Saturn

Physical characteristics A rough comparison of the sizes of Saturn and

Earth Due to a combination of its lower density rapid rotation and fluid

state Saturn is an oblate spheroid that is it is flattened at the poles

and bulges at the equator Its equatorial and polar radii differ by almost

10 60 268 km versus 54 364 km The other gas planets are also oblate but

to a lesser extent Saturn is the only planet of the Solar System that is

less dense than water Although Saturn s core is considerably denser than

water the average specific density of the planet is 0 69 g cm due to the

gaseous atmosphere Saturn is only 95 Earth masses compared to Jupiter

which is 318 times the mass of the Earth but only about 20 larger than

Saturn Internal structure Though there is no direct information about

Saturn s internal structure it is thought that its interior is similar to

that of Jupiter having a small rocky core surrounded mostly by hydrogen

and helium The rocky core is similar in composition to the Earth but

denser Above this there is a thicker liquid metallic hydrogen layer

followed by a layer of liquid hydrogen and helium and in the outermost

1000 km a gaseous atmosphere Traces of various volatile are also present

The core region is estimated to be about 922 times the mass of the Earth

Saturn has a very hot interior reaching 11 700 C at the core and it

radiates 2 5 times more energy into space than it receives from the Sun

Most of the extra energy is generated by the Kelvin Helmholtz mechanism

slow gravitational compression but this alone may not be sufficient to

explain Saturn s heat production An additional proposed mechanism by

which Saturn may generate some of its heat is the raining out of droplets

of helium deep in Saturn s interior the droplets of helium releasing heat

by friction as they fall down through the lighter hydrogen Atmosphere

Saturn s temperature emissions the prominent hot spot at the bottom of

the image is at Saturn s south pole The outer atmosphere of Saturn

consists of about 96 3 molecular hydrogen and 3 25 helium Trace amounts

of ammonia acetylene ethane phosphine and methane have also been detected

The upper clouds on Saturn are composed of ammonia crystals while the

lower level clouds appear to be composed of either ammonium hydrosulfide

NH4SH or water The atmosphere of Saturn is significantly deficient in

helium relative to the abundance of the elements in the Sun The quantity

of elements heavier than helium are not known precisely but the

proportions are assumed to match the primordial abundances from the

formation of the Solar System The total mass of these elements is

estimated to be 1931 times the mass of the Earth with a significant

fraction located in Saturn s core region Cloud layers Saturn s northern

hemisphere as seen by Cassini Note the planet s blue appearance through

the ring Saturn s celestial body atmosphere exhibits a banded pattern

similar to Jupiter s the nomenclature is the same but Saturn s bands are

much fainter and are also much wider near the equator At the bottom

extending for 10 km and with a temperature of 23 C is a layer made up of

water ice After that comes a layer of ammonium hydrosulfide ice which

extends for another 50 km and is approximately at 93 C Eighty kilometers

above that are ammonia ice clouds where the temperatures are about 153 C

Near the top extending for some 200 km to 270 km above the clouds come

layers of visible cloud tops and a hydrogen and helium atmosphere Saturn

s winds are among the Solar System s fastest Voyager data indicate peak

easterly winds of 500 m s 1800 km h Saturn s finer cloud patterns were

not observed until the Voyager flybys Since then however Earth based

telescopy has improved to the point where regular observations can be

made Saturn s usually bland atmosphere occasionally 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 and in 1994 another smaller

storm was observed The 1990 storm was an example of a Great White Spot a

unique but short lived phenomenon which occurs once every Saturnian year

or roughly every 30 Earth years around the time of the northern

hemisphere s summer solstice Previous Great White Spots were observed in

1876 1903 1933 and 1960 with the 1933 storm being the most famous If the

periodicity is maintained another storm will occur in about 2020 In

recent images from the Cassini spacecraft Saturn s northern hemisphere

appears a bright blue similar to Uranus as can be seen in the image below

This blue color cannot currently be observed from Earth because Saturn s

rings are currently blocking its northern hemisphere The color is most

likely caused by Rayleigh scattering Astronomers using infrared imaging

have shown that Saturn has a warm polar vortex and that it is the only

such feature known in the solar system This they say is the warmest spot

on Saturn Whereas temperatures on Saturn are normally 185 C temperatures

on the vortex often reach as high as 122 C North pole hexagon cloud

pattern A persisting hexagonal wave pattern around the north polar vortex

in the atmosphere at about 78N was first noted in the Voyager images

Unlike the north pole HST imaging of the south polar region indicates the

presence of a jet stream but no strong polar vortex nor any hexagonal

standing wave However NASA reported in November 2006 that the Cassini

spacecraft observed a hurricane like storm locked to the south pole that

had a clearly defined eyewall This observation is particularly notable

because eyewall clouds had not previously been seen on any planet other

than Earth including a failure to observe an eyewall in the Great Red

Spot of Jupiter by the Galileo spacecraft The straight sides of the

northern polar hexagon are each about 13 800 km long The entire structure

rotates with a period of 10h 39 m 24s the same period as that of the

planet s radio emissions which is assumed to be equal to the period of

rotation of Saturn s interior The hexagonal feature does not shift in

longitude like the other clouds in the visible atmosphere The pattern s

origin is a matter of much speculation Most astronomers seem to think it

was casued by some standing wave pattern in the atmosphere but the

hexagon might be a novel aurora Polygonal shapes have been replicated in

spinning buckets of fluid in a laboratory North polar hexagonal cloud

feature discovered by Voyager 1 and confirmed in 2006 by Cassini

Animation of hexagonal cloud feature Spring unveils Saturn s hexagon

Magnetosphere Main article Magnetosphere of Saturn Photo of Saturn by

Hubble showing both polar aurorae Saturn has an intrinsic magnetic field

that has a simple symmetric shape magnetic dipole Its strength at the

equator0 2 gauss 20 T s approximately one twentieth than that of the

field around Jupiter and slightly weaker than Earth s magnetic field As a

result the cronian magnetosphere is much smaller than the jovian and

extends slightly beyond the orbit of Titan Most probably the magnetic

field is generated similarly to that of Jupitery currents in the metallic

hydrogen layer which is called a metallic hydrogen dynamo Similarly to

those of other planets this magnetosphere is efficient at deflecting the

solar wind particles from the Sun The moon Titan orbits within the outer

part of Saturn s magnetosphere and contributes plasma from the ionized

particles in Titan s outer atmosphere Orbit and rotation The average

distance between Saturn and the Sun is over 1 400 000 000 km 9 AU With an

average orbital speed of 9 69 km s it takes Saturn 10 759 Earth days or

about 29 years to finish one revolution around the Sun The elliptical

orbit of Saturn is inclined 2 48 relative to the orbital plane of the

Earth Because of an eccentricity of 0 056 the distance between Saturn and

the Sun varies by approximately 155 000 000 km between perihelion and

aphelion which are the nearest and most distant points of the planet

along its orbital path respectively The visible features on Saturn rotate

at different rates depending on latitude and multiple rotation periods

have been assigned to various regions as in Jupiter s case System I has a

period of 10 h 14 min 00 s 844 3 d and encompasses the Equatorial Zone

which extends from the northern edge of the South Equatorial Belt to the

southern edge of the North Equatorial Belt All other Saturnian latitudes

have been assigned a rotation period of 10 h 39 min 24 s 810 76 d which

is System II System III based on radio emissions from the planet in the

period of the Voyager flybys has a period of 10 h 39 min 22 4 s 810 8 d

because it is very close to System II it has largely superseded it

However a precise value for the rotation period of the interior remains

elusive While approaching Saturn in 2004 the Cassini spacecraft found

that the radio rotation period of Saturn had increased appreciably to

approximately 10 h 45 m 45 s 36 s The cause of the change is unknownt was

thought to be due to a movement of the radio source to a different

latitude inside Saturn with a different rotational period rather than

because of a change in Saturn s rotation Later in March 2007 it was found

that the rotation of the radio emissions did not trace the rotation of

the planet but rather is produced by convection of the plasma disc which

is dependent also on other factors besides the planet s rotation It was

reported that the variance in measured rotation periods may be caused by

geyser activity on Saturn s moon Enceladus The water vapor emitted into

Saturn s orbit by this activity becomes charged and weighs down Saturn s

magnetic field slowing its rotation slightly relative to the rotation of

the planet At the time it was stated that there is no currently known

method of determining the rotation rate of Saturn s core The latest

estimate of Saturn s rotation based on a compilation of various

measurements from the Cassini Voyager and Pioneer probes was reported in

September 2007 is 10 hours 32 minutes 35 seconds Planetary rings The

rings of Saturn imaged here by Cassini in 2007 are the most conspicuous

in the Solar System Artist s impression of the Phoebe ring which dwarfs

the main rings Main article Rings of Saturn Saturn is probably best known

for its system of planetary rings which makes it the most visually

remarkable object in the solar system They extend from 6 630 km to 120

700 km above Saturn s equator average approximately 20 meters in

thickness and are composed of 93 percent water ice with a smattering of

tholin impurities and 7 percent amorphous carbon The particles that make

up the rings range in size from specks of dust to the size of a small

automobile There are two main theories regarding the origin of Saturn s

rings One theory is that the rings are remnants of a destroyed moon of

Saturn The second theory is that the rings are left over from the

original nebular material from which Saturn formed On 6 October 2009 the

discovery was announced of a tenuous outer disk of material that is in

the plane of Phoebe s orbit which is tilted 27 degrees from Saturn s

equatorial plane The ring is from 128 to 207 times the radius of Saturn

and is thought to originate from micrometeoroid impacts on Phoebe which

orbits at an average distance of 215 Saturn radii The ring material

should thus share Phoebe s retrograde orbital motion and after migrating

inward would encounter Iapetus s leading face which could help explain

the dramatic two faced nature of this satellite While the infalling

material cannot be directly responsible for the observed pattern of light

and dark regions on Iapetus it is believed to have initiated a runaway

thermal self segregation process in which ice sublimes from warmer

regions and condenses onto cooler regions This leaves contrasting areas

of dark ice depleted residue and bright ice deposits Natural satellites

Main article Moons of Saturn Four of Saturn s moons Dione Titan

Prometheus edge of rings Telesto top center Saturn has at least 62 moons

Titan the largest comprises more than 90 percent of the mass in orbit

around Saturn including the rings Saturn s second largest moon Rhea may

have a tenuous ring system of its own Many of the other moons are very

small 34 are less than 10 km in diameter and another 14 less than 50 km

Traditionally most of Saturn s moons have been named after Titans of

Greek mythology History and exploration There are three main phases of

observation and exploration of Saturn The first era was ancient

observations such as with the naked eye before the invention of the

modern telescopes Starting in the 1600s progressively more advanced

telescopic observations from earth have been made The other type is

visitation by spacecraft either by orbiting or flyby In the 21st century

observations continue from the earth or earth orbiting observatories and

from the Cassini orbiter at Saturn Ancient observations Saturn has been

known since prehistoric times In ancient times it was the most distant of

the five known planets in the solar system excluding Earth and thus a

major character in various mythologies In ancient Roman mythology the god

Saturnus from which the planet takes its name was the god of the

agricultural and harvest sector The Romans considered Saturnus the

equivalent of the Greek god Kronos The Greeks had made the outermost

planet sacred to Kronos and the Romans followed suit In Hindu astrology

there are nine astrological objects known as Navagrahas Saturn one of

them is known as Sani or Shani the Judge among all the planets and by

everyone accordingly to their own performed deeds bad or good Ancient

Chinese and Japanese





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