Vast mounds of layered material lie in numerous places throughout the giant canyon system of Valles Marineris, and especially in Candor Chasma, Ophir Chasma, and Melas Chasma. The origin of these “interior layered deposits” (ILDs) have been debated since they were discovered in the early 1970s. Theories include lakebed deposits, sub-ice eruptions, and groundwater rising to alter beds of wind-blown volcanic ash and other materials.
Two scientists — Joseph Michalski (Planetary Science Institute) and Paul Niles (NASA Johnson Space Center) — now suggest in Geology that the ILDs formed by a climate-change driven process combining ice, dust, and volcano-generated sulfuric acid. (The two also recently advocated a related origin for the giant mound in Gale Crater.)
“We propose,” the scientists write, “that the ILDs are remnants of sediments originally composed of dust, ice, and acidic aerosols that were concentrated into discrete deposits at low latitudes during periods of high obliquity.” Models of the Martian climate over millions of years show that during such times when the rotation axis tilts more with respect to the Martian orbit, ice and snow accumulate in the equatorial regions instead of the poles.
The researchers continue, “Recent spectroscopic results show that these materials contain coarse-grained hematite, sulfates, and clays. These suggest that the layered deposits are fundamentally similar to layered sulfate deposits seen elsewhere on Mars, and are therefore a key piece of the global aqueous history of Mars.”
A problem with previous explanations for making the ILDs through groundwater or lakebed deposits is that these theories require unrealistically large quantities of material to be deposited and eroded.
The atmospheric deposition model avoids these requirements, they note. “Many of the complications of explaining the ILDs as sourced from groundwater or standing bodies of water are nonfactors if the sediments originated through atmospheric sources.
“We favor an alternative model in which the ILDs form in a configuration similar to what is observed today through atmospherically driven deposition of ice, dust, and volcanogenic sulfuric acid.”