Many of the radiative processes that govern the climate system of the Earth are also important on the other planets of the Solar System. The different boundary conditions that apply, especially distance from the Sun, give rise to different histories and surface and atmospheric properties, but parallels can often be drawn that test our understanding of the physics involved, which of course is everywhere the same. Comparisons between the planets are instructive, therefore, as are attempts to understand how the climate on each has changed over the lifetime of the Solar System. It is well known, for example, that the Earth has experienced numerous periods of dramatic climate change (the ice ages) over its history. Recent evidence from planetary probes confirms that Mars had a warm, wet epoch in its (probably distant) past, and that early Venus may have had oceans comparable in extent to those on the present-day Earth. It is vital that we understand the reasons for climate change on this sort of grand scale and the processes, balances and feedbacks involved in complete quantitative detail. By comparing the processes at work on Mars, Venus and Titan to those on our own planet we may gain a deeper understanding of present-day global change on the Earth, the origin and evolution over the long term of our world and its habitability, and the processes behind threats such as greenhouse warming.

A good place to start is to examine how well radiative-transfer models of the energy budget can explain the climatic conditions on the other planets, whose atmospheric composition and large-scale thermal structures are now quite well known. We begin with the regimes on the nearby, and similarly sized planets Mars and Venus, which are the best explored and bear the closest resemblance to the Earth. Saturn's giant moon Titan, although more distant and colder, has a number of Earth-like attributes, while even hot, virtually airless Mercury and the gas giant planets offer some instructive parallels, despite their more extreme contrasts with the terrestrial environment (Table 12.1).

All of the major bodies of the Solar System except Pluto have now been visited and we know in general terms what climatic conditions prevail there and, roughly at least, what physical processes sustain them. Comparative planetary climatology addresses the detailed theory of the physics that determines the environmental conditions, the stability in each case against climate change, and the

Table 12.1 Data relevant to the climate on Venus, Earth, Mars and Titan. Composition is % by volume. For the Earth, water vapour is additional pressure of maximum of about 3% of the dry atmospheric pressure.





Distance from Sun (AU)





Radius (km)





Surface pressure (bar)





Surface temperature (K)





Day (Earth days)





Main atmospheric constituents

*CO2 96.5%

N2 79%

*CO2 95%

N2 95%

(*key greenhouse gases)

N2 3.5%

O2 21%

N2 2.7%

*CH4 5%

Ar 1%

Ar 1.6%

*H2O (3%)

*C02 0.035%

Bond albedo





Gravity (m s~2)





Effective temperature (K)





practical development of new experiments to further investigate these. The comparative aspect asks not only what the climate systems on different planets have in common and what controls their stability, but what change has taken place and why, what is the chronology of past changes, and can we make plausible predictions of future change? From a consideration of the spatial and temporal bases for climate variability we can work out the appropriate measurement objectives for future space missions that seek to address these and other climate-related questions further.

At the present time, such objectives definitely include the diversity in climatic behaviour found between the Earth and even our closest neighbours. For example, why is Venus so hot and so dry? How are its thick, sulphurous cloud layers created and maintained? Is the surface of Venus shaped by plate tectonics like the Earth's? How was Mars able to have a thick atmosphere, and free water on its surface, in the past, when this would be impossible under the conditions we observe today? What mechanism maintains the Great Red Spot on Jupiter, and what is its relationship to the Earth-sized dark spot on Neptune? To what extent do these otherworldly storms resemble terrestrial hurricanes? Simple models and calculations, or attempts to draw analogies with the behaviour of the terrestrial ecosystem, give no definite answers to questions like these. Until we have developed a more complex and complete understanding of climate systems in general, we cannot have confidence in predictions of related behaviour on the Earth.

The First Law of Thermodynamics, determining how a planet achieves radiative balance, is the primary consideration in climate physics and this, of course, is not unique to the Earth. For instance, the greenhouse effect plays a major role in determining the surface environment on the other terrestrial planets with atmospheres, Mars and Venus, and the same greenhouse gases (principally carbon dioxide and water vapour) are responsible. The existence of these planets offers an important opportunity to see how the greenhouse effect works in situations other than that we observe on the Earth. The general message that Earth-like planets apparently can be subject to extremely large and variable greenhouse warmings is an important and often under-rated one.

12.1.1 Origin of the solar system

According to most current theories, the Sun, the planets, and the small bodies and orbiting debris in the Solar System all formed about 4.5 Byr (billion years) ago when a cloud of dust and gas known as the solar nebula underwent gravitational collapse. The potential energy released during this process resulted in heating, especially at the centre where the material was most compressed, and opaque enough to inhibit cooling by the escape of radiation.

Model calculations suggest that the initial collapse took less than 100000 years. At the end of this time, most of the mass was in the hot, vaporized 'protosun' at the centre, but a significant amount of cooler material remains in orbit around it, held by the centrifugal force of the rotating system. Unlike the protosun, this accretion disk was optically thin, and cooled relatively rapidly by radiation to space.

As a result of this cooling, metal, rock and, in the outer reaches of the Solar System, ice, all condensed out into dust-sized particles. These accreted by colliding and forming first loose aggregates, and then larger particles. The growth rate accelerated once these had enough mass to attract other particles by gravity, rather than by chance collisions, their final size depending on the distance from the star and the local density and composition of the protoplanetary nebula. In the inner Solar System, these 'planetesimals' probably had a size distribution similar to that found in the current asteroid belt, with a few as large as Earth's moon. In the outer Solar System, where condensation of ice provided additional material, there may have been planetesimals several times the mass of the Earth, which took from a few hundred thousand to about twenty million years to form.

Models of the early Solar System require a total mass during formation of the planets that is considerably larger than that which remains today. During the formation of planetesimals, the disk appears to have lost material, including most of its gas, perhaps during a 'T-Tauri' phase of the Sun, so called after a young star that can be seen exhibiting this behaviour today (Fig. 12.1). This phase of stellar evolution is characterized by an intense particle flux streaming outwards into space, a more vigorous version of the modern solar wind. Only the large, icy planetesimals in the outer Solar System that later formed the gas giant planets had enough mass to retain atmospheres at this point. These atmospheres, like that of the Sun, consisted of the most common elements, principally hydrogen and helium.

flG. 12.1. The T-Tauri system, showing the accretion disk. Once thought to be a model for the early Solar System, T-Tauri is now known to be a double star. Nevertheless, the young Sun will have had a disk like this, containing material that, over several million years, either fell into the star, remained in orbit and aggregated into planetesimals, or was driven off into space by the solar wind.

Eventually, after perhaps another hundred million years, the planetesimals collided and combined until a small number of larger bodies - the planets we know today - were left in stable orbits. The process may not be entirely complete; the asteroid belt still contains planetesimals, and in the outermost reaches of the Solar System a large number of bodies of considerable size orbits in the Kuiper Belt. Some of these may eventually collide and form further planet-sized objects outside the orbit of Neptune.

In the effort to understand the details and veracity of the history outlined above, in addition to indepth exploration of our own solar system, we are now able to detect planetary systems around other stars. With many more samples to study, the theory of planetary system formation and evolution is advancing rapidly. Already, we run into difficulties when it comes to explaining why Jupiter-like planets are being found very close to their parent stars; it may be that the T-Tauri phase does not occur in all types of stars, for example.

12.1.2 Evolution of planetary atmospheres

The Earth probably retained very few of the gases that surrounded the solid bodies at the time the planetesimals condensed. Instead, the present atmosphere formed later as a result of outgassing from the crust, a process that would have been augmented by the infall of icy cometary material. Both processes are still happening, on a reduced scale, today.

The primitive atmosphere thus acquired would have had a composition quite different from that which exists now, probably consisting primarily of carbon dioxide, methane, ammonia, and water vapour with no, or very little, free nitrogen or oxygen. The large proportion of nitrogen we breathe today came about as a consequence of the photodissociation of ammonia into nitrogen and hydrogen. The latter gas is so light that it can escape from the Earth even at the present time, whereas most heavier gases are gravitationally trapped now that the Earth is cooler and the Sun less active.

12.1.3 Escape processes

Following the development of the kinetic theory of gases and Maxwell's work on the velocity distribution of the molecules in a gas of a given temperature and composition, Jeans formulated his theory of atmospheric escape rates in 1904. He reasoned that a molecule would escape if: a) the upward component of its velocity exceeded the escape velocity (11.2 km for the Earth), and b) it was above the level where the pressure is so low that collisions between molecules are negligible. The first condition applies when the thermal energy becomes greater than the gravitational potential energy, i.e.

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