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Malْnglt Comfortable t CI.IAPTER 9 MaLúnglt ComfortabLe Runn mg W ater, Temp eratur e Contr ol., arrdsunProtectron The solør system is marked by temperature extremes, from the millions of degrees of the solar corona to the near absolute zero of interplanetary space. The moon has temperature variations of -300"C between lunar night and lunar døy. Although Venus is nearly equal in size and bulk composition to Earth, the Venusian ground surface is 450"C warmer. Mars is 80oC colder. None of these environments are able to host liquid water, which is a critical need for life as we know it. Earth, in contrast, has a "goldilocki' temperature, with evidence for continuous liquid water at the surface for aII of Earth\ history recorded in the rock record. During this time the luminosity of the sun has brightened by about 35% as the sun has consumed iß hydrogen fuel. How did thß equable climate come about? When did it begin? How has it been maintained? Eørthi climate stability is dependent on its volatiles. The volatile budget of a planet depends on its accretion history and bulk composition, the amount of volatile loss to space from impacts and the solar wind, and the cycles of the volatile ele- ments between planetary interior and exterior. Once the core formed a few tens of míllions of years after accretion, Earth would have developed a magnetic field that deflected the solar Fig. 9-0: Seascape photograph by Ansel Adams of Point Sur during a storm. Earth has wind, minimizing its efects on the atmosphere, preventing had a persisent liquid ocean since at least 3.8 Ga. (Photo by Ansel Adams. collection center for creative Photograph¡ university of Arizona, @ The Ansel Adams Publish- volatile loss, and protecting the surface from ionizing radiation ing Rights Trust, with permission) harmful to life. Liquid water was present very early in Earth\ history. Sediments, formed by deposition from water, and pillow basalts, solidified lavas that formed in contact with water are present in the oldest rocks at 3.8 Ga. There is evidence for even U Wate¡ Temperature Control, and the Sun . 251 250 . ChaPter 9 that have ages as There are, of course, no simple answers to these questions. In pre- earlier liquid water from tiny crystøls of zircon characteristic vious chapters we have seen that the habitability of our planet is in part old as 4.i Ga. Climøte stability is a long-standing determined by its nebular heritage that sets its size, orbit, spin, and bulk of Earth\ eYolution. of chemical composition. It is in part determined by the evolution of its A planet's surface temperature depends on the luminosity the star' It also depends interior and crust. As we shall see in this chapter, it also critically de- its star and on the planei\ distance from "greenhouse pow,er" pends on what happened to its volatiles after planetary accretion, and on the reflectivity o¡ it, ,u'¡ot' and on the consisting of more than how they are cycled through planetary processes. of its atmosphere, caused by molecules andVenus ihree atoms;, such as CO2, H2O and CHn' Earth Most Venu- exemPlify the importance of the greenhouse fficL powerful T he P Lanetary Vo lat i [e Budget sian carbon is in the atmosphere as CO, creating a all stored thermal blanket. By contrast, Earth's carbon is nearly For life of any complexity to develop on a planet, abundant liquid water insedimentsascarbonatelnineralsøndorganicresídues.Feeà- the atmosphere must be available. Water is essential for all of life as we know it. Water is backs must have modulated greenhouse gases in a fundamental medium of transport and chemical communication that to maintøin long-term climate stability' The most likely feedback and volccÌnic make cellular processes possible. Living cells are about 70o/o water by is a "tectonic thermostat" that relates subduction high weight. The average human being is 600/o water (watermelon is )907o outgassing of COrto changes in weathering' High COror water). Water's centrality to life is reflected in the stark differences we trriprrotlrà, ,rioræ weathering that releases Ca2+ to the CO'øs see from region to region related to water availability. Given abundant orrorr. This then leads to cooling caused by removal of greater buildup rainfall, we have lush forests teeming with all manner of living organ- CaCOr. Low COror low temperatures permit ß its elf isms. Where there is little rain, we have deserts sparse in life. Where only of cO) from, olíoro rr, causing w ørming' Weathering Hence' snow falls, we have barren ice caps. These contrasts are found on a planet iifturired by plate movements and mountain building' plate tectonics' whose surface is 70o/o covered by liquid water! Earth's clímate reflects linkages among the sun' climøte stability Carbon is also essential for habitability, since it is the central element and. surface biogeochemical cycles, providing depends on for all the organic molecules (those with C-H bonds) of which life is consistent with tiquid water' The tectonic thermostat surface has just made. As we will see later in this chapter, C as CO, is also a pivotal mol- the coexistenæ oi or"on ønd continent' Earth's this is a ecule for climate stabilit¡ and its exact concentration in the atmosphere the right amount of water for this balance' Whether planetary is a fundamental control on surface temperature.T}re carbon cyclelinks hoppi accident or the result of feedbacks in early the carbon in organic molecules, the atmosphere, the oceans, the mantle history remains an unsolved puzzle' and limestone (CaCOr) in a balance that supports both life and the cli- mate that is essential for life. For a habitable planet, the right amounts lntroductton of HrO and CO, are both critical. Given the centrality of HrO and CO' the first requirement for habit- most important to ability is that the planet must have captured enough volatiles, including We have yet to consider the attribute of our planet the surface' sufficient water to make a sizable ocean. The silicate earth as a whole living organisms-a stable climate that permits water on of its surface? contains only small amounts of HrO and COr-about 700 ppm HrO fixes its water supply? What sets the temperature What (0.07 oceans? In a nut- wt.%) and 200 ppm CO, (0.02 wt.%). For HrO these small amounts What permits the lucky coexistence of continents and mean that Earth ended up with only one water molecule out of every shell, what makes our planet habitable? ? 252 . Chapter 9 Water, Temperature Control, and the Sun . 253 matter from which Earth formed. three million that were in the pool of Evîdencefor LLquLdW ater b efore 4.o G a Most of the carbon in the nebula was in the form of methane gas. Earth, one every 3,000 car- however, somehow managed to capture about in Some of the oldest rocks are sedimentary rocks, and most sediments that Earth is very volatile bon atoms. These numbers further illustrate require weathering, transport ,and deposition by liquid water. The evi- fact is that the ratio of depleted relative to the solar nebula. A curious dence for the earliest fossils comes from rocks that are 3.5 billion years ratios ]HzOlCO2in the silicate earth (-3.5) is signifrcantlyhigher than the old. The oldest sediments are those of the 3.8 Ga Isua formation in be that much of in chondrites (HrO/COr) <1.5. One solution would Greenland. These rocks include cherts, carbonates, and banded iron for- have a high HrO/CO, Earth's volatile budget comes from comets, which mations. All of these rocks require liquid water to form, and the same elements in the core. Water and ratio. Or carbon may be one of the light rock types occur in much younger rocks when we know liquid water was of the origin of Earth's atmosphere COrare further pieces tothepuzzle present. The rocks show that water was present at least as earþ as 3.8 Ga. and ocean prior to the beginning ofthe rock record. We are able to push back the presence of liquid water even earlier by as a whole lead to the second The small amounts of volatiles in Earth using evidence from a surprising source, the mineral zircon (ZrSiOn)- budget, the volatiles habitability requirement-given a modest volatile one of the highest-temperature and most stable minerals found in rocks. occurred on Earth, since at the must be concentrated at the surface. This Zircon is very low in abundance (usually less than 0.02o/o of the rock), To contrar¡ the surface a low volatile budget is not at all evident. the but also very common. Virtually all granites and sandstones contain atmosphere, ocean, and proportion of HrO and CO, in the combined some grains of zircon. Zircons are also very stable minerals and are dif- wt.o/o COr. These numbers crust is rather high-7.2 wt. % H2O and t-5 ficult to alter or dissolve. The high chemical fldelity and chemical resis- the planet and are just the reflect - 100 times enrichment relative to total tance of zircons allows them to survive during weathering and sedimen- and climate stability conducive right amounts for a large liquid ocean tarytransport; theyare so robusttheyoften survive even multiple melting to life. At the high temperatures that must have prevailed during the period of the formation of our iron core, extensive melting and active convection would have cycled mantle rocks to the surface, where their HrO and CO, would have been transported to the atmosphere in gas- eous form.
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