Saturday, 24 September 2011
As we shall see on subsequent pages, while volcanic rocks predominate on Mars, there are widespread sedimentary rocks - mostly evaporites and some carbonates. The overall surface state of Mars, judging from these full planet face views, is that it consists of three major color states, each tied to some dominant material: 1) the predominant reddish colored surface, which is now known to be regions in which the rocks, soils and dust are strongly oxidized into phases consisting of hematite, and possibly maghemite (the γ polymorph of Fe2O3) and similar minerals (including the group of hydrated iron oxides going under the name of "limonite", if water at and beneath the surface has "weathered" the hematite); 2) the dark bluish to blackish surface, presumably basaltic bedrock with less iron discoloration, and 3) the whitish areas around the poles, identified as a mix of water and carbon dioxide (the outer coating). Color phase 2) implies that the surface is volcanic bedrock with insufficient iron oxide dust cover to significantly alter the color depicted; this suggests much less onsite alteration of the basalt and the transient nature of dust cover as martian winds remove much of previously deposited red dust (but some may be cyclically deposited during strong dust storms [see next page]). As an example of this dark color phase, look at the Syrtis Major physiographic region. It probably is volcanic crust, likely basalt. It size, however, varies from time to time because winds carry red dust back and forth over its boundaries.