Preferred orientation of Ridges in Phyllosilicate Terrains, Mars: Implications for Crustal Habitability
Abstract
The nature of subsurface hydrological processes, their role in the geologic evolution of planetary bodies and implications for habitability are critically linked. Based on a synthesis of the mineralogy, stratigraphy, and chemistry of phyllosilicate deposits on Mars, Ehlmann et al [Nature, 2011] proposed that the longest-lived habitable environment on Mars was in the subsurface. Is there additional evidence for this? Small, linear ridges have been recognized on Mars and proposed to be the manifestation of breccia dikes formed during impact events [Head and Mustard, MAPS 2006]. Extending these observations, we have mapped with Context Imager (CTX) and HiRISE data, acquired by the Mars Reconnaissance Orbiter, small linear ridges in the Nili Fossae and Nilo Syrtis regions of Mars. Ridges were defined as sharply tapered linear to curvilinear features that express positive relief and produce shadows visible at CTX resolution, and are distinct from features such as dunes and scarps. When two or more ridges overlapped, individual ridge segments were defined as features that were continuous along strike even when intersecting other ridges. Ridges that appeared to abruptly change direction were counted as two segments. Ridge orientations were calculated using a vector defined by the start and end points of each ridge segment and measured relative to 0° North. A total of 4020 individual ridge segments were mapped: average length of 533 m. Longer ridges are sometimes associated with knobs tens of meters in diameter that often occur at branch nodes. These ridges have only been identified in strongly eroded terrains and appear to be restricted to the oldest exposed stratigraphic unit, the smectite-rich Noachian crust. Ridges are not expressed in the overlying mafic cap and olivine-carbonate units and are often observed to terminate at base of the olivine-bearing unit. The stratigraphic confinement of the ridges to the phyllosilicate-bearing crust suggests the ridges were emplaced prior to the capping units or that rheological differences between the two units influenced the formation and expression of ridges. The mapped ridges have a preferred orientation centered at approximately 58° azimuth coinciding with the orientation of the major troughs of Nili Fossae, suggesting the influence of a common stress field. Ridges observed on Mars have been attributed to a variety of processes including exhumed igneous dikes, impact-generated pseudotachylyte and breccia dikes, eskers and eolian landforms. We consider an additional hypotheses: the ridges are mineralized fracture zones or relict conduits of fluid flow, that were established by impact-generated deformation in the crust, or by extensional stress during flexural readjustments due to loading after the Isidis basin-forming impact. The zones of concentrated flow were sealed, and preferentially hardened relative to the clay-bearing host rock and then exposed by differential erosion. Basin-scale impacts have the potential to remain hot for millions of years after the impact, providing a mechanism to drive hydrothermal circulation through the crust. We propose these ridges are preserved elements of regional subsurface hydrologic activity that was a key and persistent habitable zone. Head J. W., Mustard J. F. Meter. & Planet. Sci. 41, 1675-1690 [2006] Ehlmann B. L. et al., Nature, 479, pg. 53-60 [2011]
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.P13E..03M
- Keywords:
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- 5420 PLANETARY SCIENCES: SOLID SURFACE PLANETS / Impact phenomena;
- cratering;
- 6225 PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS / Mars