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Our solar system is our neighborhood in space. The Sun is the center of this neighborhood, which includes at least nine planets plus hundreds of moons and thousands of asteriods. Sun Diameter: 863,750 miles Mass: 884 x 1029 lbs. Temperature: 9,980° F (surface); 28,000,000° F (core) The Sun is one of more than 100 billion stars in our galaxy. The Sun is by far the largest object in the solar system. It contains more than 99.8% of the total mass of the solar system (Jupiter contains most of the rest). At present, the Sun is about 75% hydrogen and 25% helium by mass. Everything else ("metals") amounts to only 0.1%. This changes slowly over time as the Sun converts hydrogen to helium in its core. The Sun is personified in many mythologies: the Greeks called it Helios and the Romans called it Sol. The outer layers of the Sun rotate at different rates. At the equator, the surface rotates once every 25.4 days. Near the poles the surface rotates once every 36 days. Conditions at the Sun's core are extreme. The temperature is 28,000,000° F and the core's gases are compressed to a density 150 times that of water. The Sun outputs 386 billion billion megawatts of energy produced by nuclear fusion reactions. The surface of the Sun, called the photosphere, is at a temperature of about 9,980° F. Sunspots are "cool" regions (only 6380° F) caused by complicated interactions with the Sun's magnetic field and can be very large, as much as 31,000 miles in diameter. A small region known as the chromosphere lies above the photosphere. The region above the chromosphere, called the corona, extends millions of kilometers into space but is visible only during eclipses. Temperatures in the corona are more than 1,800,000° F. In addition to heat and light, the Sun also emits a low density stream of charged particles (mostly electrons and protons) known as the solar wind, which travels throughout the solar system at about 280 km/sec. The solar wind and the much higher energy particles ejected by solar flares can dramatically affect the Earth with changes ranging from power line surges to radio interference to the beautiful aurora borealis. The solar wind affects the tails of comets and the trajectories of spacecraft. Spectacular loops and prominences are often visible on the Sun's edge. The Sun is about 4.5 billion years old. Since its birth it has used up about half of the hydrogen in its core. It will continue to radiate "peacefully" for another 5 billion years or so, but eventually it will run out of hydrogen fuel. It will then expand into a red giant, resulting in the total destruction of the Earth. Mercury Mercury is the closest planet to the Sun. Mercury is smaller in diameter than Jupiter's moons, Ganymede and Titan. Orbit: 57,910,000 km or (36,092,000 miles) (0.38 A.U.) from Sun Diameter: 4880 km or 2508 miles Mass: 3.30 x 1023 kg or 7.26 x 1023 lbs 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). Mercury was given two names by the Greeks: Apollo for its apparition as a morning star and Hermes as an evening star. Mercury has been visited by only one spacecraft, Mariner 10. It flew by three times in 1973 and 1974. Only 45% of the surface has been mapped (and, unfortunately, it is too close to the Sun to be safely imaged by the Hubble Space Telescope). 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. It is now known that Mercury rotates three times in two of its years. Temperature changes on Mercury are the most extreme in the solar system ranging from -300°F to 800°F. The temperature on Venus is slightly hotter but very stable. 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. Mercury's interior is mostly a large iron core whose radius is 2800 to 3000 miles. The silicate outer shell (the same as Earth's mantle and crust) is only 750 to 900 miles 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. Unlike the Earth and Venus whose atmospheres are stable, Mercury's atmosphere is constantly being replaced. The surface of Mercury exhibits enormous canyons, some up to hundreds of miles in length and as much as 4 ½ miles high. One of the largest features on Mercury's surface is the Caloris Basin; it is about 2000 miles in diameter. It is similar to the large basins (maria) on the Moon. It was probably caused by a very large impact early in the history of the solar system. That impact was probably also responsible for the odd terrain on the exact opposite side of the planet In addition to the heavily cratered terrain, Mercury also has regions of relatively smooth plains. Perhaps from the result of ancient volcanic activity. 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 naked eye, but it is always very near the Sun and difficult to see in the twilight sky. Venus Venus is the brightest object in the sky, second only to the Moon. It is the second planet from the Sun, and the sixth largest planet. Orbit: 108,200,000 km or (67,238,500 miles) or (0.72 AU) from Sun Diameter: 12,100 km (7521 miles) Mass: 4.869 x 1024 kg (9835 x 1024 lbs) The Romans named Venus after the goddess of love and beauty probably because it is the brightest of the planets known to the ancients. Venus was known to the Greeks as Aphrodite and to the Babylonians as Ishtar. 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, Venus was popularly thought to be two separate bodies, but Greek astronomers knew better. Since Venus is closer to the Sun than Earth, it shows phases when viewed with a telescope. Galileo's observation of this phenomenon was important evidence in favor of Copernicus's heliocentric (Sun-centered) theory of the solar system. The first spacecraft to visit Venus was Mariner 2 in 1962. It was then visited by more than 20 others. Other spacecraft included Pioneer Venus and the Soviet Venera 7, the first spacecraft to land on another planet, and Venera 9 which returned the first photographs of the surface. Most recently, orbiting U.S. spacecraft Magellan produced detailed maps of Venus' surface using radar. Venus' rotation is very slow with 243 Earth days equal to one Venus day. Venus rotation is retrograde, opposite to the Earth's rotation. Venus' year lasts 225 days, almost equal to the length of its day. Venus is sometimes regarded as Earth's sister planet. In some ways, Earth and Venus are very similar: Venus is only slightly smaller than Earth Its diameter is equal to 95% of the size of Earth's diameter, and 80% of Earth's mass. Both have few craters indicating relatively young surfaces. Their densities and chemical compositions are also similar. More detailed study of Venus reveals that in many important ways it is very different from Earth: The pressure of Venus' atmosphere at the surface is 90 atmospheres, about the same as the pressure at a depth of one kilometer in the Earth's oceans. It is made mostly of carbon dioxide and sulfuric acid. These clouds completely block our view of the surface. This dense atmosphere produces a run-away greenhouse effect that raises Venus' surface temperature by about 400 degrees to over 800 degrees, which is hot enough to melt lead. Venus' surface is actually hotter than Mercury's despite being nearly twice as far from the Sun. Winds are strong (200 mph) at the cloud tops but at the surface they are very slow, at no more than a few miles 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. Data from Magellan's imaging radar shows that much of Venus' surface is covered by lava flows. There are several large shield volcanoes (similar to Hawaii or Olympus Mons on Mars) such as Sif Mons. Venus is still volcanically active, but only in a few hot spots. For the most part, Venus been geologically quiet for the past few hundred million years. Venus has no small craters. 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. Magellan's images show a wide variety of interesting and unique features including pancake volcanoes which seem to be eruptions of very thick lava and coronae which seem to be collapsed domes over large magma chambers. The interior of Venus is probably very similar to that of Earth. It has an iron core about 1,865 miles in radius and a molten rocky mantle comprising the majority of the planet. Venus has no magnetic field, perhaps because of its slow rotation. Venus has no satellites. Venus is usually visible with the naked eye. Sometimes referred to as the morning star or the evening star, it is by far the brightest "star" in the sky. Earth Earth is the third planet from the Sun and the fifth largest planet. Orbit: 149,600,000 km (93,000,000 miles) or (1.00 AU) from Sun Diameter: 12,756.3 km or 7927 miles Mass: 5.9736 x 1024 kg or 12.066 x 1024 lbs Earth has one moon. Earth is the only planet whose English name does not come from Greek/Roman mythology. The name comes from Old English and Germanic. In Roman mythology, the goddess of Earth was Tellus - the fertile soil. It was not until the 16th century that it was understood that Earth is just another planet. Earth can be studied without the aid of spacecraft, but it was not until the 20th century that maps were made of the entire planet. Pictures of the planet taken from space are important because they help in weather prediction and in tracking and predicting hurricanes. Earth is divided into several layers which have distinct chemical and seismic properties: 0- 40 km - Crust 40- 400 km - Upper mantle 400- 650 km - Transition region 650-2700 km - Lower mantle 2700-2890 km - D-layer 2890-5150 km - Outer core 5150-6378 km - Inner core Earth's crust changes in thickness. It is thinner under the oceans and thicker under the continents. The inner core and crust are solid and the outer core and mantle layers are fluid. Most of the mass of Earth is in the mantle (94%) The part we inhabit is a tiny fraction of the whole: Atmosphere - 0.0000051% Oceans - 0.001% Crust - 0.026% Mantle - 4.043% Outer core -1.835% Inner core - 0.097% The core is composed mostly of iron, although it is possible that some lighter elements may be present. Temperatures at the center of the core may be as high as 7500 Kelvin, hotter than the surface of the Sun. Taken as a whole, Earth's chemical composition (by mass) is: Iron -34.6% Oxygen - 29.5% Silicon - 15.2% Magnesium - 12.7% Nickel - 2.4% Sulfur - 1.9% Titanium - 0.05% Earth is the most dense major body in the solar system. Unlike other planets, Earth's crust is divided into several separate solid plates which float around independently on top of the hot mantle below. This is known as plate tectonics. Presently, there are eight major plates. Earth is 4.5 to 4.6 billion years old, but the oldest known rocks are about four billion years old. Rocks older than three billion years are rare. The oldest fossils of living organisms are less than 3.9 billion years old. Seventy-one percent of Earth's surface is covered with water. Earth is the only planet on which water can exist in a liquid form on the surface. Liquid water is essential for human life The heat capacity of the oceans keeps Earth's temperature stable. Liquid water is also responsible for most of the erosion and weathering of Earth's continents. Earth's atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water. The tiny amount of carbon dioxide in the atmosphere maintains Earth's surface temperature because of the greenhouse effect. The greenhouse effect raises the temperature about 35 degrees above what it would normally be (from -1 F to +46 F). Without this, the oceans would freeze. The interaction of Earth and the Moon slows Earth's rotation by about two milliseconds per century. Research shows that days were 18-hours long and a year lasted 481 days about 900 million years ago. Mars The Bringer of War Mars is the fourth planet from the Sun and the seventh largest: Orbit: 227,940,000 km (141,641,900 miles) or (1.52 AU) from Sun Diameter: 6,794 km or 4222 miles Mass: 6.4219 x 1023 km or 12.98 x 1028 lbs 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. 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 other followed including the two Viking landers in 1976. Ending a long 20 year hiatus, Mars Pathfinder landed successfully on Mars on 1997 July 4. 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 dayside during summer. Though Mars is much smaller than Earth, its surface area is about the same as the land surface area of Earth. Except for Earth, Mars has 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; 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. The southern hemisphere of Mars is pre ntly 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 abrupt boundary are unknown (some speculate that they are due to a very large impact shortly after Mars' formation). Recently, some scientists have begun to question whether the abrupt elevation is real in the first place. Mars Global Surveyor should resolve the issue. 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. 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; there is no evidence of 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. And though Mars may have been more volcanicly active in the past, it appears to never have had any plate tectonics. 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 water on the surface There may have been large lakes or even oceans. But it seems that this occurred only briefly and very long ago; the age of the erosion channels is estimated at about nearly 4 billion years. (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. Mars has permanent ice caps at both poles composed mostly of 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. It's not known if a similar layer of water ice exists below the southern cap since its carbon dioxide layer never completely disappears. 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. The Viking landers performed experiments to determine the existence of life on Mars. The results were somewhat unclear 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 and others announced the first identification of organic compounds in a Martian meteorite. The authors further suggest that these compounds, in conjunction with a number of other mineralogical features observed in the rock, may be evidence of ancient Martian microorganisms. Exciting as this is, it is important to note while this evidence is strong it by no means establishes the fact of extraterrestrial life. There have also bee several contradictory studies published since the McKay paper. Remember, "extraordinary claims require extraordinary evidence." Much work remains to be done before we can be confident of this most extraordinary claim. 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's 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 naked eye. Its apparent brightness varies greatly according to its relative position to the Earth. Mars' Satellites: Mars has two tiny satellites which orbit very close to the surface. Satellite Distance(000km) Radius (km) Mass (kg) Discoverer Date Phobos 9 11 1.08e16 Hall 1877 Deimos 23 6 1.80e15 Hall 1877 Jupiter The Bringer of Jollity Jupiter is the fifth planet from the Sun, the largest in the solar system. Orbit: 778,330,000 km (483,654,262 miles) or (5.20 AU) from Sun Diameter: 142,984 km (88850 miles) Mass: 1.900 x 1027 kg (3.838 x 1027 lbs) In mythology, Jupiter (also known as Jove or 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). If Jupiter were hollow, more than one thousand Earths could fit inside. It also contains more matter than all of the other planets combined. It has a mass of 1.9 x 1027 kg and is 142,800 kilometers (88,736 miles) across the equator. Jupiter possesses 16 satellites, four of which - Callisto, Europa, Ganymede and Io - were observed by Galileo as long ago as 1610. There is a ring system, but it is very faint and totally invisible from the Earth. (The ring was discovered in 1979 by Voyager 1.) The atmosphere is very deep, perhaps comprising the whole planet, and is somewhat like the Sun. It is composed mainly of hydrogen and helium, with small amounts of methane, ammonia, water vapor and other compounds. Deep within Jupiter, the pressure is so great that the hydrogen atoms are broken up and the electrons are freed so that the resulting atoms consist of bare protons. This produces a state in which the hydrogen becomes metallic. Jupiter was first visited by Pioneer 10 in 1973 and later by Pioneer 11, Voyager 1, Voyager 2 and Ulysses. The spacecraft Galileo is currently in orbit around Jupiter and will be sending back data for at least the next two years. Colorful latitudinal bands, atmospheric clouds and storms illustrate Jupiter's dynamic weather systems. The cloud patterns change within hours or days. The Great Red Spot is a complex storm moving in a counter-clockwise direction. At the outer edge, material appears to rotate in four to six days; near the center, motions are small and nearly random in direction. An array of other smaller storms and eddies can be found throughout the banded clouds. Auroral emissions, similar to Earth's northern lights, were observed in the polar regions of Jupiter. The auroral emissions appear to be related to material from Io that spirals along magnetic field lines to fall into Jupiter's atmosphere. Cloud-top lightning bolts, similar to superbolts in Earth's high atmosphere, were also observed. In July 1994, Comet Shoemaker-Levy 9 collided with Jupiter with spectacular results. The effects were clearly visible even with amateur telescopes. The debris from the collision was visible for nearly a year afterward with HST. Jupiter's Ring Unlike Saturn's intricate and complex ring patterns, Jupiter has a single ring that is almost uniform in its structure. It is probably composed of dust particles less than 10 microns in diameter-about the size of cigarette smoke particles. It extends to an outer edge of about 129,000 kilometers (80,161 miles) from the center of the planet and inward to about 30,000 kilometers (18,642 miles). The origin of the ring is probably from micrometeorite bombardment of the tiny moons orbiting within the ring. Jupiter's ring and moons exist within an intense radiation belt of electrons and ions trapped in the planet's magnetic field. These particles and fields comprise the jovian magnetosphere or magnetic environment, which extends 3 to 7 million kilometers (1.9 to 4.3 million miles) toward the Sun, and stretches in a windsock shape at least as far as Saturn's orbit-a distance of 750 million kilometers (466 million miles). 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. Values for the smaller moons are approximate. Jupiter's Ring Distance(km) Width(km) Mass (kg) Halo 100000 22800 ? Main 122800 64000 le13 Gossamer 120200 850000 ? Jupiter's satellites are named for other figures in mythology. Satellite Distanct (000km) Radius (km) Mass (kg) Discoverer Date Metis 128 20 9.57e16 Synott 1979 Adrastea 129 10 1.91e16 Jewitt 1979 Amalthea 181 98 7.17e18 Barnard 1982 Thebe 222 50 7.77e17 Synott 1979 Io 422 1815 8.94e22 Galileo 1610 Europa 671 1569 4.80e22 Galileo 1610 Ganymede 1070 2631 1.48e23 Galileo 1610 Callisto 1883 2400 1.08e23 Galileo 1610 Leda 11094 8 5.68e15 Kowal 1964 Himalia 11480 93 9.56e18 Perrine 1904 Lysithea 11720 18 7.77e16 Nicholson 1938 Elara 11737 38 7.77e17 Perrine 1905 Ananke 21200 15 3.82e16 Nicholson 1951 Carme 22600 20 9.56e16 Nicholson 1938 Pasiphae 23500 25 1.91e17 Melotte 1908 Sinope 23700 18 7.77e16 Nicholson 1914 Saturn Saturn is the sixth planet from the Sun and the second largest: Orbit: 1,429,4000 km (887,220,000 miles) or (9.54 AU) from Sun Diameter: 120,536 km or 74,901 miles (equatorial) Mass: 5.68 x 1026 kg or 12.5244 x 1026 lbs Cassini on its way! The Cassini mission to explore Saturn and its moon Titan was launched successfully on Oct. 15, 1997. The first of its four, planned gravity assist maneuvers has been successfully completed. It will arrive at Saturn on July 1, 2004. 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." 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 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 determined 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 Pioneer 11 in 1979 and later by Voyager 1 and Voyager 2. Cassini, now on its way, will arrive in 2004. Saturn is visibly flattened (oblate) when viewed through a small telescope; its equatorial and polar diameters vary by almost 10 percent (74,901 miles vs. 67,563 miles). This is the result of its rapid rotation and fluid state. The other gas planets are also oblate, but not nearly as much as Saturn. Saturn is the least dense of the planets; its specific gravity (0.7) is less than that of water. If you could find a container big enough and fill it with water, Saturn would float. Like Jupiter, Saturn is about 75 percent hydrogen and 25 percent 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 and consists 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 (12,273 F at the core), and it 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 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. In 1994, another, smaller storm was observed. Two prominent rings (A and B) and one faint ring (C) can be seen from 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 Gap. The Voyager pictures show four additional faint rings. Saturn's rings, unlike the rings of the other planets, are very bright. Though they look continuous from 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 mile-sized objects are also likely. Saturn's rings are very thin. Though they're 155,350 miles or more in diameter, they're no more than one-mile 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 62 miles 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 a puzzling feature in the rings called "spokes" which were first reported by amateur astronomers. 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 is not seen in the Voyager 2 images perhaps because Voyager 2 imaged regions where the component rings are roughly parallel. 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 Gap. The whole system is very complex and as yet, poorly understood. The origin of the rings of Saturn (and the other gas planets) is unknown. Though they may have had rings since their formation, the ring systems are not stable. They must be regenerated by ongoing processes, probably the breakup of larger satellites. Like the other gas planets, Saturn has a significant magnetic field. When it is in the nighttime sky, Saturn is easily visible to the naked 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. The rings and the larger satellites are visible with a small astronomical telescope. Saturn has 18 named satellites, more than any other planet. There may also be several small ones yet to be discovered. Of those moons for which rotation rates are known, all but Phoebe and Hyperion rotate synchronously. The three pairs, Mimas-Tethys, Enceladus-Dione and TitanHyperion, interact gravitationally in such a way as to maintain stable relationships between their orbits. The period of Mimas' orbit is exactly half that of Tethys. They are thus said to be in a 1:2 resonance. Enceladus-Dione are also 1:2; Titan-Hyperion are in a 3:4 resonance. In addition to the 18 named satellites, at least a dozen more have been reported and given provisional designations. Saturn's Satellites Satellite Distance Radius Mass (miles) (miles) (lbs) Pan 83 6 ? Atlas 85 8.5 ? Prometheus 87 28.5 ? Pandora 88 28.5 ? Epimethius 98 35.5 ? Janus 98 55 ? Mimas 115.5 122 ? Enceladus 148 161.5 ? Tethys 183 329 ? Telesto 183 9 ? Calypso 184 8 ? Dione 234 348 2.394 x 10e21 Helene 234 9.5 ? Rhea 327.5 475 5.6772 x 10e21 Titan 759 1600 3.078 x 10e23 Hyperion 920 89 4.0356 x 10e19 Iapetus 2213 453 4.2864 x 10e21 Phoebe 8048 68 9.12 x 10e18 Saturn's Rings Distance Width (miles) (miles) (lbs) D 41634 4660 ? C 46294 10874 2.508 x 10e18 B 57169 15946 6.384 x 10e19 A 75935 9072 14.136 x 10e18 F 87126 311 ? G 103028 4971 2.28 x 10 e 7 E 111852 186420 ? Ring Mass Cassini division Distance is from Saturn's center to the ring's inner edge. This categorization is actually somewhat misleading as the density of particles varies in a complex way not indicated by a division into neat regions. There are variations within the rings, the gaps are not entirely empty and the rings are not perfectly circular. Uranus The Magician 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 (1,784,033,000 miles ) or (19.218 AU) from Sun Diameter: 51,118 km or 31,764 miles (equatorial) Mass: 8.683 x 1025 kg or 17.539 x 1025 lbs 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 the Voyager 2 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 percent 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 percent hydrogen, 15 percent helium and 2 percent methane. Like the other gas planets, Uranus has bands of clouds that blow around rapidly. But the clouds are extremely faint, visible only with radical image enhancement of the Voyager 2 pictures. Recent observations with HST show larger and more unced streaks. The speculation is that the difference is due to seasonal effects (the sun is now at a somewhat lower Uranian latitude which may cause more unced day/night effects). 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 to be discovered after Saturn's. 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 five 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 naked 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. Uranus' Satellites Uranus has 15 named moons plus two recently discovered ones, which have yet to be named. Unlike the other bodies in the solar system which have names from classical mythology, Uranus' moons take their names from the writings of Shakespeare and Pope. They form two distinct classes: the 10 small very dark inner ones discovered by Voyager 2 and the five large outer ones. They all have nearly circular orbits in the plane of Uranus' equator (and hence at a large angle to the plane of the ecliptic). Uranus' Satellites Satellite Radius Distance Mass (km) (000 km) (kg) Discoverer Date Cordelia 50 13 ? Voyager 2 1986 Ophelia 54 16 ? Voyager 2 1986 Bianca 59 22 ? Voyager 2 1986 Cressida 62 33 ? Voyager 2 1986 Desdmona 63 29 ? Voyager 2 1986 Juliet 64 42 ? Voyager 2 1986 Portia 66 55 ? Voyager 2 1986 Rosiland 70 27 ? Voyager 2 1986 Belinda 75 34 ? Voyager 2 1986 Puck 86 77 ? Voyager 2 1985 Miranda 130 266 6.30e19 Kuiper 1948 Ariel 191 579 1.27e21 Lassell 1851 Umbriel 266 585 1.27e21 Lassell 1851 Titania 436 789 3.49e21 Herschel 1787 Oberon 583 761 3.49e21 Herschel 1787 Caliban 7200 30 ? Gladman 1997 Sycorax 12000 60 ? Gladman 1997 Uranus' Rings Ring Distance Width (km) (km) 1986U2R 38000 2,500 6 41840 1-3 5 42230 2-3 4 42580 2-3 Alpha 44720 7-12 Beta 45670 7-12 Eta 47190 0-2 Gamma 47630 1-4 Delta 48290 3-9 1986U1R 50020 1-2 Epsilon 20-100 51140 Neptune Orbit: 4,504,000,000 km or (2,798,785,600 miles) or (30.06 AU) from Sun Diameter: 49,532 km or 30779 miles (equatorial) Mass: 1.0247 x 1026 kg or 2.069 x 1026 lbs Neptune is the eighth planet from the sun and the fourth largest (by diameter). It is smaller in diameter but larger in mass than Uranus. 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. Galle and d'Arrest first observed Neptune on Sept. 23, 1846, 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. Adams and Le Verrier 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. Neptune has been visited by only one spacecraft, Voyager 2, on Aug. 25, 1989. Almost everything we know about Neptune comes from this encounter. Because Pluto's orbit is so eccentric, it sometimes crosses the orbit of Neptune. Since 1979, Neptune has actually been the most distant planet from the sun; Pluto will again be the most distant in 1999. Neptune's composition is probably similar to Uranus': various "ices" and rock with about 15 percent hydrogen and a little helium. Like Uranus, but unlike Jupiter and Saturn, it may not have a distinct internal layering but have a more or less uniform 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 the result of absorption of red light by methane in the atmosphere. 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 2,000 km/hour (1,250 miles/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 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 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. 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 coorbital 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. Neptune's Satellites Neptune has eight known moons -- seven small ones and Triton. Radius Distance Mass Satellite (km) (000 km) (kg) Discoverer Date Naiad 48 29 ? Voyager 2 1989 Thalassa 50 40 ? Voyager 2 1989 Despina 53 74 ? Voyager 2 1989 Galatea 62 79 ? Voyager 2 1989 Larissa 96 ? Voyager 2 1989 Proteus 118 209 ? Voyager 2 1989 Triton 355 1350 2.14e22 Lassell 1946 Nereid 5509 170 ? 1949 74 Kuiper Pluto Orbit: 5,913,520,000 km (3,674,661,000 miles) or (39.5 AU) from Sun Diameter: 2274 km or 1,413 miles (?) Mass: 1.27 x 1022 kg or 2.8022 lbs (?) Pluto is the farthest Solar System body from the sun (usually). Pluto is smaller than seven of the solar system's moons (the moon, Io, Europa, Ganymede, Callisto, Titan and Triton). In Roman mythology, Pluto (Greek: Hades) is the god of the underworld. The planet received this name (after many other suggestions) perhaps because it's so far from the sun that it is in perpetual darkness. Pluto was discovered in 1930 by a “fortunate accident”. Calculations, which later turned out to be in error, had predicted a planet beyond Neptune based on the motions of Uranus and Neptune. Not aware of the error, Clyde W. Tombaugh at Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto anyway. After the discovery of Pluto, it was quickly determined that Pluto was too small to account for the discrepancies in the orbits of the other planets. The search for Planet X continued, but nothing was found at that time. Pluto has a large satellite, or moon, named Charon. It was discovered (in 1978) just before its orbital plane moved edge-on toward the inner solar system. It was therefore possible to observe many transits of Pluto over Charon and vice versa. By carefully calculating which portions of which body would be covered at what times and watching brightness curves, astronomers were able to construct a rough map of light and dark areas on both bodies. Pluto's radius is not well known. JPL's value of 706.5 is given with an error of +/8, almost one percent. Though the sum of the masses of Pluto and Charon is known pretty well (it can be determined from careful measurements of the period and radius of Charon's orbit and Kepler's Third Law), the individual masses of Pluto and Charon are difficult to determine. This would require determining their mutual motions around the center of mass of the system, which requires much finer measurements. They're so small and far away that even HST has difficulty. The ratio of their masses is probably somewhere between 0.084 and 0.157. More observations are underway. Pluto's orbit is highly eccentric. At times it is closer to the sun than Neptune (the last time was from 1979 to 1999). Pluto rotates in the opposite direction from most of the other planets. Pluto is locked in a 3:2 resonance with Neptune; i.e. Pluto's orbital period is exactly 1.5 times longer than Neptune's. Its orbital inclination is also much higher than the other planets. Thus, though it appears that Pluto's orbit crosses Neptune's, it really doesn't; and they will never collide. Like Uranus, the plane of Pluto's equator is at almost right angles to the plane of its orbit. The surface temperature on Pluto is not well known but is probably between 35 and 45 Kelvins (-378 to -396 F). Pluto's composition is unknown, but its density indicates that it is probably a mixture of 70 percent rock and 30 percent water ice much like Triton. The bright areas of the surface seem to be covered with ices of nitrogen with smaller amounts of (solid) methane and carbon monoxide. The composition of the darker areas of Pluto's surface is unknown but may be due to primordial organic material or photochemical reactions driven by cosmic rays. Little is known about Pluto's atmosphere, but it probably consists primarily of nitrogen with some carbon monoxide and methane. It is extremely tenuous, the surface pressure being only a few microbars. Pluto's atmosphere may exist as a gas only when Pluto is near its perihelion; for the majority of Pluto's long year, the atmospheric gases are frozen into ice. Near perihelion, it is likely that some of the atmosphere escapes to space perhaps even interacting with Charon. The unusual nature of the orbits of Pluto and of Triton and the similarity of bulk properties between Pluto and Triton suggest some historical connection between them. It was once thought that Pluto may have once been a satellite of Neptune's, but this now seems unlikely. A more popular theory is that Triton, like Pluto, once moved in an independent orbit around the sun and was later captured by Neptune. Perhaps Triton, Pluto and Charon are the only remaining members of a large class of similar objects the rest of which were ejected into the Oort cloud. Like the Earth's moon, Charon may be the result of a collision between Pluto and another body. Pluto can be seen with an amateur telescope, but it is not easy. Mike Harvey's planet finder charts show the current position of Pluto (and the other planets) in the sky, but much more detailed charts and careful observations over several months will be required to actually find it. Suitable charts can be created with many planetarium programs such as Starry Night. Charon Charon ("KAIR en") is Pluto's largest satellite. Orbit: 19,640 km (12,204 miles) from Pluto Diameter: 1172 km or 728 miles (?) Mass: 1.90 x 1021 or 3.821 lb.. (?) Charon is named for the mythological figure who ferried the dead across the River Styx intolly find it. Suitable charts can be created with many planetarium programs such as Starry Night. Jim Christy discovered Charon in 1978. Prior to that it was thought that Pluto was much larger since the images of Charon and Pluto were blurred together. Charon is unusual in that it is the largest moon with respect to its primary planet in the solar system (a distinction once held by Earth's moon). Charon's radius is not well known. JPL's value of 364 has an error margin of +/13, more than two percent. Its mass and density are also poorly known. Pluto and Charon are also unique in that not only does Charon rotate synchronously, but Pluto does, too. They both keep the same face toward one another. (This makes the phases of Charon as seen from Pluto very interesting.) Charon's composition is unknown, but its low density indicates that it may be similar to Saturn's icy moons (i.e. Rhea). Its surface seems to be covered with water ice. Unlike Pluto, Charon does not have large albedo features, though it may have smaller ones that have not been resolved. It has been proposed that Charon was formed by a giant impact similar to the one that formed Earth's moon. It is doubtful that Charon has a significant atmosphere. In May 2005, an important discovery was made. By studying many images sent by the Hubble Space Telescope, astronomers concluded that Pluto actually has 3 moons. The 2 additional moons were previously unknown. Later named Hydra and Nix, their diameters are close to 100 miles and are on the same orbital plane as Charon, though much farther away from Pluto. Observations of Hydra and Nix continue, with the on-going assistance of the Hubble Space Telescope, to gain further knowledge of these two distant moons of Pluto. Pluto is the only planet that has not yet been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface. So, NASA designed the first mission to explore Pluto, called New Horizons. The spacecraft of the same name was launched in January of 2006, destined to reach Pluto and it’s 3 moons by 2015. New Horizons will unlock many secrets of Pluto and its three satellites. NASA has 7 scientific instruments on board the probe. The instruments will send back much information about the surface properties, geology, interior makeup and atmosphere. There are some who think Pluto would be better classified as a large asteroid or comet rather than as a planet. Some consider it to be the largest of the Kuiper Belt objects. In August 2006, the International Astronomical Union passed a resolution and created 3 new categories to classify planets and other Solar System bodies: 1. A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. 2. A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit and (d) is not a satellite. 3. All other objects orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”. This reclassifies Pluto as “dwarf plant”, based on this new criteria. Some astronomers, educators, students and the public do not agree with this change and believe our Solar System should stay the same as it has been. Others understand that Pluto and it’s 3 satellites haven’t changed; our understanding of the physical features and attributes have changed, due to advanced technology and new discoveries. This recent information allows scientists to recognize differences and see our Solar System in a new way. What do you think about this change?