ICARUS 71, 203-224 (1987) The Case for a Wet, Warm Climate on Early Mars J. B. POLLACK AND J. F. KASTING NASA Ames Research Center, Moffett Field, California 94035 S. M. RICHARDSON Iowa State University, Ames, Iowa 50011 AND K. POLIAKOFF I.M.I. Inc., San Jose, California 95114 Received September 22, 1986; revised March 9, 1987 Theoretical arguments are presented in support of the idea that Mars possessed a dense CO2 atmosphere and a wet, warm climate early in its history. Calculations with a one-dimensional radiative-convective climate model indicate that CO2 pressures between 1 and 5 bars would have been required to keep the surface temperature above the freezing point of water early in the planet's history. The higher value corresponds to globally and orbitally averaged conditions and a 30% reduction in solar luminosity; the lower value corresponds to conditions at the equator during perihelion at times of high orbital eccentricity and the same reduced solar luminosity. The plausibility of such a COz greenhouse is tested by formulating a simple model of the CO2 geochemical cycle on early Mars. By appropriately scaling the rate of silicate weathering on present Earth, we estimate a weathering time constant of the order of several times 107 years for early Mars. Thus, a dense atmosphere could have persisted for a geologically significant time period (-109 years) only if atmospheric COz was being continuously resupplied. The most likely mechanism by which this might have been accomplished is the thermal decomposition of carbonate rocks induced directly and indirectly (through burial) by intense, global-scale volcanism. For plausible values of the early heat flux, the recycling time constant is also of the order of several times 107 years. The amount of COz dissolved in standing bodies of water was probably small; thus, the total surficiai CO2 inventory required to maintain these conditions was approximately 2 to 10 bars. The amount of CO2 in Mars' atmosphere would eventually have dwindled, and the climate cooled, as the planet's internal heat engine ran down. A test for this theory will be provided by spectroscopic searches for carbonates in Mars' crust. © 1987 Academic Press, Inc. 1. INTRODUCTION surface above the temperature at which the planet radiates to space, about 212°K (Pol- Today, Mars is a desert world. The glo- lack 1979). Although temperatures at the bally averaged atmospheric pressure at the surface rise above the freezing point of surface is only about 7 mbar. Hence, the water near the equator during the warmest dominant carbon dioxide atmosphere pro- part of the day, water ice is evaporated so duces only about a 5°K warming of the efficiently that its temperature at the sur- 203 0019-1035/87 $3.00 Copyright © 1987 by Academic Press, Inc. All rights of reproduction in any form reserved. 204 POLLACK ET AL. face, except perhaps for some very special widespread volcanism or greenhouse circumstances, cannot reach 273°K (Inger- warming within an exposed ice deposit soll 1970). (Cloy 1984). However, the climate of Mars may not Geomorphic studies of large impact have always been as severe as it is today. basins on Mars indicate that such features Geomorphic evidence and theoretical and their associated secondary craters and arguments suggest that the atmosphere of ejecta were much more heavily eroded in Mars may have been significantly denser the planet's early history--more specifi- early in the planet's history (Walker 1978, cally, prior to the formation of the Argyre Pollack 1979, Pollack and Yung 1980, Cess basin--than at later times (Schultz 1985). If et al. 1980, Toon et al. 1980, Kahn 1985, this interpretation is correct, it implies the Schultz 1985). The resulting greenhouse existence of a much denser atmosphere at warming may have been much larger than these early times. Furthermore, there that at present, perhaps large enough to appears to have been a dramatic decrease permit the common occurrence of liquid in the density of valley networks and a water at the surface (Pollack 1979). This change in drainage style after the Argyre possibility was first indicated by the impact (Schultz 1985). These correlations discovery of fluvially produced channels on are consistent with the existence of a dense images obtained by the Mariner 9 orbiting atmosphere in Mars' early history, one spacecraft (Masursky 1973). However, it capable of raising the surface temperature was soon realized that the large outflow above the freezing point of water for an channels might be generated under current extended period of time (-109 years). climate conditions. Large amounts of liquid The most promising gas for creating a water, rapidly released from a near-surface large greenhouse effect on early Mars is reservoir, could flow large distances before carbon dioxide. It is likely that much more totally freezing by forming a top, insulating of it was released into the atmosphere over layer of ice (Peale et al. 1975, Baker 1978). the planet's historywespecially at early This mechanism may be appropriate to times--than resides there today. Further- explain isolated, local channels, but it is more, carbon dioxide is relatively stable harder to make such an argument for the against UV photolysis (Pollack 1979, Pol- fluvial gullies or valley networks. Such fea- lack and Yung 1980, Cess et al. 1980). tures are ubiquitous on the old terrain of the Alternative greenhouse gases, such as southern highlands. Some corresponding ammonia, methane, and sulfur dioxide, are widespread process is needed to generate rapidly photolyzed into gases and/or par- the liquid water that produced them. ticles that either are poor greenhouse gases Careful analyses of Mariner 9 and Viking (e.g., nitrogen) or have short residence images of valley networks indicate that times in the atmosphere (e.g., sulfuric acid they were formed chiefly by sapping, rather aerosols) (Pollack and Yung 1980, Cess et than by rainfall (Sharp and Malin 1975, al. 1980). Greenhouse calculations indicate Pieri 1980). This deduction does not pro- that CO2 partial pressures in excess of vide hard evidence that Mars was warmer about 1 bar are required to raise the surface in its early history. On the other hand, there temperature on Mars above the freezing is still a requirement that thermal condi- point of water (Pollack 1979, Cess et al. tions close to the surface permitted the 1980, Toon et al. 1980). Such quantities of occurrence of liquid water over some finite carbon dioxide are consistent with interval of time at many locations in the old estimates of the planet's total CO2 cratered terrain. A warm climate would inventory, as discussed in Section 6 of this represent one such mechanism (Pollack paper. 1979). Alternative possibilities include For the time being, we assume that A WARM AND WET MARS 205 enough carbon dioxide was present in the decomposed by processes associated with early atmosphere of Mars to raise the sur- global-scale volcanism on time scales com- face temperature above the freezing point parable to the weathering time scales. This of water, at least at low latitudes. We first decomposition returns gaseous CO2 to perform greenhouse calculations with the atmosphere. Hence, dense CO2 improved opacities for carbon dioxide and atmospheres can be maintained on early water vapor to define more precisely the Mars with plausible volatile inventories. amount of carbon dioxide needed to main- However, at later times, recycling car- tain these conditions. Then, we consider bonate rocks becomes inefficient and so the stability of this dense carbon dioxide liquid water occurs on the surface far less atmosphere against the formation of car- frequently. bonate rocks via the weathering of silicate rocks. On Earth, in the presence of liquid water, chemical weathering proceeds at 2. GREENHOUSE MODELS such a rate that all the atmospheric carbon In this section of the paper, we present dioxide would be eliminated on a time scale estimates of the amount of atmospheric of I0,000 years in the absence of CO2 sour- carbon dioxide needed to raise the surface ces (e.g., volcanic outgassing) (Holland temperature on early Mars above the freez- 1978). We use what is known about the CO2 ing point of water. The greenhouse warm- geochemical cycle on Earth to estimate the ing is produced by the infrared opacity of time scale for CO2 loss on Mars. Next, we both gaseous carbon dioxide and water evaluate the time scale on which the vapor. However, the amount of atmo- atmosphere can be resupplied with gas by spheric water vapor is controlled by juvenile outgassing and volcanically driven its vapor pressure curve, as is the case for recycling of carbonate rocks. Finally, we the present atmosphere of the Earth. determine the total volatile inventory of near-surface carbon dioxide required to 2.1. Procedure maintain a dense carbon dioxide atmo- A one-dimensional radiative-convective sphere over a billion years by a quasi- model was used to determine the steady- steady-state carbon dioxide cycle that state vertical temperature profile in the involves a balance between production and atmosphere and the temperature at the sur- loss. Through such analyses we hope to face. Details of the model have been understand better the plausibility of a published elsewhere (Kasting and Acker- warm, wet climate on Mars in its early man 1986) and will not be repeated here. history. In the final sections of the paper we We assume that the lapse rate in the make such an assessment and propose a troposphere follows a moist adiabat and possible observational test of our scenario. that the tropospheric relative humidity It is perhaps useful to summarize here decreases linearly with pressure, starting the major theses of this paper as a guide to from a value of 0.77 at the surface (Manabe the following sections.