1. Introduction: solar system bodies and their atmospheres

This article is a synopsis of the author’s new book, Planetary Atmospheres (Oxford University Press, 2010), where more detail may be found. See also Elementary Climate Physics(Oxford University Press, 2005).

Figures 1 and 2 show the scale of the Solar System, and Table I lists the main physical properties of the planets. The planets fall into two groups, four small, rocky inner planets and four large, fluid, outer planets. Venus, Earth and Mars, the inner Solar System planets with substantial atmospheres, all have rocky surfaces, and interiors that contain large abundances of heavy elements such as iron and nickel. Their atmospheres are geometrically thin compared to the planetary radius, but they are dense enough to be optically thick at some wavelengths and so to have an important influence on the radiative energy balance prevailing at the solid surface and, hence, climate.

The largest satellite of Saturn, Titan, is an unique case in our planetary system of a moon with a thick atmosphere (thicker than Earth’s). It is also the only known example other than Earth itself of a body with a substantial atmosphere in which the main component is nitrogen. Thus, although it is in the outer Solar System, and a satellite not a planet, Titan offers a further example of an Earth-like atmosphere, making a total of four with Venus, Mars and Earth itself.

The innermost planet, Mercury, has only a trace of atmosphere, and yet it is not completely negligible for comparative climate studies with the Earth, since its polar regions appear to be rich in frozen volatiles. Mercury, with virtually no gaseous envelope, no obvious source of large amounts of water, and in a severe thermal environment, manages to support substantial deposits of ice at its poles.

Pluto, still thought of by many as a member of the family of planets, is now officially excluded from that definition. With a mass of only about one-fifth of Earth’s Moon, and a tenth of that of Titan, Pluto is properly considered a member of the Kuiper Belt (the outer asteroid belt). As such, it is just one of a large number of frozen bodies, some considerably more massive than Pluto, orbiting near the limits of the Sun’s influence. Because it was discovered as long ago as 1930, Pluto remains the best studied of the Kuiper belt objects. Pluto has a satellite, Charon, and a thin atmosphere that derives from frozen nitrogen and other volatiles, including methane, ethane and propane, on its surface. Because Pluto has a very eccentric orbit, its surface pressure cycles from almost nothing at the greatest distance (∼50 AU) [1 astronimcal unit (AU) ∼1.49 × 10⁸ km], to perhaps a few hundredths of a hectoPascal during its closest approach to the Sun (∼30 AU).

The last four decades has witnessed the first close-up exploration of the planets of the Solar System. American, Russian and (increasingly) European planetary space missions have now explored the atmospheres and environments of all of the planets, some many times. The data produced places the Earth in its wider context as never before among the terrestrial planets of the Solar System. Common aspects are revealed, not only of their contemporaneous origin and evolution, but also of their atmospheric structure and surface conditions.

2. The atmosphere and climate of Venus

Venus is the closest planet to the Earth, both in terms of distance and in terms of its physical character. The two planets are almost the same size and mean density, and, so far as we know and expect, have much the same solid-body composition. The largest external differences appear in the absence of a natural satellite around Venus, the slow, retrograde, rotation of the solid body of Venus, and the absence of a measurable Venusian planetary magnetic field. Internally, Venus seems to lack plate tectonics and instead to be much more volcanically active than the Earth.

Venus receives almost twice as much energy per unit area from the Sun as Earth does, and nearly four times as much as Mars. However, the surface conditions on frozen Mars and baking Venus are both sustained by roughly equal amounts of net heating, due mainly to differences in their atmospheres that determine the albedo, the fraction of incoming radiative energy that is reflected back to space.

Significant amounts of solar energy reach the ground on all of the terrestrial planets. Even on Venus with its thick cloud cover, enough sunlight diffuses through to provide about 17 W cm⁻² of surface insolation on the average, about 12% of the total absorbed by Venus as a whole. The opacity of the troposphere for long wavelengths is large and the surface temperature must rise to 730 K in order to force enough infrared cooling to balance the incoming solar energy, a more extreme example of the greenhouse effect familiar on Earth (Taylor and Grinspoon, 2009).

An airless body with the same albedo and heliocentric distance as Venus would reach equilibrium for a mean surface temperature of only about 230 K. Convection in the troposphere carries energy upwards to the base of the stratosphere, where radiative cooling to space can occur strongly. On Venus, this level (the tropopause) occurs about 40 km above the surface. The corresponding distances for Earth and Mars are 10 and 30 km, on these planets the enhancements in surface temperature due to greenhouse warming are ∼40 and ∼10 K respectively. The surface and free air temperatures on Mars can be significantly higher due to airborne dust: model calculations show the warming due to dust can be 50 or even 100 K in the middle atmosphere (Taylor, 2009).

The absence of an internal magnetic field on Venus means that the atmosphere is exposed directly to the solar wind, which constantly erodes the atmosphere. Very light atoms and ions, especially hydrogen, are particularly easily removed, after they are formed by dissociation of hydrogenous atmospheric gases by solar ultraviolet radiation. Oxygen is also removed, at a rate which even today is observed by instruments on the Venus Expressspacecraft to be about half that of hydrogen, supporting the expectation that water is the main source molecule. Over its history, Venus has lost any liquid water it may have had in its early life and, probably as a direct consequence of this, has retained in its atmosphere huge amounts of carbon dioxide. On the Earth, corresponding amounts of carbon dioxide have dissolved in the oceans and formed carbonate rocks. Earth and Venus have retained approximately equal masses of nitrogen, although on Venus this is diluted by the additional carbon dioxide and makes up only 2–3% of the total atmosphere, compared to 79% on Earth.

At visible wavelengths, the cloud cover on Venus is complete and impenetrable, with no markings that could be associated with continents, oceans or any of the surface features that abound on the other inner planets. Instead, only extremely subtle and ephemeral markings, and some ‘scalloping’ of the terminator which separates the day and night sides, have been reported by visual observers. Through an ultraviolet filter, of the type used in the television cameras on Mariner 10, which observed Venus from a distance of 10 000 km in 1973, subtle dark markings appear in the clouds. In the mid-1980s, it was discovered that much more striking contrasts can be observed at certain wavelengths in the near infrared part of the spectrum. These reveal structure in the clouds at considerably greater depths than those seen in the ultraviolet. Large-scale meteorological activity organizes the clouds into patterns, mainly convective near the equator and more laminar flow at higher latitudes (Figure 3) (Gierasch et al., 1997).

During the 1991 Venus fly-by of the Galileo spacecraft, en route for Jupiter, it was found that the principal topographical features on the surface of the planet could also be discerned in images obtained at some near infrared wavelengths. These contrasts originate in the temperature lapse rate of the atmosphere, which causes high features on Venus to appear dark in maps of the thermal emission from the surface. The very high scattering albedo of the cloud droplets in the near infrared means that the emission can diffuse to space in the spectral windows between the absorption bands of the main atmospheric constituents. Spectroscopy in these windows allows mapping of the abundance of interesting minor constituents of the atmosphere near the surface, including water vapour, halides, carbon monoxide, sulphur dioxide and carbonyl sulphide (Baines et al., 2005).

The most striking feature in infrared images of Venus is the polar ‘dipole’, a predominantly wave-2 feature often (particularly when observed with low spatial resolution) consisting of two warm regions circulating around the pole with a variable period typically about 2.9 days. At higher resolution these sometimes have the appearance of linked vortices, as shown in the left frame of Figure 4. This beautiful structure also takes on a variety of more complex shapes, however, as the rest of the figure illustrates. Clearly many disturbances and instabilities are present in addition to the basic dipole characteristic (Piccioni et al., 2007).

Why is Earth so cool and wet when Venus, a similar-sized planet which formed nearby from the same protostellar cloud, is hot and dry? A clue may be obtained by plotting typical temperature profiles for both atmospheres on a scale where the vertical dimension is represented by log (pressure), as shown in Figure 5. In the region where the two profiles overlap, the difference is fairly straightforwardly accounted for by (i) the difference in composition (mostly nitrogen on Earth, mostly carbon dioxide on Venus), (ii) the heating induced by the ozone layer on Earth, which has no equivalent on Venus, and (iii) the difference in solar heating (which is actually greater for Earth, because of Venus’ high albedo). The scorching surface temperature on Venus is caused by the additional atmospheric gas, nearly 100 times more than on Earth. It is mostly carbon dioxide, but it need not be: if the atmosphere is optically thick in the infrared, adding more of any gas, even a non-greenhouse gas, raises the surface temperature. On Venus, the temperature at 1000 hPa pressure is increasing with depth at a rate approximately equal to the adiabatic lapse rate (about 10 K km⁻¹) and it must continue to do so until the pressure reaches nearly 10 000 hPa at the surface some 45 km below (Taylor, 2007).

The Earth enjoys its mild, wet climate primarily because the atmospheric pressure is neither too high, as on Venus, nor too low, as on Mars. This in turn is due to the removal of most of the primordial carbon dioxide, leaving only the nitrogen and a few minor constituents such as argon, to which life later added the oxygen. The conversion of carbon dioxide into carbonate minerals requires liquid water, which may once have been abundant on Venus. No model of Solar System formation would allow the formation of two planets as close as Earth and Venus with such different amounts of water as those we see today. The deuterium-to-hydrogen ratio on Venus is more than 100 times that on Earth, supporting the idea that Venus has lost large amounts of hydrogen, therefore water, to space, a process that would be expected to produce fractionation between the lighter and heavier isotopes.

There has been a long-standing debate about whether lightning is present in the Venusian clouds. The standard view used to be that they are too tenuous, although we now know that localized dense clouds, storms and clouds of volcanic ejecta could provide the right conditions. Impulsive radio frequency signals in the 100 kHz to 5.6 MHz range, for which lightning is the only known source, have been detected by spacecraft. Venus Express has detected strong, low-frequency ‘whistler’ mode discharges that have been interpreted as due to a level of lightning activity that is similar to that on the Earth, but probably cloud-to-cloud rather than cloud-to-ground. Despite evidence such as this, there are some who still do not believe lightning occurs on Venus, and who point out that no flashes have been detected optically, despite scans of the dark side of the planet from orbiting spacecraft designed to search for such evidence. When this was done for Jupiter, many flashes were quickly detected. However, it may be that they are much rarer and less energetic on Venus, and more obscured and diffused by the clouds.

3. Venus exploration

Much current knowledge of the details of the Venus atmosphere and climate system was accrued by the Pioneer Venus orbiter and entry probe missions of the late 1970s and early 1980s. Four probes sounded the clouds and lower atmosphere, returning chemical, physical and meteorological data on Venus’s atmosphere. The orbiter observed the surface of Venus with a radar altimeter and sounded the atmosphere in the infrared and ultraviolet regions of the spectrum. It also provided in situ data on the upper atmosphere, ionosphere and solar wind interaction.

The Soviet Venera and VEGA missions of the early 1980s were followed by the NASA Magellan surface mapping mission which arrived at Venus in August 1990, but there was a gap of two decades before another mission with an atmospheric focus was launched. This was rectified in May 2006 when the Venus Express mission of the European Space Agency became the 28^th spacecraft to arrive successfully at Venus since Mariner 2 in 1962. Its goals were to carry out systematic climate-related remote sensing observations of the Venusian atmosphere below the clouds, producing improved greenhouse models of the energy balance in the lower atmosphere, validating and improving general circulation models of the atmosphere, with improved treatment of the zonal superrotation, the meridional Hadley circulation and the polar vortices.

The small inclination and eccentricity of Venus’s orbit means that no significant seasonal changes during the annual circuit around the Sun should be expected, and none have been observed.

4. The atmosphere and climate of Mars

The atmosphere of Mars has many similarities to that of Earth. The two planets are similar in size, axial tilt and rotation rate, and both are heated primarily by the surface, which receives the bulk of the incoming solar energy directly. The main differences are that the atmosphere of Mars is thin (about 0.7% of that of Earth by surface pressure) and, like that of Venus, consists almost entirely of carbon dioxide. Even at its low surface pressure, the Martian atmosphere is thick enough to display a wide range of analogues to Earthly dynamical phenomena, including cloud formation, fronts and storms. The atmosphere shows large-scale overturning from low to high latitudes, although (in part because of the tendency for surface temperatures to follow the Sun), the Hadley cell on Mars has a larger seasonal variation than on Earth (or Venus) and near the solstices extends from the summer to the winter hemisphere, straddling the equator (Read and Lewis, 2004).

The Martian air, although very dry by Earth or even Venus standards, is often close to saturation and water clouds do occur. Extensive hazes of water ice crystals are seen over the Martian poles in autumn and spring. The effect is greatest in the northern autumn, when the ‘polar hood’, as it is known, extends as far south as 50°N. It tends to disperse in the early winter, probably because the ice crystals making up the hood grow larger and precipitate out of the atmosphere. Orographic clouds, those that form as a result of air flowing over elevated features on the surface, form both as extensive layers over large-scale topography and as lee clouds behind tall structures such as the giant volcanoes. They can extend for hundreds of kilometres. The thin atmosphere of Mars means that the surface cools rapidly as the solar heating drops off towards nightfall and the air becomes saturated with water vapour. The resulting ground hazes of condensed water vapour can persist all night and into the morning, often precipitating onto the surface to form a thin, bright layer of frost that lasts for several hours.

Clouds of condensed carbon dioxide are also thickest over the winter poles, in the seasonal darkness where carbon dioxide freezes and snows out onto the polar cap. High hazes and wispy cirrus clouds of carbon dioxide ice crystals are also found at lower latitudes, particularly at night and typically at heights of 80–100 km above the surface where the atmosphere is extremely cold. They are most easily seen from orbiting spacecraft in images of the atmosphere at the limb of Mars, especially in the early morning after which they usually vanish as the Sun rises. More persistent examples are seen occasionally and have been photographed from landers on the surface (Figure 6).

In fact, the most important source of aerosol opacity is that due to wind-raised dust, which make a large contribution to the greenhouse effect and, therefore, the surface temperatures on Mars. During exceptionally strong global dust storms that occur typically every 2 years near the perihelion of Mars, the entire planet can be masked by huge amounts of airborne dust and the meteorological conditions are greatly changed everywhere. After a period of up to a few months, the storms subside as quickly as they begin.

Even without storms, there is dust in the atmosphere virtually all the time and the sky, as seen from the surface, is more red than blue, especially near the horizon. Some of the dust particles are very small, and remain airborne for a long time. The principal removal mechanism for all but the largest particles is freezing of water or carbon dioxide onto the grains, especially at night or over the dark winter pole. However, background dust levels are nearly always quite high and small storms and localized phenomena such as dust devils have a role in maintaining this.

As on Earth, wave motions are an important part of the Martian circulation, with a major contribution to the excitation of various modes by the topography of the surface, which is more extreme than on other planets. Mountains up to 25 km tall and valleys down to 7 km deep are more than twice the extremes on Earth on a smaller, rapidly rotating planet where the pressure scale height is about 8 km. The topography affects the circulation in two main ways: firstly by interfering mechanically with the flow and, secondly, by enhancing the temperature contrasts at the base of the atmosphere which drive the winds in the first place. The global distribution of topography and albedo differences determine which periodicities will be driven, and those that grow fastest form the natural wave modes of the atmosphere.

The very thin atmosphere of Mars means there is little opacity to prevent nearly all of the solar heating taking place at the surface, unlike Earth where it is about half and Venus where it is only 1% or so. This, and the low thermal capacity of the air for transporting heat between regions of different temperature, leads to large day/night temperature variations on Mars. The highest measured temperature at the surface of Mars is about 20 °C, and the lowest − 140 °C. The global average is about − 60 °C. The condensation of carbon dioxide in the polar night during the long period of perpetual darkness that occurs each winter results in a large swing of nearly a third in the mean surface pressure all over the planet twice each year (once for each pole).

By analogy with Earth and Venus, it would be expected that Mars formed with a much larger inventory of water and carbon dioxide than is apparent now. The oldest terrain on the surface shows ample evidence of running and standing water, clearly visible even from orbit, and supported by the observations of sedimentary deposits on the surface by the recent Mars Exploration Rovers. To produce these required not only a lot more water than can be seen on Mars at the present time, but also a much thicker atmosphere to raise the temperature enough to keep the water liquid.

The fact that the present surface temperatures and pressures are so close to the triple point of water may provide a clue. The Martian oceans would have dissolved atmospheric carbon dioxide, just as they do on Earth, producing carbonate deposits as chalk layers that have since been buried under layers of windblown dust and soil. The difference, then, is that on Mars this process went to the point where liquid water was no longer stable, shutting off further carbonate production. At certain locations, for example at the bottom of the huge impact basin called Hellas, the pressure is twice the global average and liquid water could still be found, although only during the day when the temperature is above freezing point of 0 °C. In this scenario, the water that once filled oceans and rivers is now frozen below the surface, covered up like the carbonates by centuries of wind-blown dust deposits, or in the permanent polar caps. The north cap, which seems to contain most of the water, is 1200 km across and up to 3 km thick, which corresponds to about 4% of Earth’s polar ice.

At Mars’s distance from the Sun even a thick carbon dioxide atmosphere might not be sufficient to warm the planet enough to support liquid water on the surface. Here again there is a clue: Mars has several regions dominated by enormous extinct volcanoes, and the Exploration Rovers have found copious deposits of sulphate minerals, apparently produced at the time when the volcanoes were active and filling the atmosphere with sulphurous and other gases and aerosols. Although the details are still to be filled in, the data point towards a strong greenhouse effect on Mars that lasted only as long as the active volcanism did, that is, around the first billion years of the planet’s history.

5. Mars observations

The most recent of several infrared atmospheric sounding experiments is Mars Climate Sounder (MCS), which was deployed in Mars orbit on Mars Reconnaissance Orbiter in 2006 (McCleese et al., 2007). The MCS obtains close-packed, repetitive temperature, dust, water vapour and cloud profiles, with an extended vertical range and improved altitude resolution compared to previous measurements, and with nearly continuous coverage. This allows global monitoring of the properties of the Martian climate with respect to atmospheric circulation, seasonal changes, and interannual climate variability, and the examination of the annual dust, carbon dioxide and water cycles.

6. The outer planets

The mean diameter of Jupiter is about 11 times greater than that of Earth, so Jupiter is more than 1000 times larger in volume. The rapid rotation flattens the planet by about 6% of its polar diameter, giving it a perceptibly oval shape. Jupiter’s mass of 1.9 × 10²⁷ kg is more than 300 Earth masses, and 2.5 times the combined total of all the other bodies orbiting the Sun. Since the mean density is about one-fourth that of the Earth, it follows that 95% of the mass of Jupiter is atmosphere, with the heavier elements concentrated in a relatively small core with a diameter less than 10% of that of the visible disc. At the interface between the core and the highly compressed fluid (predominantly liquid metallic hydrogen) at the base of the atmosphere, estimates from models predict temperatures in excess of 10 000 K and pressures of over 4 × 10⁹ hPa.

The visible surface of Jupiter consists of layers of cloud which appear as alternating dark and light bands parallel to the equator, the darker reddish and brown coloured bands called belts, and the lighter yellow regions known as zones. The banding is the result of convective motions forced by a large internal heat source, of approximately equal magnitude to the solar heating. This internal energy probably comes from the slow collapse of Jupiter’s fluid bulk in response to its huge gravitational field, converting potential energy into heat.

The fast rotation, the great depth of the atmosphere, and the influence of an internal source of heat in addition to that arriving from the Sun, make for a very dynamic atmosphere (Figure 7). Superimposed on the basic belt-zone cloud patterns are very high winds and wind shears, and a variety of giant eddies (Ingersoll et al., 2004). These are compact, circulating air masses roughly comparable to terrestrial hurricanes, but often much larger. The most striking of all the atmospheric features is the Great Red Spot, which has been observed off and on for at least 300 years, and exceeds the Earth in diameter. Its sense of circulation, and that in other ovals in the southern hemisphere, is counter clockwise (anticyclonic). The eddy velocities are typically a few tens of metres per second. The red colour indicates a composition different from the rest of the clouds, and it seems likely that the GRS is a deep vortex that draws up material from depths where it condenses or reacts to form a cloud cap with additional constituents not found elsewhere across the planet. Truly red cloud materials are fairly rare: the simplest possibility is elemental phosphorus, while more exotic candidates include an almost infinite range of complex organic substances of many colours.

Jupiter’s deep atmosphere probably contains hundreds of layers of cloud, each with a different composition (West et al., 2004). Moist air, rising in the cloudy zones from a great depth, contains many minor constituents, each of which condenses at the appropriate level as the temperature falls with height. Only the top three layers have actually been observed: water clouds at about 273 K and 3000 hPa, ammonium hydrosulphide (NH₄SH, formed by the combination of ammonia, NH₃ and hydrogen sulphide, H₂S) at about 230 K and 1500 hPa, and ammonia ice at 135 K and 500 hPa. All of these materials are basically white, but contain impurities, especially the ammonium hydrosulphide, which appears to be responsible for the yellow and brown colours that dominate the appearance of Jupiter. It is still not known what these ‘chromophores’ consist of: various forms of elemental sulphur and its compounds are the most likely candidates.

The Galileo entry probe confirmed in 1995 that the composition of Jupiter’s atmosphere is mostly hydrogen (about 86%) with the remainder mostly helium (Taylor et al.,2004). The hydrides of the common elements, especially methane and ammonia, are the principal minor constituents, although their relative abundances are significantly different from the mix in the Sun, typically by factors in the range from 2 to 4. Many other species, in particular more complex hydrocarbons including C₂H₂, C₂H₄, C₃H₄ and C₃H₈, are present in the thinner upper atmosphere where enough solar radiation penetrates to drive photochemical reactions. Methane is dissociated and the fragments reassemble as a range of heavier hydrocarbons. Some of these condense to form a thin haze of oily droplets that contributes to the yellowish colour of the disk, particularly in the cases of Jupiter and Saturn (and Titan).

Saturn, and to a lesser extent Uranus and Neptune, may be thought of as smaller, cooler versions of Jupiter where meteorology is concerned. The markings on the disc of Saturn resemble the banded cloud structure of Jupiter’s atmosphere, but with much less contrast and more subtle colours. Giant eddies and ribbon-shaped clouds do occur, and, on rare occasions, extensive irregular storm systems, greater in area than anything seen on Jupiter, appear. As with Jupiter, Saturn has an internal heat source that is comparable to the solar input, so the atmosphere is heated by about the same amount from above and from below, and is highly convective.

Uranus’s rotation is peculiar in that its axis is tilted 98° to the perpendicular to its orbital plane, that is, it lies almost on its side, and has retrograde rotation. The spin axis can point almost directly at the Sun, so regions near the poles spend half of the long orbit alternately in sunlight or darkness. The effect this has on the structure and global circulation of the atmosphere, compared to its similarly-sized but more normally aligned neighbour Neptune, is difficult to know at present because there are so few relevant observations. The cloud patterns suggested zonal east–west winds similar to those on the other three giant planets.

Neptune’s orbital period is almost 165 years and it has not quite completed an orbit of the Sun since its discovery in 1846. It has one large satellite, Triton (2710 km in diameter) which is in an odd retrograde orbit with an inclination of 159°. This suggests that Triton was a drifting object in the Kuiper belt before it was captured by Neptune, which is remarkable in view of its size. Neptune itself has an axial inclination of 29°, not very different from Earth’s 23.5°.

The temperature profiles in the outer planet’s atmospheres can be obtained using estimates of the internal heat source and the albedo from measurements by broad-band radiometric instruments, and temperature sounding measurements by infrared spectrometers and radio occultation profile inversions. The resulting profiles are summarized in Figure 8. As would be expected, they get progressively cooler with distance from the Sun and smaller internal sources: in the case of Uranus and Neptune these effects almost balance, giving very similar profiles except in the thermosphere.

7. Titan: a satellite with an atmosphere and a climate

Titan is Saturn’s biggest moon and is larger than the planet Mercury, with about 40% of the diameter and 2.26% of the mass of Earth. Titan’s surface temperature is about − 178 °C or 95 K (Achterberg et al., 2008). The atmosphere is predominantly composed of nitrogen, as is Earth’s, but with a higher surface pressure, despite the significantly lower gravity. Two other satellites, Io (orbiting Jupiter) and Triton (orbiting Neptune) have thin, transient atmospheres fuelled by volcanoes, expelling mainly sulphur dioxide and hydrogen sulphide on Io and nitrogen on Triton. The volcanoes are driven by tidal heating of the interior of the body in both cases, rather than by primordial and radioactive heat as with the Earth.

The nitrogen on Titan probably started out as ammonia, which is easily photochemically dissociated followed by the escape to space of most of the hydrogen. Today, nitrogen forms about 95% of the atmosphere. The remainder is mostly methane, which is also subjected to dissociation by solar ultraviolet radiation and energetic particles from Saturn’s radiation belts, the Sun, and cosmic rays. There has to be a source of methane on Titan to explain its continued presence, because at the present rate of destruction it would all vanish in only about 1 million years. Titan has a low mean density (1.88 g cm⁻³) so the interior must contain a lot of ice. Some of this would be expected to be frozen methane and ammonia, along with water and other ices, and at the high pressures in the interior, assisted by small amounts of radioactive and tidal heating, the methane could vapourize and escape though cracks and vents in the crust. The Cassini spacecraft in orbit around Saturn has obtained some visual evidence for these ‘cryovolcanoes’ during close passes over Titan.

The decomposing methane in the upper atmosphere is likely to be the main source of the thick layer of orange haze that dominates the visual appearance of Titan. Although its detailed composition is still not known, despite the attentions of several instruments on the Cassini Saturn orbiter and the Huygens Titan probe, it apparently consists of drops of oily hydrocarbons, produced by a chain of reactions that start with the dissociation of methane and nitrogen by solar ultraviolet photons. The ethane, acetylene, ethylene, hydrogen cyanide and other trace constituents that have been detected spectroscopically in the atmosphere are also part of this process. As larger and larger molecules are synthesized, condensation occurs forming aerosol particles that can grow by coalescence. Model calculations of the haze formation predict production and growth rates that are quite rapid and also irreversible, so eventually large droplets drizzle onto on the surface to form tar-like deposits of condensable hydrocarbons and nitriles. Titan also has sporadic, cumuliform methane clouds, localized in space and time, at altitudes between 15 and 18 km at tropical latitudes and more frequently and higher, but still below 30 km altitude, at both poles.

About 10% of the electromagnetic radiation emitted by the Sun at wavelengths in or near the visible part of the spectrum reaches the ground on Titan. This heats the ground sufficiently for the atmosphere to become unstable against convection. Since Titan has the same main atmospheric constituent as on Earth, the resulting lapse rate differs mainly due to the difference in gravity, and its value on Titan works out to be only about 1.4 K km⁻¹. The temperature falls with height from the surface of Titan up to the tropopause temperature, where it is 70.43 K at about 44 km where the pressure is 115 ± 1 hPa. The lower stratosphere on Titan is the shallow, quasi-isothermal region near 50 km, while the upper stratosphere is the much deeper region from about 50–300 km altitude where temperature increases with height, i.e. the region between the first temperature minimum and the first maximum above the surface (Figure 9).

In terms of its general circulation Titan resembles Venus, since both are slowly-rotating bodies with a dense atmosphere containing a deep cloud layer (Hourdin et al., 1995). There is evidence from imaging, temperature and composition field measurements, supported by oblateness measurements using stellar occultation data, that Titan has a super-rotating atmosphere and an equator to pole Hadley circulation similar to Venus (Teanby et al., 2008). An important difference from Venus is the strong seasonal cycle due to its obliquity of 26.7°, closer to Earth’s. Titan’s atmosphere has a number of other Earth-like characteristics: in addition to having the same major constituent, nitrogen, and nearly the same surface pressure, data from the Cassini spacecraft which has made many close fly-bys of Titan while orbiting Saturn, has revealed storms, precipitation, and lightning (Flasar et al., 2005).

There seem to be at least two distinct kinds of precipitation. The first is the slow drizzle of hydrocarbons, possibly including heavy, relatively complex molecules, from the planet-wide haze layers. The second is rain, sometimes voluminous, from the cumulus-type clouds of methane and ethane that form in convective updrafts where local meteorological conditions are right. Apparently, the lower atmosphere is unstable with regard to convection under conditions of relatively strong heating. The rising air can then bring large amounts of moisture up to form dense clouds that are locally thick but patchy in coverage, occurring mainly in middle and high latitudes in the summer. Thus, locally high surface temperatures, combined with a source of methane on or in the surface, leads to the rapid growth of a convective storm analogous to thunderstorms and monsoons on the Earth and accompanied by the same kind of torrential rain. The cloudy, stormy region migrates from one hemisphere to another with the seasons, producing local flooding for a short time each year in the summer. The surface accumulations of contaminated liquid methane soon drain below ground or evaporate. This inferred behaviour explains why the landscape viewed by the Huygens probe shows river valleys and other features apparently produced by copious amounts of running liquid, although the surface was dry at the time of its landing (Coustenis and Taylor, 2008).