For the gas giant planets - Jupiter and Saturn - many of the most striking questions that arise are fluid dynamical in nature. These questions include the origin of the banded multiple-jet structure of the atmospheric flow, and the dynamics of long-lived atmospheric vortices, most famously Jupiter's Great Red Spot. We shall have little to say about such fluid dynamical questions in this book. However, the thermal structure of the atmosphere provides an essential underpinning for any dynamical inquiry. Moreover, the thermal structure determines the rate of heat loss from the planet, and therefore plays a crucial role in the long-term evolution of the gas giants. The thermal structure also affects atmospheric chemistry and the nature of the colorful clouds that allow us to visualize the spectacular fluid patterns on these planets.
The gas giants present an interesting contrast to rocky terrestrial-type planets, because of the lack of a solid surface. Instead of solar radiation having the possibility of penetrating to the ground and being absorbed there, thus heating the atmosphere from below, solar radiation on the gas giants is deposited continuously throughout the upper portion of the atmosphere as the solar beam propagates downward and attenuates. Further, unlike the case of the rocky planets where heat flux from the interior is an insignificant player in climate, the fluid nature of the gas giants allows considerable heat flux from the interior to escape to space. For both Jupiter and Saturn, this heat flux is comparable to the flux of energy received from the Sun. One of our objectives in subsequent chapters will be to learn how the distinct nature of atmospheric driving on the gas giants affects the thermal structure. The gas giants also offer an interesting opportunity to test ideas about how climate is affected by atmospheric composition. These planets are mostly made of H2, with a lesser amount of He and trace amounts of a range of other substances, including ammonia (NH3), methane (CH4) and water, the latter three of which exist in both gaseous and condensed forms. The composition affects the thermodynamics of the atmosphere, as well as the optical properties for both infrared and visible light.
Uranus and Neptune are like the gas giants in that they have no distinct solid surface at any depth that could significantly affect the atmosphere. However, they are usually classified separately as ice giants because they contain a much higher proportion of ice-forming substances such as water, ammonia and methane. The composition of the outer portions of the atmospheres can be determined by spectral observations, and contain a high proportion of hydrogen and helium. The overall density of the planets, however, constrains them to be composed primarily of an ice mantle. In the case of Uranus, the ice mantle must make up between 9.3 and 13.4 Earth masses worth of the total mass of the planet, which is 14.5 Earth masses. Similar proportions apply to Neptune. The commonly used term "ice mantle" is somewhat misleading, since the substance is actually a hot, slushy mixture that would be more aptly described as a water-ammonia ocean. Whatever term is used,the very thermal structure that determines the nature of the transition between the ice mantle and the more gaseous outer atmosphere engages all the same issues of atmospheric energy balance as one encounters on other planets. A novel feature of Uranus is its axial tilt. Its axis of rotation is nearly perpendicular to the normal to the plane of the orbit. In other words, the axis lies almost in the plane of the orbit. That means that in the Uranian Northern Hemisphere summer, the North pole is pointing directly at the Sun and the entire Southern Hemisphere is in darkness. By way of contrast, the Earth's axis is only tilted by 23.4o relative to the normal at present. The high axial tilt of Uranus potentially gives that planet an extreme seasonal cycle, though it will take a long time to observe it since Uranus' year lasts 84 Earth years. The effect of axial tilt on seasonal cycles of planets are discussed in general terms in Chapter 7. The very low solar radiation received at the distant orbits of Uranus and Neptune leads to extremely cold outer atmospheres, particularly in the case of Neptune. These planets provide an opportunity to examine the novel features of an atmosphere driven by an exceedingly weak trickle of solar energy, supplemented by an equally feeble trickle of heat from the interior. Despite the weak thermal driving, Neptune has by far the strongest winds in the Solar system, as well as a variety of interesting meteorological features. We will not say much about planetary winds, but as in the case of the gas giants, a good understanding of the thermal structure is a prerequisite for any attack on the meteorology.
The gas and ice giants also challenge our notion of habitable zones - orbits where a planet has some region where there are Earthlike temperatures allowing for liquid water. The gas and ice giants have no distinct surface, but there is some depth on each of them where the temperature is Earthlike and liquid water can exist. The atmosphere also have plenty of chemical feedstocks for organic molecules, including ammonia and methane. The pressures are no greater than those seen at the bottom of the Earth's ocean. One may have some prejudice in favor of surfaces for life to live on, but it must be recalled that on Earth life first arose in the oceans and indeed stayed there for many billions of years before venturing onto land. The gas and ice giants could just as well be thought of as being "all ocean" rather than "all atmosphere" so it is far from clear that they are inhospitable, at least for chemosynthetic forms of life that don't need much sunlight. Our thinking about habitable zones is overly prejudiced toward life that carries out its existence on a rocky surface.
From the standpoint of planetary climate, one of the most interesting Solar System bodies is not a planet at all, but a satellite. Titan, which orbits Saturn, is a fairly large icy body with a radius of that is 76% of that of Mars. Because it is composed of ice rather than rock, the surface gravity is low: 1.35 m/s2, which is actually lower than the Moon's surface gravity, though the Moon is smaller than Titan. What makes Titan interesting, however is its dense atmosphere. The atmosphere of Titan consists mainly of nitrogen, with a surface pressure about 1.5 times that of Earth. What is even more interesting is that the lower portion of the atmosphere is about 30% methane. At the cold temperatures of Titan (about 95K) methane can rain out, and participates in a hydrological cycle analogous to that of water on Earth - but operating at a much colder temperature. In subsequent chapters we will develop the physics to examine the similarities and differences between the role of methane on Titan vs. water on Earth. Titan's atmosphere is also a seething organic chemical factory, with complex long-chain hydrocarbon hazes being manufactured from methane in the upper atmosphere. These hazes absorb solar radiation, shade the surface, and are a key player in Titan's climate. Such organic hazes were first discovered on Titan, but there are speculations that similar hazes could have been present in methane-dominated atmospheres of the Early Earth.
A major question about Titan is why it has an atmosphere left at all . Given the low gravity, the N2 atmosphere would be expected to escape fairly quickly (we'll have a look at the relevant physics in Chapter 8). Moreover, the chemical reactions in the atmosphere should gradually convert all the methane into a tarry sludge sequestered at the surface. In some way or another, the atmosphere of Titan must be dynamically maintained by recycling of chemicals deposted on the surface, and by outgassing of N2 (probably in the form of ammonia) and CH4 from the interior. Precisely how this happens is one of the Big Questions of Titan.
Even icy moons without an appreciable atmosphere can manifest features of considerable interest. Jupiter's moon Europa is a case in point. This satellite has a water ice crust between 10 and 50 km thick, but beneath the crust there lies a liquid water ocean. Europa shows an intriguing range of crustal features, including some that suggest melt-through of the ocean. Of course, the existence of the ocean has attracted attention as possible habitat for life. The icy moons challenge the epistemological boundaries of planetary science. At the cold temperatures of Europa's surface, as on Titan, water ice is basically a rock, just as sand can be considered an "ice" of SiO2 on Earth. The ice-rock forms minerals with ammonia and methane and other compounds, and when warm enough the ice-minerals can flow or melt and lead to cryovolcanism. When studying the crust and interior of Europa or Titan or other icy moons are we doing geology or oceanography or glaciology? Whatever one wants to call it, these moons are, as has been said, "always icy, never dull."
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