Malْnglt Comfortable
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CI.IAPTER 9
MaLúnglt ComfortabLe
Runn mg W ater, Temp eratur e Contr ol., arrdsunProtectron
The solør system is marked by temperature extremes, from the millions of degrees of the solar corona to the near absolute zero of interplanetary space. The moon has temperature variations of -300"C between lunar night and lunar døy. Although Venus is nearly equal in size and bulk composition to Earth, the Venusian ground surface is 450"C warmer. Mars is 80oC colder. None of these environments are able to host liquid water, which is a critical need for life as we know it. Earth, in contrast, has a "goldilocki' temperature, with evidence for continuous liquid water at the surface for aII of Earth\ history recorded in the rock record. During this time the luminosity of the sun has brightened by about 35% as the sun has consumed iß hydrogen fuel. How did thß equable climate come about? When did it begin? How has it been maintained? Eørthi climate stability is dependent on its volatiles. The volatile budget of a planet depends on its accretion history and bulk composition, the amount of volatile loss to space from impacts and the solar wind, and the cycles of the volatile ele- ments between planetary interior and exterior. Once the core formed a few tens of míllions of years after accretion, Earth would have developed a magnetic field that deflected the solar Fig. 9-0: Seascape photograph by Ansel Adams of Point Sur during a storm. Earth has wind, minimizing its efects on the atmosphere, preventing had a persisent liquid ocean since at least 3.8 Ga. (Photo by Ansel Adams. collection center for creative Photograph¡ university of Arizona, @ The Ansel Adams Publish- volatile loss, and protecting the surface from ionizing radiation ing Rights Trust, with permission) harmful to life. Liquid water was present very early in Earth\ history. Sediments, formed by deposition from water, and pillow basalts, solidified lavas that formed in contact with water are present in the oldest rocks at 3.8 Ga. There is evidence for even U
Wate¡ Temperature Control, and the Sun . 251 250 . ChaPter 9
that have ages as There are, of course, no simple answers to these questions. In pre- earlier liquid water from tiny crystøls of zircon characteristic vious chapters we have seen that the habitability of our planet is in part old as 4.i Ga. Climøte stability is a long-standing determined by its nebular heritage that sets its size, orbit, spin, and bulk of Earth\ eYolution. of chemical composition. It is in part determined by the evolution of its A planet's surface temperature depends on the luminosity the star' It also depends interior and crust. As we shall see in this chapter, it also critically de- its star and on the planei\ distance from "greenhouse pow,er" pends on what happened to its volatiles after planetary accretion, and on the reflectivity o¡ it, ,u'¡ot' and on the consisting of more than how they are cycled through planetary processes. of its atmosphere, caused by molecules andVenus ihree atoms;, such as CO2, H2O and CHn' Earth Most Venu- exemPlify the importance of the greenhouse fficL powerful T he P Lanetary Vo lat i [e Budget sian carbon is in the atmosphere as CO, creating a all stored thermal blanket. By contrast, Earth's carbon is nearly For life of any complexity to develop on a planet, abundant liquid water insedimentsascarbonatelnineralsøndorganicresídues.Feeà- the atmosphere must be available. Water is essential for all of life as we know it. Water is backs must have modulated greenhouse gases in a fundamental medium of transport and chemical communication that to maintøin long-term climate stability' The most likely feedback and volccÌnic make cellular processes possible. Living cells are about 70o/o water by is a "tectonic thermostat" that relates subduction high weight. The average human being is 600/o water (watermelon is )907o outgassing of COrto changes in weathering' High COror water). Water's centrality to life is reflected in the stark differences we trriprrotlrà, ,rioræ weathering that releases Ca2+ to the CO'øs see from region to region related to water availability. Given abundant orrorr. This then leads to cooling caused by removal of greater buildup rainfall, we have lush forests teeming with all manner of living organ- CaCOr. Low COror low temperatures permit ß its elf isms. Where there is little rain, we have deserts sparse in life. Where only of cO) from, olíoro rr, causing w ørming' Weathering Hence' snow falls, we have barren ice caps. These contrasts are found on a planet iifturired by plate movements and mountain building' plate tectonics' whose surface is 70o/o covered by liquid water! Earth's clímate reflects linkages among the sun' climøte stability Carbon is also essential for habitability, since it is the central element and. surface biogeochemical cycles, providing depends on for all the organic molecules (those with C-H bonds) of which life is consistent with tiquid water' The tectonic thermostat surface has just made. As we will see later in this chapter, C as CO, is also a pivotal mol- the coexistenæ oi or"on ønd continent' Earth's this is a ecule for climate stabilit¡ and its exact concentration in the atmosphere the right amount of water for this balance' Whether planetary is a fundamental control on surface temperature.T}re carbon cyclelinks hoppi accident or the result of feedbacks in early the carbon in organic molecules, the atmosphere, the oceans, the mantle history remains an unsolved puzzle' and limestone (CaCOr) in a balance that supports both life and the cli- mate that is essential for life. For a habitable planet, the right amounts lntroductton of HrO and CO, are both critical. Given the centrality of HrO and CO' the first requirement for habit- most important to ability is that the planet must have captured enough volatiles, including We have yet to consider the attribute of our planet the surface' sufficient water to make a sizable ocean. The silicate earth as a whole living organisms-a stable climate that permits water on of its surface? contains only small amounts of HrO and COr-about 700 ppm HrO fixes its water supply? What sets the temperature What (0.07 oceans? In a nut- wt.%) and 200 ppm CO, (0.02 wt.%). For HrO these small amounts What permits the lucky coexistence of continents and mean that Earth ended up with only one water molecule out of every shell, what makes our planet habitable? ?
252 . Chapter 9 Water, Temperature Control, and the Sun . 253
matter from which Earth formed. three million that were in the pool of Evîdencefor LLquLdW ater b efore 4.o G a Most of the carbon in the nebula was in the form of methane gas. Earth, one every 3,000 car- however, somehow managed to capture about in Some of the oldest rocks are sedimentary rocks, and most sediments that Earth is very volatile bon atoms. These numbers further illustrate require weathering, transport ,and deposition by liquid water. The evi- fact is that the ratio of depleted relative to the solar nebula. A curious dence for the earliest fossils comes from rocks that are 3.5 billion years ratios ]HzOlCO2in the silicate earth (-3.5) is signifrcantlyhigher than the old. The oldest sediments are those of the 3.8 Ga Isua formation in be that much of in chondrites (HrO/COr) <1.5. One solution would Greenland. These rocks include cherts, carbonates, and banded iron for- have a high HrO/CO, Earth's volatile budget comes from comets, which mations. All of these rocks require liquid water to form, and the same elements in the core. Water and ratio. Or carbon may be one of the light rock types occur in much younger rocks when we know liquid water was of the origin of Earth's atmosphere COrare further pieces tothepuzzle present. The rocks show that water was present at least as earþ as 3.8 Ga. and ocean prior to the beginning ofthe rock record. We are able to push back the presence of liquid water even earlier by as a whole lead to the second The small amounts of volatiles in Earth using evidence from a surprising source, the mineral zircon (ZrSiOn)- budget, the volatiles habitability requirement-given a modest volatile one of the highest-temperature and most stable minerals found in rocks. occurred on Earth, since at the must be concentrated at the surface. This Zircon is very low in abundance (usually less than 0.02o/o of the rock), To contrar¡ the surface a low volatile budget is not at all evident. the but also very common. Virtually all granites and sandstones contain atmosphere, ocean, and proportion of HrO and CO, in the combined some grains of zircon. Zircons are also very stable minerals and are dif- wt.o/o COr. These numbers crust is rather high-7.2 wt. % H2O and t-5 ficult to alter or dissolve. The high chemical fldelity and chemical resis- the planet and are just the reflect - 100 times enrichment relative to total tance of zircons allows them to survive during weathering and sedimen- and climate stability conducive right amounts for a large liquid ocean tarytransport; theyare so robusttheyoften survive even multiple melting to life. At the high temperatures that must have prevailed during the period of the formation of our iron core, extensive melting and active convection would have cycled mantle rocks to the surface, where their HrO and CO, would have been transported to the atmosphere in gas- eous form. As the iron migrated to the core, HrO and CO, would have migrated to the surface. It is also possible that volatiles degassed during impacts of incoming planetesimals, so that volatiles were preferentially accreted at the surface. Sufficient volatiles at the surface, however, are not all that is required for habitability. The water at the surface must be in liquid form. Can we somehow ascertain when liquid water, essential for the formation of life, might have first appeared? The answer to this question is critical for the \ Chapter 13, because it constrains the time origin of life, discussed in -:g. 9-1: Images ofzircons. On the left: A single zircon crystal in a biotite. The crystal is about interval over which the processes leading to life had time to operate. - l0 microns ( I / l0 mm) in length. On the right: A zircon viewed under cathode luminescence -t For example, if life and liquid water appeared at the same time, then at reveals its zoning and growth history. Each point in a zircon can give a precise date using U-Pb system, (see Fig. 9-2).The zircon has a complex history, with an old inner core that Iife's origin would be geologically instantaneous. If water were present =e Partially corroded, surrounded by younger generations of mineral that grew around it. to develop. '^-as from the beginning, life could have a billion years or more l:e old core has an age of 4.4 Ga, the oldest age found for any terrestrial material. (Photograph Which is it? :-.urtesy of John Valley) 7
254 . Chapter 9 Water, Temperature Control, and the Sun . 255
1.2 cient zircons are preserved in Archean sedimentary rocks. These rocks are as old as the igneous rocks that surround them, but the sedimentary
1.0 4.5 by rocks, particularly sandstones, preserve zircons that were created by a Concordia previous generation ofigneous events and then were preserved during 4.0 by weathering to survive in the younger sediment. The most famous lo- 3.5 by cality for such ancient zircons is an apparently nondescript sedimentary ì 0.6 G 3.0 by rock in Australia, calledthe lack Hills formation.T}ris formation has been 2.5 by exhaustively studied, and zircons have been recovered with concordant o.4 2.0by ages as old as 4.4 billion years. This is much older than the oldest known 1.s by rock with a reliable date associated with it-the Acasta gneiss, near 4.0 Ga. 1.0 by So how do these little minerals tell us about ancient water? 0.s by 0 Zircons provide evidence for the early presence "o 20 ' 40 60 80 loo of water through two 2o7Pbl23su fairly detailed lines of reasoning. Water has a strong influence onfreez- ing point depression and permits melting of Earth materials at low tem- Fig. 9-2: The "Concordia" diagram used to illustrate U-Pb dating that is possible with zircons. Zircons contain no Pb initially, so all the Pb is created by radiOactive decay of peratures. The lowest temperature silicate melt is granitic magma, which 23su 207Pb the two isotopes of u. Because the half-life of is much shorter, much more is can be produced by hydrous melting of basalt, sediment, or other gran- rocks. Samples that have not lost any Pb since created in older rocks than in younger ites. Higher-temperature magmas also differentiate toward granitic com- they formed will plot on the concordia, and their age is confirmed by two independent positions, as they crystallize in the presence of water. Granites imply methods. Pb loss causes the data to move directly toward the origin at ùe time the Pb loss occurred. This can also be used to constrain ages, an original age and a metamor- water. While zircons can be found in anhydrous mafic rocks, they are phic age when the Pb loss occurred. rare. In hydrous granitic rocks they are ubiquitous. The common exis- tence of zircons in the )ack Hills formation suggests granites, which re- events, creating zonedminerals where each part of the mineral formed quires water. This is not quite proof however, since some zircons also at a different time (Fig. 9-1). occur in high-temperature, anhydrous magma. Zirconhas one other key characteristic-mineral grains can be indi- Further evidence comes from the trace element contents of zircons. vidually dated. Zircon concentrates the parent element U and excludes Titanium is incorporated as a trace element in zircon because it has the the ultimate daughter element Pb. These âre the ideal initial conditions same 4* valence state as Zr.Brtce Watson and Mark Harrison showed for radiometric dating. Furthermore, 2381J and 23sU have different decay that the amount of Ti incorporated in zircon is very sensitive to tem- constants and decay to 20óPb and 207Pb, respectively, so that two ages can perature. Measured Ti contents of the |ack Hills zircons show that they be determined independently. Because of the different decay constants, formed at temperatures of about 750"C, a temperature associated with 206Pb relatively more 207Pb is formed at the beginning and more more hydrous, granitic magmas and not with anhydrous mantle-derived mag- recently. IfPb is lost during subsequent geological processes, all the Pb mas. The zircons come from low temperature granites, which requires isotopes are lost proportionall¡ and the ages no longer agree. When rvater. Granites are also the characteristic rock type of continents, so the they do agree, the age is robust and the zircon is said tobe concordant data suggest continents were present as well. (Fig. 9-2). Zircons are thus very effective messengers from the past, be- cause they record the time of their formation and they resist subsequent STABLE I SOTOPE FRACTì ONATI ON chemical change. These characteristics make ancient zircons the oldest Earth materials Here we need a brief interlude to introduce the concept of stable isotope that have not been modifled since earliest Earth's history. The most an- fractionation, which is a frnal line of evidence from zircons. In previous rr
Water, Temperature Control, and the Sun ' 257 256 . ChaPter 9
b) isotope ratios of elements re- a) chapters we n '100 is not a radioactive decay prod- I Mantle tÐ 5-18o/oo + 0.12o/oo I Kimberlite 5.32o/oo+0.17o/oo sulting from xenocrySls peridotite 20 so ú80 þ. 5. Africa uct, and all i same electron shell structure, xenoliths o. E Mauna Loa E that at low a d 1q how could the ratios of oxygen isotopes vary? It turns out 960 and Loihi basalt o processes discriminate slightly among isotopes of the temperatures, ì¿o ¡t 10 sume el"ment based on their mass. The variations are so small that they E 2zo= z5 are reported as parts per thousand (or "per mil") relative to a seawater One consequence of this isotopic fractionation is that rainwater ") standard. -2 3 4 5 6 7 I 9 3 4 5 6 7 8 9 has a slightly higher (zircon) (o/oo VSMoW) laser fluorination that falls on continents is isotopically heavy-i.e., 618O (olivine) (o/oo VSMOW) laser fluorination 6180 180/160. sta- ratio of In order to make the numbers a bit more intuitive, d) as "per mil" variations rela- ble isotope variations are always referred to + 0'690/oo I Archean EE 5.600/oo + 0'70oloo I Jack Hills 6.280/oo tive to a well-known standard, with the heavier isotope in the numera- r9neous -6 rocks o the standard is -a JU tor and the light isotope in the denominator. For oxygen E Sanukitoids E e S+ used is ô18O, so seawater has a ôl8O value E Metasedi- mean seawater. The notation E20 mentary o 18O/160 o mil. "Hearry" oxygen with higher than seawater has posi- provrnce l¡ of 0 per plutons E'¡ lo/o heavier than seawater). 10 a- tive ialues of õt8O (e.g. t0 per mil would be z= z The mantle has dI8O of about 5 per mil, as indicated by the oxygen iso- 9-3b. 0 offi7ss tope measurements of mantle materials reported in Figure 9-3a and 23456789 6r80 (zircon) (o/oo VSMOW) laser fl uorination ôr80 (zircon) (o/ooVSMOW) ion microprobe Rocks that have been influenced by a low-temperature water cycle data from the ancient evaporation and precipitation (i'e', weather with a liquid -:g. 9-3: Oxygen isotope data from various rocks compared to the involving data for ions collected in the Jack Hitls formation. The top two panels, a and b, show the oxygen, with ô18o greater than *5. This interaction ocean) have heavier -antle values of can involve either sediments in the source regions that melted to pro- r.:-5.3 values, but values of duce the rocks or interactions with migrating fluids in the crust that are i:d-ime er Hills zircons in panel d require that their source rocks had seen the influence ofa low derived from rain. In either case, a low-temperature water cycle is indi- =e Jack :::nperatufe water cycle. (Modified from f. Valley, Revíews in Minerologlt and Geochemistry' evidence of this water cycle by examining data cated. we can see the . i3, no. 1,343-385) from rocks when we know a water cycle was present. For example, some and rarest mineral grains found in rocks hold the essential clues to wa- ter's presence, providing crucial evidence for appropriate conditions for life's origin.
ControÍs onVolatlles at the SurJace
water cycle was present during the formation of the rocks whose zircons - :: nabitability over Earth's histor¡ we need to consider the long-term survived in the lack Hills formation. Otherwise, the oxygen isotopes :-r=-.1-¡ on the total amount of water and carbon at the surface-not would not be fractionateò. -j- - r r ----: -.-olatiles cycle through surface reservoirs over a few thousand The zircon evidence suggests that as far back as 4.4 billion ve ârS 5: ullL- :. but how they cycle between the interior and exterior over was a robust water cycle on Earth, and liquid water was Prese:-- -: - =:s- lri--:r::: :: of years. beautiful piece of geological detective work that some of the =-,' l.*'r =illions It
the 259 258 . Chapter 9 Wateç Temperature Control, and Sun '
If impacts caused degassing of incoming planetesimals, or core for- )ust as a space vehicle can escape Earth's gravitational field with suffi- mation led to massive early degassing of Earth's interior, volatiles would cient velocit¡ individual atoms or molecules can escape when their ve- have been concentrated to the surface in earliest Earth's history, consis- locity is high enough. velocities increase with temperature and decreas- tent with the evidence cited above for a substantial ocean at that time. If ing atomic mass. For Earth the temperature of the outer regions of the early degassing were so e Earth's vola- upper atmosphere is about l500oK, while that at the surface is only about tile budget to be initially ntinued vol- 300.K. The high temperatures of the upper atmosphere greatly increase canism over time would latiles to the the probabilrty for molecular escaPe. surface, causing most of Earth's volatile budget to be in the atmosphere The velocity required for escape depends on the planet's gravitational and ocean. Surprisingl¡ however, substantial HrO and CO, remain in field and on the mass of the molecule itself. These dependencies are Earth's interior. volcanoes today are still emitting volatiles, and these extremely strong. The escape velocity from |upiter is 60 km/sec, from permit estimates of the concentrations of volatiles in the mantle. while Earth it is 11.2 km/sec, and from the moon only 2.4 km/sec. For smaller the concentrations are low the volume of the mantle is so large relative planets, the escape velocity is much lower, and gases can escape more to the crust that about half of Earth's CO, and HrO still reside in the in- easily. Escape also depends critically on the mass of the gas molecule. A terior. Another entire ocean volume remains trapped in the solid Earth. factor-of-2 difference in mass changes the likelihood of escape by sev- Furthermore, current volcanic emissions over time wcjuld generate an eral orders of magnitude. While Earth and Venus are massive enough to ocean volume of water in only 2-3 Ga, and yet as we saw above there is prevent thermal escape of all but the lightest gases, the moon has insuf- evidence for an ocean prior to 4 Ga. Has the ocean then increased sub- ficient gravity to hold even the heaviest gas. Thus, Earth and Venus have stantially in size? substantial atmospheres while the moon has none at all. |upiter has such Studies ofrocks from the continental crust can be used to show that a high escape velocity that it has been able to retain even the lightest sea level has remained remarkably constant through Earth's history, im- gases, hydrogen and helium. Planet size and the ability to retain an at- plying near constancy of the volume of the oceans and hence water at mosphere are critical for habitability. lh. srrrfu.". Since volatiles (including Hro) are steadily being supplied There are other mechanisms of atmospheric loss. Particles of the solar from Earths interior, how can the volume of HrO at the surface have wind can have very high velocities that can strip gases from the outer remained in a narrow range? And since degassing is an inevitable result atmosphere. Impacts can also cause stripping of the atmosphere by ac- of early Earth's history and continued volcanism, how can so manv vola- celerating molecules to escape velocities. And if planets are too close to tiles remain in Earth's interior? their star, the very high temperatures can produce various effects that These questions require a consideration ofthe surface volatile budget, create atmospheric loss. The diversity of processes of atmospheric loss not just as a p so as a dynamic process involv- and how they change over planetary history may help to account for ing fluxes am space, causing volatile addition much of the diversity of planetary atmospheres and compositions in the and removal. could occur by volatile loss to solar system. space or by returning HrO and CO, to the interior during recycling of For our present PurPoses, however, we need to estimate to what ex- Earth's plates. tent HrO and CO, might be lost from Earth. By turning to the evidence from helium, we are able to estimate the extent these heavier gases have been lost. This method involves a comparison between the total number ATMOSPHERIC LOSS TO SPACE of helium atoms in the atmosphere and the number of helium atoms that (Fig. Once at the surface, any gaseous substance has the opportunity to escaPe leak into the atmosphere from Earth's interior each year 9-a). The into space. The most well known mechanism is called thermal escape. number in the air is obtained from the mass of the atmosphere and its Water, Temperature Control, and the Sun ' 261 260 . Chapter 9
Table 9-l of Earth's atmosphere today. -1* F ¡Á€4?gsp{ô4^ Composition t-" Percent ,r,rur"o Gas Name Gas Formula byVolume N2 78.08 \ ( Nitrogen WITHIN rHE 20.95 E4RI.,9 Oxygen 02 ,*rtì)2rerJ22o6Pb+B4HE Argon Ar 0.93 zot -115\)'---' Pb + 7 4H Carbon dioxide-- CO, 0.039 F b 0.0018 132Th'---+208P + a atjF Neon Ne Helium He 0.00052 oms are generated within the Earttis crust ium. By measuring the amount of heat Krlpton Kr 0.00011 ea of how much uranium and thorium Xenon Xe 0.00009 i" .JH:T;Tï: iïHîåîî;I"i:: Hydrogen H2 0.00005 of l miilion years before escaping from reach the surface, where it resides an average CHo 0.0002 thetopoftheatmosphere.Alltheheliumatomsmanufacturedbyradiodecaywithin Methane'* lost to sPace' Earth are eventualìy Nitrous oxide-- NrO 0.00005
'ln addition, the atmosphere contains water vapor (H rO) in variable amounts (up to 2% for warm air and down to a Íew parts Per at ocean ridges can be determined helium content. Helium production millionfor very cold strøtospheric air) Water is ako a greenhouse gas in ocean ridge basalts' in the " Greenhouse gases. ty -""rrrring the helium concentrations hydrothermalfluidsatdeepSeavents(discussedatlengthinChapterl2), of helium is produced by the radioactive decay and in seawater. Since as helium, hydrogen mol- in continental ferent story. As they are only half as massive 238U 23sU andz32lh,estimates of these concentrations H, caused ecules have an escape time far less than I million years. Fortunately, measuring heat flow on continents that is rocks, combined with (see Table is escaping is a very rare gas in our atmosphere 9-l). decay' permit estimates of how much helium by radioactiYe generated by bacteria living in soils survive being added to the at- Today any H, molecules continents each year' The number of atoms from only a few years in the atmosphere before being converted to water (2H, the number of helium atoms mosphere in a year is about one-millionth a planet can atoms + 02 -+ 2H2O). There is another route, however, by which in the atmosphere' This suggests that helium currently residing lose its hydrogen and hence its water. High in the atmosphere, ultravio- resideintheatmosphereforanaverageoflmillionyearsbeforethey molecules apart, creating free H atoms. escape time' the Iet light from the sun breaks Hro the atmosphere to space' Using the helium escape from These free H atoms are exceedingly vulnerable to escape. The O atoms react either iron, sulfur, or carbon. This pro- 44 for escape left behind eventually with "r.up.timefortheotherg"'""u"becalculatedfrommoleculartheory'20 fo, 28 for Nr' 321or O and With masses of t' !!,' by which Venus lost most of its water. ""on, Eartn"s history. cess is the likely mechanism long that atmospheric loãs is negligible over times are so The reason why this process has not decimated Earthb water reserves has a mass similar to neon' and With its atomic weight of 18, wutt' "water But the is that our atmosphere has a trap" that keeps almost all of Earth's molecãt", do not escape from the atmosphere' therefore water rvater in the lower atmosphere, preventing it from being carried to higher of the water molecule are a dif- hydrogen atoms that are essential parti 262 . Chapter 9 Wate¡ Temperature Control, and the Sun . 263
UV light 120 Stratosphere HrO=5 Thermosphere
Troposphere HrO = 30,000 Mesopause Ê J Mesosphere Ë60 .9ì o Stratopause I Stratosphere
- I *r.r rr"o Tropopause Troposhere -Y' 1e0 210 23o tto 330 3s0 370 "rlrlrl.r*'TT,
Fig.9-6: Temperature profile through Earth's atmosphere. of weather occurs the Fig.9-5: Most of Earth's hydrogen is in the form of water, about half of which resides All in troposphere. At the top ofthe troposphere, temperatures are so cold that all water has in the ocean. The majority of the remaining water is trapped in solids making uP the precipitated as ice, and none migrates to the upper atmosphere where it can be broken mantle and crust. Earth's freshwaters (lakes, rivers, groundwater, etc.) make up only apart, which would alÌow H atoms to escape. about 3.0% of the total- Its ice caps make up only 7.5o/o of the total. only a very small fraction of our water is at any given time stored in the atmosphere as vaPor, which is confrned almost entirely to the lower, well-stirred part of the atmosphere (referred to by meteorologists as the troposphere). Only two of every billion water molecules reside its hydrogen escape hatch very small. Over the last 4.0 billion years where in the stratosphere. This is important, because only that watel present in the strato- there is a rock record and evidence for liquid water, only a small fraction a to be dissociated by ultraviolet light. Dashed line separates Earth's sphere has chance of our hydrogen has been lost! interior from exterior. Of course, what may have happened in earliest Earth's history is not so clear. In the early Earth there was no O, and bacteria likely converted levels from which it could escape. As tallied in Figure 9-5, the bulk of hydrogen to methane (4H2+ CO, -+ CH4 + 2H2O). While methane is our water resides in the ocean, sediments, and ice. At any given time i.n very low concentration in the atmosphere today, it might have been only about one H2O molecule in 100,000 is in the atmosphere. The lower more abundant on early Earth, and there is no cold trap as there is for atmosphere is called the troposphere, and as we all exPerience, the tem- water, so methane would be able to rise to the upper atmosphere and be perature in the troposphere declines rapidly with increasing height broken apart with loss of H. Massive impacts can also cause atmospheric above sea level (Fig. 9-6). This temperature decrease with altitude causes loss even of heavier molecules, and very high atmospheric temperatures water vapor to precipitate as dense ice crystals to form the clouds and after the moon-forming impact would have had major effects on the snow. It can be a warm, sunny day in Los Angeles, California, or Nice, atmosphere. Is there another way we can determine whether Earth lost France, while snowing a short distance away in the mountains at heights a lot ofits hydrogen? of a few thousand meters. Airplanes flying at 10,000 m are at -60"C even Here once again stable isotope variations provide important evidence. lH 2H on summer days. At the top of the troposphere the temperature is so Hydrogen has two isotopes, (called H) and (deuterium, or D), cold (-60'C) (Fig. 9-6) that virtually no water vapor can exist, and only rvhich is a factor-of-2 mass difference. This leads to much easier escape 1H extremely dry air migrates from the troposphere to the next highest at- for relative to D. But Earth's D/H is about the same as chondrites, mospheric level, called The stratosphere.These attributes of Earth make suggesting little hydrogen loss. Earth was able to retain its water. tlr
264 . Chapter 9 Water, Temperature Control, and the Sun . 265
AII the evidence, therefore, suggests that Earth's atmosphere was volcanic outgassing and supply sufficient CO, and HrO to the interior to stable with respect to loss of HrO out the top, certainly by the time of explain the substantial amounts that reside there. In fact, so much HrO the advent of a stable climate and the end of the largest impacts some goes down the subduction zone that if it all remained at depth the oceans 3.8 billion years ago. We thus need to look downward as well as up- would diminish with time! ward to understand the controls on the long-term volatile budgets at It remains curious that the amount of water at the surface appears to the surface. have remained within a tight range throughout Earth's history. This im- plies a balance between outgassed water and subducted water. A possible solution to this puzzle is that most of the subducted water is efficiently CYCLING OFVOLATILES BETWEEN TI-I.E SURFACE AND processed at subduction zones and returned to the surface through the EAITTH'S INTERIOR volcanism that occurs there. As we will see in Chapter 12, the volatile- Since volatile loss to space cannot explain the relatively constant water bearing minerals of the subducting plate break down and release their budget at the surface over time, the solution must lie in recycling of volatiles at the high pressures and temperatures of Earth's interior. The volatiles from the surface to the interior. Return of volatiles to the inte- released volatiles then trigger volcanism that transfers water to the sur- rior would also account for the significant amounts of volatiles that re- face. If more or less water were subducted, more or less outgassing would side today in the mantle. occur. The net flux of water in or out of the Earth could then be small The process of volatile recycling is the subduction of Earth's tectonic enough that there would be little variation in total amount of water at plates, which will be discussed at length in chapters l0 and 12. The new the surface. ocean crust formed at ocean ridges leads to extensive circulation of sea- For CO, the budget is much less clear. Unlike water, where changes water through cracks in the crust. Interactions between the rocks and in sea level provide information about the surface water budget, there is the water cause alteration of the crust to form new minerals that con- no clear geological indicator of the amount of CO, at the surface over tain water and co, in the solid state (e.g., sheet silicates and calcium Earth's history. Carbonate minerals are also more stable than hydrous carbonate. minerals in the subducting plate, and substantial amounts of CO, in Further alteration occurs as the spreading plate moves the crust away carbonates could get past the subduction zone fllter and returned to the from the ocean ridge, and at the same time sediments fall through the mantle. Much remains to be understood concerning the overall CO, ocean to create a progressively thickening sedimentary layers, including budget related to outgassing and subduction. clays and carbonates made up of volatile-rich minerals. When the plate reaches the subduction zone, this volatile-rich package is "subducted" down into the mantle, returning volatiles to the interior. By this mecha- SurfaceTemperature nism a flux from the surface to the interior balances the volcanic flux to the atmosphere. For the HrO at the surface to be in the essential liquid state, the Earth's Precise estimates of the down-going flux are difficult to obtain be- surface temperature must have been maintained within a narrow range. cause no single drill hole has yet been able to penetrate the entire ocean Our water would be of little use to life were it tied up in massive ice crust, and only a few holes have been able to penetrate a kilometer or sheets. Nor would it be of use if it were all in the atmosphere as steam. more. Considering the vast expanses of the oceans, and the diversity of And if temperature changed from near freezing to near boiling on a ocean crust environments, our knowledge of the average composition regular basis, life could also not survive. How has Earth's surface tem- of altered ocean crust is very limited. The data that do exist suggest that perature maintained a narrow range consistent with liquid water over the ocean crust contains sufficient HrO and CO, to easily balance the billions of years? ÊSrN ËiFÈiiËå+å[nåå*r ËnY- ^-+iì E g ['? ¡ Aa; s ÞÊ¡3 u u1ËËeääiii 3 +å il i Ê Ei ; ãã ^ßLÞ- BñË äÊå äågE v#8; äi'ißá"*äíEåää$äs.Ëo-Þõdws Ë ãå€äËîiliäËi åä=: +g aå a+År Ëçù"ääiågårc¿"õ_dEBíäÉEF¿ s4 Eä" ;ãqã*ÉH=ãeã Ë,É yä Ðä äe iõ+iËãi; È*:îÀp:Fõ ÞËäË ËË sE Ê lgiHia*?,¿"¿or[gÞå$i *Ë ti Pf ii zteõ ä åãgiEFäE6 5E þ: ; EEËååõã[5B'rFä€:å+" äå¿*r äË F i'tt*'FËÐååtsqg å;
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Table 9-2 Summary of the factors influencing the surface temperatures of the terrestrial planets
Mass of Distance Solar energy Black-body Fraction atmosphere from Sun received 106 temperature sunlight Reflective Green-house Actual surface oC kglcm2 106 km watts/m2 oC reflected cooling "C warming temperature
Mercury 0 58 9t26 .068 -8 0 167
Venus I 15- 108 2614 55 .90 -t44 +553 464
Earth today r.03*. 150 1368 5 .30 -25 +35 15
Earþ Earth 150 9s8 -26 .30 (?) 21 62(?) 1s (?)
Mars 0.016* 228 589 -47 .25 -16 +3 -60
Moon 0 150 1368 5 .11 -7 0 -160 - +130
'Mostly COr. "MostlyNrl- O, Water, Temperature Control, and the Sun . 269 268 . Chapter 9
240
3 200 = .= c rou f 108 3 = 6 3- rzo ä 1n6 G x f !go G ]04 = Et o - 1o' 340 'E
400 600 800 1000 1200 1400 1600 1800 2000 Wave number (cm-r) 1O-4 1O-3 10-2 10-r 1 (cm) Wavelength :ig. 9-B: Absorption of outgoing Earthlight. The jagged curve shows an actual spectrum of The -arthlight leaving the top of the atmosphere over the island of Guam. For comparison, the Fig. 9-7: Light emitted from black bodies ofthree quite different ternperatures: snooth curves are blackbody curves that show the expected spectra ifthere were no green- hotter the body, the more energy it emits (the 6,000"K body gives offabout 10,000 gases. These curves are drawn for a series oftemperatures; that appropriate for Guam times more energy per unit area than the 300'K body and 100 million times more ---ouse '.tould be just less then 300'K. The wiggles and large dips are the resuit of absorption of the per unit area than the l'K body). The wavelength representing the peak ofenergy that by wate¡ carbon dioxide, and ozone in the atmosphere. Water creates a broad ãmission for a star whose surface temperature is 6,000'K lies in the visible range, =diation and that for :eduction in the region 400-600 cm-tAs can be seen, the dip created by CO, is especially for a planet whose surface temperature is 300'K lies in the infrared range, lange. :rominent. The wave number is a measure of the frequency of the radiation. a universe whose background glow temperature is 3oK lies in the microwave Earth's atmosphere is transparent to the short wavelengths ofthe sun's radiation but absorbs some of the energy of Earth's reflected radiation' the fact that the greenhouse warming is large enough to more than off- set reflective cooling. ozone (or). The capture of outgoing Earth light by these gases serves as Table 9-2 also shows the great importance of solar energy for a plan- a thermafblanket that keeps the planet warm. For Earth, the greenhouse et's temperature, which is the baseline upon which planetary warming more than compensates for the sunlight lost through reflection. blackbody depends. Has energy been constant through time? Earth's mean surface temperature (i.e., 15"c) is 1Ooc warmer than if it temperature solar lVhile we have no direct measurements of long-term changes in energy were a perfect blackbody (see Table 12-2). coming from the sun, there is evidence from other stars in the galaxy To assess another planet's surface temperature, we also need to know at diverse stages ofevolution that reveals how stellar energy production how much sunlight it receives, the reflective Properties of its surface and changes time. Stars similar to the sun increase their energy output the amounts of infrared-absorbing gases contained in its atmosphere. with u'ith time, as the hydrogen fuel in the interior converts to helium and Solar radiation received is a straightforward function of distance from increased temperatures are needed to balance the contracting forces of the sun. Albedo can be measured from space. Greenhouse effect can be gravity. Astronomers estimate that the sun was producing about 39% determined where the composition of the atmosphere is known or can iess heat in Hadean times than it is emitting today. Using this luminos- be adequately estimated. Table 9-2 shows how the combination of solar ity, can calculate the blackbody surface temperature of Earth for its radiation, albedo, and greenhouse effect lead to a planet's surface tem- we early shown Table and illustrated in Figure 9-9. perature. On Earth, it is clear that habitability is strongly influenced by history, in 9-2 270 . ChaPter 9
IU G
o e0ú o o s Ë --çROO È =3
| '70 1.0 0.5 0 -50 3.5 - 3.0 2.O 1.4 45 4.0 first present life day begins eukarYotic cell
Billions of Years ago
the sun's luminosity has Fig. 9-9:Illustration of how Plate 1. Sèe also figure 6-1 .Jl..qo.tt..r. The sun today is emitting 39Vo more, would In the absence of any greenhouse efect' Earth would have ll out its history' With today'satmosphere' Earth that Earths ro times oider tiran 2 billion years ago' The fact combination liquid water since at least 4 Ga requires some efectorloweralbedoontheearlyEarth.FiguremodifiedfromKastingetal.l9BS,Sci. Amer.256:90-97.
for a sun with Calculation of early Earths black-body temperature a baseline (blackbody) tempera- 610/o ofits present luminosity leads to the same as it is today' the ture of only -26oC. If Earths albedo were have been a ftigtd -47"C' temperature absent a greenhouse efect would similartoMars'stemp-eraturetodaylWithagreenhousesimilartotoday' This con- on Earth would still have been below freezing' temperatures 240"E 300"E 0"E throughout Earth's history' flicts with evidence for abundant liquid water sun can be r€solved -8 -4 O 4 8 12(km) This conflict, referred to as the faint young Paradox' cooling with an albedo more Plate2. See also Êgure 8-12. either by having substantially less reflective enhanced greenhouse effect akin to Mercury or the moån, or a much warming' This enhanced that could provide some 55oC of greenhouse of greenhouse gases' Pro- warming wãuld require higher concentrations or an atmosphere with signifr- posals ii.toa" rnoÁ hight' CO, contents an atmosphere with no O"' cant methane, which iould be possible in Hydrogen
5un's Spectrum
Blue
l)late 3. See rrlst¡ tìgr,rrc- 2-.3.
Plate 6. See ¡lso figLrre 8-4.
PI¡tc 4. See rlso fìeure.5-0 Temperature (K) 7,000
6,500
6,000
s,500
5,000
4,500
4,000
3,500
3,000
2,500
2,000
I
I O O
O 3! bJl o d O
æ O Plrrtc' 7. Scc ¿rlso figtrre 8-7- fr A Plate 11. See figure I0-14.
Plate 9. See also figure 10-5.
I i 0 05 10 2.0 25 30 35 4 0 X
1C .l I
a0 Plate 10. See also figure 10-7. 0 0.5 1.0 1.5 2.0 4.O X -:: 12. See also figure 1l-5. o f E o o -É 150 o- E o o ôo l-
200
50 100 1s0 Distance (km) Plate 13. See also figure 11-0' Plate 15. See also figure 1 1-9.
¿ Spreading center: arrows indicate ry direction of movement Subduction zone: sawteeth on upPer Plate 16. See also frgure 11-10. - Fault: arrows ìndic¿te directìon Plate of relative movement - See also frgtre 1'1'-7 ' - ¡ TriPle junction Plate 14. Þ ãt D o 4l ? j o _l o o o Þ _t Þ q c m ctq ta É N o = o m o-
O m
O m
O m o m
! O m
æ O
Metamorphism, dewatering of sediment and oceanic crust Subduction factory Mid-plate processes Mid-ocean spreading center Arcvolcanism, lntrusive Hydrothermal ,:/U Ocean ' rflâÇl'ì'lâ circu lation öaCK-afC Oasln / Seamount island
Cold lithospheric plate
Melting Partial melting Melting and dewatering
r, Earthquake Volatile-rich minerals Plate recycled into Earth's interior
Plate 19. See also figule 12-1. Clipperton Transform fault
.S 9 N overlapping 50's ioreading center
5s"s
60"s
6s'S
Plate 21. See also figrre L2-2.
45'W 35"w 25"W 1s"w 5'W -l Plate 20. See also fìgu re 1 - I 9.
Plrte 22. See also Êgur:e l2--i. Plate 23. See also frgure 12-0'
l.¿rre 25. See also figure 16-0.
¡t i tl¡ r
also figure 12-6' Plate24. See :re 26. See also figlrre l6-5. Water, Temperature Control, and the Sun ' 271
Even more notable is that Earth's surface temperature has stayed in a narrow range over billions of years of progressive change in solar lumi- nosity (Fig. 9-9). As the sun's luminosity increased over time, Earth's atmosphere precisely accommodated to maintain a stable surface tem- perature. This fact suggests a sensitive feedback mechanism, a kind of terrestrial thermostat that is able to maintain a constant surface tem- perature in the face of changes in external forcing. what could be the detailed mechanisms leading to such stability? why has this mechanism not acted on other planets?
Tem lFo --u': +o.s +1.s -2.5 -l'5 E art h's Longtr er m T her mo stat Plate27. See also frgure20-6' Despite changes in solar luminosit¡ moving continents, periods of ice ages when glaciers covered much of the globe, warm periods when rep- tiles thrived at the poles, vast changes in life, and the amount of O, in the atmosphere, Earth's temperature appears to have remained comfort- ably within the range of 0o to 100oC for most of geologic time. We now explore the likely mechanism that has permitted this steady-state con- dition in the face of large changes in conditions. Carbon dioxide plays an extremely important role in establishing a ¡lanet's surface temperature. In Earth's atmosphere co, is second only io water in its greenhouse capacity. Nonetheless the amount of carbon n the atmosphere is trivial compared to the vast quantity of carbon at -he surface. Most of this carbon resides in sediments, part as calcium ;arbonate (called limestone by geologists) and part as organic residues called kerogen by geologists). Fortunately, only a tiny fraction (about :Lxty atoms out of every million) is currently in the atmosphere as COr. rtere all Earth's carbon in the form of gaseous CO/ its amount would :sceed that of N, and O, by a factor of about 100. The pressure exerted ('c) ,.v this CO, atmosphere would be a staggering 100 atmospheres (similar :o the pressure experienced by the hull of a nuclear submarine sub- Plate 28. See also frgute20-17' rerged to a depth of I km). Since CO, plays such a central role in green- :ouse warming, the partitioning between solid CaCO, and gaseous CO, :rust play a key role in climate stability. -\ very interesting argument can be constructed to show how Earth's :-imate may have been maintained in conditions where liquid water is Wate¡ Temperature Control, and the Sun . 273 272 . Chapter 9
described in Chapter 12, lhe high temperatures and pressures of the interior cause the minerals to break down in a process called metamor- phism. Restricting ourselves to the Ca and Si components of those reac- tions, the calcite reacts with the opal to yield wollastonite and carbon dioxide gas: River water then carries these to weathering and release of chemicals. SiO2 + CaCO, -+ CaSiO, + CO2 and elements to the oceans. The Fe is immediately precipitated; the Mg The calcium silicate is returned to the mantle, compensated by the Ca much of the Na are taken up by the ocean crust. Al is kept in clay miner- and Si that are included in the magmas rising to the surface from melt- ing of the mantle. The CO, dissolves in the magma that rises to the sur- tãce, where the solubility of CO, is so low that it is released out the con- r-ergent margin volcano to the atmosphere (Fig. 9-10). sior) and carbonates (in the case of cao). Most of the ca and Si come from the breakdown of feldspars, pyroxenes, and other mafrc minerals in that are common in the continental crust. Because the major players the climate context arc ca and Si, we can simplify the discussion to breakdown of the Ca-silicate component using the mineral wollastonite' with the formula casior. This mineral is broken down by interaction with water and co, dissolved in soils to form dissolved ions calcium, bicarbonate, and neutral silicate:
Sediment 3 HzO + 2CO2* CaSiO, > Caz+ + 2HCO; + H4SiO4o
eventuallr- These ions percolate through the soil to a nearby stream and to to the sea. In the modern ocean, organisms use these constituents organ- manufacture their shells. Prior to the evolution of shell-forming isms, calcium carbonate could precipitate directly from seawater inor- ganically. In both cases the reaction can be written as:
Caz+ + 2 HCo; -+ CaCo3 + H2o + Co2
Silica can also be precipitated as opal by the reaction: ::'---0:Thesubductionofoceaniccrustbeneaththecontinentscarriespartoftheblanket -+ SiO2 + 2}{2O H4SiO4o .=*jn€Dt deep into Earth's mantle: Here they are heated and metamorphosed. During this - :=i- some of the carbonate minerals contained in the sediment are broken down, releasing (cacor) and opal (sior) hard parts fall to the seafloor:: The calcite - l}is CO, migrates back to Earth's surface and rejoins the ocean-atmosphere reservoir. on the oceanic plates =' :- it recombines with calcium contribute to the sediment that accumulates =¿]lv in the mineral calcite. This calcite is buried on the ,,-. ,.-': and starts another trip toward a subduction zone. they move to the convergent margin where it is subducted. As ¡¡-iLi --'= Wate¡ Temperature Control, and the Sun . 275 274 . ChaPter 9
aspect of this cycle is So much for the background' The interesting The basic driv- how it interacts with the ðO, content of the atmosphere' the plate motions that carry sediment ing -..n"nism for the cyclã is CO, gas' Along with the from Earth's surface to the interior, releasing down-going plate, this the amount of carbonate sediment on the _sets Precipitation of CaCO, lowers rateatwhichCOrisreturnedtotheatmosphere-oceansystem'Ifatany CO, burial given time CO, is not removed from the 6 reser- in the sediments as rapidly as it is added to t ase' If' Cooler atmosphere voir, then the CO, .on,.t'i of the atmospher Less HrO in atmosphere to remove calcite from the ocean too on the other hand, organisms were Less acidic rainwater would steadily rapidl¡ the CO, content of the ocean-atmosphere system supply of CO' t9 ocean- decline. Somehow a balairce between the lhe must be achieved' Key to this Less precipitation atmosphere reservoir and removal of CO, of CaCO, raises CO, balanceistherequirementthatinordertoformcalcite,organismsneed leaks from Earths interior must calcium as well as CO' The CO, that lig. 9-11: Illustration offeedbacks that control atmospheric CO, and surface tempera- CaO CO, (i'e'' CaCOr)' mate with CaO dissolvåd from its crust to form :ure on Earth, as described in the text. no faster than calcium Thus, calcite can accumulate in marine sediments reactions taking place in is being made available through the chemical The feedback also works in the other direction. If for any reason tem- continentalsoils.Therateofthesechemicalreactionsdependsonthe go faster when the reac- lerature in the atmosphere gets particularly low, weathering will slow, temperature of the soil (all chemical reactions diminishing Ca supplies and carbonate precipitation, causing CO, to tantsareheated),theacidityofthewater(whichcausesmineralstobreak water that runs through ::se. Or if CO2 becomes too low in the atmosphere, temperature, rainfall down more rapidly, and the rainfall (the more aad acidity will decline, limiting CO, removal from the system by cut- the soils, the more that can be carried away)' :ing off the supply of Ca. This process requires long periods of time to Nowcomethefeedbackstothiscycleofevents(Fig.9-11).Asstated' faster than it was re- :ake place, because weathering, sediment precipitation, and subduction above, if CO2 were to be added to the atmosphere .:e slow processes. It is Earth's tectonic thermostat, operating on tectonic in deep-sea sediments' then the COt conten: moved by.ul.it" deposited -:mescales make the planet even of ) l0s-107 years. of the atmosphere would increase' This would The hlpothesis of a tectonic thermostat has great appeal because some warmer(becauseoftheincreasedgreenhouseblanketing)andmaketh. i--:ong feedback mechanism is necessary to preserve climate stability (warmer air holds more water vapor and hence make- planet even wetter -,-er the water more acid: - billions of years of planetary history where the radiation from the more rain). The higher CO, content also makes increase the rate at whic: ;'::r has changed substantially and where events such as great volcanic Thus, a rise in atmospheric CO, content would permit calcite to e;- : :Ðourings, meteorite impacts, or "snowball Earth'episodes (discussed calcium dissolves from the continents and thereby :=-orr') might lead to catastrophic long-term effects. This hypothesis lacks cumulatemorerapidlyinmarinesediment'Eventuallythecalcitepr:- '-:Èct CO, could be tests, however. Quantitative estimates of the changes in weather- duction rate would become great enough that =: added' The-remo'' C' .= rates with atmospheric CO, are hard to come b¡ the fate of carbon- from the atmosphere-ocean-system as fast as it was -.=. during subduction is a subject of debate, the chemical compositions buildup would be stemmed' v
Wate¡ Temperature Control, and the Sun ' 277 276 . ChaPter 9 gradually convert the FeO in the hot Venusian crust to FerOr. Evidence changed over Earth's history in ways of atmosphere and oceans have hypothesis was obtained when an unmanned Ameri- may in support of this and thermal evolution of Earths interior that are not well defined, can space probe was dropped into the atmosphere of Venus. Before this reactions in subducting plates. Absent have influenced metamorphic probe was rendered inoperative by the high temperatures, it measured the idea is widely accepted' but on our a viable competing hypothesis, and radioed back to Earth the isotopic composition of the trace amount only a 6' scale of certainty it still merits of water present in the Venusian atmosphere. The astounding finding lH rvas that the amount of deuterium (2H) relative to in Venusian water is >100 times higher than the 2Hl1H ratio in Earth water. Because of ALESSON FROh/IVENUS their twofold larger mass, deuterium atoms have a much lower escape that the situation with regard to CO' We have a dramatic reminder probability than do hydrogen atoms. Hence, the escape of hydrogen is the planet venus, which has a could well be different. The reminder to enrich the residual water in deuterium. While times lrom Venus would tend almost entirely of CO'' with ninety whopping atmosphere made the observed hundredfold enrichment of deuterium does not prove that The greenhouse effect of this CO' atmo- the surface pressure on Ëarth' \renus once had as much water as Earth, it can only be explained if Venus surface temperature of 464"C' As Venus sft.r. girr"s V"no, the scalding once had at least a thousand times more water than it does now! size and nearly the same density' it and Earth have nearþ the sÃe Thus, it is entirely possible that Venus and Earth started with roughly of vola- also started with a similar inventory seems reasonable thatihey the same volatile ingredients. Earth for some reason evolved along a of carbon in the CO' of the Venu- tiles. Indeed, the fact that the amount safely locked up in sediments and hence avoided up in path that kept its carbon same as the amount of carbon locked sian atmosphere is about the the disastrous consequences of a so-called runaway greenhouse warm- limestoneandkerogenonEarth'ssurfaceprovidesevidencethatthisis at some point slipped and let CO, build all ilg. Venus, on the other hand, conditions that would prevail on Earth if true.l Thus, Venus has the up in its atmosphere. This buildup led to high temperatures, which would and kerogen were to be released as CO' the COr locked up in limestone have terminated life (if indeed it ever achieved a foothold on venus). to the atmosPhere. how a planet once in this very hot state could ever con- It is hard to imagine Earth and Venus' a problem arises in However, when comparing cool off. with the same component of volatiles nection with water. If ünus iarted We have little knowledge of the history of Venus. It is difficult to (or rather' at its high temPerature' it should have a sizable ocean as they did on as Earth, of imagine that astronauts will ever roam about its surface steam)'z Not only is the atmosphere an atmosphere dominated by -.he moon. While the Russians and Americans have managed to land steam; water vapor is bareiy detectable' Venus not dominated by several unmanned space probes on the hot surface of Venus, these ve- Venus as that the hydrogen initially present on Most scientists believe :,icles survived the hostile conditions only long enough to radio back Venusian atmosphere' water water escaped to space' In the very hot pressure, and composition of the dis- _nformation about the temperature, transportedto the "topl' Here it could be ,r"po. *o,rid be effåtively \ enusian atmosphere and (as we learned in chapter 4) about the potas- to form hydrogen atoms that would then associated by ultraviolet iight .ium to uranium ratio in the rock surface upon which the probe landed. atoms would be stirred back do¡¡r escape. The "left behind' oxygen Ladar beams bounced off Venus tell us that its surface has large topo- the-surface of the planet' where they would through the atmosphere to aaphic features and a young surface lacking in large-impact craters. 3ecause of the young surface, we do not know whether Venus might Also' CaCO' would deco=- it surely has no life and hence no kerogen' .açe Earth's. Certainly in the early solar that nearþ all ca-- had an earlier history more like its carbon as CO' gas' Henie' it is likely n 'l-td";l"g with the luminosity of the sun 30% less than present, Venus would b e resides as CO, gas in its atmospher =stem 2IftheEarthwereheatedtothepointwhereitsoceânwasconvertedentirelyintosteam..--j ,eem to have been in a favorable position for life within the solar system. ofthe present Earth atmosphere' would .*..t pt"'*t" 270 times that steam " "boot Wate¡ Temperature Control, and the Sun ' 279 278 . ChaPter 9
conditions permitting the In any case, after the runaway greenhouse' We can speculate that Ve- development of life were neve; reestablished' Venus is closer to the sun than nus's runaway climate occurred because increase in solar luminosity' Was Earth and was not able to manage the more slowly than Earth? Was that Venus spins hundreds of times it also com- Venus? Was it because the initial it because life never got started on than that on Earth? In any of water on Venus was much smaller fon"nt assured' us that climate stability is not case, Venus's presence reminds climate can go catastrophically off sending a message that planetary course.
SNOWBALL EAITTH try to envision what would hap- With this background in mind we can organlsms pen if Earths ocean were to freeze' could pre- to make calcite, nor would there b on' Under cipitate inorganically' There would ak from Earths hot interior would build these conditiorrr, tnJ CO, released became warm up in the atmo'sphere until the temperatures up and -..¡.rv. snowballearth. org) of this escape hatch is that CO' out- enough to melt the ice' The s"cr"t is therefore insensitive to is driven by Earths internal heat and g"rriig scenario A part of Earth's history where such a the surface temperature' :oles, since grorvth ofoceanic ice cover is far easier than growth ofcon- of the tectonic thermostat' took place *orrid be a useful test if polar ice begins to grow it increases Earth's albedo' been entirelv :¡ental ice sheets. was believed that Earth had never Until very r"..,ttly it _ausing more light to be reflected back into space. If the ice passes a criti- geologists' Paul Hoffman and Dan Schrag' frozen.But then two Harvard " _l surface area, then the albedo lowers the atmospheric temperature, iniggzby calTechs foseph Kirschvink picked up on a r"g;;;t;" * is a positive feedback leading to more sea ice that can extend rred during the Neo- =d there that during the episodes of ) udes. ago' Earth did indeed proterozoic period from 580 to 75 ffman and Schrag was the observation that e episodes as snowbali become totally ftozen.Kirschvink -- rlain by thick sequences of calcium carbon- catastroPhes. Deposits formed at this time bv The key observation is as follows' sediment' In other words' the glaciers were intermingled with marine Furthe !Iu.i.., must have reached sea level' deposits were ments demonstrate that these :_i-erent but so also is the isotopic composition of their carbon' Its More important, some of these gla that of "normal" limestones and in- latitude. Earr:- : -:rposition lies well away from rise to the possibility that the entire equator at sea level, giving -,.:.d closely matches that of average Earth carbon' more easily if oceans covered tl= was frozen. Such a siate cold arise Wate¡ Temperature Control, and the Sun ' 281 280 . ChaPter 9
on the tectonic ther- Hoffman and Schrag created a scenario based explain these observations' mostat that is an intriluing hlpothesis to descended.to the After the ocean froze àver and continental glaciers ceased' No more Ca was delivered shores of the sea, chemical erosion totheoceansbyrivers(somewouldstillbedeliveredbyhydrothermal and organic residues vents), and the deposition of calcium carbonate to operate' how- would drastically decrease' Plate tectonics continued the interior through volcanoes ever, so CO, continued to escape from As no mechanism existed for that would melt their way through the ice' in the ocean and atmosphere the removal of this CO, its concentration warmed despite the very went up and up. As it rose, Earth gradually or so million years' the CO' high albedo of t t'fter 10 t: melt' The melt- greenhouse effe "g1" provided by Earth's reflective ice and snow :.:.9-13: Graphical illustration of the protection from the Sun's solar wind ing led to a run field. the solar wind is deflected away from Earth, providing protection for the less sunlight would be --.:netic *í. ,.pl"."d by far less reflective se¿ .:::-osphere and surface. reflected,leadingtoapositivefeedbacktowardincreasedmelting.Ero- supplying calcium to sion would then have reinitiated with a vengeance' of course' led to the precipitation of massive the COr-rich ocean. This' its atmosphere and magnetic freld. In the planet's sur- Earth's sun protection is deposits' As the CO, was used up' our calcium carbonate modern atmosphere ozone absorbs much of the Sun's ultraviolet radia- toward its ambient state' face cooled down ¡ion, protecting life on land from its effects. It may be no accident that rracroscopic life was able to evolve and take over the continental surface only after there was sufficient oxygen in the atmosphere to give rise to Sun Protection ¿n effective ozone shield. the principal protection is Earth's conducive For cosmic rays and the solar wind, is needed to make the planetary surface One final ingredient We are all familiar with Earth's magnetic fleld through emits both ultra- nagnetic fleld. and long-term evolution of life' The sun to the origin "" Jompasses and their importance for navigation. Magnetic fields also exert of charged particles with very high violet radiation and wind cosmic radiation. Earth's 'olu' con- ¿ force on charged particles such as those in for early Earth' the solar wind could have velocities. Particularþ ::ragnetic fi.eld causes most of the particle radiation from the sun to be atmosphere and possibly led to loss of tributed to stripping of tn" (Fig' 9-13). "u'iy life. At the same :ir-erted around the planet ar-e critical for climate stability and the volatiles that largest magnetic field of any of the terrestrial planets. also have the po- Earth has the cosmic rays coming from distant stars time, galactic 1e magnetic fleld is generated by convection in the liquid outer core. tentialtodeliverdosesofradiationdetrimentaltolife.oncelifebegan, :ince Earth has been cooling since its early history of accretion, a liquid a high dose of radio-activity that this radiation would have created and the sun pro- the :ore would also have been present in the early Earth, to life as we know it' While the sun is would have been hazardous likely contributed to early habit- radiation ::ction provided by the magnetic freld of habitability, some protection against cosmic ultimate source :.cility and the possibilities of an origin of life. is necessarY' F
Water, Temperature Control, and the Sun ' 283 282 . ChaPter 9 the Atmosphere. New York: Mac- |ames callan Gray walker. 1977. Evolution of Stwtmary millan. a habitable planet. |ames F. Kasting and David catling. 2003. Evolution of
a surface environment - Earth's habitability is critically dependent on Annu. Rev. Astron. Astrophys. 4l:429 -63 Iiquid water' and a tem- with adequate volatiles, oceans and continents' range over billions of years' perature ihat remains within a restricted earþ concentration of vola- Because Earth as a whole has few volatiles' ocean and atmosphere' A tiles to the surface is essential to provide an atmospheric loss' and a sufficient planetary mass, protection from ..watertrap'intheloweratmosphereenabledEarthtoretainallitsvola- temperature of a planet tiles with the exception of helium' The surface dependsontheamountofsolarradiationitreceives,itsalbedo,andits an active grå.rrhoor. effect. Evidente from ancient zircons suggests prior to 4 billion years' iater cycle at the surface' including liquid water that water has been present and the record of sedimentary rocks shows atEarth,ssurfacesincethattime.ItisremarkablethatEarthhasman- despite a 39o/o change in the ageð. astable climate over billions of years' feedback mechanism that solar luminosity. This fact suggests a robust the earþ Earth' A tectonic would enable greater greenhouse warming in likely mechanism for thermostat involvingihe carbon cycle is the most high CO' would lead to climate control. A iu,m"' atmosphere with sequestered in carbonate greater weathering, causing more CO, to be apparent by comparison with rocks. The efficacy of this mechanism is carbonates' leading to mas- Venus, where CO, was not sequestered into water: making the sive greenhouse warmit'g loss of planetary ""a for life is also pt"rr"î unsuitable for life. A planetary surface suitable fleld' Earth's liquid outer greatly enhanced by the p'"t""tt of a magnetic the terrestrial planets' ãor. prouid.s the largesi magnetic field of any of to prevention of atmo- The magnetic field iould have contributed protects the planet from spheric loss in early Earth's history' and it of years of homegrown deadly cosmic rays, providing Earth with billions sun Protection.
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Our planet's leaþ atmosphere' Kevin f. Zahnle and David C' Catling' 2009. I I :29' S ci entifi c Am er ic an, MaY