Saturn Fast Facts about a Planet in Orbit

Saturn is the most remote of the planets known to ancient man.

Though its gorgeous and immense rings have made it a sort of symbol for all planets, these rings were not seen by mankind until the mid-17th century (see figure on page 107).The rings and moons form, in themselves, a sort of miniature laboratory analogous to the early solar system, with interactions among dust and particles and larger bodies all orbiting together within the same magnetic field. Now the complex interactions between its hundreds of rings and dozens of moons is the subject of intense research, especially following the June 30, 2004, arrival into Saturn's orbit of the much-awaited Cassini-Huygens space mission.

Saturn has not been studied by missions in the intensive way that Jupiter has been, and the Cassini-Huygens mission should deliver enough data and images to Earth to enable scientists to answer some of the outstanding questions about this spectacular planet. Cassini-Huygens hopes to answer questions about the internal structure of the planet, interactions among its magnetic field, moons, and rings, and most particularly, questions about its giant moon, Titan.

Each planet and some other bodies in the solar system (the Sun and certain asteroids) have been given its own symbol as a shorthand in scientific writing. The symbol for Saturn is shown on page 108.

Titan is the last object in the solar system with a mysterious surface: Its thick atmosphere prevented the two Voyager missions from seeing it, and the best images of Titan's surface are tantalizing blurry images from the Hubble Space Telescope using special technology.These images hint that Titan may have oceans and continents. If Titan does have oceans, they

Fundamental Information about Saturn W hile Saturn's diameter is more than nine times as large as the Earth's, it is the least dense of solar system objects. While Jupiter is on average about one-third more dense than water, Saturn is actually less dense than water and would float if it were possible to place it in a bath large enough. The low density of Saturn has another effect: Though the planet's radius is almost 10 times as great as the Earth's, its gravitational acceleration at a pressure equivalent to Earth's surface is just 90 percent of the Earth's. Were it possible to stand on Saturn, the fortunate astronaut would feel lighter than on Earth.

equatorial radius at the height where atmospheric pressure is one bar polar radius ellipticity volume

Fundamental Facts about Saturn

37,450 miles (60,268 km)

average density acceleration of gravity at the equator at the height where atmospheric pressure is one bar magnetic field strength at the surface rings moons

33,780 miles (54,362 km)

0.098, meaning the planet's equator is almost 10 percent longer than its polar radius 1.98 X 1014 cubic miles (8.27 X 1014 km3), or 755 times Earth's volume

1.25 X 1026 pounds (5.69 X 1026 kg), or 95 times Earth's mass

43.7 pounds per cubic foot (700 kg/m3), less than the density of water

23.6 feet per second squared (7.21 m/sec2), or 0.74 times Earth's gravity

2 X 10-5 tesla hundreds

33 presently known mass will consist of hydrocarbons and not water, which is frozen harder than rock at the temperatures of Titan's surface. Nonetheless Titan may be similar in some ways to conditions on the early Earth, and it may form a laboratory of sorts to answer questions about early life. Titan's atmosphere is thicker than the Earth's but consists primarily of nitrogen, as does the Earth's.Titan's cold temperatures may have preserved it in the conditions of the distant past, similar to Earth's before the development of life.

On average, Jupiter is 5.2 AU from the Sun, that is, 5.2 times farther than the Earth. Saturn, though it is spoken of as the twin of Jupiter because of their many similarities, is almost twice as far from the Sun as Jupiter is. Saturn averages 9.54 AU from the Sun, immensely farther away than Jupiter. That and Saturn's other orbital characteristics are listed in the table on page 108.

Seasons are caused largely by the tilt of the planet's rotational axis, called its obliquity (see figure on page 110).As a planet rotates around the Sun, its axis always points the same direction (the axis does wobble slightly, a movement called precession).The planet with the most extreme obliquity is Venus, with an obliquity of 177.3 degrees, followed by Pluto, with an obliquity of 122.53 degrees. These obliquities above 90 degrees mean that the planet's north pole has passed through its orbital plane and now points south. This is similar to Uranus, which has a rotational axis tipped until it almost lies flat in its orbital plane. Some scientists also think that Mercury's obliquity is 180 degrees, not 0 degrees, as usually reported. Earth's obliquity is 23.45 degrees, Mars's is similar, at 25.2 degrees, Jupiter's is 3.12 degrees, Saturn's is 26.7 degrees, and Neptune's is 29.56 degrees. With the exceptions of Mercury and

Saturn's gorgeous rings have made it a symbol for all planets; though the planet has been known since prehistory, its rings could not be seen until the invention of the telescope. (NASA and The Hubble Heritage Team [STScI/AURA], R G. French [Wellesley College], J. Cuzzi [NASA/Ames], L. Dones [SwRI], and J. Lissauer [NASA/Ames])

Many solar system objects have simple symbols; this is the symbol for Saturn.

ORBITAL PARAMETERS FOR SATURN

rotation on its axis ("day")

10 hours, 14 minutes at the equator, and 10 hours,

40 minutes at the poles; the second-shortest day

length in the solar system after Jupiter

rotation speed at equator

22,081 miles per hour (35,536 km/hour)

rotation direction

prograde (counterclockwise when viewed from

above the north pole)

sidereal period ("year")

29.46 Earth years

orbital velocity (average)

6.01 miles per second (9.67 km/sec)

sunlight travel time (average)

1 hour, 19 minutes, and 20 seconds to reach Saturn

average distance from the Sun

885,904,700 miles (1,426,725,400 km), or 9.54 AU

perihelion

885,519,000 miles (1,349,467,000 km), or 9.021 AU

from the Sun

aphelion

934,530,000 miles (1,503,983,000 km), or 10.054 AU

from the Sun

orbital eccentricity

0.0542

orbital inclination to the ecliptic

2.48 degrees

obliquity (inclination of equator to orbit)

26.73 degrees

Jupiter, therefore, all the planets have significant seasons caused by obliquity.

When a planet with obliquity has its north pole tipped toward the Sun, the northern hemisphere receives more direct sunlight than the southern hemisphere does. The northern hemisphere then experiences summer, and the southern hemisphere is in winter. As the planet progresses in its orbit, revolving around the Sun until it has moved 180 degrees, then the southern hemisphere gets more direct sunlight, and the northern hemisphere is in winter. The more oblique the rotation axis, the more severe the seasons: The hemisphere in summer receives even more sunlight, the other hemisphere even less. Summers are hotter and winters are colder. The obliquity of a planet may change over time, as well. Mars's obliquity may oscillate by as much as 20 degrees over time, creating seasons that are much more extreme. The Moon's stabilizing influence on the Earth has prevented large changes in obliquity and helped maintain a more constant climate, allowing life to continue and flourish.

Summer occurs when the Sun shines most directly on the hemisphere in question, and the intensity of summer must depend on when it occurs in the planet's orbit. If the planet's axis tilts such that the hemisphere has summer at perihelion, when the planet is closest to the Sun, then it will be a much hotter summer than if that hemisphere had summer at aphelion, when the planet is farthest from the Sun (summer at the aphelion will also be shorter, since the planet is moving faster). Planets' orbits do precess, that is, wobble, and so the positions of the seasons change over tens of thousands of years. This leads to a long-term cycle in seasonal severity.

In the northern hemisphere, the midpoint of summer occurs when the north pole points most directly toward the Sun. This is called the summer solstice: the longest day of the year, after which days become shorter. The northern hemisphere's winter solstice, when the south pole points most directly toward the Sun, is its shortest day of the year.This same day is the southern hemisphere's summer solstice, its longest day of the year.The planet's obliquity has the largest control over the severity of seasons (there are secondary effects that also influence the temperature differences between seasons). In the summer the Sun is higher in the sky, and so spends more time crossing the sky, and therefore the days are longer.This gives the Sun more time to heat the planet. In the winter the Sun is lower, and

Obliquity and the Seasons

Autumnal equinox ca. September 23rd

Winter solstice ca. December 21st

Perihelion January 3

Aphelion 1 July 4

Summer solstice ca. June 21st

24-hour sun

Sun overhead

No sun

Autumnal equinox ca. September 23rd

Winter solstice ca. December 21st

Perihelion January 3

Aphelion 1 July 4

Summer solstice ca. June 21st

24-hour sun

Sun overhead

No sun

Northern Hemisphere Summer

No sun

24-hour sun

Northern Hemisphere Summer

No sun

24-hour sun

A planet's obliquity (the inclination of its equator to its orbital plane) is the primary cause of seasons; this example is for the Earth.

the days are short, giving the Sun less time to heat the planet. Along with summer solstice and winter solstice, there are two other days that divide the year into quarters. The vernal equinox is the day in spring when day and night are the same length (equinox means equal night).The autumnal equinox is the day in fall when day and night are the same length.

Saturn's obliquity has been obvious for centuries from the tilt of its rings. Now, sensitive images of its weather patterns show that the planet may have storms that track the seasons created by the planet's obliquity and orbit around the Sun. Large bright storms appear and disappear on a 30-year cycle, and smaller features can now be tracked as well. Saturn's obliquity is similar to the Earth's and to Mars's, but far higher than its neighbor and most similar planet Jupiter. Because of its obliquity, Saturn may have cycles of weather and atmospheric circulation entirely different from Jupiter's.

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