Thursday, May 26, 2016

What do the decameter-scale polygons in the lower Peace Vallis fan of Gale crater mean?

Origin and significance of decameter-scale polygons in the lower Peace Vallis fan of Gale crater, Mars


Oehler et al


Decameter-scale polygons are extensively developed in the Bedded Fractured (BF) Unit of the lower Peace Vallis fan. The polygons occur in a likely extension of the Gillespie Lake Member, north of Yellowknife Bay, the section first drilled by the Mars Science Laboratory (MSL) mission. We examine hypotheses for the origin of these polygons to provide insight into the history of Gale crater.

The polygons are ∼4–30 m across, square to rectangular, and defined by ∼0.5–4 m wide, generally straight troughs with orthogonal intersections. Polygon networks are typically oriented-orthogonal systems, with occasional nearly circular patterns, hundreds of meters across. Potential origins include cooling of lava, and for sedimentary units, syneresis, unloading, weathering, desiccation, impact processes, and cold-climate thermal contraction. Cold-climate thermal contraction is the hypothesis most consistent with the sedimentary nature of the BF Unit and the polygon morphology, geometry, networks, and apparent restriction to the coarse-grained Gillespie Lake Member. A periglacial setting further provides the best analogs for the circular networks and is consistent with geologic context and MSL data.

Most of the decametric polygons appear to be ancient. They are confined to the Hesperian BF Unit, and only a few of their bounding fractures extend into younger or recently exposed units. In this regard, they differ from the majority of proposed thermal-contraction polygons on Mars, as those are generally thought to be young features, and, accordingly, the history of formation, preservation and reactivation of the decametric polygons is likely to be more complex than that of any proposed young polygons on Mars. The decametric polygons in the BF Unit may represent landforms developed in a cold but still comparatively wet interlude between a clement early Mars and the much drier and colder planet of today.

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