Effects and Lifetime of Ocean Induced CO2 Pulses on Mars: Implications for Fluvial Valley Formation

Baker et al. [ I ] proposed that a variety of anomalous geomorphological features on Mars can be explained by episodic ocean formation [2] triggered by extensive Tharsis volcanism and the associated massive hydrothermal system. The rapid outpourings of ground water from outflow channel discharges are assumed to release C02 dissolved in the ground water, adsorbed on the regolith and resident in the north polar cap into the atmosphere. These pulses of C02 relatively late in Mars' climatic history are suggested to be responsible for short greenhouse periods that Baker et al. believe to be evident in the geologic record. Here we investigate the climatic effect of instantaneous pulses of CO:! added to the martian atmosphere at 1 and 2Ga. We find that such pulses can indeed produce short-periods of modest greenhouse warming and significantly increase the saturation vapor pressure. Therefore, significantly greater snowfall at higher elevations is possible than under current climatic conditions [3]. This snowfall, in conjunction with melting of snow by localized geothermal heating, might be responsible for late periods of fluvial erosion and glacial activity. To test the impact of a single injection of C02 into Mars' atmosphere relatively late in the planet's history, we adapted the climate model of Haberle et al. [4]. This model was originally used to explore the evolution of the equilibrium surface temperature of Mars from 4.5Ga to present with various initial C02 boundary conditions. The model draws on published estimates of the main processes believed to affect the atmospheric inventory of C02 during this period: chemical weathering, regolith uptake, polar cap formation, and atmospheric escape. The model accounts for the variation of solar luminosity with time, the greenhouse effect, and a polar and equatorial energy budget. The C02 loss processes, with the exception of atmospheric escape, are controlled by the equilibrium surface temperature which is calculated using a modified version of the Gierasch and Toon [5] energy balance model that accounts for C02 condensation. Our model begins with a nominal 1 bar C02 atmosphere at 4.5Ga. We find model results to be insensitive to initial C02 abundance since weathering reduces the exchangeable inventory of C02 to very similar amounts by the time of the pulse. Haberle et al. [4] conclude that initial C 0 2 inventories of 0.5 to 1.0 bar result in best overall agreement with present day C02 reservoirs on Mars. An instantaneous 0.5, 1.0, or 2.0 bar pulse of C02 is then added to the model at either 1 or 2 Ga and the evolution allowed to continue. For each case we varied the percentage area of Mars (5, 10, 20, or 27%) that is covered with water. Pollack et al. [6] assumed 5% coverage, Baker et al. calculate a maximum ocean size of approximately 27% of Mars' surface area. After the CO2 pulse injection, the equilibrium surface temperature rapidly rises, the weathering rate increases, and the CO2 decays. In all cases, except for the injection at 1Ga with 5% water coierage, the atmosphere returns to the buffered regime Tpermanent polar ice caps) at the present epoch. In the exceptional cases, the weathering rate is too slow and the atmosphere does not collapse. Therefore, even late injections of C02 into the atmosphere are compatible with the current observed Martian climatic regime, assuming a sufficiently large areal extent of surface water. The duration of elevated temperatures is more sensitive to the size of the oceans than either the amount of C 0 2 in the pulse or the time of pulse injection. Depending upon the model, greenhouse warming induced by the C02 pulse may last for 300 to 1,000 million years. For the 0.5 bar pulse at 1 Ga with 10% water coverage, temperatures rise approximately 25 K above the background temperature to approximately 250 K and then slowly decay over a period of 6 x lo8 years before the atmosphere colla ses. For a 2.0 bar pulse, temperatures rise nearly 50 K and then ! decay more rapidly over 7 x 10 years to the background temperature. Assuming 27% water coverage, roughly equivalent to the maximum ocean size reported by Baker et al. [I], the atmosphere collapses in approximately 2 x lo8 md 3 x lo8 years respectively. For the same pulses injected at 2 Ga results are similar although peak temperatures are about 5K less owing to the fainter sun. The temperature increases triggered by the CO2 pulses are generally not sufficient to raise the equilibrium surface temperature above freezing, except for a 4 bar pulse. In this extreme case, global temperatures are elevated above 273K for several lo6 years. Unlike early Mars, such conditions are permissible at later times because the sun is brighter.