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Our solar system is our neighborhood in space.
The Sun is the center of this neighborhood,
which includes at least nine planets
plus hundreds of moons and thousands of asteriods.
Sun
Diameter: 863,750 miles
Mass: 884 x 1029 lbs.
Temperature: 9,980° F (surface); 28,000,000° F (core)
The Sun is one of more than 100 billion stars in our galaxy.
The Sun is by far the largest object in the solar system. It contains more than
99.8% of the total mass of the solar system (Jupiter contains most of the rest).
At present, the Sun is about 75% hydrogen and 25% helium by mass. Everything
else ("metals") amounts to only 0.1%. This changes slowly over time as the Sun
converts hydrogen to helium in its core.
The Sun is personified in many mythologies: the Greeks called it Helios and the
Romans called it Sol.
The outer layers of the Sun rotate at different rates. At the equator, the surface
rotates once every 25.4 days. Near the poles the surface rotates once every 36
days.
Conditions at the Sun's core are extreme. The temperature is 28,000,000° F and
the core's gases are compressed to a density 150 times that of water.
The Sun outputs 386 billion billion megawatts of energy produced by nuclear
fusion reactions.
The surface of the Sun, called the photosphere, is at a temperature of about
9,980° F. Sunspots are "cool" regions (only 6380° F) caused by complicated
interactions with the Sun's magnetic field and can be very large, as much as
31,000 miles in diameter.
A small region known as the chromosphere lies above the photosphere.
The region above the chromosphere, called the corona, extends millions of
kilometers into space but is visible only during eclipses. Temperatures in the
corona are more than 1,800,000° F.
In addition to heat and light, the Sun also emits a low density stream of charged
particles (mostly electrons and protons) known as the solar wind, which travels
throughout the solar system at about 280 km/sec.
The solar wind and the much higher energy particles ejected by solar flares can
dramatically affect the Earth with changes ranging from power line surges to
radio interference to the beautiful aurora borealis.
The solar wind affects the tails of comets and the trajectories of spacecraft.
Spectacular loops and prominences are often visible on the Sun's edge.
The Sun is about 4.5 billion years old. Since its birth it has used up about half of
the hydrogen in its core. It will continue to radiate "peacefully" for another 5
billion years or so, but eventually it will run out of hydrogen fuel. It will then
expand into a red giant, resulting in the total destruction of the Earth.
Mercury
Mercury is the closest planet to the Sun. Mercury is smaller in
diameter than Jupiter's moons, Ganymede and Titan.
Orbit: 57,910,000 km or (36,092,000 miles) (0.38 A.U.) from Sun
Diameter: 4880 km or 2508 miles
Mass: 3.30 x 1023 kg or 7.26 x 1023 lbs
In Roman mythology Mercury is the god of commerce, travel and thievery, the
Roman counterpart of the Greek god Hermes, the messenger of the Gods. The
planet probably received this name because it moves so quickly across the sky.
Mercury has been known since at least the time of the Sumerians (3rd
millennium BC).
Mercury was given two names by the Greeks: Apollo for its apparition as a
morning star and Hermes as an evening star.
Mercury has been visited by only one spacecraft, Mariner 10. It flew by three
times in 1973 and 1974. Only 45% of the surface has been mapped (and,
unfortunately, it is too close to the Sun to be safely imaged by the Hubble Space
Telescope).
Until 1962 it was thought that Mercury's "day" was the same length as its "year"
so as to keep that same face to the Sun much as the Moon does to the Earth. It
is now known that Mercury rotates three times in two of its years.
Temperature changes on Mercury are the most extreme in the solar system
ranging from -300°F to 800°F. The temperature on Venus is slightly hotter but
very stable.
Mercury is in many ways similar to the Moon: its surface is heavily cratered and
very old; it has no plate tectonics. On the other hand, Mercury is much denser
than the Moon.
Mercury's interior is mostly a large iron core whose radius is 2800 to 3000 miles.
The silicate outer shell (the same as Earth's mantle and crust) is only 750 to 900
miles thick. At least some of the core is probably molten.
Mercury actually has a very thin atmosphere consisting of atoms blasted off its
surface by the solar wind. Because Mercury is so hot, these atoms quickly escape
into space. Unlike the Earth and Venus whose atmospheres are stable, Mercury's
atmosphere is constantly being replaced.
The surface of Mercury exhibits enormous canyons, some up to hundreds of miles
in length and as much as 4 ½ miles high.
One of the largest features on Mercury's surface is the Caloris Basin; it is about
2000 miles in diameter. It is similar to the large basins (maria) on the Moon. It
was probably caused by a very large impact early in the history of the solar
system. That impact was probably also responsible for the odd terrain on the
exact opposite side of the planet
In addition to the heavily cratered terrain, Mercury also has regions of relatively
smooth plains. Perhaps from the result of ancient volcanic activity.
Amazingly, radar observations of Mercury's north pole (a region not mapped by
Mariner 10) show evidence of water ice in the protected shadows of some
craters.
Mercury has a small magnetic field whose strength is about 1% of Earth's.
Mercury has no known satellites.
Mercury is often visible with binoculars or even the naked eye, but it is always
very near the Sun and difficult to see in the twilight sky.
Venus
Venus is the brightest object in the sky, second only to the Moon. It is the second
planet from the Sun, and the sixth largest planet.
Orbit: 108,200,000 km or (67,238,500 miles) or (0.72 AU) from Sun
Diameter: 12,100 km (7521 miles)
Mass: 4.869 x 1024 kg (9835 x 1024 lbs)
The Romans named Venus after the goddess of love and beauty probably
because it is the brightest of the planets known to the ancients.
Venus was known to the Greeks as Aphrodite and to the
Babylonians as Ishtar. With a few exceptions, the surface
features on Venus are named for female figures.
Venus has been known since prehistoric times.
It is the brightest object in the sky except for the Sun and
the Moon.
Like Mercury, Venus was popularly thought to be two separate bodies, but Greek
astronomers knew better.
Since Venus is closer to the Sun than Earth, it shows phases when viewed with a
telescope. Galileo's observation of this phenomenon was important evidence in
favor of Copernicus's heliocentric (Sun-centered) theory of the solar system.
The first spacecraft to visit Venus was Mariner 2 in 1962. It was then visited by
more than 20 others. Other spacecraft included Pioneer Venus and the Soviet
Venera 7, the first spacecraft to land on another planet, and Venera 9 which
returned the first photographs of the surface.
Most recently, orbiting U.S. spacecraft Magellan produced detailed maps of
Venus' surface using radar.
Venus' rotation is very slow with 243 Earth days equal to one Venus day. Venus
rotation is retrograde, opposite to the Earth's rotation.
Venus' year lasts 225 days, almost equal to the length of its day.
Venus is sometimes regarded as Earth's sister planet. In some ways, Earth and
Venus are very similar: Venus is only slightly smaller than Earth Its diameter is
equal to 95% of the size of Earth's diameter, and 80% of Earth's mass. Both
have few craters indicating relatively young surfaces. Their densities and
chemical compositions are also similar.
More detailed study of Venus reveals that in many important ways it is very
different from Earth: The pressure of Venus' atmosphere at the surface is 90
atmospheres, about the same as the pressure at a depth of one kilometer in the
Earth's oceans. It is made mostly of carbon dioxide and sulfuric acid. These
clouds completely block our view of the surface. This dense atmosphere produces
a run-away greenhouse effect that raises Venus' surface temperature by about
400 degrees to over 800 degrees, which is hot enough to melt lead. Venus'
surface is actually hotter than Mercury's despite being nearly twice as far from
the Sun.
Winds are strong (200 mph) at the cloud tops but at the surface they are very
slow, at no more than a few miles per hour.
Venus probably once had large amounts of water like Earth, but it all boiled
away. Venus is now quite dry. Earth would have suffered the same fate had it
been just a little closer to the Sun. We may learn a lot about Earth by learning
why the basically similar Venus turned out so differently.
Data from Magellan's imaging radar shows that much of Venus' surface is
covered by lava flows. There are several large shield volcanoes (similar to Hawaii
or Olympus Mons on Mars) such as Sif Mons. Venus is still volcanically active, but
only in a few hot spots. For the most part, Venus been geologically quiet for the
past few hundred million years.
Venus has no small craters. It seems that small meteoroids burn up in Venus'
dense atmosphere before reaching the surface. Craters on Venus seem to come
in bunches indicating that large meteoroids that do reach the surface usually
break up in the atmosphere.
Magellan's images show a wide variety of interesting and unique features
including pancake volcanoes which seem to be eruptions of very thick lava and
coronae which seem to be collapsed domes over large magma chambers.
The interior of Venus is probably very similar to that of Earth. It has an iron core
about 1,865 miles in radius and a molten rocky mantle comprising the majority of
the planet.
Venus has no magnetic field, perhaps because of its slow rotation.
Venus has no satellites.
Venus is usually visible with the naked eye. Sometimes referred to as the
morning star or the evening star, it is by far the brightest "star" in the sky.
Earth
Earth is the third planet from the Sun and the fifth largest
planet.
Orbit: 149,600,000 km (93,000,000 miles) or (1.00 AU) from
Sun Diameter: 12,756.3 km or 7927 miles
Mass: 5.9736 x 1024 kg or 12.066 x 1024 lbs
Earth has one moon.
Earth is the only planet whose English name does not come from Greek/Roman
mythology. The name comes from Old English and Germanic. In Roman
mythology, the goddess of Earth was Tellus - the fertile soil.
It was not until the 16th century that it was understood that Earth is just another
planet.
Earth can be studied without the aid of spacecraft, but it was not until the 20th
century that maps were made of the entire planet. Pictures of the planet taken
from space are important because they help in weather prediction and in tracking
and predicting hurricanes.
Earth is divided into several layers which have distinct chemical and seismic
properties:







0- 40 km - Crust
40- 400 km - Upper mantle
400- 650 km - Transition region
650-2700 km - Lower mantle
2700-2890 km - D-layer
2890-5150 km - Outer core
5150-6378 km - Inner core
Earth's crust changes in thickness. It is thinner under the oceans and thicker
under the continents. The inner core and crust are solid and the outer core and
mantle layers are fluid.
Most of the mass of Earth is in the mantle (94%) The part we inhabit is a tiny
fraction of the whole:






Atmosphere - 0.0000051%
Oceans - 0.001%
Crust - 0.026%
Mantle - 4.043%
Outer core -1.835%
Inner core - 0.097%
The core is composed mostly of iron, although it is possible that some lighter
elements may be present.
Temperatures at the center of the core may be as high as 7500 Kelvin, hotter
than the surface of the Sun.
Taken as a whole, Earth's chemical composition (by mass) is:







Iron -34.6%
Oxygen - 29.5%
Silicon - 15.2%
Magnesium - 12.7%
Nickel - 2.4%
Sulfur - 1.9%
Titanium - 0.05%
Earth is the most dense major body in the solar system.
Unlike other planets, Earth's crust is divided into several separate solid plates
which float around independently on top of the hot mantle below. This is known
as plate tectonics. Presently, there are eight major plates.
Earth is 4.5 to 4.6 billion years old, but the oldest known rocks are about four
billion years old. Rocks older than three billion years are rare. The oldest fossils
of living organisms are less than 3.9 billion years old.
Seventy-one percent of Earth's surface is covered with water. Earth is the only
planet on which water can exist in a liquid form on the surface. Liquid water is
essential for human life
The heat capacity of the oceans keeps Earth's temperature stable. Liquid water is
also responsible for most of the erosion and weathering of Earth's continents.
Earth's atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon
dioxide and water. The tiny amount of carbon dioxide in the atmosphere
maintains Earth's surface temperature because of the greenhouse effect. The
greenhouse effect raises the temperature about 35 degrees above what it would
normally be (from -1 F to +46 F). Without this, the oceans would freeze.
The interaction of Earth and the Moon slows Earth's rotation by about two
milliseconds per century. Research shows that days were 18-hours long and a
year lasted 481 days about 900 million years ago.
Mars
The Bringer of War Mars is the fourth planet from the Sun and the
seventh largest:
Orbit: 227,940,000 km (141,641,900 miles) or (1.52 AU) from Sun
Diameter: 6,794 km or 4222 miles
Mass: 6.4219 x 1023 km or 12.98 x 1028 lbs
Mars (Greek: Ares) is the god of War. The planet probably got this name due to
its red color; Mars is sometimes referred to as the Red Planet. (An interesting
side note: the Roman god Mars was a god of agriculture before becoming
associated with the Greek Ares; those in favor of colonizing and terraforming
Mars may prefer this symbolism.) The name of the month March derives from
Mars.
Mars has been known since prehistoric times. It is still a favorite of science fiction
writers as the most favorable place in the Solar System (other than Earth!) for
human habitation. But the famous "canals" "seen" by Lowell and others were,
unfortunately, just as imaginary as Barsoomian princesses.
The first spacecraft to visit Mars was Mariner 4 in 1965. Several other followed
including the two Viking landers in 1976. Ending a long 20 year hiatus, Mars
Pathfinder landed successfully on Mars on 1997 July 4.
Mars' orbit is significantly elliptical. One result of this is
a temperature variation of about 30 C at the subsolar
point between aphelion and perihelion. This has a
major influence on Mars' climate. While the average
temperature on Mars is about 218 K (-55 C, -67 F),
Martian surface temperatures range widely from as
little as 140 K (-133 C, -207 F) at the winter pole to
almost 300 K (27 C, 80 F) on the dayside during summer.
Though Mars is much smaller than Earth, its surface area is about the same as
the land surface area of Earth.
Except for Earth, Mars has the most highly varied and interesting terrain of any
of the terrestrial planets, some of it quite spectacular: Olympus Mons: the largest
mountain in the Solar System rising 24 km (78,000 ft.) above the surrounding
plain. Its base is more than 500 km in diameter and is rimmed by a cliff 6 km
(20,000 ft) high. Tharsis: a huge bulge on the Martian surface that is about 4000
km across and 10 km high. - Valles Marineris: a system of canyons 4000 km long
and from 2 to 7 km deep; Hellas Planitia: an impact crater in the southern
hemisphere over 6 km deep and 2000 km in diameter. Much of the Martian
surface is very old and cratered, but there are also much younger rift valleys,
ridges, hills and plains.
The southern hemisphere of Mars is pre ntly ancient cratered highlands
somewhat similar to the Moon. In contrast, most of the northern hemisphere
consists of plains which are much younger, lower in elevation and have a much
more complex history. An abrupt elevation change of several kilometers seems to
occur at the boundary. The reasons for this global abrupt boundary are unknown
(some speculate that they are due to a very large impact shortly after Mars'
formation). Recently, some scientists have begun to question whether the abrupt
elevation is real in the first place. Mars Global Surveyor should resolve the issue.
The interior of Mars is known only by inference from data about the surface and
the bulk statistics of the planet. The most likely scenario is a dense core about
1700 km in radius, a molten rocky mantle somewhat denser than the Earth's and
a thin crust. Mars' relatively low density compared to the other terrestrial planets
indicates that its core probably contains a relatively large fraction of sulfur in
addition to iron (iron and iron sulfide).
Like Mercury and the Moon, Mars appears to lack active plate tectonics; there is
no evidence of horizontal motion of the surface such as the folded mountains so
common on Earth. With no lateral plate motion, hot-spots under the crust stay in
a fixed position relative to the surface. This, along with the lower surface gravity,
may account for the Tharis bulge and its enormous volcanoes. There is no
evidence of current volcanic activity, however. And though Mars may have been
more volcanicly active in the past, it appears to never have had any plate
tectonics.
There is very clear evidence of erosion in many places on Mars including large
floods and small river systems. At some time in the past there was clearly water
on the surface There may have been large lakes or even oceans. But it seems
that this occurred only briefly and very long ago; the age of the erosion channels
is estimated at about nearly 4 billion years. (Valles Marineris was NOT created by
running water. It was formed by the stretching and cracking of the crust
associated with the creation of the Tharsis bulge.)
Early in its history, Mars was much more like Earth. As with Earth almost all of its
carbon dioxide was used up to form carbonate rocks. But lacking the Earth's plate
tectonics, Mars is unable to recycle any of this carbon dioxide back into its
atmosphere and so cannot sustain a significant greenhouse effect. The surface of
Mars is therefore much colder than the Earth would be at that distance from the
Sun.
Mars has a very thin atmosphere composed mostly of the tiny amount of
remaining carbon dioxide (95.3%) plus nitrogen (2.7%), argon (1.6%) and
traces of oxygen (0.15%) and water (0.03%). The average pressure on the
surface of Mars is only about 7 millibars (less than 1% of Earth's), but it varies
greatly with altitude from almost 9 millibars in the deepest basins to about 1
millibar at the top of Olympus Mons. But it is thick enough to support very strong
winds and vast dust storms that on occasion engulf the entire planet for months.
Mars' thin atmosphere produces a greenhouse effect but it is only enough to raise
the surface temperature by 5 degrees (K); much less than what we see on Venus
and Earth.
Mars has permanent ice caps at both poles composed mostly of solid carbon
dioxide ("dry ice"). The ice caps exhibit a layered structure with alternating layers
of ice with varying concentrations of dark dust. In the northern summer the
carbon dioxide completely sublimes, leaving a residual layer of water ice. It's not
known if a similar layer of water ice exists below the southern cap since its
carbon dioxide layer never completely disappears. The mechanism responsible for
the layering is unknown but may be due to climatic changes related to long-term
changes in the inclination of Mars' equator to the plane of its orbit. There may
also be water ice hidden below the surface at lower latitudes. The seasonal
changes in the extent of the polar caps changes the global atmospheric pressure
by about 25% (as measured at the Viking lander sites).
Recent observations with the Hubble Space Telescope have revealed that the
conditions during the Viking missions may not have been typical. Mars'
atmosphere now seems to be both colder and dryer than measured by the Viking
landers.
The Viking landers performed experiments to determine the existence of life on
Mars. The results were somewhat unclear but most scientists now believe that
they show no evidence for life on Mars (there is still some controversy, however).
Optimists point out that only two tiny samples were measured and not from the
most favorable locations. More experiments will be done by future missions to
Mars.
A small number of meteorites (the SNC meteorites) are believed to have
originated on Mars.
On 1996 Aug 6, David McKay and others announced the first identification of
organic compounds in a Martian meteorite. The authors further suggest that
these compounds, in conjunction with a number of other mineralogical features
observed in the rock, may be evidence of ancient Martian microorganisms.
Exciting as this is, it is important to note while this evidence is strong it by no
means establishes the fact of extraterrestrial life. There have also bee several
contradictory studies published since the McKay paper. Remember,
"extraordinary claims require extraordinary evidence." Much work remains to be
done before we can be confident of this most extraordinary claim.
Large, but not global, weak magnetic fields exist in various regions of Mars. This
unexpected finding was made by Mars Global Surveyor just days after it entered
Mars orbit. They are probably remnants of an earlier global field that has since
disappeared. This may have important implications for the structure of Mars's
interior and for the past history of its atmosphere and hence for the possibility of
ancient life.
When it is in the nighttime sky, Mars is easily visible with the naked eye. Its
apparent brightness varies greatly according to its relative position to the Earth.
Mars' Satellites:
Mars has two tiny satellites which orbit very close to the surface.
Satellite Distance(000km)
Radius
(km)
Mass
(kg)
Discoverer Date
Phobos
9
11
1.08e16
Hall
1877
Deimos
23
6
1.80e15
Hall
1877
Jupiter
The Bringer of Jollity Jupiter is the fifth planet from the Sun, the
largest in the solar system.
Orbit: 778,330,000 km (483,654,262 miles) or (5.20 AU) from
Sun
Diameter: 142,984 km (88850 miles)
Mass: 1.900 x 1027 kg (3.838 x 1027 lbs)
In mythology, Jupiter (also known as Jove or Zeus) was the King of the Gods, the
ruler of Olympus and the patron of the Roman state. Zeus was the son of Cronus
(Saturn).
If Jupiter were hollow, more than one thousand Earths could fit inside. It also
contains more matter than all of the other planets combined. It has a mass of 1.9
x 1027 kg and is 142,800 kilometers (88,736 miles) across the equator. Jupiter
possesses 16 satellites, four of which - Callisto, Europa, Ganymede and Io - were
observed by Galileo as long ago as 1610. There is a ring system, but it is very
faint and totally invisible from the Earth. (The ring was discovered in 1979 by
Voyager 1.)
The atmosphere is very deep, perhaps comprising the whole planet,
and is somewhat like the Sun. It is composed mainly of hydrogen
and helium, with small amounts of methane, ammonia, water vapor
and other compounds. Deep within Jupiter, the pressure is so great
that the hydrogen atoms are broken up and the electrons are freed
so that the resulting atoms consist of bare protons. This produces a
state in which the hydrogen becomes metallic.
Jupiter was first visited by Pioneer 10 in 1973 and later by Pioneer 11, Voyager 1,
Voyager 2 and Ulysses. The spacecraft Galileo is currently in orbit around Jupiter
and will be sending back data for at least the next two years.
Colorful latitudinal bands, atmospheric clouds and storms illustrate Jupiter's
dynamic weather systems. The cloud patterns change within hours or days. The
Great Red Spot is a complex storm moving in a counter-clockwise direction. At
the outer edge, material appears to rotate in four to six days; near the center,
motions are small and nearly random in direction. An array of other smaller
storms and eddies can be found throughout the banded clouds.
Auroral emissions, similar to Earth's northern lights, were observed in the polar
regions of Jupiter. The auroral emissions appear to be related to material from Io
that spirals along magnetic field lines to fall into Jupiter's atmosphere. Cloud-top
lightning bolts, similar to superbolts in Earth's high atmosphere, were also
observed.
In July 1994, Comet Shoemaker-Levy 9 collided with Jupiter with spectacular
results. The effects were clearly visible even with amateur telescopes. The debris
from the collision was visible for nearly a year afterward with HST. Jupiter's Ring
Unlike Saturn's intricate and complex ring patterns, Jupiter has a single ring that
is almost uniform in its structure. It is probably composed of dust particles less
than 10 microns in diameter-about the size of cigarette smoke particles. It
extends to an outer edge of about 129,000 kilometers (80,161 miles) from the
center of the planet and inward to about 30,000 kilometers (18,642 miles). The
origin of the ring is probably from micrometeorite bombardment of the tiny
moons orbiting within the ring.
Jupiter's ring and moons exist within an intense radiation belt of electrons and
ions trapped in the planet's magnetic field. These particles and fields comprise
the jovian magnetosphere or magnetic environment, which extends 3 to 7 million
kilometers (1.9 to 4.3 million miles) toward the Sun, and stretches in a windsock
shape at least as far as Saturn's orbit-a distance of 750 million kilometers (466
million miles).
The Galileo atmospheric probe discovered a new intense radiation belt between
Jupiter's ring and the uppermost atmospheric layers. This new belt is
approximately 10 times as strong as Earth's Van Allen radiation belts.
Surprisingly, this new belt was also found to contain high energy helium ions of
unknown origin.
Values for the smaller moons are approximate.
Jupiter's Ring Distance(km) Width(km) Mass (kg)
Halo
100000
22800
?
Main
122800
64000
le13
Gossamer
120200
850000
?
Jupiter's satellites are named for other figures in mythology.
Satellite
Distanct (000km) Radius (km) Mass (kg) Discoverer Date
Metis
128
20
9.57e16
Synott
1979
Adrastea
129
10
1.91e16
Jewitt
1979
Amalthea
181
98
7.17e18
Barnard
1982
Thebe
222
50
7.77e17
Synott
1979
Io
422
1815
8.94e22
Galileo
1610
Europa
671
1569
4.80e22
Galileo
1610
Ganymede 1070
2631
1.48e23
Galileo
1610
Callisto
1883
2400
1.08e23
Galileo
1610
Leda
11094
8
5.68e15
Kowal
1964
Himalia
11480
93
9.56e18
Perrine
1904
Lysithea
11720
18
7.77e16
Nicholson
1938
Elara
11737
38
7.77e17
Perrine
1905
Ananke
21200
15
3.82e16
Nicholson
1951
Carme
22600
20
9.56e16
Nicholson
1938
Pasiphae
23500
25
1.91e17
Melotte
1908
Sinope
23700
18
7.77e16
Nicholson
1914
Saturn
Saturn is the sixth planet from the Sun and the second largest:
Orbit: 1,429,4000 km (887,220,000 miles) or (9.54 AU) from Sun
Diameter: 120,536 km or 74,901 miles (equatorial)
Mass: 5.68 x 1026 kg or 12.5244 x 1026 lbs
Cassini on its way! The Cassini mission to explore Saturn and its moon Titan was
launched successfully on Oct. 15, 1997. The first of its four, planned gravity
assist maneuvers has been successfully completed. It will arrive at Saturn on July
1, 2004.
In Roman mythology, Saturn is the god of agriculture. The associated Greek god,
Cronus, was the son of Uranus and Gaia and the father of Zeus (Jupiter). Saturn
is the root of the English word "Saturday."
Saturn has been known since prehistoric times. Galileo was the first to observe it
with a telescope in 1610; he noted its odd appearance but was confused by it.
Early observations of Saturn were complicated by the fact that Earth passes
through the plane of Saturn's rings every few years as Saturn moves in its orbit.
A low-resolution image of Saturn, therefore, changes drastically. It was not until
1659 that Christiaan Huygens determined the geometry of the rings. Saturn's
rings remained unique in the known solar system until 1977 when very faint rings
were discovered around Uranus and shortly thereafter around Jupiter and
Neptune.
Saturn was first visited by Pioneer 11 in 1979 and later by Voyager 1 and
Voyager 2. Cassini, now on its way, will arrive in 2004.
Saturn is visibly flattened (oblate) when viewed through a small telescope; its
equatorial and polar diameters vary by almost 10 percent (74,901 miles vs.
67,563 miles). This is the result of its rapid rotation and fluid state. The other gas
planets are also oblate, but not nearly as much as Saturn.
Saturn is the least dense of the planets; its specific gravity (0.7) is less than that
of water. If you could find a container big enough and fill it with water, Saturn
would float.
Like Jupiter, Saturn is about 75 percent hydrogen and 25 percent helium with
traces of water, methane, ammonia and "rock," similar to the composition of the
primordial Solar Nebula from which the solar system was formed.
Saturn's interior is similar to Jupiter's and consists of a rocky core, a liquid
metallic hydrogen layer and a molecular hydrogen layer. Traces of various ices
are also present.
Saturn's interior is hot (12,273 F at the core), and it radiates more energy into
space than it receives from the sun. Most of the extra energy is generated by the
Kelvin-Helmholtz mechanism, as in Jupiter. But this may not be sufficient to
explain Saturn's luminosity. Some additional mechanism may be at work,
perhaps the "raining out" of helium deep in Saturn's interior.
The bands so prominent on Jupiter are much fainter on Saturn. They are also
much wider near the equator. Details in the cloud tops are invisible from Earth,
so it was not until the Voyager encounters that any detail of Saturn's atmospheric
circulation could be studied. Saturn also exhibits long-lived ovals and other
features common on Jupiter. In 1990, the Hubble Space Telescope observed an
enormous white cloud near Saturn's equator, which was not present during the
Voyager encounters. In 1994, another, smaller storm was observed.
Two prominent rings (A and B) and one faint ring (C) can be seen from Earth.
The gap between the A and B rings is known as the Cassini division. The much
fainter gap in the outer part of the A ring is known as the Encke Gap. The
Voyager pictures show four additional faint rings. Saturn's rings, unlike the rings
of the other planets, are very bright.
Though they look continuous from Earth, the rings are actually composed of
innumerable small particles each in an independent orbit. They range in size from
a centimeter or so to several meters. A few mile-sized objects are also likely.
Saturn's rings are very thin. Though they're 155,350 miles or more in diameter,
they're no more than one-mile thick. Despite their impressive appearance,
there's really very little material in the rings. If the rings were compressed into a
single body, it would be no more than 62 miles across.
The ring particles seem to be composed primarily of water ice, but they may also
include rocky particles with icy coatings.
Voyager confirmed the existence of a puzzling feature in the rings called "spokes"
which were first reported by amateur astronomers. Their nature remains a
mystery but may have something to do with Saturn's magnetic field.
Saturn's outermost ring, the F-ring, is a complex structure made up of several
smaller rings along which "knots" are visible. Scientists speculate that the knots
may be clumps of ring material, or mini moons. The strange braided appearance
visible in the Voyager 1 images is not seen in the Voyager 2 images perhaps
because Voyager 2 imaged regions where the component rings are roughly
parallel.
There are complex tidal resonances between some of Saturn's moons and the
ring system. Some of the moons, the so-called "shepherding satellites" (i.e.
Atlas, Prometheus and Pandora), are clearly important in keeping the rings in
place. Mimas seems to be responsible for the paucity of material in the Cassini
division, which seems to be similar to the Kirkwood gaps in the asteroid belt. Pan
is located inside the Encke Gap. The whole system is very complex and as yet,
poorly understood.
The origin of the rings of Saturn (and the other gas planets) is unknown. Though
they may have had rings since their formation, the ring systems are not stable.
They must be regenerated by ongoing processes, probably the breakup of larger
satellites.
Like the other gas planets, Saturn has a significant magnetic field.
When it is in the nighttime sky, Saturn is easily visible to the naked eye. Though
it is not nearly as bright as Jupiter, it is easy to identify as a planet because it
doesn't "twinkle" like the stars. The rings and the larger satellites are visible with
a small astronomical telescope.
Saturn has 18 named satellites, more than any other planet. There may also be
several small ones yet to be discovered.
Of those moons for which rotation rates are known, all but Phoebe and Hyperion
rotate synchronously. The three pairs, Mimas-Tethys, Enceladus-Dione and TitanHyperion, interact gravitationally in such a way as to maintain stable
relationships between their orbits. The period of Mimas' orbit is exactly half that
of Tethys. They are thus said to be in a 1:2 resonance. Enceladus-Dione are also
1:2; Titan-Hyperion are in a 3:4 resonance. In addition to the 18 named
satellites, at least a dozen more have been reported and given provisional
designations.
Saturn's Satellites
Satellite
Distance Radius Mass
(miles)
(miles) (lbs)
Pan
83
6
?
Atlas
85
8.5
?
Prometheus 87
28.5
?
Pandora
88
28.5
?
Epimethius 98
35.5
?
Janus
98
55
?
Mimas
115.5
122
?
Enceladus
148
161.5
?
Tethys
183
329
?
Telesto
183
9
?
Calypso
184
8
?
Dione
234
348
2.394 x 10e21
Helene
234
9.5
?
Rhea
327.5
475
5.6772 x 10e21
Titan
759
1600
3.078 x 10e23
Hyperion
920
89
4.0356 x 10e19
Iapetus
2213
453
4.2864 x 10e21
Phoebe
8048
68
9.12 x 10e18
Saturn's Rings
Distance
Width
(miles)
(miles) (lbs)
D
41634
4660
?
C
46294
10874
2.508 x 10e18
B
57169
15946
6.384 x 10e19
A
75935
9072
14.136 x 10e18
F
87126
311
?
G
103028
4971
2.28 x 10 e 7
E
111852
186420 ?
Ring
Mass
Cassini division
Distance is from Saturn's center to the ring's inner edge. This categorization is
actually somewhat misleading as the density of particles varies in a complex way
not indicated by a division into neat regions. There are variations within the
rings, the gaps are not entirely empty and the rings are not perfectly circular.
Uranus
The Magician
Uranus is the seventh planet from the sun and the third largest
(by diameter). Uranus is larger in diameter but smaller in mass
than Neptune.
Orbit: 2,870,990,000 km (1,784,033,000 miles ) or (19.218 AU) from Sun
Diameter: 51,118 km or 31,764 miles (equatorial)
Mass: 8.683 x 1025 kg or 17.539 x 1025 lbs
Uranus is the ancient Greek deity of the Heavens, the earliest supreme god.
Uranus was the son and mate of Gaia, the father of Cronus (Saturn), and of the
Cyclopes and Titans (predecessors of the Olympian gods).
Uranus, the first planet discovered in modern times, was discovered by William
Herschel while systematically searching the sky with his telescope on March 13,
1781. It had actually been seen many times before but ignored as simply another
star (the earliest recorded sighting was in 1690 when John Flamsteed cataloged it
as 34 Tauri). Herschel named it "the Georgium Sidus" (the Georgian Planet) in
honor of his patron, the infamous (to Americans) King George III of England;
others called it "Herschel." The name "Uranus" was first proposed by Bode in
conformity with the other planetary names from classical mythology but didn't
come into common use until 1850.
Uranus has been visited by only one spacecraft, Voyager 2, on Jan. 24, 1986.
Most of the planets spin on an axis nearly perpendicular to the plane of the
ecliptic, but Uranus' axis is almost parallel to the ecliptic. At the time of the
Voyager 2 passage, Uranus' south pole was pointed almost directly at the sun.
This results in
the odd fact that Uranus' polar regions receive more energy input from the sun
than do its equatorial regions. Uranus is nevertheless hotter at its equator than
at its poles. The mechanism underlying this is unknown.
Actually, there's an ongoing battle over which of Uranus' poles is its north pole!
Either its axial inclination is a bit over 90 degrees, and its rotation is direct; or it's
a bit less than 90 degrees, and the rotation is retrograde. The problem is that
you need to draw a dividing line "somewhere" because in a case like Venus, there
is little dispute that the rotation is indeed retrograde (not a direct rotation with
an inclination of nearly 180).
Uranus is composed primarily of rock and various ices, with only about 15
percent hydrogen and a little helium (in contrast to Jupiter and Saturn, which are
mostly hydrogen). Uranus and Neptune are in many ways similar to the cores of
Jupiter and Saturn minus the massive liquid metallic hydrogen envelope. It
appears that Uranus does not have a rocky core like Jupiter and Saturn but
rather that its material is more or less uniformly distributed.
Uranus' atmosphere is about 83 percent hydrogen, 15 percent helium and 2
percent methane.
Like the other gas planets, Uranus has bands of clouds that blow around rapidly.
But the clouds are extremely faint, visible only with radical image enhancement
of the Voyager 2 pictures. Recent observations with HST show larger and more
unced streaks. The speculation is that the difference is due to seasonal effects
(the sun is now at a somewhat lower Uranian latitude which may cause more
unced day/night effects).
Uranus' blue color is the result of absorption of red light by methane in the upper
atmosphere. There may be colored bands like Jupiter's, but they are hidden from
view by the overlaying methane layer.
Like the other gas planets, Uranus has rings. Like Jupiter's, they are very dark;
but like Saturn's, they are composed of fairly large particles ranging up to 10
meters in diameter in addition to fine dust. There are 11 known rings, all very
faint; the brightest is known as the Epsilon ring. The Uranian rings were the first
to be discovered after Saturn's. This was of considerable importance since we
now know that rings are a common feature of planets,
not a peculiarity of Saturn alone.
Voyager 2 discovered 10 small moons in addition to the five large ones already
known. It is likely that there are several more tiny satellites within the rings.
Uranus' magnetic field is odd in that it is not centered on the center of the planet
and is tilted almost 60 degrees with respect to the axis of rotation. It is probably
generated by motion at relatively shallow depths within Uranus.
Uranus is sometimes just barely visible with the naked eye on a very clear night;
it is fairly easy to spot with binoculars (if you know exactly where to look). A
small astronomical telescope will show a small disk.
Uranus' Satellites
Uranus has 15 named moons plus two recently discovered ones, which have yet
to be named.
Unlike the other bodies in the solar system which have names from classical
mythology, Uranus' moons take their names from the writings of Shakespeare
and Pope. They form two distinct classes: the 10 small very dark inner ones
discovered by Voyager 2 and the five large outer ones.
They all have nearly circular orbits in the plane of Uranus' equator (and hence at
a large angle to the plane of the ecliptic).
Uranus' Satellites
Satellite
Radius
Distance
Mass
(km)
(000 km)
(kg)
Discoverer Date
Cordelia
50
13
?
Voyager 2 1986
Ophelia
54
16
?
Voyager 2 1986
Bianca
59
22
?
Voyager 2 1986
Cressida
62
33
?
Voyager 2 1986
Desdmona 63
29
?
Voyager 2 1986
Juliet
64
42
?
Voyager 2 1986
Portia
66
55
?
Voyager 2 1986
Rosiland
70
27
?
Voyager 2 1986
Belinda
75
34
?
Voyager 2 1986
Puck
86
77
?
Voyager 2 1985
Miranda
130
266
6.30e19 Kuiper
1948
Ariel
191
579
1.27e21 Lassell
1851
Umbriel
266
585
1.27e21 Lassell
1851
Titania
436
789
3.49e21 Herschel
1787
Oberon
583
761
3.49e21 Herschel
1787
Caliban
7200
30
?
Gladman
1997
Sycorax
12000
60
?
Gladman
1997
Uranus' Rings
Ring
Distance Width
(km)
(km)
1986U2R 38000
2,500
6
41840
1-3
5
42230
2-3
4
42580
2-3
Alpha
44720
7-12
Beta
45670
7-12
Eta
47190
0-2
Gamma
47630
1-4
Delta
48290
3-9
1986U1R 50020
1-2
Epsilon
20-100
51140
Neptune
Orbit: 4,504,000,000 km or (2,798,785,600 miles) or (30.06 AU)
from Sun
Diameter: 49,532 km or 30779 miles (equatorial)
Mass: 1.0247 x 1026 kg or 2.069 x 1026 lbs
Neptune is the eighth planet from the sun and the fourth largest (by diameter). It
is smaller in diameter but larger in mass than Uranus.
In Roman mythology, Neptune (Greek: Poseidon) was the god of the sea.
After the discovery of Uranus, it was noticed that its orbit was not as it should be
in accordance with Newton's laws. It was therefore predicted that another more
distant planet must be perturbing Uranus' orbit. Galle and d'Arrest first observed
Neptune on Sept. 23, 1846, very near to the locations independently predicted by
Adams and Le Verrier from calculations based on the observed positions of
Jupiter, Saturn and Uranus. An international dispute arose between the English
and French (though not apparently between Adams and Le Verrier personally)
over priority and the right to name the new planet. Adams and Le Verrier are
now jointly credited with Neptune's discovery.
Subsequent observations have shown that the orbits calculated by Adams and Le
Verrier diverge from Neptune's actual orbit fairly quickly. Had the search for the
planet taken place a few years earlier or later, it would not have been found
anywhere near the predicted location.
Neptune has been visited by only one spacecraft, Voyager 2, on Aug. 25, 1989.
Almost everything we know about Neptune comes from this encounter.
Because Pluto's orbit is so eccentric, it sometimes crosses the orbit of Neptune.
Since 1979, Neptune has actually been the most distant planet from the sun;
Pluto will again be the most distant in 1999.
Neptune's composition is probably similar to Uranus': various "ices" and rock with
about 15 percent hydrogen and a little helium. Like Uranus, but unlike Jupiter
and Saturn, it may not have a distinct internal layering but have a more or less
uniform composition. But there is most likely a small core (about the mass of the
Earth) of rocky material. Its atmosphere is mostly hydrogen and helium with a
small amount of methane.
Neptune's blue color is the result of absorption of red light by methane in the
atmosphere.
Like a typical gas planet, Neptune has rapid winds confined to bands of latitude
and large storms or vortices. Neptune's winds are the fastest in the solar system,
reaching 2,000 km/hour (1,250 miles/hour).
Like Jupiter and Saturn, Neptune has an internal heat source -- it radiates more
than twice as much energy as it receives from the sun.
At the time of the Voyager encounter, Neptune's most prominent feature was the
Great Dark Spot in the southern hemisphere. It was about half the size as
Jupiter's Great Red Spot (about the same diameter as Earth). Neptune's winds
blew the Great Dark Spot westward at 300 meters/second (700 mph). Voyager 2
also saw a smaller dark spot in the southern hemisphere and a small irregular
white cloud that zips around Neptune every 16 hours or so now known as "The
Scooter" (right). It may be a plume rising from lower in the atmosphere, but its
true nature remains a mystery.
However, HST observations of Neptune in 1994 show that the Great Dark Spot
has disappeared! It has either simply dissipated or is currently being masked by
other aspects of the atmosphere. A few months later, HST discovered a new dark
spot in Neptune's northern hemisphere. This indicates that Neptune's atmosphere
changes rapidly, perhaps due to slight changes in the temperature differences
between the tops and bottoms of the clouds.
Neptune also has rings. Earth-based observations showed only faint arcs instead
of complete rings, but Voyager 2's images showed them to be complete rings
with bright clumps. One of the rings appears to have a curious twisted structure.
Like Uranus and Jupiter, Neptune's rings are very dark; but their composition is
unknown.
Neptune's rings have been given names: the outermost is Adams (which contains
three prominent arcs now named Liberty, Equality and Fraternity), next is an
unnamed ring coorbital with Galatea, then Leverrier (whose outer extensions are
called Lassell and Arago), and finally the faint but broad Galle.
Neptune's magnetic field is, like Uranus', oddly oriented and probably generated
by motions of conductive material (probably water) in its middle layers.
Neptune can be seen with binoculars (if you know exactly where to look), but a
large telescope is needed to see anything other than a tiny disk.
Neptune's Satellites
Neptune has eight known moons -- seven small ones and Triton.
Radius
Distance
Mass
Satellite
(km)
(000 km)
(kg)
Discoverer Date
Naiad
48
29
?
Voyager 2 1989
Thalassa 50
40
?
Voyager 2 1989
Despina 53
74
?
Voyager 2 1989
Galatea 62
79
?
Voyager 2 1989
Larissa
96
?
Voyager 2 1989
Proteus 118
209
?
Voyager 2 1989
Triton
355
1350
2.14e22 Lassell
1946
Nereid
5509
170
?
1949
74
Kuiper
Pluto
Orbit: 5,913,520,000 km (3,674,661,000 miles) or (39.5 AU) from
Sun
Diameter: 2274 km or 1,413 miles (?)
Mass: 1.27 x 1022 kg or 2.8022 lbs (?)
Pluto is the farthest Solar System body from the sun (usually). Pluto is smaller
than seven of the solar system's moons (the moon, Io, Europa, Ganymede,
Callisto, Titan and Triton).
In Roman mythology, Pluto (Greek: Hades) is the god of the underworld. The
planet received this name (after many other suggestions) perhaps because it's so
far from the sun that it is in perpetual darkness.
Pluto was discovered in 1930 by a “fortunate accident”. Calculations, which later
turned out to be in error, had predicted a planet beyond Neptune based on the
motions of Uranus and Neptune. Not aware of the error, Clyde W. Tombaugh at
Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto
anyway.
After the discovery of Pluto, it was quickly determined that Pluto was too small to
account for the discrepancies in the orbits of the other planets. The search for
Planet X continued, but nothing was found at that time.
Pluto has a large satellite, or moon, named Charon. It was discovered (in 1978)
just before its orbital plane moved edge-on toward the inner solar system. It was
therefore possible to observe many transits of Pluto over Charon and vice versa.
By carefully calculating which portions of which body would be covered at what
times and watching brightness curves, astronomers were able to construct a
rough map of light and dark areas on both bodies.
Pluto's radius is not well known. JPL's value of 706.5 is given with an error of +/8, almost one percent.
Though the sum of the masses of Pluto and Charon is known pretty well (it can
be determined from careful measurements of the period and radius of Charon's
orbit and Kepler's Third Law), the individual masses of Pluto and Charon are
difficult to determine. This would require determining their mutual motions
around the center of mass of the system, which requires much finer
measurements. They're so small and far away that even HST has difficulty. The
ratio of their masses is probably somewhere between 0.084 and 0.157. More
observations are underway.
Pluto's orbit is highly eccentric. At times it is closer to the sun than Neptune (the
last time was from 1979 to 1999). Pluto rotates in the opposite direction from
most of the other planets.
Pluto is locked in a 3:2 resonance with Neptune; i.e. Pluto's orbital period is
exactly 1.5 times longer than Neptune's. Its orbital inclination is also much higher
than the other planets. Thus, though it appears that Pluto's orbit crosses
Neptune's, it really doesn't; and they will never collide.
Like Uranus, the plane of Pluto's equator is at almost right angles to the plane of
its orbit. The surface temperature on Pluto is not well known but is probably
between 35 and 45 Kelvins (-378 to -396 F).
Pluto's composition is unknown, but its density indicates that it is probably a
mixture of 70 percent rock and 30 percent water ice much like Triton. The bright
areas of the surface seem to be covered with ices of nitrogen with smaller
amounts of (solid) methane and carbon monoxide. The composition of the darker
areas of Pluto's surface is unknown but may be due to primordial organic material
or photochemical reactions driven by cosmic rays.
Little is known about Pluto's atmosphere, but it probably consists primarily of
nitrogen with some carbon monoxide and methane. It is extremely tenuous, the
surface pressure being only a few microbars. Pluto's atmosphere may exist as a
gas only when Pluto is near its perihelion; for the majority of Pluto's long year,
the atmospheric gases are frozen into ice. Near perihelion, it is likely that some
of the atmosphere escapes to space perhaps even interacting with Charon.
The unusual nature of the orbits of Pluto and of Triton and the similarity of bulk
properties between Pluto and Triton suggest some historical connection between
them. It was once thought that Pluto may have once been a satellite of
Neptune's, but this now seems unlikely. A more popular theory is that Triton, like
Pluto, once moved in an independent orbit around the sun and was later captured
by Neptune. Perhaps Triton, Pluto and Charon are the only remaining members of
a large class of similar objects the rest of which were ejected into the Oort cloud.
Like the Earth's moon, Charon may be the result of a collision between Pluto and
another body.
Pluto can be seen with an amateur telescope, but it is not easy. Mike Harvey's
planet finder charts show the current position of Pluto (and the other planets) in
the sky, but much more detailed charts and careful observations over several
months will be required to actually find it. Suitable charts can be created with
many planetarium programs such as Starry Night.
Charon Charon ("KAIR en") is Pluto's largest satellite. Orbit: 19,640 km (12,204
miles) from Pluto Diameter: 1172 km or 728 miles (?) Mass: 1.90 x 1021 or
3.821 lb.. (?) Charon is named for the mythological figure who ferried the dead
across the River Styx intolly find it. Suitable charts can be created with many
planetarium programs such as Starry Night.
Jim Christy discovered Charon in 1978. Prior to that it was thought that Pluto was
much larger since the images of Charon and Pluto were blurred together.
Charon is unusual in that it is the largest moon with respect to its primary planet
in the solar system (a distinction once held by Earth's moon).
Charon's radius is not well known. JPL's value of 364 has an error margin of +/13, more than two percent. Its mass and density are also poorly known.
Pluto and Charon are also unique in that not only does Charon rotate
synchronously, but Pluto does, too. They both keep the same face toward one
another. (This makes the phases of Charon as seen from Pluto very interesting.)
Charon's composition is unknown, but its low density indicates that it may be
similar to Saturn's icy moons (i.e. Rhea). Its surface seems to be covered with
water ice.
Unlike Pluto, Charon does not have large albedo features, though it may have
smaller ones that have not been resolved.
It has been proposed that Charon was formed by a giant impact similar to the
one that formed Earth's moon. It is doubtful that Charon has a significant
atmosphere.
In May 2005, an important discovery was made. By studying many images sent
by the Hubble Space Telescope, astronomers concluded that Pluto actually has 3
moons. The 2 additional moons were previously unknown. Later named Hydra
and Nix, their diameters are close to 100 miles and are on the same orbital plane
as Charon, though much farther away from Pluto. Observations of Hydra and Nix
continue, with the on-going assistance of the Hubble Space Telescope, to gain
further knowledge of these two distant moons of Pluto.
Pluto is the only planet that has not yet been visited by a spacecraft. Even the
Hubble Space Telescope can resolve only the largest features on its surface. So,
NASA designed the first mission to explore Pluto, called New Horizons. The
spacecraft of the same name was launched in January of 2006, destined to reach
Pluto and it’s 3 moons by 2015. New Horizons will unlock many secrets of Pluto
and its three satellites. NASA has 7 scientific instruments on board the probe.
The instruments will send back much information about the surface properties,
geology, interior makeup and atmosphere.
There are some who think Pluto would be better classified as a large asteroid or
comet rather than as a planet. Some consider it to be the largest of the Kuiper
Belt objects.
In August 2006, the International Astronomical Union passed a resolution and
created 3 new categories to classify planets and other Solar System bodies:
1. A planet is a celestial body that (a) is in orbit around the Sun, (b) has
sufficient mass for its self-gravity to overcome rigid body forces so that it
assumes a hydrostatic equilibrium (nearly round) shape, and (c) has
cleared the neighbourhood around its orbit.
2. A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b)
has sufficient mass for its self-gravity to overcome rigid body forces so that
it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not
cleared the neighbourhood around its orbit and (d) is not a satellite.
3. All other objects orbiting the Sun shall be referred to collectively as “Small
Solar System Bodies”.
This reclassifies Pluto as “dwarf plant”, based on this new criteria. Some
astronomers, educators, students and the public do not agree with this change
and believe our Solar System should stay the same as it has been. Others
understand that Pluto and it’s 3 satellites haven’t changed; our understanding of
the physical features and attributes have changed, due to advanced technology
and new discoveries. This recent information allows scientists to recognize
differences and see our Solar System in a new way.
What do you think about this change?