CASE-HARDENING OF ROCKS ON MARS: EVIDENCE FOR WATER-MEDIATED WEATHERING PROCESSES

Spirit's arrival at Hank's Hollow on the plains of Gusev Crater, Mars signaled an important transition from the volcanic geology of the younger intercrater plains and the older rocks of the Columbia Hills. This transition was marked by the appearance of unusual rock surface weathering features not previously seen along Spirit's traverse. Of special interest are the rocks, “Pot of Gold”, “Bread Box” and a number of smaller unnamed rocks, all of which exhibited case hardened exteriors, with cavernously-weathered interiors.

Case hardening of terrestrial rocks involves interactions between two water-mediated processes (Dorn 2004): 1) the selective cementation of a rock exterior and 2) the softening of the rock interior by selective leaching of soluable components. The addition of cements locally increases the induration of a rock, enhancing resistance to weathering and erosion. Subsequent physical and chemical weathering tends to differentially etch the surface of the rock, bringing the more resistant, better-cemented areas into relief. On Earth, case hardening is most commonly seen in sandstones that have undergone arid weathering (Campbell 1999). However, case hardening has been observed for a broad spectrum of other rock types (granites, basalts and metamorphic rocks), over a broad range of climatic conditions (wet tropical, arid and alpine glacial; Dorn 2004). In terrestrial settings, the common factor in case hardening is the late diagenetic migration of fluids along joints or other fractures, accompanied by the deposition of cements and fracture filling precipitates. The most common terrestrial cements involved in case-hardening are silica, aluminum, iron and/or manganese oxides.

“Pot of Gold” and “Bread Box” provide clear examples of case hardening of rocks on Mars. In these examples, the process appears to have occurred along pre-existing joints in a volcaniclastic parent rock which were later in-filled by chemical precipitates that cemented the rock matrix adjacent to fracture walls, while leaching the rock interiors. The case-hardened portions of these rocks have since been etched into relief by the wind, with the interiors having been largely removed by wind erosion. The major factor controlling this style of weathering was liquid water, an observation that holds important implications for habitability during an earlier period in Martian history.