The history of Earth’s atmosphere has been one of declining car- bon dioxide abundance from the Archean onward. Geologic evi- dence for extensive or near-global ice coverage – “glaciation” – in the middle Archean (2.9–2.7 billion years ago), and the late Archean/early Proterozoic (2.4–2.2 billion years ago), and the lack of iron carbonates in paleosols are important constraints, as is glaciation in the late Precambrian (Figure 14.8). The decline has been slow because of the buffering effects of the carbon–
silicate cycle, operating on a planet with liquid water and plate tectonics. The two Archean ice ages might, speculatively, have been triggered by rises in oxygen production due to photosynthe- sis (Chapter 17), which would have rapidly destroyed essentially all the methane in the atmosphere (and, by thus removing any protective organic haze, the ammonia too), thereby cooling the Earth. Or one or both might have been caused by the progressive decline in carbon dioxide abundance. Frustratingly, because of the paucity of the rock record throughout the Archean, we may never know.
If Earth did not possess liquid water, erosion of rock would be extraordinarily slow. On Venus, the surface pressure is 90 atmospheres of carbon dioxide. The total amount of carbon dioxide in Venus’ atmosphere is not very different from the total equivalent of carbon dioxide locked up in various forms in Earth’s crust. As discussed in Chapter 15, Venus lost whatever water it had early in its history, forcing all of the carbon dioxide to remain in the atmosphere. The resulting massive greenhouse effect has pushed the surface temperature of Venus to 730 K, much too high for liquid water to exist. Venus is trapped outside the carbonate cycle with no way to rid itself of the carbon dioxide that keeps the surface so warm.
Mars has a thin atmosphere, and its surface is too cold to support an ocean. Evidence (Chapter 15) shows that early Mars had episodes in which liquid water existed on its surface, but the crust of Mars shows no evidence of plate recycling – it is a small planet with little internal heat and hence lethar- gic tectonics. No recycling means that the carbonates formed
from carbon dioxide essentially will never yield the green- house gas again. The liquid water on early Mars would have encouraged weathering, carbonate formation, and decreasing carbon dioxide – until the temperature of the surface got so low that the water froze and the weathering process ground to a halt.
In this “Goldilocks” view, Earth has two unique things that keep the cycle going: (i) liquid water to make carbonates from carbon dioxide, and (ii) a vigorous recycling of the ocean crust, which releases much of the carbon dioxide back into the gas phase. Earth is far enough from the Sun to enable liquid water to exist without catastrophically vaporizing and escaping, the fate of Venus discussed in Chapter 15. Earth also is much larger than Mars, which is too small to have the tectonic recycling needed to keep carbon dioxide in the atmosphere. Nonetheless, the atmospheric supply of carbon dioxide on Earth has continued to decrease slowly over time, from as much as 10 atmospheres in the Hadean or Archean, through to the present value of 0.0003 atmospheres. And, in spite of the increasing output of the Sun, Earth’s climate generally has gotten cooler over time: the first glacial episode for which evidence exists was just after the close of the Archean. In these cold glacial episodes, less rainfall has meant that less carbon dioxide is lost from the atmosphere, allowing build up of the gas to warm the climate again and encouraging faster subsequent loss of carbon dioxide from the atmosphere. Therefore, the overall march toward less CO2and cooler climes appears to be a theme of Earth history up to the present.
Although the story presented above seems tidy, not all issues are resolved. It remains a serious problem that carbon dioxide alone may not have been enough to explain warm temperatures on Earth in the weak glow of the faint early Sun. Explaining warm episodes on Mars, as we show in Chapter 15, exacerbates the problem. Louisiana astronomer D. Whitmire and colleagues have proposed a quite different solution: that the early Sun was not low in luminosity after all. To get around the seemingly sim- ple solar physics that led to the faint-sun problem, they suggest that the Sun was more massive 4 billion years ago and has lost that mass through expulsion of hydrogen gas in the form of a wind.
The present solar wind, a tenuous medium of protons and other ions, is much too weak to remove the mass required to create a brighter early sun, and so, the hypothesis must rely on the notion that the mass loss was much higher earlier in the history of the solar system. The viability of the idea rests on observing such mass loss from stars similar in age to the Archean Sun elsewhere in the galaxy, an observation that currently is very difficult. Nonetheless, the proposal itself reminds us that the keys to understanding the history of Earth lie buried not only in our own planet, but in our planetary neighbors, in the Sun, and in the neighboring galactic regions illuminated by the burning of billions of other suns.
Summary
Earth during the Archean possessed liquid water on its sur- face, a situation no different from that of today. However, the basic physics of nuclear fusion dictates that the Sun was 25% less luminous 3.8 billion year ago than it is today, and hence if all else were the same, the oceans of the Earth should have been frozen over. A number of solutions to this problem have been proposed, including the possibility that the Sun was more massive (implausible), or that Earth’s atmosphere had a larger quantity of “greenhouse gases” at the time. Green- house gases refer to infrared absorbing molecules present in an atmosphere that is more transparent in the optical part of the spectrum than in the infrared. Sunlight arriving at the Earth is greatly diluted in the number of photons per unit area rel- ative to what was emitted at the surface of the Sun. As the photons are absorbed by the ground, they are re-emitted as a much larger quantity of infrared photons corresponding to a lower black-body temperature – a consequence of the second law of thermodynamics. These infrared photons are impeded in their movement outward through the atmosphere as they are absorbed and re-emitted by greenhouse gases. Since the energy coming in per second must balance the energy going out per second, the atmosphere responds to this imbalance between the transparency in the optical and opaqueness in the infrared by making the temperature profile steeper. Hence the surface temperature is elevated relative to the case of no atmosphere or a fully transparent atmosphere. The primary
greenhouse gases today in Earth’s atmosphere are water, car- bon dioxide, and methane. Water is controlled by evaporation and condensation; carbon dioxide is a small fraction of the total carbon that may have existed as carbon dioxide in the past, and so the early Earth’s atmosphere could have had more carbon dioxide to compensate for the fainter Sun. However, minerals that should be in the rock record if CO2had been vastly more abundant are absent, and this may limit the amount of carbon dioxide that can be invoked for the Archean Earth. Instead, other greenhouse gases such as methane might have played an important role. Complicating this is the role of clouds, which can cool or warm the surface depending on the altitude at which they form. The geologic record suggests that at times in the Archean and subsequent eons, the Earth plunged into deep ice ages, indicating either a quite variable Sun, or fluctuations in the amount of greenhouse gas present in the atmosphere over time. Carbon dioxide would have gradually been scrubbed from the atmosphere by the carbon–silicate cycle, ending up as carbonates on the seafloor. However, plate tectonics recycles some of the carbonates back into carbon dioxide, an essen- tial recharging mechanism for the atmosphere without which the Earth might have been much colder throughout its history.
Mars gives us an example of a planet that likely had a thick greenhouse atmosphere early in its history, which it then lost along with its surface environment capable of sustaining liquid water in a stable fashion.
Questions
1. The presence of carbon recycling on Earth, as a buffer against the faint early Sun, and excessive temperatures later, might strike some as a kind of “just right” story, such that few plan- ets other than twins of Earth could sustain life. What other kinds of processes could keep a climate habitable for life?
2. Is there any limit on planets much more massive than Earth sustaining life? Could a rocky body 10 times Earth’s mass sustain life? What might be the problems?
3. What is the dilution factor between the number of photons per unit area at the surface of the Sun and at a shell cor- responding to the radius of the Earth’s orbit? How would
you then calculate the equilibrium black-body temperature corresponding to the energy received per unit area and per time at that distance from the Sun?
4. Do a literature search on the evidence for and against the faint young Sun. How plausible is the possibility that the Sun has lost mass with time? How much mass would it need to lose?
5. What are some of the complexities associated with try- ing to reconstruct the past atmospheric composition in the Archean?
General reading
Kasting J. and Catling, D. 2003. Evolution of a habitable planet.
Annual Review of Astronics and Astrophysomy41, 429–63.
Williams, G. R. 1996.The Molecular Biology of Gaia. Columbia University Press, New York.
References
Falkowski, P., Scholes, R. J., Boyle, E. et al. 2000. The global carbon cycle: a test of our knowledge of Earth as a system, Science290, 291–6.
Haqq-Misra J. D., Domagal-Goldman S. D., Kasting P. J., and Kast- ing J. F. 2008. A revised, hazy methane greenhouse for the early Earth.Astrobiology8, 1127–37.
Houghton, J. T. 1977. The Physics of Atmospheres, 1st edn.
Cambridge University Press, Cambridge, UK.
Kasting, J. F. 1989. Long-term stability of the Earth’s climate.Pale- ogeography, Paleoclimatology, Paleoecology75, 83–95.
Kasting, J. F., and Ackeman, T. P. 1986. Climatic consequences of very high CO2levels in the Earth’s early atmosphere.Science 234, 1383–5.
Kharecha, P., Kasting, J. F., and Siefert, J. L. 2005. A coupled atmosphere–ecosystem model of the early Archean Earth.
Geobiology3, 53–76.
Knauth, L. P. 1992. Origin and diagenesis of cherts: an isotopic persective. InIsotopic Signatures and Sedimentary Records (N. Clauer and S. Chandhuri, eds). Springer-Verlag, Berlin, pp. 123–52.
Nutman, A. P., Mojzsis, S. J., and Friend, C. R. L. 1997. Recog- nition of≥3850 Ma water-lain sediments in Greenland and their significance for the early Archean Earth. Geochimica Cosmochimica Acta61, 2475–84.
Pavlov, A. A., Hurtgen, M. T., Kasting, J. F., and Arthur, M. A.
2003. Methane-rich proterozoic atmosphere? Geology 31, 87–90.
Peixoto, J. P. and Oort, A. H. 1992.Physics of Climate. AIP Press, New York.
Rosing, T., Bird, D. K., Sleep, N. H., and Bjerrum, C. L. 2010. No climate paradox under the faint early Sun.Nature464, 744–7.
Sagan, C. and Chyba, C. 1997. The early faint Sun paradox: organic shielding of ultraviolet-labile greenhouse gases.Science276, 1217–21.
Sheldon N. D. 2006. Precambrian paleosols and atmospheric CO2
levels.Precambrian Research147, 148–55.
Trenberth, K. E., Houghton, J. T., and Meira Filho, L. G. 1996.
The climate system: an overview. InClimate Change 1995:
The Science of Climate Change (J. T. Houghton, L. G.
Meira Filho, B. A. Callander, N. Harris, A. Kattenberg, and K. Maskell, eds). Cambridge University Press, Cambridge, UK, pp. 51–65.
Valley, J. W., Peck, W. H., King, E. M., and Wilde, S. A. 2002. A cool early Earth.Geology30, 351–4.
Whitmire, D. P., Doyle, L. R., Reynolds, R. T., and Matese, J. J.
1995. A slightly more massive young sun as an explanation for warm temperatures on early Mars.Journal of Geophysical Research100, 5457–64.
Wolf E. T. and Toon O. B. 2010. Fractal organic hazes provided an ultraviolet shield for early Earth.Science328, 1266–68.
Young, G. M., von Brunn, V., Gold D. J. C., and Minter, W. E. L.
1998. Earth’s oldest reported glaciation; physical and chemical evidence from the Archean Mozaan Group (2.9 Ga) of South Africa.Journal of Geology106, 523–38.
15
Climate histories of Mars and Venus, and the habitability of planets
Introduction
Earth at the close of the Archean, 2.5 billion years ago, was a world in which life had arisen and plate tectonics dominated the evolution of the crust and the recycling of volatiles. Yet oxygen (O2) still was not prevalent in the atmosphere, which was richer in CO2 than at present. In this last respect, Earth’s atmosphere was somewhat like that of its neighbors, Mars and Venus, which today retain this more primitive kind of atmo- sphere.
Speculations on the nature of Mars and Venus were, prior to the space program, heavily influenced by Earth-centered biases and the poor quality of telescopic observations (Figure 15.1).
Forty years of US and Soviet robotic missions to these two bod- ies changed that thinking drastically. The overall evolutions of Mars and Venus have been quite different from that of Earth, and very different from each other. The ability of the envi- ronment of a planet to veer in a completely different direction from that of its neighbors was not readily appreciated until the eternally hot greenhouse of Venus’ surface and the cold desolation of the Martian climate were revealed by spacecraft instruments.
However, robotic missions also revealed evidence that Mars once had liquid water flowing on its surface. It is tempting, then,
to assume that the early Martian climate was much warmer than it is at present, warm enough perhaps to initiate life on the surface of Mars. However, the difficulty of sustaining a warm Martian atmosphere in the face of the faint-early-sun problem of Chapter 14 remains a daunting puzzle, one that is highly relevant to the broader question of habitable planets beyond our solar system. What is the range of distances from any given star for which liquid water is stable on a planetary surface and life can gain a foothold?
In the temporal sequence that Part III of the book has been following, we stand near the end of the Archean eon. By this point in time, the evolution of Venus and its atmosphere almost certainly had diverged from that of Earth, and Mars was on its way to being a cold, dry world, if it had not already become one. This is the appropriate moment in geologic time, then, to consider how Earth’s neighboring planets diverged so greatly in climate, and to ponder the implications for habitable planets throughout the cosmos. In the following chapter, we consider why Earth became dominated by plate tectonics, but Venus and Mars did not. Understanding this is part of the key to under- standing Earth’s clement climate as discussed in Chapter 14.