2003 Annual Science Report
Harvard University Reporting | JUL 2002 – JUN 2003
The Planetary Context of Biological Evolution Subproject: Carbon Cycle and Climate Dyanamics of Mars
The surface of Mars preserves the record of a past climate in which liquid water was stable and apparently abundant. We seek to understand the planetary-scale control of climate through the study of climatic perturbations and their relationship to the carbon cycle on Earth, making basic geological and geochemical observations and then comparing these data with simple models of biogeochemical cycles.
Water is an essential ingredient for regulating climate. On Mars, as on Earth, there is substantial water on the surface at present, but it exists not as liquid water but as polar ice caps. The CO2-rich atmosphere of 6 mb is inadequate to keep Martian surface temperatures above freezing, (except, in the current epoch, for local equatorial areas for brief periods). What has limited the accumulation of atmospheric CO2 is not silicate weathering but the lack of a modern volcanic source of CO2. If volcanism existed on Mars today as it likely does on Venus, CO2 would accumulate in the atmosphere until the ice caps melted. This in turn would limit the continued rise of atmospheric CO2 through the silicate weathering feedback.
Although liquid water is not stable at the surface today, there are a large number of observations that suggest that liquid water existed at least episodically at various times throughout Martian history. Although most morphological evidence for liquid water on Mars is consistent either with episodic and rapid release of water at the surface, or else with liquid water in the subsurface that results in chemical weathering reactions, the presence of valley networks and the degradation of impact craters on ancient surfaces of late Noachian age imply weathering and erosion by liquid water at the surface for substantial amounts of time. One particularly interesting aspect of Martian surface geology is the apparent discrepancy between the geological evidence for water on Mars during the Noachian and the lack of calcium carbonate on the surface. If a large body of liquid water on Mars persisted for millions of years or longer, the high CO2 atmosphere would form carbonic acid, and react with the silicate crust, producing calcium carbonate. However, carbonate minerals have not yet been detected on the surface in sufficient quantities to be consistent with this hypothesis.
A potential explanation for the observations is the possibility that climate episodes warm enough to maintain an active hydrologic cycle endured only long enough to produce the erosional features, but not long enough for calcium carbonate to reach saturation. We are currently modeling this scenario for different ocean volumes and different atmospheric CO2 concentrations. The calculations assume simple calcium silicate chemistry for the crust and use equilibrium constants appropriate for the system CaO-CO2-H2O. The critical assumption made here is that volcanic outgassing at this time was not ongoing at a sufficient rate to replace whatever CO2 was lost from the atmosphere by ocean uptake. We are presenting our results in July, and a paper is in preparation.