CLIMATE HISTORIES OF MARS AND VENUS, AND THE HABITABILITY OF PLANETS In the temporal sequence that Part III of the book has ¡NTRODUCT¡ON 15.1 been following, we stand near the end of the Archean eon. Earth at the close of the Archean,2.5 billion years ago, By this point in time, the evolution of Venus and its atmo- was a world in which life had arisen and plate tectonics sphere almost certainly had diverged from that of Earth, dominated, the evolution of the crust and the recycling of and Mars was on its way to being a cold, dry world, if volatiles. Yet oxygen (Oz) still was not prevalent in the it had not already become one. This is the appropriate atmosphere, which was richer in COz than at present. In moment in geologic time, then, to consider how Earth's this last respect, Earth's atmosphere was somewhat like neighboring planets diverged so greatly in climate, and to that of its neighbors, Mars and Venus, which today retain ponder the implications for habitable planets throughout this more primitive kind of atmosphere. the cosmos. In the following chapter, we consider why Speculations on the nature of Mars and Venus were, Earth became dominated by plate tectonics, but Venus prior to the space program, heavily influenced by Earth- and Mars did not. Understanding this is part of the key centered biases and the poor quality of telescopic observa- to understanding Earth's clement climate as discussed in tions (figure 15.1). Thirty years of U.S. and Soviet robotic chapter 1.4. missions to these two bodies changed that thinking dras- tically. The overall evolutions of Mars and Venus have 15.2 VENUS been quite different from that of Earth, and very differ- ent from each other. The ability of the environment of a 15.2.1 Onigin of Venus' Thick Atmosphene planet to veer in a completely different direction from that of its neighbors was not readily appreciated until the eter- The atmosphere of Venus contains somewhat more nitro- nally hot greenhouse of Venus' surface and the cold deso- gen than does that of the Earth: 3 atmospheres of pressure lation of the Martian climate were revealed by spacecraft instead of 0.8 atmosphere. More striking, however, is the instruments. enormous surface pressure of 90 atmospheres of carbon Flowever, robotic missions also revealed evidence that dioxide. The consequence of Venus'massive atmosphere is Mars once had liquid water flowing on its surface. It is an enormous greenhouse effect: Even though the clouds of tempting, then, to assume that the early Martian climate Venus' upper atmosphere, largely sulfur compounds, re- was much warmer than it is at present, warm enough per- flect much more sunlight away than do the clouds of Earth, haps to initiate life on the surface of Mars. However, the Venus has a surface temperature of 730 K. In other words, difficulty of sustaining a warm Martian atmosphere in the even though the surface ofVenus receives less sunlight than face of the faint-early-sun problem of chapter 14 remains a does the Earth's surface, the temperature at Venus' surface daunting puzzle, one that is highly relevant to the broader is above the melting point of lead. Liquid water is not sta- question of habitable planets beyond our solar system. ble on the surface or anywhere in the atmosphere; gaseous !íhat is the range of distances from any given star for water vapor is only 10 parts per million of the atmosphere. which liquid water is stable on a planetary surface and Oxygen is not abundant either, with a pressure of 0.002 Iife can gain a foothold? atmosphere, one-hundredth that in our atmosphere. PLANETS 174 CLIMATE HISTOFìIES OF MAFìS AND VENUS, AND THE HABITABILITY OF Figune 15.1. Prior to th by hand typically showed telescopic images and ca some as a sign of intellige Lowell intenpneted these illusory features as vast to the parched equatonial deserts, a granden vers the Ari)ona and ialifo¡nia deserts south ând west of his high plateau observatony' retained' Although alternative models How Venus came to this state is still a subject of heated more likely to be proposed (for example, that the high deuterium debate. Venus is almost the same size as Earth, of similar have been from impacting comets), the density (and hence internal composition), and somewhat abundance is a contaminant at present to be the best expla- nearer to the Sun. One clue is the close correspondence water-loss model appears of the amount of carbon dioxide in Venus' atmosphere nation for the deuterium data. have liquid water early in the solar sys- with the amount of carbon dioxide that could be produced If Venus did challenge is to understand how it was from the carbonates and other carbon compounds trapped tem's histor¡ the explanation for the loss today in Earth's crust' If Earth's oceans were to boil awa¡ lost and when. The traditional runaluøy greenhouse,featured in many and the hydrological cycle of rainfall end, recycling of cat- lies in the so-called the solar heating at Venus' distance from bonates into the atmosphere might eventually build up a textbooks. Here, a sufficient amount of initial green- massive carbon dioxide atmosphere on our planet as well' the Sun, coupled with carbon dioxide, leads to an The divergent evolutionary paths that Earth and Venus house heating from water and Heating causes more evaporation of have taken apparently have to do with the lack, or early unstable situation: (because the evaporation rate and Ioss, of large quantities of water from Venus. Direct mea- water from the ocean content in the atmosphere are very surement of Venus'atmosphere fromPioneer Venus entry the total water vapor This higher water content' probes in 1,978 revealed a large abundance of deuterium sensitive to the temperature). temperature through (defined in chapter 2) rclative to light hydrogen in the in turn, increases the atmospheric which in turn causes more water atmosphere of Venus, the ratio of the two being about the greenhouse effect, the atmosphere further' The sys- 150 times that in the oceans of Earth' One interpretation to evaporate, warming leading quickly to the complete of such an overabundance is that large amounts of wa- tem enters a "rrtîaway," ter escaped from Venus early in its history; as the water boiling away oÍ the oceans. of the early history of Venus was lost in gaseous form from the atmosphere' the heav- Very careful modeling a runaway greenhouse was marginal ier deuterium atoms in HDO and D2O (versus H2O) were shows that at the time, 15,2 VENUS 179 for that planet. The reason lies again in the faint-early- transparent to infrared radiation, the amount of water va- sun problem. Although toda¡ Venus receives 1.9 times por drops very steeply. At about 10 km above the surface the amount of sunlight that Earth does at the top of the lies a boundary between the lower atmosphere, the tropo- atmosphere (remember much of this is reflected by Venus' sphere, and the stratosphere above it. This boundary, the clouds), in the earliest period of solar system history the tropopause, is defined by the altitude at which the tem- sunlight that Venus received was only 1,.4 times that re- perature stops falling and begins rising at higher altitude ceived by Earth at present. Below a certain threshold sur- as the air becomes transparent to most infrared radiation, face temperature, the greenhouse effect does not evaporate and some molecules selectively absorb sunlight in the ul- enough water to initiate a runaway. traviolet wavelengths. Above the tropopause, water vapor So how did Venus arrive at its present state? The solu- no longer decreases with increasing altitude; its minimum tion to this puzzle lies in considering the effect of water value is determined by the temperature at the tropopause. vapor on the entire atmosphere, as shown in figure 15.2. In Earth's atmosphere toda¡ the dropoff in temperature On Earth toda¡ because the temperature drops rapidly with height leads to a very sharp decline in water vapor with altituCe as the atmosphere thins and becomes more with altitude. The water vapor condenses as clouds and these eventually are lost as rain. The Earth's stratosphere is extremely dry today, about as dry as the present bulk atmosphere of Venus. What water vapor does exist in the stratosphere is subject to being broken apart by ultraviolet photons from the Sun to form oxygen (Oz) and hydrogen; É because hydrogen is a light molecule, it moves upward in -: 100 the atmosphere and eventually is lost to space. The ul- õ traviolet radiation is restricted to high altitudes precisely because it is absorbed there by molecules such as water 50 and ozone; the vast majority of Earth's water is protected from such destruction by being resident in the oceans and lower atmosphere. Consider now what would happen if Earth's surface temperature were increased, simulating what might have happened on Venus if it once had had liquid water oceans. More water vapor is put into the troposphere, allowing formation of more massive cloud decks. Clouds can warm or cool the climate, depending on their altitude, but their v formation by condensation always releases heat, which o¡: 100 E causes the temperature profile to fall more gently with altitude. Because of this effect, the temperature profile for higher surface temperatures declines more gradually 50 than for lower surface temperatures, and the tropopause boundary between the troposphere and the stratosphere 0L moves upward as the surface warms (figure15.2).