Geologists James Greenwood of Wesleyan University and Ruth Blake of Yale University examined various explanations for the similar soils sampled by both Mars Rovers as well as at the two Viking and the Mars Pathfinder landing sites. Each contains at least some so-called bright dust or soil that has proved to contain much sulfur, chlorine and phosphorus along with the typical ferric oxide that gives Mars its red tinge. The researchers examined Martian meteorites here on Earth to determine that the sulfur and chlorine likely resulted from ancient volcanic explosions.
But phosphorus is a different story. "The ultimate source of phosphate in Martian fluids is likely from the weathering of calcium-phosphate minerals in igneous rocks," the researchers write. Unlike on Earth, where phosphorus rarely dissolves in water because of its uptake by life, it appears to be behaving like a soluble element on the Red Planet, leached from rock by a surrounding acidic ocean. Such high levels of correlation between phosphorus and sulfur, for example, "can likely only be obtained under acidic conditions," they note.
Such a broad dispersal of similar soils argues for an acidic sea stretching across large swaths of Mars, or perhaps lakes with the same inherent chemistry. Overall levels of the elements argue for such a sea to have been short-lived, as deep ocean waters on Earth typically mix every 1,000 years or so. "Evaporation of a global acidic ocean could lead to [nanophase ferric oxide] particles with constant sulfur, chlorine and phosphorus, as seen in the Martian bright soils," the researchers note in the paper. Such a global acidic ocean would also help explain the lack of carbonates in the soil, a key indicator of the presence of former seas on Earth. But such a sea might not have been a boon to life, even though acid-loving microbes persist here; high levels of phosphorus argue against a living presence. "High concentrations of phosphorus in a Martian ocean," Greenwood and Blake conclude, "would not be expected if Mars had an active biosphere during the ocean's existence."