Active sulfur vents.
A sulfur spring, or fumarole was an opening in a planet's crust, often in the neighbourhood of volcanoes, which emits steam and gases such as carbon dioxide, sulfur dioxide, hydrogen chloride, and hydrogen sulfide. The steam is created when superheated water turns to steam as its pressure drops when it emerges from the ground. The name solfatara, from the Italian solfo, sulfur (via the Sicilian dialect), is given to fumaroles that emit sulfurous gases.