Giant
Planets
Between the region of the small inner planets and the sparsely
inhabited region of the comets there are 4 giant planets: Jupiter
(with a mass equivalent to 318 Earths, a diameter of 142984 km and orbiting the
Sun at 778 million km from it), Saturn (95, 120536, 1427 million), Uranus (15,
51118, 2871 million) and Neptune (17, 49528, 4497).
Clockwise, from the upper
left corner: Jupiter, Saturn, Uranus and Neptune (Calvin J. Hamilton)
Interiors
Composition
Among the giant planets, Jupiter is the one that displays the most
similar composition in regard to the primitive solar nebula, consisting in 74% of
hydrogen, 24% of helium and 2% of heavier elements. Hydrogen and helium also
mainly compose Saturn, but the latter is scarce at the upper layers. On the
other hand, Uranus and Neptune appear to be essentially composed by ice (water,
ammonia and methane) and rock.
In the case of Uranus and Neptune, it's noticeable that the ratio
diameter/mass is not very different from the one that would be expected in
planets entirely composed by water. On the other hand, Jupiter and Saturn
display ratios that would be expected in planets with compositions similar to
the Sun's. Although Jupiter has more than 3 times the mass of Saturn, its
diameter is just slightly bigger, which is due to the higher compression that
subjects its interior.
Structure of the Interiors
Under the pressure of 1 bar (close to the atmospheric pressure at the Earth's surface), the
temperature is 165 ºK in Jupiter, 135 ºK in Saturn and 76 ºK in Uranus. At this
level, the hydrogen appears under the form of a molecular gas (H2).
At deeper levels, where the pressures are higher, the temperature and
density of the gas gradually increase. Under the pressure of 100 000 bars, the
gas starts to look like a hot liquid. The transition is gradual, given that the
temperatures are well above the critical point of the hydrogen, which is equal
to 13 bars and 33 ºK.
20 000 km below the top of the Jupiter's clouds, the liquid hydrogen is
submitted to a pressure of more than 4 million bars and to a temperature of 10 000
ºK. The molecular connections disappear, giving place to another form: the
liquid "metallic" hydrogen. Just as in the interior of the Sun, this
form of hydrogen consists in a plasma of stripped protons and electrons, though
at a temperature that is considerably inferior to the solar. The hydrogen then
behaves like a molten metal, being an electric conductor and opaque to the
visible radiation.
Saturn is less massive and, for that reason, the internal pressures are
lower. Even so, it's thought that Saturn also contains liquid metallic hydrogen
in its interior. On the other hand, in Uranus and Neptune the hydrogen seems to
be confined to the upper layers. So, it is unlikely that they contain any
metallic hydrogen, although the opposite shall happen in regard to the liquid
molecular hydrogen.
The Saturn's atmosphere seems to be stripped from helium, comparatively
to its estimated proportion in the primitive solar nebula. This fact may be due to
the insolubility of the helium in the metallic hydrogen, when the temperatures
are low enough. It's thought that these conditions occur in Saturn. If this
separation actually occurred in Saturn's interior, then the helium should have
migrated (rained out) to the metallic hydrogen region. This region would then
have been enriched with this element.
It is thought that Jupiter and Saturn possess rocky and icy cores, with
masses ranging between 10 and 15 times the Earth's mass. The exterior surfaces
of these cores must be subjected to a temperature of 20 000 ºK in Jupiter and
12 000 ºK in Saturn. At those surfaces, the pressure must be 42 megabars
(million bars) in Jupiter and 12 megabars in Saturn.
It's thought that Uranus (14,5 terrestrial masses) and Neptune (17,2
terrestrial masses) are basically cores of rock and ice, which are present
there in "solar" proportions (similar to the ones existing in the
primitive nebula). In other words, they shall be composed by little more than ¼
of rock and almost ¾ of ice (water – H2O, methane - CH4
and ammonia - NH3). Under the conditions that prevail there, this
ice shall, however, behave like a liquid and hot soup, rich in chemical species
derived from those molecules.
Diagram of the interior
of the giant planets (The New Solar System - 3rd edition)
Internal Heat, Convection and Magnetism
Contrarily to what happens in the inner planets, the internal heat
generated inside the giant planets is due, along with the radioactive
decay, to the kinetic energy caused by the gravitational contraction. However, Saturn seems
to generate more heat than what would be expected in regard to its mass. That
may be due to the sinking of dense components, like helium, towards the
planetary centre.
In the deep interiors of Jupiter and Saturn, the convection creates and
sustains powerful dynamos in the zones of metallic hydrogen, which in turn
generate strong magnetic fields. In Uranus and Neptune, the slow convection of
their internal fluids shall be the responsible factor for the observed magnetic
fields.
Another source of internal heat in the giant planets is the tides
provoked by their satellites. This fact triggers the creation of currents in
the internal fluids, which very gradually slow down the rotation of the planet.
A portion of the energy thus generated also helps moving the satellite into
farther orbits.
Structure of the several
layers of a giant planet (The New Solar System - 3rd Edition)
Atmospheres
Chemical Unbalance
"Temperate" species (like the CO and the N2) that
are only stable in the deep regions of very high temperatures and pressures
can, nevertheless, be observed in the zone of the atmosphere where the
temperature assumes comparatively modest values of 1000 ºK or less. What gives
birth to this phenomenon is the fact that, in that region, the duration of the
chemical reactions until the balance is reached can become longer than the time
taken by these species to attain those altitudes (mixing convective time).
Not only the convection, but also the condensation in clouds, the
photo-dissociation (under the action of the light), the bombardment by charged
particles (auroras) and the electric discharge (lightning) are responsible
factors for the formation of unstable molecules in the several atmospheric
layers where they occur.
Clouds
It's thought that close to the lower limit of the described zone, the
condensations (clouds) are mainly composed by iron, magnesium and silicon
trioxide (SiO3).
In the troposphere, the dominant gases, apart from the molecular
hydrogen and the helium, are the water (H2O), the ammonia (NH3)
and the o methane (CH4). In this layer and descending in the
atmosphere, the clouds are essentially composed by ammonia (which condenses at
cooler temperatures, in the higher layers of the atmosphere), ammonium
hydrosulphide (NH4SH) and water (which condenses at warmer
temperatures, in the lower layers of the atmosphere). In Uranus and Neptune,
the temperature of the high atmosphere is so low that it also allows the
condensation of methane, which is the responsible molecule for the blue colour
of these two planets.
The Galileo probe,
penetrating in Jupiter's atmosphere (NASA - JPL)
Atmospheric Compression
Finally, it's noticeable that the compression degree of the giant
planets' atmospheres is proportional to the gravitational acceleration exerted on each one of
them. Therefore, the Jupiter's atmosphere is the most compressed and the
thinnest, or in other words, the distance between two given layers is lower,
just like the distance between a region where the atmospheric pressure assumes
a determined value and another region where that variable reaches twice that
value.
Winds and Dynamics
In the giant planets, the way the temperature changes according to the
latitude is an indicator of how effectively the winds distribute the heat from
the zone that the Sun targets more strongly (sub-solar zone). In Jupiter,
Saturn and Neptune the Sun targets more strongly the equator and in Uranus,
given the tilt of its rotation axis, the Sun heats more strongly the poles. In
these 4 cases the temperature differences between the latitudes are modest,
smaller than they would be without the winds.
This doesn't mean that the heat is transported by the winds themselves
(that doesn't actually seem to happen, given the structure of the winds in
stream lines that are confined between two fixed latitudes), but this
redistribution may rather take place in the fluid and hot interiors of the
planets, although it's not yet clearly known how does that happen.
Stream lines and eddies
at the atmosphere of Jupiter (HST - NASA)
Duality Structure-Dynamics
Besides the heat, the atmospheric flow transports mass and chemical
species, thus changing the atmospheric structure. The structure, in turn,
controls the light absorption, its re-emission as infrareds and the release of
heat during the condensation. These factors determine the temperature and,
therefore, the winds' pattern.
The Role of the Internal Heat
On the other hand, the internal heat is transported through convection
until it reaches a zone of the atmosphere that is transparent to the infrared
radiation, which can irradiate the heat directly into space and, this way, get
cooler. It's thought that the convective flows provoked by the generation of
internal heat hold a very important role in the dynamics of the giant planets'
winds.
The Role of the External Heat at the Upper Atmosphere
Above the zone where the pressure is 100 mb (1/10 of the pressure at the
sea level on Earth), the temperature rises with the altitude, because up there
the atmosphere absorbs the solar radiation. A photochemically produced haze
(under the action of the solar rays) can be the cause for the absorption of the
additional light that concurs to the heating of the surrounding zones.
Higher up, in the thermosphere, even small quantities of energy may heat
the atmosphere up to very high temperatures. In Jupiter, along with the solar
light, also the downpour of charged particles (ions) proceeding from the
satellite Io contribute to the heating of
this region (up to 1000 ºK).
Planetary
Rings
Almost all the planetary rings are found inside the Roche limit, or in
other words, they are placed at so short distances from the planets that the
tidal forces exerted by them prevent the formation of big fluid objects. For
the solid objects, the equivalent to this limit is placed at an even shorter
distance from the planet.
What may have provoked the formation of the rings may have been the
destruction of the objects when they overtook the Roche limit or the prevention
of the accumulation in bigger objects of the material existent inside that
limit. Besides that, the primordial material that existed at a short distance
from the planet could have been lost on its surface through the
Poynting-Robertson drag (due to impacts with the solar photons) or through the
atmospheric drag.
The satellites perform an important role in the structure of the rings,
since they gravitationally disturb the ring's particles, confining them to
determined orbits.
Detail of the rings of
Saturn (Calvin J. Hamilton)
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