The evolution of our Sun has one more consequence for life on Earth. From now to the end of its stable hydrogen-fusing stage, the Sun will continue to increase in luminosity. As it does so, the climate of the Earth will edge closer to the point at which a moist greenhouse is initiated and rapid loss of the Earth’s water ensues, as apparently occurred early in Venus’ history.
A natural delaying tactic is the weathering feedback process described in Chapter 14 wherein, as the brighter Sun warms Earth, more rainfall and more erosion will occur, and hence the carbon dioxide budget of our atmosphere will decrease.
However, a point will come when rising temperatures cannot be buffered by the decreasing amounts of atmospheric carbon dioxide, and rapid loss of Earth’s oceans to space will begin.
Models of the Sun’s luminosity history and the response of Earth’s atmosphere suggest that this crisis will be reached in
1 billion to 2 billion years from now. At that point, if the bio- sphere has not collapsed already from decreasing amounts of atmospheric carbon dioxide, the lack of liquid water will finally kill off all living organisms. On the other hand Mars will enjoy more clement conditions, if enough water and carbon dioxide are stored in the crust to be partially liberated into a thicker atmosphere by the brighter sun.
Life began on Earth some 3.8 billion to 4 billion years ago, and complex eukaryotic cells appear in the fossil record from
2 billion years ago. Therefore, we are more than halfway through the time period during which life, even complex life, can flourish on Earth. Our time here is not forever. After the brightening Sun drives water, and hence life, from Earth, it will continue to shine by hydrogen fusion for another 2 billion to 4 billion years. For those last several billion years of the Sun’s history, Earth’s sur- face might hold a fossil record of its long springtime of clement conditions, during which it teemed with living organisms that eventually looked upward to contemplate the stars.
Summary
Earth’s two neighboring planets are very different from each other in size and climate history. Venus, not too different from the Earth in size, orbits 30% closer to the Sun than does the Earth. One would expect Venus to be hotter than Earth, but it is the enormous atmosphere of carbon dioxide and conse- quently massive greenhouse effect that is most striking about this planet. Indeed, though less sunlight arrives at the surface of Venus than arrives on our home world’s surface, thanks to Venus’ global layer of clouds, the temperature there is 730 K – hot enough to melt lead. Isotopic evidence suggests that Venus was once clement enough to support large amounts of liquid water at or near its surface, water that was lost in an environ- mental crisis precipitated by our sister planet’s proximity to the Sun. When that happened is not known, but it left Venus with no water, a surface too hot to support liquid water and life, and a very different geologic style than the Earth’s plate tectonics.
Venus is an example of what happens to an Earth-like body at the inner edge of the “habitable zone”. Mars, on the other hand, is farther from the Sun than is the Earth, and is ten times smaller. It has a very thick carbon dioxide atmosphere with essentially no greenhouse effect, and a surface that is bom- barded by ultraviolet light and is on average much too cold to
support liquid water. But there is ample evidence from images, spectra, and chemical data, and from orbiters and rovers on the surface, that liquid water existed on the Martian surface and just below the surface in the ancient past. How Mars could have sustained clement conditions early on when the Sun was significantly fainter than today, and for how long such condi- tions existed, remain unsolved problems. If Mars was clement early in its history, life might have begun on the surface and then found refuge in the deep crust, where liquid water is still stable today. The detection of small amounts of methane in Mars’ atmosphere for a period of several years suggests either biological activity or the action of water on carbon dioxide in the deep crust, but the transient nature of the detection makes difficult its eventual follow up by future missions to determine the source. As Venus and Mars define in a coarse way the outer edges of the habitable zone, the continuing evolution of the Sun toward higher and higher luminosities means that, in 1 to 2 billion years, Earth may lose its water and become uninhab- itable. Studying Mars and Venus to understand the limits of habitability will help us to understand how precarious is the habitability of our own planet.
Questions
1. How and where would you search for evidence of past, and present, life on Mars?
2. Can you think of any refugia for carbon-based life on present- day Venus?
3. Speculate on what the evolution of an Earth-mass planet might be if it were placed at the orbit of Mars around the Sun. Would it be better able to retain greenhouse gases than did Mars? What would be the tradeoff between more mass allowing plate tectonics to occur, against the much weaker
amount of sunlight in a Mars-like orbit? Might such a planet start out frozen, become habitable, and avoid the runaway crisis the Earth will face?
4. Sulfur compounds have been suggested as a possible green- house gas for early Mars. What sulfur molecules might be suitable? Would they condense? Could they form a smog that might impede sunlight? What is the difficulty with methane as an additional greenhouse gas?
General reading
Nimmo, F. and McKenzie, D. 1998. Volcanism and tectonics on Venus.Annual Reviews of Earth and Planetary Sciences26 DOI: 10.1146.
Pierrehumbert, R. T. 2010. Principles of Planetary Climate.
Cambridge University Press, Cambridge.
Squyres, S. W. 2006. Roving Mars: Spirit, Opportunity and the Exploration of the Red Planet. Hyperion Press, New York.
References
Ehlmann, B. L. 2010. Diverse aqueous environments during Mars’
first billion years: the emerging view from orbital visible-near infrared spectroscopy.Geochemical News, 142.
Caldeira, K. and Kasting, J. F. 1992. The life span of the biosphere revisited.Nature360, 721–3.
Forget, F. and Pierre Humbert, R. T. 1997. Warming early Mars with carbon dioxide clouds that scatter infrared radiation.Science 278, 1273–6.
Holt, J. W., Safeinili, A., Plaut, J. J.et al.2008. Radar sounding evidence for buried glaciers in the southern mid-latitudes of Mars.Science322, 1235–8.
Kargel, J. S., Baker, V. R., Beg´e, J. E. et al.1995. Evidence of ancient continental glaciation in the Martian northern plains.
Journal of Geophysical Research100, 5351–68.
Kasting, J. F. 1988. Runaway and moist greenhouse atmospheres and the evolution of Earth and Venus.Icarus74, 472–94.
Kasting, J. F. 1991. CO2condensation and the climate of early Mars.
Icarus94, 1–13.
Kasting, J. F. 1997. Planetary atmosphere evolution: do other hab- itable planets exist and can we detect them? InThe Search for Extra-solar Terrestrial Planets. Techniques and Technology (M. Shull, H. Thronsoan and A. Stern, eds). Kluwer, Dor- drecht, pp. 3–24.
Kasting, J. F., Whitmire, D. P., and Reynolds, R. T. 1993. Habitable zones around main sequence stars.Icarus101, 108–28.
Lineweaver, C. H. 2004. Martian life: stuck somewhere between inevitable biochemistry and quirky biology. Microbiology Australia25(1) 20–1.
McKay, C. P. and David, W. L. 1991. Duration of liquid water habitats on early Mars.Icarus90, 214–21.
Mumma, M. J. Villanueva, G. L., Novak, R. E.et al.2009. Strong release of methane on Mars in northern summer 2003.Science 323, 1041–5.
Phillips, R. J. and Hansen, V. L. 1994. Tectonic and magmatic evolu- tion of Venus.Annual Review of Earth and Planetary Sciences 22, 597–654.
Rampino, M. R. and Caldeira, K. 1994. The Goldilocks problem: cli- matic evolution and long-term habitability of terrestrial plan- ets.Annual Reviews of Earth and Planetary Sciences32, 83–
114.
Sackmann, I-J., Boothroyd, A. I., and Kramer, K. E. 1993. Our Sun.
III Present and future.Astrophysics Journal418, 457–68.
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Squyres, S. W. and Knoll, A. H. 2005. Sedimentary rocks at Merid- iani Planum: origin, diagenesis, and implications for life on Mars.Earth and Planetary Science Letters240, 1–10.
Toon, O. B., Segura, T., and Zahnle, K. 2010. The formation of Martian river valleys by impacts.Annual Review of Earth and Planetary Science38, 303–22.
van Thienen, P., Vlaar, N. J., and van den Berg, A. P. 2004. Plate tectonics on the terrestrial planets.Physics of the Earth and Planetary Interiors142, 61–74.
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16
Earth in transition: from the Archean to the Proterozoic
Introduction
The beginning of the Proterozoic eon is set formally by geolo- gists at 2.5 billion years before present. However, the transition between the Archean and the Proterozoic is not a sharp one.
From about 3.2 billion to 2.5 billion years ago, rocks with a modern granitic composition made a widespread appearance in the geologic record. Prior to this time, rocks making up the Archean continents had a composition different from mod- ern granites in several important respects. Beginning around 3.2 billion years ago in what is now Africa, and extending to 2.6 billion years ago on the Canadian shield, large quantities of modern-type granites were produced. We can collect these rocks today and date them by use of radioisotopes. How did the original Archean continents form? Why was there a transition in chemical composition of the rocks roughly halfway through the Archean? What might Earth have been like today if this eruption of new rock types had not occurred? As we see, the transformation wrought on Earth’s primitive continents may have been an inevitable consequence of their increasing cover- age of Earth’s surface.
What might have been inevitable on Earth was apparently difficult or impossible on the other terrestrial planets. No evi- dence for large granitic masses exists on any other planet.
Venus bears two crustal masses that resemble continents, but the details of their geology suggest that they are more similar to primitive Archean continents than to our modern ones and,
even then, the connection is a weak one. Venusian geology, taking place as it did on a planet similar in size and composi- tion to Earth, might teach us about the conditions under which Earth’s tectonic regime couldnotbe achieved.
The formation of continental masses, standing above the level of the seas on an otherwise watery world, had a profound influence on the subsequent history of Earth. The weather- ing of continental granites provides the essential first step in the transport of atmospheric carbon dioxide into the oceans and sequestration as carbonates. The continents as buoyant regions of the crust probably largely determined the pattern of plate tectonics. They modulate the climate and interior heat flow through cycles of merging into single supercontinents and breaking up into dispersed land areas. Unlike the oceanic crust, which is destroyed and recreated on timescales of a few percent of Earth’s history, the continents preserve an ancient record of the geologic and biological history of Earth. Finally, the con- tinents provided a new frontier for colonization by complex life some half-billion years ago, an environment in which the nature of survival differed drastically from that in the sea.
It is in the Archean and its transition to the Proterozoic that we see Earth, once and for all, diverge in its evolution from that of its sister planets and become the planet with which we are familiar. Understanding how this happened is the subject of the present chapter.