The iron (Fe)-oxide terraces at Echinus Geyser form from the oxidation of ferrous Fe, and the temperature across the terraces ranges from ~ 60-70ºC, while pH values vary from 3.4 to 3.6. A very thin (1-2 mm) layer of water flowing over the Fe-oxide terraces from the outflow channel at Echinus Geyser, located in Yellowstone National Park’s Norris Geyser Basin, is thought to provide the needed oxygen to create habitats suitable for the Marsarchaeota. (Bill Inskeep)