CONSTRAINTS ON THE GEOLOGIC HISTORY AND TRANSPORT OF MATERIALS AT FIVE MARS LANDING SITES PROVIDED BY CLAST MORPHOLOGY AND TEXTURE

Loose rock fragments on the surface (clasts, here defined as pebble- to cobble-sized) can provide important clues to bedrock geology where bedrock outcrop is either poorly exposed or cannot be readily accessed. This is especially important on Mars, where access to primary sources of information (outcrop, laboratory sample analysis) is highly limited. Morphologic parameters such as size, shape (roundness and sphericity), sorting (range and proportion of clast sizes), and clast distribution (size-frequency and distances between clasts) all may be quantified and used to interpret clast history.

An analysis of quantitative morphologic characteristics of surface particles at five different landing sites (Viking 1 and 2, MPF, Spirit at Columbia Hills, and Curiosity at Gale Crater) yields the following results:

(1) Morphologic characteristics are sufficiently diverse at these sites that some separation of populations is possible based on lithology or mode of transport. This methodology, however, has constraints unique to Mars. For example, roundness, influenced by transport, is commonly used on terrestrial clast populations as a proxy for time in persistent transport. But measuring corner sharpness using on two-dimensional image data is not as accurate as physically measuring corners of representative clasts. Thus, the results of quantitatively assessing morphologic characteristics are most useful for discriminating populations primarily shaped by turbulent, intermittent processes such as mass wasting, ballistic impact, or sediment-gravity flows (including those involving limited water such as debris flows), from those shaped by longer-acting forces such as sustained fluvial transport. If both transport and lithology vary among analyzed clasts, discriminating populations transported by different mechanisms can be problematic.

(2) Clast morphology tends to persist because wear occurs slowly by terrestrial standards. Thus, the prior presence of outcrop may be inferred by the existence of a clast population with characteristics shared by that particular outcrop (e.g., Home Plate units in Columbia Hills, conglomerate-rich units in Gale). This ability is limited to those outcrops that have morphologies (qualitative, quantitative or a combination) that can be separated out from the background.