MAKING SENSE OF CHAOS: GEOMORPHIC INVESTIGATIONS OF MARTIAN CHAOTIC TERRAIN FRACTURE BLOCKS AND FILL UNITS

Martian chaotic terrain (sometimes simply called chaos) is a globally-distributed surface morphology characterized by polygonally-shaped mesas and massifs, separated by deep canyons that form a pattern of interlocking buttes and mesas. Chaos terrain is globally distributed, but is concentrated in the Tharsis/Valles Marineris region of Mars and is spatially associated with outflow channels; as a consequence, it is widely held to be related to outflow channel formation mechanisms (e.g., fracture of the surface and release of groundwater.

To address competing hypotheses regarding the origin of chaos terrain and to help constrain the potential volatile budget associated with their formation and evolution, we conducted geomorphic mapping of 20 chaos units and combined these maps with analysis of merged MOLA-HRSC DEM data to determine the spatial distribution of chaos, the details of its structure and fabric, and the volume of “missing” (eroded or removed) material.

Several notable trends emerge from our measurements. For most mapped units, chaos mesas and massifs get smaller in area the farther they are from the center of the chaos. This suggests either non-uniform fractured unit thickness or spatially controlled stress. In most chaos terrains, mesa or massif area increases with increasing massif relief (a proxy for thickness), however, it is notable that representative block diameter (approximate fracture spacing) can be several orders of magnitude greater than block relief—in contrast to fractures in terrestrial rock units that are commonly spaced within a factor of 1-2 of cracking unit thickness. This suggests that chaos terrains reflect preservation of underlying fill material beneath a cracked cap unit.

Finally, calculations of “missing volume” produce a surprising result: most chaoses are mostly empty space, with volumes above the mesas/massifs but below the rim of the chaos exceeding the volume of the mesas/massifs by a factor of 10 or more. Even within the mesas/massifs, void areas between blocks commonly account for 50% or more of chaos unit volume. This suggests either very large volume changes associated with chaos formation (shrinkage or drainage) or extensive subsequent erosion of inter-block materials, which is at odds with the preservation of fine surface textures on block tops.