Comet NEOWISE (16 July 2020) 1 : 無料・フリー素材/写真
Comet NEOWISE (16 July 2020) 1 / James St. John
| ライセンス | クリエイティブ・コモンズ 表示 2.1 |
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| 説明 | The fuzzy star with the tail near the bottom of the picture is Comet NEOWISE, a horribly named, long-period comet that first appeared in spring 2020.Locality: view from the western side of Newark, Licking County, east-central Ohio, USA-------------------------See info. at:en.wikipedia.org/wiki/C/2020_F3_(NEOWISE)-------------------------Comets are often described as "dirty snowballs" or "icy dirtballs". They are small to moderately small, ice-rich bodies that usually have highly elliptical orbits and occasionally get near the Sun. They appear in the sky without warning and move relatively quickly. They last weeks to months, then disappear. Some are bright enough to see during daytime. Thousands of comets have been cataloged, but it's been speculated that trillions of them occur in the far-distant areas of the Solar System.The "fuzzy star" portion is called the comet's head - it contains a solid nucleus. The fuzzy halo around the nucleus is the coma, which is really the comet's atmosphere. Comas develop when the comet is sufficiently close to the Sun to warm up and ice starts volatilizing. The comet's low gravity keeps some of this material close to the nucleus. The coma of some comets can be larger than the Sun itself. Comet heads often have a greenish glow to them, caused by C2 and CN molecules.The comet's tail also only develops relatively close to the Sun and always points away from it. The tail of a comet is basically a coma that has been stretched out by solar wind. Some comet tails are over 3.5 AU (astronomical units) long. The morphology of comet tails and tail streamers vary with time; sections of comet tails can be severed and reconnected by irregularities in solar wind and magnetic field lines.Two comet tails are often visible - one is straight and bluish-colored and the other is curved and light-colored. The straight, bluish tail is the ion tail - it is composed of electrically charged gas atoms or molecules. The curved, light-colored tail is the dust tail - it is composed of tiny solid particles (mineral and rock fragments). The degree of curvature varies (for a spectacular example, see Comet McNaught: www.flickr.com/photos/98389309@N02/10055241504 ).Based on trajectory studies, comets are known to have two sources. Short-period comets (also known as periodic comets) orbit the Sun regularly, on time scales of 200 years or less. Their orbits are usually prograde (= counter-clockwise movement around the Sun, when viewed from above the Sun's North Pole) and are close to the plane of the ecliptic. Most short-period comets appear to originate from beyond Planet Pluto. They are estimated to last a few hundred thousand years before they self-distintegrate or plunge into the Sun. A good example of a short-period comet is Halley's Comet, which returns every 76 years.Long-period comets have very long-duration orbits, on time scales of thousands to millions of years. They come in from any and all directions (= not restricted to orbiting close to the plane of the ecliptic). A good example is Comet Hale-Bopp, which was visible in daytime in 1997. Long-period comets appear to originate from the Oort Cloud, a hypothetical, extremely distant source area that forms a more-or-less spherical halo around the Solar System. No direct evidence for the Oort Cloud exists.The geology of comet nuclei was speculative until spacecraft visited, imaged, and sampled some of them. Six cometary nuclei have been visited so far: Halley, Tempel I, Wild 2, Hartley 2, Borrelly, and Churyumov-Gerasimenko. Based on this small sample set, comet nuclei have irregular shapes - their low gravity does not result in subspherical shapes (they're not "round"). Three imaged nuclei are bilobed, each of which probably formed by low-velocity impacts of two separate objects. Nuclei vary in size from tens of kilometers to about one-tenth of a kilometer across. All are low-density, about half that of water - they're fluffy, similar to cotton candy. The surfaces of comet nuclei are dark-colored, organic-rich crusts. One lander sent to a comet did obtain outcrop-scale imagery showing an agglomerate rock face, composed of pebbles. When close to the Sun, ice volatilization results in comet nuclei releasing material in the form of geyser-like jets. Individual jets can be narrow or wide. Some are short-lived; some occur due to cliff collapse events. Cometary jetting involves water vapor (H2O) and carbon dioxide gas (CO2) escaping at high velocity from subcircular pits similar to sinkholes.Dust samples captured by a spacecraft show that cometary jets shoot out gas and solid, aggregate particles (composed of many smaller grains). Individual dust grains disaggregate during ejection and also disaggregate when impacting a spacecraft's collector.Based in particular on data from Comet Wild 2 and Comet Churyumov-Gerasimenko, comets are known to be a mixture of various types of ice and trapped gases and mineral grains and rock fragments. These have multiple origins: some are Solar System materials, some are materials from the interstellar medium, and some are presolar materials derived from other stars.Minerals identified in comet samples include forsterite olivine, diopside pyroxene, anorthite plagioclase feldspar, pyrrhotite, pentlandite, spinel, and osbornite. All of these are high-temperature minerals, co-occurring with ice.Multiple varieties of ice have been identified in comets: three types of water ice (H2O, HDO, and D2O), hydrogen peroxide ice (H2O2), dry ice (CO2), carbon monoxide ice (CO), and ammonia ice (NH3) (+ varieties with D instead of H).Other chemicals identified in comets include molecular oxygen (O2), molecular nitrogen (N2), sulfur chemicals (H2S, HDS, S2, S3, S4, S8, SO, SO2, SCO, CS2, and organic sulfur compounds), hydrogen cyanide (HCN), cyanogen (C2N2), ammonium cyanide (NH4CN), and cyanamide (NH2CN). Five types of alcohol are in comets: methanol (CH3OH / CH3OD), ethanol (C2H5OH), propanol (H3H7OH), butanol (H4H9OH), and pentanol (C5H11OH). Seven, possibly eight types of petroleum natural gas are present: methane (CH4), ethane (C2H6), propane (H3H8), butane (H4H10), pentane (C5H12), hexane (C6H14), heptane (C7H16), and probably octane (C8H18). Nasty acids are also in comet nuclei: formic acid (HCOOH), acetic acid (C2H4O2), benzoic acid (C7H6O2), hydrobromic acid (HBr), hydrochloric acid (HCl), and hydrofluoric acid (HF). On top of all that, there are noble gases: argon (Ar), krypton (Kr), and 7 isotopes of xenon (Xe).Why not give the rest of the list - comets have LOTS of chemicals: acetaldehyde, acetonitrile, acetylene, benzene, butanamide, chloromethane, ethanethiol, ethylamine, ethylene glycol, formaldehyde, glycine (= an amino acid), magnesium, methanethiol, methylamine, naphthalene, phosphorus, phosphorus oxide, potassium, propylenglycol, silicon, sodium, thioformaldehyde, toluene, and xylene.Intriguingly, one possible rock sample from a comet is available. "Hypatia" is the nickname for a unique rock found in southwestern Egypt. It is black, hard, lustrous, and diamond-rich. It was recovered from the Libyan Desert Glass strewn field. Libyan Desert glass consists of (usually) yellowish-colored tektites formed during the Oligocene, about 28.5 million years ago. The lack of a crater suggests they may have formed during a bolide airburst, similar to the Tunguska, Siberia event of 1908. "Hypatia" may be a rock sample from that impacting object, which may have been a comet.So What?Comets are not only visually appealing, they do have significance in Earth history. Comets have certainly arrived on Earth in the geologic past and they have long been suspected to be an important source of Earth's water. Most comets do not have the same deuterium to hydrogen ratio as Earth's water, but at least one comet has been identified that does have just about the same D/H ratio as seawater.It's been estimated that comets delivered about 1 to 2% of all water on Earth, plus about 10 to the 17th power kilograms of organic chemicals. About 22% of Earth's atmospheric xenon seems to be cometary in origin.If you find comets intriguing, but don't have the patience to wait for the next visible example, remember that most meteors ("shooting stars") are interpreted to be dust-sized to pebble-sized particles, most of which are derived from comets (Examples: Comet Encke and Comet Swift-Tuttle). |
| 撮影日 | 2020-07-16 20:53:31 |
| 撮影者 | James St. John |
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