The Layered Units, Sinus Meridiani, Mars, and the Fluvial Megafan Analog
River-reach scale. Numerous, sinuous linear features of varying width are suggestive of fluvial channels. Sets of linear features can be interpreted as braided reaches. Cross-cutting relationships, a common feature of channels on terrestrial fluvial plains, are ubiquitous. Eroded desert megafans show cemented paleo-channels as inverted topography, a major characteristic of the ridged features in Meridiani.
Fluvial mesoscale. We propose the terrestrial megafan as the prime analog for the layered units. Megafans are fluvially emplaced, fan-shaped sediment bodies of low slope, with radii up to hundreds of km. The prime continental environment for networks of stream channels on Earth is the megafan. Areas covering 10^4 km2 of intercrater plains in Meridiani host networks of ridges. The well-known layering and internal discontinuities of the Meridiani rocks are consistent with a megafan interpretation. Low slopes typical of megafans are conducive to the development of water bodies, which are numerous on some terrestrial megafans. The lacustrine theory of the hematite-bearing unit is thus consistent with a megafan analog. Abundant near-surface hydrogen in Meridiani, in gamma-ray spectroscopy, suggests fluvial or lacustrine environments.
Regional/basin scale. The regional setting of the Meridiani units parallels many continental basins which are occupied by sequences of nested megafans. Contiguous megafan surfaces cover large areas1.25 million km2 in S America. Megafans lie at the foot of a backing highland, from which rivers supply sediment. Similarly, the layered units lie at the foot of the Sinus Meridiani highland where numerous river valleys are oriented towards Meridiani. The low slopes of the Meridiani units mirror the typically low regional slopes of terrestrial megafans.