Episodic warming of early Mars by punctuated volcanism

The widespread evidence for liquid water on the surface of early Mars is difficult to reconcile with a dimmer early Sun. Many geomorphological features suggestive of aqueous activity, such as valley networks and open-basin lakes, date to approximately 3.7 billion years ago, coincident with a period of high volcanic activity. This suggests that volcanic emissions of greenhouse gases could have sustained a warmer and wetter climate on early Mars. However, models that consider only CO₂ and H 2 O emissions fail to produce such climates, and the net climatic effect of the sulphur-bearing gases SO 2 and H 2 S is debated. Here we investigate the atmospheric response to brief and strong volcanic eruptions, including sulphur emissions and an evolving population of H 2 SO 4 -bearing aerosols, using a microphysical aerosol model. In our simulations, strong greenhouse warming by SO 2 is accompanied by modest cooling by sulphate aerosol formation in a presumably dusty early Martian atmosphere. The simulated net positive radiative effect in an otherwise cold climate temporarily increases surface temperatures to permit above-freezing peak daily temperatures at low latitudes. We conclude that punctuated volcanic activity can repeatedly lead to warm climatic conditions that may have persisted for decades to centuries on Mars, consistent with evidence for transient liquid water on the Martian surface.