Comparing Quantitative Morphology of Clasts at Three Landing Sites on Mars: Mars

Background. Much of the sedimentary transport and depositional history of Mars may be recorded in the morphologic characteristics of the individual rocks or particles that make up its sedimentary clast population (defined as a rock fragment or grain resulting from the breakdown of larger rocks or bedrock). Measuring morphology is crucial; it retains the best record of sorting and abrasive effects [1, 2]. Because it can be determined to a high degree of accuracy by relatively simple and objective means, morphology is also a property that may be studied quantitatively and statistically, using indices calculated from clast geometry. This quantifiability is important, as a scheme of measuring and expressing numerically the morphology of a clast is necessary for descriptive purposes and for discriminating between the several factors involved in the evolution of clast shape. Approach. Assessment of clast morphology is a standard analysis technique (e.g. [3-8]). On Earth, the method is often used at sites where the overall recent geologic history is constrained (a glacial environment, for example), but the number of clast populations and the general transport mechanism represented by each — such as fluvial vs. marine vs. intertidal vs. terrestrial — is not (e.g. [9-12]). This mirrors the situation for Mars, where the primary processes of various localities have yet to be identified. Previously we reported on the results of an assessment of size, sphericity, elongation and roundness for rocks within the Mars Pathfinder Rock Garden [13, 14]. Here we compare these results to similar indices for the Viking 1 and 2 landing sites. Method. The rocks at the MPF site have been classified by roundness, flatness/tabularity or angularity (e.g. [15-17]). Similarly, quantitative indices appropriate for rocks in this investigation include size, sphericity (how closely rock shape resembles a sphere), roundness (how sharp the corners of a rock are) and elongation (a measure of major to minor axis length). Rock size was derived from previously published work [18, 19]. Elongation was calculated by dividing the minor by the major axis of each rock [20], while sphericity was estimated by taking the outline of each individual rock and determining the largest inscribed and smallest circumscribed circle of the resulting polygon. These values yielded sphericity by using ρ = D ins /D cir , where D ins is the largest inscribed circle and D cir is the