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Standard planetary models suggest that the interior of
Saturn is similar to that of Jupiter, having a small rocky
core surrounded by hydrogen and helium, with trace
[42]
amounts of various volatiles. Analysis of the distortion
shows that Saturn is substantially more centrally
condensed than Jupiter and therefore contains much more
material denser than hydrogen near its center. Saturn's
central regions are about 50% hydrogen by mass, and
[43]
Jupiter's are about 67% hydrogen.
This core is similar in composition to Earth, but is more
dense. The examination of Saturn's gravitational moment,
in combination with physical models of the interior, has
allowed constraints to be placed on the mass of Saturn's
core. In 2004, scientists estimated that the core must be
[44][45]
9–22 times the mass of Earth, which corresponds to
[46]
a diameter of about 25,000 km (16,000 mi). However,
measurements of Saturn's rings suggest a much more
diffuse core, with a mass equal to about 17 Earths and a
[47]
radius equal to about 60% of Saturn's entire radius.
This is surrounded by a thicker, liquid metallic hydrogen
layer, followed by a liquid layer of helium-saturated
molecular hydrogen, which gradually transitions to a gas
as altitude increases. The outermost layer spans about
[48][49][50]
1,000 km (620 mi) and consists of gas.
Saturn has a hot interior, reaching 11,700 °C (21,100 °F)
at its core, and radiates 2.5 times more energy into space
than it receives from the Sun. Jupiter's thermal energy is
generated by the Kelvin–Helmholtz mechanism of slow
gravitational compression; but such a process alone may
not be sufficient to explain heat production for Saturn,
because it is less massive. An alternative or additional
mechanism may be the generation of heat through the
"raining out" of droplets of helium deep in Saturn's interior.
As the droplets descend through the lower-density
hydrogen, the process releases heat by friction and leaves
[51][52]
Saturn's outer layers depleted of helium. These
descending droplets may have accumulated into a helium
[42]
shell surrounding the core. Rainfalls of diamonds have
been suggested to occur within Saturn, as well as in
[53] [54]
Jupiter and ice giants Uranus and Neptune.
Atmosphere
The outer atmosphere of Saturn contains 96.3% molecular
hydrogen and 3.25% helium by volume. The proportion of
helium is significantly deficient compared to the
[42]
abundance of this element in the Sun. The quantity of
elements heavier than helium (metallicity) is 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 heavier elements is
estimated to be 19–31 times the mass of Earth, with a
[55]
significant fraction located in Saturn's core region.
Trace amounts of ammonia, acetylene, ethane, propane,
phosphine, and methane have been detected in Saturn's
[56][57][58]
atmosphere. The upper clouds are composed of
ammonia crystals, while the lower level clouds appear to
consist of either ammonium hydrosulfide (NH4SH) or
[59]
water. Ultraviolet radiation from the Sun causes
methane photolysis in the upper atmosphere, leading to a
series of hydrocarbon chemical reactions with the resulting
products being carried downward by eddies and diffusion.
This photochemical cycle is modulated by Saturn's annual
[58]
seasonal cycle. Cassini observed a series of cloud
features found in northern latitudes, nicknamed the "String
of Pearls". These features are cloud clearings that reside
[60]
in deeper cloud layers.
