Introduction To The Solar System Environmental Sciences Essay

Introduction To The Solar System Environmental Sciences Essay A. This essay will briefly describe the planets and how they relate to the planet Earth. The surface and inner geology, the atmosphere, and other general properties will show how the other planets are not unlike the Earth. B. How do the unique characteristics of each major solar system body compare with the planet Earth primarily the mass and density, and the composition? 2. The Planets Other Objects. The charted regions of the Solar System consist of the Sun, four terrestrial inner planets, an asteroid belt composed of small rocky bodies, four gas giant outer planets, and a second belt, called the Kuiper belt, composed of icy objects. Beyond the Kuiper belt is hypothetical Oort cloud. The inner Solar System is the traditional name for the region comprising the terrestrial planets and asteroids. Composed mainly of silicates and metals, the objects of the inner Solar System crowd very closely to the Sun; the radius of this entire region is shorter than the distance between Jupiter and Saturn. The four inner or terrestrial planets have dense, rocky compositions, few or no moons, and no ring systems. They are composed largely of minerals with high melting points, such as the silicates which form their solid crusts and semi-liquid mantles, and metals such as iron and nickel, which form their cores. Three of the four inner planets (Venus, Earth and Mars) have significant atmospheres; all have impact craters and tectonic surface features such as rift valleys and volcanoes. Our probe, the ESP begins the exploration of the solar system with the third planet from the sun, the Earth and the fifth largest in our solar system. Astronomers usually measure distances within the Solar System in astronomical units (AU). One AU is the approximate distance between the Earth and the Sun or roughly 149,598,000 km (93,000,000 mi). A. The Earth. The mass of the Earth is 5.98 E24 kg with a mean density of 5,520 kg/m3 and the densest of any planet in the solar system. Earths diameter is just a few hundred kilometers larger than that of Venus, and considered our sister planet. Earth is the largest of the inner planets, the only one planet known to have current geological activity, although there are moons of Jupiter and Saturn that have seismic activity, and the only planet known to have life. Its liquid hydrosphere is unique among the terrestrial planets, and it is also the only planet where plate tectonics has been observed, unlike Venus where there is no evidence of plate tectonics. Earths atmosphere is radically different from those of the other planets, having been altered by the presence of life (in two oxygen generating events) to contain 21% free oxygen. It has one satellite, the Moon, the only large satellite of a terrestrial planet in the Solar System so large as compared to its planet. No other moon-pla net has this size ratio. The four seasons are a result of Earths axis of rotation being tilted 23.45 degrees with respect to the plane of Earths orbit around the sun. During part of the year, the northern hemisphere is tilted toward the sun and the southern hemisphere is tilted away, producing summer in the north and winter in the south. Six months later, the situation is reversed. During March and September, when spring and fall begin in the northern hemisphere, both hemispheres receive nearly equal amounts of solar illumination. Earths global ocean, which covers nearly 70 percent of the planets surface, has an average depth of about 4 km (2.5 miles). Fresh water exists in the liquid phase only within a narrow temperature span, 32 to 212 degrees Fahrenheit (0 to 100 degrees Celsius). The presence and distribution of water vapor in the atmosphere is responsible for much of Earths weather. The Earths rapid rotation and molten nickel-iron core create the magnetic field which prevents the solar wind from reaching the surface (the solar wind is a stream of charged particles continuously ejected from the sun.) The Earths magnetic field does not fade off into space, but has definite boundaries. When charged particles from the solar wind become trapped in Earths magnetic field, they collide with air molecules above our planets magnetic poles. These air molecules then begin to glow, and are known as the aurora the northern and southern lights. Earths lithosphere, which includes the crust (both continental and oceanic) and the upper mantle, is divided into huge plates that are constantly moving, and the movement is accurately determined via radio telescopes from a stationary point such as a star . Earthquakes result when plates grind past one another, ride up over one another, collide to make mountains, or split and separate. The theory of motion of the large plates of the li thosphere is known as plate tectonics. Developed within the last 40 years, this explanation has unified the results of centuries of study of our planet. The Earths atmosphere consists of 78 percent nitrogen, 21 percent oxygen and 1 percent argon and other trace ingredients. The atmosphere affects Earths long-term climate and short-term local weather, shields us from much of the harmful radiation coming from the sun and protects us from meteors as well, most of which burn up before they can strike the surface as meteorites. Before the ESP leaves the immediate vicinity of the Earth, ESP will begin the journey starting with Earths Moon approximately 250,000 miles away. B. The Moon. The Earths moon provides a more livable planet by moderating our home planets wobble on its axis, leading to a relatively stable climate, and creating a rhythm that has guided humans for thousands of years. The Moon was likely formed after a Mars-sized body collided with Earth approximately 4.5 billion years ago, and the resulting debris accumulated (or accreted) to form our natural satellite. The newly formed Moon was in a molten state. Within about 100 million years, most of the global magma ocean had crystallized, with less dense rocks floating upward and eventually forming the lunar crust. The moons surface shows four significant impact structures and are used to date objects on the Moon; are called the Nectaris and Imbrium basins and the craters Eratosthenes and Copernicus. The Moon was first visited by the USSRs Luna 1 and Luna 2 in 1959. These were followed by a number of U.S. and Soviet robotic spacecraft. The U.S. sent three classes of robotic missions to prepare the way for human exploration, the Rangers (1961-1965) were impact probes, the Lunar Orbiters (1966-1967) mapped the surface to find landing sites and the Surveyors (1966-1968) were soft landers. The first human landing on the Moon was on 20 July 1969. During the Apollo missions of 1969-1972, 12 American astronauts walked on the Moon and used a Lunar Roving Vehicle to travel on the surface to investigate soil mechanics, meteoroids, lunar ranging, magnetic fields and the solar wind. The Apollo astronauts brought back 382 kg (842 pounds) of rock and soil to Earth for study. The Moon has no internally generated magnetic field, although areas of magnetism are preserved in the lunar crust, but how this occurred remains a mystery to science. The early Moon appears not to have had the right conditions to develop an internal dynamo, the mechanism for global magnetic fields for the terrestrial planets; so an iron-core did not form or have the ability for motion. In retrospect, no magnetic field may be a good thing as perhaps there would be some interactions between the Earths magnetic filed and the moons, when considering the abnormal size ratio between these bodies. With no atmosphere to impede impacts, a steady rain of asteroids, meteoroids and comets strike the surface. Over billions of years, the surface has been ground up into fragments ranging from huge boulders to powder. Nearly the entire Moon is covered by a rubble pile of gray, powdery dust and rocky debris called the lunar regolith. Beneath the regolith is a region of fractured bedrock referred to as the megaregolith. The ESP now leaves the Earth to journey toward the sun and visit the second closet to the sun, Venus our sister planet. C. Venus. From the Earth, the distance to Venus is about 23 million miles, and 0.723 AU from the sun. The orbital period of Venus is about 225 Earth days long, while the planets sidereal rotation period is 243 Earth days, making a Venus solar day about 117 Earth days long. Venus has no natural satellites. The mass of Venus is 4.87 E24 kg and close in size to Earth (0.815 Earth masses) and, like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere and evidence of internal geological activity. Because of the similar silicate mantle around an iron corer, the density is not unlike the Earths at 5,250 kg/m2. The slow rotation of Venus cannot generate a magnetic field similar to Earths, though its iron core is similar to that of the Earth and approximately 3,000 km (1,900 miles) in radius. Venus rotates retrograde (east to west) compared with Earths (west to east) rotation. Seen from Venus, the sun would rise in the west and set in the east. Current thinking suggests that Venus was completely resurfaced by volcanic activity 300 to 500 million years ago. More than 1,000 volcanoes or volcanic centers larger than 20 km (12 miles) in diameter dot the surface. Volcanic flows have produced long, channels extending for hundreds of kilometers. Venus has two large highland areas: Ishtar Terra, about the size of Australia, in the North Polar Region; and Aphrodite Terra, about the size of South America, straddling the equator and extending for almost 10,000 km (6,000 miles). Maxwell Montes, the highest mountain on Venus and comparable to Mount Everest on Earth, is at the eastern edge of Ishtar Terra. No definitive evidence of current geological activity has been detected on Venus, but as mentioned it has no magnetic field that would prevent depletion of its substantial atmosphere, which suggests that its atmosphere is regularly replenished by volcanic eruptions. Venus atmosphere consists mainly of carbon dioxide, with clouds of sulfuric acid droplets with trace amounts of water detected in the atmosphere (96% carbon dioxide, 3% nitrogen, and 0.1% water vapor.) The atmosphere is much drier than Earth and ninety times as dense. It is the hottest planet, with surface temperatures over 400 Â °C, most likely due to the amount of greenhouse gases in the atmosphere. The thick atmosphere traps the suns heat, resulting in surface temperatures higher than 880 degrees Fahrenheit (471 degrees Celsius). Probes that have landed on Venus survived only a few hours before being destroyed by the incredible temperatures. Sulfur compounds are abundant in Venus clouds. The corrosive chemistry and dense, moving atmosphere cause significant surface weathering and erosion. Atmospheric lightning bursts were confirmed in 2007 by the European Venus Express orbiter. On Earth, Jupiter and Saturn, lightning is associated with water clouds, but on Venus, it is associated with clouds of sulfuric acid. As we leave the Venusian orbit, Earths probe ESP continues toward the sun and onward Mercury. D. Mercury. The closest planet to the Sun and the smallest planet (0.055 Earth masses), Mercury is 0.387 AU from the sun. Mercury has no natural satellites, and its mass is 3.30 E23 kg with an average density of 5,420 kg/m3. The similarity of the rocky terrestrial planets is apparent. Mercurys surface resembles that of Earths Moon, scarred by many impact craters resulting from collisions with meteoroids and comets. While there are areas of smooth terrain, there are also scarps or cliffs, some hundreds of miles long and soaring up to a mile high, formed by contraction of the crust. Mercury is the second densest planet after Earth, with a large metallic core having a radius of 1,800 to 1,900 km (1,100 to 1,200 miles), about 75 percent of the planets radius (Earths core is many times smaller compared to the planets diameter). In 2007, researchers using ground-based radars to study the core found evidence that it is molten (liquid). Mercurys outer shell, comparable to Earths outer shell (called the mantle), is only 500 to 600 km (300 to 400 miles) thick. The only known geological features besides impact craters are wrinkle-ridges, probably produced by a period of contraction early in its history. The Caloris Basin, one of the largest features on Mercury, is about 1,550 km (960 miles) in diameter. It was the result of a possible asteroid impact on the planets surface early in the solar systems history. Mercurys almost negligible atmosphere consists of atoms blasted off its surface by the solar wind. Though Mercurys magnetic field has just 1 percent the strength of Earths, the field is very active. The magnetic field in the solar wind creates intense magnetic tornadoes that channel the fast, hot solar wind plasma down to the surface. When these ions strike the surface, they knock off neutral atoms and send them high into the sky where other processes may fling them back to the surface or accelerate them away from Mercury. As we leave Mercury before heading out to the deepest regions of the solar system, the ESP will make a fly-by of the sun, as the voyager probes did around Jupiter and Saturn to increase the velocity. E. Our Sun. The principal component of the Solar System is the Sun that contains 99.86% of the systems known mass and dominates it gravitationally. Jupiter and Saturn, the Suns two largest orbiting bodies, account for more than 90% of the systems remaining mass. Most large objects in orbit around the Sun lie near the plane of Earths orbit, known as the ecliptic. The planets are very close to the ecliptic while comets and Kuiper belt objects are usually at significantly greater angles to it. The orbits of the planets are nearly circular, but many comets, asteroids and objects of the Kuiper belt follow highly-elliptical orbits. The probe ESP circles the sun picking up velocity to begin the voyage to Mars again passing the terrestrial planets. F. The Red Planet, Mars. Mars is smaller than Earth and Venus (0.107 Earth masses) has a mass of 6.42 E23 kg and a mean density of 3,940 kg/m3 (lower than that of the other terrestrial planets,) and is 1.524 AU from the sun. Mars is a cold desert-like world similar to our Southwestern States, and has the same amount of dry land. Like Earth, Mars has seasons, polar ice caps, volcanoes, canyons and weather, but its atmosphere is too thin for liquid water to exist for long on the surface. There are signs of ancient floods on Mars, but evidence for water now exists mainly in icy soil and thin clouds. Mars has two tiny natural satellites Deimos and Phobos thought to be captured asteroids. Mars experiences seasons because of the tilt of its rotational axis (in relation to the plane of its orbit). Mars orbit is slightly elliptical, so its distance to the sun changes, affecting the Martian seasons that last longer than those of Earth. The polar ice caps on Mars grow and recede with the seaso ns; layered areas near the poles suggest that the planets climate has changed more than once. Mars is a rocky body about half the size of Earth. As with the other terrestrial planets (Mercury, Venus and Earth) the surface of Mars has been altered by volcanism, impacts, crustal movement, and atmospheric effects such as dust storms. Volcanism in the highlands and plains was active more than 3 billion years ago, but some of the giant shield volcanoes are younger, having formed between 1 and 2 billion years ago. Mars has the largest volcanic mountain in the solar system, Olympus Mons, as well as a spectacular equatorial canyon system, Valles Marineris. Mars has no global magnetic field, but NASAs Mars Global Surveyor orbiter found that areas of the Martian crust in the southern hemisphere are highly magnetized. Evidently, these are traces of a magnetic field that remain in the planets crust from about 4 billion years ago. Mars often appears reddish due to a combination of the fact that its surface is comprised of iron-rich minerals that rust (or oxidize) and that the dust made of these minerals is kicked up into the atmosphere, giving the atmosphere a reddish hue as well. Mars possesses an atmosphere of mostly carbon dioxide (seems like a natural tendency of the terrestrial planets), and other gases (nitrogen 3%, and argon 1.6 %.) The thin atmosphere on Mars does not allow liquid water to exist at the surface for long, and the quantity of water required to carve Mars great channels and flood plains is not obvious today. Unraveling the story of water on Mars is important to unlocking its climate history, which will help us understand the evolution of all the planets. Water is believed to be an essential ingredient for life; evidence of past or present water on Mars is expected to hold clues about whether Mars could ever have been a habitat for life. In summary, there is evidence and good science that large quantities of water may still be present below the surface. Scientists believe that Mars experienced huge floods about 3.5 billion years ago, though it is not know where the ancient flood water came from, how long it lasted or where it went, recent missions to Mars have uncovered exciting evidence. In 2002, NASAs Mars Odyssey orbiter detected hydrogen-rich polar deposits, indicating large quantities of water ice close to the surface. Further observations found hydrogen in other areas as well. If water ice permeated the entire planet, Mars could have substantial subsurface layers of frozen water, and if true, the long-term colonization of Mars is probable. In 2004, the Mars Exploration Rover named Opportunity found structures and minerals indicating that liquid water was once present at its landing site. The rovers twin, Spirit, also found the signature of ancient water near its landing site halfway around Mars from Opportunitys location. Recently, in August 2012, the probe Curiosity made another surface landing in a crater and being the first nuclear-powered probe. Leaving Mars orbit and the terrestrial planets, ESP moves further from the sun to explore the left-over remains from the formation of the solar system, the Asteroid belt. G. The Asteroids Belt. These small Solar System bodies are mostly composed of rocky and metallic non-volatile minerals. Tens of thousands of these minor planets and small rocky bodies are gathered in the main asteroid belt, a vast doughnut-shaped ring between the orbits of Mars and Jupiter. Asteroids that pass close to Earth are called Near-Earth Objects (NEOs). The main asteroid belt occupies the orbit between Mars and Jupiter, and is between 2.3 and 3.3 AU from the Sun. It is thought to be remnants from the Solar Systems formation that failed to coalesce because of the gravitational interference of Jupiter. Asteroids range in size from hundreds of kilometers across to microscopic. Despite this, the total mass of the main belt is unlikely to be more than a thousandth of that of the Earth. The main belt is very sparsely populated; spacecraft routinely pass through without incident. Asteroids with diameters between 10 and 10-4 m are called meteoroids. Asteroid groups in the main belt are divided into groups and families based on their orbital characteristics. Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. The asteroid belt also contains main-belt comets which may have been the source of Earths water. The inner Solar System is also dusted with rogue asteroids, many of which cross the orbits of the inner planets. The three broad composition classes of asteroids are C-, S- and M-types. The C-type asteroids (carbonaceous) are most common, and probably consist of clay and silicate rocks and are dark in appearance. C-type asteroids are among the most ancient objects in our solar system. The S-types (silicaceous) are made up of silicate (stony) materials and nickel-iron. M-types (metallic) are made up of nickel-iron. The asteroids compositional differences are related to how far from the sun they formed. Some experienced high temperatures after they formed and partly melted, with iron sinking to the center and forcing basaltic (volcanic) lava to the surface. One such asteroid, Vesta, survives to this day. Ceres is 2.77 AU from the sun, is the largest body in the asteroid belt, and considered a dwarf planet. It has a diameter of slightly less than 1000 km, large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in the 19th century, bu t was reclassified as an asteroid in the 1850s as further observation revealed additional asteroids. It was again reclassified in 2006 as a dwarf planet along with Pluto. Leaving the left-over rubble of the Asteroid belt ESP now begins s very long journeys as did the Voyager, and Cassini probes and visit the four outer planets, or gas giants (sometimes called Jovian planets), and collectively make up 99 percent of the mass known to orbit the Sun. H. The Gas giants Jupiter. Jupiter and Saturns atmospheres are largely hydrogen and helium. Uranus and Neptunes atmospheres have a higher percentage of ices, such as water, ammonia and methane. Some astronomers suggest they belong in their own category, ice giants. All four gas giants have rings, although only Saturns ring system is easily observed from Earth. Our probe ESP approaches Jupiter at an average distance of 5.203 AU from the sun we are now in the region of deep space. Jupiter at 318 Earth masses has 2.5 times the mass of all the other planets put together, and an average density of 1,314 kg/m3. It is composed largely of hydrogen and helium. Jupiters internal heat creates semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. On 7 January 1610, using a telescope (probably the first) he constructed, astronomer Galileo Galilei saw four small stars as he first thought near Jupiter. He had discovered Jupiters four largest moons, now called Io, Europa, Ganymede, and Callisto. These four moons are known today as the Galilean satellites. In retrospect, Jupiter has sixty-three known satellites, and show similarities to the terrestrial planets, such as volcanism and internal heating. Galileos surprise and illumination is understood. In 2004, while looking through a small Meade reflecting telescope, Jupiters four largest moons were visible as they were all in a straight line moving around the planets equatorial plane. For the first time ever, I gazed at four moons in the solar system other than our own, and it was an amazing sight. Looking at Jupiter from an Earth or near-orbit telescope or planetary probe, the apparent surface and appearance is a blend of striking colors and atmospheric features. Most visible clou ds are composed of ammonia, and water vapor exists deep below and can sometimes be seen through clear spots in the clouds. The planets stripes are dark belts and light zones are created by strong east-west winds in Jupiters upper atmosphere. The Great Red Spot, a giant spinning storm, has been observed since the 1800s, and in recent years, three storms merged to form the Little Red Spot, about half the size of the Great Red Spot. In December 1995, NASAs Galileo spacecraft dropped a probe into Jupiters atmosphere, which made the first direct measurements of the planets atmosphere, and began a multiyear study of Jupiter and the largest moons. The magnetic field of Jupiter and is nearly 20,000 times as powerful as Earths. Trapped within Jupiters magnetosphere (the area in which magnetic field lines encircle the planet from pole to pole) are swarms of charged particles. Jupiters rings and moons are embedded in an intense radiation belt of electrons and ions trapped by the magnetic field, and perhaps a moon landing is possible in the future, but protection from this radiation will be necessary. Jupiters atmosphere is similar to that of the sun, and the composition is mostly hydrogen and helium. Deep in the atmosphere, the pressure and temperature increase, compressing the hydrogen gas into a liquid. At further depths, the hydrogen becomes metallic and electrically conducting. In this metallic layer, Jupiters powerful magnetic field is generated by electrical currents driven by Jupiters fast rotation (9.8 Earth hours.) At the center, the immense pressure may support a solid core of rock about the size of Earth. Jupiters Galilean Satellites. Io is the most volcanically active body in the solar system and the surface is covered by sulfur in different multi-colored forms. As Io travels in its slightly elliptical orbit, Jupiters immense gravity causes tides in the solid surface that rise 100 m (300 feet) high on Io, generating enough heat for volcanic activity and to drive off any water. Ios volcanoes are driven by hot silicate magma. Europas surface is mostly water ice, and there is evidence that it may be covering an ocean of water or ice beneath. Europa is thought to have twice as much water as does Earth, and intrigues scientists because of its potential for having a habitable zone. Life forms have been found thriving near subterranean volcanoes on Earth and in other extreme locations that may be analogues to what may exist on Europa. Given the right chance and some basic conditions, life is possible on so many different levels. Ganymede is the largest moon in the solar system (larger than the planet Mercury), and is the only moon known to have its own internally generated magnetic field. Callistos surface is extremely heavily cratered and ancient, a visible record of events from the early history of the solar system. However, the very few small craters on Callisto indicate a small degree of current surface activity. The interiors of Io, Europa and Ganymede have a layered structure similar to the Earth). Io, Europa and Ganymede all have cores and mantles partially molten rock or a solid rock envelope around the core. The surface of Europa and Ganymede is a thick, soft ice layer and a thin crust of impure water ice. In the case of Europa, a subsurface water layer probably lies just below the icy crust and may cover the entire moon. This makes Europa a candidate for moon landing, but in the movie 2001 A Space Odyssey, mankind was forbidden to land on Europa, however, we will of course disregard. Layering at Callisto is less well defined and appears to be mainly a mixture of ice and rock. As ESP leaves the Jovian world and once more, as the voyager space probes successfully navigated, rounds the giant planet to pick up additional speed for the voyage to Saturn, and beyond. I. Saturn. At 9.5 AU from the sun Saturn has a mass of 5.69 E26 kg. With an average density of 690 kg/m3, Saturn is far less massive than any planet in the solar system, being only 95 Earth masses and could be floated in water since its density is less than that of water. Famous for its extensive ring system, Saturn has similarities to Jupiter, such as its atmospheric composition, as Saturn is mostly a massive ball of hydrogen and helium. Saturn is unique among the planets. All four gas giant planets have rings, made of chunks of ice and rock, but none are as spectacular or as complicated as Saturns. Saturns magnetic field is not as huge as Jupiters, however; it is still 578 times as powerful as the Earths. Saturn, its rings and many of its satellites lie totally within Saturns own enormous magnetosphere (the region of space in which the behavior of electrically charged particles is influenced more by Saturns magnetic field) than by the solar wind. Jupiter shares the magnetic field s imilarity. Saturn has sixty known satellites; two of which, Titan and Enceladus, show signs of geological activity, though they are largely made of ice. Titan is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere. In 1610, Italian astronomer Galileo Galilei was the first to gaze at Saturn through a telescope, and in 2004, after seeing Jupiters Galilean satellites; I saw the outline of Saturns rings. My image was not unlike Galileos where I could resolve the rings, not their structure or color, and noticed a dark space between the ring system and the planet was visible. Although a fascinating sight, nothing compared to seeing the Galilean satellites. However, to credit Galileo, my modern-day meade-reflector was equal to Galileos very first refractor; a testament to the mind of a genius. He would probably say, they dont build them like they used too. Winds in the upper atmosphere reach 500 m (1,600 feet) per second near the equatorial region. These super-fast winds, combined with heat rising from within the planets interior, cause the yellow and gold bands visible in the atmosphere. In the early 1980s, NASAs Voyager 1 and Voyager 2 spacecraft revealed that Saturns rings are made mostly of water ice and the ring system extends hundreds of thousands of kilometers from the planet, however surprising, the vertical depth is typically only about 10 m (30 feet) in the main rings. Saturns Moons. The largest moon, Titan, is a bit bigger than the planet Mercury (Titan is the second-largest moon in the solar system; only Jupiters moon Ganymede is bigger.) Titan is so large that it affects the orbits of other near-by moons. At 5,150 km (3,200 miles) across, it is the second largest moon in the solar system. Titan hides its surface with a thick nitrogen-rich atmosphere. Titans atmosphere is similar to the Earths atmosphere of long ago, before biology took hold on our home planet and changed the composition from carbon dioxide to oxygen. Titans atmosphere is approximately 95% nitrogen, 3% helium with traces of methane. While the Earths atmosphere extends about 60 km (37 miles) into space, Titans extends nearly 600 km (ten times that of the Earths atmosphere) into space. The moon Iapetus has one side as bright as snow and one side as dark as black velvet, with a huge ridge running around most of its dark-side equator. Phoebe is odd as the moon orbits the planet in a direction opposite that of Saturns larger moons, as do several of the more recently discovered moons. The result of an impact that nearly split the moon Mimas apart has an enormous crater on one side providing evidence that the solar system still contains left-over debris and can cause substantial impacts. The probe Cassini observed warm fractures on Enceladus where evaporating ice clearly escapes and forms a huge cloud of water vapor over the South Pole. Scientists have seen evidence of active ice volcanism on Enceladus. Hyperion has an odd flattened shape and rotates chaotically, probably due to a recent collision, and probably due to the space junk being tossed out from the ring-system due to collisions there. The moon Pan orbits within the main rings and helps sweep materials out of a narrow space known as the Encke Gap (have to do a better job of sweeping with the many impacts on-going.) Finally, Tethys has a huge rift zone called the Ithaca Chasma that runs nearly three-quarters of the way around the moon. Four additional moons orbit in stable places around Saturn they tag along with their larger sisters. These moons lie 60 degrees ahead of or behind a larger moon and in the same orbit. Telesto and Calypso move along with the larger moon Tethys in its orbit; Helene and Polydeuces occupy similar orbits with Dione. A collision with any of these smaller moons within the same orbit can cause catastrophic consequences with Saturns larger moons. Uranus is next as our probe moves on from Saturn. J. Uranus This strange upside-down world is 19.6 AU from the sun, and at 14 Earth masses, has a mass of 8.68 E25 kg with a mean density of 1,290 kg/m3. Uniquely among the planets is the only gas-giant whose equator is nearly at right angles to its orbit (its axial tilt is over ninety degrees to the eclip