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Abstract
Gully formation on Earth is generally well understood and typically involves landscape erosion through the action of liquid water. The discovery of gullies on Mars formed in geologically recent times suggested that liquid water had flowed on the surface, an event considered difficult under present climatic conditions. This thesis investigates the geomorphology and inferred evolution of gullies located in the Martian southern mid-latitudes and similar gullies located within: (1) a temperate site on the Lake George escarpment south of Gearys Gap, New South Wales; (2) a semi-arid region of Island Lagoon, South Australia; and, (3) a periglacial setting at Pasture Hill, New Zealand. The terrestrial gullies were analysed using a combination of Total Station and Real Time Kinematic (RTK) GPS surveying, in situ observations, Digital Elevation Models (DEMs) and orbital satellite, remotely sensed imagery. For analysis of the Martian gullies a range of remotely sensed satellite data were exploited including High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) imagery, HiRISE derived DEMs, in addition to Mars Express, Mars Global Surveyor (MGS), Mars Odyssey and Viking datasets.
These investigations demonstrated that the terrestrial and Martian gullies were similar in geomorphology, subjected to multiple erosion events, and that water was a dominant erosive agent. In the case of the terrestrial gullies, water erosion was predominantly surficial runoff-based, though subsurface piping was observed at the Lake George site. In the case of the Pasture Hill gullies, an additional source of water-based erosion was derived by seasonal snowmelt. Water-based erosion for the Martian gullies had probably been sourced from degradation of ice-rich, loosely deposited air-fall sediment, as indicated by the location of all studied gullies overprinting or in close proximity to ice-related activity. Evidence of additional mechanisms such as frost, dry mass wasting and debris flows was also found within some Martian and terrestrial gullies, suggesting gully formation may not have been restricted to a single process, but instead evolved under a more complex regime of transportation and deposition.
A further conclusion of the study was that the morphology and evolution of the studied gullies were greatly influenced by their local environments and climate. Gully morphology at the study sites on Earth and Mars was found to be greatly influenced by the thickness of readily erodible regolith, local slope and the presence or absence of bedrock exposures in the gullies. Although the Martian gullies often possessed greater volumes of eroded sediment than the studied terrestrial analogues, they had not eroded to underlying bedrock. This contrasted with the terrestrial gully channels where numerous bedrock exposures were observed, affecting their slope and overall morphology. Similarly, although dominated by dry processes, multiple bedrock exposures were present within the equator-facing Martian ravines, affecting their cross-sectional area and hence sediment transport. In addition, gully morphology appeared to be strongly influenced by composition and changes in local regolith on both Earth and Mars. Examples on Earth included alcove locations controlled by the presence of a resistant rock layer, and the presence of bedrock influencing channel shape and breaks in slope. The presence of bedrock was also observed to influence the shape and slope of Martian gullies, and gully channel shape was found to vary dramatically across transitions between regolith types.
All of the studied sites on Earth and Mars showed significant influence from initial slope angles, being consistently inherited from the local environment. Gully slopes, including those of studied terrestrial gullies, were not necessarily concave up. Profiles seemed to be dependant on those of the host escarpment, which in turn relied on composition of host material as well as the type of erosive process acting on them. This suggested that gully slope profiles were inherited from their host features, and traditional indicators of fluvial action, such as angle of repose and angles of deposition need to be taken in context of the regional environment. Effects of aspect were evident at the mesoscale level, with gullies and ravines co-located within the same alcove of a single crater. This finding, along with semi-fluidised equator facing regimes, re-enforces the complex interaction between volatiles, slope, geology and climate. This analysis can be fruitfully applied to other regions of Mars and Earth and provide a greater understanding of how geomorphological processes operate on both worlds. The methodology can also be applied to other extraterrestrial fluid-formed gullies, for example on Titan, and dry ravines, such as those on the Moon and potentially on Venus.