Physical characteristics of Comets and its effects...
Nucleus
The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia. As such, they are popularly described as "dirty snowballs" after Fred Whipple's model. However, some comets may have a higher dust content, leading them to be called "icy dirtballs".
Research conducted in 2014 suggests that comets are like "deep fried
ice cream", in that their surfaces are formed of dense crystalline ice
mixed with organic compounds, while the interior ice is colder and less dense.
The surface of the nucleus is generally dry, dusty or rocky,
suggesting that the ices are hidden beneath a surface crust several
metres thick. In addition to the gases already mentioned, the nuclei
contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane and perhaps more complex molecules such as long-chain hydrocarbons and amino acids. In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA's Stardust mission. In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine, and related organic molecules) may have been formed on asteroids and comets.
 |
| Nucleus of Comet 103P/Hartley as imaged during a spacecraft flyby. The nucleus is about 2 km in length. |
Coma
he streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the "coma", and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous "tail" to form pointing away from the Sun.The coma is generally made of H2O and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within 3 to 4 astronomical units (450,000,000 to 600,000,000 km; 280,000,000 to 370,000,000 mi) of the Sun. The H2O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization, with the solar wind playing a minor role in the destruction of water compared to photochemistry. Larger dust particles are left along the comet's orbital path whereas smaller particles are pushed away from the Sun into the comet's tail by light pressure.
 |
| Hubble image of Comet ISON shortly before perihelion. |
Tails
n the outer Solar System,
comets remain frozen and inactive and are extremely difficult or
impossible to detect from Earth due to their small size. Statistical
detections of inactive comet nuclei in the Kuiper belt have been reported from observations by the Hubble Space Telescope but these detections have been questioned.[ As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them.
The streams of dust and gas each form their own distinct tail,
pointing in slightly different directions. The tail of dust is left
behind in the comet's orbit in such a manner that it often forms a
curved tail called the type II or dust tail.
At the same time, the ion or type I tail, made of gases, always points
directly away from the Sun because this gas is more strongly affected by
the solar wind than is dust, following magnetic field lines rather than
an orbital trajectory.
On occasions - such as when the Earth passes through a comet's orbital
plane, and we see the track of the comet edge-on, a tail pointing in the
opposite direction to the ion and dust tails may be seen – the antitail.
 |
| Diagram of a comet showing the dust trail, the dust tail (or antitail) and the ion gas tail, which is formed by the solar wind flow. |
Jets
Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet's nucleus, like a geyser. These streams of gas and dust can cause the nucleus to spin, and even split apart. In 2010 it was revealed dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus.
This is known because a spacecraft got so close that it could see where
the jets were coming out, and then measure the infrared spectrum at
that point which shows what some of the materials are.
Gas and snow jets on Comet Hartley 2
Effects...
Connection to meteor showers
As a result of outgassing, comets leave in their wake a trail of solid debris too large to be swept away by radiation pressure and the solar wind. If the comet's path crosses the path the Earth follows in orbit around the Sun, then at that point there are likely to be meteor showers as Earth passes through the trail of debris. The Perseid meteor shower, for example, occurs every year between August 9 and August 13, when Earth passes through the orbit of Comet Swift–Tuttle. Halley's Comet is the source of the Orionid shower in October.
Comets and impact on life
Many comets and asteroids collided into Earth in its early stages.
Many scientists believe that comets bombarding the young Earth about 4
billion years ago brought the vast quantities of water that now fill the
Earth's oceans, or at least a significant portion of it. Other
researchers have cast doubt on this theory. The detection of organic molecules, including polycyclic aromatic hydrocarbons, in significant quantities in comets has led some to speculate that comets or meteorites may have brought the precursors of life—or even life itself—to Earth. In 2013 it was suggested that impacts between rocky and icy surfaces, such as comets, had the potential to create the amino acids that make up proteins through shock synthesis.
It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to the Earth's Moon, some of which may have survived as lunar ice. Comet and meteoroid impacts are also believed responsible for the existence of tektites and australites.
Comments