PERSPECTIVE https://doi.org/10.1038/s41550-018-0529-6 Inventory of CO2 available for terraforming Mars Bruce M. Jakosky 1,2* and Christopher S. Edwards3 We revisit the idea of ‘terraforming’ Mars — changing its environment to be more Earth-like in a way that would allow terres- trial life (possibly including humans) to survive without the need for life-support systems — in the context of what we know about Mars today. We want to answer the question of whether it is possible to mobilize gases present on Mars today in non- atmospheric reservoirs by emplacing them into the atmosphere, and increase the pressure and temperature so that plants or humans could survive at the surface. We ask whether this can be achieved considering realistic estimates of available volatiles, without the use of new technology that is well beyond today’s capability. Recent observations have been made of the loss of Mars’s atmosphere to space by the Mars Atmosphere and Volatile Evolution Mission probe and the Mars Express space- craft, along with analyses of the abundance of carbon-bearing minerals and the occurrence of CO2 in polar ice from the Mars Reconnaissance Orbiter and the Mars Odyssey spacecraft. These results suggest that there is not enough CO2 remaining on Mars to provide significant greenhouse warming were the gas to be emplaced into the atmosphere; in addition, most of the CO2 gas in these reservoirs is not accessible and thus cannot be readily mobilized. As a result, we conclude that terraforming Mars is not possible using present-day technology. he concept of terraforming Mars has been a mainstay of sci- Could the remaining planetary inventories of CO2 be mobi- ence fiction for a long time, but it also has been discussed from lized and emplaced into the atmosphere via current or plausible 1 a scientific perspective, initially by Sagan and more recently near-future technologies? Would the amount of CO2 that could T 2 by, for example, McKay et al. The idea has resurfaced lately in pub- be emplaced into the atmosphere provide any significant green- lic discussions of possible human missions to Mars and long-term house warming? human colonization3. Here we take ‘terraforming’ to mean chang- Our analysis follows that of ref. 2, but benefits from 20 years of ing the Martian environment into one that could be more suitable additional spacecraft observations of Mars. These data have pro- for the existence of terrestrial life. Following McKay et al.2, this has vided substantial new information on the volatile history of the two possible approaches. First, we can consider increasing the pres- planet, the abundance of volatiles locked up on and below the sur- sure enough to allow humans to work without the use of pressure face, and the loss of gas from the atmosphere to space. suits; this approach does not necessarily require an oxygen atmo- sphere that can be breathed directly. Second, we can increase the Reservoirs and sinks for CO2 abundance of an atmospheric greenhouse gas enough such that the Non-atmospheric CO2 reservoirs or sinks that we examine here temperature will increase to the point at which liquid water can be include: polar CO2 ice or water-ice clathrate, CO2 adsorbed onto stable or persist. Having warmer temperatures and stable liquid mineral grains in the regolith, carbon-bearing minerals (in carbon- water could greatly simplify all aspects of operations in the Martian ate-bearing rocks), and the permanent loss of CO2 from the atmo- environment. Both aspects require the addition of significant quan- sphere to space. Below we will discuss the amount of CO2 necessary tities of gas to the present atmosphere. to raise temperatures sufficiently to allow liquid water; for now, CO2 and H2O are the only greenhouse gases that are likely to be however, take a nominal requirement as being about 1 bar of CO2, –2 present on Mars today in sufficient abundance to provide any sig- which is equivalent to about 2,500 g CO2 cm in the atmosphere. nificant greenhouse warming. While other gases such as introduced For comparison, the present-day atmosphere has an average pres- –2 chlorofluorocarbons have been proposed as ways to raise the atmo- sure of about 6 mbar — equivalent to about 15 g CO2 cm at the spheric temperature2,4–6, these are short-lived and without a feasible surface. source using current technologies; they are therefore not considered further here. Previous models of atmospheric warming have dem- Polar CO2 ice and clathrate. The Martian polar caps contain a sea- onstrated that water cannot provide significant warming by itself; sonal covering of CO2 ice that deposits in winter when the surface temperatures do not allow enough water to persist as vapour with- drops below the condensation temperature of atmospheric CO2, 7 out first having significant warming by CO2 (ref. ). CH4 and H2 and disappears in summer when sunlight causes it to sublimate back have been proposed as possible greenhouse gases that contributed into the atmosphere. This seasonal covering can contain as much as to warming early Mars7, but these are not expected to be present in perhaps a third of the atmosphere at any given time, or the equiva- an accessible location on Mars today. Thus, we focus on determin- lent of about 2 mbar (ref. 8). ing the inventory of CO2 on present-day Mars, and whether the CO2 The Martian north polar cap loses its entire covering of CO2 ice that is present in non-atmospheric reservoirs can be put back into during summer, exposing a residual polar cap. This cap consists the atmosphere. broadly of water ice with intermixed dust in various proportions to To understand the availability and impact of CO2 gas on atmo- form layers in a stack up to several kilometres thick. The ice cap is spheric warming, we address the following questions. Where has surrounded by layered deposits that are thought to contain little to 9 the CO2 from an ancient, thicker greenhouse atmosphere gone? no water ice, down to a latitude of about 80° (ref. ). 1Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA. 2Department of Geological Sciences, University of Colorado, Boulder, CO, USA. 3Department of Physics and Astronomy, Northern Arizona University, Flagstaff, AZ, USA. *e-mail: [email protected] 634 NATURE ASTronomY | VOL 2 | AUGUST 2018 | 634–639 | www.nature.com/natureastronomy NATURE ASTRONOMY PERSPECTIVE The south polar cap retains a thin and spatially discontinuous 0.020 T = 200 K cover of CO2 ice after the seasonal frost has disappeared during summer. This ice is not thought to be substantial, containing only the equivalent of a fraction of the present-day atmospheric CO2 8 Palagonite abundance . Water ice is observed to be present beneath that CO2, and much of the volume of the polar deposits is thought to be water ice with intermixed dust. Within the volume of the polar deposits, however, radar measurements show what appears to be a substan- 10 tial volume of clean CO2 ice buried at depth . This volume of ice, 0.015 if emplaced into the atmosphere, would provide enough CO2 to double the atmospheric pressure, to an estimated 12 mbar (ref. 11). H2O and CO2 can also occur in a clathrate structure, in which 12 the H2O molecules form a ‘cage’ that can hold CO2 (ref. ). The H2O shell contains six molecules for every CO2 molecule trapped inside. Clathrates can be stable in the Martian polar regions13, although there’s no evidence to suggest that they are actually present. If the full volume of the polar ice deposits consisted of H2O–CO2 clath- rate, they could contain up to the equivalent of about 150 mbar of 0.010 CO2. Even if clathrate is present, however, it’s unlikely that the entire per gram adsorbent (g) polar deposit could have formed in a way that allowed them to be 2 fully populated with CO2. Such a deposit would require high CO2 pressures to have been present throughout the formation process14. Although it’s not possible to obtain a more realistic estimate of the Adsorbed CO possible CO2 content of the polar deposit, 150 mbar should be con- sidered as an extreme upper limit, and it’s possible that there is no clathrate present at all. Nontronite Furthermore, liquid CO2 is not considered to be a viable reser- 0.005 voir for polar-region CO2. The pressure is above the triple-point pressure for liquid CO2 at a depth of only a few kilometres in the polar regions15. However, temperatures are too low to allow liquid CO2 until a depth that would be well below the ice and deep within the crust. CO2 clathrate or liquid CO2 can exist within the crust in non- polar regions, based solely on their stability phase diagram14. However, for either to be present in significant quantities, the Basalt CO2 pressure in pore spaces in the crust would have to be equal to the lithostatic pressure from the overlying materials; this is not 0 5 10 likely and, were the pores sealed in a way that allowed this, the Pressure (mbar) CO2 would be effectively sequestered in the subsurface and thus 14 rendered inaccessible . Fig. 1 | CO2 adsorption isotherms for palagonite, nontronite and basalt. Credit: adapted from ref. 16, Elsevier Adsorbed CO2. Both CO2 and H2O can be adsorbed onto individual grains in the regolith and megaregolith16,17. This process involves a physical (as opposed to chemical) bonding at the level of the indi- vidual molecule. The amount of gas that can adsorb onto a grain some instances to depths of 8 km (ref.