Mars, the red planet
Mars is the fourth planet from the Sun. It is also the second nearest planet to the Earth and will probably be the first planet visited by humans. It has an orbital period equal to 687 Earth days and a rotational period equal to 24hrs 37mins and 22.6 seconds. Therefore there are 668 Martian days in a Martian Year. Mars has a very eccentric orbit that can vary from 249 million km to 207 million km. As a result, it also experiences seasons.
When it is nearest to Earth – 59 million km away – Mars can be seen in great detail even with small telescopes. The polar ice caps are visible; the southern ice cap can extend down to 50° latitude or become quite small depending on the season. There are many bright areas on the red planet, e.g. 'Hellas', that are in fact deep basins on the planet's surface. There are also dark areas on the planet's surface, e.g. 'Sirtis Major' (in the shape of a large 'V'), that were once thought to be seas, but when the atmospheric pressure was discovered to be too low for liquid water, the dark areas were then thought to be old sea beds filled with vegetation. However all this was disproved after the first fly-by mission by Mariner 4 in 1965. And two NASA rovers, Spirit and Opportunity, have since beamed back images of the Martian surface, which they explored.
The highest known surface point on Mars is a huge volcano known as 'Olympus Mons'. It stretches 24km high above the lava plains around it and it has a base measuring 600km. Mars has an average surface temperature of about -23°C. Its atmospheric content includes 95% carbon dioxide, 3% nitrogen and 1.6% argon. Mars is not as dense or as large as Earth and has an escape velocity of 5km/sec, only enough to sustain a thin, transparent atmosphere. However, some clouds can be seen and from time to time the occasional dust storm can completely cover the Martian surface. The storms occur when wind speed increases to 50-100 meters per second as dust from the surface is lifted up and bounces along the ground, colliding with other dust particles and forcing a cataclysmic reaction that could encompass the entire Martian globe. The technical name for this is 'saltation'.
panoramic view of Mars - part one panoramic view of Mars - part two panoramic view of Mars - part three Mars has two moons, Phobos and Deimos, both discovered in 1877 by Asaph Hall. These two moons are shaped irregularly and are probably asteroids that were caught by Mars' gravitational pull a long time ago. Neither is large enough to become spherical, and both have synchronous rotations enabling them to always keep the same face toward their parent planet.
Phobos orbits at a distance of less than 6000 km from the surface of Mars and, with a maximum diameter of 27 km, is larger than Deimos. Phobos has been falling very slowly toward the planet at a rate of 10 km every century, because of which it will collide with Mars in forty million years. Phobos' surface is covered with craters (the largest is the 10 km-long "Stickney," which was named after Asaph Hall's wife). Phobos has an orbital period of 7 hours and 39 minutes. Deimos is even smaller than Phobos. Its longest diameter is 15 km and it orbits 23400 km from the planet's centre and, unlike Phobos, has a stable orbit.
If you want to find out more about missions to Mars why not check out this article on ESA's project, Mars Express spacecraft.
When it is nearest to Earth – 59 million km away – Mars can be seen in great detail even with small telescopes. The polar ice caps are visible; the southern ice cap can extend down to 50° latitude or become quite small depending on the season. There are many bright areas on the red planet, e.g. 'Hellas', that are in fact deep basins on the planet's surface. There are also dark areas on the planet's surface, e.g. 'Sirtis Major' (in the shape of a large 'V'), that were once thought to be seas, but when the atmospheric pressure was discovered to be too low for liquid water, the dark areas were then thought to be old sea beds filled with vegetation. However all this was disproved after the first fly-by mission by Mariner 4 in 1965. And two NASA rovers, Spirit and Opportunity, have since beamed back images of the Martian surface, which they explored.
The highest known surface point on Mars is a huge volcano known as 'Olympus Mons'. It stretches 24km high above the lava plains around it and it has a base measuring 600km. Mars has an average surface temperature of about -23°C. Its atmospheric content includes 95% carbon dioxide, 3% nitrogen and 1.6% argon. Mars is not as dense or as large as Earth and has an escape velocity of 5km/sec, only enough to sustain a thin, transparent atmosphere. However, some clouds can be seen and from time to time the occasional dust storm can completely cover the Martian surface. The storms occur when wind speed increases to 50-100 meters per second as dust from the surface is lifted up and bounces along the ground, colliding with other dust particles and forcing a cataclysmic reaction that could encompass the entire Martian globe. The technical name for this is 'saltation'.
panoramic view of Mars - part one panoramic view of Mars - part two panoramic view of Mars - part three Mars has two moons, Phobos and Deimos, both discovered in 1877 by Asaph Hall. These two moons are shaped irregularly and are probably asteroids that were caught by Mars' gravitational pull a long time ago. Neither is large enough to become spherical, and both have synchronous rotations enabling them to always keep the same face toward their parent planet.
Phobos orbits at a distance of less than 6000 km from the surface of Mars and, with a maximum diameter of 27 km, is larger than Deimos. Phobos has been falling very slowly toward the planet at a rate of 10 km every century, because of which it will collide with Mars in forty million years. Phobos' surface is covered with craters (the largest is the 10 km-long "Stickney," which was named after Asaph Hall's wife). Phobos has an orbital period of 7 hours and 39 minutes. Deimos is even smaller than Phobos. Its longest diameter is 15 km and it orbits 23400 km from the planet's centre and, unlike Phobos, has a stable orbit.
If you want to find out more about missions to Mars why not check out this article on ESA's project, Mars Express spacecraft.
Venus, Second Planet from the Sun, Brightest Planet in Solar System
In ancient times, Venus was once thought to be two different stars, the evening star and the morning star — that is, the ones that first appeared at sunset and sunrise. In Latin, they were respectively known as Vesper and Lucifer. In Christian times, Lucifer, or "light-bringer," became known as the name of Satan before his fall.
Physical Characteristics of the Planet Venus
Venus and Earth are often called twins because they are similar in size, mass, density, composition and gravity. However, the similarities end there. [Photos of Venus, the Mysterious Planet Next Door]
Venus is the hottest world in the solar system. Although Venus is not the planet closest to the sun, its dense atmosphere traps heat in a runaway version of the greenhouse effect that warms up the Earth. As a result, temperatures on Venus reach 870 degrees F (465 degrees C), more than hot enough to melt lead. Probes that scientists have landed there have survived only a few hours before getting destroyed.
Venus has a hellish atmosphere as well, consisting mainly of carbon dioxide with clouds of sulfuric acid, and scientists have only detected trace amounts of water in the atmosphere. The atmosphere is heavier than that of any other planet, leading to a surface pressure 90 times that of Earth.
Venus' southern hemisphere, as seen in the ultraviolet.
Venus' southern hemisphere, as seen in the ultraviolet.
CREDIT: ESA
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The surface of Venus is extremely dry. There is no liquid water on its surface because the scorching heat would cause any to boil away. Roughly two-thirds percent of the Venusian surface is covered by flat, smooth plains that are marred by thousands of volcanoes, ranging from about 0.5 to 150 miles (0.8 to 240 kilometers) wide, with lava flows carving long, winding canals up to more than 3,000 miles (5,000 kilometers) in length, longer than on any other planet.
Six mountainous regions make up about one-third percent of the Venusian surface. One mountain range, called Maxwell, is about 540 miles (870 kilometers) long and reaches up to some 7 miles (11.3 kilometers) high, making it the highest feature on the planet.
Venus also possesses a number of surface features unlike anything on the Earth. For example, Venus has coronae, or crowns — ringlike structures that range from roughly 95 to 360 miles (155 to 580 kilometers) wide. Scientists believe these formed when hot material beneath the crust rises up, warping the planet’s surface. Venus also has tesserae, or tiles — raised areas in which many ridges and valleys have formed in different directions.
Not only are conditions on Venus infernal, an ancient name for Venus was even Lucifer. This name did not carry any fiendish connotations, however — Lucifer means "light-bringer," and when seen from Earth, Venus is brighter than any other planet or even any star in the night sky because of its highly reflective clouds and its closeness to our planet.
Venus takes 243 Earth days to rotate on its axis, by far the slowest of any of the major planets, and because of this sluggish spin, its metal core cannot generate a magnetic field similar to Earth's.
Venus's Orbital Characteristics
If viewed from above, while most planets rotate the same way on their axes, Venus rotates the opposite way. While on Earth, the sun appears to rise in the east and set in the west, if on Venus, the sun would rise in the west and set in the east. [Venus Transit Of 2004: 51 Photos Of Rare Celestial Sight]
The Venusian year — the time it takes to orbit the sun — is about 225 Earth days long. At the same time, it takes Venus 243 Earth days to rotate on its axis, which normally would mean that days on Venus would be longer than years. However, because of Venus' curious retrograde rotation, the time from one sunrise to the next is only about 117 Earth days long.
Composition & Structure
* Atmospheric composition (by volume)
96.5 percent carbon dioxide, 3.5 percent nitrogen, with minor amounts of sulfur dioxide, argon, water, carbon monoxide, helium and neon. ()
* Magnetic Field
Venus magnetic field is 0.000015 times that of Earth's field.
* Internal structure
Venus' metallic iron core is roughly 2,400 miles (6,000 kilometers) wide.
Venus' molten rocky mantle is roughly 1,200 miles (3,000 kilometers) thick.
Venus' crust is mostly basalt, and is estimated to be six to 12 miles (10 to 20 kilometers) thick on average.
Orbit & Rotation
Average Distance from the Sun
English: 67,237,910 miles
Metric: 108,208,930 km
By Comparison: 0.723 times that of Earth
Perihelion (closest)
English: 66,782,000 miles
Metric: 107,476,000 km
By Comparison: 0.730 times that of Earth
Aphelion (farthest)
English: 67,693,000 miles
Metric: 108,942,000 km
By Comparison: 0.716 times that of Earth
(Source: NASA)
* Atmospheric composition (by volume)
96.5 percent carbon dioxide, 3.5 percent nitrogen, with minor amounts of sulfur dioxide, argon, water, carbon monoxide, helium and neon. ()
* Magnetic Field
Venus magnetic field is 0.000015 times that of Earth's field.
* Internal structure
Venus' metallic iron core is roughly 2,400 miles (6,000 kilometers) wide.
Venus' molten rocky mantle is roughly 1,200 miles (3,000 kilometers) thick.
Venus' crust is mostly basalt, and is estimated to be six to 12 miles (10 to 20 kilometers) thick on average.
Orbit & Rotation
Average Distance from the Sun
English: 67,237,910 miles
Metric: 108,208,930 km
By Comparison: 0.723 times that of Earth
Perihelion (closest)
English: 66,782,000 miles
Metric: 107,476,000 km
By Comparison: 0.730 times that of Earth
Aphelion (farthest)
English: 67,693,000 miles
Metric: 108,942,000 km
By Comparison: 0.716 times that of Earth
(Source: NASA)
Venus's Climate
The very top layer of Venus' clouds zip around the planet every four Earth days, propelled by hurricane-force winds traveling roughly 224 miles (360 kilometers) per hour. This super-rotation of the planet's atmosphere, some 60 times faster than Venus itself rotates, may be one of Venus' biggest mysteries. The winds at the planet's surface are much slower, estimated to be just a few miles per hour.
The Venus Express spacecraft the European Space Agency launched in 2005 intriguingly found evidence of lightning on the planet. This lightning is unique from that found on the other planets in the solar system that possess it, in that it is the only lightning known that is not associated with water clouds. Instead, on Venus, the lightning is associated with clouds of sulfuric acid. Scientists are excited by these electrical discharges, because they can break molecules into fragments that can then combine with other fragments in unexpected ways.
Unusual stripes in the upper clouds of Venus are dubbed "blue absorbers" or "ultraviolet absorbers" because they strongly absorb light in the blue and ultraviolet wavelengths. These are soaking up a huge amount of energy — nearly half of the total solar energy the planet absorbs. As such, they seem to play a major role in keeping Venus as hellish as it is. Their exact composition remains uncertain.
Venus's Moons or Rings
Venus has no moons or rings.
Research & Exploration
The United States, Soviet Union, and European Space Agency have deployed many spacecraft to Venus, more than 20 in all so far. NASA's Mariner 2 came within 21,600 miles (34,760 kilometers) of Venus in 1962, making it the first planet to be observed by a passing spacecraft. The Soviet Union's Venera 7 was the first spacecraft to land on another planet, and Venera 9 which returned the first photographs of the Venusian surface. The first Venusian orbiter, NASA's Magellan, generated maps of 98 percent of the planet's surface using radar, showing details of features as small as 330 feet (100 meters) across.
The European Space Agency's Venus Express is now in orbit around Venus with a large variety of instruments, and has confirmed the presence of lightning there. The next mission to Venus, Japan's Akatsuki, was launched in 2010, and will follow Venus' thick cloud layers as they are whipped around the planet by hurricane-force winds.
Earth: The Planet's Composition, Atmosphere & Science Facts
Earth – Overview
Earth, our home, is the third planet from the sun. It is the only planet known to have an atmosphere containing free oxygen, oceans of liquid water on its surface, and, of course, life. Earth is the fifth largest of the planets in the solar system — smaller than the four gas giants, Jupiter, Saturn, Uranus and Neptune, but larger than the three other rocky planets, Mercury, Mars and Venus.
Earth has a diameter of roughly 8,000 miles (13,000 kilometers), and is round because gravity pulls matter into a ball, although it is not perfectly round, instead being more of an "oblate spheroid" whose spin causes it to be squashed at its poles and swollen at the equator. Roughly 71 percent of Earth's surface is covered by water, most of it in the oceans. About a fifth of its atmosphere is made up of oxygen, produced by plants. While scientists have been studying our planet for centuries, much has been learned in recent decaded by studying pictures of Earth from space, like the one below.
Orbital Characteristics
The Earth spins on an imaginary line called an axis that runs from the north pole to the south pole, while also orbiting the sun. It takes Earth 24 hours to complete a rotation on its axis, and roughly 365 days to complete an orbit around the sun.
The Earth's axis of rotation is tilted in relation to the ecliptic plane, an imaginary surface through Earth's orbit around the sun. This means the northern and southern hemispheres will sometimes point toward or away from the sun depending on the time of year, varying the amount of light they receive and causing the seasons.
Earth's orbit is not a perfect circle, but is rather an oval-shaped ellipse, like that of the orbits of all the other planets. Earth is a bit closer to the sun in early January and farther away in July, although this variation has a much smaller effect than the heating and cooling caused by the tilt of Earth's axis. Earth happens to lie within the so-called "Goldilocks zone" around its star, where temperatures are just right to maintain liquid water on its surface.
The Earth spins on an imaginary line called an axis that runs from the north pole to the south pole, while also orbiting the sun. It takes Earth 24 hours to complete a rotation on its axis, and roughly 365 days to complete an orbit around the sun.
The Earth's axis of rotation is tilted in relation to the ecliptic plane, an imaginary surface through Earth's orbit around the sun. This means the northern and southern hemispheres will sometimes point toward or away from the sun depending on the time of year, varying the amount of light they receive and causing the seasons.
Earth's orbit is not a perfect circle, but is rather an oval-shaped ellipse, like that of the orbits of all the other planets. Earth is a bit closer to the sun in early January and farther away in July, although this variation has a much smaller effect than the heating and cooling caused by the tilt of Earth's axis. Earth happens to lie within the so-called "Goldilocks zone" around its star, where temperatures are just right to maintain liquid water on its surface.
History: Earth's formation and evolution
Earth probably formed at roughly the same time as the sun and other planets some 4.6 billion years ago, when the solar system coalesced from a giant, rotating cloud of gas and dust known as the solar nebula. As the nebula collapsed because of its gravity, it spun faster and flattened into a disk. Most of the material was pulled toward the center to form the sun. Other particles within the disk collided and stuck together to form ever-larger bodies, including the Earth. The solar wind from the sun was so powerful that it swept away most of the lighter elements, such as hydrogen and helium, from the innermost worlds, rendering Earth and its siblings into small, rocky planets.
Scientists think Earth started off as a waterless mass of rock. Radioactive materials in the rock and increasing pressure deep within the Earth generated enough heat to melt Earth's interior, causing some chemicals to rise to the surface and form water, while others became the gases of the atmosphere. Recent evidence suggests that Earth's crust and oceans may have formed within about 200 million years after the planet had taken shape.
The history of Earth is divided into four eons — starting with the earliest, these are the Hadean, Archean, Proterozoic, and Phanerozoic. The first three eons, which together lasted nearly 4 billion years, are together known as the Precambrian. Evidence for life has bee found in the Archaean about 3.8 billion years ago, but life did not become abundant until the Phanerozoic.
The Phanerozoic is divided into three eras — starting with the earliest, these are the Paleozoic, Mesozoic, and Cenozoic. The Paleozoic Era saw the development of many kinds of animals and plants in the seas and on land, the Mesozoic Era was the age of dinosaurs, and the Cenozoic Era we are in currently is the age of mammals.
Most of the fossils seen in Paleozoic rocks are invertebrate animals lacking backbones, such as corals, mollusks and trilobites. Fish are first found about 450 million years ago, while amphibians appear roughly 380 million years ago. By 300 million years ago, large forests and swamps covered the land, and the earliest fossils of reptiles appear during this period as well.
Earth probably formed at roughly the same time as the sun and other planets some 4.6 billion years ago, when the solar system coalesced from a giant, rotating cloud of gas and dust known as the solar nebula. As the nebula collapsed because of its gravity, it spun faster and flattened into a disk. Most of the material was pulled toward the center to form the sun. Other particles within the disk collided and stuck together to form ever-larger bodies, including the Earth. The solar wind from the sun was so powerful that it swept away most of the lighter elements, such as hydrogen and helium, from the innermost worlds, rendering Earth and its siblings into small, rocky planets.
Scientists think Earth started off as a waterless mass of rock. Radioactive materials in the rock and increasing pressure deep within the Earth generated enough heat to melt Earth's interior, causing some chemicals to rise to the surface and form water, while others became the gases of the atmosphere. Recent evidence suggests that Earth's crust and oceans may have formed within about 200 million years after the planet had taken shape.
The history of Earth is divided into four eons — starting with the earliest, these are the Hadean, Archean, Proterozoic, and Phanerozoic. The first three eons, which together lasted nearly 4 billion years, are together known as the Precambrian. Evidence for life has bee found in the Archaean about 3.8 billion years ago, but life did not become abundant until the Phanerozoic.
The Phanerozoic is divided into three eras — starting with the earliest, these are the Paleozoic, Mesozoic, and Cenozoic. The Paleozoic Era saw the development of many kinds of animals and plants in the seas and on land, the Mesozoic Era was the age of dinosaurs, and the Cenozoic Era we are in currently is the age of mammals.
Most of the fossils seen in Paleozoic rocks are invertebrate animals lacking backbones, such as corals, mollusks and trilobites. Fish are first found about 450 million years ago, while amphibians appear roughly 380 million years ago. By 300 million years ago, large forests and swamps covered the land, and the earliest fossils of reptiles appear during this period as well.
The Mesozoic saw the ascendence of dinosaurs, although mammals also appear in the fossil record about 200 million years ago. During this time, flowering plants became the dominant plant group and continue to be so today.
The Cenozoic began about 65 million years ago with the end of the age of dinosaurs, which many scientists think was caused by a cosmic impact. Mammals survived to become the dominant land animals of today.
Composition & Structure
* Atmosphere
The atmosphere is roughly 78 percent nitrogen, 21 percent oxygen, with trace amounts of water, argon, carbon dioxide and other gases. Nowhere else in the solar system can one find an atmosphere loaded with free oxygen, which ultimately proved vital to one of the other unique features of Earth — us.
Air surrounds Earth and becomes thinner farther from the surface. Roughly 100 miles (160 kilometers) above Earth, the air is so thin that satellites can zip through with little resistance. Still, traces of atmosphere can be found as high as 370 miles (600 kilometers) above the surface.
The lowest layer of the atmosphere is known as the troposphere, which is constantly in motion, causing the weather. Sunlight heats the Earth's surface, causing warm air to rise. This air ultimately expands and cools as air pressure decreases, and because this cool air is denser than its surroundings, it then sinks, only to get warmed by the Earth once again.
Above the troposphere, some 30 miles (48 kilometers) above the Earth's surface, is the stratosphere. The still air of the stratosphere contains the ozone layer, which was created when ultraviolet light caused trios of oxygen atoms to bind together into ozone molecules. Ozone prevents most of the sun's harmful ultraviolet radiation from reaching Earth's surface.
The Cenozoic began about 65 million years ago with the end of the age of dinosaurs, which many scientists think was caused by a cosmic impact. Mammals survived to become the dominant land animals of today.
Composition & Structure
* Atmosphere
The atmosphere is roughly 78 percent nitrogen, 21 percent oxygen, with trace amounts of water, argon, carbon dioxide and other gases. Nowhere else in the solar system can one find an atmosphere loaded with free oxygen, which ultimately proved vital to one of the other unique features of Earth — us.
Air surrounds Earth and becomes thinner farther from the surface. Roughly 100 miles (160 kilometers) above Earth, the air is so thin that satellites can zip through with little resistance. Still, traces of atmosphere can be found as high as 370 miles (600 kilometers) above the surface.
The lowest layer of the atmosphere is known as the troposphere, which is constantly in motion, causing the weather. Sunlight heats the Earth's surface, causing warm air to rise. This air ultimately expands and cools as air pressure decreases, and because this cool air is denser than its surroundings, it then sinks, only to get warmed by the Earth once again.
Above the troposphere, some 30 miles (48 kilometers) above the Earth's surface, is the stratosphere. The still air of the stratosphere contains the ozone layer, which was created when ultraviolet light caused trios of oxygen atoms to bind together into ozone molecules. Ozone prevents most of the sun's harmful ultraviolet radiation from reaching Earth's surface.
Water vapor, carbon dioxide and other gases in the atmosphere trap heat from the sun, warming Earth. Without this so-called "greenhouse effect," Earth would probably be too cold for life to exist, although a runaway greenhouse effect led to the hellish conditions now seen on Venus.
Earth-orbiting satellites have shown that the upper atmosphere actually expands during the day and contracts at night due to heating and cooling.
* Magnetic field
Earth-orbiting satellites have shown that the upper atmosphere actually expands during the day and contracts at night due to heating and cooling.
* Magnetic field
The Earth's magnetic field is generated by currents flowing in Earth's outer core. The magnetic poles are always on the move, with the magnetic north pole recently accelerating its northward motion to 24 miles (40 km) annually, likely exiting North America and reaching Siberia in a few decades.
Earth's magnetic field is changing in other ways, too — globally, the magnetic field has weakened 10 percent since the 19th century. These changes are mild compared to what Earth's magnetic field has done in the past — sometimes the field completely flips, with the north and the south poles swapping places.
When charged particles from the sun get trapped in Earth's magnetic field, they smash into air molecules above the magnetic poles, causing them to glow, a phenomenon known as the aurorae, the northern and southern lights.
* Chemical composition
Oxygen is the most abundant element in rocks in Earth's crust, composing roughly 47 percent of the weight of all rock. The second most abundant element is silicon at 27 percent, followed by aluminum at 8 percent, iron at 5 percent, calcium at 4 percent, and sodium, potassium, and magnesium at about 2 percent each.
The Earth's core consists mostly of iron and nickel and potentially smaller amounts of lighter elements such as sulfur and oxygen. The mantle is made of iron and magnesium-rich silicate rocks. (The combination of silicon and oxygen is known as silica, and minerals that contain silica are known as silicate minerals.)
* Internal structure
The Earth's core is about 4,400 miles (7,100 kilometers) wide, slightly larger than half the Earth's diameter and roughly the size of Mars. The outermost 1,400 miles (2,250 kilometers) of the core are liquid, while the inner core — about four-fifths as big as Earth's moon at some 1,600 miles (2,600 kilometers) in diameter — is solid.
Above the core is Earth's mantle, which is about 1,800 miles (2,900 kilometers) thick. The mantle is not completely stiff, but can flow slowly. Earth's crust floats on the mantle much as a wood floats on water, and the slow motion of rock in the mantle shuffles continents around and causes earthquakes, volcanoes, and the formation of mountain ranges.
Above the mantle, Earth has two kinds of crust. The dry land of the continents consists mostly of granite and other light silicate minerals, while the ocean floors are made up mostly of a dark, dense volcanic rock called basalt. Continental crust averages some 25 miles (40 kilometers) thick, although it can be thinner or thicker in some areas. Oceanic crust is usually only about 5 miles (8 kilometers) thick. Water fills in low areas of the basalt crust to form the world's oceans. Earth has more than enough water to completely fill the ocean basins, and the rest of it spreads onto edges of the continents, areas known as the continental shelf.
Earth's magnetic field is changing in other ways, too — globally, the magnetic field has weakened 10 percent since the 19th century. These changes are mild compared to what Earth's magnetic field has done in the past — sometimes the field completely flips, with the north and the south poles swapping places.
When charged particles from the sun get trapped in Earth's magnetic field, they smash into air molecules above the magnetic poles, causing them to glow, a phenomenon known as the aurorae, the northern and southern lights.
* Chemical composition
Oxygen is the most abundant element in rocks in Earth's crust, composing roughly 47 percent of the weight of all rock. The second most abundant element is silicon at 27 percent, followed by aluminum at 8 percent, iron at 5 percent, calcium at 4 percent, and sodium, potassium, and magnesium at about 2 percent each.
The Earth's core consists mostly of iron and nickel and potentially smaller amounts of lighter elements such as sulfur and oxygen. The mantle is made of iron and magnesium-rich silicate rocks. (The combination of silicon and oxygen is known as silica, and minerals that contain silica are known as silicate minerals.)
* Internal structure
The Earth's core is about 4,400 miles (7,100 kilometers) wide, slightly larger than half the Earth's diameter and roughly the size of Mars. The outermost 1,400 miles (2,250 kilometers) of the core are liquid, while the inner core — about four-fifths as big as Earth's moon at some 1,600 miles (2,600 kilometers) in diameter — is solid.
Above the core is Earth's mantle, which is about 1,800 miles (2,900 kilometers) thick. The mantle is not completely stiff, but can flow slowly. Earth's crust floats on the mantle much as a wood floats on water, and the slow motion of rock in the mantle shuffles continents around and causes earthquakes, volcanoes, and the formation of mountain ranges.
Above the mantle, Earth has two kinds of crust. The dry land of the continents consists mostly of granite and other light silicate minerals, while the ocean floors are made up mostly of a dark, dense volcanic rock called basalt. Continental crust averages some 25 miles (40 kilometers) thick, although it can be thinner or thicker in some areas. Oceanic crust is usually only about 5 miles (8 kilometers) thick. Water fills in low areas of the basalt crust to form the world's oceans. Earth has more than enough water to completely fill the ocean basins, and the rest of it spreads onto edges of the continents, areas known as the continental shelf.
Earth gets warmer toward its core. At the bottom of the continental crust, temperatures reach about 1,800 degrees F (1,000 degrees C), increasing about 3 degrees F per mile (1 degrees C per kilometer) below the crust. Geologists think the temperature of Earth's outer core is about 6,700 to 7,800 degrees F (3,700 to 4,300 degrees C), and the inner core may reach 12,600 degrees F (7,000 degrees C), hotter than the surface of the sun. Only the enormous pressures found at the super-hot inner core keep it solid.
Orbit & Rotation
Average Distance from the Sun
English: 92,955,820 miles
Metric: 149,597,890 km
Perihelion (closest)
English: 91,400,000 miles
Metric: 147,100,000 km
Aphelion (farthest)
English: 94,500,000 miles
Metric: 152,100,000 km
Average Length of Solar Day
24 hours
Length of Year
365.24 Earth days
Equatorial Inclination to Orbit
23.45 degrees
(Source: NASA.)
Orbit & Rotation
Average Distance from the Sun
English: 92,955,820 miles
Metric: 149,597,890 km
Perihelion (closest)
English: 91,400,000 miles
Metric: 147,100,000 km
Aphelion (farthest)
English: 94,500,000 miles
Metric: 152,100,000 km
Average Length of Solar Day
24 hours
Length of Year
365.24 Earth days
Equatorial Inclination to Orbit
23.45 degrees
(Source: NASA.)
Moon
Earth's moon is 2,159 miles (3,474 kilometers) wide, about one-fourth of Earth's diameter. Earth has one moon, while Mercury and Venus have none and all the other planets in our solar system have two or more.
The leading explanation for how the moon formed was that a giant impact knocked off the raw ingredients for the moon off the primitive molten Earth and into orbit. Scientists have suggested the impactor was roughly 10 percent the mass of Earth, about the size of Mars.
Species Overview
Earth is the only planet in the universe known to possess life. There are several million known species of life, ranging from the bottom of the deepest ocean to a few miles into the atmosphere, and scientists think far more remain to be discovered. Scientists figure there are between 5 million an 100 million species on Earth, but science has only identified about 2 million of them.
* 50 Amazing Earth Facts
www.space.com
Earth's moon is 2,159 miles (3,474 kilometers) wide, about one-fourth of Earth's diameter. Earth has one moon, while Mercury and Venus have none and all the other planets in our solar system have two or more.
The leading explanation for how the moon formed was that a giant impact knocked off the raw ingredients for the moon off the primitive molten Earth and into orbit. Scientists have suggested the impactor was roughly 10 percent the mass of Earth, about the size of Mars.
Species Overview
Earth is the only planet in the universe known to possess life. There are several million known species of life, ranging from the bottom of the deepest ocean to a few miles into the atmosphere, and scientists think far more remain to be discovered. Scientists figure there are between 5 million an 100 million species on Earth, but science has only identified about 2 million of them.
* 50 Amazing Earth Facts
www.space.com
Mercury, the Sun's Closest Planetary Neighbor
Mercury's History & Naming
Mercury is the closest planet to the sun. As such, it circles the sun faster than all the other planets, which is why Romans named it after the swift-footed messenger god Mercury.
Mercury was known since at least Sumerian times roughly 5,000 years ago, where it was often associated with Nabu, the god of writing. Mercury was also given separate names for its appearance as both a morning star and as an evening star. Greek astronomers knew, however, that the two names referred to the same body. Heraclitus believed that both Mercury and Venus orbited the Sun, not the Earth. [Latest Photos: Mercury Seen by NASA's Messenger Probe]
Mercury's Physical Characteristics
As the planet nearest the sun, the surface of Mercury can reach a scorching 840 degrees F (450 degrees C). However, since this world doesn't have a real atmosphere to entrap any heat, at night temperatures can plummet to minus 275 degrees F (minus 170 degrees C), a more than 1,100 degrees F (600 degree C) temperature swing that is the greatest in the solar system.
Mercury is the smallest planet — it is only slightly larger than Earth's moon. Since it has no significant atmosphere to stop impacts, the planet is pockmarked with craters. For instance, about 4 billion years ago, a roughly 60-mile-wide (100-kilometer-wide) asteroid struck Mercury with an impact equal to 1 trillion 1-megaton bombs, creating a vast impact crater roughly 960 miles (1,550 kilometers) wide. Known as the Caloris Basin, this crater could hold the entire state of Texas.
As the planet nearest the sun, the surface of Mercury can reach a scorching 840 degrees F (450 degrees C). However, since this world doesn't have a real atmosphere to entrap any heat, at night temperatures can plummet to minus 275 degrees F (minus 170 degrees C), a more than 1,100 degrees F (600 degree C) temperature swing that is the greatest in the solar system.
Mercury is the smallest planet — it is only slightly larger than Earth's moon. Since it has no significant atmosphere to stop impacts, the planet is pockmarked with craters. For instance, about 4 billion years ago, a roughly 60-mile-wide (100-kilometer-wide) asteroid struck Mercury with an impact equal to 1 trillion 1-megaton bombs, creating a vast impact crater roughly 960 miles (1,550 kilometers) wide. Known as the Caloris Basin, this crater could hold the entire state of Texas.
Amazing, as close to the sun as Mercury is, ice may exist in its craters. In 1991, astronomers using radar observations discovered that water ice may lurk at Mercury's north and south poles inside deep craters that are perpetually shadowed and cold. Comets or meteorites might have delivered ice there, or water vapor might have outgassed from the planet's interior and frozen out at the poles.Mercury apparently shrank about 0.6 to 1.2 miles (1 to 2 kilometers) as it cooled in the billions of years after its birth. This caused its surface to crumple, creating lobe-shaped scarps or cliffs, some hundreds of miles long and soaring up to a mile high.
Mercury is the second densest planet after Earth, with a huge metallic core roughly 2,200 to 2,400 miles (3,600 to 3,800 kilometers) wide, or about 75 percent of the planet's diameter. In comparison, Mercury's outer shell is only 300 to 400 miles (500 to 600 kilometers) thick.
A completely unexpected discovery Mariner 10 made was that Mercury possessed a magnetic field. Planets theoretically generate magnetic fields only if they spin quickly and possess a molten core. But Mercury takes 59 days to rotate and is so small — just roughly one-third Earth's size — that its core should have cooled off long ago. The recent discovery from 2007 Earth-based radar observations that Mercury's core may still be molten could help explain its magnetism.
Although Mercury's magnetic field is just 1 percent the strength of Earth's, it is very active. The magnetic field in the solar wind — the charged particles streaming off the sun — periodically touches upon Mercury's field, creating powerful magnetic tornadoes that channel the fast, hot plasma of the solar wind down to the planet's surface.
Instead of a substantial atmosphere, Mercury possesses an ultra-thin "exosphere" made up of atoms blasted off its surface by solar radiation, the solar wind and micrometeoroid impacts. These quickly escape into space, forming a tail of particles.
Mercury's Orbital Characteristics
Mercury speeds around the sun every 88 Earth days, traveling through space at nearly 112,000 miles per hour (180,000 kilometers per hour), faster than any other planet. Its oval-shaped orbit is highly elliptical, taking Mercury as close as 29 million miles (47 million kilometers) and as far as 43 million miles (70 million kilometers) from the sun. If one could stand on Mercury when it is nearest to the sun, it would appear more than three times as large as it does when viewed from Earth.
Oddly, due to Mercury's highly elliptical orbit and the 59 Earth days or so it takes to rotate on its axis, when on the scorching surface of the planet, the sun appears to rise briefly, set, and rise again before it travels westward across the sky. At sunset, the sun appears to set, rise again briefly, and then set again.
Mercury speeds around the sun every 88 Earth days, traveling through space at nearly 112,000 miles per hour (180,000 kilometers per hour), faster than any other planet. Its oval-shaped orbit is highly elliptical, taking Mercury as close as 29 million miles (47 million kilometers) and as far as 43 million miles (70 million kilometers) from the sun. If one could stand on Mercury when it is nearest to the sun, it would appear more than three times as large as it does when viewed from Earth.
Oddly, due to Mercury's highly elliptical orbit and the 59 Earth days or so it takes to rotate on its axis, when on the scorching surface of the planet, the sun appears to rise briefly, set, and rise again before it travels westward across the sky. At sunset, the sun appears to set, rise again briefly, and then set again.
* Atmospheric composition (by volume)
No atmosphere: Mercury possesses an exosphere containing 42 percent oxygen, 29 percent sodium, 22 percent hydrogen, 6 percent helium, 0.5 percent potassium, with possible trace amounts of argon, carbon dioxide, water, nitrogen, xenon, krypton, neon. [Inside Planet Mercury (Infographic)]
* Magnetic Field
Roughly 1 percent the strength of Earth's.
* Internal structure
Iron core roughly 2,200 to 2,400 miles (3,600 to 3,800 kilometers) wide. Outer silicate shell about 300 to 400 miles (500 to 600 kilometers) thick.
Orbit & Rotation
Average Distance from the Sun
English: 35,983,095 miles
Metric: 57,909,175 km
By Comparison: 0.38 Earth's distance from the Sun.
Perihelion (closest)
English: 28,580,000 miles
Metric: 46,000,000 km
By Comparison: 0.313 times that of Earth
Aphelion (farthest)
English: 43,380,000 miles
Metric: 69,820,000 km
By Comparison: 0.459 times that of Earth
Length of Day:
58.646 Earth days
(Source: NASA.)
Mercury's Moons or Rings
Mercury has no moons or rings.
Research & Exploration
The first spacecraft to visit Mercury was Mariner 10, which imaged about 45 percent of the surface and detected its magnetic field. NASA's MESSENGER orbiter is the second spacecraft to visit Mercury, and has made three flybys as it slowly works its way into orbit. When it arrived in March 2011, MESSENGER became the first spacecraft to orbit Mercury, where studied Mercury for one year, mapping nearly the entire planet in color. NASA has extended the mission beyond the initial one-year science plan to continue to study Mercury. [First Photos of Mercury from Orbit]
Please see this website, http://www.nasa.gov/worldbook/mercury_worldbook.html. There is a section about the phases of Mercury that provides additional information/content about Mercury.
The first spacecraft to visit Mercury was Mariner 10, which imaged about 45 percent of the surface and detected its magnetic field. NASA's MESSENGER orbiter is the second spacecraft to visit Mercury, and has made three flybys as it slowly works its way into orbit. When it arrived in March 2011, MESSENGER became the first spacecraft to orbit Mercury, where studied Mercury for one year, mapping nearly the entire planet in color. NASA has extended the mission beyond the initial one-year science plan to continue to study Mercury. [First Photos of Mercury from Orbit]
Please see this website, http://www.nasa.gov/worldbook/mercury_worldbook.html. There is a section about the phases of Mercury that provides additional information/content about Mercury.
Neptune: The Other Blue Planet in Our Solar System
Neptune’s History & NamingThe planet Neptune was discovered on Sept. 23, 1846. Neptune was the first planet to get its existence predicted by mathematical calculations before it was actually seen by a telescope. Irregularities in the orbit of Uranus led French astronomer Alexis Bouvard to suggest that the gravitational pull from another celestial body might be responsible. German astronomer Johann Galle then relied on subsequent calculations to help spot Neptune via telescope. In accordance with all the other planets seen in the sky, this new world was given a name from Greek and Roman mythology — Neptune, the Roman god of the sea.
Physical Characteristics of the Planet Neptune
Neptune’s cloud cover has an especially vivid blue tint that is partly due to an as-yet-unidentified compound and the result of the absorption of red light by methane in the planets mostly hydrogen-helium atmosphere. Photos of Neptune reveal a blue planet, and it is often dubbed an ice giant, since it possesses a thick, slushy fluid mix of water, ammonia and methane ices under its atmosphere and is roughly 17 times Earth's mass and nearly 58 times its volume. Neptune's rocky core alone is thought to be roughly equal to Earth's mass.
Despite its great distance from the sun, which means it gets little sunlight to help warm and drive its atmosphere, Neptune's winds can reach up to 1,500 miles per hour (2,400 kilometers per hour), the fastest detected yet in the solar system. These winds were linked with a large dark storm that Voyager 2 tracked in Neptune's southern hemisphere in 1989. This oval-shaped, counterclockwise-spinning "Great Dark Spot" was large enough to contain the entire Earth, and moved westward at nearly 750 miles per hour (1,200 kilometers per hour). This storm seemed to have vanished when the Hubble Space Telescope later searched for it. Hubble has also revealed the appearance and then fading of two other Great Dark Spots over the last decade.
Neptune's magnetic poles are tipped over by roughly 47 degrees compared with the poles along which it spins. As such, the planets magnetic field, which is about 27 times more powerful than Earth's, undergoes wild swings during each rotation.
Neptune’s Orbital Characteristics
Neptune is the eighth and most distant planet from the sun. Its elliptical, oval-shaped orbit makes it keep an average distance from the sun of almost 2.8 billion miles (4.5 billion kilometers), or roughly 30 times as far away as Earth, making it invisible to the naked eye. Neptune goes around the sun once roughly every 165 Earth years, and will complete its first orbit, since being discovered, in 2011.
Every 248 years, Pluto moves inside Neptune's orbit for 20 years or so, during which time it is closer to the sun than Neptune. Nevertheless, Neptune remains the farthest planet from the sun, since Pluto was reclassified as a dwarf planet in 2006.
Every 248 years, Pluto moves inside Neptune's orbit for 20 years or so, during which time it is closer to the sun than Neptune. Nevertheless, Neptune remains the farthest planet from the sun, since Pluto was reclassified as a dwarf planet in 2006.
Composition & Structure
* Atmospheric composition (by volume)
Hydrogen - 80 percent; Helium - 19.0 percent; Methane - 1.5 percent
* Magnetic Field
Roughly 27 times more powerful than Earth's.
* Composition
The overall composition of Neptune is, by mass, thought to be about 25 percent rock, 60 to 70 percent ice, and 5 to 15 percent hydrogen and helium. (Tristan Guillot, "Interiors of Giant Planets Inside and Outside the Solar System." Science Vol. 286 (5437), p. 72-77, October 1, 1999.)
* Internal structure
Mantle of water, ammonia and methane ices
Core of iron and magnesium-silicate
(Tristan Guillot, "Interiors of Giant Planets Inside and Outside the Solar System." Science Vol. 286 (5437), p. 72-77, October 1, 1999.)
Orbit & Rotation
Average Distance from the Sun
Metric: 4,498,252,900 km
English: 2,795,084,800 miles
By Comparison: 30.069 x Earth
Perihelion (closest)
Metric: 4,459,630,000 km
English: 2,771,087,000 miles
By Comparison: 29.820 x Earth
Aphelion (farthest)
Metric: 4,536,870,000 km
English: 2,819,080,000 miles
By Comparison: 30.326 x Earth
(Source: NASA)
Neptune’s Moons
Neptune has 13 known moons, named after lesser sea gods and nymphs from Greek mythology, just as Neptune itself was named after the Roman god of the sea. The largest by far is Triton, whose discovery on Oct. 10, 1846 was in a sense enabled by beer — amateur astronomer William Lassell used the fortune he made as a brewer to finance his telescopes.
Triton is unique in being the only large moon in the solar system to circle its planet in a direction opposite to its planet's rotation — this "retrograde orbit" suggests that Triton may once have been a dwarf planet that Neptune captured rather than forming in place. Neptune's gravity is dragging Triton closer to the planet, meaning that millions of years from now, Triton will come close enough for gravitational forces to rip it apart.
Triton is extremely cold, with temperatures on its surface reaching about minus 391 degrees F (minus 235 degrees C), making it one of the coldest places in the solar system. Nevertheless, Voyager 2 detected geysers spewing icy matter upward more than 5 miles (8 kilometers), showing its interior appears warm. Recently, seasons have been discovered on Triton.
Triton is the only spherical moon of Neptune — the planet’s other twelve moons are irregularly shaped.
The Rings of Neptune
Triton is unique in being the only large moon in the solar system to circle its planet in a direction opposite to its planet's rotation — this "retrograde orbit" suggests that Triton may once have been a dwarf planet that Neptune captured rather than forming in place. Neptune's gravity is dragging Triton closer to the planet, meaning that millions of years from now, Triton will come close enough for gravitational forces to rip it apart.
Triton is extremely cold, with temperatures on its surface reaching about minus 391 degrees F (minus 235 degrees C), making it one of the coldest places in the solar system. Nevertheless, Voyager 2 detected geysers spewing icy matter upward more than 5 miles (8 kilometers), showing its interior appears warm. Recently, seasons have been discovered on Triton.
Triton is the only spherical moon of Neptune — the planet’s other twelve moons are irregularly shaped.
The Rings of Neptune
Neptune's unusual rings are not uniform, but possess bright thick clumps of dust called arcs. The rings are thought to be relatively young and short-lived. Earth-based observations announced in 2005 found that Neptune's rings are apparently far more unstable than previously thought, with some dwindling away rapidly. (Imke de Pater et al., "The dynamic neptunian ring arcs: evidence for a gradual disappearance of Liberté and resonant jump of courage." Icarus, Volume 174, Issue 1, March 2005, Pages 263-272.)
Research & Exploration
NASA's Voyager 2 space satellite was the first and as yet only spacecraft to visit Neptune on Aug. 25, 1989. The satellite discovered Neptune's rings and six of the planet's moons — Despina, Galatea, Larissa, Naiad, Proteus, and Thalassa. An international team of astronomers relying on ground telescopes announced the discovery of five new moons orbiting Neptune in 2003.
Formation of Neptune
Neptune is generally thought to have formed with the initial buildup of a solid core followed by the capture of surrounding hydrogen and helium gas in the nebula surrounding the early sun. In this model, proto-Neptune formed over the course of 1 to 10 million years.
NASA's Voyager 2 space satellite was the first and as yet only spacecraft to visit Neptune on Aug. 25, 1989. The satellite discovered Neptune's rings and six of the planet's moons — Despina, Galatea, Larissa, Naiad, Proteus, and Thalassa. An international team of astronomers relying on ground telescopes announced the discovery of five new moons orbiting Neptune in 2003.
Formation of Neptune
Neptune is generally thought to have formed with the initial buildup of a solid core followed by the capture of surrounding hydrogen and helium gas in the nebula surrounding the early sun. In this model, proto-Neptune formed over the course of 1 to 10 million years.
Saturn, the Solar System's Major Ring Bearer
Saturn's History and Naming
Saturn was the Roman name for Cronus, the lord of the titans in Greek mythology. Saturn happens to be the root of the English word "Saturday."
Physical Characteristics of the Planet Saturn
Saturn is a gas giant made up mostly of hydrogen and helium. Saturn is the second largest planet, big enough to hold more than 760 Earths, and is more massive than any other planet except Jupiter, roughly 95 times Earth's mass. However, Saturn has the lowest density of all the planets, and is the only one less dense than water — if there were a bathtub big enough to hold it, Saturn would float on top.
Although the other gas giants in the solar system — Jupiter, Uranus and Neptune — also have rings, those of Saturn are without a doubt the most extraordinary. The largest one to date spans up to 200 times the diameter of the planet.
Saturn is the farthest planet from Earth visible to the naked human eye. The yellow and gold bands seen in the planet's atmosphere are the result of super-fast winds in the upper atmosphere, which can reach up to 1,100 miles per hour (1,800 kilometers per hour) around its equator, combined with heat rising from the planet's interior.
Saturn spins faster than any other planet except Jupiter, completing a rotation roughly every 10-and-a-half hours. This rapid spinning causes Saturn to bulge at its equator and flatten at its poles — the planet is 8,000 miles (13,000 kilometers) wider at its equator than between the poles.
Saturn's most recent curiosity may be the giant hexagon circling its north pole, with each of its sides nearly 7,500 miles (12,500 kilometers) across — big enough to fit nearly four Earths inside. Thermal images show it reaches some 60 miles (100 kilometers) down into the planet's atmosphere. It remains uncertain what causes it.
Orbital Characteristics of the Planet Saturn
* Composition & Structure
o Atmospheric composition (by volume)
+ 96.3 percent molecular hydrogen, 3.25 percent helium, minor amounts of methane, ammonia, hydrogen deuteride, ethane, ammonia ice aerosols, water ice aerosols, ammonia hydrosulfide aerosols
o Magnetic Field
+ Saturn has a magnetic field about 578 times more powerful than Earth's.
o Chemical composition
+ Saturn seems to have a hot solid inner core of iron and rocky material surrounded by an outer core probably composed of ammonia, methane, and water. Next is a layer of highly compressed, liquid metallic hydrogen, followed by a region of viscous hydrogen and helium. This hydrogen and helium becomes gaseous near the planet's surface and merges with its atmosphere.
o Internal structure
+ Saturn seems to have a core between about 10 to 20 times as massive as the Earth.
* Orbit & Rotation
Average Distance from the Sun
English: 885,904,700 miles
Metric: 1,426,725,400 km
By Comparison: 9.53707 times that of Earth
Perihelion (closest)
English: 838,519,000 miles
Metric: 1,349,467,000 km
* Composition & Structure
o Atmospheric composition (by volume)
+ 96.3 percent molecular hydrogen, 3.25 percent helium, minor amounts of methane, ammonia, hydrogen deuteride, ethane, ammonia ice aerosols, water ice aerosols, ammonia hydrosulfide aerosols
o Magnetic Field
+ Saturn has a magnetic field about 578 times more powerful than Earth's.
o Chemical composition
+ Saturn seems to have a hot solid inner core of iron and rocky material surrounded by an outer core probably composed of ammonia, methane, and water. Next is a layer of highly compressed, liquid metallic hydrogen, followed by a region of viscous hydrogen and helium. This hydrogen and helium becomes gaseous near the planet's surface and merges with its atmosphere.
o Internal structure
+ Saturn seems to have a core between about 10 to 20 times as massive as the Earth.
* Orbit & Rotation
Average Distance from the Sun
English: 885,904,700 miles
Metric: 1,426,725,400 km
By Comparison: 9.53707 times that of Earth
Perihelion (closest)
English: 838,519,000 miles
Metric: 1,349,467,000 km
Aphelion (farthest)
English: 934,530,000 miles
Metric: 1,503,983,000 km
By Comparison: 9.886 times that of Earth
(Source: NASA.)
Saturn's Moons
Saturn has at least 62 moons. Since the planet was named after Cronus, lord of the titans in Greek mythology, most of Saturn's moons are named after other titans, their descendants, as well as after giants from Gallic, Inuit and Norse myths.
Saturn's largest moon, Titan, is slightly larger than Mercury, and is the second-largest moon in the solar system behind Jupiter's moon Ganymede. Titan is veiled under a very thick, nitrogen-rich atmosphere that might be like what Earth's was long ago, before life. While the Earth's atmosphere extends only about 37 miles (60 kilometers) into space, Titan's reaches nearly 10 times as far.
These moons can possess bizarre features. Pan and Atlas are shaped like flying saucers, Iapetus has one side as bright as snow and one side as dark as coal, and Enceladus shows evidence of "ice volcanism," spewing out water and other chemicals. A number of these satellites, such as Prometheus and Pandora, are shepherd moons, interacting with ring material to keep rings in their orbits.
Saturn's Rings
Galileo was the first to see Saturn's rings in 1610, although from his telescope they resembled handles or arms. It took Dutch astronomer Christiaan Huygens, who had a more powerful telescope, to propose that Saturn had a thin, flat ring.
Saturn actually has many rings made of billions of particles of ice and rock, ranging in size from a grain of sugar to the size of a house. The rings are believe to be debris left over from comets, asteroids or shattered moons. Although they extend thousands of miles from the planet, the main rings are typically only about 30 feet thick. Cassini revealed vertical formations in some of the rings, with particles piling up in bumps and ridges more than 2 miles (3 kilometers) high.
Saturn has at least 62 moons. Since the planet was named after Cronus, lord of the titans in Greek mythology, most of Saturn's moons are named after other titans, their descendants, as well as after giants from Gallic, Inuit and Norse myths.
Saturn's largest moon, Titan, is slightly larger than Mercury, and is the second-largest moon in the solar system behind Jupiter's moon Ganymede. Titan is veiled under a very thick, nitrogen-rich atmosphere that might be like what Earth's was long ago, before life. While the Earth's atmosphere extends only about 37 miles (60 kilometers) into space, Titan's reaches nearly 10 times as far.
These moons can possess bizarre features. Pan and Atlas are shaped like flying saucers, Iapetus has one side as bright as snow and one side as dark as coal, and Enceladus shows evidence of "ice volcanism," spewing out water and other chemicals. A number of these satellites, such as Prometheus and Pandora, are shepherd moons, interacting with ring material to keep rings in their orbits.
Saturn's Rings
Galileo was the first to see Saturn's rings in 1610, although from his telescope they resembled handles or arms. It took Dutch astronomer Christiaan Huygens, who had a more powerful telescope, to propose that Saturn had a thin, flat ring.
Saturn actually has many rings made of billions of particles of ice and rock, ranging in size from a grain of sugar to the size of a house. The rings are believe to be debris left over from comets, asteroids or shattered moons. Although they extend thousands of miles from the planet, the main rings are typically only about 30 feet thick. Cassini revealed vertical formations in some of the rings, with particles piling up in bumps and ridges more than 2 miles (3 kilometers) high.
The rings are generally named alphabetically in the order they were discovered. They are usually relatively close to each other, with one key exception caused by the Cassini Division, a gap some 2,920 miles (4,700 kilometers) wide. The main rings, working out from the planet, are known as C, B and A, with the Cassini Division separating B and A. The innermost is the extremely faint D ring, while the outermost to date, revealed in 2009, could fit a billion Earths within it.
Mysterious spokes have been seen in Saturn's rings, which might form and disperse over a few hours. Scientists have conjectured these spokes might be composed of electrically charged sheets of dust-sized particles created by small meteors impacting the rings or electron beams from the planet's lightning. Saturn's F Ring also has a curious braided appearance — it is composed of several narrow rings, and bends, kinks, and bright clumps in them can give the illusion that these strands are braided.
Research & Exploration
The first spacecraft to reach Saturn was Pioneer 11 in 1979, flying within 13,700 miles (22,000 kilometers) of it, which discovered the planet's two of its outer rings as well as the presence of a strong magnetic field. The Voyager spacecraft discovered the planet's rings are made up of ringlets, and sent back data that led to the discovery or confirmation of the existence of nine moons.
The Cassini spacecraft now in orbit around Saturn is the largest interplanetary spacecraft ever built, a two-story-tall probe that, at 6 tons in weight (5,650 kilograms), is roughly equal in mass to an empty 30-passenger school bus. It discovered plumes on the icy moon Enceladus, and carried the Huygens probe, which plunged through Titan's atmosphere to successfully land on its surface.
Mysterious spokes have been seen in Saturn's rings, which might form and disperse over a few hours. Scientists have conjectured these spokes might be composed of electrically charged sheets of dust-sized particles created by small meteors impacting the rings or electron beams from the planet's lightning. Saturn's F Ring also has a curious braided appearance — it is composed of several narrow rings, and bends, kinks, and bright clumps in them can give the illusion that these strands are braided.
Research & Exploration
The first spacecraft to reach Saturn was Pioneer 11 in 1979, flying within 13,700 miles (22,000 kilometers) of it, which discovered the planet's two of its outer rings as well as the presence of a strong magnetic field. The Voyager spacecraft discovered the planet's rings are made up of ringlets, and sent back data that led to the discovery or confirmation of the existence of nine moons.
The Cassini spacecraft now in orbit around Saturn is the largest interplanetary spacecraft ever built, a two-story-tall probe that, at 6 tons in weight (5,650 kilograms), is roughly equal in mass to an empty 30-passenger school bus. It discovered plumes on the icy moon Enceladus, and carried the Huygens probe, which plunged through Titan's atmosphere to successfully land on its surface.
Saturn's Gravitational Impact on the Solar System
As the most massive planet in the solar system after Jupiter, the pull of Saturn's gravity has helped shape the fate of our system. It might have helped violently hurl Neptune and Uranus outward. It, along with Jupiter, might alos have slung a barrage of debris toward the inner planets early in the system's history.
Possibility of Life
The life found on Earth could not live on Saturn, and scientists doubt that any form of life exists there.
As the most massive planet in the solar system after Jupiter, the pull of Saturn's gravity has helped shape the fate of our system. It might have helped violently hurl Neptune and Uranus outward. It, along with Jupiter, might alos have slung a barrage of debris toward the inner planets early in the system's history.
Possibility of Life
The life found on Earth could not live on Saturn, and scientists doubt that any form of life exists there.
Pluto, the Ninth Planet That Was a Dwarf
Pluto's History & Naming
Pluto is the only dwarf planet to once have been considered a major planet. Once thought of as the ninth planet and the one most distant from the sun, Pluto is now seen as one of the largest known members of the Kuiper belt, a shadowy disk-like zone beyond the orbit of Neptune populated by a trillion or more comets. Pluto was reclassified as a dwarf planet in 2006, a change widely thought of as a demotion that has attracted controversy and debate.
American astronomer Pervical Lowell first caught hints of Pluto's existence in 1905 from odd deviations he observed in the orbits of Neptune and Uranus, suggesting that another world's gravity was tugging at them from beyond. He predicted its location in 1915, but died without finding it. Its discovery came in 1930 from Clyde Tombaugh at the Lowell Observatory, based on predictions from Lowell and other astronomers.
Pluto is the only world named by an 11-year-old girl, Venetia Burney of Oxford, England, who suggested to her grandfather that it get its name from the Roman god of the underworld. Her grandfather then passed the name on to Lowell Observatory. The name also honors Percival Lowell, whose initials are the first two letters of Pluto.
Physical Characteristics of the Dwarf Planet Pluto
Since Pluto is so far from Earth, little is known about the planet’s size or surface conditions. Pluto has an estimated diameter less than one-fifth that of Earth or only about two-thirds as wide as Earth's moon. The planets’ surface conditions probably consist of a rocky core surrounded by a mantle of water ice, with more exotic ices such as methane and nitrogen frost coating its surface.
Pluto's orbit is highly eccentric, or far from circular, which means its distance from the sun can vary considerably and at times, Pluto’s orbit will take within the orbit of the planet Neptune. When Pluto is closer to the sun, its surface ices thaw and temporarily form a thin atmosphere, mostly of nitrogen, with some methane. Pluto's low gravity, which is a little more than one-twentieth that of Earth's, causes this atmosphere to extend much higher in altitude than Earth's. When traveling farther away from the Sun, most of Pluto's atmosphere is thought to freeze and all but disappear. Still, in the time that it does have an atmosphere, Pluto can apparently experience strong winds.
Pluto's surface is one of the coldest places in the solar system at roughly minus 375 degrees F (minus 225 degrees C). For a long time, astronomers knew little about its surface because of its distance from Earth, but more is coming, bit by bit, with the Hubble Space Telescope returning images of a planet that appears reddish, yellowish and grayish in places, with a curious bright spot near the equator that might be rich in carbon monoxide frost. When compared with past images, the Hubble pictures revealed that Pluto had apparently grown redder over time, apparently due to seasonal changes.
Pluto's Orbital Characteristics
Pluto's highly elliptical orbit can take it more than 49 times as far out from the sun as Earth. It actually gets closer to the sun than Neptune for 20 years out of Pluto's 248-Earth-years-long orbit, providing astronomers a rare chance to study this small, cold, distant world. So after 20 years as the 8th planet (in order going out from the sun), in 1999, Pluto crossed Neptune's orbit to become the farthest planet from the sun (until it was demoted to the status of dwarf planet).
Composition & Structure
* Atmospheric composition
Methane, nitrogen
* Magnetic Field
It remains unknown whether Pluto has a magnetic field, but its small size and slow rotation suggest it has little to none.
* Chemical composition
Probably a mixture of 70 percent rock and 30 percent water ice.
* Internal structure.
Probably a rocky core surrounded by a mantle of water ice, with more exotic ices such as methane and nitrogen frost coating its surface.
Average Distance from the Sun
English: 3,670,050,000 miles
Metric: 5,906,380,000 km
By Comparison: 39.482 times that of Earth
Perihelion (closest)
English: 2,756,902,000 miles
Metric: 4,436,820,000 km
By Comparison: 30.171 times that of Earth
Aphelion (farthest)
English: 4,583,190,000 miles
Metric: 7,375,930,000 km
By Comparison: 48.481 times that of Earth
(Source: NASA.)
Pluto's Moons
In 1978, astronomers discovered Pluto had a very large moon nearly half its size, dubbed Charon, named for the mythological demon who ferried souls to the underworld in Greek mythology. The huge size of Charon sometimes leads scientists to refer to Pluto and Charon as a double dwarf planet or binary system.
Pluto and Charon are just 12,200 miles (19,640 kilometers) apart, less than the distance by flight between London and Sydney. Charon's orbit around Pluto takes 6.4 Earth days, and one Pluto rotation — a Pluto day — also takes 6.4 Earth days. This is because Charon hovers over the same spot on Pluto's surface, and the same side of Charon always faces Pluto, a phenomenon known as tidal locking.
While Pluto appears reddish, Charon seems grayish. Scientists suggest Pluto is covered with nitrogen and methane while Charon is covered with ordinary water ice.
Compared with most of solar system's planets and moons, the Pluto-Charon system is tipped on its side in relation to the sun. Also, Pluto's rotation is retrograde compared to the other worlds — it spins backwards, from east to west.
In 2005, as scientists photographed Pluto with the Hubble Space Telescope in preparation for the New Horizons mission — the first spacecraft to visit Pluto and the Kuiper Belt — they discovered two other tiny moons of Pluto, now dubbed Nix and Hydra. These are two to three times farther away from Pluto than Charon, and they are thought to be just 31 to 62 miles (50 to 100 kilometers) wide.
Compared with most of solar system's planets and moons, the Pluto-Charon system is tipped on its side in relation to the sun. Also, Pluto's rotation is retrograde compared to the other worlds — it spins backwards, from east to west.
In 2005, as scientists photographed Pluto with the Hubble Space Telescope in preparation for the New Horizons mission — the first spacecraft to visit Pluto and the Kuiper Belt — they discovered two other tiny moons of Pluto, now dubbed Nix and Hydra. These are two to three times farther away from Pluto than Charon, and they are thought to be just 31 to 62 miles (50 to 100 kilometers) wide.
Research & Exploration of the Dwarf Planet
Pluto's distance from Earth has made it hard to see with telescopes and a daunting challenge to explore with spacecraft — NASA's New Horizons mission will be the first probe to study Pluto, its moons, and other worlds within the Kuiper belt. It was launched on January 2006, making its closest approach to Pluto on July 2015, and carries some of the ashes of Pluto's discoverer, Clyde Tombaugh.
Pluto's Formation & Origins
The leading theory for the formation of Pluto and Charon is that a nascent Pluto was struck with a glancing blow by another Pluto-sized object. Most of the combined matter became Pluto, while the rest spun off to become Charon.
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Jupiter, Largest Planet of the Solar System
Jupiter was the king of the gods in Roman mythology — a fitting name for the largest of the planets. In a similar manner, the ancient Greeks named the planet after Zeus, the king of the Greek pantheon.
Jupiter helped revolutionize the way we saw the universe and ourselves in 1610, when Galileo discovered Jupiter's four large moons — Io, Europa, Ganymede and Callisto, now known as the Galilean moons. This was the first time celestial bodies were not seen circling the Earth, major support of the Copernican view that Earth was not the center of the universe.
Physical Characteristics of the Planet Jupiter
Jupiter is the most massive planet in our solar system, more than twice as massive as all the other planets combined, and had it been about 80 times more massive, it would have actually become a star instead of a planet. Its atmosphere resembles that of the sun, made up mostly of hydrogen and helium, and with four large moons and many smaller moons in orbit around it, Jupiter by itself forms a kind of miniature solar system. All told, the immense volume of Jupiter could hold more than 1,300 Earths.
The colorful bands of Jupiter are arranged in dark belts and light zones created by strong east-west winds in the planet's upper atmosphere traveling more than 400 miles per hour (640 kilometers per hour). The white clouds in the zones are made of crystals of frozen ammonia, while darker clouds of other chemicals are found in the belts. At the deepest visible levels are blue clouds.
The most extraordinary feature on Jupiter is undoubtedly the Great Red Spot, a giant hurricane-like storm seen for more than 300 years. At its widest, the Great Red Spot is three times the diameter of the Earth, and its edge spins counterclockwise around its center at a speed of about 225 miles (360 kilometers) per hour. The color of the storm, which usually varies from brick red to slightly brown, may come from small amounts of sulfur and phosphorus in the ammonia crystals in Jupiter's clouds. Every now and again, the Great Red Spot seems to fade entirely.
Jupiter's gargantuan magnetic field is the strongest of all the planets in the solar system at nearly 20,000 times the strength of Earth's. It traps electrically charged particles in an intense belt of electrons and other electrically charged particles that regularly blasts the planet's moons and rings with a level of radiation more than 1,000 times the lethal level for a human, damaging even heavily-shielded spacecraft such as NASA's Galileo probe. The magnetosphere of Jupiter, which is comprised of these fields and particles, swells out some 600,000 to 2 million miles (1 million to 3 million kilometers) toward the sun and tapers to a tail extending more than 600 million miles (1 billion kilometers) behind Jupiter.
Jupiter spins faster than any other planet, taking a little under 10 hours to complete a turn on its axis, compared with 24 hours for Earth. This rapid spin actually makes Jupiter bulge at the equator and flatten at the poles, making the planet about 7 percent wider at the equator than at the poles.
Jupiter broadcasts radio waves strong enough to detect on Earth. These come in two forms — strong bursts that occur when Io, the closest of Jupiter's large moons, passes through certain regions of Jupiter's magnetic field, and continuous radiation from Jupiter's surface and high-energy particles in its radiation belts.
upiter's Orbital Characteristics
Composition & Structure
* Atmospheric composition (by volume)
89.8 percent molecular hydrogen, 10.2 percent helium, minor amounts of methane, ammonia, hydrogen deuteride, ethane, water, ammonia ice aerosols, water ice aerosols, ammonia hydrosulfide aerosols
* Magnetic Field
Nearly 20,000 times stronger than Earth's
* Chemical composition
A dense core of uncertain composition, surrounded by a helium-rich layer of fluid metallic hydrogen, wrapped up in an atmosphere primarily made of molecular hydrogen.
* Internal structure
A core less than 10 times Earth's mass (Tristan Guillot, "Interiors of Giant Planets Inside and Outside the Solar System." Science Vol. 286 (5437), p. 72-77, October 1, 1999.), surrounded by a layer of fluid metallic hydrogen extending out to 80 to 90 percent of the diameter of the planet, enclosed in an atmosphere mostly made of gaseous and liquid hydrogen.
Composition & Structure
* Atmospheric composition (by volume)
89.8 percent molecular hydrogen, 10.2 percent helium, minor amounts of methane, ammonia, hydrogen deuteride, ethane, water, ammonia ice aerosols, water ice aerosols, ammonia hydrosulfide aerosols
* Magnetic Field
Nearly 20,000 times stronger than Earth's
* Chemical composition
A dense core of uncertain composition, surrounded by a helium-rich layer of fluid metallic hydrogen, wrapped up in an atmosphere primarily made of molecular hydrogen.
* Internal structure
A core less than 10 times Earth's mass (Tristan Guillot, "Interiors of Giant Planets Inside and Outside the Solar System." Science Vol. 286 (5437), p. 72-77, October 1, 1999.), surrounded by a layer of fluid metallic hydrogen extending out to 80 to 90 percent of the diameter of the planet, enclosed in an atmosphere mostly made of gaseous and liquid hydrogen.
Orbit & Rotation
Average Distance from the Sun
English: 483,682,810 miles
Metric: 778,412,020 km
By Comparison: 5.203 times that of Earth
Perihelion (closest)
English: 460,276,100 miles
Metric: 740,742,600 km
By Comparison: 5.036 times that of Earth
Aphelion (farthest)
English: 507,089,500 miles
Metric: 816,081,400 km
By Comparison: 5.366 times that of Earth
(Source: NASA.)
Jupiter's Moons
Jupiter has at least 63 moons, which are often named after the Roman god's many lovers. The four largest moons of Jupiter, now called Io, Europa, Ganymede, and Callisto, were discovered by Galileo Galilei himself, and are appropriately known today as the Galilean satellites.
Ganymede is the largest moon in our solar system, larger even than Mercury and Pluto. It is also the only moon known to have its own magnetic field.
Io is the most volcanically active body in our solar system. The sulfur these volcanoes spew out gives Io a blotted yellow-orange appearance that is often compared to a pepperoni pizza. As Io orbits Jupiter, the planet's immense gravity causes 'tides' in Io's solid surface that rise 300 feet (100 meters) high, generating enough heat for volcanic activity.
The frozen crust of Europa is made up mostly of water ice, and it might hide a liquid ocean holding twice as much water as Earth does. Icy oceans might also exist beneath the crusts of Callisto and Ganymede.
Callisto has the lowest reflectivity, or albedo, of the four Galilean moons. This suggests that its surface might be composed of dark, colorless rock.
Jupiter's Rings
Jupiter's three rings came as a surprise when NASA's Voyager 1 spacecraft discovered them around the planet's equator in 1979. Each are much fainter than Saturn's rings.
Average Distance from the Sun
English: 483,682,810 miles
Metric: 778,412,020 km
By Comparison: 5.203 times that of Earth
Perihelion (closest)
English: 460,276,100 miles
Metric: 740,742,600 km
By Comparison: 5.036 times that of Earth
Aphelion (farthest)
English: 507,089,500 miles
Metric: 816,081,400 km
By Comparison: 5.366 times that of Earth
(Source: NASA.)
Jupiter's Moons
Jupiter has at least 63 moons, which are often named after the Roman god's many lovers. The four largest moons of Jupiter, now called Io, Europa, Ganymede, and Callisto, were discovered by Galileo Galilei himself, and are appropriately known today as the Galilean satellites.
Ganymede is the largest moon in our solar system, larger even than Mercury and Pluto. It is also the only moon known to have its own magnetic field.
Io is the most volcanically active body in our solar system. The sulfur these volcanoes spew out gives Io a blotted yellow-orange appearance that is often compared to a pepperoni pizza. As Io orbits Jupiter, the planet's immense gravity causes 'tides' in Io's solid surface that rise 300 feet (100 meters) high, generating enough heat for volcanic activity.
The frozen crust of Europa is made up mostly of water ice, and it might hide a liquid ocean holding twice as much water as Earth does. Icy oceans might also exist beneath the crusts of Callisto and Ganymede.
Callisto has the lowest reflectivity, or albedo, of the four Galilean moons. This suggests that its surface might be composed of dark, colorless rock.
Jupiter's Rings
Jupiter's three rings came as a surprise when NASA's Voyager 1 spacecraft discovered them around the planet's equator in 1979. Each are much fainter than Saturn's rings.
Research & Exploration
Seven missions have flown by Jupiter — Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Ulysses, Cassini and New Horizons — while another, NASA's Galileo, actually orbited the planet.
Pioneer 10 revealed how dangerous Jupiter's radiation belt is, while Pioneer 11 provided data on the Great Red Spot and close-up pictures of its polar region. Voyager 1 and 2 helped astronomers create the first detailed maps of the Galilean satellites, discovered Jupiter's rings, revealed sulfur volcanoes on Io, and saw lightning in Jupiter's clouds. Ulysses discovered the solar wind has a much greater impact on Jupiter's magnetosphere than before suggested. New Horizons took close-up pictures of Jupiter and its largest moons.
In 1995, Galileo sent a probe plunging towards Jupiter, making the first direct measurements of its atmosphere and measuring the amount of water and other chemicals there. When Galileo ran low on fuel, the craft was intentionally crashed into Jupiter's atmosphere to avoid any risk of it slamming into and contaminating Europa, which might have an ocean below its surface capable of supporting life.
NASA is now planning a mission named Juno scheduled to launch in 2011. It will study Jupiter from a polar orbit to figure out how it and the rest of the solar system formed, which could shed light on how alien planetary systems might have developed.
Jupiter's Gravitational Impact on the Solar System
As the most massive body in the solar system after the sun, the pull of Jupiter's gravity has helped shape the fate of our system. It might have violently hurled Neptune and Uranus outward. Jupiter, along with Saturn, might have slung a barrage of debris toward the inner planets early in the system's history. It might even nowadays help keep asteroids from bombarding Earth, and recent events certainly have shown that it can absorb potentially deadly impacts.
Currently, Jupiter's gravitational field influences numerous asteroids that have clustered into the regions preceding and following Jupiter in its orbit around the sun. These are known as the Trojan asteroids, after three large asteroids there, Agamemnon, Achilles and Hector, names drawn from the Iliad, Homer's epic about the Trojan War.
Possibility of Life on the Planet Jupiter
If one were to dive into Jupiter's atmosphere, one would discover it to grow warmer with depth, reaching room temperature, or 70 degrees F (21 degrees C), at an altitude where the atmospheric pressure is about 10 times as great as it is on Earth. Scientists have conjectured that if Jupiter has any form of life, it might dwell at this level, and would have to be airborne. However, researchers have found no evidence for life on Jupiter.
Seven missions have flown by Jupiter — Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Ulysses, Cassini and New Horizons — while another, NASA's Galileo, actually orbited the planet.
Pioneer 10 revealed how dangerous Jupiter's radiation belt is, while Pioneer 11 provided data on the Great Red Spot and close-up pictures of its polar region. Voyager 1 and 2 helped astronomers create the first detailed maps of the Galilean satellites, discovered Jupiter's rings, revealed sulfur volcanoes on Io, and saw lightning in Jupiter's clouds. Ulysses discovered the solar wind has a much greater impact on Jupiter's magnetosphere than before suggested. New Horizons took close-up pictures of Jupiter and its largest moons.
In 1995, Galileo sent a probe plunging towards Jupiter, making the first direct measurements of its atmosphere and measuring the amount of water and other chemicals there. When Galileo ran low on fuel, the craft was intentionally crashed into Jupiter's atmosphere to avoid any risk of it slamming into and contaminating Europa, which might have an ocean below its surface capable of supporting life.
NASA is now planning a mission named Juno scheduled to launch in 2011. It will study Jupiter from a polar orbit to figure out how it and the rest of the solar system formed, which could shed light on how alien planetary systems might have developed.
Jupiter's Gravitational Impact on the Solar System
As the most massive body in the solar system after the sun, the pull of Jupiter's gravity has helped shape the fate of our system. It might have violently hurled Neptune and Uranus outward. Jupiter, along with Saturn, might have slung a barrage of debris toward the inner planets early in the system's history. It might even nowadays help keep asteroids from bombarding Earth, and recent events certainly have shown that it can absorb potentially deadly impacts.
Currently, Jupiter's gravitational field influences numerous asteroids that have clustered into the regions preceding and following Jupiter in its orbit around the sun. These are known as the Trojan asteroids, after three large asteroids there, Agamemnon, Achilles and Hector, names drawn from the Iliad, Homer's epic about the Trojan War.
Possibility of Life on the Planet Jupiter
If one were to dive into Jupiter's atmosphere, one would discover it to grow warmer with depth, reaching room temperature, or 70 degrees F (21 degrees C), at an altitude where the atmospheric pressure is about 10 times as great as it is on Earth. Scientists have conjectured that if Jupiter has any form of life, it might dwell at this level, and would have to be airborne. However, researchers have found no evidence for life on Jupiter.
