From the Archives 3

April 12, 2015

Taken August 25, 2002, this photograph shows a layer of the yellow mineral schwertmannite atop grey-blue shale at the edge of Ohio’s Vermilion River. As I was checking before I posted to make sure schwertmannite has two n’s, I came across the abstract of a book chapter that raises the question of whether schwertmannite—a compound composed of iron, oxygen, hydrogen, and sulfur—occurs on Mars. I don’t understand much of the abstract, but I do get that the book-chapter’s author believes the mineral’s possible occurrence on Mars could be a result of “chemical weathering processes as acidic groundwater came in contact with the [Mars] atmosphere.” Biogeologist Norrie Robbins, whom I consulted when writing They Breathe Iron has another idea about the formation of schwertmannite, which adds excitement. She says that schwertmannite accumulates as tiny spheres around the cells of a still-unnamed bacterium. So here’s a thought: If schwertmannite exists on Mars, it could be in association with a life form. Pretty thrilling, eh? . . . The tea-colored stuff at the top left in this photograph, by the way, is how iron oxide looks when it dissolves in water. Here’s what I wrote in the book next to this photograph:

“Occasionally a reddish-brown liquid resembling strong tea collects in small puddles along the edge of the Vermilion River. Geochemists have nicknamed this fluid “cherry juice” or “prune juice.” Really, it is dissolved oxidized iron usually mixed with reduced iron (iron that used to be iron oxide but that has had its oxygen removed). Oxidized iron only dissolves under highly acidic conditions, so it often occurs near schwertmannite.”