CHAPTER 1

INTRODUCTION:THE DIVERSITY OFPLANETARY CLIMATES

Climate is the average weather—long-termproperties of the atmosphere like temperature, wind,cloudiness, and precipitation, and properties of the surfacelike snow, glaciers, rivers, and oceans. Earth has awide range of climates, but the range among the planetsis much greater. Studying the climates of other planetshelps us understand the basic physical processes in alarger context. One learns which factors are important insetting the climate and how they interact.

Earth is the only planet with water in all three phases—solid,liquid, and gas. Mars has plenty of water, but it's almostall locked up in the polar caps as ice. There's a smallamount of water vapor in its atmosphere but no standingbodies of liquid water. Venus has a small amount ofwater vapor in its atmosphere, but the Venus surface ishot enough to melt lead and is too hot for solid or liquidwater. Thus by human standards, Venus is too hot andMars is too cold. The classic "habitable zone," whereEarth resides and life evolved, lies in between.

Things get strange in the outer solar system. Titan, amoon of Saturn, has rivers and lakes, but they're madeof methane, which we know as natural gas. The giantplanets have no solid or liquid surfaces, so you wouldneed a balloon or an airplane to visit them. The climatesthere range from terribly cold at the tops of the cloudsto scorching hot in the gaseous interiors, with warm,wet, rainy layers in between. Some of the moons in theouter solar system have oceans of liquid water beneaththeir icy crusts. The solar system's habitable zone couldbe an archipelago that includes these icy moons, but thecrusts could be tens of kilometers thick. Their subsurfaceoceans are beyond the scope of this book.

The diversity of planetary climates is huge, but thebasic ingredients are the same—the five elements H,O, C, N, and S. A fundamental difference is the relativeabundance of hydrogen and oxygen. In the innersolar system—Earth, Mars, and Venus—the elementsare combined into compounds like oxygen (O₂), carbondioxide (CO₂), nitrogen (N₂), sulfur dioxide (SO₂), andwater (H₂O). In the outer solar system—Jupiter, Saturn,Uranus, and Neptune—the elements are combined intocompounds like methane (CH₄), ammonia (NH₃), hydrogensulfide (H₂S), and water. Saturn's moon Titan hasan atmosphere of nitrogen and methane, and Jupiter'smoon Io has an atmosphere of sulfur dioxide. The compositionof a planetary atmosphere has a profound effecton its climate, yet many of the processes that control thecomposition are poorly understood.

The underlying physical processes are the same aswell. Temperature is a crucial variable, and it is largelybut not entirely controlled by distance to the Sun. Thetemperature of the planet adjusts to maintain thermalequilibrium—to keep the amount of outgoing infraredradiation equal to the amount of absorbed sunlight.

Clouds and ice reflect sunlight, leading to cooler temperatures,but clouds also block outgoing infrared radiation,leading to warmer temperatures down below. Manygases like water vapor, carbon dioxide, methane, ammonia,and sulfur dioxide do the same. They are calledgreenhouse gases, although an actual greenhouse trapsthe warm air inside by blocking the wind outside. Anatmosphere has nothing outside, just space, so the greenhousegases trap heat by blocking the infrared radiationto space. Venus has clouds of sulfuric acid and a massivecarbon dioxide atmosphere that together reflect 75% ofthe incident sunlight. Yet enough sunlight reaches thesurface, and enough of the outgoing radiation is blocked,to make the surface of Venus hotter than any other surfacein the solar system. Gases like nitrogen (N₂) andoxygen (O₂) do not block infrared radiation and are notsignificant contributors to the greenhouse effect.

The wind speeds on other planets defy intuition. Athigh altitudes on Venus, the winds blow two or threetimes faster than the jet streams of Earth, which blowat hurricane force although they usually don't touch theground. In fact, Earth has the slowest winds of any planetin the solar system. Paradoxically, wind speed seemsto increase with distance from the Sun. Jupiter has jetstreams that blow three times faster than those on Earth,and Neptune has jet streams that blow ten times faster.

The weather is otherworldly. At least it is unlikewhat we are used to on Earth. Mars has two kinds ofclouds—water and carbon dioxide. And Jupiter has threekinds—water, ammonia, and a compound of ammoniaand hydrogen sulfide. Mars has dust storms that occasionallyenshroud the planet. Jupiter and Saturn haveno oceans and no solid surfaces, but they have lightningstorms and rain clouds that dwarf the largest thunderstormson Earth. Saturn stores its energy for decades andthen erupts into a giant thunderstorm that sends out atail that wraps around the planet.

Many of these processes are not well understood.Our Earth-based experience has proved inadequate toprepare us for the climates we have discovered on otherplanets. The planets have surprised us, and scientistsoften emerge from a planetary encounter with morequestions than answers. But surprises tell us somethingnew, and new questions lead to new approaches andgreater understanding. If we knew what we would findevery time a spacecraft visited a planet, then we wouldn'tbe learning anything. In the chapters that follow, we willsee how much we know and don't know about climate,using the planets to provide a broader context than whatwe experience on Earth.

We will visit the planets in order of distance from theSun, starting with Venus and ending with planets aroundother stars. Most planets get one or two chapters. Usuallythe first chapter is more descriptive—what the planet islike and how it got that way. The second chapter is moremechanistic—describing the physical processes thatcontrol the present climate of that planet. The chaptersare augmented by sections called boxes, which containequations and constitute a brief textbook-type introductionto climate science.

Chapter 2 is about the greenhouse effect and climateevolution, for which Venus is the prime example. Chapter3 is about basic physical processes like convection,radiation, Hadley cells, and the accompanying winds,with Venus as the laboratory. Mars illustrates the "faintyoung Sun paradox," in which evidence of ancient rivers(chapter 4) contradicts results from astronomy that theSun's output in the first billion years of the solar systemwas 70% of its current value. Mars also allows us to talkabout the fundamental physical processes of condensationand evaporation (chapter 5), since exchanges ofwater vapor and CO₂ between the atmosphere and polarice determine the climate of Mars. Titan allows us tostudy a hydrologic cycle in which the working fluid isnot water (sections 6.1–6.3). Titan is an evolving atmosphere,close to the lower size limit of objects that canretain a sizeable atmosphere over geologic time (section6.4). Below this limit, the atmospheres are tenuous andtransient (section 6.5).

Jupiter is almost a cooled-down piece of the Sun, butthe departures from solar composition tell a crucial storyabout how the solar system formed (chapter 7). The giantplanets are laboratories for studying the effect of planetaryrotation on climate (chapter 8), including the high-speedjet streams and storms that last for centuries. Chapter 9 isabout Saturn, a close relative of Jupiter, although the differencesare substantial and hard to understand. Uranusspins on its side, which allows us to compare sunlightand rotation for their effects on weather patterns (section10.1). Neptune has the strongest winds of any planet(section 10.2), and we speculate about why this might be.The field of exoplanets—planets around other stars (section10.3) is full of new discoveries, and we only give abrief introduction to this rapidly expanding field.

This book was written for a variety of readers. One is anundergraduate science major or a nonspecialist scientistwho knows little about planets or climate. This reader willlearn a lot about the planets and something about the fundamentalphysical processes that control climate. We gofairly deep into the physical processes, but the emphasis ison intuitive understanding. We touch on convection, radiation,atmospheric escape, evaporation, condensation,atmospheric chemistry, and the dynamics of rotating fluids.There are good textbooks and popular science bookson planetary science and there are multiauthoredspecialized books about individual planets. Thereare also good textbooks on atmospheric science.Therefore another potential reader is a student of atmosphericscience who has learned the relevant equationsand wants to step back and think about the fundamentalprocesses in a broader planetary context. Finally, thereare the climate specialists and planetary specialists whowant to know about the mysteries and unsolved problemsin planetary climate. Such readers might solve some ofthe many mysteries about planetary climates and therebyhelp us understand climate in general.

CHAPTER 2

VENUS:ATMOSPHERIC EVOLUTION

2.1 EARTH'S SISTER PLANET GONE WRONG

Until the beginning of the space age, Venus wasconsidered Earth's sister planet. In terms of size, mass, anddistance from the Sun, it is the most Earth-like planet, andpeople assumed it had an Earth-like climate—a humid atmosphere,liquid water, and warm temperatures beneathits clouds, which were supposed to be made of condensedwater. This benign picture came apart in the 1960s whenradio telescopes peering through the clouds measuredbrightness temperatures close to 700 K. Also in the 1960s,the angular distribution of reflected sunlight—the existenceof a rainbow in the clouds—revealed that they weremade of sulfuric acid droplets. The Soviet Venera probesshowed that the atmosphere was a massive reservoir ofcarbon dioxide, exceeding the reservoir of limestonerocks on Earth. The U.S. Pioneer Venus radar imagesshowed a moderately cratered volcanic landscape withno trace of plate tectonics.

We now know that Venus mostly has an Earth-likeinventory of volatiles—the basic ingredients of atmospheresand oceans—but with one glaring exception,and that is water. Earth's ocean is three hundred times asmassive as its atmosphere. Water is more abundant thanall the other volatiles combined, including carbon dioxide,nitrogen, and oxygen. In contrast, Venus has only asmall amount of water, and it is all in the atmosphere.Relative to the mass of the planet, the amount of wateron Venus is ~4 × 10^-6 times the amount on Earth. Thisraises some fundamental questions: Was Venus born dry,and if so, how? Or, was Venus born with an Earth-likeinventory of water that it somehow lost? These questionsare the theme of this chapter. In section 2.1 we reviewthe volatile inventories for the terrestrial planets.In section 2.2 we discuss the possibility that Venus losta large amount of water, perhaps an ocean's worth, overgeologic time. The evidence is found in the isotopes ofhydrogen—the anomalously high deuterium to hydrogenratio, which could come about when the lighter hydrogenescaped the planet's gravity at a higher rate thanthe heavier deuterium. Finally, in section 2.3 we discusshow Venus might have lost its ocean while Earth heldon. The effect of extra sunlight on Venus is amplifiedby feedback—the warmer it gets, the more water vaporenters the atmosphere, which makes it even warmer becausewater vapor is a potent greenhouse gas. Somewherebetween Earth and Venus, the theory goes, an Earth-likeplanet cannot exist. Inside this critical zone, the oceansboil away and are eventually lost to space.

Venus comes closer to the Earth than any otherplanet—its orbit around the Sun is 72% the size ofEarth's orbit. The planet itself is 95% the size of Earth,and its surface gravity is 90% of Earth's. The bulk densitiesare nearly the same. The density of a terrestrialplanet tells us about the proportions of rock and metalinside. The metal, mostly iron, is denser and resides inthe core. The rocks are oxidized metals, mostly silicon,magnesium, and iron combined with oxygen, and havedensities one-third that of the metallic core. From thebulk densities, one infers that the mass of rock relativeto the mass of metal is about 2.2 for Earth and about 3.0for Venus. Thus Venus and Earth are rocky terrestrialplanets with similar amounts of metals locked away intheir metallic cores.

Given these similarities in size, distance from the Sun,and bulk composition, one might expect the two planetswould have similar climates and similar inventories ofthe lighter elements H, O, C, N, and S. These elementsare the main ingredients of atmospheres, oceans, andice sheets, which are the basic elements of climate. Insome ways this expectation is correct, but in other waysit fails miserably. Venus has a small amount of water, allin vapor form, a massive atmosphere of carbon dioxide,and a surface temperature of ~730 K (457 °C , 855 °F).Clouds of sulfuric acid enshroud the planet (fig. 2.1). Thepressure at the ground is 92 bars, or 92 times the Earth'ssea-level pressure. Since pressure (force per unit area) isthe weight of overlying atmosphere (gravity g times massper unit area), the mass per unit area of the atmosphereof Venus is about 100 times that of Earth when one takesthe lower gravity of Venus into account. Table 2.1 givesthe volatile inventories of Venus, Earth, and Mars.

Earth actually has a lot of carbon dioxide, but it'snot in the atmosphere. Instead it resides mostly inlimestone—calcium and magnesium sediments withchemical formulas like CaCO₃ and CaMg(CO₃)₂. Thesecarbonate compounds have accumulated over the lifeof the Earth when igneous rocks (rocks formed from amelt) containing calcium and magnesium have weathered—grounddown and dissolved—and then combinedwith carbon dioxide in the oceans where they precipitateout. Calcium is found in many kinds of igneous rock, buta simple example is CaSiO₃. Dissolved in water, CO₂ is aweak acid, which helps to dissolve the rock. The net resultis that the CaSiO₃ combines with CO₂ to make SiO₂(silica) and CaCO₃ (calcium carbonate).

The silica precipitates out as quartz and as hydratedsilica (clay). The calcium carbonate precipitates out aslimestone. As CO₂ is released from inside the Earth involcanoes and fumaroles (volcanic vents), and as silicaterocks are weathered, the limestone accumulates. Biologicalprocesses aid the precipitation. Some plankton andother species use calcium carbonate in their shells, andmuch of the limestone deposits on Earth are derived fromthe shells of marine organisms. Other plankton speciesuse silica in their shells. When one adds up the limestonedeposits on Earth, the mass of CO₂ sequestered is60 to 180 times the total mass of Earth's atmosphere. Thelarger number includes estimates of carbonates that weresubducted into the mantle, and it is highly uncertain. Inany case the mass of CO₂ in carbonate rocks on Earthis comparable to—within a factor of two—the mass ofCO₂ in the atmosphere of Venus. Formation of carbonatesdepends on liquid water, so the fact that the CO₂on Venus is in the atmosphere and the CO₂ on Earth isin carbonate rocks is probably tied to the question ofwhy only Earth has oceans. That question hinges on thegreenhouse effect and climate, as we shall see.

Before discussing water, let us discuss two other gaseswhose abundances are comparable on Earth and Venus.The first is nitrogen (N₂), which makes up 3.5% of Venus's92-bar atmosphere and 78% of Earth's 1-bar atmosphere.These percentages refer to the numbers of molecules, nottheir masses. The ratio of the atmospheric masses of nitrogenon Venus and Earth is 3.0, which means they arecomparable. The other gas is argon, which, on Earth atleast, is mostly ⁴⁰Ar. The number 40 refers to the mass ofthe nucleus, which is controlled by the number of protonsand neutrons. ⁴⁰Ar is produced from the decay ofradioactive potassium, ⁴⁰K. The amount of ⁴⁰Ar in a terrestrialplanet atmosphere is a measure of the amountof radioactive potassium in the crust and the degreeof outgassing—the extent to which the gaseous argonis released from the rock and conveyed to the surface.Relative to the planet's mass, the ratio of the mass of ⁴⁰Arin the atmosphere of Venus to that of the Earth is ~0.5,which means the amount of outgassing on the two planetsis comparable as well.