Earth and Mars - Evolution of Atmospheres and Surface Temperatures - Sagan and Mullen

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Earth and Mars: Evolution of Atmospheres and Surface Temperatures

Carl Sagan; George Mullen

Science, New Series, Vol. 177, No. 4043. (Jul. 7, 1972), pp. 52-56.

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ReportsEarth and Mars: Evolution oE Atmospheresand Surface Temperatures

Abstract . Solar evolution imp lies, for contetnporary albedos and a tmosphericcomposit ion, global mean temp eratures below the freezing point o f seawater lessthan 2.3 aeons ago, contrary to geologic a! ~ d aleontological evidence. Amn zoniamix ing ratios o f the order of a few ports per m rllion in th e tniddle Preranlbrianatnrosphere resolve this and other problems. Possible teinperature evolutionarytracks /or Earth and Mar s are described. A runaway greenhouse eflect wil l occuron Earth about 4 .5 aeons f ro111 now, when clen ~en tconditions will prevail onMars.

The present surface temperature ofEarth represents an energy balance be-tween the visible and near-infrared sun-light that fal ls on the planet and themiddle-infrared thermal emission thatleaves. In the absence of an atmo-sphere this equillbriurn is written ?ASX( 1 - 2 ) = eoTe4 , where S is the solarconstant ; 2 is the Russell-Bond spheri-cal albedo of Earth, a reflectivity inte-grated over al l frequencies; e is themean ernissivity of Earth's surface inthe middle infrared; u is the Stefan-Bol tzmann constant ; and T , is the ef-fective equillbriurn temperature of (at-mosphereless) Ear th. The factor '/4 isthe ratio of the area ;;R2 that interceptssunlight to the area 4~;R"hat emitsthermal infrared radiation to space.When the best est imates of these param-eters are used, a value for T , of 250'to 25 SSK is obtaine d; this is far lessthan the observed mean surface tem-perature , T,, of Ear th, 286' to288K. The di f ference i s due to thegreenhouse effect , in which visible andnear-infra red sunlight penetrates throughEarth's atmosphere relat ively unim-peded, but thermal emission by Earth'ssurf ace is absorbed by atmospheric con-sti tuents that have strong absorptionbands in the middle infrared. Thus, t imevariations in S , 2, e , or a tmospher iccomposi t ion may induce impor tantchanges in T, . The present heat flowfrom the inter ior of Ear th i s about2 x 10-5 the solar constant and plays anegligible role in determining T,.

To calculate T,, allowing for thegreenhouse effect, we divide the emer-gent flux into two parts, one emitted bythe surface at temperature T , directlyinto space through atmospheric win-

dows, and the other emitted by theatmosphere into space in wavelengthregions of strong atmospheric absorp-tion. In the lat ter case we consider theemission to occur from the skin tern-perature of the approximately isother-ma1 outer boundary of an atmospherein radiative equil ibrium, which is, inthe Eddington approximat ion, a t atemp erature of 2-1/4Te. T hus,

x S ( 1 - 2 ) = C e ~ h t ( * , ) ~ ~ 'B (2 l 4 ( 1 )

Here BAi3 the Planck specific intensity,and the wavelength intervals a rechosen to pack with adequate densitythose wavelength regions where B h ischan ging rapidly. The equation is solvediteratively for TS on an electronic corn-Puter. T he adopted s tep-funct ion aP-proximation to the actual nongraY ab-sorption spectrum, which is due to rota-tion-vibration transitions in Earth'satmosphere, is compared with the mea-s u r d t ransmi s si on spec trum i n ( 1 ) .Theresulting values of T Sare shown in Table1. The correct value of e is, from studiesof a wide variety of minerals (2)s closerto 0.9 than to 1.0. Extensive calculations( 3 ) based on measurements made f romEart h yield values for 2 of 0.33 to 0.35,and an analysis of observations madeover 5 years by meteorological satellites( 4 ) yields 0.30 for 2. Since we areconc erned with differential effects, wehave adopted 2 = 0.35 to secure agree-ment with the observed T , in our ap-proximation (5).The solar constant is varying; thel un~ i nos i t y ,L, of the sun has increasedby about 40 percent in geologic t ime

(6). This variat ion has profound conse-quences for the surface temperatures ofthe terrestrial planets (7). T h e m a i n -sequence brightening of the sun is oneof the most reliable conclusions drawnfrom the mod ern theory of stel lar evolu-tion, which explains in considerable de-tal l the observed FIertzsprungRusselldiagram. T he models of solar evolutionused in this report give an age for thesun in excellent agreement with the agedetermined on independent grounds forEarth, the moon, and the meteorites.The principal uncertainties in such cal-culat ions are in the age of the sun andits init ial abun dan ce of helium. ~~~hlarger possible errors in such parametersas thermonuclear reaction rates or opac-it ies have much smaller effects ondL / d t ( 8 ) . Ka t z (8 ) conc l udes t ha t(1 /L ) (dL /dt ) i s in er ror by at most25 percent fo r the bes t contemporaryevolut ionary models . A variety of calcu-lations of main-seqLlence solar evo lu-t ion give a variat ion, AL, of 30 to 60percent over geologic t ime (6). The bestpresent estimate of AL is 40 rt 10 per-cent. For most of the following calcula-t ions, we conservatively adopt AL = 3 0percent.

We then run the sun backwardthrough t ime and assume init ial ly thatthe terrestrial atmospheric composition,e , an d 2 remain constant. The resultsf rom E q. 1 are sbown in F ig. 1 . We seethat the global temperature of Earthdropped below the freezing point ofseawater less than 2.3 aeons ago (1 aeonis 109 yea rs); 4 0 to 4.5 aeons agoglobal temperatures were about 263OK.Had we used 50 percent for AL , th efreezing point of seawater would havebeen reached abo ut 1.4 aeons ago, andtemperatures 4 0 to 4.5 aeons agowould have been about 245K. Be-cause of albedo instabilities (discussedbelow) it is unlikely that extensiveliquid water could have existed any-where on Earth with such global meantemperatures.

Th e presence of pillow lavas, mu dcracks, and ripple marks in rocks fromthe Swaziland supergroup stronglyimplies abundant l iquid water 3.2 aeonsag o (9) . The earl iest known microfos-sils (10, 1I ) , 3 .2 & 0.1 aeons old, in-clude blue-green algae, which would bevery dificult to imagine on a frozenEarth. Algal stromatoli tes, 2.0 to 2.8aeons old, exist in various parts of theworld (12, 13). If they are intert idal(13), there must have been at leastmeters of liquid water; if they are sub-tidal (I#), much greater depths are im-

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p l ied . The t ime requ ired fo r su rface may be somewhat less , bu t the f ract ion than the con temporary g lobal average,water to accumulate sed imen ts in geo-synclinal trough suggests (15) the pres-ence of extensive bodies of water onEar th 4 aeons ago o r more. F inal ly ,liquid water is almost certainly neces-sary for the origin of l ife; if we believethat l i fe began shor t ly af ter the fo rma-t i on o f E a r th (1 6 ) , li q uid w a te r m u s thave been p resen t fo r most o f the per iodbetween 3 .5 and 4 .5 aeons ago . Thus,even using our most conservat ive valueof A L, we find a serious discrepancybetween theory and observat ion .

Th is d iscrepancy ind icates a n erro rin a t leas t one o f ou r in i t ia l assump-t ions . Ther e a re on ly th ree l ikely sourcesof erro r : S, 2 , a n d t h e a t m o sp h e ri ccomposi t ion . The so lar constan t i s un -likely to be sufficiently in error to ac-coun t fo r the d iscrepancy ; the mostp robab le value o f S considerab lyw ~ d e n s t h e d i sc repan cy . F o r sm a l lg reenhouse co rrect ions , the v ar ia t ion ofST, with S x c a n be w ri tt en 8 2 =- 4 ( 1 - z )S T , /T , . T h u s , fo r T , 3 .5to 4 aeons ago to be increased f romthe valucs shown in F ig . 1 to the f reez-ing po in t o f seawater requ ires analbedo decremen t o f abou t 0 .06 to 0 .09 ;fo r A L = 5 0 p e rcen t , an a lb ed o d ec re -men t o f more than 0 .20 is requ ired .Such albedos are unaccep tab ly smal l . Atg lobal temperatu res 10" o r more belowcon temporary values , the c loud cover

o f E ar th covere d by ice , snow, andg laciat ion wi ll be very m uch larger . Th ealbedos of thick deposits of ice or snowa r e 0.50 to 0.70. A declinc in the glo-bal tem pera ture of E ar th is l ikely to in-crease ra ther than decrease the a lbedo ,bu t in any case the a lbedo decl ine re-qu ired to exp lain the d iscrepancy ap -pears to be ou t o f the quest ion . Indeed ,detai led g lobal c l imat ic models (17)suggest that a relative increase in 2 ofon ly 2 percen t i s enough to induce ex -tensive g lacia t ion on Ear th , wh ich im-plies that the present climate is ex-tremely sensitive to albedo.

Th is leaves changes in a tmospher iccomposition as a possible explanation.Majo r var ia t ions in the CO, abundancewill have only minor greenhouse effectsbecause the s t rongest bands are near lysatu rated . A change in the p resen t CO,abundance by a facto r o f 2 h i l l p ro d u cedirectly a 2 ' variation in surface tem-p e ra tu re (1 8 ) . T h e C O , ab u n d an ce i shighly controlled by ii l icate-carbonateequilibria; by buffering with seawater,wh ich con tains a lmost 1 00 times theatmospher ic CO, ; and by the resp i ra-t ion and pho tosyn thesis feedback loop(19). The negat ive exponen t ia l depen-dence o f the vapor p ressu re o f water onrecip rocal temperatu re impl ies that fo ra lower g lobal temperatu re there i s nol ikel ihood o f gain ing more water vapor

a b o u t 1 g c n ~ - ~ .h e on ly s ur viv in gal ternat ive appears to be that theatmosphe re of Ea r th 1 o r 2 aeonsag o co n ta in ed so m e co n s t i t u en t o r co n -stituents, not now present, with signifi-can t absorp t ion in the midd le in frared ,in the vicinity of the Wien peak ofEar th ' s thermal emiss ion . A large num-ber o f cand idate molecu les were in -vest igated . The ideal molecu le shou ldprov ide s ign if ican t absorp t ion in thepresen t window from 8 t o 1 3 F m , ev enin l o w ab u n d an ces . L a rg e am o u n t s o fCO, SO,, O, , and the variou s oxidesof n i t rogen arc inadequate , as are manyt imes the con temporary abundances o fthe homonuclear d ia tomic molecu les 0,and N,, wh ich have no permit ted v ib ra-tion-rotation transitions.

Bu t , among the more reducing gases ,NH, , i s very appropr ia te . A vo lume mix-ing ratio , [NH,], o f ab o u t 10-5 a t ap ressu re o f 1 bar p rov ides appreciab leabsorp t ion at 8 t o 1 3 p m . N o o t h e rp lausib le reduced gas ( fo r example, CH,or H,S) p rov ides comp arab le absorp -t ion . The p resen t value o f [NH,] is stillin some quest ion , bu t i t appears to beless than (20). At thermody namicequ i l ib r ium in the p resen t ox id izingatmosphere th is mix ing ra t io wou ld bemuc h less than lo -" (21) . As is nowthe case fo r CH 4, a smal l s teady-s ta teabun danc e cou ld be main tained if the

Earth_,ntttal 1 bar H2 atmosphereevolutonary trackEqulhbr~um NH,evout~onary rack

Freeztng point of---eawaterNow

&L-i-i-30 -2.0 -1.0 0.0 10 I 1 3;o 14.0 2.0 5.0 6.0 Ttme (4) Time (A)

Fig. 1 (le ft). Calculated time-dependent model greenhouse effects for Russell-Bond albedo2 about 0.35 and two surface infraredemissivities, e = 0.9 and e = 1.0, the former being more nearly valid ( 2 ) and giving the correct present global temperatures. Theatmosphere of CO, an d HiO is assumed to contain the present abun dance s of these gases, pressure broadened by 1 bar of aforeign gas. The slightly reducing atmosphere has the same constituents with the addition of a 10." volume mixing ratio of NHn,CHI, and H,S. In this case NH3 is the dominant absorber. At the top is the greenhouse resulting from the addition of 1 bar ofH, to the constituents already mentioned. The evidence for liquid water at 2. 7 to 4.0 aeons (A )ago comes from a varietyof geologic and paleontological data (9-16). Th e time evolution of the effective tem pera ture is also displayed. All calculation s arefo r AL = 30 percent; for larger time-derivatives of the solar luminosity, the freezing point of seawater is reached in yet more recenttimes. Fig. 2 (right). Three derived evolutionary tracks for the temperature of Ea rth. As described in the text, the subequi-librium NHa track ( less than 10-"olume mixing rati o), while schematic, is though t to be most likely. Th e amplitu de of globaltemperature oscillations in Mesozoic and Paleozoic glaciations is smaller than the width of the temperature curve shown. A run-away greenhouse effect occurs several aeons in our future.7 JULY 1972 53


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molecule were generated at a high rate,as by biological activity (21), but thiscalculation does indicate the great ther-mod ynam ic instabil i ty of N H 3 in thepresent 0, atmosphere. On the other'hand, Bada and Miller (2 2) calculatedNH, mixing ratios on the preoxygenicEarth by using clay mineral equil ibriain the oceans and, independently, byassuming that the deam ination of as-part ic acid is reversed, since this a minoacid is required fo r the origin andearly evolution of l ife. They foundthe following approximate values for[NH,]: 10-7 a t OC, 3 x 10-5 a t2SC, and 3 X 10-4 at 50C, in goodaccord with our requirements. Lowervalues of the NH, mixing ratio are pos-sible because of ultraviolet photolysis(23 ) , but the s teady-sta te NH, abun-dance would have been maintained byan equil ibrium between photolysis andproduction. An NH, mixing ratio evenas small as 10-6 will produce a high-temperature equil ibrium mesopauselayer, somewhat an alogous to the terres-trial ozone layer, and will also serve toprotect oth er gases closer to the surface,notably water vapor, from photodissoci-ation (24).Accordingly, we calculated (1) green-house temperatures for atmospheres inwhich NH, is a minor consti tuent. Weassumed that water vapor is present inapproximately i ts present abundance,determined largely by T , and by meteo-rology; that CO, is present in approxi-mately i ts present abundance, main-tained by mineral equil ibria and oceanicbuffering reactions; and that CH, andH,S are present in amounts comparableto that of NH,. Because CH, a ndH,S are not as effective absorb ers,their pre sence does no t significantlyaffect the results. T he surface tem-perature in this (slightly) reducingatmosp here is shown as a function oftime in Fig. I . We see that such anatmosphere is entirely adequate to re-solve the discrepancy and keep the glo-bal temperature of Earth well above thefreezing point of water, which confirmsa conjecture (25) made by one of ussome years ago (26).If even small quantities of hydrogenwere present in the early atmosphere,the primitive exosphere would, by dif-fusive equil ibrium, be dominated by H.Because of the high therma l condu c-tivity of hydrogen, such an exospherewould cool very efficiently by conduc-tion do wnw ard; in addit ion, the presenceof polyatomic reduced molecules withbtrong emission features in the middle

Table 1. Calculated contemporary surfacetemperatures , T,, for Ear th; e is the meanemissivity o_f Earth's surface in the middleinfrared; A is its Russell-Bond sphericalalbedo.T, (OK)e -- --- -- --

A-= 0.35 A = 0.30

and far inf rared would thermostat suchan exosphere efficiently. A primitive re-du c~ ng t rnosphere would have been s ta-ble against gravitational escape for pe-riods approaching l aeon. Accordingly,there may have been a significant epochin the early history of Earth in which W,was an important consti tuent of theatmosphere. I t is est imated (24) thatthe maximum H, mixing ratio in thelower atmosphere was 0.10 to 0.15 aftera possible initial period in which exo-spheric blowoff occurred because thegas kinetic energy of H, exceeded itsgravitat ional potential energy. We haveno way of knowing the [H,] history inearly t imes; for heurist ic purposes wecalculate the addit ional greenhouse ef-fect due to 1 bar of H,. At such apressure, permitted qua drupole and pres-sure-induced dipole transit ions producemajor absorptions at longer wavelengthsthan 7.5 /Im. On e bar of H, fills In thelong-wavelength windows in the atmo-s p h e r ~ c greenhouse, producing nearlycomplete absorption at all wavelengthslonger than 4.9 /Lm and increasing T ,well above the normal boil ing pointof water.Accordingly, we are lef t w ~ th hreeevolutionary tracks for the temperaturehistory of primitive Farth (Fig. 2):

1) With an initial extensive H, atmo-sphere, Earth originates at temperaturesabove the norm al boil ing point of water,even in the absence of endogenous heatsources. The temperature rapldly de-clines in the first aeon becduse of theescape of hyd rogen into space, and then,about 3.5 aeons ago, enters a mildercl imate dominated by the NH, andwater greenhouses. Th e photodls-sociation, reaction, and oxidation ofthe reduced gases of such an atmo-sphere produce a gradual decline Intemperatures, and the planet ap-proaches the evolutionary track forthe present greenhouse consti tuents,perhaps 1 to 2 aeons ago. We do notknow whether as much as 1 bar of H,could have been retained by Earth dur-ing its formation, but we are skepttcalabout this evolutionary track, because it

prohibits temperatures suitable for theorigin of life even close to epochs whenthe prokaryotes-organisms requiringa major t ime interval for their evolu-tion ary antecedents-were presen t inabundance.

2) An insignificant amount of H, ispresent init ial ly and the atmosphericgreenhouse is dominated by H,O and byNH, in i ts calculated equil ibrium abun-dance, which then declines as beforetoward the CO,/H,O track.

3 ) Am mon ia is init ial ly present insubequil ibrium abundances, because ofphotodissociation and reaction with oth-er atmospheric consti tuents. The NH,absorption, pressure-broadened with aforeign gas at 1 bar, declines appreci-ably between [NH,] 10-5 and [NH,]5 10--6. The flatness of this third trackrepresents a rough balance between theslow decline of [NH,] and the slow in-crease of L. Calculations (27) of thethermo dynam ic stabil i ty of a num ber ofamino acids in aqueous phase show avariation of several orders of magnitudein half-life for a decline of l o0 o r 20.The subequil ibrium NH, in the thirdtrack therefore (i ) keeps T , abovethe freezing point of water; (ii) isresponsive to comments (23) on thephotodissociation of NH,; and (iii)provides increases of many ordersof magnitude in the concentra-tions of organic constituents in theprimitive seas, thus enhancing thelikelihood of the origin of life on primi-t ive Earth. The subsequent decline inNH, abundance is most l ikely due tooxidation by 0, produced in greenplant photosynthesis. The evolution ofgreen plants could have significantlycooled off Earth.Our conclusions would not be signi-ficantly different if we had used largervalues of AL. With such values, how-ever, the argument requires small quan-tities of NH,, in Earth's atmosphere al-most up to the Precambrian-Cambrianboun dary. Because of the thermody-namic instability of NH, in an excess of0, (21), such results suggest the absenceof con tempo rary values of [O,] for a ma-jor fraction of the history of Earth. Thisis consistent. with a v ariety of othe revidence-including data on band ediron formation s (13, 19, 28), on the oxi-dation of uraninite (19, 28, 29), and onthe relatively small US enrichment ofSwaziland barites (3'0)-and makesquite implausible the suggestion (31)of an early oxidizing atmosphere onEar th. A long epoch in which smallquantities of such reduced gases as NH,

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might coexist with small quantities ofphotosynthetically o r photolytically pro-duced 0, is not excluded. Our conclu-sion that significant quantities of 0, didnot arise unti l 1 to 2 aeons ago isin excellent accord with the conclu-sions of Cloud (13 ) , based o n the chro-nology of banded iron formations andthe. oldest fossil eukaryotes. By thisperiod an extensive evolutionary devel-opm ent of catalase, peroxidases, an dperoxisomes to defend the cells againstoxidation products must have occurred(16, 32), and selection pressures fo rdefenses against or avoidance ( at ocean-ic depth s) of solar ultraviolet l ightgradually eased (16, 33 ) .Th e presence of NH, in Earth'satmosphere for most of the Precam-brian has a range of biological implica-tions. It is, of course, a very usefulprecursor co ~n po un d or prebiologicalorganic chemistry. There is strong evi-dence (34) that NH, i s the key interme-diary in the fixation of atmospheric N2.By the Horowitz (35) hypothesis on thebackward evolution of enzymatic reac-t ion chains, this must imply an earl ierevolutionary stage in which NH, wasavailable. which is consistent with thepresent argument. While most micro-organisms can util ize N H , directly asa source of N,, it is largely the pro-karyotes that are N, fixers (36). Th e hy-drogenase-ferredoxin system for N,fixation is not specific for N,, and canreduce C2H,, NO,, azides, and cyanides(34); its original function, in times ofexcess NH,, may not have been Ngfixing.Analogous calculations have beenper lormed for M ars . The present green-house effect on Mars is due almost en-tirely to CO,; the water-vapor abun-dance of a few tens of precipitablemicrometers makes no significant con-tribution. Th e total greenhouse effect isonly a few degrees Kelvin; T, on M ar s i sroughly 210 to 220K, depending onthe choice of 2. The mer idional anddiurnal temperature gradients on Marsare both extreme because of the paucityof liquid water and the thin atmosphere.Thus, temperatures much warmer thanthe global mean exist on Mars, butcould not have existed extensively onprimitive Earth. Earlier condit ions onMars may have been much moreclement. After very rapid blowoff ofa possible initial H, atmosphere, theequilibrium NH, evolutionary tracksgive T , near the freezing point of sea-water. There appears to be an epochin the first aeon after the origin of Mars

in which global temperatures were notfar from the freezing point of seawaterand in which the origin of l ife mayhave occurred. as i t did on primitiveEarth in the same period. Because ofthe smaller martian gravilat ional ac-celeration, photodissociation and escape~ h o u l dhave changed the intermediateoxidation state atmosphere much morerapidly on Mars than on Ear th. Anymartian organisms would have had toface low temperatures and increasinglyinaccessible liquid water. Over geologictime, Mars could hav e lost meters to tensof meters of liquid water by photodis-sociation, escape of hydrogen, and oxi-dation of surface material (3 7) ; theseare not oceans, but they are respectabledepths. I t is a debatable but hardly q uix-otic contention that martian organismsmay have been able to adapt to the in-creasingly rigorous martian environmentand may st i l l be present ( 3 8 ) . Fairlyabundant l iquid water early in the his-tory of Mars would also be helpful inexplaining the puzzling large-scaleerosion of the oldest martian craters-a phenomenon dist inctly different fromlunar crater eros ion (39) .

These calculations have also been ex-tended into the future of Earth andMars. As the sun continues to evolve,the surface temperature of Earth willincrease; more water vapor will be putinto the atmosphere, enhancing theatmospheric absorption; and eventually arunaway greenhouse ef fect w ~ l l ccur , aspreviously discussed for Venus (40, 41).Acccrding to calculations by Pollack(41) a value of the solar constant 1.5times the present terrestrial value isadequatz to cause such a runaway fora planet with 50 percent cloud cover.F o r A L = 30 percent, this event oc-curs about 4.5 aeons in our future; for4 L = 50 percent, 3 aeons in our fu-ture. Edrth will then resemble contem-pGrary Ven us, but with an atmosphericpressure of 3 00 bars of steam. It isdifficult to imagine what could be doneto prevent this runaway, even with avery advanced technology (perhaps aprogressive Increase in atmosphericaerosol content), but at the same epochthe global temperature of Mnrs will be-come very similar to that of present-day Earth. If there are any organismsleft on our planet in that remote epoch,they m ay w ~ s ho take advantage of thiscoincidence. C A R LSAGAN

GEORGEM U L L E N'Laboratory for. Plarzetauy St~lclies,Corrzeli U~ziveuti ty,Ithoca, N rw Y o r k

References and NotesI . C. Sagan and G. Mullen, Repo rt 460, Center for Radiophysics and Space Research. Cor-n e l l ~ n i v ~ r s i t y 1971).2. R. J. P. Lyon, NASA Contractor Report CR-100. Stanford Research Insti tute (1964). 3. J. ond don, Final Report, contract ~ ~' 1 9( 1 22 )-165, Departme nt of Meteorology and Ocean-ography, New York University (1957);-nd T. Sasamori, in Space Research, K .Rondratyev, R. Rycroft, C. Sagan, Eds.

(Akademie Verlag, Berlin, 19711, vol. 11. Instandard textbooks, R. M . Goody [ A t m o -s ph er ic Radia t ion (Clarendon Press, Oxford,1964)l adopts a value for A of about 0.4,and K . Ya. Kondratyev [ Radia t ion in th eAtinosphere (Academic Press, New York,1969)l chooses A about 0.35.4. T. H. von dcr Haar and V. Suomi, J . A t w ~ o s .Sci. 28, 305 (1971).5. While we believe this step-function approxi-mation to be ade quate for the present dis-cussion, we ar e pursuing a more detailed set of calcu lations, which allows for much finer wavelength and altitude grids. 6. Estimated values of the increase in so larluminosity, AL, over geologic time are: 60percent [ M . Schwarzschild, R. Howard, R.Harm, As tr oph ys . J. 125, 233 (1957); (7)J;30 percent [F. Hoyle, in Stellar Poprtlations ,D. J . K . O'Connell, Ed. (Specola Vaticana,Vatican City, 1958), p. 223; C. B. Haselgroveand F. Hoyle, M o n . N o t . R o y . A s t r o n . S o c .119, 112 (1959); D. Ezer and A. G. W.Cameron, C a n . J . Plzys. 43, 1497 (1965)l; 50percent [I . Iben, in Stellar Evolution, R. F .Stein and A. G. W. Cameron, Eds. (Plenum,New York, 19661, p. 2371; 35 percent [J. H.Bahcall and G. Shaviv, As tr op lS s . J. 153, 113(1968)l; 40 percent [I. Iben, A n n . P h y s . N e wY o r k 54, 164 (1969)l. A weighted mean ofthese values is 40 percent -C 10 percent.7. One of the earli est recognitions of this possi-bility occurs in M . Schwarzschild's S tr ~r c tu r ean d E vo lu t io~z o f th e S tar s [(Princeton Univ.Press, Princ eton , N.J., 1958), p. 2071 "We maythus conclude that the solar luminosity musthave increased by about a factor 1.6 duringthe past five billion years. Can this change inthe brightness of the sun have had some geo-physical or geological consequences that mightbe detectable?"8. The discrepancy between theory and experi-ment in the SB solar n eutrino flux depends insensitively on the paramete rs which strong-ly affect d L l d t . A wide range of po stulated convective cores, extending to 0.4 sola r radii, if essentially const ant in time, do not sig-nificantly affect d L / d t . J. Katz has examined many othe r conceivable sources of e rror in the theory of solar evolution, including the possibility that the sun may be burning othe r elements in addition to hydrogen; influences cf rotationa l and magnetic energy and of quarks; errors in the weak-interaction theory; varia-tions with time of the Newtonian gravita-tional constant, the fine-structure constant, the strong and weak coupling constants, the ratio of elect ron to proton masses, and other physical constants; and the time dependence of the size of a solar convective c ore. 9. J. G. Ramsay, T r a ~z s . Geo l . S oc . S . Af r. . 66, 353 (1963). 10. J. W . Schopf and E. S. Barghoorn, Science156, 508 (1967).

11. J. W. Schopf, Bio l . Rev . Co in br idge P h i l .S oc . , in press.12. P. F. Hoffman, Geo l . S l r rv . Car ? . P ap . 68-42(1969); J. W. Schopf (11).13. P. Cloud, S c i e ~ ~ c e60, 729 (1968).14. M. R. Walter, Scieizce 170, 1331 (1970).15. W. L. Donn, B. D. Donn, W. G . Valentine,Bi t il . G eo l . S oc . A i i l e r . 76, 287 (1965).16. C. Sagan, R a d i a t . R e s . 15, 174 (1961).17. W. D. Sellers, .7. A p p l . M e t e o r o l . 8 , 392(1969); M. I. Budyko, T e l h ~ s21, 611 (1969); .M. I. Budyko, .I.A p p l . M e t e o r o l . 9, 310(1970). .18. S. Manabe, in G l o b a l E f f e c t s o f E n v i r o n-nrental Pollzrtion, S. F. Singer, Ed. (Springer-Verlag, New York, 19701, p. 25.19. H. C. Urey, The Plarzets (Yale Univ. Press,New Haven, 1952); H. D. Holland, in prep-aration; F. S. Johnson, in G l o b a l E f f e c t s o fEn v i r on n ~en ta l P o l l r ~ t ion , S. F . Singer, Ed.(Springer-Verlag, New York, 19701, p. 4.20. G. E. Hutchinson, in T h e Ear th as a P lan e t ,


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G. P. Kuiper, Ed. (Univ. of Chicago Press,Chicago, 1954), p. 371.E. R. Lippincott, R. V. Eck, M. 0 . Dayhoff,C. Sagan, Astrophys . J . 147, 753 (1967).J. L. Bada and S. L. Miller, Science 159, 423(1968).P. H. Abelson. Pr oc . Nu t . Acad . S c i . U . S . A.55, 1365 (1966).W. E. McGovem, J. Atmos . S c i . 26, 623(1969).C. Sagan, in I. S. Shklovskii and C. Sagan,Intelligent Life in the Univ erse (Holden-Day,San Francisco, 1966), pp. 222-223.Donn et al . (15) unaccountably concludedthat the amount of gas-phase NH, in equi-librium with seawater would yield an inaig-nificant greenhouse effect.J. Bada, in Proceedings of the 5th Conferenceon th e Or ig ins o f L i fe , Be lmon t , Md. , Apr i l1971, L. Margulis, Ed. (Gordon & Breach,New York, in press).M. G. Rutten, The Geological Aspects of theOrigin o f Life on Earth (Elsevier, Amster-dam, 1962). But R. T. Cannon [Natu r e 205,586 (1965)l has described occasional oxidizedred beds more than 2 aeons old, which sug-gests at least local oxidizing conditions inThis epoch.P. Ramdohr, AbR. Deu t . Akad . Wis s . Ber linKl . Ch en t . Geol . Bio l . 3, 35 (1958); D. R .Derry, Bull . Geol . Soc. Amer. 70, 1587 (1959).E. C. Perry, J. Monster, T. Reimer, Science171. 1015 (1971).L. ;an ~ al e n,~ c i e n c e171, 439 (1971).See also J. M. Olson, i b id . 168, 438 (1970).See also L. V. Berkner and L. C. Marshall,J . Atmos . S c i . 22 . 225 (1965).See, for example, A. L. Lehninger, Biochem-istry (Worth, New York, 1970), pp. 560-561.N. H. Horowitz, Pr oc . Nat . Acad . S c i . U . S . A .31, 153 (1945).R. Y. Stanier, M. Doudoroff, E. A. Adelberg,Th e Microbia l W or ld (Prentice-Hall, NewYork, ed. 2, 1963).C . Sagan, in International Dict ionary o f Geo-

physics, S . K . Runcorn, Ed. (Pergamon, Eon-don, 1967), p. 97; C. Barth, A. Stewart, C.Hord, A . Lane, Icarus, in press.See, for example, W. Vishniac, K. C. At-wood, R. M. Bock, H. Gaffron, T. H. Jukes,A. D. McLaren, C. Sagan, H. Spinrad, inBiology and the Explorat ion of Mars , C. S.Pittendrigh, W. Vishniac, J. P. T. Pearman,Eds. (Publication No. 1296, National Acad-emy of Sciences-National Research Council,Washington. D.C., 1966); J. Lederberg andC. Sagan, Pr oc . Nu t . Acad . S c i . U . S . A. 48,1473 (1962); C. Sagan, "Life," in Encyclo-paedia Britannica (Benton, Chicago, 1970);Icarus 15, 511 (1971).B. C. Murray, L. A. Soderblom, R. P. Sharp,J. A. Cutts, J . Geoph ys . Res . 76, 313 (1971);C. R. Chapman, J. B. Pollack, C. Sagan,As t r on . J . 74, 1039 (1969); A. Dollfus, R.Fryer, C. Titulaer, C . R . H. Acad. Sci . Ser . B270, 424 (1970); C. Sagan and J. Veverka,Icarus 14, 222 (1971); W. K. Hartmann,i b id . 15, 410 (1971).C. Sagan, Technical Report TR 32-34, JetPropulsion Laboratory, Pasadena (1960); A.P. Ingersoll, J. Atnlos. Sci. 26, 1191 (1969);S. I. Rasool and C. deBerg, Nature 226, 1037(1970).J. B. Pollack, Icarus 14, 295 (1971).We are grateful to E. E. Salpeter, J. W.Schopf, M. Schwarzschild, and J. Bahcall forstimulating conversations; to B. N. Khare forthe laboratory infrared spectra; to L. D. G.Young for the calculated Ha spectra; and toJ. Katz for a comprehensive discussion ofthe reliabi lity of current models of main-sequence solar evolution. This research wassupported by the Atmospheric Science Section,National Science Foundation, under grantGA 23945.Present address: Department of Physics,Mansfield State College, Mansfield, Pennsyl-vania 16993.December 1971; revised 21 April 1972

A Binding Protein for Fatty Acids in Cytosol ofIntestinal Mucosa, Liver, Myocardium, and Other Tissues

Abstract. A protein of molecular weight -.12,000 which binds long-chain fattyacids and certain other lipids has been identified in cytosol of intestinal mucosa,liver, myocardium, adipose tissue, and kidney. Binding is noncovalent and isgreater for unsaturated than for saturated and medium-chain fatty acids. Thisprotein appears to be identical with the smaller of two previously described cyto-plasmic anion-bindirzg proteins. Binding of long-chain fatty acids by this proteinis greater than that of other anions tested, including sulfobromophthalein, anddoes not depend on negative charge alone. The presence o f this binding proteinmay explain previously observed differences in intestinal absorption amortg fattyacids, and the protein tnay participate in the utilization o f long-chain fa tty acidsby many mammalian tissues.

Translocation of fatty acids fromcell surface to endoplasmic reticulumand mitochondria is fundamental to theintestinal absorption of lipids and tothe utilization of free fatty acids inplasma by liver, muscle, and other tis-sues. However, although long-chainfatty acids are at best poorly soluble inaqueous media, a mechanism to ac-count for the apparent facility withwhich they traverse the cytosol (aque-ous cytoplasm) has not been identified.In studies of the intestinal absorptionof long-chain fatty acids ( I , 2 ) , we ob-served that although saturated and un-saturated fatty acids were taken up by

everted jejunal sacs at equal rates, un-saturated fatty acids were esterifiedmore rapidly. However, our studies andthose of others ( 3 ) indicated that theseresults could not be explained by cor-responding differences in the activationof fatty acids by microsomal fatty acid-coenzyme A (CoA) ligase (4). Ac -cordingly, we postulated (2) that ap-parent differences in rates of esterifica-tion might be due to different rates oftranslocation of fatty acids from themicrovillus membrane to the site oftheir activation in the endoplasmic retic-ulum. As a result, we discovered abinding protein for long-chain fatty

acids in the cytosol of jejunal mucosaand of other mammalian tissues (5 ) .Male Sprague-Dawley rats-fasting ifintestine was to be studied, otherwisenonfasting-were killed by decap itation.The proxim al half of the sm all intestine,distal to the ligament of Treitz, was re-moved an d flushed with 40 ml of 0.01Mphosphate buffer in 0.154 M KC1 (pH7.4, 4O.C). Mucosa was extruded,weighed, homogenized in three volumesof buffer, and centrifuged at 105,000gfor 2 hours. T he supernatant, exclusiveof floating fat, was used for gel filtra-tion. The liver was perfused in situthrough the portal vein with cold bufferbefore homogenization and ultracentrif-ugation as just described. Appropriateligands (Figs. 1 and 2) were added invitro to 105,000g supernatants, and themixture was subjected immediately togel filtration on Sephadex G-75. Proteinconcentration in the column effluentwas measured as absorbance at 280nm; radioactivity was determined byliquid scintillation spectrom etry; sulfo-bromophthalein (BSP) was measuredas absorbance at 58 0 nm after alkalini-zation.Sephadex G-75 chromatography ofrat jejunal supernatant with [l4C1oleateshowed association of radioactivity witha protein of low molecular weight,which we have designated "fatty acidbinding protein" (FABP) (Fig. 1).Variable radioactivity was also associ-ated with macromolecules (includinglipoproteins) in the excluded (void)volume, and with residual albumin inthe tissue homogenate. By lipid extrac-tion (6 ) and thin-layer chrom atographyof the FABP peak, more than 95 per-cent of 14C was recovered as free fattyacid, a result indicating that bindingwas noncovalent and not the result ofprior conversion to fatty acyl-CoA orother derivatives, A FABP with virtu-ally identical elution characteristics wasdemonstrated in liver supernatant.An estimation of the molecularweight of FABP was obtained by com-paring its relative elution volume(Ve/V,) with that of proteins of knownmolecular weight on Sephadex G-75.Both jejunal and hepatic FABP wereconsistently eluted in a volume (Ve/V,= 2.10 5 .02) slightly greater than thatof cytochrome c (horse heart, Sigma,molecular weight 12,400, Ve/ V, = 2.08k .02); this indicates a molecularweight of about 12,000. This valuemust be regarded as an approximation,however, because elution characteristicsof proteins on gel filtration show a

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Earth and Mars - Evolution of Atmospheres and Surface Temperatures - Sagan and Mullen