Why Do You Think That The Speeds Of Comets Increase As They Near The Sun?

Comet Tails

Second, comet tails were observed to stream out radially from the Sun. 3rd, the extent of the corona during solar eclipses was surprisingly big and looked very piddling like a gravitationally bound, spherically symmetrical temper.

From: International Geophysics , 2004

Archaic Solar Arrangement Objects: Asteroids and Comets

Lucy-Ann McFadden , Daniel T. Britt , in Encyclopedia of Concrete Science and Technology (3rd Edition), 2003

III.B.3 Tails

Comet tails come up in ii types. The big and often curved tails, an example of which is shown in Fig. 8, are blazon II tails and are composed of grit. Grit particles are typically ejected from the surface of a comet by the gas jets created past warming frozen volatiles. This gas-powered ejection, every bit well as forces from the dominicus's gravity, solar radiation pressure, and the differing masses and ejection directions of the dust particles, puts the dust into orbits of their own that steadily diverge from the parent comet. The orbital/velocity differences between the comet and it ejected dust tend to produce a curved tail relative to the comet–sun line. Comet tails volition always point away from the dominicus because of the radiation force per unit area of sunlight. The strength from sunlight on the small grit particles pushing them away from the sun is greater than the force of gravity interim in the direction toward the sun. As a effect, during its inbound passage a comet'due south tail streams backside the nucleus, only on its outbound passage back to the outer solar system the tail is in forepart of the nucleus.

The dust analyzers on the Soviet VEGA spacecraft revealed the existence of at to the lowest degree three classes of dust grains. Ane grade is composed of depression-atomic-number elements, primarily carbon, hydrogen, oxygen, and nitrogen, chosen CHON particles. A second is similar to the composition of CI carbonaceous chondrite meteorites but enriched in carbon. The third type is equivalent to a hydrogen-enriched CHON particle. A significant finding from the spacecraft explorations of comet Halley was that the "parents" of some of the gaseous species might be small dust grains rather than larger, stable molecules.

Additionally, the dust detectors revealed large quantities of very pocket-sized particles. The smallest size detected was 0.01 g, bold a density of 1.0 1000 cm⁻³. Their distribution was unanticipated, as well. The smallest particles were detected much farther from the nucleus than expected based on classical theory where radiation pressure decelerates the particles. Possible explanations of this phenomenon include (i) the grains do not absorb radiation, (2) larger grains break up after leaving the nucleus, and (three) the grains are electrically accelerated by virtue of their electric accuse and their presence in the magnetized plasma environs.

Some comets also take tails pointing straight back from the comet–dominicus line, as can be seen in Fig. viii, illustrating blazon I tails. These are composed of molecules that take physically interacted with charged particles emitted from the sun called the solar wind. Such interactions consequence in charged molecules (called plasma) that emit calorie-free. Plasma tails follow the path of the solar wind (which travels radially away from the lord's day). Thus, plasma tails class straight lines backside the comet head post-obit the direction of the solar wind. They eventually finish to receive plenty light to be seen and dissipate after leaving the inner solar system. Sometimes these tails grade kinks or ropey structures, or break up entirely. This is due to changes in the flux of particles leaving the sunday in the direction of the comet or to jetting from the comet itself. Studying the dynamics of these tails tells united states of america almost the interaction of the lord's day with other particles in the solar arrangement.

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Volume i

Dina Prialnik , in Encyclopedia of Geology (Second Edition), 2022

Historical Record

The oldest, most extensive and most authentic records of comet observations originate in China. Remarkably, equally far back equally the seventh century BCE, Chinese astronomers were already aware that comet tails bespeak away from the Sunday. The effort to understand comets started in ancient Greece: Were comets celestial objects or rather atmospheric phenomena? The Greek philosophers of the sixth and 5th centuries BCE believed them to exist celestial bodies, akin to planets. However, the idea that prevailed for virtually 2000 years was that of Aristotle (384–322 BCE), who regarded comets equally sudden atmospheric phenomena, ruling out the possibility of a planetary origin, because the irregular features and unpredictable apparitions of these objects contradicted his philosophical concept of the unchanging nature of the universe. Moreover, the atmospheric origin could apparently explain the widespread belief that comets were omens of droughts, high winds and diverse other disasters (see Fig. 1). A potent support to this idea was provided by the fact that comets are observable with the naked eye merely about the horizon (near the surface of the Globe), at dusk or dawn.

During the Heart Ages comets were oftentimes recorded, but the commencement attempts to measure the distance to comets were made only during the 15th century. The beginning catalogues of comets were issued around 1550. Only by the beginning of the 17th century, when Aristotle's views were finally abandoned, it became accepted that comets are afar objects.

The next puzzle concerned cometary orbits. It was solved towards the end of the 17th century, mainly attributable to Isaac Newton's theory of gravitation. The shapes of the orbits were recognized every bit ellipses, parabolas or hyperbolas. Thus Newton concluded that comets were a sort of planet, revolving in very eccentric orbits effectually the Sun. He also postulated that comets polish by reflected sunlight and comet tails ascend by vapors emanating from their surfaces.

British astronomer Edmond Halley (1656–1742) played a crucial office in Newton's involvement in comets. Halley found that the orbital elements for the comet discovered in 1682 showed close correspondence with the comets of 1607 and of 1531 (and probably too the comet of 1456). The periods (75 to 76 years) were besides similar. His proposition that this was 1 and the same comet—and his prediction of its return in 1758—was the first application of Newton's laws of move. When indeed information technology reappeared as predicted, subsequently Halley'southward death, the comet was named after him. Thus, in the 18th century, comets were finally established every bit members of the solar system.

The mass of a comet was the new puzzle, the huge size of the tail misleading in the estimate of the nucleus size. For example, Immanuel Kant, in his 'Universal Natural History and Theory of the Heavens' published in 1755, considered comets to be basically like to planets, differing from them only in their place of origin. However, in 1770, comet Lexell passed very close to Earth, but had no perceptible effect on World's move. Based on this observation, French mathematician Pierre-Simon Laplace was able to derive an upper limit to the mass of the comet, as depression as 1/5000 the mass of the World. Comets are indeed among the smallest bodies in the solar system.

Some other phenomenon related to comets was revealed by an intriguing event that took place in 1846: comet Biela—discovered in 1826 and detected again in 1832—was observed to suddenly split into 2 singled-out fragments, which connected to journey adjacent along the same orbit. Eventually, both parts of Biela developed into complete comets with tails and both reappeared in 1852, but they were never seen again. Yet, in 1867 and over again in 1872 meteor showers were detected, identified with Biela'due south orbit, and these showers were considered to be the droppings of the comet. Past now, the connexion betwixt the orbits of meteor streams and those of comets has become a well-established fact.

The major effect related to comets at the showtime of the 20th century was the render of Halley'due south comet in 1910. The comet returned again in 1986, when information technology was met by an armada of spacecraft designed to probe and measure out all aspects of its activeness. One of the major achievements of the close encounter observations of comet Halley was the first close-upward image of a comet nucleus taken past the spacecraft Giotto, revealing its shape and surface structure (see Fig. 2). The image, showing a highly irregular shape, a nighttime surface and jets of gas and grit emitted from localized areas, took astronomers by surprise and instigated an unprecedented interest in the study of comets that led to a serial of space missions of growing composure.

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Comet Assay

Solange Costa , João Paulo Teixeira , in Encyclopedia of Toxicology (Tertiary Edition), 2022

Emerging Applications

Fluorescence in situ Hybridization

Fluorescence in situ hybridization (FISH) is a method used to locate specific DNA sequences within interphase chromatin and metaphase chromosomes and to place both structural and numerical chromosome changes. FISH coupled with Comet Assay can provide unique knowledge about the Deoxyribonucleic acid damage and repair in specific genes and DNA sequences. By hybridizing fluorescently labeled probes to Deoxyribonucleic acid after electrophoresis, Deoxyribonucleic acid damage in particular genes and DNA sequences can be measured by the Comet Assay. If the DNA of the gene is constitute in the comet tail, this indicates that a Deoxyribonucleic acid interruption had occurred in the proximity of the gene. In improver, Comet-FISH can be practical as a starting time look of the three-dimensional arrangement of genomic loci and elucidation of mechanisms of comet formation and DNA arrangement in comets.

Chromosome

This recent application is based on the chromosome isolation protocols used for whole chromosome mounting in electron microscopy, in combination with Comet Assay, to visualize putative DNA harm in subnuclear structures. The results show that migrant Deoxyribonucleic acid fragments can exist visualized in whole nuclei and isolated chromosomes and that they exhibit patterns of DNA migration that depend on the level of DNA impairment produced.

DNA Repair

In add-on to evaluate DNA harm, the Comet Assay can be used in different ways to measure the Deoxyribonucleic acid repair chapters at cellular level.

One uncomplicated arroyo is to treat cells with a DNA-damaging agent, incubate them in a culture medium (37 °C, CO_ii bedroom), and monitor the speed with which they remove the lesions. At intervals, samples are taken for assay with Comet Assay. The charge per unit at which the damage is removed indicates the efficiency of repair.

Alternatively the repair chapters of cells extract can be assessed in an in vitro assay and practical in population studies.

Dna repair is a major factor in individual susceptibility to cancer. Even so, little is known about the degree of variation of intra- and interindividual capacity for DNA repair, which can be explained past the limitations of the available methods. One option is to measure the expression of Dna repair genes, but results often do non correlate with the enzymatic repair activity.

Modified versions of the Comet Assay are able to measure enzyme repair activity and hence be used in human biomonitoring studies.

Dna repair chapters is evaluated by extracting Dna repair enzymes from cells and measure the excerpt'southward power to repair Dna lesions in a substrate. The substrate is composed of cells previously treated with a Deoxyribonucleic acid-dissentious agent, appropriate for the type of repair existence measured. After lysis the cells with induced Deoxyribonucleic acid damage are incubated with the excerpt containing DNA repair enzymes. The excerpt'south incision activity (the start pace in the repair process) on Deoxyribonucleic acid impairment is measured by the Comet Analysis. Thus the efficiency of both base excision repair and nucleotide excision repair pathways can be assessed.

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MICROSCOPY TECHNIQUES | Sample Preparation for Calorie-free Microscopy

C. Hammond , P.J. Evennett , in Encyclopedia of Analytical Science (Second Edition), 2005

Surface Treatments

For many materials (eastward.g., rocks, ceramics) no farther treatment is required and the elective phases may exist sufficiently well revealed past their differences in color, bireflectance, and surface relief. For metals and alloys, etching treatments are required, but in any event all materials should be examined in the as-polished condition to check for artifacts (e.g., localized smears or 'comet-tails') and to notice fibroid inclusions and porosity that may exist afflicted by carving processes.

Etching may exist divers equally the selective set on of grain and interphase boundaries and/or the selective assault of the phases themselves. In complex microstructures no single etchant may be effective and unlike etchants may exist required in order to reveal unlike elements of the microstructure. Etchants usually consist of aqueous or alcoholic solutions and mixtures of salts, acids, and alkalis; the more than corrosion-resistant metals and alloys (due east.one thousand., stainless steels) naturally require more aggressive solutions. The etching process consists simply of dipping or swabbing the specimen in the etchant, or of electrolysis, with the specimen acting as the anode of the electrolytic cell. The range of etchants and etching weather practically equals the range of materials and microstructures to be examined. Similar staining, it is very much a affair of feel and experimental skill – incorrect choice of etchants, etching time, etc., but leads to an overall assault on the specimen surface, revealing no detail at all. Ion bombardment is another method for carving.

The carving process may exist partially combined with the final stage of polishing: the etchant is added to the polishing chemical compound and contributes to the overall material removal rate as well as reducing any tendency to smearing. However, a postpolishing carving phase is normally required.

Finally, contrast between phases may be revealed or enhanced past developing a thin (interference) film on the every bit-polished or polished-and-etched specimen surface. The technique is based on multiple-beam interference betwixt the light reflected from the air–film and moving-picture show–substrate surfaces, and the colors that occur are determined past such factors as pic thickness, refractive index, and absorption coefficient. The film may be developed by anodizing (in which case it is restricted to oxides of the metal specimen) or it may be deposited by vacuum evaporation or reactive-ion sputtering (in which case there is no brake either to the deposited film or to the specimen). Anodizing is carried out using a range of oxidizing reagents and is a process analogous to electroetching except that the cell conditions are adjusted to develop an oxide film on the specimen (anode) surface. In do, the ii processes may exist carried out sequentially.

Contrast enhancement by deposition of a metal oxide, sulfide, selenide, etc., film is a relatively new technique (interference film microscopy) and also lends itself to quantitative estimation.

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The Outer Heliosphere: The Next Frontiers

E.North. Parker , in COSPAR Colloquia Series, 2001

1 INTRODUCTION

Mayhap the identify to begin is the realization that almost all stars obviously take "astrospheres", all also transparent to be seen merely undoubtedly as complex as our own heliosphere. The stellar wind is the creator of the astrosphere, but as the solar wind sweeps out the cavity in interstellar space that we telephone call the heliosphere. Thus the origin of the heliosphere and the astrosphere traces back to the hydrodynamics of the meg degree solar and stellar coronas. The solar corona appears to be created by the dissipation of mechanical and magnetic free energy in the tenuous gas to a higher place the dense photosphere. It is that dissipation, evidently in the form of the microflaring in the magnetically "quiet" regions of the Dominicus, that creates the heliosphere. The staggering complexity of the convective and magnetic machinations on all scales down into the unresolved microstructure of the solar activeness gives some idea of the mystery of the stellar corona and astrosphere. Indeed, the mystery does non cease with the microflaring, for we are in the dark equally to the origin of the fibril magnetic fields that seem to drive the system from beneath the visible surface. With the variety of stellar types and circumstances that may be presumed to create stellar winds and astrospheres, the inquiry into the heliosphere and the extrapolation to other stars is bewildering.

The kickoff archaic model of the heliosphere was sketched some 45 years ago, and the subject has come a long style since that fourth dimension with the advent of the infinite age. We brainstorm, then, by noting that the heliosphere evidently has been in identify since the germination of the Sun and Earth some 4.6 × 10⁹ years ago. Unknown to classical astronomy, the heliosphere remained "silent" until the advance of engineering science and science first began to uncover its effects. Only in the last half century accept we appreciated its existence. So in one case we ventured into infinite the "silent" heliosphere became noisy indeed. There are a number of terrestrial effects, but in the early on years they were more puzzling than informative. Some furnishings are obvious, e.g. the aurora, while others east.chiliad. geomagnetic fluctuations, cosmic ray variations, etc. are detected only by scientific instruments. It was the geomagnetic tempest that a century ago first suggested bursts of "solar corpuscular radiation" from the Lord's day, consisting mainly of protons and an equal number of electrons to provide electrical neutrality. Otherwise space was regarded as a difficult vacuum capable of supporting unlimited electric potential differences, at the same time that the zodiacal light was interpreted as sunlight scattered from about 500 complimentary electrons/cm^three at the distance of World (1 AU). Then about half a century ago Biermann's ([1], [2] ) studies of the anti-solar acceleration of comet tails led to his fundamental pronouncement of the perpetual universal emission of solar corpuscular radiation. The velocity of the solar corpuscular radiation had long been estimated at 10 ³ km/sec, from the time delay of a couple of days between the flaring on the Sun and the affect of the corpuscular radiation against the outer boundary of the geomagnetic field. Biermann inferred from the measured anti-solar dispatch of gaseous comet tails that the number density of the solar corpuscular radiations at the orbit of Earth is in excess of 103 electrons and ions per cm 3, after revised downward to perhaps as little every bit 500/cm a based on resonant charge exchange with the cometary atoms. This density seemed to exist confirmed by the comparable interplanetary electron density inferred from the intensity of the zodiacal low-cal, considered at that time to be Thomson scattering of sunlight by gratis electrons. So the solar corpuscular radiation was powerful stuff. Its impact against the geomagnetic dipole field was calculated to confine the field to a distance of about five Globe's radii on the sunward side.

Leverett Davis ([6]) conceived the first sketches of the heliosphere, reproduced in Fig. 1, based on Biermann'southward annunciation of universal solar corpuscular radiations. Davis referred to it as the "cavity in the galactic magnetic field", the term heliosphere originating only thirteen years later in an article by A. J. Dessler. From the existing estimates of the density and velocity of the solar corpuscular radiation Davis suggested that the corpuscular radiation pushed back the interstellar gas and field to a radius of the order of 200 AU. He recognized that the radius of the heliosphere would vary with the 11-year magnetic cycle of the Sun, and he suggested that the varying size of the heliosphere was responsible for the observed variation of the catholic ray intensity within the heliosphere.

It should be noted hither that the origin of the solar corpuscular radiation at the Sun was a mystery at that time, with vague ideas almost acceleration in or around the magnetic fields of active regions, sunspots, and flares. Thus the origin was fabricated fifty-fifty more than mysterious by Biermann's basic point that the Sun emitted corpuscular radiation in all directions at all times, regardless of the presence or absence of magnetic agile regions.

At present by 1956 John Simpson ([25], [thirteen]) had succeeded in determining the free energy spectrum of the variation of the catholic ray intensity with the varying level of activity of the Sun. The variations were first detected by Scott Forbush, using ion chambers, which are sensitive to the muons produced in the atmosphere by catholic ray protons with energies of 10-20 Gev and up. Simpson invented the cosmic ray neutron monitor which responds to the nucleonic component in the atmosphere, thereby registering the effect of catholic ray protons down to near 1 Gev, where the time variations are much larger. Using five neutron monitors distributed from the geomagnetic equator to Chicago (at 55° geomagnetic latitude) he exploited the geomagnetic field of World equally a magnetic spectrometer. He showed that the variations had an energy spectrum that could not exist a consequence of an electrostatic potential difference in space, which would exist presumed to subtract the energy of each particle by the same amount. Instead, the variations, apart from the bursts of solar cosmic rays from the occasional big flare, showed simply a removal of particles that increased with declining cosmic ray particle energy. He noted that the variations suggested time varying magnetic fields in space.

The great cosmic ray flare of 23 February 1956 showed direct passage of the solar catholic rays from their origin on the Sun to Earth, arriving promptly at Earth from the management of the Sun ([12]). Thereafter the solar catholic ray intensity was observed to decline slowly every bit if escaping by diffusing through a magnetic barrier get-go at about the orbit of Mars and extending outward to the orbit of Jupiter. The simplest model suggested by the observations was a radial magnetic field extending from the Sun out to the orbit of Mars, with a matted nonradial magnetic field beyond.

Collectively this indicated a dynamical state of the solar corpuscular radiation and magnetic field in interplanetary space. The challenge, then, was to understand how the corpuscular radiation and interplanetary magnetic field were created by the Dominicus.

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Physics and Chemistry of the Solar System

In International Geophysics, 2004

Discovery of the Solar Current of air

The beginning evidence suggestive of the beingness of a radial outflow of very hot ionized gas (plasma) from the Sun was the observation well-nigh a century ago that there was a close correlation betwixt solar flares and terrestrial auroral activeness, which frequently became prominent a few days afterward a major flare. The agonizing influence therefore traveled at a typical speed of about ane AU per three days, or (1.5 × 10^eight km)/(3 × 10^five s) = 500 km s⁻¹. These events also were sometimes seen to exist associated with episodes of erratic magnetic activity on World (magnetic storms). Also, at about the same time observations of the corona during solar eclipses showed that the intensity of continuum light from regions out to several solar radii was startlingly strong. This was clearly not molecular emission and was attributed to scattering of visible sunlight either by dust particles (Lord Kelvin) or by electrons (Becquerel, Fitzgerald, and Sir Oliver Guild). The latter example of Thomson scattering required a very substantial electron density and, considering of the requirement of electrical neutrality, a big positive ion concentration. Because there was no visible emission from positive ions and because hydrogen is and so enormously abundant, the logical candidate was complimentary protons; with no bound electrons, hydrogen ions could contribute both scattering and free–gratis emission without any distinctive line emission.

The dynamical nature of this dumbo solar plasma was brought forcibly to the attention of astronomers in the early 1950s by Ludwig Biermann, who showed that the streaming of comet tails radially outward from the Lord's day was accompanied by a marked acceleration of the cometary plasma up to speeds of several hundred kilometers per second. Biermann correctly argued that corpuscular radiation with nearly that speed must be emitted by the Dominicus. Observations of the aberration of the tail management with variation in the speed of the comet and the dispatch of plasma "knots" in the tail both yielded the same decision.

In the 1960s radio and radar techniques were practical to the study of the solar air current plasma. Pointlike extra-galactic radio sources were observed to scintillate every bit a result of the passage of their emissions through the dumbo, turbulent solar plasma. Conscientious timing of pulsar signals near the ecliptic plane showed timing shifts when the Sun was nearly in the Globe–pulsar line of sight that could exist attributed simply to the finite refractive index of the plasma at long radio wavelengths.

But the near extensive trunk of data on the solar wind was produced by spacecraft launched on lunar and interplanetary missions, which carried them well articulate of the Earth'due south magnetosphere and permitted directly in situ observations of the plasma. The speed distribution, direction, temperature, composition, and spatial structure of the solar wind were mapped from a number of spacecraft, most of which did not deviate more than a few degrees from the plane of the ecliptic. They thus mostly sampled the solar air current at low solar latitudes.

The flow of the solar wind was constitute to average 300 to 400 km due south^−ane, directed radially outward from the center of the Sun. At any time, the magnetic field embedded in the solar current of air flow was found to showroom sectorial structure, with "gores" of alternating polarity carrying spiral field lines rooted in the Sun at one end, but apparently open at the other. The plasma conspicuously originates in some kind of delinquent expansion of the corona, but the mechanism and energetics of that expansion were not at all obvious.

The discovery of the Globe'south trapped radiation belts by James A. Van Allen in 1958, and the later creation of bogus radiation belts past high-altitude nuclear explosions, caused greatly enhanced interest in the behavior of space plasmas. There was then renewed interest in a model of the behavior of the solar wind that had been proposed past Eugene N. Parker of the Academy of Chicago several years prior to the beginning straight studies of the solar wind by lunar and planetary spacecraft. Because of the major function played by radiophysics and space plasma investigations in the early history of the exploration of the Solar Arrangement, nosotros shall devote a little more attention to both the basic radiophysics of the solar plasma and the attempts to construct a theory of the dynamical behavior of the solar current of air.

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Contaminants

L. Skipperud , B. Salbu , in Encyclopedia of the Anthropocene, 2022

Environmental Furnishings: Radioecotoxicology

The radiation characteristics of radionuclides, their environmental mobility, and bioaccumulation are important determinants of the magnitude of the consequences following deposition in the environment. Characteristics of the receiving ecosystems also influence the calibration of the consequences, since some are more susceptible to incorporating radionuclides into exposure chains than others. Relationships between exposure (accumulation, external and internal doses) and short- and long-term effects (biological endpoints) are difficult to document and quantify in the field, although biological responses from molecular to ecosystem level have been identified for different test organisms in laboratory experiments. When biological systems are exposed to ionizing radiations, deleterious effects are caused by the consecration of free radicals and the subsequent formation of reactive oxygen species (ROS). ROS species may affect membrane integrity and damage biomolecules such equally proteins and nucleic acids (Dna, RNA), and a series of downstream responses may occur affecting key biological endpoints such as reproduction failure, allowed organization deficiency, mutagenesis, morbidity, and bloodshed. All effects may, withal, not exist negative as Deoxyribonucleic acid repair can be stimulated, and stress tolerance and adaptation can develop.

A series of new biomarker techniques (e.g., proteomics, metabolomics, transcriptomics, epigenetic changes including miRNA profiling) providing information on responses, for instance gene regulation, accept recently been utilized within radioecotoxicology. Such techniques were applied when juvenile Atlantic salmon (Salmo salar) were exposed to gamma radiations and depleted uranium, individually and in combination. Effects on the molecular level could exist observed based on global transcriptional assay; differentially expressed genes (DEGs) and functional analysis of DEGs revealed that the stressors displayed a similar mode of action induced by gamma and depleted uranium. This suggest induction of oxidative stress, Deoxyribonucleic acid damage and disturbance of oxidative phosphorylation, but also stressor-specific mechanisms such as cellular stress and metabolic disorder (Song et al., 2022). When radioactive particles are retained in nonhuman organisms, such as the filter-feeding marine mollusc Mytilus edulis, several effects are besides observed, such as burn mark (necrosis), increase in the Comet tail-DNA percent, and increase in the micronucleus frequency observed in the hemolymph collected from the muscle, implying that nontargeted effects of radiation were induced past radiation from the retained particle (Jaeschke et al., 2022). Notwithstanding, the human relationship between uptake, accumulation, doses, and upshot responses at molecular and individual levels and furthermore to population and ecosystem levels represents a key scientific challenge.

Dose–effect relationships are based on exposure characteristics (dose distribution) and a variety of biological endpoints ranging from molecular to ecosystem level. Systems for dosimetry measurements in radiation biological science experiments both for external and internal exposures are much better developed for human exposures, than for animals and plants, which are nonetheless based on simplified geometry bold that the dose is evenly distributed within organisms. In the field, organisms are externally (gamma radiation) and internally (accumulated radionuclides) exposed to radiation. If radioactive particles carrying a substantial corporeality of radioactive decay are retained in for instance filtering organisms, doses volition be unevenly distributed. Furthermore, the Relative Biological Effectiveness (RBE) for flora and fauna seems to be different to those calculated for humans where high linear energy transfer (Allow) radiation (east.g., alpha radiation) causes a greater caste of biological damage than depression Permit radiation (eastward.grand., gamma radiation) for a given absorbed dose. The RBE depends on many factors in improver to the radiation quality: how radiations is delivered, the dose and dose rate, the cell, tissue type or life phase beingness irradiated, the endpoint and private radiosensitivity. Thus, in dissimilarity to radiation protection of humans, no issue unit is provided for nonhuman organisms, and the estimation of dose to wildlife still represents a challenge.

As biological systems are complex, extrapolating toxicity data from single experiments are frequently used to cover gaps in noesis such as extrapolation from 1 stressor to others, from 1 organism to others, from event concentration to no-effect concentration, from astute to chronic effects, from laboratory to field weather condition, from isolated test species to complex systems. Examination systems, where selected organisms are exposed to individual stressors under defined conditions, have been designed especially for regulatory purposes. The uncertainties associated with extrapolations are hard to account for, and poorly defined condom factors are used to substitute proper uncertainty estimates (Brechignac et al., 2022). The propagation of radiation effects from individuals to populations and ecosystems too depends on factors such every bit the species' life-history traits, as indirect effects of radiation (i.e., influence on habitat, foraging, competition, predator–prey interactions/nutrient supply) in add-on to individual radiosensitivity (Hinton et al., 2022). In improver, field organisms are exposed to a cocktail of contaminants (radionuclides, trace metals, and organics), that is, multiple stressors, where multiple types of interactions and interactions with multiple targets occur, and condiment, antagonistic, and synergistic effects may arise ( Fig. half dozen ). Thus, the link between exposure and associated responses for nonhuman organisms is all the same hard to establish and estimates have large uncertainties. It is recognized that these are important research areas in the future (Hinton et al., 2022).

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Future climates of the world: a modelling perspective

Michael R. Rampino , in World Survey of Climatology, 1995

Introduction

Long before comets were known to exist large bodies travelling through the Solar System, they were feared as bad omens, with changes in the weather condition as one of their possible effects. The discovery of the truthful nature of comets in the 1600s soon led to dramatic theories of the Earth involving comets and their catastrophic effects on climate. For example, in his "New Theory of the Earth" (1696) William Whiston proposed a cosmogeny in which our planet originated when a comet was transformed into an ideal world, with a circular orbit, and without tilt or rotation. After, God sent another comet towards the Globe, and its collision changed the planet's orbit and started it rotating. The impact cracked the chaff, releasing the waters of the Flood, while the vapours of the comet's tail condensed into torrential rainfall. By the 18th century, calculations showed that large comets might commonly cross the path of the Earth, and scholarly reports of the possible catastrophic climatic effects of comet collisions or well-nigh-collisions produced panics among the full general populace ( Heuer, 1953).

In the uniformitarian view of the geologic record, proposed past James Hutton, and codified by Charles Lyell in his "Principles of Geology" (1830–1833), the use of such extraordinary events to explain geologic changes was rejected. Past climatic fluctuations were attributed by Lyell to gradual geologic processes, such every bit changes in the distribution of land and sea resulting from erosion and uplift. Lyell specifically maintained that desperate shifts in climate could occur "without help from a comet, or any astronomical change" (quoted in Marvin, 1990).

Nosotros now know that standoff of extraterrestrial bodies with the World represents a natural process that tin can be observed during meteorite falls and airbursts ( Chapman and Morrison, 1994). The size distribution of comets and asteroids that move through the Solar System on Earth-crossing orbits and the tape of bear on craters on the terrestrial planets can be used to determine the boilerplate intervals between impacts of various sizes. Like many such natural processes, impacts follow an inverse power law distribution, and big bodies, ∼five–x km in diameter, are expected to hit the Earth virtually every 20–100 million years ( Barlow, 1990; Shoemaker et al., 1990).

Although a number of modern studies prior to 1980 discussed the possible climatic, biological and geologic effects of a large planetesimal touch (eastward.g. DeLaubenfels, 1956; Urey, 1973; Napier and Clube, 1979), little attending was paid to this work until the publication of geochemical testify for a major asteroid or comet impact at the end of the Cretaceous Period (∼65 million years agone) by the Alvarez group at Berkeley ( Alvarez et al., 1980) and others ( Smit and Hertogen, 1980; Ganapathy, 1980). The major geological purlieus between the Cretaceous and the subsequent Tertiary Periods (the and so-chosen K/T boundary) is marked by the mass extinction of some 75% of the species of marine organisms ( Raup, 1992) including more than 95% of marine plankton and the apparently sudden extinction of the dinosaurs ( Sheehan et al., 1991) and other terrestrial animals and plants ( Johnson, 1992a).

Discovery of anomalous concentrations of iridium and other trace elements in a sparse globally distributed clay-rich layer that occurs in geologic sections coincident with the One thousand/T boundary (Fig. 1) was followed by reports of microspherules of diverse composition, diagnostic stupor-deformed minerals, glassy microtektites and touch-wave deposits (e.one thousand. Alvarez, 1986; Alvarez and Asaro, 1990; Sigurdsson, 1990; Smit, 1990; Smit et al., 1992). The evidence is now overwhelming that the basal few millimetres of the clay layer represents ejecta from the impact of a big asteroid or comet on the World (e.g. Glen, 1990). A large (>200 km bore) candidate crater, the Chicxulub impact structure in northern Yucatan ( Hildebrand et al., 1991) dates from 65.2 ± 0.four Ma ( Sharpton et al., 1992).

The apparent coincidence of the impact (or impacts) with the abrupt extinctions, including that of the dinosaurs and the possibility that climatic and environmental changes caused by the impact led to the extinctions, revived interest in the physical furnishings of large impact events. Although much attending has been focused on the relatively brusque-term (firsthand to several hundred one thousand years) furnishings of impacts on the environment, the possibility also exists that touch on perturbations tin trigger or ready in move longer term environmental and geological changes, such as water ice ages (eastward.1000. Kyte et al., 1988) or pulses of volcanic and/or tectonic activity that might, in themselves, affect long-term global climate ( Urey, 1973; Napier and Clube, 1979; Rampino and Stothers, 1984a,b, 1988; Rampino and Caldeira, 1993).

The possible connection betwixt climatic change and mass extinctions has been a much-debated bailiwick, with some arguing that extinctions are generally related to cooling of the climate (e.g. Stanley, 1984, 1988), while others have implicated warming in mass-extinction scenarios (e.g. McLean, 1978). Estimates of climatic and ecology changes that might exist caused past the impact of large extraterrestrial bodies have been approached in two basic means: theoretical studies that attempt to guess the kinds and magnitudes of climatic and geologic changes that impacts of various sizes might induce and study of proxy ecology and climate indicators in the geologic tape at times of documented or suspected large-trunk impacts and associated mass extinctions.

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The Solar Wind

J.T. Gosling , in Encyclopedia of the Solar System (Third Edition), 2022

1 Discovery

1.1 Early Indirect Observations

In 1859 R. Carrington fabricated 1 of the first white light observations of a solar flare. He noted that a major geomagnetic storm began approximately 17 h afterward the flare and tentatively suggested that a causal relationship might exist between the solar and geomagnetic events. Subsequent observations revealed numerous examples of associations between solar flares and big geomagnetic storms. In the early 1900s F. Lindemann suggested that this could be explained if big geomagnetic storms result from an interaction between the geomagnetic field and plasma clouds ejected into interplanetary infinite by solar activity. Early studies of geomagnetic activity also noted that some geomagnetic storms tend to recur at the ∼27 twenty-four hour period rotation period of the Sun every bit observed from Earth, particularly during declining years of solar activity. This ascertainment led to the suggestion that certain regions on the Sun, commonly called G (for magnetic)-regions, occasionally produce long-lived charged particle streams in interplanetary infinite. Furthermore, because some course of auroral and geomagnetic activity is well-nigh always nowadays at high geomagnetic latitudes, it was inferred that charged particles from the Dominicus near continuously impact and perturb the geomagnetic field.

Observations of modulations in galactic catholic rays in the 1930s too suggested that plasma and magnetic fields are ejected from the Lord's day during intervals of high solar activeness. For example, S. Forbush noted that cosmic ray intensity often decreases suddenly during large geomagnetic storms, and and then recovers slowly over a period of several days. Moreover, cosmic ray intensity varies in a cycle of ∼11 years, but roughly 180° out of phase with the solar activeness cycle. 1 possible explanation of these observations was that magnetic fields embedded in plasma clouds from the Sunday sweep cosmic rays away from the vicinity of Earth.

In the early 1950s, L. Biermann concluded that there must be a continuous outflow of charged particles from the Sun to explain the fact that ionic tails of comets e'er indicate away from the Sun. He estimated that a continuous particle flux of the gild of x^ten protons/cm² s was needed at 1 AU to explain the comet tail observations. He later revised his approximate downward to a value of ∼ten⁹ protons/cm² southward, closer to the boilerplate observed solar wind proton flux of ∼three.8 × 10⁸ protons/cm² s at ane AU.

1.2 Parker's Solar Current of air Model

Manifestly inspired by these various observations and interpretations, E. Parker, in 1958, formulated a radically new model of the solar corona in which the solar temper is continually expanding outward. Before Parker'due south work nigh theories of the solar atmosphere treated the corona as static and gravitationally spring to the Sun except for sporadic outbursts of material into space at times of loftier solar activity. South. Chapman had constructed a model of a static solar corona in which rut transport was dominated by electron thermal conduction. For a 10^{half dozen} K corona Chapman institute that fifty-fifty a static solar corona must extend far out into space. Parker realized, yet, that a static model leads to pressures at large distances from the Sun that are vii to viii orders of magnitude larger than estimated pressures in the interstellar plasma. Because of this mismatch at large heliocentric distances, he reasoned that the solar corona could not be in hydrostatic equilibrium and must therefore be expanding. His consideration of the hydrodynamic (i.e. fluid) equations for mass, momentum, and free energy conservation for a hot solar corona led him to unique solutions for the coronal expansion that depended on the coronal temperature close to the surface of the Sun. Parker'south model produced low menstruum speeds close to the Sun, supersonic flow speeds far from the Sun, and vanishingly small pressures at big heliocentric distances. In view of the fluid grapheme of the solutions, Parker called this continuous, supersonic coronal expansion the "solar wind". The region of space filled past the solar wind is now known as the "heliosphere".

ane.3 First Directly Observations of the Solar Wind

Several Russian and American infinite probes in the 1959–1961 era penetrated interplanetary space and found tentative evidence for a solar wind. House proof of the wind's existence was provided by C. Snyder and Chiliad. Neugebauer, who flew a plasma experiment on Mariner 2 during its epic iii-month journey to Venus in belatedly 1962. Their experiment detected a continual outflow of plasma from the Sun that was highly variable, being structured into alternating streams of loftier- and low-speed flows that lasted for several days each. Several of the high-speed streams recurred at roughly the rotation period of the Sun. Boilerplate solar wind proton densities (normalized for a 1 AU heliocentric distance), flow speeds, and temperatures during this 3-month interval were v.4 cm⁻³, 504 km/s, and 1.7 × 10^v K respectively, in essential agreement with Parker'south predictions. The Mariner ii observations besides showed that helium, in the grade of alpha particles, is present in the solar wind in variable amounts; the average alpha particle abundance relative to protons of 4.six% being about a factor of 2 lower than estimates of the helium abundance within the Sun. Finally, measurements made by Mariner two confirmed that the solar current of air carried a magnetic field whose strength and orientation in the ecliptic aeroplane were much equally predicted past Parker (come across Section iii).

Despite the practiced agreement of observations with Parker's model, we still do non fully understand the processes that oestrus the solar corona and accelerate the solar current of air. Parker simply assumed that the corona is heated to a very high temperature, but he did not explain how the heating was accomplished. Moreover, it is now known that electron heat conduction is bereft to power the coronal expansion. Nowadays models for heating the corona and accelerating the solar air current generally fall into two classes: heating and dispatch by waves generated by convective motions beneath the photosphere; and bulk dispatch and heating associated with transient events in the solar temper such as magnetic reconnection. Nowadays observations are incapable of distinguishing between these and other alternatives.

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Dust in the Solar Arrangement

Harald Krüger , Eberhard Grün , in Encyclopedia of the Solar System (Tertiary Edition), 2022

2.5 Comet Dust

The inner coma of a comet (encounter Physics and Chemical science of Comets) is one of the most grit-rich environments in the solar organisation. Almost everything we see from a comet with the naked heart is grit. Both the coma and the tail are seen as sunlight scattered by micron-sized dust (cf. Figure 29.1). Particles ejected from a comet form unlike dust populations. Submicron-sized grains have high ejection velocity (∼1 km/s), and on ejection from the comet they may rapidly presume hyperbolic orbits and escape from the solar system. Grains of size 1–100 μm are ejected at a speed of several hundred meters per 2nd and nether the action of solar radiation pressure they grade the comet tail which disperses them far from the comet orbit within a short time flow. Even bigger particles stay in a trail shut to the comet's orbit. If the Globe crosses such a trail, it is observed as a meteor stream (Table 29.i).

One of the problems in characterizing the dust surround of a comet is that information on the nucleus, its dust, and gas release is very limited. Before 1986, observations of cometary grit were the domain of astronomers. Loftier-resolution images of cometary comae revealed jets and other structures in the inner parts. Some of these structures formed spirals which rotated similar water from a lawn sprinkler, indicating detached dust emissions from localized active parts of the nucleus surface. A effect of observing in visible lite is that the results are biased by particle sizes in the range of 1–10 μm, considering much smaller and much larger particles practice not contribute significantly to the scattered light in the visible range. With the extension of the appreciable spectral range to infrared wavelengths using space-based telescopes, the thermal emission of dust besides became attainable to astronomers. It revealed information on the affluence of larger grains and on the mineralogical composition of the grit.

A breakthrough in understanding cometary constituents came with space missions to several comets: Giotto and ii VeGa spacecraft to comet 1P/Halley in 1986, Deep Infinite 1 to comet 19P/Borelly in 1999, Stardust to comet 81P/Wild 2 in 2004, Deep Impact to comets 9P/Tempel one in 2005 and 103P/Hartley 2 in 2010, and nearly recently Stardust to 9P/Tempel 1 in 2022. Water and CO were identified as the main species in the gas, and dust particles fabricated of carbonaceous and silicate materials ranging from nanometer to millimeter sizes were detected. Active areas on resolved images of cometary nuclei and corresponding grit jets were identified for some of the visited comets.

The Stardust mission was the first space mission designed to return extraterrestrial material from interplanetary space. The primary goal of Stardust was to collect grit samples during its flyby of comet Wild two. The spacecraft flew through the coma at a speed of 6.1 km/southward within 236 km from the nucleus and comet particles were collected in aerogel of 50 and 20 kg/m³ density. The collector consisted of 0.one thou² aerogel and of 0.015 1000ⁱⁱ aluminum foil. Micrometer-sized dust particles were decelerated gradually by the aerogel, forming several millimeter long tracks, and minimizing the damage to the dust grains. The collector was stored in a Sample Return Capsule which was released from the spacecraft just before reentry into Earth's atmosphere, for a landing on a parachute.

In January 2006, the Stardust sample capsule returned safely to the World with thousands of particles from comet 81P/Wild 2 for laboratory study. Affect tracks in aerogel created past particles ranging from dense mineral grains to loosely bound, polymineralic aggregates ranging from 0.01 to 100 μm in size displayed diverse impact features. Residues in impact craters on the structure supporting the aerogel were also analyzed.

The collected particles are chemically heterogeneous; however, the mean elemental composition of comet Wild 2 particles is consequent with CI meteorite composition (cf. chapter meteorites). The particles are weakly synthetic mixtures of nanometer-scale grains with occasionally much larger Iron–Mg silicates, Atomic number 26–Ni sulfides, and Atomic number 26–Ni metal phases. A very wide range of olivine and depression-Ca pyroxene compositions was too found.

The nerveless samples are dominated by loftier-temperature materials that closely resemble meteoritic components. These materials include chondrule and calcium-aluminum-rich inclusion (CAI)-like fragments (Figure 29.eight). The affluence of loftier-temperature minerals such as forsterite and enstatite appears to take formed in the hot inner regions of the solar nebula. From at that place they were transported beyond the orbit of Neptune where they accreted together with ice and organic components to course comet Wild 2.

Hydrogen, carbon, nitrogen, and oxygen isotopic compositions are heterogeneous amidst particle fragments; notwithstanding, extreme isotopic anomalies are rare, indicating that this comet is non a pristine aggregate of presolar materials. An extreme oxygen ratio ¹⁷O/¹⁶O = x^−three was plant which is a gene 2.6 higher than the Solar Arrangement value and is similar to that of some presolar grains found in meteorites. Merely five presolar grains have been discovered in the Wild two samples then far. The presolar grain content appears to be lower than in chondrites and in almost IDPs.

The organics are rich in oxygen and nitrogen compared with meteoritic organics. Aromatic compounds are present only less abundant than in meteorites and IDPs. The organics institute in comet Wild 2 shows a heterogeneous and unequilibrated distribution in abundance and limerick. Even glycine, a fundamental building block of life, was plant in samples of comet Wild 2 which supports the idea that these precursors of life are prevalent in infinite (see Physics and Chemistry of Comets; Meteorites).

Small bodies in the inner solar system were traditionally classified equally either asteroids or comets. Comets are agile objects, with activity showing up as a prominent coma and tail(s) when they approach the Lord's day. Cometary activeness is largely driven by the sublimation of near-surface water ice. On the contrary, asteroids lack such signatures of action. Recently, a minor number of intermediate-blazon objects were discovered which are chosen "main belt comets" or "active asteroids". Dynamically their orbits resemble those of asteroids, while they show clear signs of activeness similar to comets. The activity is usually evidenced by the light handful off dust particles emitted from the object, forming a comalike construction and sometimes a tail. Their mass loss, nevertheless, is likely driven by a surprising multifariousness of mechanisms. Besides sublimation of h2o ice, impact ejection, rotational instability due to spin upwardly, dehydration stresses and thermal fracture, besides as electrostatic repulsion and solar radiations pressure sweeping of dust particles from the surfaces of these objects have been suggested. Fifty-fifty though no single mechanism can explain the varied examples of activity observed, blackout and tail morphology as evidenced by the emitted grit particles tin can give valuable information nigh the ascendant processes. Coma and tail structures may constrain the duration of activeness, e.g. whether the emission event was impulsive due to an impact or long lasting or even repetitive. Astronomers may also be able to constrain particle sizes and ejection velocities from the coma and tail morphology (see Master-Belt Asteroids; Comet Populations and Cometary Dynamics).

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