Comets Essays (2385 words) - Comets, Minor Planets,

Comets The first written records of comets date back to nearly 3,000 years ago from China and Europe. The accounts of these comets were believed to be the causes of terrible events that occurred afterwards. In more recent times, however, astronomers have found out what they really are. A comet is basically a mixture of ices, from both water and frozen gases, and dust. They have also been given the names ?dirty snowballs? or ?icy mud balls.? The typical comet is less than 10 kilometers across. They spend most of their time frozen solid in the outer parts of our solar system. Comets are composed of five parts: the nucleus, coma, hydrogen cloud, dust tail, and ion tail. The nucleus is pretty solid and stable, composed mostly of ice and gas with a small amount of dust and other solids. The surface of the nucleus is best described as a black crust. Comet nuclei can range from 1 kilometer to about 50 kilometers across. The black crust on the surface of the nuclei helps the comet to absorb heat, which causes some of the ices under the crust to turn to a gas. Pressure builds up underneath the crust and causes the surface to bubble up in some places. Eventually, the weak spots of the crust break open from the pressure, and the gas shoots outward; astronomers refer this to as a jet. Dust that had been mixed in with the gas is also pushed out, and as more jets appear, a small gas and dust shell forms around the nucleus, and this is called the coma. The coma, also called the head, is a dense cloud of water, carbon dioxide and other gases and comes off of the nucleus. They can be several thousand kilometers in diameter, depending on the comet's distance from the sun and the size of the nucleus. The size of the nucleus is important because since large nuclei have a greater surface area facing the sun, which is the side that is the warmest, hence the side where most of the jets are coming from, it means more jets and greater amounts of gas and dust go into the coma. Even though the coma can get to be very large, its size can actually decrease about the time it crosses the orbit of Mars. At this distance the particles that drift out from the sun act as a powerful wind which blows the gas and dust particles away from the nucleus and coma. This is the process, which mak es the comet's tail. The hydrogen cloud is very large at millions of kilometers in diameter. But it is only a very sparse body of neutral hydrogen. It was discovered from spectroscopy that was carried out by satellites in 1970. Hydrogen was discovered in comets Tago-Sato-Kosaka and Bennett. It is ionized hydrogen that forms the light that goes past the coma. The reason why the hydrogen cloud was not discovered for a long time is because it is not visible from Earth. Atomic hydrogen emits in the ultraviolet, but the ozone layer stops the waves from entering. The hydrogen cloud can only be observed from space, with satellites. The dust tail is usually up to 10 million kilometers long, and is composed of smoke-sized dust particles that come off the nucleus by escaping gases. The dust tail is also the most visible part of a comet to the naked eye. The tail has a potential to be long when it enters the orbit of Earth. The record for the longest tail is the length of the Great Comet of 1843; its tail extended more than 250 million kilometers. The ion tail, known as type I or plasma, is made up of ions. It can be up to 100 million kilometers long and 100,000 kilometers wide. The tail is straight and always is oppo sed to the direction of the Sun. The color of it, through a spectrum, is mostly blue. The reason why the tail is ionized is because of solar wind. Solar wind, which flows at about 400 kilometers per second, is filled with charged particles that are around the solar magnetic field. The gases in the tail are ionized

Emerald Ash Borer (Agrilus planipennis)

Emerald Ash Borer (Agrilus planipennis) Emerald ash borer (EAB), a native beetle of Asia, invaded North America in the 1990s by way of wooden packing material. In a decades time, these pests killed tens of millions of trees throughout the Great Lakes region. Get to know this pest, so you can sound the alarm if it makes its way to your neck o the woods. Description: The adult emerald ash borer is a striking metallic green, with an iridescent purple abdomen hidden beneath the forewings. This elongate beetle reaches about 15 mm in length and just over 3 mm in width. Look for adults from June to August, when they fly in search of mates. Creamy white larvae reach lengths of 32 mm at maturity. The prothorax nearly obscures its tiny, brown head. EAB pupae also appear creamy white. The eggs are white at first, but turn deep red as they develop. To identify emerald ash borer, you should learn to recognize the signs of an infestation. Unfortunately, symptoms of emerald ash borer dont become obvious until two or more years after borers enter a tree. D-shaped exit holes, just 1/8 in diameter, mark the emergence of adults. Split bark and foliage dieback may also portend pest trouble. Just under the bark, S-shaped larval galleries will confirm the presence of EAB. Classification: Kingdom - AnimaliaPhylum - ArthropodaClass – InsectaOrder – ColeopteraFamily - BuprestidaeGenus - AgrilusSpecies - planipennis Diet: Emerald ash borer larvae feed only on ash trees. Specifically, EAB feeds on the vascular tissues between the bark and sapwood, a habit that interrupts the flow of nutrients and water required by the tree. Life Cycle: All beetles, including the emerald ash borer, undergo complete metamorphosis. Egg – Emerald ash borers lay eggs singly, in crevices in the bark of host trees. A single female can lay up to 90 eggs. Eggs hatch within 7-9 days.Larva – Larvae tunnel through the trees sapwood, feeding on the phloem. Emerald ash borers overwinter in the larval form, sometimes for two seasons.Pupa – Pupation occurs in mid-spring, just under the bark or phloem.Adult – After emerging, adults remain within the tunnel until their exoskeletons properly harden. Special Adaptations and Defenses: The emerald ash borers green color acts as camouflage within the forest foliage. The adults fly quickly, fleeing from danger when needed. Most buprestids can produce a bitter chemical, buprestin, to deter predators. Habitat: Emerald ash borer requires only their host plant, ash trees (Fraxinus spp.). Range: Emerald ash borers native range includes parts of China, Korea, Japan, Taiwan, as well as small areas of Russia and Mongolia. As an invasive pest, EAB now lives in Ontario, Ohio, Indiana, Illinois, Maryland, Pennsylvania, West Virginia, Wisconsin, Missouri, and Virginia. Other Common Names: EAB