Jupiter

Jupiter's structure completely differs from that of our planet. Earth is solid and can be divided, at least conceptually, into an internal region made of iron and an external region, comprising the mantle and the crust, made of silicates. Jupiter is a gas giant; most of its mass is not made up of silicates or iron. Rather, Jupiter is mostly hydrogen and helium. The pressure is so large within the planet's interior that these gases are squeezed into a molecular liquid and even a 'metallic' liquid phase.

Jupiter is covered by several cloud layers that hide anything lying beneath. While early observations of Jupiter's atmosphere indicated that it was mostly made of methane (CH₄) and ammonia (NH₃), more recent infrared data have brought scientists to the conclusion that its main components are molecular hydrogen (H₂) and helium (He). It is now clear that the composition of Jupiter's atmosphere reflects that of the solar nebula from which it formed, and this is true for the planet as a whole.

We do not have any direct observation of Jupiter under its cloud layer. However, there is plenty of indirect evidence to support the view that Jupiter does not have a well-defined solid or liquid surface with a gaseous atmosphere just above it. Computer models indicate that Jupiter has rocky core of iron and silicates, surrounded by hydrogen and helium in 'liquid metallic' and 'liquid molecular' phases, extending to the base of a gaseous atmosphere made of the same compounds as the underlying layers. But temperature and pressure at the base of the atmosphere are such that the transition between gaseous and liquid hydrogen is gradual, so that there is no well-defined 'surface' to Jupiter underneath its atmosphere.

Hydrogen and helium make up most of the mass of Jupiter's atmosphere. Other elements, like oxygen, nitrogen and carbon, are present in relatively small quantities, and they combine with hydrogen to form methane (CH₄), ammonia (NH₃) and water (H₂O). Other elements, notably sulfur (S) and phosphorus (P) are also present in Jupiter's atmosphere. They play an important side role in the formation of clouds. Both elements also form compounds that could give the reddish-brownish coloration to the clouds.

Clouds form when a chemical compound condenses from a vapor into a liquid or ice. The process of condensation depends mostly on temperature, but also on atmospheric pressure. By using a combination of data and theory, the following picture of the distribution of clouds on Jupiter has emerged. There are three main layers of clouds, corresponding to altitudes where ammonia (NH₃), ammonium hydrosulfide (NH₄SH) and water (H₂O) vapors condense. Jupiter's atmosphere is not cold enough to lead to the condensation of methane (CH₄), though this compound is present as a vapor.

The white ammonia clouds constitute the top cloud deck, i.e. the white bands so prominent in images of the planet. The temperature at that altitude in the atmosphere is about 140 K and the pressure is about 0.6 bars. The interesting point is that the other two decks of clouds underneath the ammonia layer are also made of droplets conferring them a white color. We would expect the cloud layers covering Jupiter to be virtually indistinguishable from each other, if not for the altitude of the different cloud decks. But Jupiter's clouds display a beautiful array of colors, ranging from white to brown to red. These colored clouds seem to be lower in altitude than the ammonia clouds. They probably correspond to the ammonium hydrosulfide (NH₄SH) cloud deck, which forms at a temperature of above 160 K, a pressure of about 1 bar and an altitude 30 kilomters lower than the ammonia clouds. In general, it is not easy to analyze the composition of clouds from a distance, just like it is not easy to find the composition of liquids or solids by just analyzing the light they reflect. /PARGRAPH The composition of the chemicals coloring Jupiter's clouds is still a matter of speculation. One possibility is that traces of sulphur or complex organic molecules react with the ammonium hydrosulfide (NH₄SH) in the presence of lightning, yielding compounds such as ammonium polysulfides, (NH_4)2S_x (where x = 2, 3,... ) and others. Phosphorus compounds could also be involved. Such compounds would be present only in trace amounts, nonetheless sufficient to explain the cloud colors we see.

In 1995 the Galileo spacecraft launched a probe into Jupiter's atmosphere with the explicit mission of measuring various atmospheric parameters and the composition of gas and clouds. The probe was successful its mission, and slowly descended under the various cloud layers after deploying a parachute. The temperature and pressure it detected at various altitudes broadly confirmed the predictions of computer models. But the probe failed in finding the water cloud layer predicted by the models. It is probable that we were unlucky and the probe descended through a break in the clouds. As of today, the riddle of their exact composition is not completely solved.

The overall circulation of Jupiter's atmosphere is somewhat simpler than the Earth's because it is strongly affected by the transport of energy from its interior. On Earth the atmospheric circulation transports heat from the warmer equatorial regions to the Poles, and the difference in temperature between them is large enough to create a wavelike pattern of cold and warm fronts so familiar from the weather forecast maps.

The difference in temperature measured between equator and poles on Jupiter is quite small, despite the difference in solar irradiation (the poles receive less sunlight). Calculations show that the interior heat sources are more important than the Sun in powering Jupiter's atmosphere, while on Earth almost all the heat comes from the Sun.

Unlike on Earth, on Jupiter the overall structure of the cloud layers simply follows the parallels, and the prevailing winds are in the East-West direction. On Earth this is true only in average. The direction of Jupiter's prevailing winds depends on the planet's rotation, as it does on Earth. The period of rotation of the planet was measured using the periodic wobbling of its magnetic field, which is oriented at an angle of 10 degrees from the rotation axis. The rotation period of the planet's interior thus measured is 9 hours 55 minutes, and obviously differs from the period measured by using the planet's clouds. The prevailing winds on Jupiter are West-to-East and move at km/h compared to the interior.

If one takes into account all forms of energy flux, from visible reflected light to infrared heat radiation, Jupiter emits double the power than it receives from the Sun. This energy comes from the planet's interior and the main source is gravitational, as we will discuss. The atmospheric circulation on Jupiter is dominated by a convection mechanism that extends well into the planet's interior. In other words, the rise and descent of vast air masses transfer heat from the Jupiter's interior to its surface.

A comparison with Earth may clarify how different Jupiter really is. On Earth, sunlight hits the ground and raises its temperature. The ground, in turn, heats up the layer of air in contact with it. Warmer air tends to rise, because it has a lower density. As the air reaches a higher altitude it gradually cools down by radiating heat to outer space, and once cooler it sinks back down. A circulation ensues wherein air masses rise while others replace them. The net effect is the transfer of heat from the ground to space, mediated by the atmosphere.

On Jupiter the heat comes from an interior energy source, rather than from a solar-heated ground. The white ammonia clouds are associated with ascending air masses, and they tend to dissipate when they re-descend. This mechanism is similar to what happens on Earth, when upsweeping air cells carry water vapor until clouds condense: when the cells descend they are much drier, because the vapor they carried has already condensed during the ascent.

Scientists think that the brownish clouds are associated with descending air masses. In places were air masses descend we can see the lower cloud decks and we have visual access to warmer regions of the planet's atmosphere. The brownish cloud deck is almost certainly made of ammonium hydrosulfide (NH₄SH) plus the coloring molecules we previously mentioned.

While it is true that Jupiter's atmosphere is largely divided into bands along its parallels, there are also several storms, similar to Earth's hurricanes. These storms are only partly correlated with the band circulation pattern in their surroundings, just as hurricanes are affected only partially by the surrounding general circulation of the Earth's atmosphere. The most famous of such storms is the great Red Spot, an oval region that measures 40,000 kilometers across, three times the size of our planet. The Red Spot has been present for over 300 years, periodically changing in color and shape, but otherwise a constant feature. The origin of its distinctive color remains the subject of speculation, as is the case for many other features of Jupiter's atmosphere.