daftar hadir santri pondok MA miftahussalam banyumas
മാറി കിട രുബത് ഇമെയില് kita
Welcome to Gmail
Once you complete this form, your Gmail username will become your new default email address as well as the primary address identifying your Google Account. Your previous email address will remain a part of your Google Account and you may continue to log in with that address if you prefer.
If you wish to create a separate Google Account for use with the Gmail service then click here to first log out and create a new Google Account. (Note: you can only be logged into one Google Account at a time).
israel
Israel
israel sebagai negara teroristis
Jump to: navigation, search
Semi-protected
This is related to a current event: 2008–2009 Israel–Gaza conflict.
Information may change rapidly as the event progresses.
For a topic outline on this subject, see List of basic Israel topics. For other uses, see Israel (disambiguation).
Featured article
מְדִינַת יִשְרָאֵל
Medīnat Yisrā'el دولة إسرائيل
Dawlat Isrā'īl
State of Israel
Flag of Israel Emblem of Israel
Flag Emblem
Anthem: Hatikvah
The Hope
Location of Israel
Capital
(and largest city) Jerusalem[1]
[show location on an interactive map] 31°47′N 35°13′E / 31.783, 35.217
Official languages Hebrew, Arabic[2]
Ethnic groups 76% Jewish, 19% Arab, 5% minority groups
Demonym Israeli
Government Parliamentary democracy[2]
- President Shimon Peres
- Prime Minister Ehud Olmert
- Knesset Speaker Dalia Itzik
- Supreme Court President Dorit Beinisch
Independence from British Mandate of Palestine
- Declaration May 14, 1948 (05 Iyar 5708)
Area
- Total 1 20,770 / 22,072 km2 (151st)
8,019 / 8,522 sq mi
- Water (%) ~2%
Population
- 2008 estimate 7,282,0002[3] (96th)
- 1995 census 5,548,523
- Density 324/km2 (34th)
839/sq mi
GDP (PPP) 2007 estimate
- Total $188.936 billion[4] (52nd)
- Per capita $27,146[4] (32nd)
GDP (nominal) 2007 estimate
- Total $164.103 billion[4]
- Per capita $23,578[4]
Gini (2005) 38.6[2]
HDI (2007) ▬ 0.932 (high) (23rd)
Currency Israeli new sheqel (₪) (ILS)
Time zone IST (UTC+2)
- Summer (DST) (UTC+3)
Drives on the right
Internet TLD .il
Calling code 972
1 Excluding / Including the Golan Heights and East Jerusalem; see below.
2 Includes all permanent residents in proper Israel, the Golan Heights and East Jerusalem. Also includes Israeli population in the West Bank.
Israel (Hebrew: יִשְרָאֵל, Yisra'el; Arabic: إسرائيل, Isrā'īl) officially the State of Israel (Hebrew: He-Medinat Israel.ogg מְדִינַת יִשְרָאֵל (help·info), Medinat Yisra'el; Arabic: دَوْلَةْ إِسْرَائِيل, Dawlat Isrā'īl), is a country in Western Asia located on the eastern edge of the Mediterranean Sea. It borders Lebanon in the north, Syria in the northeast, Jordan in the east, and Egypt on the southwest, and contains geographically diverse features within its relatively small area.[5] The West Bank and Gaza Strip are also adjacent. With a population of about 7.28 million,[3] the majority of whom are Jews, Israel is the world's only Jewish state.[6] It is also home to other ethnic groups, including most numerously Arab citizens of Israel, as well as many religious groups including Muslims, Christians, Druze, Samaritans and others.
The modern state of Israel has its roots in the Land of Israel (Eretz Yisrael), a concept central to Judaism since ancient times,[7] and the heartland of the ancient Kingdom of Judah to which modern Jews are usually attributed. After World War I, the League of Nations approved the British Mandate of Palestine with the intent of creating a "national home for the Jewish people."[8] In 1947, the United Nations approved the partition of Palestine into two states, one Jewish and one Arab.[9] On May 14, 1948 the state of Israel declared independence and this was followed by a war with the surrounding Arab states, which refused to accept the plan. The Israelis were subsequently victorious in a series of wars confirming their independence and expanding the borders of the Jewish state beyond those in the UN Partition Plan. Since then, Israel has been in conflict with many of the neighboring Arab countries, resulting in several major wars and decades of violence that continue to this day.[10] Since its foundation, Israel's boundaries and the State's right to exist have been subject to dispute, especially among its Arab neighbors and their many Palestinian refugees. Israel has signed peace treaties with Egypt and Jordan, though efforts for a long-lasting peace with the Palestinians have so far been unsuccessful.
Israel is a representative democracy with a parliamentary system and universal suffrage.[11][12] The Prime Minister serves as head of government and the Knesset serves as Israel's legislative body. In terms of nominal gross domestic product, the nation's economy is estimated as being the 44th-largest in the world.[13] Israel ranks highest among Middle Eastern countries on the bases of human development,[14] freedom of the press,[15] and economic competitiveness.[16] Jerusalem is the country's capital, seat of government, and largest city, while Israel's main financial center is Tel Aviv.[1]
Frog
From Wikipedia, the free encyclopedia
Jump to: navigation, search
Semi-protected
For other uses, see Frog (disambiguation).
Frog
Fossil range: Triassic–present
PreЄ
Є
O
S
D
C
P
T
J
K
Pg
N
White's Tree Frog (Litoria caerulea)
White's Tree Frog (Litoria caerulea)
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Amphibia
Order: Anura
Merrem, 1820
Native distribution of frogs (in black)
Native distribution of frogs (in black)
Suborders
Archaeobatrachia
Mesobatrachia
Neobatrachia
-
List of Anuran families
The frog is an amphibian in the order Anura (meaning "tail-less", from Greek an-, without + oura, tail), formerly referred to as Salientia (Latin saltare, to jump). The name frog derives from Old English frogga,[1] (compare Old Norse frauki, German Frosch, older Dutch spelling kikvorsch), cognate with Sanskrit plava (frog), probably deriving from Proto-Indo-European praw = "to jump".[2]
Most frogs are characterized by long hind legs, a short body, webbed digits (fingers or toes), protruding eyes and the absence of a tail. Most frogs have a semi-aquatic lifestyle, but move easily on land by jumping or climbing. They typically lay their eggs in puddles, ponds or lakes, and their larvae, called tadpoles, have gills and develop in water. Adult frogs follow a carnivorous diet, mostly of arthropods, annelids and gastropods. Frogs are most noticeable by their call, which can be widely heard during the night or day, mainly in their mating season.
The distribution of frogs ranges from tropic to subarctic regions, but most species are found in tropical rainforests. Consisting of more than 5,000 species described, they are among the most diverse groups of vertebrates. However, populations of certain frog species are significantly declining.
A distinction is often made between frogs and toads on the basis of their appearance, caused by the convergent adaptation among so-called toads to dry environments; however, this distinction has no taxonomic basis. The only family exclusively given the common name "toad" is Bufonidae, but many species from other families are also called "toads," and the species within the toad genus Atelopus are referred to as "harlequin frogs".
Contents
[hide]
* 1 Taxonomy
* 2 Morphology and physiology
o 2.1 Feet and legs
o 2.2 Jumping
o 2.3 Skin
o 2.4 Poison
o 2.5 Respiration and circulation
o 2.6 Digestion and excretion
o 2.7 Nervous system
* 3 Natural history
o 3.1 Life cycle
o 3.2 Reproduction of frogs
o 3.3 Parental care
o 3.4 Call
* 4 Distribution and conservation status
* 5 Evolution
* 6 Uses in agriculture and research
* 7 Cultural beliefs
* 8 See also
* 9 Cited references
* 10 General references
* 11 External links
o 11.1 Media
Taxonomy
European Fire-bellied Toad (Bombina bombina)
For more details on this topic, see List of Anuran families.
The order Anura contains 4,810 species[3] in 33 families, of which the Leptodactylidae (1100 spp.), Hylidae (800 spp.) and Ranidae (750 spp.) are the richest in species. About 88% of amphibian species are frogs.
The use of the common names "frog" and "toad" has no taxonomic justification. From a taxonomic perspective, all members of the order Anura are frogs, but only members of the family Bufonidae are considered "true toads". The use of the term "frog" in common names usually refers to species that are aquatic or semi-aquatic with smooth and/or moist skins, and the term "toad" generally refers to species that tend to be terrestrial with dry, warty skin that seems as though it has been mauled by another animal. An exception is the fire-bellied toad (Bombina bombina): while its skin is slightly warty, it prefers a watery habitat.
Frogs and toads are broadly classified into three suborders: Archaeobatrachia, which includes four families of primitive frogs; Mesobatrachia, which includes five families of more evolutionary intermediate frogs; and Neobatrachia, by far the largest group, which contains the remaining 24 families of "modern" frogs, including most common species throughout the world. Neobatrachia is further divided into the Hyloidea and Ranoidea.[4] This classification is based on such morphological features as the number of vertebrae, the structure of the pectoral girdle, and the morphology of tadpoles. While this classification is largely accepted, relationships among families of frogs are still debated. Future studies of molecular genetics should soon provide further insights to the evolutionary relationships among Anuran families.[5]
Some species of anurans hybridise readily. For instance, the Edible Frog (Rana esculenta) is a hybrid of the Pool Frog (R. lessonae) and the Marsh Frog (R. ridibunda). Bombina bombina and Bombina variegata similarly form hybrids, although these are less fertile, giving rise to a hybrid zone.
Morphology and physiology
Skeleton of Rana
The morphology of frogs is unique among amphibians. Compared with the other two groups of amphibians, (salamanders and caecilians), frogs are unusual because they lack tails as adults and their legs are more suited to jumping than walking. The physiology of frogs is generally like that of other amphibians (and differs from other terrestrial vertebrates) because oxygen can pass through their highly permeable skin. This unique feature allows frogs to "breathe" largely through their skin.[citation needed] Because the oxygen is dissolved in an aqueous film on the skin and passes from there to the blood, the skin must remain moist at all times; this makes frogs susceptible to many toxins in the environment, some of which can similarly dissolve in the layer of water and be passed into their bloodstream. This may be cause of the decline in frog populations.[citation needed]
Many characteristics are not shared by all of the approximately 5,250 described frog species. However, some general characteristics distinguish them from other amphibians. Frogs are usually well suited to jumping, with long hind legs and elongated ankle bones. They have a short vertebral column, with no more than ten free vertebrae, followed by a fused tailbone (urostyle or coccyx), typically resulting in a tailless phenotype.[citation needed]
Frogs range in size from 10 mm (0.39 in) (Brachycephalus didactylus of Brazil and Eleutherodactylus iberia of Cuba) to 300 mm (12 in) (goliath frog, Conraua goliath, of Cameroon). The skin hangs loosely on the body because of the lack of loose connective tissue. Skin texture varies: it can be smooth, warty or folded. Frogs have three eyelid membranes: one is transparent to protect the eyes underwater, and two vary from translucent to opaque. Frogs have a tympanum on each side of the head, which is involved in hearing and, in some species, is covered by skin. Most frogs do in fact have teeth of a sort. They have a ridge of very small cone teeth around the upper edge of the jaw. These are called maxillary teeth. Frogs often also have what are called vomerine teeth on the roof of their mouth. They do not have anything that could be called teeth on their lower jaw, so they usually swallow their food whole. The so-called "teeth" are mainly used to hold the prey and keep it in place till they can get a good grip on it and squash their eyeballs down to swallow their meal. Toads, however, do not have any teeth.[citation needed]
Tyler's Tree Frog (Litoria tyleri) illustrates large toe pads and webbed feet.
Feet and legs
The structure of the feet and legs varies greatly among frog species, depending in part on whether they live primarily on the ground, in water, in trees, or in burrows. Frogs must be able to move quickly through their environment to catch prey and escape predators, and numerous adaptations help them do so.
Many frogs, especially those that live in water, have webbed toes. The degree to which the toes are webbed is directly proportional to the amount of time the species lives in the water. For example, the completely aquatic African dwarf frog (Hymenochirus sp.) has fully webbed toes, whereas the toes of White's tree frog (Litoria caerulea), an arboreal species, are only a half or a quarter webbed.
Arboreal frogs have "toe pads" to help grip vertical surfaces. These pads, located on the ends of the toes, do not work by suction. Rather, the surface of the pad consists of interlocking cells, with a small gap between adjacent cells. When the frog applies pressure to the toe pads, the interlocking cells grip irregularities on the substrate. The small gaps between the cells drain away all but a thin layer of moisture on the pad, and maintain a grip through capillarity. This allows the frog to grip smooth surfaces, and does not function when the pads are excessively wet.[6]
In many arboreal frogs, a small "intercalary structure" in each toe increases the surface area touching the substrate. Furthermore, since hopping through trees can be dangerous, many arboreal frogs have hip joints that allow both hopping and walking. Some frogs that live high in trees even possess an elaborate degree of webbing between their toes, as do aquatic frogs. In these arboreal frogs, the webs allow the frogs to "parachute" or control their glide from one position in the canopy to another.[7]
Ground-dwelling frogs generally lack the adaptations of aquatic and arboreal frogs. Most have smaller toe pads, if any, and little webbing. Some burrowing frogs have a toe extension—a metatarsal tubercle—that helps them to burrow. The hind legs of ground dwellers are more muscular than those of aqueous and tree-dwelling frogs.
Jumping
Frogs are generally recognized as exceptional jumpers, and the best jumper of all vertebrates. The Australian rocket frog, Litoria nasuta, can leap over 50 times their body length (5.5 cm), resulting in jumps of over 2 meters. The acceleration of the jump may be up to twice gravity. There are tremendous differences between species in jumping capability, but within a species, jump distance increases with increasing size, but relative jumping distance (body-lengths jumped) decreases.
While frog species can use a variety of locomotor modes (running, walking, gliding, swimming, and climbing), more are either proficient at jumping or descended from ancestors who were, with much of the musculo-skeletal morphology modified for this purpose. The tibia, fibula and tarsals have been fused into a single, strong bone, as have the radius and ulna in the forelimbs (which must absorb the impact of landing). The metatarsals have become elongated to add to the leg length and allow the frog to push against the ground for longer during a jump. The illium has elongated and formed a mobile joint with the sacrum which, in specialist jumpers such as Ranids or Hylids, functions as an additional limb joint to further power the leaps. This elongation of the limbs results in the frog being able to apply force to the ground for longer during a jump, which in turn results in a longer, faster jump.[citation needed]
The muscular system has been similarly modified. The hind limbs of the ancestor of frogs presumably contained pairs of muscles which would act in opposition (one muscle to flex the knee, a different muscle to extend it), as is seen in most other limbed animals. However, in modern frogs, almost all muscles have been modified to contribute to the action of jumping, with only a few small muscles remaining to bring the limb back to the starting position and maintain posture. The muscles have also been greatly enlarged, with the muscles involved in jumping accounting for over 17% of the total mass of the frog.
In some extremely capable jumpers, such as the cuban tree frog, the peak power exerted during a jump can exceed what muscle is capable of producing. Currently, it is hypothesized that frogs are storing muscular energy by stretching their tendons like springs, then triggering the release all at once, allowing the frog to increase the energy of its jump beyond the limits of muscle-powered acceleration. A similar mechanism has already been documented in locusts and grasshoppers.[citation needed]
Skin
Pouched Frog (Assa darlingtoni) camouflaged against leaf litter.
Many frogs are able to absorb water and oxygen directly through the skin, especially around the pelvic area. However, the permeability of a frog's skin can also result in water loss. Some tree frogs reduce water loss with a waterproof layer of skin. Others have adapted behaviours to conserve water, including engaging in nocturnal activity and resting in a water-conserving position. This position involves the frog lying with its toes and fingers tucked under its body and chin, respectively, with no gap between the body and substrate. Some frog species will also rest in large groups, touching the skin of the neighbouring frog. This reduces the amount of skin exposed to the air or a dry surface, and thus reduces water loss. These adaptations only reduce water loss enough for a predominantly arboreal existence, and are not suitable for arid conditions.
Camouflage is a common defensive mechanism in frogs. Most camouflaged frogs are nocturnal, which adds to their ability to hide. Nocturnal frogs usually find the ideal camouflaged position during the day to sleep. Some frogs have the ability to change colour, but this is usually restricted to shades of one or two colours. For example, White's tree frog varies in shades of green and brown. Features such as warts and skin folds are usually found on ground-dwelling frogs, where a smooth skin would not disguise them effectively. Arboreal frogs usually have smooth skin, enabling them to disguise themselves as leaves.[citation needed]
Certain frogs change colour between night and day, as light and moisture stimulate the pigment cells and cause them to expand or contract.
Oophaga pumilio, a poison dart frog, contains numerous alkaloids which deter predators.
Poison
Many frogs contain mild toxins that make them unpalatable to potential predators. For example, all toads have large poison glands—the parotoid glands—located behind the eyes on the top of the head. Some frogs, such as some poison dart frogs, are especially toxic. The chemical makeup of toxins in frogs varies from irritants to hallucinogens, convulsants, nerve poisons, and vasoconstrictors. Many predators of frogs have adapted to tolerate high levels of these poisons. Others, including humans, may be severely affected.
Some frogs obtain poisons from the ants and other arthropods they eat;[8] others, such as the Australian Corroboree Frogs (Pseudophryne corroboree and Pseudophryne pengilleyi), can manufacture an alkaloid not derived from their diet.[9] Some native people of South America extract poison from the poison dart frogs and apply it to their darts for hunting,[10] although few species are toxic enough to be used for this purpose. It was previously a misconception the poison was placed on arrows rather than darts. The common name of these frogs was thus changed from "poison arrow frog" to "poison dart frog" in the early 1980s. Poisonous frogs tend to advertise their toxicity with bright colours, an adaptive strategy known as aposematism. There are at least two non-poisonous species of frogs in tropical America (Eleutherodactylus gaigei and Lithodytes lineatus) that mimic the colouration of dart poison frogs' coloration for self-protection (Batesian mimicry).[11][12]
Because frog toxins are extraordinarily diverse, they have raised the interest of biochemists as a "natural pharmacy". The alkaloid epibatidine, a painkiller 200 times more potent than morphine, is found in some species of poison dart frogs. Other chemicals isolated from the skin of frogs may offer resistance to HIV infection.[13] Arrow and dart poisons are under active investigation for their potential as therapeutic drugs.[14]
The skin secretions of some toads, such as the Colorado River toad and cane toad, contain bufotoxins, some of which, such as bufotenin, are psychoactive, and have therefore been used as recreational drugs. Typically, the skin secretions are dried and smoked. Skin licking is especially dangerous, and appears to constitute an urban myth. See psychoactive toad.
Respiration and circulation
The skin of a frog is permeable to oxygen and carbon dioxide, as well as to water. There are a number of blood vessels near the surface of the skin. When a frog is underwater, oxygen is transmitted through the skin directly into the bloodstream. On land, adult frogs use their lungs to breathe. Their lungs are similar to those of humans, but the chest muscles are not involved in respiration, and there are no ribs or diaphragm to support breathing. Frogs breathe by taking air in through the nostrils (which often have valves which close when the frog is submerged), causing the throat to puff out, then compressing the floor of the mouth, which forces the air into the lungs. In August 2007 an aquatic frog named Barbourula kalimantanensis was discovered in a remote part of Indonesia. The Bornean Flat-headed Frog (B. kalimantanensis) is the first species of frog known to science without lungs.
Frogs are known for their three-chambered heart, which they share with all tetrapods except birds and mammals. In the three-chambered heart, oxygenated blood from the lungs and de-oxygenated blood from the respiring tissues enter by separate atria, and are directed via a spiral valve to the appropriate vessel—aorta for oxygenated blood and pulmonary vein for deoxygenated blood. This special structure is essential to keeping the mixing of the two types of blood to a minimum, which enables frogs to have higher metabolic rates, and to be more active than otherwise.
Digestion and excretion
The frog's digestive system begins with the mouth. Frogs have teeth along their upper jaw called the maxillary teeth, which are used to grind food before swallowing. These teeth are very weak, and cannot be used to catch or harm agile prey. Instead, the frog uses its sticky tongue to catch food (such as flies or other insects). The food then moves through the esophagus into the stomach. The food then proceeds to the small intestine (duodenum and ileum) where most digestion occurs. Frogs carry pancreatic juice from the pancreas, and bile (produced by the liver) through the gallbladder from the liver to the small intestine, where the fluids digest the food and extract the nutrients. When the food passes into the large intestine, the water is reabsorbed and wastes are routed to the cloaca. All wastes exit the body through the cloaca and the cloacal vent.
Nervous system
The frog has a highly developed nervous system which consists of a brain, spinal cord and nerves. Many parts of the frog's brain correspond with those of humans. The medulla oblongata regulates respiration, digestion, and other automatic functions. Muscular coordination and posture are controlled by the cerebellum. The relative size of the cerebrum of a frog is much smaller than that of a human. Frogs have ten cranial nerves (nerves which pass information from the outside directly to the brain) and ten pairs of spinal nerves (nerves which pass information from extremities to the brain through the spinal cord). By contrast, all amniotes (mammals, birds and reptiles) have twelve cranial nerves. Frogs do not have external ears; the eardrums (tympanic membranes) are directly exposed. As in all animals, the ear contains semicircular canals which help control balance and orientation.
Natural history
The life cycle of frogs, like that of other amphibians, consists of four main stages: egg, tadpole, metamorphosis and adult. The reliance of frogs on an aquatic environment for the egg and tadpole stages gives rise to a variety of breeding behaviours that include the well-known mating calls used by the males of most species to attract females to the bodies of water that they have chosen for breeding. Some frogs also look after their eggs—and in some cases even the tadpoles—for some time after laying.
Life cycle
Frogspawn
Frogspawn development
Tadpole of Haswell's Froglet (Paracrinia haswelli
Adult leopard frog
Sister project Wikibooks has a book on the topic of
Adventist Youth Honors Answer Book/Nature/Life cycle of a frog
The life cycle of a frog starts with an egg. A female generally lays frogspawn, or egg masses containing thousands of eggs, in water. The eggs are highly vulnerable to predation, so frogs have evolved many techniques to ensure the survival of the next generation. In colder areas the embryo is black to absorb more heat from the sun, which speeds up the development. Most commonly, this involves synchronous reproduction. Many individuals will breed at the same time, overwhelming the actions of predators; the majority of the offspring will still die due to predation, but there is a greater chance some will survive. Another way in which some species avoid the predators and pathogens eggs are exposed to in ponds is to lay eggs on leaves above the pond, with a gelatinous coating designed to retain moisture. In these species the tadpoles drop into the water upon hatching. The eggs of some species laid out of water can detect vibrations of nearby predatory wasps or snakes, and will hatch early to avoid being eaten.[15] Some species, such as the Cane Toad (Bufo marinus), lay poisonous eggs to minimise predation. While the length of the egg stage depends on the species and environmental conditions, aquatic eggs generally hatch within one week. Other species goes through their whole larval phase inside the eggs or the mother, or they have direct development.
Eggs hatch and continue life as tadpoles (occasionally known as polliwogs), which typically have oval bodies and long, vertically-flattened tails. At least one species (Nannophrys ceylonensis) has tadpoles that are semi-terrestrial and lives among wet rocks,[16][17] but as a general rule, free living larvae are fully aquatic. They lack lungs, eyelids, front and hind legs, and have a cartilaginous skeleton, a lateral line system, gills for respiration (external gills at first, internal gills later) and tails with dorsal and ventral folds of skin for swimming[18]. Some species which go through the metamorphosis inside the egg and hatch to small frogs never develop gills, instead there are specialised areas of skin that takes care of the respiration. Tadpoles also lack true teeth, but the jaws in most species usually have two elongate, parallel rows of small keratinized structures called keradonts in the upper jaw while the lower jaw has three rows of keradonts, surrounded by a horny beak, but the number of rows can be lower or absent, or much higher[1]. Tadpoles are typically herbivorous, feeding mostly on algae, including diatoms filtered from the water through the gills. Some species are carnivorous at the tadpole stage, eating insects, smaller tadpoles, and fish. Tadpoles are highly vulnerable to predation by fish, newts, predatory diving beetles and birds such as kingfishers. Cannibalism has been observed among tadpoles. Poisonous tadpoles are present in many species, such as Cane Toads. The tadpole stage may be as short as a week, or tadpoles may overwinter and metamorphose the following year in some species, such as the midwife toad (Alytes obstetricans) and the common spadefoot (Pelobates fuscus). In the Pipidae, with the exception for Hymenochirus, the tadpoles have paired anterior barbels which make them resemble small catfish[19].
With the exception of the base of the tail, where a few vertebral structures develop to give rise to the urostyle later in life, the tail lacks the completely solid, segmental, skeletal elements of cartilage or bony tissue that are so typical for other vertebrates, although it does contain a notochord
At the end of the tadpole stage, frogs undergo metamorphosis, in which they transition into adult form. Metamorphosis involves a dramatic transformation of morphology and physiology, as tadpoles develop hind legs, then front legs, lose their gills and develop lungs. Their intestines shorten as they shift from an herbivorous to a carnivorous diet. Eyes migrate rostrally and dorsally, allowing for binocular vision exhibited by the adult frog. This shift in eye position mirrors the shift from prey to predator, as the tadpole develops and depends less upon a larger and wider field of vision and more upon depth perception. The final stage of development from froglet to adult frog involves apoptosis (programmed cell death) and resorption of the tail.
After metamorphosis, young adults may leave the water and disperse into terrestrial habitats, or continue to live in the aquatic habitat as adults. Almost all species of frogs are carnivorous as adults, eating invertebrates such as arthropods, annelids and gastropods. A few of the larger species may eat prey such as small mammals, fish and smaller frogs. Some frogs use their sticky tongues to catch fast-moving prey, while others capture their prey and force it into their mouths with their hands. However, there are a very few species of frogs that primarily eat plants.[20] Adult frogs are themselves preyed upon by birds, large fish, snakes, otters, foxes, badgers, coatis, and other animals. Frogs are also eaten by people (see section on uses in agriculture and research, below).
Frogs and toads can live for many years; though little is known about their life span in the wild, captive frogs and toads are recorded living up to 40 years.[21]
Although it is not common knowledge, some species of frog in their tadpole stage are known to be carnivorous. Early developers who gain legs may be eaten by the others, so the late bloomers survive longer. This has been observed in England in the species Rana temporaria (common frog).[22]
Unlike salamanders and newts, frogs and toads never become sexually mature while still in their larval stage.
Reproduction of frogs
Once adult frogs reach maturity, they will assemble at a water source such as a pond or stream to breed. Many frogs return to the bodies of water where they were born, often resulting in annual migrations involving thousands of frogs. In continental Europe, a large proportion of migrating frogs used to die on roads, before special fences and tunnels were built for them.
Male and female Common toad (Bufo bufo) in amplexus
Once at the breeding ground, male frogs call to attract a mate, collectively becoming a chorus of frogs. The call is unique to the species, and will attract females of that species. Some species have satellite males who do not call, but intercept females that are approaching a calling male.
The male and female frogs then undergo amplexus. This involves the male mounting the female and gripping her (sometimes with special nuptial pads) tightly. Fertilization is external: the egg and sperm meet outside of the body. The female releases her eggs, which the male frog covers with a sperm solution. The eggs then swell and develop a protective coating. The eggs are typically brown or black, with a clear, gelatin-like covering.
Most temperate species of frogs reproduce between late autumn and early spring. In the UK, most common frog populations produce frogspawn in February, although there is wide variation in timing. Water temperatures at this time of year are relatively low, typically between four and 10 degrees Celsius. Reproducing in these conditions helps the developing tadpoles because dissolved oxygen concentrations in the water are highest at cold temperatures. More importantly, reproducing early in the season ensures that appropriate food is available to the developing frogs at the right time.
Parental care
Colour plate from Ernst Haeckel's 1904 Kunstformen der Natur, depicting frog species that include two examples of parental care.
Although care of offspring is poorly understood in frogs, it is estimated that up to 20% of amphibian species may care for their young in one way or another, and there is a great diversity of parental behaviours.[23] Some species of poison dart frog lay eggs on the forest floor and protect them, guarding the eggs from predation and keeping them moist. The frog will urinate on them if they become too dry. After hatching, a parent (the sex depends upon the species) will move them, on its back, to a water-holding bromeliad. The parent then feeds them by laying unfertilized eggs in the bromeliad until the young have metamorphosed. Other frogs carry the eggs and tadpoles on their hind legs or back (e.g. the midwife toads, Alytes spp.). Some frogs even protect their offspring inside their own bodies. The male Australian Pouched Frog (Assa darlingtoni) has pouches along its side in which the tadpoles reside until metamorphosis. The female Gastric-brooding Frogs (genus Rheobatrachus) from Australia, now probably extinct, swallows its tadpoles, which then develop in the stomach. To do this, the Gastric-brooding Frog must stop secreting stomach acid and suppress peristalsis (contractions of the stomach). Darwin's Frog (Rhinoderma darwinii) from Chile puts the tadpoles in its vocal sac for development. Some species of frog will leave a 'babysitter' to watch over the frogspawn until it hatches.
Call
Some frog calls are so loud, they can be heard up to a mile away.[24] The call of a frog is unique to its species. Frogs call by passing air through the larynx in the throat. In most calling frogs, the sound is amplified by one or more vocal sacs, membranes of skin under the throat or on the corner of the mouth that distend during the amplification of the call. The field of neuroethology studies the neurocircuitry that underlies frog audition.
Some frogs lack vocal sacs, such as those from the genera Heleioporus and Neobatrachus, but these species can still produce a loud call. Their buccal cavity is enlarged and dome-shaped, acting as a resonance chamber that amplifies their call. Species of frog without vocal sacs and that do not have a loud call tend to inhabit areas close to flowing water. The noise of flowing water overpowers any call, so they must communicate by other means.
The main reason for calling is to allow males to attract a mate. Males call either individually or in a group called a chorus. Females of many frog species, for example Polypedates leucomystax, produce calls reciprocal to the males', which act as the catalyst for the enhancement of reproductive activity in a breeding colony.[25] A male frog emits a release call when mounted by another male. Tropical species also have a rain call that they make on the basis of humidity cues prior to a rain shower. Many species also have a territorial call that is used to chase away other males. All of these calls are emitted with the mouth of the frog closed.
A distress call, emitted by some frogs when they are in danger, is produced with the mouth open, resulting in a higher-pitched call. The effectiveness of the call is unknown; however, it is suspected the call intrigues the predator until another animal is attracted, distracting them enough for its escape.
Many species of frog have deep calls, or croaks. The English onomatopoeic spelling is "ribbit". The croak of the American bullfrog (Rana catesbiana) is sometimes spelt "jug o' rum".[26] Other examples are Ancient Greek brekekekex koax koax for probably Rana ridibunda, and the description in Rigveda 7:103.6 gómāyur éko ajámāyur ékaħ = "one [has] a voice like a cow's, one [has] a voice like a goat's".
Distribution and conservation status
Golden toad (Ollotis periglenes) - last seen in 1989
The Red-eyed Tree Frog (Litoria chloris) is a species of tree frog native to eastern Australia.
The habitat of frogs extends almost worldwide, but they do not occur in Antarctica and are not present on many oceanic islands.[27][28] The greatest diversity of frogs occurs in the tropical areas of the world, where water is readily available, suiting frogs' requirements due to their skin. Some frogs inhabit arid areas such as deserts, where water may not be easily accessible, and rely on specific adaptations to survive. The Australian genus Cyclorana and the American genus Pternohyla will bury themselves underground, create a water-impervious cocoon and hibernate during dry periods. Once it rains, they emerge, find a temporary pond and breed. Egg and tadpole development is very fast in comparison to most other frogs so that breeding is complete before the pond dries up. Some frog species are adapted to a cold environment; for instance the wood frog, whose habitat extends north of the Arctic Circle, buries itself in the ground during winter when much of its body freezes.
Frog populations have declined dramatically since the 1950s: more than one third of species are believed to be threatened with extinction and more than 120 species are suspected to be extinct since the 1980s.[29] Among these species are the golden toad of Costa Rica and the Gastric-brooding frogs of Australia. Habitat loss is a significant cause of frog population decline, as are pollutants, climate change, the introduction of non-indigenous predators/competitors, and emerging infectious diseases including chytridiomycosis. Many environmental scientists believe that amphibians, including frogs, are excellent biological indicators of broader ecosystem health because of their intermediate position in food webs, permeable skins, and typically biphasic life (aquatic larvae and terrestrial adults).[30] It appears that it is the species with both aquatic eggs and aquatic larvae that are most affected by the decline, while those with direct development are the most resistant .[31]
A Canadian study conducted in 2006 proposed heavy traffic near frog habitats as a large threat to frog populations.[32]
In a few cases, captive breeding programs have been attempted to alleviate the pressure on frog populations, and these have proved successful.[33][34] In May 2007, it was reported the application of certain probiotic bacteria could protect amphibians from chytridiomycosis.[35]
Zoos and aquariums around the world have named 2008 the Year of the Frog, to draw attention to the conservation issues.[36]
Evolution
A fossilized frog from the Czech Republic, possibly Palaeobatrachus gigas.
Until the discovery of the Early Permian Gerobatrachus hottoni, a stem-batrachian with many salamander-like characteristics, the earliest known proto-frog was Triadobatrachus massinoti, from the 250 million year old early Triassic of Madagascar.[37] The skull is frog-like, being broad with large eye sockets, but the fossil has features diverging from modern amphibia. These include a different ilium, a longer body with more vertebrae, and separate vertebrae in its tail (whereas in modern frogs, the tail vertebrae are fused, and known as the urostyle or coccyx). The tibia and fibula bones are unfused and separate, making it probable Triadobatrachus was not an efficient leaper.
Another fossil frog, discovered in Arizona and called Prosalirus bitis, was uncovered in 1985, and dates from roughly the same time as Triadobatrachus. Like Triadobatrachus, Prosalirus did not have greatly enlarged legs, but had the typical three-pronged pelvic structure. Unlike Triadobatrachus, Prosalirus had already lost nearly all of its tail.
The earliest true frog is Vieraella herbsti, from the early Jurassic (188–213 million years ago). It is known only from the dorsal and ventral impressions of a single animal and was estimated to be 33 mm (1.3 in) from snout to vent. Notobatrachus degiustoi from the middle Jurassic is slightly younger, about 155–170 million years old. It is likely the evolution of modern Anura was completed by the Jurassic period. The main evolutionary changes involved the shortening of the body and the loss of the tail.
The earliest full fossil record of a modern frog is of sanyanlichan, which lived 125 million years ago[38] and had all modern frog features, but bore 9 presacral vertebrae instead of the 8 of modern frogs.[39]
Frog fossils have been found on all continents, including Antarctica.
Uses in agriculture and research
For more details on this topic, see animal testing on frogs.
Frogs are raised commercially for several purposes. Frogs are used as a food source; frog legs are a delicacy in China, France, the Philippines, the north of Greece and in many parts of the American South, especially Louisiana. Dead frogs are sometimes used for dissections in high school and university anatomy classes, often after being injected with coloured plastics to enhance the contrast between the organs. This practice has declined in recent years with the increasing concerns about animal welfare.
Frogs have served as important model organisms throughout the history of science. Eighteenth-century biologist Luigi Galvani discovered the link between electricity and the nervous system through studying frogs. The African clawed frog or platanna (Xenopus laevis) was first widely used in laboratories in pregnancy assays in the first half of the 20th century. When human chorionic gonadotropin, a hormone found in substantial quantities in the urine of pregnant women, is injected into a female X. laevis, it induces them to lay eggs. In 1952, Robert Briggs and Thomas J. King cloned a frog by somatic cell nuclear transfer, the same technique later used to create Dolly the Sheep, their experiment was the first time successful nuclear transplantation had been accomplished in metazoans.[40]
Frogs are used in cloning research and other branches of embryology because frogs are among the closest living relatives of man to lack egg shells characteristic of most other vertebrates, and therefore facilitate observations of early development. Although alternative pregnancy assays have been developed, biologists continue to use Xenopus as a model organism in developmental biology because it is easy to raise in captivity and has a large and easily manipulatable embryo. Recently, X. laevis is increasingly being displaced by its smaller relative X. tropicalis, which reaches its reproductive age in five months rather than one to two years (as in X. laevis),[41] facilitating faster studies across generations. The genome sequence of X. tropicalis will probably be completed by 2015 at the latest.[42]
Cultural beliefs
Moche Frog 200 A.D. Larco Museum Collection Lima, Peru.
For more details on this topic, see Frogs in popular culture.
Frogs feature prominently in folklore, fairy tales and popular culture. They tend to be portrayed as benign, ugly, clumsy, but with hidden talents. Examples include Michigan J. Frog, The Frog Prince, and Kermit the Frog. Michigan J. Frog, featured in the Warner Brothers cartoon One Froggy Evening, only performs his singing and dancing routine for his owner. Once another person looks at him, he will return to a frog-like pose. "The Frog Prince" is a fairy tale of a frog who turns into a handsome prince once kissed. Kermit the Frog, on the other hand, is a conscientious and disciplined character of The Muppet Show and Sesame Street; while openly friendly and greatly talented, he is often portrayed as cringing at the fanciful behaviour of more flamboyant characters.
Saturn
From Wikipedia, the free encyclopedia
Jump to: navigation, search
This article is about the planet. For other uses, see Saturn (disambiguation).
Semi-protected
Saturn Astronomical symbol for Saturn The planet Saturn
Saturn, as seen by Cassini
Designations
Adjective Saturnian
Orbital characteristics[1][2]
Epoch J2000
Aphelion 1 513 325 783 km
10.115 958 04 AU
Perihelion 1 353 572 956 km
9.048 076 35 AU
Semi-major axis 1 433 449 370 km
9.582 017 20 AU
Eccentricity 0.055 723 219
Orbital period 10 832.327 days
29.657 296 yr
Synodic period 378.09 days[3]
Average orbital speed 9.69 km/s[3]
Mean anomaly 320.346 750°
Inclination 2.485 240°
5.51° to Sun's equator
Longitude of ascending node 113.642 811°
Argument of perihelion 336.013 862°
Satellites ~ 200 observed (60 with secure orbits)
Physical characteristics
Equatorial radius 60 268 ± 4 km[4][5]
9.4492 Earths
Polar radius 54 364 ± 10 km[4][5]
8.5521 Earths
Flattening 0.097 96 ± 0.000 18
Surface area 4.27×1010 km²[6][5]
83.703 Earths
Volume 8.2713×1014 km³[3][5]
763.59 Earths
Mass 5.6846×1026 kg[3]
95.152 Earths
Mean density 0.687 g/cm³[3][5]
(less than water)
Equatorial surface gravity 8.96 m/s²[3][5]
0.914 g
Escape velocity 35.5 km/s[3][5]
Sidereal rotation
period 0.439 – 0.449 day[7]
(10 h 32 – 47 min)
Equatorial rotation velocity 9.87 km/s[5]
35 500 km/h
Axial tilt 26.73°[3]
North pole right ascension 2 h 42 min 21 s
40.589°[4]
North pole declination 83.537°[4]
Albedo 0.342 (bond)
0.47 (geom.)[3]
Surface temp.
1 bar level
0.1 bar
min mean max
134 K[3]
84 K[3]
Apparent magnitude +1.2 to -0.24[8]
Angular diameter 14.5" — 20.1"[3]
(excludes rings)
Atmosphere[3]
Scale height 59.5 km
Composition
~96% Hydrogen (H2)
~3% Helium
~0.4% Methane
~0.01% Ammonia
~0.01% Hydrogen deuteride (HD)
0.000 7% Ethane
Ices:
Ammonia
water
ammonium hydrosulfide(NH4SH)
Saturn (en-us-Saturn.ogg /ˈsætɚn/ (help·info)[9]) is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant. Together, these four planets are sometimes referred to as the Jovian planets, where Jovian is the adjectival form of Jupiter.
Saturn is named after the Roman god Saturnus (that became the namesake of Saturday), equated to the Greek Kronos (the Titan father of Zeus) the Babylonian Ninurta and to the Hindu Shani. Saturn's symbol represents the god's sickle (Unicode: ♄).
The planet Saturn is composed of hydrogen, with small proportions of helium and trace elements.[10] The interior consists of a small core of rock and ice, surrounded by a thick layer of metallic hydrogen and a gaseous outer layer. The outer atmosphere is generally bland in appearance, although long-lived features can appear. Wind speeds on Saturn can reach 1800 km/h, significantly faster than those on Jupiter. Saturn has a planetary magnetic field intermediate in strength between that of Earth and the more powerful field around Jupiter.
Saturn has a prominent system of rings, consisting mostly of ice particles with a smaller amount of rocky debris and dust. Sixty known moons orbit the planet. Titan, Saturn's largest and the Solar System's second largest moon (after Jupiter's Ganymede), is larger than the planet Mercury and is the only moon in the Solar System to possess a significant atmosphere.[11]
Contents
[hide]
* 1 Physical characteristics
o 1.1 Internal structure
o 1.2 Atmosphere
o 1.3 Cloud layers
o 1.4 Magnetosphere
* 2 Orbit and rotation
* 3 Planetary rings
o 3.1 History
o 3.2 Physical characteristics
o 3.3 Spokes of the rings
* 4 Natural satellites
* 5 History and exploration
o 5.1 Ancient times and observation
o 5.2 Pioneer 11 flyby
o 5.3 Voyager flybys
o 5.4 Cassini orbiter
* 6 Best viewing
* 7 See also
* 8 References
* 9 External links
Physical characteristics
A rough comparison of the sizes of Saturn and Earth.
Due to a combination of its lower density, rapid rotation, and fluid state, Saturn is an oblate spheroid; that is, it is flattened at the poles and bulges at the equator. Its equatorial and polar radii differ by almost 10%—60 268 km vs. 54 364 km.[3] The other gas planets are also oblate, but to a lesser extent. Saturn is the only planet of the Solar System that is less dense than water. Although Saturn's core is considerably denser than water, the average specific density of the planet is 0.69 g/cm³ due to the gaseous atmosphere. Saturn is only 95 Earth masses,[3] compared to Jupiter, which is 318 times the mass of the Earth[12] but only about 20% larger than Saturn.[13]
Internal structure
Though there is little direct information about Saturn's internal structure, it is thought that its interior is similar to that of Jupiter, having a small rocky core surrounded mostly by hydrogen and helium. The rocky core is similar in composition to the Earth, but denser. Above this, there is a thicker liquid metallic hydrogen layer, followed by a layer of liquid hydrogen and helium, and in the outermost 1000 km a gaseous atmosphere.[14] Traces of various ices are also present. The core region is estimated to be about 9–22 times the mass of the Earth.[15] Saturn has a very hot interior, reaching 11 700 °C at the core, and it radiates 2.5 times more energy into space than it receives from the Sun. Most of the extra energy is generated by the Kelvin-Helmholtz mechanism (slow gravitational compression), but this alone may not be sufficient to explain Saturn's heat production. An additional proposed mechanism by which Saturn may generate some of its heat is the "raining out" of droplets of helium deep in Saturn's interior, the droplets of helium releasing heat by friction as they fall down through the lighter hydrogen.[16]
Atmosphere
Saturn's temperature emissions: the prominent hot spot at the bottom of the image is at Saturn's south pole.
The outer atmosphere of Saturn consists of about 93.2% molecular hydrogen and 6.7% helium. Trace amounts of ammonia, acetylene, ethane, phosphine, and methane have also been detected.[17] The upper clouds on Saturn are composed of ammonia crystals, while the lower level clouds appear to be composed of either ammonium hydrosulfide (NH4SH) or water.[18] The atmosphere of Saturn is significantly deficient in helium relative to the abundance of the elements in the Sun.
The quantity of elements heavier than helium are not known precisely, but the proportions are assumed to match the primordial abundances from the formation of the Solar System. The total mass of these elements is estimated to be 19–31 times the mass of the Earth, with a significant fraction located in Saturn's core region.[19]
Cloud layers
Saturn's celestial body atmosphere exhibits a banded pattern similar to Jupiter's (the nomenclature is the same), but Saturn's bands are much fainter and are also much wider near the equator. At the bottom, extending for 10 km and with a temperature of -23 °C, is a layer made up of water ice. After that comes a layer of ammonium hydrosulfide ice, which extends for another 50 km and is approximately at -93 °C. Eighty kilometers above that are ammonia ice clouds, where the temperatures are about -153 °C. Near the top, extending for some 200 km to 270 km above the clouds, come layers of visible cloud tops and a hydrogen and helium atmosphere.[20] Saturn's winds are among the Solar System's fastest. Voyager data indicate peak easterly winds of 500 m/s (1800 km/h).[10] Saturn's finer cloud patterns were not observed until the Voyager flybys. Since then, however, Earth-based telescopy has improved to the point where regular observations can be made.
Saturn's usually bland atmosphere occasionally exhibits long-lived ovals and other features common on Jupiter. In 1990, the Hubble Space Telescope observed an enormous white cloud near Saturn's equator which was not present during the Voyager encounters, and, in 1994, another smaller storm was observed. The 1990 storm was an example of a Great White Spot, a unique but short-lived phenomenon which occurs once every Saturnian year, or roughly every 30 Earth years, around the time of the northern hemisphere's summer solstice.[21] Previous Great White Spots were observed in 1876, 1903, 1933, and 1960, with the 1933 storm being the most famous. If the periodicity is maintained, another storm will occur in about 2020.[22]
In recent images from the Cassini spacecraft, Saturn's northern hemisphere appears a bright blue, similar to Uranus, as can be seen in the image below. This blue color cannot currently be observed from Earth, because Saturn's rings are currently blocking its northern hemisphere. The color is most likely caused by Rayleigh scattering.
Saturn's northern hemisphere, as seen by Cassini. Note the planet's blue appearance through the ring.
North polar hexagonal cloud feature, discovered by Voyager 1 and confirmed in 2006 by Cassini.[23]
Astronomers using infrared imaging have shown that Saturn has a warm polar vortex and that it is the only such feature known in the solar system. This, they say, is the warmest spot on Saturn. Whereas temperatures on Saturn are normally -185 °C, temperatures on the vortex often reach as high as -122 °C.[24]
A persisting hexagonal wave pattern around the north polar vortex in the atmosphere at about 78°N was first noted in the Voyager images.[25][26] Unlike the north pole, HST imaging of the south polar region indicates the presence of a jet stream, but no strong polar vortex nor any hexagonal standing wave.[27] However, NASA reported in November 2006 that the Cassini spacecraft observed a 'hurricane-like' storm locked to the south pole that had a clearly defined eyewall.[28] This observation is particularly notable because eyewall clouds had not previously been seen on any planet other than Earth (including a failure to observe an eyewall in the Great Red Spot of Jupiter by the Galileo spacecraft).[29]
The straight sides of the northern polar hexagon are each about 13 800 km long. The entire structure rotates with a period of 10h 39 m 24s, the same period as that of the planet's radio emissions, which is assumed to be equal to the period of rotation of Saturn's interior. The hexagonal feature does not shift in longitude like the other clouds in the visible atmosphere.
The pattern's origin is a matter of much speculation. Most astronomers seem to think some sort of standing-wave pattern in the atmosphere; but the hexagon might be a novel sort of aurora. Polygon shapes have been replicated in spinning buckets of fluid in a laboratory.[30]
Magnetosphere
Saturn has an intrinsic magnetic field that has a simple, symmetric shape—a magnetic dipole. Its strength at the equator—0.2 gauss (20 µT)—is approximately one twentieth than that of the field around Jupiter and slightly weaker than Earth's magnetic field.[31] As a result the cronian magnetosphere is much smaller than the jovian and extends slightly beyond the orbit of Titan.[32] Most probably, the magnetic field is generated similarly to that of Jupiter—by currents in the metallic-hydrogen layer, which is called a metallic-hydrogen dynamo.[32] Similarly to those of other planets, this magnetosphere is efficient at deflecting the solar wind particles from the Sun. The moon Titan orbits within the outer part of Saturn's magnetosphere and contributes plasma from the ionized particles in Titan's outer atmosphere.[31]
Orbit and rotation
Animation of hexagonal cloud feature.
The average distance between Saturn and the Sun is over 1 400 000 000 km (9 AU). With an average orbital speed of 9.69 km/s,[3] it takes Saturn 10 759 Earth days (or about 29½ years), to finish one revolution around the Sun.[3] The elliptical orbit of Saturn is inclined 2.48° relative to the orbital plane of the Earth.[3] Because of an eccentricity of 0.056, the distance between Saturn and the Sun varies by approximately 155 000 000 km between perihelion and aphelion,[3] which are the nearest and most distant points of the planet along its orbital path, respectively.
The visible features on Saturn rotate at different rates depending on latitude, and multiple rotation periods have been assigned to various regions (as in Jupiter's case): System I has a period of 10 h 14 min 00 s (844.3°/d) and encompasses the Equatorial Zone, which extends from the northern edge of the South Equatorial Belt to the southern edge of the North Equatorial Belt. All other Saturnian latitudes have been assigned a rotation period of 10 h 39 min 24 s (810.76°/d), which is System II. System III, based on radio emissions from the planet in the period of the Voyager flybys, has a period of 10 h 39 min 22.4 s (810.8°/d); because it is very close to System II, it has largely superseded it.
However, a precise value for the rotation period of the interior remains elusive. While approaching Saturn in 2004, the Cassini spacecraft found that the radio rotation period of Saturn had increased appreciably, to approximately 10 h 45 m 45 s (± 36 s).[33] The cause of the change is unknown—it was thought to be due to a movement of the radio source to a different latitude inside Saturn, with a different rotational period, rather than because of a change in Saturn's rotation.
Later, in March 2007, it was found that the rotation of the radio emissions did not trace the rotation of the planet, but rather is produced by convection of the plasma disc, which is dependent also on other factors besides the planet's rotation. It was reported that the variance in measured rotation periods may be caused by geyser activity on Saturn's moon Enceladus. The water vapor emitted into Saturn's orbit by this activity becomes charged and "weighs down" Saturn's magnetic field, slowing its rotation slightly relative to the rotation of the planet itself. At the time it was stated that there is no currently known method of determining the rotation rate of Saturn's core.[34][35][36]
The latest estimate of Saturn's rotation based on a compilation of various measurements from the Cassini, Voyager and Pioneer probes was reported in September 2007 is 10 hours, 32 minutes, 35 seconds.[37]
Planetary rings
The rings of Saturn (as imaged here by Cassini in 2007) are the most conspicuous in the Solar System.[14]
Main article: Rings of Saturn
Saturn is probably best known for its system of planetary rings, which makes it the most visually remarkable object in the solar system.[14]
History
The rings were first observed by Galileo Galilei in 1610 with his telescope, but he was unable to identify them as such. He wrote to the Duke of Tuscany that "The planet Saturn is not alone, but is composed of three, which almost touch one another and never move nor change with respect to one another. They are arranged in a line parallel to the zodiac, and the middle one (Saturn itself) is about three times the size of the lateral ones [the edges of the rings]." He also described Saturn as having "ears." In 1612 the plane of the rings was oriented directly at the Earth and the rings appeared to vanish. Mystified, Galileo wondered, "Has Saturn swallowed his children?", referring to the myth of the god Saturn eating his own children to prevent them from overthrowing him.[38] Then, in 1613, they reappeared again, further confusing Galileo.[39]
In 1655, Christiaan Huygens became the first person to suggest that Saturn was surrounded by a ring. Using a telescope that was far superior to those available to Galileo, Huygens observed Saturn and wrote that "It [Saturn] is surrounded by a thin, flat, ring, nowhere touching, inclined to the ecliptic."[39]
In 1675, Giovanni Domenico Cassini determined that Saturn's ring was composed of multiple smaller rings with gaps between them; the largest of these gaps was later named the Cassini Division. This division in itself is a 4800 km-wide region between the A Ring and B Ring.[40]
In 1859, James Clerk Maxwell demonstrated that the rings could not be solid or they would become unstable and break apart. He proposed that the rings must be composed of numerous small particles, all independently orbiting Saturn.[41] Maxwell's theory was proven correct in 1895 through spectroscopic studies of the rings carried out by James Keeler of Lick Observatory.
Physical characteristics
Saturn's rings cut across an eerie scene that is ruled by Titan's luminous crescent and globe-encircling haze, broken by the small moon Enceladus, whose cryovolcanos are dimly visible at its south pole. North is up. Imaged by Cassini in 2006.
The rings can be viewed using a quite modest modern telescope or with good binoculars. They extend from 6 630 km to 120 700 km above Saturn's equator, average approximately 20 meters in thickness, and are composed of 93 percent water ice with a smattering of tholin impurities, and 7 percent amorphous carbon.[42] They range in size from specks of dust to the size of a small automobile.[43] There are two main theories regarding the origin of Saturn's rings. One theory, originally proposed by Édouard Roche in the 19th century, is that the rings were once a moon of Saturn whose orbit decayed until it came close enough to be ripped apart by tidal forces (see Roche limit). A variation of this theory is that the moon disintegrated after being struck by a large comet or asteroid. The second theory is that the rings were never part of a moon, but are instead left over from the original nebular material from which Saturn formed.
While the largest gaps in the rings, such as the Cassini Division and Encke Gap, can be seen from Earth, both Voyager spacecraft discovered that the rings have an intricate structure of thousands of thin gaps and ringlets. This structure is thought to arise, in several different ways, from the gravitational pull of Saturn's many moons. Some gaps are cleared out by the passage of tiny moonlets such as Pan, many more of which may yet be discovered, and some ringlets seem to be maintained by the gravitational effects of small shepherd satellites such as Prometheus and Pandora. Other gaps arise from resonances between the orbital period of particles in the gap and that of a more massive moon further out; Mimas maintains the Cassini division in this manner. Still more structure in the rings consists of spiral waves raised by the moons' periodic gravitational perturbations.
Data from the Cassini space probe indicate that the rings of Saturn possess their own atmosphere, independent of that of the planet itself. The atmosphere is composed of molecular oxygen gas (O2) produced when ultraviolet light from the Sun interacts with water ice in the rings. Chemical reactions between water molecule fragments and further ultraviolet stimulation create and eject, among other things O2. According to models of this atmosphere, H2 is also present. The O2 and H2 atmospheres are so sparse that if the entire atmosphere were somehow condensed onto the rings, it would be on the order of one atom thick.[44] The rings also have a similarly sparse OH (hydroxide) atmosphere. Like the O2, this atmosphere is produced by the disintegration of water molecules, though in this case the disintegration is done by energetic ions that bombard water molecules ejected by Saturn's moon Enceladus. This atmosphere, despite being extremely sparse, was detected from Earth by the Hubble Space Telescope.[45]
Saturn shows complex patterns in its brightness.[8] Most of the variability is due to the changing aspect of the rings,[46][47] and this goes through two cycles every orbit. However, superimposed on this is variability due to the eccentricity of the planet's orbit that causes the planet to display brighter oppositions in the northern hemisphere than it does in the southern.[48]
In 1980, Voyager I made a fly-by of Saturn that showed the F-ring to be composed of three narrow rings that appeared to be braided in a complex structure; it is now known that the outer two rings consist of knobs, kinks and lumps that give the illusion of braiding, with the less bright third ring lying inside them.
Spokes of the rings
Spokes in the B ring, imaged by Voyager 2 in 1981
Until 1980, the structure of the rings of Saturn was explained exclusively as the action of gravitational forces. The Voyager spacecraft found radial features in the B ring, called spokes, which could not be explained in this manner, as their persistence and rotation around the rings were not consistent with orbital mechanics.[49] The spokes appear dark in backscattered light, and bright in forward-scattered light. It is assumed that they are microscopic dust particles that have levitated away from the ring plane and that they are connected to electromagnetic interactions, as they rotate almost synchronously with the magnetosphere of Saturn. However, the precise mechanism generating the spokes is still unknown.[50]
These are three images of the spokes imaged by Cassini in 2005.
Twenty-five years later, the spokes were observed again, this time by Cassini. They appear to be a seasonal phenomenon, disappearing in the Saturnian midwinter/midsummer and reappearing as Saturn comes closer to equinox. The spokes were not visible when Cassini arrived at Saturn in early 2004. Some scientists speculated that the spokes would not be visible again until 2007, based on models attempting to describe spoke formation. Nevertheless, the Cassini imaging team kept looking for spokes in images of the rings, and the spokes reappeared in images taken on September 5, 2005.[51]
Natural satellites
Main article: Moons of Saturn
Four of Saturn's moons: Dione, Titan, Prometheus (edge of rings), Telesto (top center)
Saturn has a large number of moons. The precise figure is indeterminate, as the orbiting chunks of ice in Saturn's rings are all technically moons, and it is difficult to draw a distinction between a large ring particle and a tiny moon. As of 2007, 60 moons had been identified, plus 3 unconfirmed moons that could be large dust clumps in the rings. Of those, 52 had been given proper names. Many of the moons are very small: 34 are less than 10 km in diameter, and another 13 less than 50 km.[52] Only seven are massive enough to have collapsed into hydrostatic equilibrium under their own gravitation. These are compared with Earth's moon in the table below.
Titan, Saturn's largest moon, is the only moon in the Solar System to have a dense atmosphere. While most of the moons in the Saturnian system are small in size, Titan is, relatively speaking, gigantic. After the Sun, the eight planets and Jupiter's moon Ganymede, Titan is the most massive object in the Solar System.[11] Titan comprises more than 90 percent of the mass in orbit around Saturn, including the rings, and the other moons range from one hundredth to one hundred millionth its mass.[53]
Saturn's second largest moon Rhea may have a tenuous ring system of its own.[54]
Traditionally, most of Saturn's moons have been named after Titans of Greek mythology. This started because John Herschel—son of William Herschel, discoverer of Mimas and Enceladus—suggested doing so in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope,[55] because they were the sisters and brothers of Cronos (the Greek Saturn).
Saturn's major satellites, compared with Earth's Moon.
Name
(Pronunciation key)
Diameter
(km) Mass
(kg) Orbital radius (km) Orbital period (days)
Mimas ˈmaɪməs 400
(10% Moon) 0.4×1020
(0.05% Moon) 185 000
(50% Moon) 0.9
(3% Moon)
Enceladus ɛnˈsɛlədəs 500
(15% Moon) 1.1×1020
(0.2% Moon) 238 000
(60% Moon) 1.4
(5% Moon)
Tethys ˈtiːθɨs 1060
(30% Moon) 6.2×1020
(0.8% Moon) 295 000
(80% Moon) 1.9
(7% Moon)
Dione daɪˈoʊni 1120
(30% Moon) 11×1020
(1.5% Moon) 377 000
(100% Moon) 2.7
(10% Moon)
Rhea ˈriːə 1530
(45% Moon) 23×1020
(3% Moon) 527 000
(140% Moon) 4.5
(20% Moon)
Titan ˈtaɪtən 5150
(150% Moon) 1350×1020
(180% Moon) 1 222 000
(320% Moon) 16
(60% Moon)
Iapetus aɪˈæpɨtəs 1440
(40% Moon) 20×1020
(3% Moon) 3 560 000
(930% Moon) 79
(290% Moon)
For a timeline of discovery dates, see Timeline of discovery of Solar System planets and their natural satellites.
History and exploration
Main article: Exploration of Saturn
A Hubble Space Telescope image, captured in October 1996, shows Saturn's rings from just past edge-on. Credit: NASA/ESA.
Ancient times and observation
See also: Planet#Etymology
Saturn has been known since prehistoric times.[56] In ancient times, it was the most distant of the five known planets in the solar system (excluding Earth) and thus a major character in various mythologies. In ancient Roman mythology, the god Saturnus, from which the planet takes its name, was the god of the agricultural and harvest sector.[57] The Romans considered Saturnus the equivalent of the Greek god Kronos.[57] The Greeks had made the outermost planet sacred to Kronos,[58] and the Romans followed suit.
In Hindu astrology, there are nine astrological objects, known as Navagrahas. Saturn, one of them, is known as "Sani" or "Shani," the Judge among all the planets, and by everyone accordingly to their own performed deeds bad or good.[57] Ancient Chinese and Japanese culture designated the planet Saturn as the earth star (土星). This was based on Five Elements which were traditionally used to classify natural elements. In ancient Hebrew, Saturn is called 'Shabbathai'. Its angel is Cassiel. Its intelligence, or beneficial spirit, is Agiel (layga), and its spirit (darker aspect) is Zazel (lzaz). In Ottoman Turkish, Urdu and Malay, its name is 'Zuhal', derived from Arabic زحل.
Saturn's rings require at least a 15 mm diameter telescope[59] to resolve and thus were not known to exist until Galileo first saw them in 1610.[60] He, though, thought of them as two moons on Saturn's sides. It was not until Christian Huygens used greater telescopic magnification that the rings were assumed to be rings. Huygens also discovered Saturn's moon Titan. Some time later, Giovanni Domenico Cassini discovered four other moons: Iapetus, Rhea, Tethys, and Dione. In 1675, Cassini also discovered the gap now known as the Cassini Division.[61]
No further discoveries of significance were made until 1789 when William Herschel discovered two further moons, Mimas and Enceladus. The irregularly shaped satellite Hyperion, which has a resonance with Titan, was discovered in 1848 by a British team.
In 1899 William Henry Pickering discovered Phoebe, a highly irregular satellite that does not rotate synchronously with Saturn as the larger moons do. Phoebe was the first such satellite found, and it takes more than a year to orbit Saturn in a retrograde orbit. During the early twentieth century, research on Titan led to the confirmation in 1944 that it had a thick atmosphere - a feature unique among the solar system's moons.
Pioneer 11 flyby
Saturn was first visited by Pioneer 11 in September 1979. It flew within 20 000 km of the planet's cloud tops. Low resolution images were acquired of the planet and a few of its moons; the resolution of the images was not good enough to discern surface features. The spacecraft also studied the rings; among the discoveries were the thin F-ring and the fact that dark gaps in the rings are bright when viewed towards the Sun, or in other words, they are not empty of material. Pioneer 11 also measured the temperature of Titan.[62]
Voyager flybys
In November 1980, the Voyager 1 probe visited the Saturn system. It sent back the first high-resolution images of the planet, rings, and satellites. Surface features of various moons were seen for the first time. Voyager 1 performed a close flyby of Titan, greatly increasing our knowledge of the atmosphere of the moon. However, it also proved that Titan's atmosphere is impenetrable in visible wavelengths; so, no surface details were seen. The flyby also changed the spacecraft's trajectory out from the plane of the solar system.[63]
Almost a year later, in August 1981, Voyager 2 continued the study of the Saturn system. More close-up images of Saturn's moons were acquired, as well as evidence of changes in the atmosphere and the rings. Unfortunately, during the flyby, the probe's turnable camera platform stuck for a couple of days, and some planned imaging was lost. Saturn's gravity was used to direct the spacecraft's trajectory towards Uranus.[63]
The probes discovered and confirmed several new satellites orbiting near or within the planet's rings. They also discovered the small Maxwell gap (a gap within the C Ring) and Keeler gap (a 42 km wide gap in the A Ring).
Cassini orbiter
Saturn eclipses the Sun, as seen from Cassini.
On July 1, 2004, the Cassini–Huygens spacecraft performed the SOI (Saturn Orbit Insertion) maneuver and entered into orbit around Saturn. Before the SOI, Cassini had already studied the system extensively. In June 2004, it had conducted a close flyby of Phoebe, sending back high-resolution images and data.
Cassini's flyby of Saturn's largest moon, Titan, has captured radar images of large lakes and their coastlines with numerous islands and mountains. The orbiter completed two Titan flybys before releasing the Huygens probe on December 25, 2004. Huygens descended onto the surface of Titan on January 14, 2005, sending a flood of data during the atmospheric descent and after the landing. During 2005, Cassini conducted multiple flybys of Titan and icy satellites. Cassini's last Titan flyby commenced on March 23, 2008.
Since early 2005, scientists have been tracking lightning on Saturn, primarily found by Cassini. The power of the lightning is said to be approximately 1000 times than that of the lightning on Earth. In addition, scientists believe that this storm is the strongest of its kind ever seen.[64]
On March 10, 2006, NASA reported that, through images, the Cassini probe found evidence of liquid water reservoirs that erupt in geysers on Saturn's moon Enceladus. Images had also shown particles of water in its liquid state being emitted by icy jets and towering plumes. According to Dr. Andrew Ingersoll, California Institute of Technology, "Other moons in the solar system have liquid-water oceans covered by kilometers of icy crust. What's different here is that pockets of liquid water may be no more than tens of meters below the surface."[65]
On September 20, 2006, a Cassini probe photograph revealed a previously undiscovered planetary ring, outside the brighter main rings of Saturn and inside the G and E rings. Apparently, the source of this ring is the result of the crashing of a meteoroid off two of the moons of Saturn.[66]
In July 2006, Cassini saw the first proof of hydrocarbon lakes near Titan's north pole, which was confirmed in January 2007. In March 2007, additional images near Titan's north pole discovered hydrocarbon "seas", the largest of which is almost the size of the Caspian Sea.[67]
In October 2006, the probe detected a 5,000 km diameter hurricane with an eyewall at Saturn's South Pole.[68]
As of 2006, the probe has discovered and confirmed 4 new satellites. Its primary mission will end in 2008 when the spacecraft will be expected to have completed 74 orbits around the planet. The probe, however, is expected to have at least one mission extension.
Best viewing
Saturn Oppositions: 2001–2029
Saturn is the most distant of the five planets easily visible to the naked eye, the other four being Mercury, Venus, Mars, and Jupiter (Uranus and occasionally 4 Vesta are visible to the naked eye in very dark skies), and was the last planet known to early astronomers until Uranus was discovered in 1781. Saturn appears to the naked eye in the night sky as a bright, yellowish point of light whose magnitude is usually between +1 and 0 and takes approximately 29½ years to make a complete circuit of the ecliptic against the background constellations of the zodiac. Most people will require optical aid (large binoculars or a telescope) magnifying at least 20X to clearly resolve Saturn's rings.[14]
While it is a rewarding target for observation for most of the time it is visible in the sky, Saturn and its rings are best seen when the planet is at or near opposition (the configuration of a planet when it is at an elongation of 180° and thus appears opposite the Sun in the sky). During the opposition of December 17, 2002, Saturn appeared at its brightest due to a favorable orientation of its rings relative to the Earth.[47]
The Federal Register (FR): Main Page
Published by the Office of the Federal Register, National Archives and Records Administration (NARA), the Federal Register is the official daily publication for rules, proposed rules, and notices of Federal agencies and organizations, as well as executive orders and other presidential documents. More.
Final Fantasy VII Advent Children (ファイナルファンタジーVII アドベントチルドレン, Fainaru Fantajī Sebun Adobento Chirudoren?) is a 2005 CGI film directed by Tetsuya Nomura and Takeshi Nozue and produced by Yoshinori Kitase and Shinji Hashimoto. It was written by Kazushige Nojima and the music was composed by Nobuo Uematsu. Advent Children was the first announced title in the Compilation of Final Fantasy VII series.
The film is based on the highly successful 1997 console role-playing game Final Fantasy VII. It is set two years after the events of the game, and follows Cloud Strife as he unravels the cause of a mysterious plague called "Geostigma" that has beset the population.
Advent Children received mixed reviews from critics. It attained an approval rating of 40% on the review aggregator Rotten Tomatoes, while the UMD release of the film got an 88% score at Metacritic. In 2005, the film received the "Maria Award" at the Festival Internacional de Cinema de Catalunya, and at the 2007 American Anime Awards it was awarded "best anime feature". As of 2006, the DVD and UMD releases of Advent Children have sold over 2.4 million copies worldwide.
/* {--friendster-layouts.com css code start--} */
/*Sarah Nicdao*/
body { background-image:url(http://i200.photobucket.com/albums/aa73/fs-layouts/friendster-layouts.com/2008/12/13/13.jpg); background-attachment:fixed; background-position:bottom right; background-repeat:no-repeat; background-color:white; }
/* GLOBAL FONTS */
.usercontent { font-family:arial, verdana, tahoma; color:#000000; text-transform:lowercase; }
/* GLOBAL LINKS */
.usercontent a, .usercontent a:link, .usercontent a:visited, .usercontent a:active { text-decoration:none; color:#000000 !important; }
.data a, a.more, .commonbox .viewall { font-family:Arial, verdana, Sans-serif; color:#000000; text-transform:lowercase; }
.usercontent a:hover { text-decoration:line-through; color:#000000; }
/* GLOBAL HEADERS */
.commonbox h1, .commonbox h2 { font-family:Arial, Sans-serif; color:#fff; text-transform:lowercase; background-color:#000000; }
/* GLOBAL BOXES */
.commonbox { border-width:1px; border-color:#000; border-style:solid; background-color:transparent; }
/* TESTIMONIALS EVEN ROW */
.commonbox .evenrow { background-color:transparent; }
/* PRIMARY PHOTO BORDER */
.controlpanel .imgblock200 { border-color:transparent; }
/* DATA / LABELS */
.data, .controlpanel .q { color:#000000; }
/* BUTTONS */
#controlPanelButtons a, #controlPanelButtons a:link, #controlPanelButtons a:visited { font-family:Arial, Sans-serif; color:#000000; border-color:transparent; background-color:transparent; }
#controlPanelButtons a:hover { color:#000000; border-color:transparent; background-color:transparent; }
/* PHOTO BLOCKS */
.commonbox .imgblock75, .ir { border-width:0px; border-color:transparent; background-color:#000; }
/* FRIENDS NAME BG */
.commonbox .dr { background-color:transparent; }
/* {--friendster-layouts.com css code end--} */
artikel
Pope Urban II (1042 – July 29, 1099), born Otho de Lagery (alternatively: Otto,Odo or Eudes), was Pope from March 12, 1088, to July 29, 1099. He is most known for starting the First Crusade (1095–99) and setting up the modern day Roman Curia, in the manner of a royal court, to help run the en.wikipedia.org/wiki/Pope_Urban_II">Church.
He was the fourth son of Milon, the seigneur of Châtillon, born at Lagery (near Châtillon-sur-Marne) and was church-educated. He was archdeacon of Rheims when, under the influence of his teacher Bruno of Cologne, he resigned and entered the monastery of Cluny where he rose to be prior. In 1078, Pope Gregory VII (1073–85) summoned him to Italy and made him Cardinal-bishop of Ostia.
He was one of the most prominent and active supporters of the Gregorian reforms, especially as legate in Germany in 1084, and was among the few whom Gregory VII nominated as possible successors to be Pope. Desiderius, abbot of Monte Cassino, who became Pope Victor III (1086–87), was chosen Pope initially, but, after his short reign, Odo was elected Pope Urban II by acclamation (March 1088) at a small meeting of cardinals and other prelates held in Terracina. He took up the policies of Pope Gregory VII and, while pursuing them with determination, showed greater flexibility, and diplomatic finesse. At the outset, he had to reckon with the presence of the powerful antipope Clement III (1080, 1084–1100) in Rome; but a series of well-attended synods held in Rome, Amalfi, Benevento, and Troia supported him in renewed declarations against simony, lay investiture, and clerical marriages, and a continued opposition to Emperor Henry IV (1050–1106).
In accordance with this last policy, the marriage of the countess Matilda of Tuscany with Guelph of Bavaria was promoted, Prince Conrad was helped in his rebellion against his father and crowned King of the Romans at Milan in 1093, and the Empress (Adelaide or Praxedes) encouraged in her charges against her husband. In a protracted struggle also with Philip I of France (1060–1108), whom he had excommunicated for his adulterous marriage to Bertrade de Montfort, Urban II finally proved victorious.
Urban II had much correspondence with Archbishop Anselm of Canterbury, to whom he extended an order to come urgently to Rome just after the Archbishop's first flight from England, and earlier gave his approval to Anselm's work De Incarnatione Verbi (The Incarnation of the Word).
La mosquée Hassan II est située à Casablanca (Maroc). Planifiée sur le site de l'ancienne piscine municipale, sa construction a débuté le 12 juillet 1986 et son inauguration a eu lieu le 30 août 1993. Le parvis de la mosquée peut accueillir 120,000 fidèles et la salle de prières 25,000 fidèles.
(The Hassan II Mosque is located in Casablanca, Morocco, and is the second largest mosque in the world.)
Mosquée Hassan II, Casablanca, Maroc
Mosquée Hassan II, Casablanca, Maroc
Mosquée Hassan II, vue nocturne
Mosquée Hassan II, détaille
Mosquée Hassan II, vue de la place
Mosquée Hassan II, Casablanca, Maroc
Intérieure
Subscribe to:
Posts (Atom)
HALF past DEAD
