【thatll】iBT备考日志

thatllUID: 2538832

                       星  球  大  战
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【星球大战之水星篇】

Mercury is the closest planet to the Sun and the eighth largest. Mercury is slightly smaller in diameter than the moons Ganymede and Titan but more than twice as massive.
        orbit:    57,910,000 km (0.38 AU) from Sun        
        diameter: 4,880 km        
        mass:     3.30e23 kg
In Roman mythology Mercury is the god of commerce, travel and thievery, the Roman counterpart of the Greek god Hermes, the messenger of the Gods. The planet probably received this name because it moves so quickly across the sky.
Mercury has been known since at least the time of the Sumerians (3rd millennium BC). It was sometimes given separate names for its apparitions as a morning star and as an evening star. Greek astronomers knew, however, that the two names referred to the same body. Heraclitus even believed that Mercury and Venus orbit the Sun, not the Earth.
Since it is closer to the Sun than the Earth, the illumination of Mercury's disk varies when viewed with a telescope from our perspective. Galileo's telescope was too small to see Mercury's phases but he did see the phases of Venus. Mercury has been now been visited by two spacecraft, Mariner 10 and MESSENGER. Marriner 10 flew by three times in 1974 and 1975. Only 45% of the surface was mapped (and, unfortunately, it is too close to the Sun to be safely imaged by HST). MESSENGER was launched by NASA in 2004 and will orbit Mercury starting in 2011 after several flybys. Its first flyby in Jan 2008 provided new high quality images of some of the terrain not seen by Marriner 10.
Mercury's orbit is highly eccentric; at perihelion it is only 46 million km from the Sun but at aphelion it is 70 million. The position of the perihelion precesses around the Sun at a very slow rate. 19th century astronomers made very careful observations of Mercury's orbital parameters but could not adequately explain them using Newtonian mechanics. The tiny differences between the observed and predicted values were a minor but nagging problem for many decades. It was thought that another planet (sometimes called Vulcan) slightly closer to the Sun than Mercury might account for the discrepancy. But despite much effort, no such planet was found. The real answer turned out to be much more dramatic: Einstein's General Theory of Relativity! Its correct prediction of the motions of Mercury was an important factor in the early acceptance of the theory.
Until 1962 it was thought that Mercury's "day" was the same length as its "year" so as to keep that same face to the Sun much as the Moon does to the Earth. But this was shown to be false in 1965 by doppler radar observations. It is now known that Mercury rotates three times in two of its years. Mercury is the only body in the solar system known to have an orbital/rotational resonance with a ratio other than 1:1 (though many have no resonances at all).
This fact and the high eccentricity of Mercury's orbit would produce very strange effects for an observer on Mercury's surface. At some longitudes the observer would see the Sun rise and then gradually increase in apparent size as it slowly moved toward the zenith. At that point the Sun would stop, briefly reverse course, and stop again before resuming its path toward the horizon and decreasing in apparent size. All the while the stars would be moving three times faster across the sky. Observers at other points on Mercury's surface would see different but equally bizarre motions.
Temperature variations on Mercury are the most extreme in the solar system ranging from 90 K to 700 K. The temperature on Venus is slightly hotter but very stable.

Mercury craters

Mercury is in many ways similar to the Moon: its surface is heavily cratered and very old; it has no plate tectonics. On the other hand, Mercury is much denser than the Moon (5.43 gm/cm3 vs 3.34). Mercury is the second densest major body in the solar system, after Earth. Actually Earth's density is due in part to gravitational compression; if not for this, Mercury would be denser than Earth. This indicates that Mercury's dense iron core is relatively larger than Earth's, probably comprising the majority of the planet. Mercury therefore has only a relatively thin silicate mantle and crust.
Mercury's interior is dominated by a large iron core whose radius is 1800 to 1900 km. The silicate outer shell (analogous to Earth's mantle and crust) is only 500 to 600 km thick. At least some of the core is probably molten.
Mercury actually has a very thin atmosphere consisting of atoms blasted off its surface by the solar wind. Because Mercury is so hot, these atoms quickly escape into space. Thus in contrast to the Earth and Venus whose atmospheres are stable, Mercury's atmosphere is constantly being replenish ed.

Southwest Mercury

The surface of Mercury exhibits enormous escarpments, some up to hundreds of kilometers in length and as much as three kilometers high. Some cut thru the rings of craters and other features in such a way as to indicate that they were formed by compression. It is estimated that the surface area of Mercury shrank by about 0.1% (or a decrease of about 1 km in the planet's ra dius).

Caloris Basin

One of the largest features on Mercury's surface is the Caloris Basin (right); it is about 1300 km in diameter. It is thought to be similar to the large basins (maria) on theMoon. Like the lunar basins, it was probably caused by a very large impact early in the history of the solar  system.

Weird terrain opposite Caloris Basin

That impact was probably also responsible for the odd terrain on the exact opposite side of the planet (left).
In addition to the heavily cratered terrain, Mercury also has regions of relatively smooth plains. Some may be the result of ancient volcanic activity but some may be the result of the deposition of ejecta from cratering impacts.
A reanalysis of the Mariner data provides some preliminary evidence of recent volcanism on Mercury. But more data will be needed for confirmation.


Amazingly, radar observations of Mercury's north pole (a region not mapped by Mariner 10) show evidence of water ice in the protected shadows of some craters.
Mercury has a small magnetic field whose strength is about 1% of Earth's.
Mercury has no known satellites.
Mercury is often visible with binoculars or even the unaided eye, but it is always very near the Sun and difficult to see in the twilight sky. There are several Web sites that show the current position of Mercury (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

thatllUID: 2538832

【星球大战之金星】

Venus is the second planet from the Sun and the sixth largest. Venus' orbit is the most nearly circular of that of any planet, with an eccentricity of less than 1%.

orbit: 108,200,000 km (0.72 AU) from Sun

diameter: 12,103.6 km

mass: 4.869e24 kg

Venus (Greek: Aphrodite; Babylonian: Ishtar) is the goddess of love and beauty. The planet is so named probably because it is the brightest of the planets known to the ancients. (With a few exceptions, the surface features on Venus are named for female figures.)

Venus has been known since prehistoric times. It is the brightest object in the sky except for the Sun and the Moon. Like Mercury, it was popularly thought to be two separate bodies: Eosphorus as the morning star and Hesperus as the evening star, but the Greek astronomers knew better. (Venus's apparition as the morning star is also sometimes called Lucifer.)

Since Venus is an inferior planet, it shows phases when viewed with a telescope from the perspective of Earth. Galileo's observation of this phenomenon was important evidence in favor of Copernicus'sheliocentric theory of the solar system.

The first spacecraft to visit Venus was Mariner 2 in 1962. It was subsequently visited by many others (more than 20 in all so far), including Pioneer Venus and the SovietVenera 7 the first spacecraft to land on another planet, and Venera 9 which returned the first photographs of the surface. The first orbiter, the US spacecraft Magellan  produced detailed maps of Venus' surface using radar. ESA's Venus Express is now in orbit with a large variety of instruments.

Venus' rotation is somewhat unusual in that it is both very slow (243 Earth days per Venus day, slightly longer than Venus' year) and retrograde. In addition, the periods of Venus' rotation and of its orbit are synchronized such that it always presents the same face toward Earth when the two planets are at their closest approach. Whether this is a resonance effect or merely a coincidence is not known.

Venus is sometimes regarded as Earth's
sister planet. In some ways they are very similar:

·Venus is only slightly smaller than Earth (95% of Earth's diameter, 80% of Earth's mass).

·Both have few craters indicating relatively young surfaces.

·Their densities and chemical compositions are similar.

Because of these similarities, it was thought that below its dense clouds Venus might be very Earthlike and might even have life. But, unfortunately, more detailed study of Venus reveals that in many important ways it is radically different from Earth. It may be the least hospitable place for life in the solar system.

The pressure of Venus' atmosphere at the surface is 90 atmospheres (about the same as the pressure at a depth of 1 km in Earth's oceans). It is composed mostly of carbon dioxide. There are several layers of clouds many kilometers thick composed of sulfuric acid. These clouds completely obscure our view of the surface. This dense atmosphere produces a run-awaygreenhouse effect that raises Venus' surface temperature by about 400 degrees to over 740 K (hot enough to melt lead). Venus' surface is actually hotter than Mercury's despite being nearly twice as far from the Sun.

There are strong (350 kph) winds at the cloud tops but winds at the surface are very slow, no more than a few kilometers per hour.

Venus probably once had large amounts of water like Earth but it all boiled away. Venus is now quite dry. Earth would have suffered the same fate had it been just a little closer to the Sun. We may learn a lot about Earth by learning why the basically similar Venus turned out so differently.

Most of Venus' surface consists of gently rolling plains with little relief. There are also several broad depressions: Atalanta Planitia, Guinevere Planitia, Lavinia Planitia. There two large highland areas: Ishtar Terra in the northern hemisphere (about the size of Australia) and Aphrodite Terra along the equator (about the size of South America). The interior of Ishtar consists mainly of a high plateau, Lakshmi Planum, which is surrounded by the highest mountains on Venus including the enormous Maxwell Montes.

Data from Magellan's imaging radar shows that much of the surface of Venus is covered by lava flows. There are several large shield volcanoes (similar to Hawaii or Olympus Mons) such as Sif Mons. Recently announced findings indicate that Venus is still volcanically active, but only in a few hot spots; for the most part it has been geologically rather quiet for the past few hundred million years.

There are no small craters on Venus. It seems that small meteoroids burn up in Venus' dense atmosphere before reaching the surface. Craters on Venus seem to come in bunches indicating that large meteoroids that do reach the surface usually break up in the atmosphere.

The oldest terrains on Venus seem to be about 800 million years old. Extensive volcanism at that time wiped out the earlier surface including any large craters from early in Venus' history.

Magellan's images show a wide variety of interesting and unique features including pancake volcanoes (left) which seem to be eruptions of very thick lava and coronae(right) which seem to be collapsed domes over large magma chambers.

The interior of Venus is probably very similar to that of Earth: an iron core about 3000 km in radius, a molten rocky mantle comprising the majority of the planet. Recent results from the Magellan gravity data indicate that Venus' crust is stronger and thicker than had previously been assumed. Like Earth, convection in the mantle produces stress on the surface. However on Venus the stress is relieved in many relatively small regions instead of being concentrated at the boundaries of large plates as is the case on Earth.

Venus has no magnetic field, perhaps because of its slow rotation.

Venus has no satellites, and thereby hangs a tale.

Venus is usually visible with the unaided eye. Sometimes (inaccurately) referred to as the "morning star" or the "evening star", it is by far the brightest "star" in the sky. There are several Web sites that show the current position of Venus (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

On June 8 2004, Venus passed directly between the Earth and the Sun, appearing as a large black dot travelling across the Sun's disk. This event is known as a "transit of Venus" and is very rare: the last one was in 1882, the next one is in 2012 but after than you'll have to wait until 2117. While no longer of great scientific importance as it was in the past, this event was the impetus for a major journey for many amateur astronomers.

thatllUID: 2538832

【星球大战之地球】

Earth is the third planet from the Sun and the fifth largest:
        orbit:    149,600,000 km (1.00 AU) from Sun        
        diameter: 12,756.3 km      
        mass:     5.972e24 kg

Earth is the only planet whose English name does not derive from Greek/Roman mythology. The name derives from Old English and
Germanic. There are, of course, hundreds of other names for the planet in other languages. In Roman Mythology, the goddess of the Earth was Tellus - the fertile soil (Greek: Gaia, terra mater - Mother Earth).

thatllUID: 2538832

【星球大战之火星】

Mars is the fourth planet from the Sun and the seventh largest:
      orbit: 227,940,000 km (1.52 AU) from Sun      
      diameter: 6,794 km      
      mass: 6.4219e23 kg

Mars (Greek: Ares) is the god of War. The planet probably got this name due to its red color; Mars is sometimes referred to as the Red Planet. (An interesting side note: the Roman god Mars was a god of agriculture before becoming associated with the Greek Ares; those in favor of colonizing and terraforming Mars may prefer this symbolism.) The name of the month March derives from Mars.

Mars has been known since prehistoric times. Of course, it has been extensively studied with ground-based observatories. But even very large telescopes find Mars a difficult target, it's just too small. It is still a favorite of science fiction writers as the most favorable place in the Solar System (other than Earth!) for human habitation. But the famous "canals" "seen" by Lowell and others were, unfortunately, just as imaginary as Barsoomian princesses.

The first spacecraft to visit Mars was Mariner 4 in 1965. Several others followed including Mars 2, the first spacecraft to land on Mars and the two Viking landers in 1976. Ending a long 20 year hiatus, Mars Pathfinder landed successfully on Mars on 1997 July 4. In 2004 the Mars Expedition Rovers "Spirit" and "Opportunity" landed on Mars sending back geologic data and many pictures; they are still operating after more than three years on Mars. In 2008, Phoenix landed in the northern plains to search for water. Three Mars orbiters (Mars Reconnaissance Orbiter, Mars Odyssey, and Mars Express) are also currently in operation.

Mars' orbit is significantly elliptical. One result of this is a temperature variation of about 30 C at the subsolar point between aphelion and perihelion. This has a major influence on Mars' climate. While the average temperature on Mars is about 218 K (-55 C, -67 F), Martian surface temperatures range widely from as little as 140 K (-133 C, -207 F) at the winter pole to almost 300 K (27 C, 80 F) on the day side during summer.

Though Mars is much smaller than Earth, its surface area is about the same as the land surface area of Earth.

Mars has some of the most highly varied and interesting terrain of any of the terrestrial planets, some of it quite spectacular:

·Olympus Mons: the largest mountain in the Solar System rising 24 km (78,000 ft.) above the surrounding plain. Its base is more than 500 km in diameter and is rimmed by a cliff 6 km (20,000 ft) high.

·Tharsis: a huge bulge on the Martian surface that is about 4000 km across and 10 km high.

·Valles Marineris: a system of canyons 4000 km long and from 2 to 7 km deep (top of page);

·Hellas Planitia: an impact crater in the southern hemisphere over 6 km deep and 2000 km in diameter.

Much of the Martian surface is very old and cratered, but there are also much younger rift valleys, ridges, hills and plains. (None of this is visible in any detail with a telescope, even the Hubble Space Telescope; all this information comes from the spacecraft that we've sent to Mars.)

The southern hemisphere of Mars is predominantly ancient cratered highlands somewhat similar to the Moon. In contrast, most of the northern hemisphere consists of plains which are much younger, lower in elevation and have a much more complex history. An abrupt elevation change of several kilometers seems to occur at the boundary. The reasons for this global dichotomy and abrupt boundary are unknown (some speculate that they are due to a very large impact shortly after Mars' accretion). Mars Global Surveyor has produced a nice 3D mapof Mars that clearly shows these features.

The interior of Mars is known only by inference from data about the surface and the bulk statistics of the planet. The most likely scenario is a dense core about 1700 km in radius, a molten rocky mantle somewhat denser than the Earth's and a thin crust. Data from Mars Global Surveyor indicates that Mars' crust is about 80 km thick in the southern hemisphere but only about 35 km thick in the north. Mars' relatively low density compared to the other terrestrial planets indicates that its core probably contains a relatively large fraction of sulfur in addition to iron (iron and iron sulfide).

Like Mercury and the Moon, Mars appears to lack active plate tectonics at present; there is no evidence of recent horizontal motion of the surface such as the folded mountains so common on Earth. With no lateral plate motion, hot-spots under the crust stay in a fixed position relative to the surface. This, along with the lower surface gravity, may account for the Tharis bulge and its enormous volcanoes. There is no evidence of current volcanic activity. However, data from Mars Global Surveyor indicates that Mars very likely did have tectonic activity sometime in the past.

There is very clear evidence of erosion in many places on Mars including large floods and small river systems. At some time in the past there was clearly some sort of fluid on the surface. Liquid water is the obvious fluid but other possibilities exist. There may have been large lakes or even oceans; the evidence for which was strenghtened by some very nice images of layered terrain taken by Mars Global Surveyor and the mineralology results from MER Opportunity. Most of these point to wet episodes that occurred only briefly and very long ago; the age of the erosion channels is estimated at about nearly 4 billion years. However, images from Mars Express released in early 2005 show what appears to be a frozen sea that was liquid very recently (maybe 5 million years ago). Confirmation of this interpretation would be a very big deal indeed! (Valles Marineris was NOT created by running water. It was formed by the stretching and cracking of the crust associated with the creation of the Tharsis bulge.)

Early in its history, Mars was much more like Earth. As with Earth almost all of its carbon dioxide was used up to form carbonate rocks. But lacking the Earth's plate tectonics, Mars is unable to recycle any of this carbon dioxide back into its atmosphere and so cannot sustain a significant greenhouse effect. The surface of Mars is therefore much colder than the Earth would be at that distance from the Sun.

Mars has a very thin atmosphere composed mostly of the tiny amount of remaining carbon dioxide (95.3%) plus nitrogen (2.7%), argon (1.6%) and traces of oxygen (0.15%) and water (0.03%). The average pressure on the surface of Mars is only about 7 millibars (less than 1% of Earth's), but it varies greatly with altitude from almost 9 millibars in the deepest basins to about 1 millibar at the top of Olympus Mons. But it is thick enough to support very strong winds and vast dust storms that on occasion engulf the entire planet for months. Mars' thin atmosphere produces a greenhouse effect but it is only enough to raise the surface temperature by 5 degrees (K); much less than what we see on Venus and Earth.

Early telescopic observations revealed that Mars has permanent ice caps at both poles; they're visible even with a small telescope. We now know that they're composed of water ice and solid carbon dioxide ("dry ice"). The ice caps exhibit a layered structure with alternating layers of ice with varying concentrations of dark dust. In the northern summer the carbon dioxide completely sublimes, leaving a residual layer of water ice. ESA's Mars Express has shown that a similar layer of water ice exists below the southern cap as well. The mechanism responsible for the layering is unknown but may be due to climatic changes related to long-term changes in the inclination of Mars' equator to the plane of its orbit. There may also be water ice hidden below the surface at lower latitudes. The seasonal changes in the extent of the polar caps changes the global atmospheric pressure by about 25% (as measured at the Viking lander sites).

Recent observations with the Hubble Space Telescope have revealed that the conditions during the Viking missions may not have been typical. Mars' atmosphere now seems to be both colder and dryer than measured by the Viking landers (more details from STScI).

The Viking landers performed experiments to determine the existence of life on Mars. The results were somewhat ambiguous but most scientists now believe that they show no evidence for life on Mars (there is still some controversy, however). Optimists point out that only two tiny samples were measured and not from the most favorable locations. More experiments will be done by future missions to Mars.

A small number of meteorites (the SNC meteorites) are believed to have originated on Mars.

On 1996 Aug 6, David McKay et al announced what they thought might be evidence of ancient Martian microorganisms in the meteorite ALH84001. Though there is still some controversy, the majority of the scientific community has not accepted this conclusion. If there is or was life on Mars, we still haven't found it.

Large, but not global, weak magnetic fields exist in various regions of Mars. This unexpected finding was made by Mars Global Surveyor just days after it entered Mars orbit. They are probably remnants of an earlier global field that has since disappeared. This may have important implications for the structure of Mars' interior and for the past history of its atmosphere and hence for the possibility of ancient life.

When it is in the nighttime sky, Mars is easily visible with the unaided eye. Mars is a difficult but rewarding target for an amateur telescope though only for the three or four months each martian year when it is closest to Earth. Its apparent size and brightness varies greatly according to its relative position to the Earth. There are several Web sites that show the current position of Mars (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

ppbboUID: 2537437

佩服下 同战26 祈福

thatllUID: 2538832

【星球大战之木星】

Jupiter is the fifth planet from the Sun and by far the largest. Jupiter is more than twice as massive as all the other planets combined (the mass of Jupiter is 318 times that of Earth).
        orbit:    778,330,000 km (5.20 AU) from Sun        
        diameter: 142,984 km (equatorial)      
        mass:     1.900e27 kg

Jupiter (a.k.a. Jove; Greek Zeus) was the King of the Gods, the ruler of Olympus and the patron of the Roman state. Zeus was the son of Cronus (Saturn).

Jupiter is the fourth brightest object in the sky (after the Sun, the Moon and Venus). It has been known since prehistoric times as a bright "wandering star". But in 1610 when Galileo first pointed a telescope at the sky he discovered Jupiter's four large moons Io, Europa, Ganymede and Callisto (now known as the Galilean moons) and recorded their motions back and forth around Jupiter. This was the first discovery of a center of motion not apparently centered on the Earth. It was a major point in favor of Copernicus's heliocentric theory of the motions of the planets (along with other new evidence from his telescope: the phases of Venus and the mountains on the Moon). Galileo's outspoken support of the Copernican theory got him in trouble with the Inquisition. Today anyone can repeat Galileo's observations (without fear of retribution :-) using binoculars or an inexpensive telescope.

Jupiter was first visited by Pioneer 10 in 1973 and later by Pioneer 11, Voyager 1, Voyager 2 and Ulysses. The spacecraft Galileo orbited Jupiter for eight years. It is still regularly observed by the Hubble Space Telescope.

The gas planets do not have solid surfaces, their gaseous material simply gets denser with depth (the radii and diameters quoted for the planets are for levels corresponding to a pressure of 1 atmosphere). What we see when looking at these planets is the tops of clouds high in their atmospheres (slightly above the 1 atmosphere level).

Jupiter is about 90% hydrogen and 10% helium (by numbers of atoms, 75/25% by mass) with traces of methane, water, ammonia and "rock". This is very close to the composition of the primordial Solar Nebula from which the entire solar system was formed. Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium.

Our knowledge of the interior of Jupiter (and the other gas planets) is highly indirect and likely to remain so for some time. (The data from Galileo's atmospheric probe goes down only about 150 km below the cloud tops.)

Jupiter probably has a core of rocky material amounting to something like 10 to 15 Earth-masses.

Above the core lies the main bulk of the planet in the form of liquid metallic hydrogen. This exotic form of the most common of elements is possible only at pressures exceeding 4 million bars, as is the case in the interior of Jupiter (and Saturn). Liquid metallic hydrogen consists of ionized protons and electrons (like the interior of the Sun but at a far lower temperature). At the temperature and pressure of Jupiter's interior hydrogen is a liquid, not a gas. It is an electrical conductor and the source of Jupiter's magnetic field. This layer probably also contains some helium and traces of various "ices".

The outermost layer is composed primarily of ordinary molecular hydrogen and helium which is liquid in the interior and gaseous further out. The atmosphere we see is just the very top of this deep layer. Water, carbon dioxide, methane and other simple molecules are also present in tiny amounts.

Recent experiments have shown that hydrogen does not change phase suddenly. Therefore the interiors of the jovian planets probably have indistinct boundaries between their various interior layers.

Three distinct layers of clouds are believed to exist consisting of ammonia ice, ammonium hydrosulfide and a mixture of ice and water. However, the preliminary results from the Galileo probe show only faint indications of clouds (one instrument seems to have detected the topmost layer while another may have seen the second). But the probe's entry point (left) was unusual -- Earth-based telescopic observations and more recent observations by the Galileo orbiter suggest that the probe entry site may well have been one of the warmest and least cloudy areas on Jupiter at that time.

Data from the Galileo atmospheric probe also indicate that there is much less water than expected. The expectation was that Jupiter's atmosphere would contain about twice the amount of oxygen (combined with the abundant hydrogen to make water) as the Sun. But it now appears that the actual concentration much less than the Sun's. Also surprising was the high temperature and density of the uppermost parts of the atmosphere.

Jupiter and the other gas planets have high velocity winds which are confined in wide bands of latitude. The winds blow in opposite directions in adjacent bands. Slight chemical and temperature differences between these bands are responsible for the colored bands that dominate the planet's appearance. The light colored bands are called zones; the dark ones belts. The bands have been known for some time on Jupiter, but the complex vortices in the boundary regions between the bands were first seen by Voyager. The data from the Galileo probe indicate that the winds are even faster than expected (more than 400 mph) and extend down into as far as the probe was able to observe; they may extend down thousands of kilometers into the interior. Jupiter's atmosphere was also found to be quite turbulent. This indicates that Jupiter's winds are driven in large part by its internal heat rather than from solar input as on Earth.

The vivid colors seen in Jupiter's clouds are probably the result of subtle chemical reactions of the trace elements in Jupiter's atmosphere, perhaps involving sulfur whose compounds take on a wide variety of colors, but the details are unknown.

The colors correlate with the cloud's altitude: blue lowest, followed by browns and whites, with reds highest. Sometimes we see the lower layers through holes in the upper ones.

The Great Red Spot (GRS) has been seen by Earthly observers for more than 300 years (its discovery is usually attributed to Cassini, or Robert Hooke in the 17th century). The GRS is an oval about 12,000 by 25,000 km, big enough to hold two Earths. Other smaller but similar spots have been known for decades. Infrared observations and the direction of its rotation indicate that the GRS is a high-pressure region whose cloud tops are significantly higher and colder than the surrounding regions. Similar structures have been seen on Saturn and Neptune. It is not known how such structures can persist for so long.

Jupiter radiates more energy into space than it receives from the Sun. The interior of Jupiter is hot: the core is probably about 20,000 K. The heat is generated by the Kelvin-Helmholtz mechanism, the slowgravitational compression of the planet. (Jupiter does NOT produce energy by nuclear fusion as in the Sun; it is much too small and hence its interior is too cool to ignite nuclear reactions.) This interior heat probably causesconvection deep within Jupiter's liquid layers and is probably responsible for the complex motions we see in the cloud tops. Saturn and Neptune are similar to Jupiter in this respect, but oddly, Uranus is not.
Jupiter is just about as large in diameter as a gas planet can be. If more material were to be added, it would be compressed by gravity such that the overall radius would increase only slightly. A star can be larger only because of its internal (nuclear) heat source. (But Jupiter would have to be at least 80 times more massive to become a star.)

Jupiter has a huge magnetic field, much stronger than Earth's. Its magnetosphere extends more than 650 million km (past the orbit of Saturn!). (Note that Jupiter's magnetosphere is far from spherical -- it extends "only" a few million kilometers in the direction toward the Sun.) Jupiter's moons therefore lie within its magnetosphere, a fact which may partially explain some of the activity on Io. Unfortunately for future space travelers and of real concern to the designers of the Voyager and Galileo spacecraft, the environment near Jupiter contains high levels of energetic particles trapped by Jupiter's magnetic field. This "radiation" is similar to, but much more intense than, that found within Earth's Van Allen belts. It would be immediately fatal to an unprotected human being.


The Galileo atmospheric probe discovered a new intense radiation belt between Jupiter's ring and the uppermost atmospheric layers. This new belt is approximately 10 times as strong as Earth's Van Allen radiation belts. Surprisingly, this new belt was also found to contain high energy helium ions of unknown origin.

Jupiter has rings like Saturn's, but much fainter and smaller (right). They were totally unexpected and were only discovered when two of the Voyager 1 scientists insisted that after traveling 1 billion km it was at least worth a quick look to see if any rings might be present. Everyone else thought that the chance of finding anything was nil, but there they were. It was a major coup. They have since been imaged in the infra-red from ground-based observatories and by Galileo.

Unlike Saturn's, Jupiter's rings are dark (albedo about .05). They're probably composed of very small grains of rocky material. Unlike Saturn's rings, they seem to contain no ice.

Particles in Jupiter's rings probably don't stay there for long (due to atmospheric and magnetic drag). The Galileo spacecraft found clear evidence that the rings are continuously resupplied by dust formed by micrometeor impacts on the four inner moons, which are very energetic because of Jupiter's large gravitational field. The inner halo ring is broadened by interactions with Jupiter's magnetic field.

In July 1994, Comet Shoemaker-Levy 9 collided with Jupiter with spectacular results (left). The effects were clearly visible even with amateur telescopes. The debris from the collision was visible for nearly a year afterward with HST.

When it is in the nighttime sky, Jupiter is often the brightest "star" in the sky (it is second only to Venus, which is seldom visible in a dark sky). The four Galilean moons are easily visible with binoculars; a few bands and the Great Red Spot can be seen with a small astronomical telescope. There are several Web sites that show the current position of Jupiter (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

thatllUID: 2538832

【星球大战之土星】


Saturn is the sixth planet from the Sun and the second largest:
        orbit:    1,429,400,000 km (9.54 AU) from Sun        
        diameter: 120,536 km (equatorial)        
        mass:     5.68e26 kg

In Roman mythology, Saturn is the god of agriculture. The associated Greek god, Cronus, was the son of Uranus and Gaia and the father of Zeus (Jupiter). Saturn is the root of the English word "Saturday" (see Appendix 5).

Saturn has been known since prehistoric times. Galileo was the first to observe it with a telescope in 1610; he noted its odd appearance but was confused by it. Early observations of Saturn were complicated by the fact that the Earth passes through the plane of Saturn's rings every few years as Saturn moves in its orbit. A low resolution image of Saturn therefore changes drastically. It was not until 1659 that Christiaan Huygens correctly inferred the geometry of the rings. Saturn's rings remained unique in the known solar system until 1977 when very faint rings were discovered around Uranus (and shortly thereafter around Jupiter and Neptune).

Saturn was first visited by NASA's Pioneer 11 in 1979 and later by Voyager 1 and Voyager 2. Cassini (a joint NASA / ESA project) arrived on July 1, 2004 and will orbit Saturn for at least four years.

Saturn is visibly flattened (oblate) when viewed through a small telescope; its equatorial and polar diameters vary by almost 10% (120,536 km vs. 108,728 km). This is the result of its rapid rotation and fluid state. The other gas planets are also oblate, but not so much so.

Saturn is the least dense of the planets; its specific gravity (0.7) is less than that of water.

Like Jupiter, Saturn is about 75% hydrogen and 25% helium with traces of water, methane, ammonia and "rock", similar to the composition of the primordial Solar Nebula from which the solar system was formed.

Saturn's interior is similar to Jupiter's consisting of a rocky core, a liquid metallic hydrogen layer and a molecular hydrogen layer. Traces of various ices are also present.

Saturn's interior is hot (12000 K at the core) and Saturn radiates more energy into space than it receives from the Sun. Most of the extra energy is generated by the Kelvin-Helmholtz mechanism as in Jupiter. But this may not be sufficient to explain Saturn's luminosity; some additional mechanism may be at work, perhaps the "raining out" of helium deep in Saturn's interior.

The bands so prominent on Jupiter are much fainter on Saturn. They are also much wider near the equator. Details in the cloud tops are invisible from Earth so it was not until the Voyager encounters that any detail of Saturn's atmospheric circulation could be studied. Saturn also exhibits long-lived ovals (red spot at center of image at right) and other features common on Jupiter. In 1990, HST observed an enormous white cloud near Saturn's equator which was not present during the Voyager encounters; in 1994 another, smaller storm was observed (left).

Two prominent rings (A and B) and one faint ring (C) can be seen from the Earth. The gap between the A and B rings is known as the Cassini division. The much fainter gap in the outer part of the A ring is known as the Encke Division (but this is somewhat of a misnomer since it was very likely never seen by Encke). The Voyager pictures show four additional faint rings. Saturn's rings, unlike the rings of the other planets, are very bright (albedo 0.2 - 0.6).

Though they look continuous from the Earth, the rings are actually composed of innumerable small particles each in an independent orbit. They range in size from a centimeter or so to several meters. A few kilometer-sized objects are also likely.

Saturn's rings are extraordinarily thin: though they're 250,000 km or more in diameter they're less than one kilometer thick. Despite their impressive appearance, there's really very little material in the rings -- if the rings were compressed into a single body it would be no more than 100 km across.

The ring particles seem to be composed primarily of water ice, but they may also include rocky particles with icy coatings.

Voyager confirmed the existence of puzzling radial inhomogeneities in the rings called "spokes" which were first reported by amateur astronomers (left). Their nature remains a mystery, but may have something to do with Saturn's magnetic field.

Saturn's outermost ring, the F-ring, is a complex structure made up of several smaller rings along which "knots" are visible. Scientists speculate that the knots may be clumps of ring material, or mini moons. The strange braided appearance visible in the Voyager 1 images (right) is not seen in the Voyager 2 images perhaps because Voyager 2 imaged regions where the component rings are roughly parallel. They are prominent in the Cassini images which also show some as yet unexplained wispy spiral structures.

There are complex tidal resonances between some of Saturn's moons and the ring system: some of the moons, the so-called "shepherding satellites" (i.e. Atlas, Prometheus and Pandora) are clearly important in keeping the rings in place; Mimas seems to be responsible for the paucity of material in the Cassini division, which seems to be similar to the Kirkwood gaps in the asteroid belt; Pan is located inside the Encke Division and S/2005 S1 is in the center of the Keeler Gap. The whole system is very complex and as yet poorly understood.

The origin of the rings of Saturn (and the other jovian planets) is unknown. Though they may have had rings since their formation, the ring systems are not stable and must be regenerated by ongoing processes, perhaps the breakup of larger satellites. The current set of rings may be only a few hundred million years old.

Like the other jovian planets, Saturn has a significant magnetic field.

When it is in the nighttime sky, Saturn is easily visible to the unaided eye. Though it is not nearly as bright as Jupiter, it is easy to identify as a planet because it doesn't "twinkle" like the stars do. The rings and the larger satellites are visible with a small astronomical telescope. There are several Web sites that show the current position of Saturn (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

thatllUID: 2538832

【星球大战之天王星】

Uranus is the seventh planet from the Sun and the third largest (by diameter). Uranus is larger in diameter but smaller in mass than Neptune.
        orbit:    2,870,990,000 km (19.218 AU) from Sun        
        diameter: 51,118 km (equatorial)        
        mass:     8.683e25 kg

Uranus is the ancient Greek deity of the Heavens, the earliest supreme god. Uranus was the son and mate of Gaia the father of Cronus (Saturn) and of the Cyclopes and Titans (predecessors of the Olympian gods).

Uranus, the first planet discovered in modern times, was discovered by William Herschel while systematically searching the sky with his telescope on March 13, 1781. It had actually been seen many times before but ignored as simply another star (the earliest recorded sighting was in 1690 when John Flamsteed cataloged it as 34 Tauri). Herschel named it "the Georgium Sidus" (the Georgian Planet) in honor of his patron, the infamous (to Americans) King George III of England; others called it "Herschel". The name "Uranus" was first proposed by Bode in conformity with the other planetary names from classical mythology but didn't come into common use until 1850.

Uranus has been visited by only one spacecraft, Voyager 2 on Jan 24 1986.

Most of the planets spin on an axis nearly perpendicular to the plane of the ecliptic but Uranus' axis is almost parallel to the ecliptic. At the time of Voyager 2's passage, Uranus' south pole was pointed almost directly at the Sun. This results in the odd fact that Uranus' polar regions receive more energy input from the Sun than do its equatorial regions. Uranus is nevertheless hotter at its equator than at its poles. The mechanism underlying this is unknown.

Actually, there's an ongoing battle over which of Uranus' poles is its north pole! Either its axial inclination is a bit over 90 degrees and its rotation is direct, or it's a bit less than 90 degrees and the rotation is retrograde. The problem is that you need to draw a dividing line *somewhere*, because in a case like Venus there is little dispute that the rotation is indeed retrograde (not a direct rotation with an inclination of nearly 180).

Uranus is composed primarily of rock and various ices, with only about 15% hydrogen and a little helium (in contrast to Jupiter and Saturn which are mostly hydrogen). Uranus (and Neptune) are in many ways similar to the cores of Jupiter and Saturn minus the massive liquid metallic hydrogen envelope. It appears that Uranus does not have a rocky core like Jupiter and Saturn but rather that its material is more or less uniformly distributed.

Uranus' atmosphere is about 83% hydrogen, 15% helium and 2% methane.

Like the other gas planets, Uranus has bands of clouds that blow around rapidly. But they are extremely faint, visible only with radical image enhancement of the Voyager 2 pictures (right). Recent observations with HST (left) show larger and more pronounced streaks. Further HST observations show even more activity. Uranus is no longer the bland boring planet that Voyager saw! It now seems clear that the differences are due to seasonal effects since the Sun is now at a lower Uranian latitude which may cause more pronounced day/night weather effects. By 2007 the Sun will be directly over Uranus's equator.

Uranus' blue color is the result of absorption of red light by methane in the upper atmosphere. There may be colored bands like Jupiter's but they are hidden from view by the overlaying methane layer.

Like the other gas planets, Uranus has rings. Like Jupiter's, they are very dark but like Saturn's they are composed of fairly large particles ranging up to 10 meters in diameter in addition to fine dust. There are 11 known rings, all very faint; the brightest is known as the Epsilon ring. The Uranian rings were the first after Saturn's to be discovered. This was of considerable importance since we now know that rings are a common feature of planets, not a peculiarity of Saturn alone.

Voyager 2 discovered 10 small moons in addition to the 5 large ones already known. It is likely that there are several more tiny satellites within the rings.

Uranus' magnetic field is odd in that it is not centered on the center of the planet and is tilted almost 60 degrees with respect to the axis of rotation. It is probably generated by motion at relatively shallow depths within Uranus.

Uranus is sometimes just barely visible with the unaided eye on a very clear night; it is fairly easy to spot with binoculars (if you know exactly where to look). A small astronomical telescope will show a small disk. There are several Web sites that show the current position of Uranus (and the other planets) in the sky, but much more detailed charts will be required to actually find it. Such charts can be created with a planetarium program.

thatllUID: 2538832

【星球大战之海王星】

Neptune is the eighth planet from the Sun and the fourth largest (by diameter). Neptune is smaller in diameter but larger in mass than Uranus.
        orbit:    4,504,000,000 km (30.06 AU) from Sun        
        diameter: 49,532 km (equatorial)        
        mass:     1.0247e26 kg

In Roman mythology Neptune (Greek: Poseidon) was the god of the Sea.

After the discovery of Uranus, it was noticed that its orbit was not as it should be in accordance with Newton's laws. It was therefore predicted that another more distant planet must be perturbing Uranus' orbit. Neptune was first observed by Galle and d'Arrest on 1846 Sept 23 very near to the locations independently predicted by Adams and Le Verrier from calculations based on the observed positions of Jupiter, Saturn and Uranus. An international dispute arose between the English and French (though not, apparently between Adams and Le Verrier personally) over priority and the right to name the new planet; they are now jointly credited with Neptune's discovery. Subsequent observations have shown that the orbits calculated by Adams and Le Verrier diverge from Neptune's actual orbit fairly quickly. Had the search for the planet taken place a few years earlier or later it would not have been found anywhere near the predicted location.

More than two centuries earlier, in 1613, Galileo observed Neptune when it happened to be very near Jupiter, but he thought it was just a star. On two successive nights he actually noticed that it moved slightly with respect to another nearby star. But on the subsequent nights it was out of his field of view. Had he seen it on the previous few nights Neptune's motion would have been obvious to him. But, alas, cloudy skies prevented obsevations on those few critical days.

Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. Much of we know about Neptune comes from this single encounter. But fortunately, recent ground-based and HST observations have added a great deal, too.

Because Pluto's orbit is so eccentric, it sometimes crosses the orbit of Neptune making Neptune the most distant planet from the Sun for a few years.

Neptune's composition is probably similar to Uranus': various "ices" and rock with about 15% hydrogen and a little helium. Like Uranus, but unlike Jupiter and Saturn, it may not have a distinct internal layering but rather to be more or less uniform in composition. But there is most likely a small core (about the mass of the Earth) of rocky material. Its atmosphere is mostly hydrogen and helium with a small amount of methane.

Neptune's blue color is largely the result of absorption of red light by methane in the atmosphere but there is some additional as-yet-unidentified chromophore which gives the clouds their rich blue tint.

Like a typical gas planet, Neptune has rapid winds confined to bands of latitude and large storms or vortices. Neptune's winds are the fastest in the solar system, reaching 2000 km/hour.

Like Jupiter and Saturn, Neptune has an internal heat source -- it radiates more than twice as much energy as it receives from the Sun.

At the time of the Voyager encounter, Neptune's most prominent feature was the Great Dark Spot (left) in the southern
hemisphere. It was about half the size as Jupiter's Great Red Spot (about the same diameter as Earth). Neptune's winds blew the Great Dark Spot westward at 300 meters/second (700 mph). Voyager 2 also saw a smaller dark spot in the southern hemisphere and a small irregular white cloud that zips around Neptune every 16 hours or so now known as "The Scooter" (right). It may be a plume rising from lower in the atmosphere but its true nature remains a mystery.

However, HST observations of Neptune (left) in 1994 show that the Great Dark Spot has disappeared! It has either simply dissipated or is currently being masked by other aspects of the atmosphere. A few months later HST discovered a new dark spot in Neptune's northern hemisphere. This indicates that Neptune's atmosphere changes rapidly, perhaps due to slight changes in the temperature differences between the tops and bottoms of the clouds.

Neptune also has rings. Earth-based observations showed only faint arcs instead of complete rings, but Voyager 2's images showed them to be complete rings with bright clumps. One of the rings appears to have a curious twisted structure (right).

Like Uranus and Jupiter, Neptune's rings are very dark but their composition is unknown.

Neptune's rings have been given names: the outermost is Adams (which contains three prominent arcs now named Liberty, Equality and Fraternity), next is an unnamed ring co-orbital with Galatea, then Leverrier (whose outer extensions are called Lassell and Arago), and finally the faint but broad Galle.

Neptune's magnetic field is, like Uranus', oddly oriented and probably generated by motions of conductive material (probably water) in its middle layers.

Neptune can be seen with binoculars (if you know exactly where to look) but a large telescope is needed to see anything other than a tiny disk. There are several Web sites that show the current position of Neptune (and the other planets) in the sky, but much more detailed charts will be required to actually find it. Such charts can be created with a planetarium program.

thatllUID: 2538832

【星球大战之图展】

thatllUID: 2538832

【torpor----hibernate----estivate】
estivate: to pass the summer in a state of torpor---compare HIBERNATE
所以这个词条其实很好的解释了这三者之间的关系。事实上：
torpor : a state of lowered physiological activity typically characterized by reduced metabolism, heart rate, respiration, and body temperature that occurs in varying degrees especially in hibernating and estivating animals

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How the Sun Affects Climate: Solar and Milankovitch Cycles

Earth gets all its energy from the Sun and it is the Sun's energy that keeps Earth warm. But the amount of energy Earth receives is not always the same. Changes in the Sun and changes in Earth's orbit affect the amount of energy that reaches the Earth.
The 11-Year Solar Cycle
When the Sun has fewer sunspots, it gives off less energy, less energy makes its way to Earth, and our planet cools down. More than three hundred years ago, when the climate was cooler for a time called the "Little Ice Age", people noticed there were no sunspots for several decades. Over time, scientists have noticed a pattern in the number of sunspots. About every 11 years the number of sunspots reaches a high and then decreases again.
Milankovitch Cycles
Over thousands of years, changes in Earth's orbit cause changes in the amount of the Sun's energy that gets to the planet. Over the past several million years these changes have caused cycles of global warming and cooling.
There are three ways that Earth's orbit changes over time.
•Eccentricity: The shape of Earth's orbit around the Sun becomes slightly more and then less oval every 100,000 years.
•Precession: Earth wobbles on it axis as it spins, completing a full wobble every 23,000 years.
•Tilt: The angle of the Earth's axis relative to the plane of its orbit changes about three degrees every 41,000 years.
Once the Sun's energy reaches the Earth, several things can happen. The energy can be absorbed by the planet, reflected back into space, or become trapped in the atmosphere.

                                                                             
The shape of Earth’s orbit becomes more or less oval (eccentricity), Earth wobbles as it spins (precession), and Earth's axis changes too (tilt). All these changes, over thousands of years, causes Earth's climate to change.

注： In the 1920s, a mathematician named Milutin Milankovitch worked out why summers would be cooler by looking at the factors that limit sunlight's reach to Earth. He identified three factors: the tilt in the Earth's axis, the way the Earth wobbles on its axis and how close the Earth gets to the sun. By combining these factors in a mathematical formula, he was able to predict that ice ages would occur every 22,000, 41,000 and 100,000 years. These rhythms became known as Milankovitch cycles.

thatllUID: 2538832

The Nile, the Moon and Sirius:
The Ancient Egyptian Calendar

By
Richard Weininger

The star-sprinkled Egyptian night sky that not only stuns visitors to Egyptwas also studied intensely by special temple priests who soon discovered that the appearance of a star they named sepdet (which we know as Sirius) was associated with the beginning of the Nile flood . This was the start of the world's first calendar, invented over 5000 years ago.
                                                            

To develop a calendar, you need a regular event that is predictable. And what was more regular and important to the ancient Egyptians than the rise and fall of the River Nile?

The waters started rising around the end of June, and the flood period (achet) lasted until October, covering the land with rich black mud and preparing it for the sowing and growing period (peret). The harvest time (schemu) started at the end of February and ended with the new Nile flood.

This predictable, ongoing cycle defined the agricultural year.

But there was a problem! The flood came within a range of 80 days with variable intensities .... all in all, not very accurate timing.
Sirius, or Sothis as it was called by the ancient Egyptians, the star who's heliacal rising was in early July 3000 years ago, but due to the wobble of the earth on its axis is now a few weeks later, turned out to be a very reliable predictor of the recurring flood and defined the exact length of the trip of the earth around the sun . (Sirius also revealed the entrance to the tomb of Akhenaton in the novel The Watch Gods by Barbara Wood. )

From their mythology, the Egyptians saw a connection between Sepdet's appearance and the beginning of the Nile flood. They believed the Nile flood was caused by the tears that Isis ( the Great Mother of All Gods and Nature ) shed, after her husband Osiris was murdered by his brother Seth. Sepdet was the cosmic appearance of Isis.

The first new moon following the reappearance of Sirius after it disappeared under the horizon for 70 days was established as the first day of the New Year ( Egypt: wepet senet) and of the achet (flood) period--even if the Nile had not yet started to rise.

The priests also observed there were four moon periods that fit into each of the three seasons --or rather didn't quite fit! The lunar month has 29 �days, resulting in "short" or "long" years of 12 or 13 new moons.

It didn't really matter because the appearance of Sirius and the next NewYear put the calendar back to baseline.

But, as in our times, this calendar was not accurate enough for the central administration; taxes and other things have to be paid on time. So in the Old Kingdom, a standard calendar with 12 months of 30 days each was introduced . Each month was divided into decades of 10 days.

Because this public calendar with 360 days was too short to coordinate with the agricultural and lunar calendar, five extra days called the heriu renpet were added at the end of the year and celebrated with religious festivities.

With this last calendar reform before Roman times, the ancient Egyptians missed the true length of the solar year by only � day. But the missing days added up and the gap between the lunar calendar and the public calendar increased by one day every four years . So, in 1460 years the calendar slipped through a whole year, meaning that in between, according to one calendar, it could be harvest time, although in reality the floodwaters were just receding!

This little "big problem" wasn't solved until Augustus introduced the 憀eap year� in Egypt around 30 B.C.

Winter and spring are the best time to watch Sirius. In February, Siriusstands low in the southeast and is the brightest star in the sky ----- go and look ------ and imagine the Egyptian priests doing the same 5000 years ago.

thatllUID: 2538832

Principal Types of Volcanoes
Geologists generally group volcanoes into four main kinds--cinder cones, composite volcanoes, shield volcanoes, and lava domes.


Cinder cones
Cinder cones are the simplest type of volcano. They are built from particles and blobs of congealed lava ejected from a single vent. As the gas-charged lava is blown violently into the air, it breaks into small fragments that solidify and fall as cinders around the vent to form a circular or oval cone. Most cinder cones have a bowl-shaped crater at the summit and rarely rise more than a thousand feet or so above their surroundings. Cinder cones are numerous in western North America as well as throughout other volcanic terrains of the world.

Schematic representation of the internal structure of a typical cinder cone.

In 1943 a cinder cone started growing on a farm near the village of Parícutin in Mexico. Explosive eruptions caused by gas rapidly expanding and escaping from molten lava formed cinders that fell back around the vent, building up the cone to a height of 1,200 feet. The last explosive eruption left a funnel-shaped crater at the top of the cone. After the excess gases had largely dissipated, the molten rock quietly poured out on the surrounding surface of the cone and moved downslope as lava flows. This order of events--eruption, formation of cone and crater, lava flow--is a common sequence in the formation of cinder cones.
During 9 years of activity, Parícutin built a prominent cone, covered about 100 square miles with ashes, and destroyed the town of San Juan. Geologists from many parts of the world studied Parícutin during its lifetime and learned a great deal about volcanism, its products, and the modification of a volcanic landform by erosion.

Parícutin Volcano, Mexico, is a cinder cone rising approximately 1,200 feet above the surrounding plain.

Composite volcanoes
Some of the Earth's grandest mountains are composite volcanoes--sometimes called stratovolcanoes. They are typically steep-sided, symmetrical cones of large dimension built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs and may rise as much as 8,000 feet above their bases. Some of the most conspicuous and beautiful mountains in the world are composite volcanoes, including Mount Fuji in Japan, Mount Cotopaxi in Ecuador, Mount Shasta in California, Mount Hood in Oregon, and Mount St. Helens and Mount Rainier in Washington.
Most composite volcanoes have a crater at the summit which contains a central vent or a clustered group of vents. Lavas either flow through breaks in the crater wall or issue from fissures on the flanks of the cone. Lava, solidified within the fissures, forms dikes that act as ribs which greatly strengthen the cone.
The essential feature of a composite volcano is a conduit system through which magma from a reservoir deep in the Earth's crust rises to the surface. The volcano is built up by the accumulation of material erupted through the conduit and increases in size as lava, cinders, ash, etc., are added to its slopes.

Schematic representation of the internal structue of a typical composite volcano.

When a composite volcano becomes dormant, erosion begins to destroy the cone. As the cone is stripped away, the hardened magma filling the conduit (the volcanic plug) and fissures (the dikes) becomes exposed, and it too is slowly reduced by erosion. Finally, all that remains is the plug and dike complex projecting above the land surface--a telltale remnant of the vanished volcano.

Shishaldin Volcano, an imposing composite cone, towers 9,372 feet above sea level in the Aleutian Islands, Alaska.

An interesting variation of a composite volcano can be seen at Crater Lake in Oregon. From what geologists can interpret of its past, a high volcano--called Mount Mazama- probably similar in appearance to present-day Mount Rainier was once located at this spot. Following a series of tremendous explosions about 6,800 years ago, the volcano lost its top. Enormous volumes of volcanic ash and dust were expelled and swept down the slopes as ash flows and avalanches. These large-volume explosions rapidly drained the lava beneath the mountain and weakened the upper part. The top then collapsed to form a large depression, which later filled with water and is now completely occupied by beautiful Crater Lake. A last gasp of eruptions produced a small cinder cone, which rises above the water surface as Wizard Island near the rim of the lake. Depressions such as Crater Lake, formed by collapse of volcanoes, are known as calderas. They are usually large, steep-walled, basin-shaped depressions formed by the collapse of a large area over, and around, a volcanic vent or vents. Calderas range in form and size from roughly circular depressions 1 to 15 miles in diameter to huge elongated depressions as much as 60 miles  long.

Crater Lake, Oregon; Wizard Island, a cinder cone, rises above the lake surface.

The Evolution of a Composit e Volcano

A. Magma, rising upward through a conduit, erupts at the Earth's surface to form a volcanic cone. Lava flows spread over the surrounding area. B. As volcanic activity continues, perhaps over spans of hundreds of years, the cone is built to a great height and lava flows form an extensive plateau around its base. During this period, streams enlarge and deepend their valleys. C. When volcanic activity ceases, erosion starts to destroy the cone. After thousands of years, the great cone is stripped away to expose the hardened "volcanic plug" in the conduit. During this period of inactivity, streams broaden their valleys and dissect the lava plateau to form isolated lava-capped mesas. D. Continued erosion removes all traces of the cone and the land is worn down to a surface of low relief. All that remains is a projecting plug or "volcanic neck," a small lava-capped mesa, and vestiges of the once lofty volcano and its surrounding lava plateau.

Shield volcanoes

The internal structure of a typical shield volcano

Shield volcanoes, the third type of volcano, are built almost entirely of fluid lava flows. Flow after flow pours out in all directions from a central summit vent, or group of vents, building a broad, gently sloping cone of flat, domical shape, with a profile much like that of a warrior's shield. They are built up slowly by the accretion of thousands of highly fluid lava flows called basalt lava that spread widely over great distances, and then cool as thin, gently dipping sheets. Lavas also commonly erupt from vents along fractures (rift zones) that develop on the flanks of the cone. Some of the largest volcanoes in the world are shield volcanoes. In northern California and Oregon, many shield volcanoes have diameters of 3 or 4 miles and heights of 1,500 to 2,000 feet. The Hawaiian Islands are composed of linear chains of these volcanoes including Kilauea and Mauna Loa on the island of Hawaii-- two of the world's most active volcanoes. The floor of the ocean is more than 15,000 feet deep at the bases of the islands. As Mauna Loa, the largest of the shield volcanoes (and also the world's largest active volcano), projects 13,677 feet above sea level, its top is over 28,000 feet above the deep  ocean floor.

Mauna Loa Volcano, Hawaii, a giant among the active volcanoes of the world; snow-capped Mauna Kea Volcano in the distance.

In some eruptions, basaltic lava pours out quietly from long fissures instead of central vents and floods the surrounding countryside with lava flow upon lava flow, forming broad plateaus. Lava plateaus of this type can be seen in Iceland, southeastern Washington, eastern Oregon, and southern Idaho. Along the Snake River in Idaho, and the Columbia River in Washington and Oregon, these lava flows are beautifully exposed and measure more than a mile in total thickness.




Lava domes

Schematic representation of the internal structure of a typical volcanic dome.

Volcanic or lava domes are formed by relatively small, bulbous masses of lava too viscous to flow any great distance; consequently, on extrusion, the lava piles over and around its vent. A dome grows largely by expansion from within. As it grows its outer surface cools and hardens, then shatters, spilling loose fragments down its sides. Some domes form craggy knobs or spines over the volcanic vent, whereas others form short, steep-sided lava flows known as "coulees." Volcanic domes commonly occur within the craters or on the flanks of large composite volcanoes. The nearly circular Novarupta Dome that formed during the 1912 eruption of Katmai Volcano, Alaska, measures 800 feet across and 200 feet high. The internal structure of this dome--defined by layering of lava fanning upward and outward from the center--indicates that it grew largely by expan sion from within.

The Novarupta Dome formed during the 1912 eruption of Katma Volcano, Alaska.

Mont Pelée in Martinique, Lesser Antilles, and Lassen Peak and Mono domes in California are examples of lava domes. An extremely destructive eruption accompanied the growth of a dome at Mont Pelée in 1902. The coastal town of St. Pierre, about 4 miles downslope to the south, was demolished and nearly 30,000 inhabitants were killed by an incandescent, high-velocity ash flow and associated hot gases and volcanic dust.
Only two men survived; one because he was in a poorly ventilated, dungeon-like jail cell and the other who somehow made his way safely throug h the burning city.

A sketch of the havoc wrought in St. Pierre Harbor on Martinique during the eruption of Mont Pelée in 1902.

thatllUID: 2538832

A nebula is a cloud of dust and gas, composed primarily of hydrogen (97%) and helium (3%). Within a nebula, there are varying regions when gravity causes this dust and gas to “clump” together. As these “clumps” gather more atoms (mass), their gravitational attraction to other atoms increases, pulling more atoms into the “clump.”

What causes these “gravitational centers” to form in these huge clouds?

If you knew that, you’d have a Nobel Prize!

Adding atoms to the center of a protostar is a process astronomers call
accretion. Because numerous reactions occur within the mass of forming star material, a protostar is not very stable.

In order to achieve life as a star, the protostar will need to achieve and maintain
equilibrium. What is equilibrium? It is a balance, in this case a balance between gravity pulling atoms toward the center and gas pressure pushing heat and light away from the center. Achieving and keeping this balance is tough to do. When a star can no longer maintain equilibrium, it dies.

Equilibrium: How it Works!Equilibrium is a battle between gravity and gas pressure. It works like this:


1. Gravity pulls gas and dust inward toward the core.

2. Inside the core, temperature increases as gas atom collisions increase.

3. Density of the core increases as more atoms try to share the same space.

4. Gas pressure increases as atomic collisions and density (atoms/space) increase.

5. The protostar’s gas pressure RESISTS the collapse of the nebula.

6. When gas pressure = gravity, the protostar has reached equilibrium and accretion stops




Equilibrium for a protostar occurs when gas pressure equals gravity. Gravity remains constant, so what changes the gas pressure in a protostar? Gas pressure depends upon two things to maintain it: a very hot temperature (keep those atoms colliding!) and density (lots of atoms in a small space).


There are two options for a protostar at this point:



• Option 1:
  If a critical temperature in the core of a protostar is not reached, it ends up a
  brown dwarf. This mass never makes “star status.”
  
• Option 2:
  If a critical temperature in the core of a
  protostar
  is reached, then nuclear fusion begins. We identify the birth of a star as the moment that it begins fusing hydrogen in the core into helium.

So, what is a star? A star is a really hot ball of gas, with hydrogen fusing into helium at its core. Stars spend the majority of their lives fusing hydrogen, and when the hydrogen fuel is gone, stars fuse helium into carbon. The more massive stars can fuse carbon into even heavier elements, which is where most of the heavy elements in the universe are made. Throughout this whole process is that battle between gravity and gas pressure, known as equilibrium. It’s crucial to keep this battle in your mind when trying to understand how stars live and die.
The Main SequenceStars live out the majority of their lives in a phase termed as the Main Sequence. Once achieving nuclear fusion, stars radiate (shine) energy into space. The star slowly contracts over billions of years to compensate for the heat and light energy lost. As this slow contraction continues, the star’s temperature, density, and pressure at the core continue to increase. The temperature at the center of the star slowly rises over time because the star radiates away energy, but it is also slowly contracting. This battle between gravity pulling in and gas pressure pushing out will go on over the entire life span of the star.

Equilibrium: Life Goal of a StarLook at the diagram. There are essentially two sections of a star: the core (where fusion occurs), and an outer gaseous shell. The core serves as the gravitational “center” of the star. It is very hot and very dense. The outer shell is made of hydrogen and helium gas. This shell helps move heat from the core of the star to the surface of the star where energy in the form of light and heat is released into space.

The star’s main goal in life is to achieve stability, or equilibrium. The term equilibrium does not mean that there isn’t any change in the star. It just means that there is not a net overall change in the star. In a stable star, the gas pressure pushing out from the center is equal with the gravity pulling atoms inward to the center – when these forces are equal, the star is at equilibrium.
Once a star reaches equilibrium for the first time, it will start burning (fusing) hydrogen into helium.


This 5-step process works like this:

1，Nuclear fusion. Gravity = gas pressure (equilibrium)
2，Out of fuel.
3，Fusion stops, temperature drops.
4，Core contracts (gravity pulling atoms in).
5，Increased temperature (more atoms, more collisions) and density in the core reinitiates nuclear fusion, equilibrium is achieved, and the cycle begins again at step 1.

Because interstellar medium is 97% hydrogen and 3% helium, with trace amounts of dust, etc., a star primarily burns hydrogen during its lifetime. A medium-size star will live in the hydrogen phase, called the main sequence phase, for about 50 million years. Once hydrogen fuel is gone, the star has entered “old age.”
Let’s see if you understand the relationships between gas pressure, temperature, and gravity as it relates to equilibrium. Consider it apractice quiz so you are ready for the one your teacher will undoubtedly give to you.
After Main SequenceWhat happens to a star after the main sequence phase? Old age and death! How long it takes for a star to die depends upon its initial mass. A lower-mass star like the sun can survive for billions of years, but after the hydrogen and helium fuel is gone it cannot get hot enough to fuse carbon.

This type of star dies by puffing off its outer layers to produce expanding planetary nebulae. These nebulae, which are the remains of dying stars, serve as the birthplace for future protostars.

In contrast with our sun, which is really a main sequence star, massive stars live very short lives, perhaps only millions of years, before they develop dead iron cores and explode as a supernova. The core of a dying massive star may form a neutron star or black hole, but the outermost parts of the exploded star return to the interstellar medium from which they came.

Let’s look at the relationship between initial mass and length of star life. How long do most stars survive? Millions to billions of years, depending upon the star’s “birth-mass.” Is bigger always better? Not with stars. The more mass a star has at birth, the harder it is to keep that fusion reaction going. It may have more atoms, but the fusion reaction goes faster and therefore burns the star out faster than smaller stars.
Bigger is not better in this case!
Keep in mind that fusion is what allows a star to maintain equilibrium. If a star cannot achieve a hot enough temperature to initiate fusion, then it’s dying already. Fusion reactions need a fuel, and there are three main fuels that a star uses for fusion: hydrogen, helium, and carbon.


HELIUM BURNING: The Beginning of the EndFor stars that live most of their lives in the main sequence, helium burning is the beginning of the end. The overall thermonuclear reaction for helium burning is as follows: 3 He -> 1 C + energy released



Helium in the core of the star is still burning hot. Gravity keeps contractingthe core to maintain equilibrium, and as the core contracts the atoms are packed together even tighter than before. The outer shell has expanded in an effort to help heat from the core escape into space. At this point, the star is often termed a
red giant.
The red giant is the first step in old age.
Fusion is releasing more energy during helium burning than at the main sequence stage, so the star is bigger, but less stable. Eventually, the core will run out of helium fuel, and in order to maintain equilibrium, the core will contract again to initiate the last type of fusion – carbon burning.


CARBON BURNING: DeathUp to this point, most of the events of stellar evolution are well documented.
What happens to a star after the red-giant phase is not certain.
We do know that a star, regardless of its size, must eventually run out of fuel and collapse. In theory, GRAVITY WINS. With this in mind, we will consider the death of stars and group them into three categories according to mass:


• Low-Mass Stars (0.5 solar mass or less)
• Medium-Mass Stars (0.5 solar mass to 3.0 solar mass)
• Massive Stars (3.0 solar masses or larger)


Low-mass stars
A low mass star becomes a white dwarfLow mass stars (0.08-5 SM during main sequence) will go the planetary nebula route. A low mass core (,1.4 SM) shrinks to white dwarf. Electrons prevent further collapse. The size of the white dwarf is close to that of earth, and the outer layers are planetary nebula.

Medium-mass stars become neutron stars
A higher mass core (between 1.4-3 SM) shrinks to neutron star. Supernova happens when a neutron star is created. Neutrons prevent further collapse. The size of a neutron star is about that of a large city.


The overall reactions that occur for carbon burning occur so rapidly and with so much energy that the star blows apart in an explosion called a supernova. The outer layers of the star blast into space, and the core is crushed to immense densities. Carbon burning occurs when the helium in the core is gone. The core needs to maintain temperature to keep the gas pressure up; otherwise the star cannot resist gravity.
When carbon burning does occur, iron is formed.
Iron is the most stable of all nuclei, and ends the nuclear fusion process within a star. When these heavier elements form in the core, they take away energy rather than release it. With the decrease in fuel for fusion, the temperature decreases and the rate of collapse increases. Just before the star totally collapses, there is a sudden increase in temperature, density, and pressure. The pressure and energy compact the core further, squeezing it like “Charmin.” The compact core becomes a rapidly whirling ball of neutrons, and that’s why now this star is termed a neutron star.
The largest mass stars may become black holesThe highest mass star has a core that shrinks to a point. On the way to total collapse it may momentarily create a neutron star and the resulting supernova rebound explosion. Gravity finally wins. Nothing holds it up. Space so warped around the object that it effectively leaves our space – black  hole!

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thatllUID: 2538832

【detract】

transitive verb
1 archaic : to speak ill of
2 archaic : to take away
3 : DIVERT *detract attention*
intransitive verb : to diminish the importance, value, or effectiveness of something---often used with from *small errors that do not seriously detract from the book*

v.tr.
To draw or take away; divert: They could detract little from so solid an argument.
Archaic. To speak ill of; belittle.
v.intr.
To reduce the value, importance, or quality of something. Often used with from: testimony that only detracts from the strength of the plaintiff's case.

我们一注意到这个detract就是一个及物的意思和一个不及物的意思，及物的就是divert的意思，转移注意力，detract attention。不及物就是减少重要性，价值，有效性，跟from连用，sth detract from sth，就表示什么东西减少了什么东西的重要性等。
detraction：
1 : a lessening of reputation or esteem especially by envious, malicious, or petty criticism  : BELITTLING, DISPARAGEMENT
2 : a taking away  *it is no detraction from its dignity or prestige---J. F. Golay*
那这个名词形式就是表达了detract的两个已经过时不用的含义的名词形式，即to speak ill of和to take away。意即诽谤，贬低；还有就是带走，取走。

【distract】

1 a : to turn aside : DIVERT *refused to be distracted from her purpose* b : to draw or direct (as one's attention) to a different object or in different directions at the same time *was distracted by a sudden noise*
2 : to stir up or confuse with conflicting emotions or motives

conflicting : being in conflict, collision, or opposition  : INCOMPATIBLE  *conflicting theories*

pull in：
1. Arrive at a destination, as in The train pulled in right on time. [c. 1900]
2. Rein in, restrain, as in She pulled in her horse, or The executives did not want to pull in their most aggressive salesmen. [c. 1600]
3. Arrest a suspect, as in The police said they could pull him in on lesser charges. [Late 1800s]