An Overview of the Solar System

Orbits The solar system consists of the Sun; the eight official planets, at least three "dwarf planets", more than 130 satellites of the planets, a large number of small bodies (the comets and asteroids), and the interplanetary medium. (There are probably also many more planetary satellites that have not yet been discovered.) The inner solar system contains the Sun, Mercury, Venus, Earth and Mars:

The main asteroid belt (not shown) lies between the orbits of Mars and Jupiter. The planets of the outer solar system are Jupiter, Saturn, Uranus, and Neptune (Pluto is now classified as a dwarf planet): The first thing to notice is that the solar system is mostly empty space. The planets are very small compared to the space between them. Even the dots on the diagrams above are too big to be in proper scale with respect to the sizes of the orbits. The orbits of the planets are ellipses with the Sun at one focus, though all except Mercury are very nearly circular. The orbits of the planets are all more or less in the same plane (called the ecliptic and defined by the plane of the Earth's orbit). The ecliptic is inclined only 7 degrees from the plane of the Sun's equator. The above diagrams show the relative sizes of the orbits of the eight planets (plus Pluto) from a perspective somewhat above the ecliptic (hence their non- circular appearance). They all orbit in the same direction (counter-clockwise looking down from above the Sun's north pole); all but Venus, Uranus and Pluto also rotate in that same sense. (The above diagrams show correct positions for October 1996 as generated by the excellent planetarium program Starry Night; there are also many other similar programs available, some free.) Sizes The above composite shows the eight planets and Pluto with approximately correct relative sizes (see another similar composite and a comparison of the terrestrial planets or Appendix 2 for more). One way to help visualize the relative sizes in the solar system is to imagine a model in which everything is reduced in size by a factor of a billion. Then the model Earth would be about 1.3 cm in diameter (the size of a grape). The Moon would be about 30 cm (about a foot) from the Earth. The Sun would be 1.5 meters in diameter (about the height of a man) and 150 meters (about a city block) from the Earth. Jupiter would be 15 cm in diameter (the size of a large grapefruit) and 5 blocks away from the Sun. Saturn (the size of an orange) would be 10 blocks away; Uranus and Neptune (lemons) 20 and 30 blocks away. A human on this scale would be the size of an atom but the nearest star would be over 40000 km away. Not shown in the above illustrations are the numerous smaller bodies that inhabit the solar system: the satellites of the planets; the large number of asteroids (small rocky bodies) orbiting the Sun, mostly between Mars and Jupiter but also elsewhere; the comets (small icy bodies) which come and go from the inner parts of the solar system in highly elongated orbits and at random orientations to the ecliptic; and the many small icy bodies beyond Neptune in the Kuiper Belt. With a few exceptions, the planetary satellites orbit in the same sense as the planets and approximately in the plane of the ecliptic but this is not generally true for comets and asteroids. Classification Hardcopy The New Solar System Summarizes what we've learned from interplanetary explorations in the last 25 years. My primary reference for The Nine Planets. The Compact NASA Atlas of the Solar System This 'road map' of the solar system is the definitive guide for beginning planetary science. The classification of these objects is a matter of minor controversy. Traditionally, the solar system has been divided into planets (the big bodies orbiting the Sun), their satellites (a.k.a. moons, variously sized objects orbiting the planets), asteroids (small dense objects orbiting the Sun) and comets (small icy objects with highly eccentric orbits). Unfortunately, the solar system has been found to be more complicated than this would suggest: • there are several moons larger than Pluto and two larger than Mercury; • there are many small moons that are probably started out as asteroids and were only later captured by a planet; • comets sometimes fizzle out and become indistinguishable from asteroids; • the Kuiper Belt objects (including Pluto) and others like Chiron don't fit this scheme well • The Earth/Moon and Pluto/Charon systems are sometimes considered "double planets". Other classifications based on chemical composition and/or point of origin can be proposed which attempt to be more physically valid. But they usually end up with either too many classes or too many exceptions. The bottom line is that many of the bodies are unique; the actual situation is too complicated for simple categorization. In the pages that follow, I will use the conventional categorizations. The eight bodies officially categorized as planets are often further classified in several ways: • by composition: ○ terrestrial or rocky planets: Mercury, Venus, Earth, and Mars:  The terrestrial planets are composed primarily of rock and metal and have relatively high densities, slow rotation, solid surfaces, no rings and few satellites. ○ jovian or gas planets: Jupiter, Saturn, Uranus, and Neptune:  The gas planets are composed primarily of hydrogen and helium and generally have low densities, rapid rotation, deep atmospheres, rings and lots of satellites. • by size: ○ small planets: Mercury, Venus, Earth, Mars.  The small planets have diameters less than 13000 km. ○ giant planets: Jupiter, Saturn, Uranus and Neptune.  The giant planets have diameters greater than 48000 km. ○ The giant planets are sometimes also referred to as gas giants. • by position relative to the Sun: ○ inner planets: Mercury, Venus, Earth and Mars. ○ outer planets: Jupiter, Saturn, Uranus, Neptune. ○ The asteroid belt between Mars and Jupiter forms the boundary between the inner solar system and the outer solar system. • by position relative to Earth: ○ inferior planets: Mercury and Venus.  closer to the Sun than Earth.  The inferior planets show phases like the Moon's when viewed from Earth. ○ Earth. ○ superior planets: Mars thru Neptune.  farther from the Sun than Earth.  The superior planets always appear full or nearly so. • by history: ○ classical planets: Mercury, Venus, Mars, Jupiter, and Saturn.  known since prehistorical times  visible to the unaided eye  in ancient times this term also refered to the Sun and the Moon; the order was usually specificied as: Saturn, Jupiter, Mars, Sun, Venus, Mercury and Moon, based on the time for them to go "all the way round" the sphere of the "fixed" stars). ○ modern planets: Uranus, Neptune.  discovered in modern times  visible only with optical aid ○ Earth. ○ The IAU has recently decided that "classical" should refer to all eight planets (Mercury thru Neptune, including Earth but not Pluto). This is contrary to historical usage but makes some sense from a 21st century perspective. Pictures Note: most of the images in The Nine Planets are not true color. Most of them were created by combining several black and white images taken thru various color filters. Though the colors may look "right" chances are they aren't exactly what your eye would see. • The Nine Planets montage (larger version of the above) 36k jpg • Another relative size comparison (from LANL) 93k gif • Sun and large planet comparison (from Extrema) 15k jpg • Earth and small body comparison (from Extrema) 13k jpg • Voyager 1 mosaic of the solar system from 4 billion miles out 36k jpg; html (caption) • Voyager 1 images of 6 planets from 4 billion miles out 123k jpg; html • Pale Blue Dot , reflections on the above image by Carl Sagan. More General Overview • The largest, smallest, brightest, etc bodies • The history of solar system discovery • Solar System Introduction from LANL • Solar System Family Portrait from NSSDC • Solar System Live , the interactive Orrery of the Web. • notes about the most distant object in the solar system and the surface temperatures of the planets from RGO • scale models of the solar system ○ A Solar System Scale Model Meta Page (links to many others) ○ Lakeview Museum Community Solar System , the world's largest model of the solar system ○ scale model from LPI ○ Sagan Planet Walk in Ithaca, NY ○ Build a Solar System , a neat scale model calculator ○ Silver City, NM Sidewalk Solar System ○ Solar System Walk in Gainesville, Florida ○ Eugene Oregon Scale Model Solar System ○ Bonsall Elementary ○ PlanetTrek , a solar system scale model for Pasadena • Walk the Solar System , a nice size comparison calculator from the Exploratorium • Your Weight on Other Worlds , another neat calculator from the Exploratorium • ConverTable Planets , a nice little Macintosh app to calculate your weight on other planets • Galileo SSI Education Module on Planetary Surfaces • a good bibliography of print material about the solar system The Big Questions • What is the origin of the solar system? It is generally agreed that it condensed from a nebula of dust and gas. But the details are far from clear. • How common are planetary systems around other stars? There is now good evidence of Jupiter-sized objects orbiting several nearby stars. What conditions allow the formation of terrestrial planets? It seems unlikely that the Earth is totally unique but we still have no direct evidence one way or the other. • Is there life elsewhere in the solar system? If not, why is Earth special? • Is there life beyond the solar system? Intelligent life? • Is life a rare and unusual or even unique event in the evolution of the universe or is it adaptable, widespread and common? Answers to these questions, even partial ones, would be of enormous value. Answers to the lesser questions on the pages that follow may help answer some of these big ones. Our Sun is a normal main-sequence G2 star, one of more than 100 billion stars in our galaxy. diameter: 1,390,000 km. mass: 1.989e30 kg temperature: 5800 K (surface) 15,600,000 K (core) 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). It is often said that the Sun is an "ordinary" star. That's true in the sense that there are many others similar to it. But there are many more smaller stars than larger ones; the Sun is in the top 10% by mass. The median size of stars in our galaxy is probably less than half the mass of the Sun. The Sun is personified in many mythologies: the Greeks called it Helios and the Romans called it Sol. The Sun is, at present, about 70% hydrogen and 28% helium by mass everything else ("metals") amounts to less than 2%. This changes slowly over time as the Sun converts hydrogen to helium in its core. The outer layers of the Sun exhibit differential rotation: at the equator the surface rotates once every 25.4 days; near the poles it's as much as 36 days. This odd behavior is due to the fact that the Sun is not a solid body like the Earth. Similar effects are seen in the gas planets. The differential rotation extends considerably down into the interior of the Sun but the core of the Sun rotates as a solid body. Conditions at the Sun's core (approximately the inner 25% of its radius) are extreme. The temperature is 15.6 million Kelvin and the pressure is 250 billion atmospheres. At the center of the core the Sun's density is more than 150 times that of water. The Sun's energy output (3.86e33 ergs/second or 386 billion billion megawatts) is produced by nuclear fusion reactions. Each second about 700,000,000 tons of hydrogen are converted to about 695,000,000 tons of helium and 5,000,000 tons (=3.86e33 ergs) of energy in the form of gamma rays. As it travels out toward the surface, the energy is continuously absorbed and re-emitted at lower and lower temperatures so that by the time it reaches the surface, it is primarily visible light. For the last 20% of the way to the surface the energy is carried more by convection than by radiation. The surface of the Sun, called the photosphere, is at a temperature of about 5800 K. Sunspots are "cool" regions, only 3800 K (they look dark only by comparison with the surrounding regions). Sunspots can be very large, as much as 50,000 km in diameter. Sunspots are caused by complicated and not very well understood interactions with the Sun's magnetic field. A small region known as the chromosphere lies above the photosphere. The highly rarefied region above the chromosphere, called the corona, extends millions of kilometers into space but is visible only during a total solar eclipse (left). Temperatures in the corona are over 1,000,000 K. It just happens that the Moon and the Sun appear the same size in the sky as viewed from the Earth. And since the Moon orbits the Earth in approximately the same plane as the Earth's orbit around the Sun sometimes the Moon comes directly between the Earth and the Sun. This is called a solar eclipse; if the alignment is slighly imperfect then the Moon covers only part of the Sun's disk and the event is called a partial eclipse. When it lines up perfectly the entire solar disk is blocked and it is called a total eclipse of the Sun. Partial eclipses are visible over a wide area of the Earth but the region from which a total eclipse is visible, called the path of totality, is very narrow, just a few kilometers (though it is usually thousands of kilometers long). Eclipses of the Sun happen once or twice a year. If you stay home, you're likely to see a partial eclipse several times per decade. But since the path of totality is so small it is very unlikely that it will cross you home. So people often travel half way around the world just to see a total solar eclipse. To stand in the shadow of the Moon is an awesome experience. For a few precious minutes it gets dark in the middle of the day. The stars come out. The animals and birds think it's time to sleep. And you can see the solar corona. It is well worth a major journey. The Sun's magnetic field is very strong (by terrestrial standards) and very complicated. Its magnetosphere (also known as the heliosphere) extends well beyond Pluto. 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 propagates throughout the solar system at about 450 km/sec. The solar wind and the much higher energy particles ejected by solar flares can have dramatic effects on the Earth ranging from power line surges to radio interference to the beautiful aurora borealis. Recent data from the spacecraft Ulysses show that during the minimum of the solar cycle the solar wind emanating from the polar regions flows at nearly double the rate, 750 kilometers per second, than it does at lower latitudes. The composition of the solar wind also appears to differ in the polar regions. During the solar maximum, however, the solar wind moves at an intermediate speed. Further study of the solar wind will be done by the recently launched Wind, ACE and SOHO spacecraft from the dynamically stable vantage point directly between the Earth and the Sun about 1.6 million km from Earth. The solar wind has large effects on the tails of comets and even has measurable effects on the trajectories of spacecraft. Spectacular loops and prominences are often visible on the Sun's limb (left). The Sun's output is not entirely constant. Nor is the amount of sunspot activity. There was a period of very low sunspot activity in the latter half of the 17th century called the Maunder Minimum. It coincides with an abnormally cold period in northern Europe sometimes known as the Little Ice Age. Since the formation of the solar system the Sun's output has increased by about 40%. 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 (although its luminosity will approximately double in that time). But eventually it will run out of hydrogen fuel. It will then be forced into radical changes which, though commonplace by stellar standards, will result in the total destruction of the Earth (and probably the creation of a planetary nebula). The Sun's satellites There are eight planets and a large number of smaller objects orbiting the Sun. (Exactly which bodies should be classified as planets and which as "smaller objects" has been the source of some controversy, but in the end it is really only a matter of definition. Pluto is no longer officially a planet but we'll keep it here for history's sake.) Distance Radius Mass Planet (000 km) (km) (kg) Discoverer Date ------Mercury 57,910 2439 3.30e23 Venus 108,200 6052 4.87e24 Earth 149,600 6378 5.98e24 Mars 227,940 3397 6.42e23 Jupiter 778,330 71492 1.90e27 Saturn 1,426,940 60268 5.69e26 Uranus 2,870,990 25559 8.69e25 Herschel 1781 Neptune 4,497,070 24764 1.02e26 Galle 1846 Pluto 5,913,520 1160 1.31e22 Tombaugh 1930

More detailed data and definitions of terms can be found on the data page. More about the Sun Top of Form

w w w .nineplanet Search

Web www.nineplanets.org pub-5844607605 1 9930379608 ISO-8859-1 ISO-8859-1

active GALT:#555555;G en

Bottom of Form • more Sun images • from NSSDC • Stanford Solar Center • Yohkoh Public Outreach Project , lots of good info, images and movies • The University of Michigan Solar and Heliospheric Research Group's Web Space for Kids and Non-Scientists • Today's Space Weather and index of solar images • Solar Data Analysis Center • Elemental abundances in the Sun • Solar Eclipse info ○ from Fred Espenak, Mr. Eclipse ○ from Solar Data Analysis Center ○ Sky Online's Eclipse Page ○ Dale Ireland's Eclipses ○ images by Bob Yen ○ Umbraphilia , a personal account of the 1998 eclipse in the Caribbean • National Solar Observatory / Sacramento Peak Image Index • more info and links about sunspots • historical info about sunspots • Virtual Tour of the Sun by Michael Berger • The Sun: a Pictorial Introduction , a slide set by P. Charbonneau and O.R. White • The HK Project • Ulysses Home Page • Spartan 201 , NASA's mission to explore the Sun's corona • IACG Campaign IV : including lots of good references • Lives and Deaths of Stars ; notes by Nick Strobel of the University of Washington • ESA/NASA's SOHO - Solar and Heliospheric Observatory home page • articles by John Bahcall, many focusing on the solar neutrino problem • Interview with Sol by Robert J. Nemiroff • Solar Folklore , various myths about the Sun 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 The New Solar System Summarizes what we've learned from interplanetary explorations in the last 25 years. My primary reference for The Nine Planets. In Search of Planet Vulcan An account of the non-discovery of a planet interior to Mercury. A much more interesting tale than you might imagine. 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 replenished. 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 radius).

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 the Moon. 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. More about Mercury Top of Form

Bottom of Form • more Mercury images • from NSSDC • Mariner 10 Image Project • from StarDate • from RGO • from NSSDC • Mercury Unveiled , a new look at the Mariner 10 data • Mercury Nomenclature Table • (low res) images from the 0.5 m Swedish Vacuum Solar Telescope • ground based images from Boston University • more ground-based images • Mercury Studies , ongoing research • Degas Crater and some links Open Issues • Mercury's density (5.43 gm/cm3) is nearly as high as Earth's. Yet in most other respects it more closely resembles the Moon. Did it lose its light rocks in some early catastrophic impact? • No trace of iron has been seen in spectroscopic studies of Mercury's surface. Given its presumably large iron core this is very odd. Is Mercury much more completely differentiated than the other terrestrial planets? • What processes produced Mercury's smooth plains? • Are there any surprises on the other half of the surface we've not seen? Low resolution radar images obtained from Earth show no surprises, but you never know. • ESA may also build a Mercury orbiter called BepiColombo but it will launch no sooner than 2012. 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 The New Solar System

Summarizes what we havve learned from interplanetary explorations in the last 25 years. My primary reference for The Nine Planets. Venus Revealed The latest results from Magellan in an accessible and easygoing book. Covers mythology and history of our "sister planet" as well as up to date science and a history of the Magellan project. Venus in Transit

Fascinating account of past transits and the woes that befell those involved.

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's heliocentric theory of the solar system.

Venera 9 surface photo 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 Soviet Venera 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

Magellan radar map (false color) 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.

Venus in visible light from Galileo 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-away greenhouse 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.

Venus in ultra-violet light 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.

Sif Mons (Magellan radar) 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.

Coronae

Pancake volcanoes 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. More about Venus Top of Form Bottom of Form • more Venus images • from NSSDC • Venus Revealed Images by David Grinspoon • Magellan mission home page • Venusian Craters and Venusian Volcanic Features from LANL • Magellan Venus Explorer's Guide from JPL; about Magellan and Venus prior to the Magellan data • Guide to Magellan Image Interpretation from JPL • The Magellan Spacecraft at Venus by Andrew Fraknoi of ASP • Venus Nomenclature Table • Magellan Image Browser • maps of Venus, Mars and the Moon Open Issues • There is some evidence of spreading and folding on Venus' surface and of recent volcanic flows. But there is no evidence of plate tectonics as seen on Earth. Is this a result of the higher surface temperature? • The greenhouse effect is much stronger on Venus than Earth because of Venus' dense carbon dioxide atmosphere. But why did Venus evolve so differently from Earth?