astrobiology is increasingly driven by large quantities of data. Such data have been provided by the robotic exploration of the Solar System, the first observations of extrasolar planets, laboratory experiments into prebiotic chemistry, spectroscopic measurements of organic molecules in extraterrestrial environments, analytical advances in the biogeochemistry and paleobiology of very ancient rocks, surveys of Earth’s microbial diversity and ecology, and experiments to delimit the capacity of organisms to survive and thrive in extreme conditions.
Edward R. D. Scott
Iron meteorites are thought to be samples of metallic cores and pools that formed in diverse small planetary bodies. Their great diversity offers remarkable insights into the formation of asteroids and the early history of the solar system. The chemical compositions of iron meteorites generally match those predicted from experimental and theoretical considerations of melting in small bodies. These bodies, called planetesimals, were composed of mixtures of grains of silicates, metallic iron-nickel, and iron sulfide with compositions and proportions like those in chondrite meteorites. Melting in planetesimals caused dense metal to sink through silicate so that metallic cores formed. A typical iron meteorite contains 5–10% nickel, ~0.5% cobalt, 0.1–0.5% phosphorus, 0.1–1% sulfur and over 20 other elements in trace amounts. A few percent of iron meteorites also contain silicate inclusions, which should have readily separated from molten metal because of their buoyancy. They provide important evidence for impacts between molten or partly molten planetesimals. The major heat source for melting planetesimals was the radioactive isotope 26Al, which has a half-life of 0.7 million years. However, a few iron meteorites probably formed by impact melting of chondritic material. Impact processes were also important in the creation of many iron meteorites when planetesimals were molten. Chemical analysis show that most iron meteorites can be divided into 14 groups: about 15% appear to come from another 50 or more poorly sampled parent bodies. Chemical variations within all but three groups are consistent with fractional crystallization of molten cores of planetesimals. The other three groups are richer in silicates and probably come from pools of molten metal in chondritic bodies. Isotopic analysis provides formation ages for iron meteorites and clues to their provenance. Isotopic dating suggests that the parent bodies of iron meteorites formed before those of chondrites, and some irons appear to be the oldest known meteorites. Their unexpected antiquity is consistent with 26Al heating of planetesimals. Bodies that accreted more than ~2 million years after the oldest known solids (refractory inclusions in chondrites) should not have contained enough 26Al to melt. Isotopic analysis also shows that iron meteorites, like other meteorite types, display small anomalies due to pre-solar grains that were not homogenized in the solar nebula (or protoplanetary disk). Although iron meteorites are derived from asteroids, their isotopic anomalies provide the best clues that some come from planetesimals that did not form in the asteroid belt. Some may have formed beyond Jupiter; others show isotopic similarities to Earth and may have formed in the neighborhood of the terrestrial planets. Iron meteorites therefore contain important clues to the formation of planetesimals that melted and they also provide constraints on theories for the formation of planets and asteroids.
Larry W. Esposito
Saturn’s rings are not only a beautiful and enduring symbol of space, but astronomers’ best local laboratory for studying phenomena in thin cosmic disks like those where planets formed. All the giant planets have ring systems. Saturn’s are the biggest and brightest. Saturn’s rings are made of innumerable icy particles, ranging from the size of dust to that of football stadiums. Galileo discovered Saturn’s rings with his newly invented telescope, but they were not explained until Huygens described them as thin, flat disks surrounding the planet. In the space age, rings were found around the other giant planets in our solar system. Rings have been seen around asteroids and likely exist around exoplanets. Many of the ring structures seen are created by gravity from Saturn’s moons. Rings show both ongoing aggregation and disaggregation. After decades of study from space and by theoretical analysis, some puzzles still remain unexplained. There is evidence for youthful rings from Cassini results, but no good theory to explain their recent origin. A future Saturn Ring Observer mission would be able to determine the direct connections between the individual ring physical properties and the origin and evolution of larger structures.