P12C-01
Martian valley networks 3D geometry from the HRSC data in the Echus Chasma, Huygens and Aeolis regions
Valley networks on Mars have been the subject of considerable debates about their formation processes since three decades. MOLA altimetry data (Mars Observer Laser Altimeter) gives the possibility to study the geometry and topography of valley networks at spatial resolution of typically 500m -1 km. This resolution is sufficient to map large valleys but not small tributaries that we can map on high resolution imagery. The Mars Express High Resolution Stereo Camera (HRSC) allows us to extract DTM (Digital terrain Model) from stereoscopic images with typical spatial resolution <50 m, and vertical resolution typically 10 to 100 m depending on terrains roughness and image quality. We compare the organization of valley networks in two regions (Aeolis region, Huygens Crater region and West Echus Chasma plateau) using both DTMs (MOLA and HRSC). Compared to MOLA, HRSC DTMs improve the quantification of valley networks properties such as drainage densities and aspect ratios. In Aeolis region, MOLA drainage density gives between 0.1 and 0.2 km-1 with Strahler order of 3 whereas HRSC DTM gives between 0.1 and 0.3 km-1 with Strahler's order of 4. The three studied regions enable us to compare two Noachian terrains (Aeolis and Huygens) and one Hesperian terrain (Echus plateau). The comparison shows that the West Echus plateau younger drainages are much more shallow with less incision (20 m to 80 m) than in Aeolis (20 m to 200 m) and Huygens (50 to 400 m), despite that the drainage densities is higher and their maturity is large. The longitudinal profile appears also more concave in the Noachian regions than in the Hesperian one. This difference is probably the effect of three parameters: (1) Noachian terrains are not as well preserved as late episodes of activity on freshly formed Hesperian rocks, possibly explaining small tributaries are missing over Noachian terrains, (2) the duration of Noachian networks formation might be longer to explain the larger incision of valleys, (3) the bedrock composition is uncertain and might be different.
P12C-02
Martian Stepped Fans: Analysis of the apron and associated channel
Martian stepped fans have a distinctive terraced profile and are often fed by deep, first order canyons with small drainage basins. Using flume experiments, we find that distinct terraces form as a fan deposits into a crater basin filling with water. Terrace preservation implies no long-term deposition at a stable water level and an inactive fluvial system during water drainage. Based on scaling from our experiments, we estimate that the formation time for Martian stepped-fans and channel erosion is on the order of decades and conclude that stepped fans on Mars likely formed during a single, basin-filling event driven by a short-lived, intense hydrologic cycle. The estimated discharge rates are comparable to some of the largest rivers on Earth and these rates highlight the challenges of determining the water source.
P12C-03
Fluvial Interpretation of Ridged Units, Northern Sinus Meridiani/Southwest Arabia Terra, Mars
THEMIS, MOC, and HiRISE imagery shows features at various scales that suggest fluvial emplacement of the ridge-forming rock units exposed in northern Sinus Meridiani and southwestern Arabia Terra. The study area -- 10 N to 2 S latitude and 10 W to 8 E longitude -- spans the interface from the southern highlands to the northern plains. Numerous, linear ridges of varying width, orientation and sinuosity (mainly lower sinuosity) are suggestive of fluvial channels. Sets of features can be interpreted as braided channel reaches. Cross-cutting relationships, a common feature of channels on terrestrial fluvial plains, are ubiquitous. Many sinuous features appear as twinned parallel lines, suggesting preferential cementing of coarser channel-bank sediments. A few examples exist of features that can be interpreted as scroll bars and channel augmentation in locally narrow reaches. Layering and internal discontinuities of the Meridiani rocks are consistent with a fluvial interpretation. The regional setting of study-area units accords closely with many terrestrial basins which are occupied by fluvially emplaced sediment bodies known as megafans. Contiguous megafan surfaces (characterized by numerous channel traces, of varied orientation) cover large areas -- 1.25 million sq. km. in S. America -- with radii of hundreds of km. Megafans characteristically lie at the foot of a backing highland, from which rivers supply sediment. The ridged units on Mars lie at the foot of the southern highlands from which numerous river valleys have drained towards Meridiani Planum/southwest Arabia Terra. Further, the present regional slope is apparently away from the highlands, with downslope dimensions of hundreds of km. The low slopes of the northern Meridiani units mirror the typically low regional slopes of terrestrial megafans. Low slopes are conducive to the development of water bodies, which are numerous on some terrestrial megafans. The lacustrine model for the formation of the hematite-bearing unit is thus consistent with a megafan analog. Eroded desert landscapes on Earth show chemically cemented paleo-channels as inverted topography. Paleo- megafans in terrestrial deserts, displaying inverted topography, are the closest morphologic analogs to the ridges of northern Meridiani/southwest Arabia Terra. Further studies -- including quantitative comparisons of ridge characteristics and spectroscopic investigation of cementing minerals -- are underway to determine the origin of these widespread martian units.
P12C-04
A Survey of Raised Curvilinear Features in Aeolis Mensae, Mars
Raised curvilinear features (RCFs) are seen on the surface of Mars in the Aeolis Mensae/western Medussae Fossae Formation. The sinuous plan form of these RCFs and their frequently networked appearance suggest their formation by some manner of aqueous flow. In order to gain a better understanding of the formation mechanisms and the hydroclimatic implications of these RCFs, we conducted a survey focused on a region spanning -8N to 4 N and 138E to 159E, centered on the two western lobes of the Medussae Fossae Formation. All THEMIS-VIS data within this region (478 images) up to the July 2007 release were examined. RCFs were then noted with a location and elevation and put into a database. Where required, THEMIS-VIS images were mosaicked to provide coverage of aerially broad or extensive RCFs and RCF networks. Based on our initial characterizations, we developed a morphological classification scheme to categorize more than 150 RCFs found in this area. The classification scheme includes five types of individual morphologies and five types of network morphologies, which have been cross-correlated, allowing us to determine the most common morphologic combinations. The RCFs range in size from small features, less than 10km in length, to large features spanning several hundred square kilometers. Their location in and around the Amazonian-aged Medusae Fossae Formation suggests that these RCFs formed during the Amazonian epoch, Mars' youngest time-stratigraphic epoch. However, possible clustering of RCFs around the margins of the western Medusae Fossae Formation lobes implies that a significant portion of these features may be located in the time- stratigraphic unit beneath the Medusae Fossae Formation, and therefore may predate the Medusae Fossae Formation. Elevational data derived from gridded Mars Orbiter Laser Altimeter (MOLA) topography indicate that these RCFs are located throughout an elevational range spanning ~2300 m, suggesting that, whenever their formation occurred, it occurred over a geologically significant period of time. Current hypotheses for the formation mechanisms of these RCFs include inverted fluvial channels and glaciofluvial eskers. Some association with impact craters is also noted, although the exact nature of that association is still unclear. Continued work will focus on determining the formation mechanisms for each class of RCF and more accurate determination of their time-stratigraphic epoch(s) of formation.