A satellite image of Olympus Mons, Mars, the largest volcano in the solar system at about three times the height of Mount Everest. Around 3.5 to 4.0 billion years ago, the release of volcanic gases, especially the greenhouse gas sulfur-dioxide, may have warmed the surface of Mars episodically, thereby explaining the presence of geomorphological features indicative of the flow of water on the planet's ancient surface.Weizmann Institute of ScienceWhy does the cold, barren surface of Mars contain geological features that appear to have been formed by flowing water: river valleys, lake basins and deltas? A new model, which was published online in Nature Geoscience this week, suggests that sulfur spewed into the Martian atmosphere by ancient volcanoes could have periodically warmed the surface enough for the ice to melt and water to flow.

 

Indeed, the signs of flowing water have been a puzzle, as the latest generation of climate models portrays Mars as an eternally ice-cold planet with all of its water frozen solid, especially early in its history, when the Sun was weaker than it is today. Today, most of that water is locked in the polar caps. Drs. Itay Halevy of the Weizmann Institute’s Earth and Planetary Science Department and James Head III of Brown University, Rhode Island, thought the answer might lie in the now dormant volcanoes on the planet’s surface, which could have played a larger role than thought in shaping its climate.
 

On Earth, volcanic emissions – sulfur compounds and ash – tend to cool the climate. But in the presumably dusty early atmosphere of Mars, the net effects might have been different. To understand their impact, Halevy and Head first calculated the size of ancient volcanic eruptions, based on the volcanic rock formations observed on the planetary surface today. Their estimations show that the eruptions were violent – hundreds of times the force of the average eruption on Earth – and may have lasted up to a decade. This means that the amounts of gases spewed from the mouths of these volcanoes, from what we know of Earthly eruptions, must have been enormous.
 

The team’s simulations showed large amounts of the greenhouse gas sulfur-dioxide mixing into the atmosphere. But warming by the sulfur-dioxide was thought to be outweighed by cooling due to the creation of sun-blocking sulfuric acid particles, which form as the sulfur-dioxide reacts in the atmosphere. Halevy and Head showed that in an atmosphere already as dusty as that of Mars, the sulfuric acid mostly forms thin coatings around particles of mineral dust and volcanic ash, subduing the added cooling. The net effect, according to the model the scientists created, was modest warming – just enough to allow water to flow at low latitudes on either side of the planet’s equator.
 

Liquid water may have flowed in these regions for tens to hundreds of years during and immediately after volcanic eruptions. The model suggests that during these brief, but intense wet periods, the surface of the planet could have been carved by flowing rivers and streams.