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. Earth’s 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. Earth’s 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 it’s planet. No other moon-planet has this size ratio.

The four seasons are a result of Earth’s axis of rotation being tilted 23.45 degrees with respect to the plane of Earth’s 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. Earth’s global ocean, which covers nearly 70 percent of the planet’s 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 Earth’s weather.

The Earth’s 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 Earth’s magnetic field does not fade off into space, but has definite boundaries. When charged particles from the solar wind become trapped in Earth’s magnetic field, they collide with air molecules above our planet’s magnetic poles. These air molecules then begin to glow, and are known as the aurora — the northern and southern lights. Earth’s 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 lithosphere 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 Earth’s atmosphere consists of 78 percent nitrogen, 21 percent oxygen and 1 percent argon and other trace ingredients. The atmosphere affects Earth’s 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 Earth’s Moon approximately 250,000 miles away.

B. The Moon. The Earth’s moon provides a more livable planet by moderating our home planet’s 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 moon’s 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 USSR’s 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 Earth’s 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 planet’s 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 Earth’s at 5,250 kg/m2. The slow rotation of Venus cannot generate a magnetic field similar to Earth’s, 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 Earth’s (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 °C, most likely due to the amount of greenhouse gases in the atmosphere. The thick atmosphere traps the sun’s 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, Earth’s 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. Mercury’s surface resembles that of Earth’s 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 planet’s radius (Earth’s core is many times smaller compared to the planet’s diameter). In 2007, researchers using ground-based radars to study the core found evidence that it is molten (liquid). Mercury’s outer shell, comparable to Earth’s 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 planet’s surface early in the solar system’s history.

Mercury’s almost negligible atmosphere consists of atoms blasted off its surface by the solar wind. Though Mercury’s magnetic field has just 1 percent the strength of Earth’s, 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 system’s known mass and dominates it gravitationally. Jupiter and Saturn, the Sun’s two largest orbiting bodies, account for more than 90% of the system’s remaining mass. Most large objects in orbit around the Sun lie near the plane of Earth’s 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 seasons; layered areas near the poles suggest that the planet’s 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 NASA’s 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 planet’s 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, NASA’s 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 rover’s twin, Spirit, also found the signature of ancient water near its landing site halfway around Mars from Opportunity’s location. Recently, in August 2012, the probe Curiosity made another surface landing in a crater and being the first nuclear-powered probe. Leaving Mar’s 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 System’s 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 Earth’s 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, but 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 Saturn’s atmospheres are largely hydrogen and helium. Uranus and Neptune’s 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 Saturn’s 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. Jupiter’s 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 Jupiter’s 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. Galileo’s surprise and illumination is understood. In 2004, while looking through a small Meade reflecting telescope, Jupiter’s 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 clouds are composed of ammonia, and water vapor exists deep below and can sometimes be seen through clear spots in the clouds. The planet’s “stripes” are dark belts and light zones are created by strong east-west winds in Jupiter’s 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, NASA’s Galileo spacecraft dropped a probe into Jupiter’s atmosphere, which made the first direct measurements of the planet’s 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 Earth’s. Trapped within Jupiter’s magnetosphere (the area in which magnetic field lines encircle the planet from pole to pole) are swarms of charged particles. Jupiter’s 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.

Jupiter’s 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, Jupiter’s powerful magnetic field is generated by electrical currents driven by Jupiter’s fast rotation (9.8 Earth hours.) At the center, the immense pressure may support a solid core of rock about the size of Earth.

Jupiter’s 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, Jupiter’s 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. Io’s volcanoes are driven by hot silicate magma.

Europa’s 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. Callisto’s 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 mantle’s 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 Saturn’s. Saturn’s magnetic field is not as huge as Jupiter’s, however; it is still 578 times as powerful as the Earth’s. Saturn, its rings and many of its satellites lie totally within Saturn’s own enormous magnetosphere (the region of space in which the behavior of electrically charged particles is influenced more by Saturn’s magnetic field) than by the solar wind. Jupiter shares the magnetic field similarity.

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 Jupiter’s Galilean satellites; I saw the outline of Saturn’s rings. My image was not unlike Galileo’s 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 Galileo’s very first refractor; a testament to the mind of a genius. He would probably say, they don’t 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 planet’s interior, cause the yellow and gold bands visible in the atmosphere. In the early 1980s, NASA’s Voyager 1 and Voyager 2 spacecraft revealed that Saturn’s 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.

Saturn’s Moon’s. The largest moon, Titan, is a bit bigger than the planet Mercury (Titan is the second-largest moon in the solar system; only Jupiter’s 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. Titan’s atmosphere is similar to the Earth’s atmosphere of long ago, before biology took hold on our home planet and changed the composition from carbon dioxide to oxygen. Titan’s atmosphere is approximately 95% nitrogen, 3% helium with traces of methane. While the Earth’s atmosphere extends about 60 km (37 miles) into space, Titan’s extends nearly 600 km (ten times that of the Earth’s 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 Saturn’s 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 Saturn’s 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 ecliptic,) and like Venus, rotates east to west. Scientists’ believe a collision with an Earth-sized object may explain Uranus’ unique tilt. Because of Uranus’ unusual orientation, the planet experiences extreme variations in sunlight during each 20-year-long season. Uranus has more methane in it’s mainly hydrogen and helium atmosphere than Jupiter or Saturn. Methane gives Uranus its blue tint. It has a much colder core than the other gas giants, and radiates very little heat into space. Uranus has twenty-seven known satellites, the largest ones being Titania, Oberon, Umbriel, Ariel and Miranda. Scientists have now identified 13 known rings around Uranus. The inner system of nine rings, discovered in 1977, consists mostly of narrow, dark rings. Voyager 2 found two additional inner rings. An outer system of two more-distant rings was discovered by the Hubble Space Telescope in 2003.

Uranus is one of the two ice-giants of the outer solar system (the other is Neptune). Sunlight passes through the atmosphere and is reflected back out by Uranus’ cloud tops. Methane gas absorbs the red portion of the light, resulting in a blue-green color. The bulk (80 percent or more) of the mass of Uranus is contained in an extended liquid core consisting mostly of icy materials (water, methane and ammonia). Magnetic fields are usually aligned with a planet’s rotation, however, Uranus’ magnetic field is tipped over (the magnetic axis is tilted nearly 60 degrees from the planet’s axis of rotation.) The magnetic fields of both Uranus and Neptune are very irregular.

Uranus has 27 known moons and unique in being named for Shakespearean characters, along with a couple of the moons being named for characters from the works of Alexander Pope, whereas most of the satellites orbiting other planets take their names from Greek mythology. The Voyager 2 spacecraft visited the Uranian system in 1986 and tripled the number of known moons. Voyager 2 found an additional ten moons, just 16-96 miles in diameter: Juliet, Puck, Cordelia, Ophelia, Bianca, Desdemona, Portia, Rosalind, Cressida and Belinda. Since then, astronomers using the Hubble Space Telescope and improved ground-based telescopes have raised the total to 27 known moons. All of Uranus’s inner moons (those observed by Voyager 2) appear to be roughly half water ice and half rock. The composition of the moons outside the orbit of Oberon remains unknown, but they are likely captured asteroids.

The largest moons of Uranus. Miranda is the innermost and smallest of the five major satellites. It has giant canyons as much as 12 times as deep as the Grand Canyon, with surfaces that appear very old, and others that look much younger. The brightest and possibly the youngest surface among all the moons of Uranus is Ariel. It has few large craters and many small ones, indicating that fairly recent impact collisions wiped out the large craters that would have been left by much earlier, bigger collisions. Intersecting valleys pitted with craters scars its surface. Saturn’s moon Umbriel is ancient, and the darkest of the five large moons. It has many old, large craters and shows a mysterious bright ring on one side. Oberon, the outermost of the five major moons, is old, heavily cratered and shows little signs of internal activity. The shepherd moons, Cordelia and Ophelia keep Uranus’ thin, outermost “epsilon” ring well defined. Between them and Miranda is a group of eight small satellites unlike any other system of planetary moons. Astronomers don’t yet understand how the little moons have managed to avoid crashing into each other within this crowded region. Leaving Uranus to rotate on it’s side, the ESP plots a course to Neptune, and begins the venture to the outter regions of the solar system.

K. Neptune. An immense distance of 30 AU from the sun (4.5 billion km, 2.8 billion miles,) more than 30 times as far from the sun as Earth and invisible to the naked eye, the planet takes almost 165 Earth years to orbit our sun. In 2011 Neptune completed its first orbit since its discovery in 1846, and portrays the immense size of the solar system. Though slightly smaller than Uranus, is more massive (equivalent to 17 Earths) and therefore denser, and radiates more internal heat, but not as much as Jupiter or Saturn. The mass of Neptune is 1.02 E26 kg and has a density of 1,640 kg/m3. Neptune is the last of the hydrogen and helium gas giants (although called an ice-giant) in our solar system. Neptune has thirteen known satellites. Neptune was the first planet located through mathematical predictions rather than through regular observations of the sky because Uranus didn’t travel exactly as astronomers expected it to, therefore it was hypothesized the position and mass of another unknown planet may be the cause of the observed changes to Uranus’ orbit.

The magnetic field of Neptune is about 27 times more powerful than that of Earth. Like Uranus, whose magnetic axis is tilted about 60 degrees from the axis of rotation; Neptune’s magnetosphere undergoes wild variations during each rotation because of a similar 47 degrees misalignment with the planet’s rotational axis.

Neptune’s atmosphere extends to great depths, gradually merging into water and other melted ices over a heavier, approximately Earth-size solid core. Neptune’s blue color is the result of methane in the atmosphere, but Neptune’s more vivid, brighter blue is the result of an unknown component that causes the more intense color. Despite its great distance and low energy input from the sun, Neptune’s winds are estimated at three times stronger than Jupiter’s and nine times stronger than Earth’s. In 1989, Voyager 2 tracked a large, oval-shaped, dark storm (Great Dark Spot) in Neptune’s southern hemisphere, which was large enough to contain the entire Earth, spun counterclockwise and moved westward at almost 750 miles per hour. Voyager 2’s observations confirmed that Neptune has six known rings that are considered to be unusual, have four thick regions (clumps of dust) called arcs, and thought to be relatively young and short-lived. Voyager 2’s observations also discovered 6 moons at Neptune, 13 that are known today. Voyager 2 also discovered geysers spewing icy material upward more than 8 km (5 miles) on Neptune’s moon Triton.

Neptune’s Moons. The largest moon, Triton, is geologically active, with geysers of liquid nitrogen. Triton (not to be confused with Saturn’s moon, Titan), orbits the planet in the opposite direction compared with the rest of the moons, suggesting that it may have been captured by Neptune in the distant past. Triton is extremely cold with temperatures on its surface about -391degrees Fahrenheit (-235 degrees Celsius). Triton’s thin atmosphere, also discovered by Voyager, has been detected from Earth several times since, and is growing warmer, although scientists do not yet know why. Voyager 2 revealed fascinating details about Triton, such as ice volcanoes that spout, what is probably a mixture of liquid nitrogen, methane and dust, and which instantly freezes and then snows back down to the surface. One image from Voyager 2 shows a plume shooting 5 miles into the sky and drifting 87 miles downwind.

Neptune’s gravity acts as a drag on the counter-orbiting Triton, slowing it down and making it drop closer and closer to the planet. Millions of years from now, Triton will come close enough for gravitational forces to break it apart, possibly forming a ring around Neptune bright enough to be seen with a telescope from the Earth. Proteus and five other moons had to wait for Voyager 2 to make them known. All six are among the darker objects found in the solar system. Astronomers using improved ground-based telescopes found more satellites in 2002 and 2003, bringing the known total to 13.

L. Trans-Neptunian Region. The area beyond Neptune, often called the outer Solar System or the “trans-Neptunian region” is still unexplored. It appears to consist primarily of small worlds (the largest having a diameter only a fifth that of the Earth and a mass far smaller than that of the Moon) composed mainly of rock and ice. Our probe, The Earth Science Probe (ESP) has travelled billions of miles and explored the eight known planets, and now embarks to the edge of the solar system to explore the Kuiper belt and the Oort Cloud, and beyond the influence of the sun to the heliosphere.

Comets, friend or antagonist. Comets are leftovers from the formation of the solar system around 4.6 billion years ago, and consist mostly of ice coated rocky material, referred to as dirty snowballs, and yield important clues about the formation of our solar system. Comets may have brought water and organic compounds, the building blocks of life, to the early Earth and other parts of the solar system. Most comets travel a safe distance from the sun, comet Halley comes no closer than 89 million km (55 million miles). However, some comets, called sun-grazers, crash straight into the sun or get so close that they break up and evaporate.

A disc-like belt of icy bodies exists just beyond Neptune, as theorized by astronomer Gerard Kuiper (the so called Kuiper Belt), where a population of dark comets orbits the sun in the realm of Pluto. These icy objects, occasionally pushed by gravity into orbits bringing them closer to the sun, become the so-called short-period comets. They take less than 200 years to orbit the sun, and their appearance is predictable because they have passed by before. Comets are small Solar System bodies, usually only a few kilometers across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to sublimate and ionize, creating a coma: a long tail of gas and dust often visible to the naked eye. Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are believed to originate in the Kuiper belt, while long-period comets, such as Hale-Bopp, are believed to originate in the Oort cloud, however, these long-period comets are less predictable as many arrive from a region called the Oort Cloud about 100,000 AU from the sun. These Oort Cloud comets can take as long as 30 million years to complete one trip around the sun.

NASA’s Stardust mission successfully flew within 236 km (147 miles) of the nucleus of Comet Wild 2 in January 2004, collecting particles and interstellar dust for a sample return to Earth in 2006. Analysis of the Stardust samples suggests that comets may be more complex than originally thought. Minerals that formed near the sun or other stars were found in the samples, and suggest that materials from the inner regions of the solar system traveled to the outer regions where comets formed. Another NASA mission, called Deep Impact, consisted of a flyby spacecraft and an impactor. In July 2005, the impactor was released into the path of comet “Tempel 1” in a planned collision, which vaporized the impactor and ejected massive amounts of fine, powdery material from beneath the comet’s surface.

M. The Kuiper Belt. The Kuiper belt, the region’s first formation, is a great ring of debris similar to the asteroid belt, but composed mainly of ice. It extends between 30 and 50 AU from the Sun. This region is thought to be the source of short-period comets. It is composed mainly of small Solar System bodies (Kuiper Belt Object, or KBO for short,) but many of the largest KBOs, such as Quaoar, Varuna, and Orcus, may be reclassified as dwarf planets. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km, but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of the Earth. Many Kuiper belt objects have multiple satellites, and most have orbits that take them outside the plane of the ecliptic.

The Demoted Pluto is now considered a dwarf planet and is the largest known object in the Kuiper belt at an average distance of 39 AU. When discovered in 1930 it was considered to be the ninth planet; this changed in 2006 with the adoption of a formal definition of planet. Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane (the Earth’s moon is 5 degrees) and ranging from 29.7 AU from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU at aphelion. It is unclear whether Charon, Pluto’s largest moon, will continue to be classified as such or as a dwarf planet itself.

In July 2005, a team of scientists announced the discovery of a KBO that was initially thought to be about 10 percent larger than Pluto. The object later named Eris, orbits the sun about once every 560 years, its distance varying from about 38 to 98 AU. (For comparison, Pluto travels from 29 to 49 AU in its solar orbit.) Eris has a small moon named Dysnomia. More recent measurements show it to be slightly smaller than Pluto. The discovery of Eris orbiting the sun and similar in size to Pluto (which was then designated the ninth planet), forced astronomers to consider whether Eris should be classified as the tenth planet. Instead, in 2006, the International Astronomical Union created a new class of objects called dwarf planets, and placed Pluto, Eris and the asteroid Ceres in this category. While no spacecraft has yet traveled to the Kuiper Belt, NASA’s New Horizons spacecraft is scheduled to arrive at Pluto in 2015. The New Horizons mission team hopes to study one or more KBOs after its Pluto mission is complete.

N. Farthest regions. The point at which the Solar System ends and interstellar space begins is not precisely defined, since its outer boundaries are shaped by two separate forces, the solar wind and the Sun’s gravity. The solar wind is believed to yield to the interstellar medium at roughly four times Pluto’s distance.

The Scattered Disc. The scattered disc overlaps the Kuiper belt but extends much further outwards. Scattered disc objects are believed to come from the Kuiper belt, having been ejected by the gravitational influence of Neptune’s early outward migration. Most scattered disc objects (SDOs) move from within the Kuiper belt and as far as 150 AU from the Sun. SDOs’ orbits are also highly inclined to the ecliptic plane, and are often almost perpendicular to it. Eris (68 AU average) is the largest known scattered disc object, and caused a debate about what constitutes a planet, since it is at least 5% larger than Pluto with an estimated diameter of 2400 km (1500 mi). It is the largest of the known dwarf planets. It has one moon, Dysnomia. Like Pluto, its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto’s distance from the Sun) and an aphelion of 97.6 AU, and steeply inclined to the ecliptic plane.

The Heliopause. The heliosphere is divided into two separate regions. The solar wind travels at its maximum velocity out to about 95 AU, or three times the orbit of Pluto. The edge of this region is the termination shock, the point at which the solar wind collides with the opposing winds of the interstellar medium. Here the wind slows, condenses and becomes more turbulent, forming a great oval structure known as the heliosheath that looks and behaves very much like a comet’s tail, extending outward for a further 40. The outer boundary of the heliosphere, the heliopause, is the point at which the solar wind finally terminates, and is the beginning of interstellar space. No spacecraft, not even the Voyager probes have yet passed beyond the heliopause, so it is impossible to know for certain the conditions in local interstellar space.

O. Oort cloud. The hypothetical Oort cloud is a great mass of up to a trillion icy objects that is believed to be the source for all long-period comets and to surround the Solar System at around 50,000 AU, and possibly to as far as 100,000 AU. It is believed to be composed of comets which were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events such as collisions.

Sedna and the inner Oort cloud. In March 2004, a team of astronomers announced the discovery of a planet-like object orbiting the sun at an extreme distance. The object, since named Sedna for an Inuit goddess who lives at the bottom of the frigid Arctic ocean, approaches the sun only briefly during its 10,500-year solar orbit. Sedna travels in a long, elliptical orbit between 76 and nearly 1,000 AU from the sun. Since Sedna’s orbit takes it to such an extreme distance, its discoverers have suggested that it is the first observed body belonging to the inner Oort Cloud. Sedna is a large, reddish Pluto-like object, and discovered by Mike Brown in 2003, asserts that it cannot be part of the scattered disc or the Kuiper Belt, he and other astronomers consider it to be the first in an entirely new population. Brown terms this population the “Inner Oort cloud,” as it may have formed through a similar process, although it is far closer to the Sun. Sedna is very likely a dwarf planet, though its shape has yet to be determined with certainty.

P. Solar System Boundaries. Much of our Solar System is still unknown. The Sun’s gravitational field is estimated to dominate the gravitational forces of surrounding stars out to about two light years (125,000 AU). The outer extent of the Oort cloud may not extend farther than 50,000 AU. Despite discoveries such as Sedna, the region between the Kuiper belt and the Oort cloud, an area tens of thousands of AU in radius, is still virtually unmapped. There are also ongoing studies of the region between Mercury and the Sun. Objects may yet be discovered in the Solar System’s uncharted regions.

3. Our Galactic Context. Our Solar System is located in the Milky Way galaxy, a spiral galaxy with a diameter of about 100,000 light years containing about 200 billion stars. Our Sun resides in one of the Milky Way’s outer spiral arms, known as the Orion Arm. The Sun lies between 25,000 and 28,000 light years from the Galactic Center completing one revolution about the galactic center every 225-250 million years, and is known as the Solar System’s galactic year.

A. The Solar System’s location. The evolution of life on Earth in the galaxy is likely a factor in as we inhabit a relatively quite area less dense than one would expect nearer to the galactic center where events are more violent. The Solar System’s orbit is close to being circular and roughly the same speed as that of the spiral arms, which means it passes through them only rarely, so wandering space debris (asteroids) does not typically venture into the influence of the suns gravitational pull.

B. Objects orbiting the Sun. All objects are divided into three classes; planets (their 166 known moons), three dwarf planets (Ceres, Pluto, and Eris and their four known moons), and billions of small Solar System bodies. A planet is any body in orbit around the Sun that has enough mass to form itself into a spherical shape. There are eight known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. On August 24 2006 the International Astronomical Union defined the term “planet” for the first time, excluding Pluto and reclassifying it under the new category of dwarf planet along with Eris and Ceres.

C. The Solar System Formation. Is believed to have formed according to the nebular hypothesis, which says that 4.6 billion years ago the Solar System formed from the gravitational collapse of a giant molecular cloud several light-years across. As gravity, acted on the contracting cloud, it began to flatten into a spinning disk with a diameter of roughly 200 AU and a hot, dense protostar at the center began to form. After 100 million years, the pressure and density of hydrogen in the centre of the collapsing nebula became great enough for the young-sun to begin thermonuclear fusion eventually becoming a full-fledged star.

D. The remaining cloud of gas and dust. They are believed to have formed by accretion, the planets began as dust grains in orbit around the central protostar; then gathered by direct contact into clumps; then collided to form larger bodies (planetesimals); then gradually increased by further collisions over the course of the next few million years. The planetesimals which formed the inner Solar System were relatively small and composed largely of compounds with high melting points, such as silicates and metals. These rocky bodies eventually became the terrestrial planets. Farther out beyond the asteroid belt, and beyond the frost line, where icy compounds could remain solid, Jupiter and Saturn became the gas giants. Uranus and Neptune captured much less material and are known as ice giants because their cores are believed to be made mostly of ices (hydrogen compounds).

C. Planet Summary. Terrestrial planets all have approximately the same type of structure: a central metallic core, mostly iron, with a surrounding silicate mantle. The Moon is similar, but has a much smaller iron core. Terrestrial planets have canyons, craters, mountains, and volcanoes. Terrestrial planets possess secondary atmospheres – atmospheres generated through internal volcanism or comet impacts, as opposed to the gas giants, which possess primary atmospheres – atmospheres captured directly from the original solar nebula.

A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune.

The terrestrial planets primarily composed of dense silicates formed closer to the sun and retained their solid structure because of the close proximity to the sun. In contrast, the gas giants initially formed from nebular-gases far from the sun became planets and retained gas-like structures cold enough to condense to liquids and ice.