Evolution of Atmospheres and Surface Temperatures Author(S): Carl Sagan and George Mullen Source: Science, New Series, Vol

Home , Faint young Sun paradox

Earth and Mars: Evolution of Atmospheres and Surface Temperatures Author(s): Carl Sagan and George Mullen Source: Science, New Series, Vol. 177, No. 4043 (Jul. 7, 1972), pp. 52-56 Published by: American Association for the Advancement of Science Stable URL: http://www.jstor.org/stable/1733927 . Accessed: 10/12/2014 08:53

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp

. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected].

American Association for the Advancement of Science is collaborating with JSTOR to digitize, preserve and extend access to Science.

http://www.jstor.org

This content downloaded from 128.103.149.52 on Wed, 10 Dec 2014 08:53:18 AM All use subject to JSTOR Terms and Conditions (6). This variation has profound consequences for the surface temperatures of the terrestrial planets (7). The main- sequence brightening of the sun is one Reports of the most reliable conclusions drawn from the modern theory of stellar evolution, which explains in considerable de- tail the observed Hertzsprung-Russell Earth and Mars: Evolution of Atmospheres diagram. The models of solar evolution used in this report give an age for the and Surface Temperatures sun in excellent agreement with the age determined on independent grounds for Abstract. Solar evolution implies, for contemporary albedos and atmospheric Earth, the moon, and the meteorites. composition, global mean temperatures below the freezing point of seawater less The principal uncertainties in such cal- than 2.3 aeons ago, contrary to geologic and paleontological evidence. Ammonia culations are in the age of the sun and mixing ratios of the order of a few parts per million in the middle Precambrian its initial abundance of helium. Much atmosphere resolve this and other problems. Possible temperature evolutionary larger possible errors in such parameters tracks for Earth and Mars are described. A runaway greenhouse effect will occur as thermonuclear reaction rates or opac- on Earth about 4.5 aeons from now, when clement conditions will prevail on ities have much smaller effects on Mars. dL/dt (8). Katz (8) concludes that (1/L) (dL/dt) is in error by at most The present surface temperature of dows, and the other emitted by the 25 percent for the best contemporary Earth represents an energy balance be- atmosphere into space in wavelength evolutionary models. A variety of calcu- tween the visible and near-infrared sun- regions of strong atmospheric absorp- lations of main-sequence solar evolu- light that falls on the planet and the tion. In the latter case we consider the tion give a variation, AL, of 30 to 60 middle-infrared thermal emission that emission to occur from the skin tem- percent over geologic time (6). The best leaves. In the absence of an atmo- perature of the approximately isother- present estimate of AL is 40 ? 10 per- sphere this equilibrium is written /4S X mal outer boundary of an atmosphere cent. For most of the following calcula- (1 - A) = eaTe4, where S is the solar in radiative equilibrium, which is, in tions, we conservatively adopt AL = 30 constant; A is the Russell-Bond spheri- the Eddington approximation, at a percent. cal albedo of Earth, a reflectivity inte- temperature of 2-1/4Te. Thus, We then run the sun backward grated over all frequencies; e is the through time and assume initially that - Ax, mean emissivity of Earth's surface in /45(I A-) = EeBi (Ts) + the terrestrial atmospheric composition, the middle infrared; a is the Stefan- e, and A remain constant. The results Boltzmann constant; and Te is the ef- BjHee pec(2-/h ) (1) from Eq. 1 are shown in Fig. 1. We see fective equilibrium temperature of (at- that the global temperature of Earth mosphereless) Earth. The factor 1/4 is Here BAis the Planck specific intensity, dropped below the freezing point of the ratio of the area 7rR2 that intercepts and the wavelength intervals AX are seawater less than 2.3 aeons ago (1 aeon sunlight to the area 47rR2 that emits chosen to pack with adequate density is 109 years); 4.0 to 4.5 aeons ago thermal infrared radiation to space. those wavelength regions where BA is global temperatures were about 263?K. When the best estimates of these param- changing rapidly. The equation is solved Had we used 50 percent for AL, the eters are used, a value for Te of 250? iteratively for Ts on an electronic com- freezing point of seawater would have to 255?K is obtained; this is far less puter. The adopted step-function ap- been reached about 1.4 aeons ago, and than the observed mean surface tem- proximation to the actual nongray ab- temperatures 4.0 to 4.5 aeons ago which is due to rota- perature, T,, of Earth, 286? to sorption spectrum, would have been about 245?K. Be- 288 K. The difference is due to the tion-vibration transitions in Earth's cause of albedo instabilities (discussed with greenhouse effect, in which visible and atmosphere, is compared the mea- below) it is unlikely that extensive near-infrared sunlight penetrates through sured transmission spectrum in (1). The liquid water could have existed any- Earth's atmosphere relatively unim- resulting values of T, are shown in Table where on Earth with such global mean The value of e from studies peded, but thermal emission by Earth's 1. correct is, temperatures. surface is absorbed by atmospheric con- of a wide variety of minerals (2), closer The presence of pillow lavas, mud stituents that have strong absorption to 0.9 than to 1.0. Extensive calculations cracks, and ripple marks in rocks from bands in the middle infrared. Thus, time (3) based on measurements made from the Swaziland supergroup strongly variations in S, A, e, or atmospheric Earth yield values for A of 0.33 to 0.35, implies abundant liquid water 3.2 aeons made composition may induce important and an analysis of observations ago (9). The earliest known microfos- changes in Ts. The present heat flow over 5 years by meteorological satellites sils (10, 11), 3.2 ? 0.1 aeons old, in- from the interior of Earth is about (4) yields 0.30 for A. Since we are clude blue-green algae, which would be 2 X 10-5 the solar constant and plays a concerned with differential effects, we very difficult to imagine on a frozen - negligible role in determining Ts. have adopted A 0.35 to secure agree- Earth. Algal stromatolites, 2.0 to 2.8 To calculate Ts, allowing for the ment with the observed Ts in our ap- aeons old, exist in various parts of the greenhouse effect, we divide the emer- proximation (5). world (12, 13). If they are intertidal gent flux into two parts, one emitted by The solar constant is varying; the (13), there must have been at least the surface at temperature Ts directly luminosity, L, of the sun has increased meters of liquid water; if they are sub- into space through atmospheric win- by about 40 percent in geologic time tidal (14), much greater depths are im- 52 SCIENCE, VOL. 177

This content downloaded from 128.103.149.52 on Wed, 10 Dec 2014 08:53:18 AM All use subject to JSTOR Terms and Conditions ...

plied. The time required for surface may be somewhat less, but the fraction than the contemporary global average, water to accumulate sediments in geo- of Earth covered by ice, snow, and about 1 g cm-2. The only surviving synclinal trough suggests (15) the pres- glaciation will be very much larger. The alternative appears to be that the ence of extensive bodies of water on albedos of thick deposits of ice or snow atmosphere of Earth 1 or 2 aeons Earth 4 aeons ago or more. Finally, are 0.50 to 0.70. A decline in the glo- ago contained some constituent or con- liquid water is almost certainly neces- bal temperature of Earth is likely to in- stituents, not now present, with signifi- sary for the origin of life; if we believe crease rather than decrease the albedo, cant absorption in the middle infrared, that life began shortly after the forma- but in any case the albedo decline re- in the vicinity of the Wien peak of tion of Earth (16), liquid water must quired to explain the discrepancy ap- Earth's thermal emission. A large num- have been present for most of the period pears to be out of the question. Indeed, ber of candidate molecules were in- between 3.5 and 4.5 aeons ago. Thus, detailed global climatic models (17) vestigated. The ideal molecule should even using our most conservative value suggest that a relative increase in A of provide significant absorption in the of AL, we find a serious discrepancy only 2 percent is enough to induce ex- present window from 8 to 13 ,um, even between theory and observation. tensive glaciation on Earth, which im- in low abundances. Large amounts of This discrepancy indicates an error plies that the present climate is ex- CO, SO,, 03, and the various oxides in at least one of our initial assump- tremely sensitive to albedo. of nitrogen are inadequate, as are many tions. There are only three likely sources This leaves changes in atmospheric times the contemporary abundances of of error: S, A, and the atmospheric composition as a possible explanation. the homonuclear diatomic molecules 02 composition. The solar constant is un- Major variations in the CO, abundance and N2, which have no permitted vibra- likely to be sufficiently in error to ac- will have only minor greenhouse effects tion-rotation transitions. count for the discrepancy; the most because the strongest bands are nearly But, among the more reducing gases, probable value of S considerably saturated. A change in the present CO2 NH, is very appropriate. A volume mix- widens the discrepancy. For small abundance by a factor of 2 will produce ing ratio, [NH3], of about 10-5 at a greenhouse corrections, the variation of directly a 2? variation in surface tem- pressure of 1 bar provides appreciable ST, with 8A can be written 8A = perature (18). The CO2 abundance is absorption at 8 to 13 t,m. No other -4(1 -A)3Ts/Ts. Thus, for T, 3.5 highly controlled by silicate-carbonate plausible reduced gas (for example, CH4 to 4 aeons ago to be increased from equilibria; by buffering with seawater, or H,S) provides comparable absorp- the values shown in Fig. 1 to the freez- which contains almost 100 times the tion. The present value of [NHa] is still ing point of seawater requires an atmospheric CO,; and by the respira- in some question, but it appears to be albedo decrement of about 0.06 to 0.09; tion and photosynthesis feedback loop less than 10-8 (20). At thermodynamic for AL - 50 percent, an albedo decre- (19). The negative exponential depen- equilibrium in the present oxidizing ment of more than 0.20 is required. dence of the vapor pressure of water on atmosphere this mixing ratio would be Such albedos are unacceptably small. At reciprocal temperature implies that for much less than 10-35 (21). As is now global temperatures 10? or more below a lower global temperature there is no the case for CH4, a small steady-state contemporary values, the cloud cover likelihood of gaining more water vapor abundance could be maintained if the

I 450-? .- Earth 450 -I;.....

420 - ': Swaziland supergroup, Y -:: evidence for liquid water o 400- - : . Initial 1 bar H2 atmosphere . a 3 I .-- evolutionary track (L) 370. - Normal of water Q n 'i- Bulawayan strmatolites boiling point -i E E ...350 - EquilibriumNH3 S evolutionary track . . o 320 32 32 : A0.35 ICn . . cn - - 'i- ...... Subequilibrium NH3 .. . :300[- . ::300 evolutionary track .. 2.:.j:.;:::.j..i?I,:.;.::? A -{ ...... , /i?. 272--a: ;;??*;,<^';{:ri":! lFreezing point of seawater 270 250 Origin of Now Earth 230 -W4. -3.U -Z.U -1.0 QO .0 2.0 3.0 4.0 5.0 6.0 Time (4) Time /E) Fig. 1 (left). Calculated time-dependent model greenhouse effects for Russell-Bond albedo A about 0.35 and two surface infrared e = emissivities, 0.9 and e = 1.0, the former being more nearly valid (2) and giving the correct present global temperatures. The atmosphere of CO2 and H20 is assumed to contain the present abundances of these gases, pressure broadened 1 bar of a The by foreign gas. slightly reducing atmosphere has the same constituents with the addition of a 10-5 volume mixing ratio of NHa, CH,, and H>S. In this case NH3 is the dominant absorber. At the top is the greenhouse resulting from the addition of 1 bar of H, to the constituents already mentioned. The evidence for liquid water at 2.7 to 4.0 aeons (E) ago comes from a variety of geologic and paleontological data (9-16). The time evolution of the effective temperature is also All calculations are AL displayed. for = 30 percent; for larger time-derivatives of the solar luminosity, the freezing point of seawater is reached in more recent times. 2 yet Fig. (right). Three derived evolutionary tracks for the temperature of Earth. As described in the text, the librium NH3 track subequi- (less than 10-" volume mixing ratio), while schematic, is thought to be most likely. The amplitude of global temperature oscillations in Mesozoic and Paleozoic glaciations is smaller than the width of the temperature curve shown. A runaway greenhouse effect occurs several aeons in our future. 7 JULY 1972

This content downloaded from 128.103.149.52 on Wed, 10 Dec 2014 08:53:18 AM All use subject to JSTOR Terms and Conditions molecule were generated at a high rate, Table 1. Calculated contemporarysurface prohibits temperatures suitable for the for e is but this temperatures,T,, Earth; the mean of life even close to when as by biological activity '(21), emissivity of Earth's surface in the middle origin epochs calculation does indicate the great ther- infrared; A is its Russell-Bond spherical the prokaryotes-organisms requiring modynamic instability of NH3 in the albedo. a major time interval for their evolu- present 02 atmosphere. On the other T, (?K) tionary antecedents-were present in Bada and Miller calculated abundance. 'hand, (22) A =0.35 A- 0.30 NH3 mixing ratios on the preoxygenic 2) An insignificant amount of H2 is Earth by using clay mineral equilibria 0.9 286 292 present initially and the atmospheric 1.0 278 284 in the oceans and, independently, by greenhouse is dominated by H20 and by assuming that the deamination of as- NH3 in its calculated equilibrium abun- partic acid is reversed, since this amino dance, which then declines as before acid is required for the origin and and far infrared would thermostat such toward the CO2/H0O track. early evolution of life. They found an exosphere efficiently. A primitive re- 3) Ammonia is initially present in the following approximate values for ducing atmosphere would have been sta- subequilibrium abundances, because of [NH3]: 10-7 at 0?C, 3 X 10-5 at ble against gravitational escape for pe- photodissociation and reaction with oth- 25?C, and 3 X 10-4 at 50?C, in good riods approaching 1 aeon. Accordingly, er atmospheric constituents. The NH3 accord with our requirements. Lower there may have been a significant epoch absorption, pressure-broadened with a values of the NH3 mixing ratio are pos- in the early history of Earth in which H, foreign gas at 1 bar, ideclines appreci- sible because of ultraviolet photolysis was an important constituent of the ably between [NH3] 10-5 and [NH3] (23), but the steady-state NH3 abun- atmosphere. It is estimated (24) that s 10-6. The flatness of this third track dance would have been maintained by the maximum H2 mixing ratio in the represents a rough balance between the an equilibrium between photolysis and lower atmosphere was 0.10 to 0.15 after slow decline of [NH3] and the slow in- production. An NH3 mixing ratio even a possible initial period in which exo- crease of L. Calculations (27) of the as small as 10-6 will produce a high- spheric blowoff occurred because the thermodynamic stability of a number of temperature -equilibrium mesopause gas kinetic energy of H2 exceeded its amino acids in aqueous phase show a layer, somewhat analogous to the terres- gravitational potential energy. We have variation of several orders of magnitude trial ozone layer, and will also serve to no way of knowing the [H2] history in in half-life for a decline of 10? or 20?. protect other gases closer to the surface, early times; for heuristic purposes we The subequilibrium NH3 in the third notably water vapor, from photodissoci- calculate the additional greenhouse ef- track therefore (i) keeps T. above ation (24). fect due to 1 bar of H2. At such a the freezing point of water; (ii) is Accordingly, we calculated (1) green- pressure, permitted quadrupole and pres- responsive to comments (23) on the house temperatures for atmospheres in sure-induced dipole transitions produce photodissociation of NH3; and (iii) which NH3 is a minor constituent. We major absorptions at longer wavelengths provides increases of many orders assumed that water vapor is present in than 7.5 Itm. One bar of H, fills in the of magnitude in the concentra- approximately its present abundance, long-wavelength windows in the atmo- tions of organic constituents in the determined largely by Ts and by meteo- spheric greenhouse, producing nearly primitive seas, thus enhancing the rology; that CO2 is present in approxi- complete absorption at all wavelengths likelihood of the origin of life on primi- mately its present abundance, main- longer than 4.9 /um and increasing Ts tive Earth. The subsequent decline in tained by mineral equilibria and oceanic well above the normal boiling point NH3 abundance is most likely due to buffering reactions; and that CH4 and of water. oxidation by 02 produced in green H,S are present in amounts comparable Accordingly, we are left with three plant photosynthesis. The evolution of to that of NH3. Because CH4 and evolutionary tracks for the temperature green plants could have significantly H2S are not as effective absorbers, history of primitive Earth (Fig. 2): cooled off Earth. their presence does not significantly 1) With an initial extensive H. atmo- Our conclusions would not be signi- affect the results. The surface tem- sphere, Earth originates at temperatures ficantly different if we had used larger perature in this (slightly) reducing above the normal boiling point of water, values of AL. With such values, how- atmosphere is shown as a function of even in the absence of endogenous heat ever, the argument requires small quan- al- time in Fig. 1. We see that such an sources. The temperature rapidly de- tities of NH3 in Earth's atmosphere atmosphere is entirely adequate to re- clines in the first aeon because of the most up to the Precambrian-Cambrian solve the discrepancy and keep the glo- escape of hydrogen into space, and then, boundary. Because of the thermody- of in an excess of bal temperature of Earth well above the about 3.5 aeons ago, enters a milder namic instability NH3 results the absence freezing point of water, which confirms climate dominated by the NH3 and ,0 (21), such suggest values of for a ma- a conjecture (25) made by one of us water greenhouses. The photodis- of contemporary [O,] fraction of the of Earth. This some years ago (26). sociation, reaction, and oxidation of jor history with a of If even small quantities of hydrogen the reduced gases of such an atmo- is consistent, variety lother data on banded were present in the early atmosphere, sphere produce a gradual decline in evidence-including formations on the oxi- the primitive exosphere would, by dif- temperatures, and the planet ap- iron (13, 19, 28), uraninite and on fusive equilibrium, be dominated by H. proaches the evolutionary track for dation of (19, 28, 29), Because of the high thermal conduc- the present greenhouse constituents, the relatively small 34S enrichment of tivity of hydrogen, such an exosphere perhaps 1 to 2 aeons ago. We do not Swaziland barites (310)-and makes would cool very efficiently by conduc- know whether as much as 1 bar of H2 quite implausible the suggestion (31) tion downward; in addition, the presence could have been retained by Earth dur- of an early oxidizing atmosphere on in which small of polyatomic reduced molecules with ing its formation, but we are skeptical Earth. A long epoch strong emission features in the middle about this evolutionary track, because it quantities of such reduced gases as NH3 54 SCIENCE, VOL. 177

This content downloaded from 128.103.149.52 on Wed, 10 Dec 2014 08:53:18 AM All use subject to JSTOR Terms and Conditions might coexist with small quantities of in which global temperatures were not References and Notes photosynthetically or photolytically pro- far from the freezing point of seawater 1. C. Sagan and G. Mullen, Report 460, Center duced is not excluded. Our conclu- and in which the of life for Radiophysics and Space Research, Cor- 02 origin may nell University (1971). sion that significant quantities of O2 did have occurred, as it did on primitive 2. R. J. P. Lyon, NASA Contractor Report not arise until to 2 CR-100, Stanford Research Institute (1964). 1 aeons ago is Earth in the same period. Because of 3. J. London, Final Report, contract AF 19(122)- in excellent accord with the conclu- the smaller martian gravitational ac- 165, Department of Meteorology and Ocean- ography, New York University (1957); - sions of Cloud (13), based on the chro- celeration, photodissociation and escape and T. Sasamori, in Space Research, K. nology of banded iron formations and should have changed the intermediate Kondratyev, R. Rycroft, C. Sagan, Eds. (Akademie Verlag, Berlin, 1971), vol. 11. In the oldest fossil eukaryotes. By this oxidation state atmosphere much more standard textbooks, R. M. Goody [Atno- an extensive devel- on Mars than on Earth. spheric Radiation (Clarendon Press, Oxford, period evolutionary rapidly Any 1964)] adopts a value for A of about 0.4, opment of catalase, peroxidases, and martian organisms would have had to and K. Ya. Kondratyev [Radiation in the Atmosphere (Academic Press, New York, peroxisomes to defend the cells against face low temperatures and increasingly 1969)] chooses A about 0.35. oxidation products must have occurred inaccessible liquid water. Over geologic 4. T. H. von der Haar and V. Suomi, J. Atmos. Sci. 28, 305 (1971). (16, 32), and selection pressures for time, Mars could have lost meters to tens 5. While we believe this step-function approxi- defenses against or avoidance (at ocean- of meters of liquid water mation to be adequate for the present dis- by photodis- cussion, we are pursuing a more detailed set ic depths) of solar ultraviolet light sociation, escape of hydrogen, and oxi- of calculations, which allows for much finer eased dation of surface material these wavelength and altitude grids. gradually (16, 33). (37); 6. Estimated values of the increase in solar The presence of NH3 in Earth's are not oceans, but they are respectable luminosity, AL, over geologic time are: 60 for most of the Precam- It is a debatable but percent [M. Schwarzschild, R. Howard, R. atmosphere depths. hardly quix- Harm, Astrophys. J. 125, 233 (1957); (7)]; brian has a range of biological implica- otic contention that martian organisms 30 percent [F. Hoyle, in Stellar Populations, tions. It of a useful D. J. K. O'Connell, Ed. (Specola Vaticana, is, course, very may have been able to adapt to the in- Vatican City, 1958), p. 223; C. B. Haselgrove precursor compound for prebiological creasingly rigorous martian environment and F. Hoyle, Mon. Not. Roy. Astron. Soc. 119, 112 (1959); D. Ezer and A. G. W. organic chemistry. There is strong evi- and may still be present (38). Fairly Cameron, Can. J. Phys. 43, 1497 (1965)]; 50 dence (34) that NH3 is the key interme- abundant liquid water early in the his- percent [I. Iben, in Stellar Evolution, R. F. Stein and A. G. W. Cameron, Eds. (Plenum, diary in the fixation of atmospheric N2. tory of Mars would also be helpful in New York, 1966), p. 237]; 35 percent [J. H. the Horowitz on the the Bahcall and G. Shaviv, Astropys. J. 153, 113 By (35) hypothesis explaining puzzling large-scale (1968)]; 40 percent [I. Iben, Ann. Phys. New backward evolution of enzymatic reac- erosion of the oldest martian craters- York 54, 164 (1969)]. A weighted mean of tion this must an earlier a these values is 40 percent ? 10 percent. chains, imply phenomenon distinctly different from. 7. One of the earliest recognitions of this possi- evolutionary stage in which NH3 was lunar crater erosion (39). bility occurs in M. Schwarzschild's Structure and Evolution of the Stars [(Princeton Univ. available, which is consistent with the These calculations have also been ex- Press, Princeton, N.J., 1958), p. 207] "We may present argument. While most micro- tended into the future of Earth and thus conclude that the solar luminosity must have increased by about a factor 1.6 during organisms can utilize NH3 directly as Mars. As the sun continues to evolve, the past five billion years. Can this change in a source of N3, it is largely the pro- the surface temperature of Earth will the brightness of the sun have had some geo- physical or geological consequences that might karyotes that are N2 fixers (36). The hy- increase; more water vapor will be put be detectable?" for N2 into the the 8. The discrepancy between theory and experi- drogenase-ferredoxin system atmosphere, enhancing ment in the sB solar neutrino flux depends fixation is not specific for N2, and can atmospheric absorption; and eventually a insensitively on the parameters which strong- affect A reduce and effect will ly dL/dt. wide range of postulated C2H2, NO2, azides, cyanides runaway greenhouse occur, as convective cores, extending to 0.4 solar radii, (34); its original function, in times of previously discussed for Venus (40, 41). if essentially constant in time, do not significantly affect dL/dt. J. Katz has examined excess NH3, may not have been N2 According to calculations by Pollack many other conceivable sources of error in fixing. (41) a value of the solar constant 1.5 the theory of solar evolution, including the possibility that the sun may be burning other Analogous calculations have been times the present terrestrial value is elements in addition to hydrogen; influences of for Mars. The to cause such a for rotational and magnetic energy and of quarks; performed present green- adequate runaway errors in the weak-interaction theory; varia- house effect on Mars is due almost en- a planet with 50 percent cloud cover. tions with time of the Newtonian gravita- to the abun- For AL = 30 this event oc- tional constant, the fine-structure constant, tirely CO2; water-vapor percent, the strong and weak coupling constants, the dance of a few tens of precipitable curs about 4.5 aeons in our future; for ratio of electron to proton masses, and other physical constants; and the time dependence micrometers makes no significant con- AL = 50 percent, 3 aeons in our fu- of the size of a solar convective core. tribution. The total greenhouse effect is ture. Earth will then resemble contem- 9. J. G. Ramsay, Trans. Geol. Soc. S. Afr. 66, 353 (1963). a few on Mars is but only degrees Kelvin; T, porary Venus, with an atmospheric 10. J. W. Schopf and E. S. Barghoorn, Science roughly 210? to 220?K, depending on pressure of 300 bars of steam. It is 156, 508 (1967). meridional 11. J. W. Schopf, Biol. Rev. Cambridge Phil. the choice of A. The and difficult to imagine what could be done Soc., in press. diurnal temperature gradients on Mars to prevent this runaway, even with a 12. P. F. Hoffman, Geol. Surv. Can. Pap. 68-42 are both extreme because of the advanced a (1969); J. W. Schopf (11). paucity very technology (perhaps 13. P. Cloud, Science 160, 729 (1968). of liquid water and the thin atmosphere. progressive increase in atmospheric 14. M. R. Walter, Science 170, 1331 (1970). Thus, temperatures much warmer than aerosol content), but at the same epoch 15. W. L. Donn, B. D. Donn, W. G. Valentine, Bull. Geol. Soc. Amer. 76, 287 (1965). the global mean exist on Mars, but the global temperature of Mars will be- 16. C. Sagan, Radiat. Res. 15, 174 (1961). could not have existed extensively on come very similar to that of present- 17. W. D. Sellers, J. Appl. Meteorol. 8, 392 Earth. (1969); M. I. Budyko, Tellus 21, 611 (1969); primitive Earth. Earlier conditions on day If there are any organisms M. I. Budyko, J. Appl. Meteorol. 9, 310 Mars may have been much more left on our planet in that remote epoch, (1970). clement. After blowoff of wish to take of 18. S. Manabe, in Global Effects of Environ- very rapid they may advantage this mental Pollution, S. F. Singer, Ed. (Springer- a possible initial H2 atmosphere, the coincidence. Verlag, New York, 1970), p. 25. 19. H. C. The Planets Univ. tracks CARL SAGAN Urey, (Yale Press, equilibrium NH3 evolutionary New Haven, 1952); H. D. Holland, in prep- give T, near the freezing point of sea- GEORGE MULLEN* aration; F. S. Johnson, in Global Effects of Environmental Pollution, S. F. Singer, Ed. water. There appears to be an epoch Laboratory for Planetary Studies, (Springer-Verlag, New York, 1970), p. 4. in the first aeon after the origin of Mars Cornell University, Ithaca, New York 20. G. E. Hutchinson, in The Earth as a Planet, 7 JULY 1972 55

This content downloaded from 128.103.149.52 on Wed, 10 Dec 2014 08:53:18 AM All use subject to JSTOR Terms and Conditions G. P. Kuiper, Ed. (Univ. of Chicago Press, physics, S. K. Runcorn, Ed. (Pergamon, Lon- acids in the of mucosa Chicago, 1954), p. 371. don, 1967), p. 97; C. Barth, A. Stewart, C. cytosol jejunal 21. E. R. Lippincott, R. V. Eck, M. O. Dayhoff, Hord, A. Lane, Icarus, in press. and of other mammalian tissues (5). C. Sagan, Astrophys. J. 147, 753 (1967). 38. See, for example, W. Vishniac, K. C. At- Male if 22. J. L. Bada and S. L. Miller, Science 159, 423 wood, R. M. Bock, H. Gaffron, T. H. Jukes, Sprague-Dawleyrats-fasting (1968). A. D. McLaren, C. Sagan, H. Spinrad, in intestine was to be studied, otherwise 23. P. H. Abelson, Proc. Nat. Acad. Sci. U.S.A. Biology and the Exploration of Mars, C. S. 55, 1365 (1966). Pittendrigh, W. Vishniac, J. P. T. Pearman, nonfasting-were killed by decapitation. 24. W. E. McGovern, J. Atmos. Sci. 26, 623 Eds. (Publication No. 1296, National Acad- The proximalhalf of the small intestine, (1969). emy of Sciences-National Research Council, distal to 25. C. Sagan, in I. S. Shklovskii and C. Sagan, Washington. D.C., 1966); J. Lederberg and the ligament of Treitz, was re- Intelligent Life in the Universe (Holden-Day, C. Sagan, Proc. Nat. Acad. Sci. U.S.A. 48, moved and flushedwith 40 ml of 0.01M San Francisco, 1966), pp. 222-223. 1473 (1962); C. Sagan, "Life," in Encyclo- 26. Donn et al. (15) unaccountably concluded paedia Britannica (Benton, Chicago, 1970); phosphate buffer in 0.154M KC1 (pH that the amount of gas-phase NH3 in equi- Icarus 15, 511 (1971). 7.4, 4?C). Mucosa was librium with seawater would yield an insig- 39. B. C. Murray, L. A. Soderblom, R. P. Sharp, extruded, nificant greenhouse effect. J. A. Cutts, J. Geophys. Res. 76, 313 (1971); weighed, homogenizedin three volumes 27. J. Bada, in Proceedings of the 5th Conference C. R. Chapman, J. B. Pollack, C. Sagan, of and at on the Origins of Life, Belmont, Md., April Astron. J. 74, 1039 (1969); A. Dollfus, R. buffer, centrifuged 105,000g 1971, L. Margulis, Ed. (Gordon & Breach, Fryer, C. Titulaer, C. R. H. Acad. Sci. Ser. B for 2 hours. The supernatant,exclusive New York, in press). 270, 424 (1970); C. Sagan and J. Veverka, of 28. M. G. Rutten, The Geological Aspects of the Icarus 14, 222 (1971); W. K. Hartmann, floating fat, was used for gel filtra- Origin of Life on Earth (Elsevier, Amster- ibid. 15, 410 (1971). tion. The liver was perfused in situ dam, 1962). But R. T. Cannon [Nature 205, 40. C. Sagan, Technical Report TR 32-34, Jet 586 (1965)] has described occasional oxidized Propulsion Laboratory, Pasadena (1960); A. throughthe portal vein with cold buffer red beds more than 2 aeons old, which sug- P. Ingersoll, J. Atmos. Sci. 26, 1191 (1969); before homogenizationand ultracentrif- gests at least local oxidizing conditions in S. I. Rasool and C. deBerg, Nature 226, 1037 this epoch. (1970). ugation as just described. Appropriate P. Abh. 29. Ramdohr, Deut. Akad. Wiss. Berlin 41. J. B. Pollack, Icarus 295 (1971). Kl. Chem. Geol. Biol. 35 D. 14, ligands (Figs. 1 and 2) were added in 3, (1958); R. 42. We are to E. E. Salpeter, J. W. Bull. Geol. Soc. Amer. 1587 grateful Derry, 70, (1959). Schopf, M. Schwarzschild, and J. Bahcall for vitro to 105,000g supernatants,and the 30. E. C. J. T. Science Perry, Monster, Reimer, stimulating conversations; to B. N. Khare for mixture was to 171, 1015 (1971). the infrared to L. D. G. subjected immediately L. van Science 439 laboratory spectra; 31. Valen, 171, (1971). for the calculated H2 spectra; and to gel filtrationon G-75. Protein 32. See also J. M. ibid. 438 Young Sephadex Olson, 168, (1970). J. Katz for a comprehensive discussion of concentration in the 33. See also L. V. Berkner and L. C. Marshall, the of current models of main- column effluent J. Atmos. Sci. 225 reliability 22, (1965). sequence solar evolution. This research was was measured as absorbance at 280 34. See, for example, A. L. Lehninger, Biochem- the Science New supported by Atmospheric Section, istry (Worth, York, 1970), pp. 560-561. National Science Foundation, under grant nm; radioactivity was determined by 35. N. H. Horowitz, Proc. Nat. Acad. Sci. U.S.A. GA 23945. 31, 153 liquid scintillation spectrometry;sulfo- (1945). * Present address: Department of Physics, 36. R. Y. M. E. A. Stanier, Doudoroff, Adelberg, Mansfield State College, Mansfield, Pennsyl- bromophthalein (BSP) was measured The Microbial World (Prentice-Hall, New vania 16993. as absorbanceat 580 nm York, ed. 2, 1963). after alkalini- 37. C. Sagan, in International Dictionary of Geo- 17 December 1971; revised 21 April 1972 * zation. Sephadex G-75 chromatography of rat jejunal supernatantwith [14C]oleate showed associationof radioactivitywith A Binding Protein for Fatty Acids in Cytosol of a protein of low molecular weight, which we have designated "fatty acid Intestinal Mucosa, Liver, Myocardium, and Other Tissues binding protein" (FABP) (Fig. 1). Variable radioactivity was also associ- Abstract. A protein of molecular weight ~ 12,000 which binds long-chain fatty ated with macromolecules (including acids and certain other lipids has been identified in cytosol of intestinal mucosa, lipoproteins) in the excluded (void) liver, myocardium, adipose tissue, and kidney. Binding is noncovalent and is volume, and with residual albumin in greater for unsaturated than for saturated and medium-chain fatty acids. This the tissue homogenate. By lipid extrac- protein appears to be identical with the smaller of two previously described cyto- tion (6) and thin-layerchromatography plasmic anion-binding proteins. Binding of long-chain fatty acids by this protein of the FABP peak, more than 95 per- is greater than that of other anions tested, including sulfobromophthalein, and cent of 14Cwas recovered as free fatty does not depend on negative charge alone. The presence of this binding protein acid, a result indicating that binding may explain previously observed differences in intestinal absorption among fatty was noncovalent and not the result of acids, and the protein may participate in the utilization of long-chain fatty acids prior conversion to fatty acyl-CoA or by many mammalian tissues. other derivatives. A FABP with virtu- ally identical elution characteristicswas Translocation of fatty acids from everted jejunal sacs at equal rates, un- demonstratedin liver supernatant. cell surface to endoplasmic reticulum saturated fatty acids were esterified An estimation of the molecular and mitochondriais fundamentalto the more rapidly. However, our studies and weight of FABP was obtained by com- volume intestinal absorption of lipids and to those of others (3) indicated that these paring its relative elution the utilization of free fatty acids in results could not be explained by cor- (Ve/Vo) with that of proteins of known molecular on G-75. plasma by liver, muscle, and other tis- responding differencesin the activation weight Sephadex were sues. However, although long-chain of fatty acids by microsomalfatty acid- Both jejunal and hepatic FABP eluted in a volume fatty acids are at best poorly soluble in coenzyme A (CoA) ligase (4). Ac- consistently (Ve/JV = ? than that aqueous media, a mechanism to accordingly, we postulated (2) that ap- 2.10 .02) slightly greater c count for the apparent facility with parent differencesin rates of esterifica- of cytochrome (horse heart, Sigma, molecular V = 2.08 which they traverse the cytosol (aque- tion might be due to different rates of weight 12,400, Ve/ ? .02); this indicates a molecular ous cytoplasm) has not been identified. translocation of fatty acids from the In studies of the intestinal absorption microvillus membrane to the site of weight of about 12,000. This value must be as an of long-chain fatty acids (1, 2), we ob- their activationin the endoplasmicretic- regarded approximation, elution characteristics served that although saturated and un- ulum. As a result, we discovered a however, because of on filtration show a saturated fatty acids were taken up by binding protein for long-chain fatty proteins gel SCIENCE, VOL. 177 56

This content downloaded from 128.103.149.52 on Wed, 10 Dec 2014 08:53:18 AM All use subject to JSTOR Terms and Conditions