General Astronomy/Comets

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Comets are described by astronomers as "dirty snowballs." They have a nucleus of dust and ice surrounded by a coma (thin atmosphere of evaporated ice) [1]. The nucleus is mainly made up of H₂O (water), with some parts NH₃ (ammonia), CH₄ (methane), and CO₂ (carbon dioxide).

Comets have two tails: the ion tail (blue, composed of gas), and the dust tail (yellow). Meteor showers, like the Perseid or Leonids, occur when the Earth travels through a comet's dust tail. Comets come from the Oort Cloud or the Kuiper Belt. [2] The Oort Cloud contains 10¹¹ comets.

One of the most famous comets is Halley's Comet, which was discovered by Edmond Halley in 1705. The comet had last been sighted in 1682; Halley predicted it would return in 1758. Unfortunately, he died in 1742, so he was unable to see his prediction come true. The comet was subsequently named after him.

Halley's Comet has a period of about 76 years. It came by the Earth last in 1986, and will return in 2061.

[edit] Diversity of Comets

About 4.5 billion years Comet was formed along with the Solar system from the collapse of a giant, diffuse cloud of gas and dust. The cloud started to heat up and whirl faster as it shrank. The material in the fast-spinning cloud spread out into a flattened disk and the temperature in the dense, central core was so hot that it ignited nuclear fusion creating the Sun and the disk's outer region was cold. Low temperatures allowed water to freeze onto dust grains, which grew in size to make clumps. The clumps merged by collisions, and formed the planets. But some of the matter did not merge into planets and resides beyond Neptune called comet ^[1]. Comet moved randomly through the solar system and when it strays too close to a planet, the larger body's gravity pushed the comet into the inner solar system. Then the comet grew a tail and became visible to observers on Earth ^[2].

It has been postulated that the chemical diversity observed in the population of comets found in the Kuiper Belt (region extended after Neptune orbit) as well as Oort cloud(spherical cloud lie roughly 50 000 AU from the Sun) is primordial or due evolution effects since formation.

The chemical abundances of comets were observed between both classes of comets: Oort cloud comets and the Jupiter-Family comet (population consisting of short-period comets formed in the Kuiper belt), and a difference was noted between the two groups. This can be attributed to several factors including differences in the chemical and physical environments in comet-forming regions, chemical evolution during their long storage in the Oort cloud and Kuiper belt, and thermal processing by the Sun when entering the inner Solar System. The chemical composition of comets is investigated by remote sensing using spectroscopy. This investigation is indirect, since only the gas and dust coma was observed after the nucleus ices have sublimated from the nucleus.

Observations were made at millimeter/sub-millimeter wavelengths with the IRAM 30-m, JCMT, CSO and SEST telescopes, which was based on molecular abundances in comets by Biver et al. 2007. Six Jupiter-family, three Halley-family, and fifteen long-period comet were observed from 1986 to 2001. The eight molecular species were HCN, HNC, CH3CN, CH3OH, H2CO, CO, CS, and H2S. HCN were detected in all comets, while at least two molecules were detected in nineteen comets. It was inferred that the HCN abundance relative to water varies from 0.08% to 0.25% from the sub-sample of comets for which contemporary H2O production rates are available. HCN is the molecule which exhibits the lowest abundance variation from comet to comet with respect to other species and was found in all the 24 Comets. HCN was found only in the 4 Comets and low CO abundances measured in Jupiter-family comets, while several Oort cloud comets exhibit high CO abundances ^[3]. According to the observation 23% of CO molecule with respect to water was observed in 5 comets. While 0.035% of HNC relative to water was observed in 5 comets.15 comets exibits CH3CN upto maximum 6.2%.And H2CO, H2S and CS was found in 13,11,9 comets respectively. They were found maximum upto 1.3%,1.5% and 0.17% respectively relative to water^[4].

[edit] Comet C/2001 Q4 (NEAT)

Comet C/2001 Q4 NEAT was discovered on 2001 August in the course of the Near Earth Asteroid Tracking (NEAT) program. The comet was found on Charged Coupled Device (CCD) images and the astronomers were able to confirm the discovery on August 24. The comet was described as a round nebulosity measuring about 8 arc seconds across and was at the distance of 10.1 AU from the Sun. The total visual magnitude was of 20.0. Its perihelion date was 2004 May 26 and perihelion distance was 1.00 AU. It has an unusual, almost perpendicular retrograde orbit which brings it into the inner solar system by a deeply southward path. It initially emerged from its remote home spending most of its time near the south celestial pole^[5]. The comet brightened slowly during the remainder of 2001 and throughout 2002. The comet appeared fainter than magnitude 15 at the beginning of the 2003, but it steadily brightened as the year progressed, magnitude 12 around mid-September. In September amateur astronomers could easily mark the time of comet in the Southern Hemisphere began supplying regular visual observations of this comet. The diameter of the coma was typically measured as 0.6 to 1.2 arc minutes. In CCD images amateurs began observing a short fan-shaped tail pointing northward in July as it approaches near the Sun. In the late September, the tail extended about 0.8 arc minutes toward the northwest. As 2003 came to an end, the comet had become slightly brighter than magnitude 10, with a coma diameter of about 2 arc minutes. A faint, fan-shaped tail extended about 4 arc minutes toward the east. Finally Comet C/2001 Q4 (NEAT) entered the inner solar system in 2004, reaching perihelion of 1 AU. It had the high magnitude of 3.3 clearly visible. C/2001 Q4 is referred to as an Oort cloud comet (originated from Oort cloud). It is composed of HCN, CO, CH3OH, H2S and other species^[6].

[edit] References