Mars' color and albedo variations are dictated not only by composition but also by grain size. Terrestrial materials are classified based on size, ranging from dust to rocks (Table 4.2). Martian materials are subdivided into rocks (pebbles, cobbles, and larger material), drift material (fine-grained and cohesive material, primarily composed of clay-sized particles), crusty-to-cloddy material (clay-sized grains weakly cemented by salts), and blocky material (composed of sand-sized and
Surface characteristics Table 4.2 Grade scale for small particles
0.0004-0.00625 0.00625-0.125 0.125-0.25 0.25-0.5 0.5-1.0 1.0-2.0 2.0-4.0 4.0-64 64-256
Very fine sand Fine sand Medium sand Coarse sand Very coarse sand Granule Pebble Cobble smaller grains which are strongly cemented by salts) (Moore and Jakosky, 1989). The sizes of surface materials at a specific location depend on the geologic processes which have operated in that region. Bedrock is broken into rocks that are weathered into the smaller materials.
Weathering processes are divided into physical and chemical weathering. Physical weathering is the mechanical breakage of larger materials into smaller pieces. For example, when water seeps into cracks in a rock and then expands upon freezing, the forces it exerts on the rock during expansion can eventually fracture the rock into smaller pieces. Chemical weathering is the alteration of one mineral into another, which often weakens the chemical bonds holding the rock together. Chemical weathering also includes the dissolution of minerals - thus, the vugs seen in rocks at the Opportunity landing site indicate that dissolution of salt crystals, and thus chemical weathering, has occurred.
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