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Article

Henry Hsieh

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.

Article

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.

Article

Alexander T. Basilevsky

Lunar and planetary geology can be described using examples such as the geology of Earth (as the reference case) and geologies of the Earth’s satellite the Moon; the planets Mercury, Mars and Venus; the satellite of Saturn Enceladus; the small stony asteroid Eros; and the nucleus of the comet 67P Churyumov-Gerasimenko. Each body considered is illustrated by its global view, with information given as to its position in the solar system, size, surface, environment including gravity acceleration and properties of its atmosphere if it is present, typical landforms and processes forming them, materials composing these landforms, information on internal structure of the body, stages of its geologic evolution in the form of stratigraphic scale, and estimates of the absolute ages of the stratigraphic units. Information about one body may be applied to another body and this, in particular, has led to the discovery of the existence of heavy “meteoritic” bombardment in the early history of the solar system, which should also significantly affect Earth. It has been shown that volcanism and large-scale tectonics may have not only been an internal source of energy in the form of radiogenic decay of potassium, uranium and thorium, but also an external source in the form of gravity tugging caused by attractions of the neighboring bodies. The knowledge gained by lunar and planetary geology is important for planning and managing space missions and for the practical exploration of other bodies of the solar system and establishing manned outposts on them.

Article

Comets  

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.

Article

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.