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Martian and terrestrial paleoclimatology: Relevance of solar variability
ICARUS 22, 301-311 (1974)
Martian and Terrestrial Paleoclimatology: Relevance of Solar Variability W I L L I A M K. HARTMANN Planetary Science Institute, 252 W. I n a Road, Suite D, Tucson, Arizona 85704 R e c e i v e d : F e b r u a r y 13, 1974; r e v i s e d M a r c h 27, 1974 E v i d e n c e for p a s t M a r t i a n r i v e r s is p e r h a p s t h e m o s t p u z z l i n g a n d i n c o n s i s t e n t p r e s e n t p l a n e t a r y p r o b l e m , conflicting as it does w i t h c u r r e n t c o n d i t i o n s o n t h e p l a n e t . T h i s p a p e r e m p h a s i z e s t h e s i m i l a r p u z z l i n g e v i d e n c e t h a t t h e E a r t h , as Well as Mars, was w a r m e r in t h e p a s t . N e u t r i n o e v i d e n c e raises t h e s u g g e s t i o n t h a t t h e s u n was also " w a r m e r " in t h e p a s t . A h y p o t h e t i c a l cause reconciling all effects is episodic c h a n g e in solar l u m i n o s i t y o n a t i m e - s c a l e of a few h u n d r e d m i l l i o n years. T h i s p a p e r o u t l i n e s r e q u i r e m e n t s a n d c o n s e q u e n c e s of s u c h a w o r k i n g h y p o t h e s i s , w i t h p r o a n d c o n a r g u m e n t s . I t is i m p o r t a n t to t r y to p r o v e or disp r o v e t h i s h y p o t h e s i s b e c a u s e it h a s r a d i c a l i m p l i c a t i o n s for c u r r e n t science. F o r e x a m p l e , it suggests (1) c l i m a t e s of all t h e p l a n e t s h a v e b e e n m a r k e d l y a l t e r e d s i m u l t a n e o u s l y b y solar c h a n g e s , s o m e t i m e s c a t a s t r o p h i c a l l y ; (2) solar c h a n g e s h a v e b e e n a d o m i n a n t " f o r c i n g f u n c t i o n " d r i v i n g biological s p e c u l a t i o n a n d evol u t i o n o n E a r t h ; (3) t h e c o n c e p t of geologic u n i f o r m i t a r i a n i s m is s o m e w h a t m o d i f i e d b y cosmic v a r i a b l e s ; (4) e v o l u t i o n o f w a t e r - u t i l i z i n g i n t e l l i g e n t c r e a t u r e s n e a r o t h e r s t a r s m a y b e less likely t h a n h a s b e e n t h o u g h t , d u e to catast r o p h i c p l a n e t a r y c l i m a t e r e a c t i o n s to c h a n g e in stellar l u m i n o s i t y .
l . ]NATURE OF THE PROBLEM
Mariner 9 has impressed a curious observational fact on scientists: Mars, a planet noted for extreme aridity, has arroyo-like features which have not been adequately explained other than by flowing water (Milton, 1973; Sagan, Toon and Gierasch, 1973; Carr, 1974; Schumm, 1974). Braiding and other deposition patterns observed at resolutions of a few hundred meters give strong evidence for water flow. While some arroyos appear to spring from thermokarst topography, suggesting local melting of permafrost and thus internal sources of heat and water, systems of minor sinuous arroyos exist, particularly in equatorial regions. Some major arroyos have secondary and tertiary tributary systems. These latter features suggest a widespread, probably atmospheric, mechanism to deliver liquid water to the surface. Further, H a r t m a n n (1973a) gives evidence for a much higher past Martian erosion rate (this may or may Copyright © 1974by AcademicPress, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
not be only the last cycle of an episodic Martian erosion history). At recent conferences the consequences of the "rivers" have been little discussed. This is perhaps because of their radical implications and not simply because of lack of tractable investigation, since geomorphological analyses, sinuosity indices, crater counts in river floors and crater counts in background regions are all underway in various institutions. In this paper, the nature of the problem is assumed to lie in four assertions: 1. The Martian arroyos exist and imply t h a t Mars was wetter and warmer in the past. 2. Simultaneously with this discovery, a deficiency in solar neutrinos has been found, indirectly implying t h a t the sun was more luminous in the past than now. 3. A large body of geological literature indicates t h a t the earth's climate is "normally" warmer than in the present. 4. A small increase in solar luminosity in the past or future would convert Mars 301
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to a more favorable climate for liquid water, by releasing more C02 and increasing advective heat transport to the poles (Sagan, Toon, and Gierasch, 1973). Assertions (1) and (4) will not be further discussed, as they are dealt with in the papers already mentioned. Some discussion of recent debate about assertions (2) and (3) will be given, and in particular, a review will be given of the terrestrial geological evidence for climatic changes driven by solar changes, because this portion of the problem has not been advanced in the recent Martian literature. A working hypothesis of solar variation is developed by adopting terrestrial and Martian observations, and the paper will discuss whether this hypothesis can be firmly rejected on the basis of present data.
:[I. HISTORICAL FACTORS IN THE QUESTION OF SOLAR VARIABILITY
The problem of solar variations has been bound up in a special way with the geological problem of plate tectonics or "continental drift." As is well-known (see Wyllie, 1971), the first half of this century saw a raging debate on this problem. One of the empirical facts was a set of geological data showing such curiosities as ancient widespread glaciation in Africa and northern South America and ancient palms, figs, and corals in locations such as Alaska, Greenland, and Spitzbergen Island. Of the two schools, "drifters" asserted t h a t these data showed the continents had moved in latitude, while some "fixists" asserted t h a t they showed t h a t solar luminosity had varied in such a way as to cause world-wide climate changes. The compelling evidence in the mid-1960's for continental and sea-floor rifting and plate motion led to a widespread assumption t h a t the paleoclimatic data had been adequately explained. However, several analysts have indicated t h a t the plate tectonic theory in itself is inadequate to explain all paleoclimatic data, as will be discussed. This paragraph is intended simply to warn readers t h a t
the discovery of plate tectonics, though irrelevant to solar variability, has for historical reasons had the sociological effect of "suppressing" discussion of the latter during the last ten years. III. REVIEW OF ()PIK'S PALEOCLIMATIC THEORY
One of the analysts of terrestrial paleoclimatic data in pre-Mariner years deserves special attention here because of his direct concern with planetary problems. This is E. J. 0pik, whose work over m a n y decades has shown remarkable prescience and intuition in the light of recent results. 0pik (1958) reviewed paleoclimatic data and concluded : " I t seems t h a t variations in the amount of heat received by the earth as a whole can only account for the major variations in past c l i m a t e s . . . " " F o r a long time there were doubts about the globe-wide character of the cold periods. Scientists were more inclined to consider them of hemispherical extent, taking place alternately in the northern and southern hemispheres and caused by minor changes in the eccentricity of the orbit and obliquity of the axes of the earth (also an early and current attempt to explain the Martian features--WKH). (0pik) showed t h a t this cause is inadequate. Only a simultaneous decrease of solar heat all over the globe can lead to such results, in which case the temperature will decrease everywhere." "All doubts in this respect have been removed by the determination of 'fossil' temperatures of the surface of tropical oceans, based on Urey's method of oxygen isotopes . . . . " Reviewing such data and geologic reports, 0pik (1958) presented a table of geologic observations which implied recurrent, dated cold waves every 250 m.y., including the present time and traced by geologic data back to 10O0m.y. ago. He asserted at least three older undated ice ages occurred. Theoretically, 0pik calculated
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t h a t a 9% higher rate of solar radiation would account for an increased global average temperature of about 8°C, accounting for the observations• A drop of 5% in present solar luminosity, according to this calculation, would drop the temperature b y about 6°K, causing the very recent ice ages when glaciers advanced from the poles to low latitudes• (Budyko, 1970, refers to two additional models t h a t predict complete glaciation of the earth with a solar luminosity only 2 to 5% below the present value•) I n the same 1958 paper, ()pik hypothesized t h a t these changes result from disturbances in radiation transfer rate in the sun's core, causing t e m p o r a r y expansions of the sun, lowering surface temperature, and decreasing luminosity. The durations of low-luminosity would equal the H e l m hol t z- K el vi n contraction time for the sun to re-adjust its structure• This is about 6m.y., taken by ()pik to be consistent with the duration of the "cool periods" noted in earth history. Opik estimated the time between "cool periods" to be "several hundred million years," consistent with the geologic record. As shown below, these ideas are quite consistent with recent observations. ~pik (1965) updated this model, producing exactly the same basic results, and additionally calling for a "flickering" of the sun due to convective processes in the outer sun, not amenable to a detailed, predictive theory. The "flickers" were advanced to account for fine structure in recent glacial paleoclimatology, and could cause, for example, " a rapid rise in solar luminosity, followed by a gradual decline. In a recent note Opik (1973) reiterated his belief in the recurrent variations of terrestrial climate driven b y solar coolings separated b y intervals theoretically estimated at 600m.y. and observationally estimated at 250m.y. He commented t h a t the recent neutrino work "agrees completely with ~pik's conclusions . . ." I V . TERRESTRIAL EVIDENCE OF RECURRENT CLIMATIC CHANGES
The geological literature abounds with evidence t h a t the earth was once warmer
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t h a n at present, just as Mars appears to have been• This evidence has been underemphasized in recent literature on the Mars problem. Furthermore, it causes some concern among traditional geologists• For example, Kummel (1970) states in his detailed t e x t on historical geology: " T h e doctrine of uniformitarianism has been such a strong guiding principle in the reconstruction of the earth's history t h a t it m a y come as a surprise to state t h a t the world t o d a y is in a very abnormal condition . . . . The world's climate at the m o m e n t . . . is quite exceptional, for the 'normal' climate of the geologic past appears to have varied from moderate to warm." Precisely the same could be said of Mars. Figure 1 presents some examples of the terrestrial data in question• The top record is after (~pik's, Kummel's (1970, p. 542), and Dunbar's (1963) figures and tables, based on paleontological and geological data. This record is qualitative, in t h a t no absolute temperature measures are made apart from " w a r m e r " or "colder" than present• The essential question about the paleontological and geological data is whether continental drift can remove all the discrepancies. For example, much evidence for a Permian cold period about 260m.y. ago comes from glacial evidence in Africa, South America, and India, at sites presently located near latitudes - 2 0 °, - 1 0 ° and +10 °. However, utilizing a widely accepted plate tectonic reconstruction (Dietz and Holden, 1970) we find these areas in the Permian were near - 6 5 ° , - 3 5 ° , and - 7 0 ° , respectively, so t h a t the glaciation does not appear so anomalous. Thus, uncritical listing of fossil evidence for low-latitude glaciation or high-latitude tropical plants is not sufficient to establish world-wide climatic variations. The writer has thus gone through a list or more t han two dozen reported fossil flora and fauna (Kummel, 1970; Dunbar, 1963) and reconstructed the sites' original latitudes from Dietz and Holden (1970). About 25 of the sites involve fossil corals.
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FIo. l. Example of three types of published paleoclimatie terrestrial data, all suggesting major climatic fluctuations with several 10Syr timescales. Black arrows {bottom) show suggested major cold periods; open arrows (top) show suggested major warm periods. Other less marked maxima and minima m ay reflect nonglobal climate variations, depending on technique used. See text for discussion and sources.
Corals now have a global distribution symmetric with the equator, primarily limited to latitudes ±30 °, dependent on water temperature and coastal conditions (originally shown by Darwin, 1897). About 70% of these coral cases are brought by continental drift to original positions within the 30 ° latitude range, and about 80% are brought within a 35 ° range. In other words, most of the problem is solved b y continental drift, but some marked anomalies remain. For example, Cretaceous corals are reported in Argentina at a site which should have been about 49 ° south of the equator, and Triassic corals are reported in Alaska when it should have been about 60 ° north of the equator. Cretaceous figs and palms in Greenland come from a site estimated to have been near 42°N. Tertiary figs and magnolias are reported in Alaska at sites reconstructed to lie near 70°N. All these anomalies suggest t h a t some other factor besides continental drift has affected paleoclimatology, and has caused the earth to be globally warmer in the past t h a n at present, as widely reported in geological literature. The middle graph in Fig. 1 is quantitative and serves as an example of the isotopic data. I t is a measure of sulphur isotope ratios based on a world wide inventory of sulphate sediments by Holser
and Kaplan (1966). Here, m any sources of fine structure could be present, such as changes in sediment delivery rates of ancient rivers emptying into the ocean, bacteriological activity, and mixing of bottom sediments by currents. In spite of all these local sources of fine structure which might wipe out long term trends, Holser and Kaplan find, " F o r sulphates of oceanic origin, variation with geological age is so large t h a t it nearly masks all other factors." The bottom record in Fig. 1 is t h a t of Meyerhoff (1970) and Meyerhoff and Teichert (1971) who made a comprehensive inventory of evaporites, desert eolian deposits, and glacial tillite beds from around the world, and produced maps of them for different geologic periods. Latitudinal belts were found, and as Meyerhoff notes, "When the evaporite beds have the greatest latitudinal spread (up to 125 °) the earth must have been very warm . . . . The great fluctuation of the widths of these belts illustrates t h e . . , time-related pattern: specifically, the average world temperature has changed considerably during earth history, and these changes are epi-
SOLARVARIABILITY sodic; ultimately a periodicity may be established." Meyerhoff and Teichert (1971) confirm this and stress t h a t the widths of these climatic belts are not constant: " . . . average earth temperature (or climate zonation)--not latitude-is a very critical element in determining whether or not glaciation will take place." The graph produced by this last study is semiquantitative; the original ordinate is the width of the evaporite zone in degrees. This can be called semiquantitative, since the measured width depends to some extent on the observer's judgement of the data from the given geologic period. Sources of noise or fine structure in this graph include statistical limitations on available field data, and local climatic factors on the ancient continents, such as coastal ranges. The tie between climatic data and continental drift data is illustrated, incidentally, by Meyerhoff's assertion t h a t his climatic bands are so well defined with respect to the present equatorial regions t h a t the continents must have not moved in latitude nearly as much as in the widely accepted models, such as t h a t of Dietz and Holden (1970). To the extent t h a t Meyerhoff's conclusion is correct, the data in the graphs become even stronger in arguing for world-wide climate shifts. In conclusion, it would appear t h a t just as on Mars, geological evidence on earth indicates t h a t the world-wide average climate varied in the past. A warm period occurred (centered) about 150m.y. ago and a still warmer period occurred about 500m.y. ago. Cold periods occurred now, about 260 and about 650m.y. ago. A great variety of data, including historical records, sunspot correlations, 0pik's "flickering" theory, planetary dynamical studies, and the very recent (30 000yr BP) glacial advances and retreats (e.g. Gribbin, 1973), also suggest t h a t periodic or nonperiodic variations of climate occur on all timeseales as implied by 0pik's proposed "flickering," and t h a t the further back in
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time we go, the greater the excursions of climate. In any ease, noticeable major excursions toward unusually warm or unusually cold conditions occur at intervals on the order of a few hundred million years, and these may not be precisely periodic. In addition to the above data, the evidence for several ("at least t h r e e " - 0pik, 1958) preCambrian epochs of widespread glaciation should be noted, although the dating and spacing of these is very uncertain (R. Siever, private communication).
V. ASTROPHYSICALEVIDENCE Any attempt to disprove solar-induced Martian climatic change by establishing the stability of the earth's paleoclimate is hardly successful, as the above section shows. Rather, the terrestrial evidence is parallel to the Martian evidence in suggesting a cold period now and noticeably warmer periods earlier. Hence the question becomes, can we rule out solar variations as a cause of these phenomena on the basis of astrophysical evidence? Note t h a t we are here discussing episodic variations; Sagan and Mullen (1972) have already reviewed astrophysical evidence t h a t a long-term increase of solar luminosity by 40% occurred over geologic time. We are discussing smaller superimposed oscillations in luminosity, not necessarily sinusoidal. The lack of solar neutrinos, as compared to current theoretical models, and the interpretation t h a t the sun was more luminous in the past, are well-known (Dilke and Gough, 1973; Cameron, 1972) and the applications to the problem of Mars and earth paleoclimates has been more widely discussed than the terrestrial geological evidence itself (Cameron, 1973 ; Fowler, 1973; Sagan and Young, 1973). This material will not be reviewed here except to consider whether any facets of it are strong enough to disprove the idea t h a t solar variations caused the Martian climate changes. Direct evidence for solar variation would be very shaky if the neutrino
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deficiency could be explained without the sun's luminosity being affected. No such models have been entirely successful (Fowler, 1973; Cameron, 1973), although Demarque et al. (1973) assert t h a t a Dicke-type fast-rotating solar core would explain the neutrino observations. This model has a drawback of predicting much lower early solar luminosities than other theories, for example <0.2L e during the first b.y., and about 0.85L o in the Cambrian. This prediction, which would imply early freezing of sea water (Budyko, ]970; Sagan and Mullen, 1972), has not been confirmed by available geologic data. The most popular model is similar to 0pik's original suggestion, and calls for episodic mixing which changes the energy production of the sun. A new equilibrium configuration is then reached in a time governed by Helmholtz-Kelvin contraction. Cameron (1973) has given perhaps the most complete review of the theories and derives a sudden-mixing model calling for a 6m.y. drop in luminosity to 0.7 its normal value. This model is said to be an improvement over other sudden-mixing models, but all sudden-mixing models have the virtue of getting minimum luminosities t h a t readily account for the neutrino deficiency. However, such drastic changes in solar luminosity appear to be inconsistent with the terrestrial paleoclimatic data. ()pik (1958) estimated t h a t a reduction of solar luminosity to only 0.84 the normal value would cause the coldest ice ages; Budyko (1970) and Opik invoke reductions of only a few percent to account for the recent ice ages. If, as in this paper, we t r y to account for the paleoclimatic data by solar theory, we would prefer a model with a longer and less pronounced cool period. Along these lines, Cameron notes evidence by Fenyves et al. (1973), t h a t the earth's ocean temperatures are anomalously low and have been low, not for 6m.y., but for 50m.y. Cameron points out t h a t slow mixing in the sun can be invoked arbitrarily to extend the cool periods. But current versions of such slow-mixing models make it more difficult or impossible to account for the low observed neutrino
flux, since the neutrino minimum becomes less deep. I f solar-type stars vary in brightness, a broadening of the main sequence for G stars could result. Such a broadening has been sought and found by Sagan and Young (1973) in the careful photometry by Johnson (1952) of the Praesepe star cluster. This cluster is said by Sagan and Young to be the optimum cluster for this test. However, Cameron (1973) earlier pointed out t h a t according to the best models of the variability process, the sun changes size, surface temperature, and luminosity in such a way as to move primarily along the main sequence, thus producing very little apparent broadening. Cameron (private communication, 1973) suggests that this detracts from the applicability of the Sagan-Young results. The strongest case against the SaganYoung conclusion is t h a t (1) Johnson had to estimate and discard other sources of dispersion, particularly the known influence of stars with unresolved companions t h a t affect photometry, and (2) Johnson's result was expressed merely as an upper limit on intrinsic, or "cosmic" dispersion of 0.03 magnitude, equivalent to excursions of individual stars of about 0.07 in luminosity), whereas Sagan and Young, re-analyzing Johnson's data, propose actual excursions of 0.15 in luminosity. These arguments are not strong enough to dispose of the Sagan-Young conclusion, because the fact remains t h a t dispersion in the Praesepe main sequence is equivalent to 0.07 to 0.15 excursions in luminosity and is unaccounted for by any known sources of error. Further, as Sagan and Young point out in reversing Cameron's argument, if we empirically detect excursions of 0.07 to 0.15, the true excursions of an individual star are still considerably larger because of the large theoretical component of evolution along the main sequence. Smith (1973) has responded to the SaganYoung conclusion by confirming the dispersion with newer data, but pointing out t h a t broadening of the main sequence can occur due to varying rotation rates among stars. Rotation rates affect lumin-
SOLAR VARIABILITY
osity. In the case of Praesepe, Smith concludes t h a t a normal spread of uniform rotations (as in solid rotators) ammag stars would account for less t h a n half the observed luminosity spread. However, nonuniform rotations (fast-rotating cores) of the type hypothesized by Dicke could be invoked to account for all the observed spread. We conclude here that, in view of the controversial nature of Dicke's model, the luminosity spread in Praesepe is still unaccounted for, and may indicate luminosity variations. Smith (1973) makes the further interesting point t h a t the scatter of luminosities in Praesepe is inconsistent with the sudden-mixing models t h a t call for 6m.y. dim periods and much longer bright periods. Instead, the scatter is more uniform and would be consistent with more sinusoidal variations in luminosity among individual stars. In summary, the present case against solar luminosity variations is certainly no stronger than the case for it. Neutrinorelated sudden-mixing models do not provide the time durations t h a t are consistent with terrestrial or Praesepe evidence, and they invoke purely ad hoc choices of mixing rates inside the sun. Further modified models are called for. Given the present state of flux in solar models, it seems most prudent simply to say t h a t present-day solar theory and observations suggest intermittent cold periods, including the present cold periods observed on earth and Mars. Indeed, Dilke and Gough (1973) have already proposed a solar periodicity of 250m.y. on grounds of identification with terrestrial climate cycles, and Opik (1973) has claimed a theoretical estimate of solar periodicity of about 600m.y., based on his own model of the sun. V I . I~ECOI~CILING INTERPLANETARY PALEOCLIMATIC TIMESCALES
The remaining avenue for disproving the idea t h a t solar variations cause simultaneous planetary climate variations is to show t h a t the time-scales of observed or suspected variations are incompatible.
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Here, the hypothesis faces greatest vulnerability because a superficial reading of the most popular published interpretations of the hitherto independent lines of evidence reveals some inconsistencies: 1. The sun is said to be cool now and stay cool for about 6m.y. (sudden-mixing model) or longer at unknown intervals. 2. The earth appears to be cool now and to have been cool for about 50m.y., to have been warmer roughly 150m.y. ago, and to have had cool episodes roughly 260 and 650m.y. ago. 3. Mars is cool now. The Martian timescale is extremely uncertain, although H a r t m a n n (1973a) derived a "best" estimate of the end of the last high-erosion interval on Mars of roughly 600m.y. (factor three uncertainty). Using the same best estimate of timescale, H a r t m a n n (1973b) has suggested t h a t some Martian river flows have occurred in the last 300m.y. At first glance, one is tempted to argue t h a t the timescales of cool intervals on the sun are much shorter than permissible to account for terrestrial cold intervals, and t h a t cool intervals on Mars are too long to be correlated with the sun or earth. However, it is just as instructive to consider the state of uncertainty of all these results, and to see whether a synthesis can be achieved by considering t h e m not independently. For example, the 6m.y. cool period of the sun is only the most currently popular result of the simplest model of mixing: a sudden mixing. I t is not completely satisfying. Cameron suggests t h a t slower mixing and longer cool periods might occur, Opik argues for rather unpredictable "flickering" superimposed on this due to not-easily-tractable convection processes, and the Praesepe data remind us t h a t empirical evidence may be at hand for luminosity excursions not wholly accounted for by any of the theoretical models. As for the Martian timescales, the ages derived by H a r t m a n n depend on an assumed cratering rate and are the shortest of a range of ages permitted by rough cratering rate estimates based on statistical dynamic theory (Wetherill, 1973, private
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communication). The oldest Martian ages permitted on this view are about five times those quoted and occur if comets are the primary source of Martian cratering. Chapman (1974, in press) has concluded t h a t only one erosive episode occurred on Mars, probably 1 to 2b.y. ago. Should these old ages be proven correct, the synchronism of earth and Mars climatic changes could probably be ruled out and solar variations as the prime cause be disproven. However, a qualifier is in order on the use of statistical dynamical meteoritic theories to estimate Martian cratering rates and ages. We know t ha t a substantial percentage of our own meteorites result from a small number of specific parentbody fragmentation events within the last l09 years. Thus, we are at the mercy of small-number statstics; it is always conceivable t h a t the Martian cratering rate is man y times different from t ha t predicted by statistical models. This could be the case if a major fragmentation in the last few 108yr produced a family of Mars-crossing asteroid fragments t h a t do not cross the earth's orbit. In such a case, the cratering rate in the last few 10Syr could be anomalously high on Mars (a "spike" with some 10Syr decay time), and features be younger than suspected. I t is unlikely t h a t features could be man y times older t ha n suspected, due to such a mechanism, because the high cratering rate "spikes" due to collisions decay back to a normal background rate of meteorite impacts ; the rate should never drop m a ny times below normal. Alternative hypotheses on the cause of Martian climatic variations face an even greater difficulty with timescale than the solar variation hypothesis. The most prominent example is Ward's (1973) analysis of obliquity variations in the history of Mars. He concludes t h a t obliquity is now near its median value but has varied widely in the past, causing greater possible polar heating and strongly affecting the Martian climate. Ward, as well as Sagan, Toon, and Gieraseh (1973) have considered this a possible explanation of the Martian
climate variations, in which case solar variations would be irrelevant. The proble m is t h a t obliquity variations occur on a 1.2m.y. period, implying a timescale for Martian river history of 106yr. This conflicts with the stratigraphic timescales proposed by investigators of Martian geology. Median river ages appear to compare to ages of the major volcanic systems of the Tharsis area; major channels do not post-date Tharsis. But as noted above, the youngest currently-acceptable ages for these features are several 108yr, over 100 times as ancient as obliquity variations alone would suggest. To put it another way, finite numbers of distinct craters overlie features t hat should be only some 106yr old on the obliquity hypothesis, and it is difficult to imagine such craters forming in such short time periods (Fig. 2). V I I . CONCLUSIONS : A WORKING HYPOTHESIS
To form a working hypothesis t h a t solar variations are the dominant, but not the only, cause of climatic variations on the earth and Mars, we need only bring the three timescales into agreement. Terrestrial data is most reliable for this purpose. Such a working hypothesis thus states : 1. Superimposed on any secular change in solar luminosity are variations occurring on a time-scale of several l08 yr and having an amplitude of 7 to 35% in total bolometric luminosity. 2. The sun's luminosity has been reduced to 0.65 to 0.93 of its normal values, the range suggested from solar, ice age, and Praesepe data, for the last 6-50m.y. 3. (~pik's slow "flickering" probably occurs on various timescales, but it is not possible to distinguish it in the planetary records from other variable effects, such as obliquity changes, changes in ocean current patterns, continental drift, etc. Vestigial Martian river activity might be associated with some of Ward's recent obliquity cycles. 4. About 150m.y. ago, the sun was nearer its normal or peak luminosity, and Mars and earth were warmer than today.
SOLAR VARIABILITY
Fro. 2. E x a m p l e of a c r a t e r ( d i a m e t e r a b o u t 2.5kin) o v e r l a p p i n g d e p o s i t s i n a M a r t i a n c h a n n e l s y s t e m . T h i s is a n a r g u m e n t for cons i d e r a b l e age of t h e s y s t e m , Vallis M a n g a l a , n e a r 151 °, - 7 °. T h e c r a t e r a p p e a r s n o t to postd a t e t h e m o s t r e c e n t , c e n t r a l c h a n n e l , a n d is t h u s a n e x a m p l e of e v i d e n c e for long t i m e i n t e r v a l s b e t w e e n episodes of fluvial a c t i v i t y .
Some fluvial activity may have occurred on Mars at this time. 5. About 260m.y. ago, the sun, earth, and Mars were again cooler than normal, a result suggested b y all three records in Fig. 1. 6. About 500m.y. ago, for an interval of several 10Sm.y., the sun was near its peak luminosity. Higher luminosities m a y have been reached in this period than at any subsequent time, according to the sulphur isotope and evaporite data in Fig. 1. The ancient high rates of Martian erosion detected b y Hartmann (1973a), Soderblom et al. (1973), and Chapman (1974), may have occurred at, or prior to, this warm period.
309
7. Earlier cold and warm cycles occurred, including a probable cold period around 660m.y. ago suggested b y all there records in Fig. 1. 8. The Martian arroyos, or river-like channels, represent episodic vestigial fluvial activity (Fig. 2) that occurred after even higher rates of erosion ceased. This statement is independent of the solar data, and rests only on the Martian crater-count and morphological evidence that the channels saw activity more recently than the end of the high-erosion period (Hartmann, 1973a; Chapman, 1974; Soderblom, 1974). As already emphasized by Fowler (1973), Cameron (1973), Sagan, Toon and Gierasch (1973), and Hartmann (1973b), this working hypothesis has several extraordinary consequences and is worthy of further efforts towards disproof or confirmation. Among the consequences are: 1. Planetary climate changes are primarily controlled by solar changes. These planetary changes are not the trivial grey-body changes that might be expected, but are catastrophic "flips" from one equilibrium state to another through advective instabilities (Sagan, Toon, and Gierasch, 1973). These reversible effects deserve further theoretical attention to determine their magnitudes for planetary atmospheres composed of various constituents, representing different planets and different evolutionary times. 2. Speciation and biological evolution on earth may be driven by these solarinduced climatic changes. Cameron (1973) assuming the 6-m.y. time-scale, suggests that the recent decline in temperature may have driven the natural selection of intelligence in man, and Hartmann (1973b), assuming a longer timescale more compatible with Martian evidence, suggests that it caused the transition from dominance of large cold-blooded reptiles to warm-blooded mammals. Further analysis of paleontological evidence is warranted. 3. The geologic tenet, "uniformitarianism," m a y have to be modified due to planetological progress. The present is not necessarily the key to the past if solar
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WILLIAM K. HARTMANN
variations cause the main climate variations on planets. 4. If planets commonly "flip" from states with liquid volatiles to arid and/or frozen conditions, as Mars appears to have done, the evolution of substantial, H20-based intelligent life-forms on planets around most stars may be hindered, even though speciation of more primitive lifeforms could be encouraged. Periodic 10S-yr disappearances of liquid water would not favor evolution of large, intelligent beasts, a process which took more than 500m.y. on earth. This could be a factor in our c u r r e n t loneliness in the universe. Note added in proof: Additional data are in accord with this paper's hypothesis that the sun ma y be driving simultaneous climatic changes on all the planets. In a paper being published simultaneously (J. Geophys. Res., in press), I have described Martian arroyo crater counts that imply ages of the order 10 s yr (not the 10~ yr t h a t would be implied by climate variations driven only by Ward's obliquity changes). The hypothesis that speciation is governed by major climatic change is supported by Van Valen and Sloan (1972, Abstr. 24th Internat. Geol. Cong.), who report Montana fossil sequences that show dinosaur extinction due to faunal changes caused by cooling winters in an "environmental c ha ng e. . . p ro b ab l y world-wide" about 60 m.y. ago. Newell (1972, Sei. Amer., June, p. 54) gives a nontechnical example of the appeal to plate tectonics alone to explain "episodes of sweeping mass extinctions (that) simultaneously affected such disparate organisms as ammonites at sea and dinosaurs ashore," suggesting "simultaneous world-wide evolutions." Such data are more readily explainable if solar variations on a timescale of some l0 s yr produce climatic instabilities. Recent papers grappling with the neutrino problem continue to suggest room for improvement in traditional solar models.
ACKNOWLEDGMENTS The writer thanks various colleagues, especially A. Cameron, C. Chapman, G. Hartmann, F. Herbert, C. Sagan, L. Soderblom, and an anonymous referee for discussions. The writer also also thanks E. Anders for encouragement. This work was supported by private funds and through the Planetary Science Institute of Science Applications, Inc.
REFERENCES BUDYKO, M. I. ~970). Comments on " A global climatic model~Sased on the energy balance of the earth-atmosphere system." J. Appl. Met. 9, 310. CAMERSON, A. G. W. (1973). Major variations in solar luminosity? Rev. Geophys. Space Phys. l l , 505. C~ma~, M. H. (1974). The role of lava erosion in the formation of lunar and Martian channels. Icarus 22, 1-23. C H K e Z ~ , C. R. (1974). Cratering on Mars, I. Cratering and obliteration history. Icarus 22, 272-291. D/mwIN, C. R. (1897). "The Structure and Distribution of Coral Reefs." D. Appleton, New York. DEMARQUE, P., MENGEL, J., A~D SWEIGERT, A. (1973). Solar rotation and the neutrino flux. Nature Phys. Sci. 246, 33. DILKE, F., AND GOUGH, D. (1972). The solar spoon. Nature 240, 262. FENYVES, E., JANI, Y., SozoKI, K., AND LEE, C. (1973). The solar neutrino puzzle and the temperature history of the earth. I n "Proe. 6th Texas Symp. on Rel. Astrophys.", N.Y. Acad. Sci., in press. FOWLER, W. A. (1973). " W h a t SNU? The Case of the Missing Solar Neutrinos." Cal. Tech. preprint OAP-339. GRIBBIN, J. (1973). Planetary alignments, solar activity, and climatic change. Nature 246, 453. H~mT~ANN, W. K. (1973a). Martian cratering. I V : Mariner 9 initial analysis of cratering chronology. J G R 78, 4096. H ~ A N Z % W. K. (1973b). Martian surface history. Paper presented at International Mars Colloquium, Pasadena, Nov. 30, 1973. HOLSER, M., AND KAPLAI~, I. (1966). Isotope geochemistry of sedimentary sulfates. Chemical Geology l, 93. JOHNSON, H. L. (1952). Praesepe: magnitudes and colors. Astrophys. J. 116, 640. KUMMEL, B. (1970). " H i s t o r y of the E a r t h . " W. Freeman, San Francisco. MEYERHOFF, A. A. (1970). Continental drift: Implications ofpaleomagnetic studies, meteorology, physical oceanography, and climatology. J. Geology 1, 78. MEYERHOFF, A., AND TEICHERT, C. (1971). Continental drift. I I I : Late Paleozoic glacial centers, and Devonian-Eocene coal distribution. J. Geol. 79, 285. MILTOn, D. (1973). W at er and processes of degradation in the Martian landscape. J G R 78, 4037.
SOLAR VARIABILITY MImI~AY, B., WARD, W., and YEUNC, S. (1973). Periodic isolation variations on Mars. Science 180, 638. -" ~3PIK, E. J. (1958). Solar variability and paleoclimatic changes. I r i s h A . J . 5, 97. 0PIK, E. J. (1965). Climatic change in cosmic perspective. I c a r u s 4, 289. OI'IK, E. J. (1973). Terrestrial climate and solar evolution. I r i s h A . J . 10, 295. SAGAN, C., AND MULLEN, G. (1972). E a r t h and Mars: evolution of atmospheres and surface temperatures. Science 177, 52. SAGAN, C., Too~, O., A~D GIERASCH, P. {1973). Climatic change on Mars. Science 181, 1045, SAGAN, C., AND YOUNG, A. (1973). Solar neut-
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rinos, Martian rivers, and Praesepe. N a t u r e 243, 459. SCHUM~t, S. A. (1974). Martian "channels." I c a r u s 22, 371-384. SODERBLOI~I,L. A., CONDIT, C. D., WEST, R. A., HERMAN, B. M., AND KREIDLER, T. J. (1974). Martian planetwide crater distributions: Implications for geologic history and surface processes. I c a r u s 22, 239-263. SlvilrI~, R. C. (1973). Praesepe and the solar neutrino problem. _Nature 244, 560. WARD, WIImlA~ R. (1973). Large scale variations in the obliquity of Mars. Science 181, 260. WYLLIE, P. J. (1971). "The Dynamic E a r t h . " Wiley, New York.
Martian and Terrestrial Paleoclimatology: Relevance of Solar Variability W I L L I A M K. HARTMANN Planetary Science Institute, 252 W. I n a Road, Suite D, Tucson, Arizona 85704 R e c e i v e d : F e b r u a r y 13, 1974; r e v i s e d M a r c h 27, 1974 E v i d e n c e for p a s t M a r t i a n r i v e r s is p e r h a p s t h e m o s t p u z z l i n g a n d i n c o n s i s t e n t p r e s e n t p l a n e t a r y p r o b l e m , conflicting as it does w i t h c u r r e n t c o n d i t i o n s o n t h e p l a n e t . T h i s p a p e r e m p h a s i z e s t h e s i m i l a r p u z z l i n g e v i d e n c e t h a t t h e E a r t h , as Well as Mars, was w a r m e r in t h e p a s t . N e u t r i n o e v i d e n c e raises t h e s u g g e s t i o n t h a t t h e s u n was also " w a r m e r " in t h e p a s t . A h y p o t h e t i c a l cause reconciling all effects is episodic c h a n g e in solar l u m i n o s i t y o n a t i m e - s c a l e of a few h u n d r e d m i l l i o n years. T h i s p a p e r o u t l i n e s r e q u i r e m e n t s a n d c o n s e q u e n c e s of s u c h a w o r k i n g h y p o t h e s i s , w i t h p r o a n d c o n a r g u m e n t s . I t is i m p o r t a n t to t r y to p r o v e or disp r o v e t h i s h y p o t h e s i s b e c a u s e it h a s r a d i c a l i m p l i c a t i o n s for c u r r e n t science. F o r e x a m p l e , it suggests (1) c l i m a t e s of all t h e p l a n e t s h a v e b e e n m a r k e d l y a l t e r e d s i m u l t a n e o u s l y b y solar c h a n g e s , s o m e t i m e s c a t a s t r o p h i c a l l y ; (2) solar c h a n g e s h a v e b e e n a d o m i n a n t " f o r c i n g f u n c t i o n " d r i v i n g biological s p e c u l a t i o n a n d evol u t i o n o n E a r t h ; (3) t h e c o n c e p t of geologic u n i f o r m i t a r i a n i s m is s o m e w h a t m o d i f i e d b y cosmic v a r i a b l e s ; (4) e v o l u t i o n o f w a t e r - u t i l i z i n g i n t e l l i g e n t c r e a t u r e s n e a r o t h e r s t a r s m a y b e less likely t h a n h a s b e e n t h o u g h t , d u e to catast r o p h i c p l a n e t a r y c l i m a t e r e a c t i o n s to c h a n g e in stellar l u m i n o s i t y .
l . ]NATURE OF THE PROBLEM
Mariner 9 has impressed a curious observational fact on scientists: Mars, a planet noted for extreme aridity, has arroyo-like features which have not been adequately explained other than by flowing water (Milton, 1973; Sagan, Toon and Gierasch, 1973; Carr, 1974; Schumm, 1974). Braiding and other deposition patterns observed at resolutions of a few hundred meters give strong evidence for water flow. While some arroyos appear to spring from thermokarst topography, suggesting local melting of permafrost and thus internal sources of heat and water, systems of minor sinuous arroyos exist, particularly in equatorial regions. Some major arroyos have secondary and tertiary tributary systems. These latter features suggest a widespread, probably atmospheric, mechanism to deliver liquid water to the surface. Further, H a r t m a n n (1973a) gives evidence for a much higher past Martian erosion rate (this may or may Copyright © 1974by AcademicPress, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
not be only the last cycle of an episodic Martian erosion history). At recent conferences the consequences of the "rivers" have been little discussed. This is perhaps because of their radical implications and not simply because of lack of tractable investigation, since geomorphological analyses, sinuosity indices, crater counts in river floors and crater counts in background regions are all underway in various institutions. In this paper, the nature of the problem is assumed to lie in four assertions: 1. The Martian arroyos exist and imply t h a t Mars was wetter and warmer in the past. 2. Simultaneously with this discovery, a deficiency in solar neutrinos has been found, indirectly implying t h a t the sun was more luminous in the past than now. 3. A large body of geological literature indicates t h a t the earth's climate is "normally" warmer than in the present. 4. A small increase in solar luminosity in the past or future would convert Mars 301
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to a more favorable climate for liquid water, by releasing more C02 and increasing advective heat transport to the poles (Sagan, Toon, and Gierasch, 1973). Assertions (1) and (4) will not be further discussed, as they are dealt with in the papers already mentioned. Some discussion of recent debate about assertions (2) and (3) will be given, and in particular, a review will be given of the terrestrial geological evidence for climatic changes driven by solar changes, because this portion of the problem has not been advanced in the recent Martian literature. A working hypothesis of solar variation is developed by adopting terrestrial and Martian observations, and the paper will discuss whether this hypothesis can be firmly rejected on the basis of present data.
:[I. HISTORICAL FACTORS IN THE QUESTION OF SOLAR VARIABILITY
The problem of solar variations has been bound up in a special way with the geological problem of plate tectonics or "continental drift." As is well-known (see Wyllie, 1971), the first half of this century saw a raging debate on this problem. One of the empirical facts was a set of geological data showing such curiosities as ancient widespread glaciation in Africa and northern South America and ancient palms, figs, and corals in locations such as Alaska, Greenland, and Spitzbergen Island. Of the two schools, "drifters" asserted t h a t these data showed the continents had moved in latitude, while some "fixists" asserted t h a t they showed t h a t solar luminosity had varied in such a way as to cause world-wide climate changes. The compelling evidence in the mid-1960's for continental and sea-floor rifting and plate motion led to a widespread assumption t h a t the paleoclimatic data had been adequately explained. However, several analysts have indicated t h a t the plate tectonic theory in itself is inadequate to explain all paleoclimatic data, as will be discussed. This paragraph is intended simply to warn readers t h a t
the discovery of plate tectonics, though irrelevant to solar variability, has for historical reasons had the sociological effect of "suppressing" discussion of the latter during the last ten years. III. REVIEW OF ()PIK'S PALEOCLIMATIC THEORY
One of the analysts of terrestrial paleoclimatic data in pre-Mariner years deserves special attention here because of his direct concern with planetary problems. This is E. J. 0pik, whose work over m a n y decades has shown remarkable prescience and intuition in the light of recent results. 0pik (1958) reviewed paleoclimatic data and concluded : " I t seems t h a t variations in the amount of heat received by the earth as a whole can only account for the major variations in past c l i m a t e s . . . " " F o r a long time there were doubts about the globe-wide character of the cold periods. Scientists were more inclined to consider them of hemispherical extent, taking place alternately in the northern and southern hemispheres and caused by minor changes in the eccentricity of the orbit and obliquity of the axes of the earth (also an early and current attempt to explain the Martian features--WKH). (0pik) showed t h a t this cause is inadequate. Only a simultaneous decrease of solar heat all over the globe can lead to such results, in which case the temperature will decrease everywhere." "All doubts in this respect have been removed by the determination of 'fossil' temperatures of the surface of tropical oceans, based on Urey's method of oxygen isotopes . . . . " Reviewing such data and geologic reports, 0pik (1958) presented a table of geologic observations which implied recurrent, dated cold waves every 250 m.y., including the present time and traced by geologic data back to 10O0m.y. ago. He asserted at least three older undated ice ages occurred. Theoretically, 0pik calculated
SOLAR VARIABILITY
t h a t a 9% higher rate of solar radiation would account for an increased global average temperature of about 8°C, accounting for the observations• A drop of 5% in present solar luminosity, according to this calculation, would drop the temperature b y about 6°K, causing the very recent ice ages when glaciers advanced from the poles to low latitudes• (Budyko, 1970, refers to two additional models t h a t predict complete glaciation of the earth with a solar luminosity only 2 to 5% below the present value•) I n the same 1958 paper, ()pik hypothesized t h a t these changes result from disturbances in radiation transfer rate in the sun's core, causing t e m p o r a r y expansions of the sun, lowering surface temperature, and decreasing luminosity. The durations of low-luminosity would equal the H e l m hol t z- K el vi n contraction time for the sun to re-adjust its structure• This is about 6m.y., taken by ()pik to be consistent with the duration of the "cool periods" noted in earth history. Opik estimated the time between "cool periods" to be "several hundred million years," consistent with the geologic record. As shown below, these ideas are quite consistent with recent observations. ~pik (1965) updated this model, producing exactly the same basic results, and additionally calling for a "flickering" of the sun due to convective processes in the outer sun, not amenable to a detailed, predictive theory. The "flickers" were advanced to account for fine structure in recent glacial paleoclimatology, and could cause, for example, " a rapid rise in solar luminosity, followed by a gradual decline. In a recent note Opik (1973) reiterated his belief in the recurrent variations of terrestrial climate driven b y solar coolings separated b y intervals theoretically estimated at 600m.y. and observationally estimated at 250m.y. He commented t h a t the recent neutrino work "agrees completely with ~pik's conclusions . . ." I V . TERRESTRIAL EVIDENCE OF RECURRENT CLIMATIC CHANGES
The geological literature abounds with evidence t h a t the earth was once warmer
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t h a n at present, just as Mars appears to have been• This evidence has been underemphasized in recent literature on the Mars problem. Furthermore, it causes some concern among traditional geologists• For example, Kummel (1970) states in his detailed t e x t on historical geology: " T h e doctrine of uniformitarianism has been such a strong guiding principle in the reconstruction of the earth's history t h a t it m a y come as a surprise to state t h a t the world t o d a y is in a very abnormal condition . . . . The world's climate at the m o m e n t . . . is quite exceptional, for the 'normal' climate of the geologic past appears to have varied from moderate to warm." Precisely the same could be said of Mars. Figure 1 presents some examples of the terrestrial data in question• The top record is after (~pik's, Kummel's (1970, p. 542), and Dunbar's (1963) figures and tables, based on paleontological and geological data. This record is qualitative, in t h a t no absolute temperature measures are made apart from " w a r m e r " or "colder" than present• The essential question about the paleontological and geological data is whether continental drift can remove all the discrepancies. For example, much evidence for a Permian cold period about 260m.y. ago comes from glacial evidence in Africa, South America, and India, at sites presently located near latitudes - 2 0 °, - 1 0 ° and +10 °. However, utilizing a widely accepted plate tectonic reconstruction (Dietz and Holden, 1970) we find these areas in the Permian were near - 6 5 ° , - 3 5 ° , and - 7 0 ° , respectively, so t h a t the glaciation does not appear so anomalous. Thus, uncritical listing of fossil evidence for low-latitude glaciation or high-latitude tropical plants is not sufficient to establish world-wide climatic variations. The writer has thus gone through a list or more t han two dozen reported fossil flora and fauna (Kummel, 1970; Dunbar, 1963) and reconstructed the sites' original latitudes from Dietz and Holden (1970). About 25 of the sites involve fossil corals.
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WILLIAM K. HARTMANN
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FIo. l. Example of three types of published paleoclimatie terrestrial data, all suggesting major climatic fluctuations with several 10Syr timescales. Black arrows {bottom) show suggested major cold periods; open arrows (top) show suggested major warm periods. Other less marked maxima and minima m ay reflect nonglobal climate variations, depending on technique used. See text for discussion and sources.
Corals now have a global distribution symmetric with the equator, primarily limited to latitudes ±30 °, dependent on water temperature and coastal conditions (originally shown by Darwin, 1897). About 70% of these coral cases are brought by continental drift to original positions within the 30 ° latitude range, and about 80% are brought within a 35 ° range. In other words, most of the problem is solved b y continental drift, but some marked anomalies remain. For example, Cretaceous corals are reported in Argentina at a site which should have been about 49 ° south of the equator, and Triassic corals are reported in Alaska when it should have been about 60 ° north of the equator. Cretaceous figs and palms in Greenland come from a site estimated to have been near 42°N. Tertiary figs and magnolias are reported in Alaska at sites reconstructed to lie near 70°N. All these anomalies suggest t h a t some other factor besides continental drift has affected paleoclimatology, and has caused the earth to be globally warmer in the past t h a n at present, as widely reported in geological literature. The middle graph in Fig. 1 is quantitative and serves as an example of the isotopic data. I t is a measure of sulphur isotope ratios based on a world wide inventory of sulphate sediments by Holser
and Kaplan (1966). Here, m any sources of fine structure could be present, such as changes in sediment delivery rates of ancient rivers emptying into the ocean, bacteriological activity, and mixing of bottom sediments by currents. In spite of all these local sources of fine structure which might wipe out long term trends, Holser and Kaplan find, " F o r sulphates of oceanic origin, variation with geological age is so large t h a t it nearly masks all other factors." The bottom record in Fig. 1 is t h a t of Meyerhoff (1970) and Meyerhoff and Teichert (1971) who made a comprehensive inventory of evaporites, desert eolian deposits, and glacial tillite beds from around the world, and produced maps of them for different geologic periods. Latitudinal belts were found, and as Meyerhoff notes, "When the evaporite beds have the greatest latitudinal spread (up to 125 °) the earth must have been very warm . . . . The great fluctuation of the widths of these belts illustrates t h e . . , time-related pattern: specifically, the average world temperature has changed considerably during earth history, and these changes are epi-
SOLARVARIABILITY sodic; ultimately a periodicity may be established." Meyerhoff and Teichert (1971) confirm this and stress t h a t the widths of these climatic belts are not constant: " . . . average earth temperature (or climate zonation)--not latitude-is a very critical element in determining whether or not glaciation will take place." The graph produced by this last study is semiquantitative; the original ordinate is the width of the evaporite zone in degrees. This can be called semiquantitative, since the measured width depends to some extent on the observer's judgement of the data from the given geologic period. Sources of noise or fine structure in this graph include statistical limitations on available field data, and local climatic factors on the ancient continents, such as coastal ranges. The tie between climatic data and continental drift data is illustrated, incidentally, by Meyerhoff's assertion t h a t his climatic bands are so well defined with respect to the present equatorial regions t h a t the continents must have not moved in latitude nearly as much as in the widely accepted models, such as t h a t of Dietz and Holden (1970). To the extent t h a t Meyerhoff's conclusion is correct, the data in the graphs become even stronger in arguing for world-wide climate shifts. In conclusion, it would appear t h a t just as on Mars, geological evidence on earth indicates t h a t the world-wide average climate varied in the past. A warm period occurred (centered) about 150m.y. ago and a still warmer period occurred about 500m.y. ago. Cold periods occurred now, about 260 and about 650m.y. ago. A great variety of data, including historical records, sunspot correlations, 0pik's "flickering" theory, planetary dynamical studies, and the very recent (30 000yr BP) glacial advances and retreats (e.g. Gribbin, 1973), also suggest t h a t periodic or nonperiodic variations of climate occur on all timeseales as implied by 0pik's proposed "flickering," and t h a t the further back in
305
time we go, the greater the excursions of climate. In any ease, noticeable major excursions toward unusually warm or unusually cold conditions occur at intervals on the order of a few hundred million years, and these may not be precisely periodic. In addition to the above data, the evidence for several ("at least t h r e e " - 0pik, 1958) preCambrian epochs of widespread glaciation should be noted, although the dating and spacing of these is very uncertain (R. Siever, private communication).
V. ASTROPHYSICALEVIDENCE Any attempt to disprove solar-induced Martian climatic change by establishing the stability of the earth's paleoclimate is hardly successful, as the above section shows. Rather, the terrestrial evidence is parallel to the Martian evidence in suggesting a cold period now and noticeably warmer periods earlier. Hence the question becomes, can we rule out solar variations as a cause of these phenomena on the basis of astrophysical evidence? Note t h a t we are here discussing episodic variations; Sagan and Mullen (1972) have already reviewed astrophysical evidence t h a t a long-term increase of solar luminosity by 40% occurred over geologic time. We are discussing smaller superimposed oscillations in luminosity, not necessarily sinusoidal. The lack of solar neutrinos, as compared to current theoretical models, and the interpretation t h a t the sun was more luminous in the past, are well-known (Dilke and Gough, 1973; Cameron, 1972) and the applications to the problem of Mars and earth paleoclimates has been more widely discussed than the terrestrial geological evidence itself (Cameron, 1973 ; Fowler, 1973; Sagan and Young, 1973). This material will not be reviewed here except to consider whether any facets of it are strong enough to disprove the idea t h a t solar variations caused the Martian climate changes. Direct evidence for solar variation would be very shaky if the neutrino
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WILLL~M K. HARTMANN
deficiency could be explained without the sun's luminosity being affected. No such models have been entirely successful (Fowler, 1973; Cameron, 1973), although Demarque et al. (1973) assert t h a t a Dicke-type fast-rotating solar core would explain the neutrino observations. This model has a drawback of predicting much lower early solar luminosities than other theories, for example <0.2L e during the first b.y., and about 0.85L o in the Cambrian. This prediction, which would imply early freezing of sea water (Budyko, ]970; Sagan and Mullen, 1972), has not been confirmed by available geologic data. The most popular model is similar to 0pik's original suggestion, and calls for episodic mixing which changes the energy production of the sun. A new equilibrium configuration is then reached in a time governed by Helmholtz-Kelvin contraction. Cameron (1973) has given perhaps the most complete review of the theories and derives a sudden-mixing model calling for a 6m.y. drop in luminosity to 0.7 its normal value. This model is said to be an improvement over other sudden-mixing models, but all sudden-mixing models have the virtue of getting minimum luminosities t h a t readily account for the neutrino deficiency. However, such drastic changes in solar luminosity appear to be inconsistent with the terrestrial paleoclimatic data. ()pik (1958) estimated t h a t a reduction of solar luminosity to only 0.84 the normal value would cause the coldest ice ages; Budyko (1970) and Opik invoke reductions of only a few percent to account for the recent ice ages. If, as in this paper, we t r y to account for the paleoclimatic data by solar theory, we would prefer a model with a longer and less pronounced cool period. Along these lines, Cameron notes evidence by Fenyves et al. (1973), t h a t the earth's ocean temperatures are anomalously low and have been low, not for 6m.y., but for 50m.y. Cameron points out t h a t slow mixing in the sun can be invoked arbitrarily to extend the cool periods. But current versions of such slow-mixing models make it more difficult or impossible to account for the low observed neutrino
flux, since the neutrino minimum becomes less deep. I f solar-type stars vary in brightness, a broadening of the main sequence for G stars could result. Such a broadening has been sought and found by Sagan and Young (1973) in the careful photometry by Johnson (1952) of the Praesepe star cluster. This cluster is said by Sagan and Young to be the optimum cluster for this test. However, Cameron (1973) earlier pointed out t h a t according to the best models of the variability process, the sun changes size, surface temperature, and luminosity in such a way as to move primarily along the main sequence, thus producing very little apparent broadening. Cameron (private communication, 1973) suggests that this detracts from the applicability of the Sagan-Young results. The strongest case against the SaganYoung conclusion is t h a t (1) Johnson had to estimate and discard other sources of dispersion, particularly the known influence of stars with unresolved companions t h a t affect photometry, and (2) Johnson's result was expressed merely as an upper limit on intrinsic, or "cosmic" dispersion of 0.03 magnitude, equivalent to excursions of individual stars of about 0.07 in luminosity), whereas Sagan and Young, re-analyzing Johnson's data, propose actual excursions of 0.15 in luminosity. These arguments are not strong enough to dispose of the Sagan-Young conclusion, because the fact remains t h a t dispersion in the Praesepe main sequence is equivalent to 0.07 to 0.15 excursions in luminosity and is unaccounted for by any known sources of error. Further, as Sagan and Young point out in reversing Cameron's argument, if we empirically detect excursions of 0.07 to 0.15, the true excursions of an individual star are still considerably larger because of the large theoretical component of evolution along the main sequence. Smith (1973) has responded to the SaganYoung conclusion by confirming the dispersion with newer data, but pointing out t h a t broadening of the main sequence can occur due to varying rotation rates among stars. Rotation rates affect lumin-
SOLAR VARIABILITY
osity. In the case of Praesepe, Smith concludes t h a t a normal spread of uniform rotations (as in solid rotators) ammag stars would account for less t h a n half the observed luminosity spread. However, nonuniform rotations (fast-rotating cores) of the type hypothesized by Dicke could be invoked to account for all the observed spread. We conclude here that, in view of the controversial nature of Dicke's model, the luminosity spread in Praesepe is still unaccounted for, and may indicate luminosity variations. Smith (1973) makes the further interesting point t h a t the scatter of luminosities in Praesepe is inconsistent with the sudden-mixing models t h a t call for 6m.y. dim periods and much longer bright periods. Instead, the scatter is more uniform and would be consistent with more sinusoidal variations in luminosity among individual stars. In summary, the present case against solar luminosity variations is certainly no stronger than the case for it. Neutrinorelated sudden-mixing models do not provide the time durations t h a t are consistent with terrestrial or Praesepe evidence, and they invoke purely ad hoc choices of mixing rates inside the sun. Further modified models are called for. Given the present state of flux in solar models, it seems most prudent simply to say t h a t present-day solar theory and observations suggest intermittent cold periods, including the present cold periods observed on earth and Mars. Indeed, Dilke and Gough (1973) have already proposed a solar periodicity of 250m.y. on grounds of identification with terrestrial climate cycles, and Opik (1973) has claimed a theoretical estimate of solar periodicity of about 600m.y., based on his own model of the sun. V I . I~ECOI~CILING INTERPLANETARY PALEOCLIMATIC TIMESCALES
The remaining avenue for disproving the idea t h a t solar variations cause simultaneous planetary climate variations is to show t h a t the time-scales of observed or suspected variations are incompatible.
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Here, the hypothesis faces greatest vulnerability because a superficial reading of the most popular published interpretations of the hitherto independent lines of evidence reveals some inconsistencies: 1. The sun is said to be cool now and stay cool for about 6m.y. (sudden-mixing model) or longer at unknown intervals. 2. The earth appears to be cool now and to have been cool for about 50m.y., to have been warmer roughly 150m.y. ago, and to have had cool episodes roughly 260 and 650m.y. ago. 3. Mars is cool now. The Martian timescale is extremely uncertain, although H a r t m a n n (1973a) derived a "best" estimate of the end of the last high-erosion interval on Mars of roughly 600m.y. (factor three uncertainty). Using the same best estimate of timescale, H a r t m a n n (1973b) has suggested t h a t some Martian river flows have occurred in the last 300m.y. At first glance, one is tempted to argue t h a t the timescales of cool intervals on the sun are much shorter than permissible to account for terrestrial cold intervals, and t h a t cool intervals on Mars are too long to be correlated with the sun or earth. However, it is just as instructive to consider the state of uncertainty of all these results, and to see whether a synthesis can be achieved by considering t h e m not independently. For example, the 6m.y. cool period of the sun is only the most currently popular result of the simplest model of mixing: a sudden mixing. I t is not completely satisfying. Cameron suggests t h a t slower mixing and longer cool periods might occur, Opik argues for rather unpredictable "flickering" superimposed on this due to not-easily-tractable convection processes, and the Praesepe data remind us t h a t empirical evidence may be at hand for luminosity excursions not wholly accounted for by any of the theoretical models. As for the Martian timescales, the ages derived by H a r t m a n n depend on an assumed cratering rate and are the shortest of a range of ages permitted by rough cratering rate estimates based on statistical dynamic theory (Wetherill, 1973, private
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communication). The oldest Martian ages permitted on this view are about five times those quoted and occur if comets are the primary source of Martian cratering. Chapman (1974, in press) has concluded t h a t only one erosive episode occurred on Mars, probably 1 to 2b.y. ago. Should these old ages be proven correct, the synchronism of earth and Mars climatic changes could probably be ruled out and solar variations as the prime cause be disproven. However, a qualifier is in order on the use of statistical dynamical meteoritic theories to estimate Martian cratering rates and ages. We know t ha t a substantial percentage of our own meteorites result from a small number of specific parentbody fragmentation events within the last l09 years. Thus, we are at the mercy of small-number statstics; it is always conceivable t h a t the Martian cratering rate is man y times different from t ha t predicted by statistical models. This could be the case if a major fragmentation in the last few 108yr produced a family of Mars-crossing asteroid fragments t h a t do not cross the earth's orbit. In such a case, the cratering rate in the last few 10Syr could be anomalously high on Mars (a "spike" with some 10Syr decay time), and features be younger than suspected. I t is unlikely t h a t features could be man y times older t ha n suspected, due to such a mechanism, because the high cratering rate "spikes" due to collisions decay back to a normal background rate of meteorite impacts ; the rate should never drop m a ny times below normal. Alternative hypotheses on the cause of Martian climatic variations face an even greater difficulty with timescale than the solar variation hypothesis. The most prominent example is Ward's (1973) analysis of obliquity variations in the history of Mars. He concludes t h a t obliquity is now near its median value but has varied widely in the past, causing greater possible polar heating and strongly affecting the Martian climate. Ward, as well as Sagan, Toon, and Gieraseh (1973) have considered this a possible explanation of the Martian
climate variations, in which case solar variations would be irrelevant. The proble m is t h a t obliquity variations occur on a 1.2m.y. period, implying a timescale for Martian river history of 106yr. This conflicts with the stratigraphic timescales proposed by investigators of Martian geology. Median river ages appear to compare to ages of the major volcanic systems of the Tharsis area; major channels do not post-date Tharsis. But as noted above, the youngest currently-acceptable ages for these features are several 108yr, over 100 times as ancient as obliquity variations alone would suggest. To put it another way, finite numbers of distinct craters overlie features t hat should be only some 106yr old on the obliquity hypothesis, and it is difficult to imagine such craters forming in such short time periods (Fig. 2). V I I . CONCLUSIONS : A WORKING HYPOTHESIS
To form a working hypothesis t h a t solar variations are the dominant, but not the only, cause of climatic variations on the earth and Mars, we need only bring the three timescales into agreement. Terrestrial data is most reliable for this purpose. Such a working hypothesis thus states : 1. Superimposed on any secular change in solar luminosity are variations occurring on a time-scale of several l08 yr and having an amplitude of 7 to 35% in total bolometric luminosity. 2. The sun's luminosity has been reduced to 0.65 to 0.93 of its normal values, the range suggested from solar, ice age, and Praesepe data, for the last 6-50m.y. 3. (~pik's slow "flickering" probably occurs on various timescales, but it is not possible to distinguish it in the planetary records from other variable effects, such as obliquity changes, changes in ocean current patterns, continental drift, etc. Vestigial Martian river activity might be associated with some of Ward's recent obliquity cycles. 4. About 150m.y. ago, the sun was nearer its normal or peak luminosity, and Mars and earth were warmer than today.
SOLAR VARIABILITY
Fro. 2. E x a m p l e of a c r a t e r ( d i a m e t e r a b o u t 2.5kin) o v e r l a p p i n g d e p o s i t s i n a M a r t i a n c h a n n e l s y s t e m . T h i s is a n a r g u m e n t for cons i d e r a b l e age of t h e s y s t e m , Vallis M a n g a l a , n e a r 151 °, - 7 °. T h e c r a t e r a p p e a r s n o t to postd a t e t h e m o s t r e c e n t , c e n t r a l c h a n n e l , a n d is t h u s a n e x a m p l e of e v i d e n c e for long t i m e i n t e r v a l s b e t w e e n episodes of fluvial a c t i v i t y .
Some fluvial activity may have occurred on Mars at this time. 5. About 260m.y. ago, the sun, earth, and Mars were again cooler than normal, a result suggested b y all three records in Fig. 1. 6. About 500m.y. ago, for an interval of several 10Sm.y., the sun was near its peak luminosity. Higher luminosities m a y have been reached in this period than at any subsequent time, according to the sulphur isotope and evaporite data in Fig. 1. The ancient high rates of Martian erosion detected b y Hartmann (1973a), Soderblom et al. (1973), and Chapman (1974), may have occurred at, or prior to, this warm period.
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7. Earlier cold and warm cycles occurred, including a probable cold period around 660m.y. ago suggested b y all there records in Fig. 1. 8. The Martian arroyos, or river-like channels, represent episodic vestigial fluvial activity (Fig. 2) that occurred after even higher rates of erosion ceased. This statement is independent of the solar data, and rests only on the Martian crater-count and morphological evidence that the channels saw activity more recently than the end of the high-erosion period (Hartmann, 1973a; Chapman, 1974; Soderblom, 1974). As already emphasized by Fowler (1973), Cameron (1973), Sagan, Toon and Gierasch (1973), and Hartmann (1973b), this working hypothesis has several extraordinary consequences and is worthy of further efforts towards disproof or confirmation. Among the consequences are: 1. Planetary climate changes are primarily controlled by solar changes. These planetary changes are not the trivial grey-body changes that might be expected, but are catastrophic "flips" from one equilibrium state to another through advective instabilities (Sagan, Toon, and Gierasch, 1973). These reversible effects deserve further theoretical attention to determine their magnitudes for planetary atmospheres composed of various constituents, representing different planets and different evolutionary times. 2. Speciation and biological evolution on earth may be driven by these solarinduced climatic changes. Cameron (1973) assuming the 6-m.y. time-scale, suggests that the recent decline in temperature may have driven the natural selection of intelligence in man, and Hartmann (1973b), assuming a longer timescale more compatible with Martian evidence, suggests that it caused the transition from dominance of large cold-blooded reptiles to warm-blooded mammals. Further analysis of paleontological evidence is warranted. 3. The geologic tenet, "uniformitarianism," m a y have to be modified due to planetological progress. The present is not necessarily the key to the past if solar
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variations cause the main climate variations on planets. 4. If planets commonly "flip" from states with liquid volatiles to arid and/or frozen conditions, as Mars appears to have done, the evolution of substantial, H20-based intelligent life-forms on planets around most stars may be hindered, even though speciation of more primitive lifeforms could be encouraged. Periodic 10S-yr disappearances of liquid water would not favor evolution of large, intelligent beasts, a process which took more than 500m.y. on earth. This could be a factor in our c u r r e n t loneliness in the universe. Note added in proof: Additional data are in accord with this paper's hypothesis that the sun ma y be driving simultaneous climatic changes on all the planets. In a paper being published simultaneously (J. Geophys. Res., in press), I have described Martian arroyo crater counts that imply ages of the order 10 s yr (not the 10~ yr t h a t would be implied by climate variations driven only by Ward's obliquity changes). The hypothesis that speciation is governed by major climatic change is supported by Van Valen and Sloan (1972, Abstr. 24th Internat. Geol. Cong.), who report Montana fossil sequences that show dinosaur extinction due to faunal changes caused by cooling winters in an "environmental c ha ng e. . . p ro b ab l y world-wide" about 60 m.y. ago. Newell (1972, Sei. Amer., June, p. 54) gives a nontechnical example of the appeal to plate tectonics alone to explain "episodes of sweeping mass extinctions (that) simultaneously affected such disparate organisms as ammonites at sea and dinosaurs ashore," suggesting "simultaneous world-wide evolutions." Such data are more readily explainable if solar variations on a timescale of some l0 s yr produce climatic instabilities. Recent papers grappling with the neutrino problem continue to suggest room for improvement in traditional solar models.
ACKNOWLEDGMENTS The writer thanks various colleagues, especially A. Cameron, C. Chapman, G. Hartmann, F. Herbert, C. Sagan, L. Soderblom, and an anonymous referee for discussions. The writer also also thanks E. Anders for encouragement. This work was supported by private funds and through the Planetary Science Institute of Science Applications, Inc.
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