2010 Annual Research Report

Principal Investigator Williams documented evidence for episodic alluvial fan formation based on morphological, thermal and spectral attributes in Harris crater, located in western Terra Tyrrhena, Mars (Figure 1, located at conclusion of report).  The study adopted an interdisciplinary analysis of CTX, HiRISE and THEMIS data for the alluvial fan located in this Late Noachian-aged crater.  An accepted Icarus manuscript will be published in early 2011 that documents distinct fan depositional lobes and discrete source regions.  This paper documents the first example of a transition in fluvial style on a martian alluvial fan from early fluvial flows to late-stage debris flows, recording a change in sediment supply and/or decline in water availability.   We applied simple modeling to a well preserved subsection of the fan complex to quantify the developmental history.  Our results indicate that significant water was involved in fan construction over an extensive period of millennia or longer.

The greatest concentration of sinuous ridges on Mars is in the Aeolis/Zephyria Plana (AZP) Region.  In collaboration with Dr. Devon Burr (University of Tennessee-Knoxville) and colleagues, we have been examining formation mechanisms for these landforms.  Many of the sinuous ridges are interpreted as paleochannels, formed by precipitation, indurated by chemical cementation and/or clast armoring, buried by subsequent deposition and finally exhumed.  In addition, we applied paleodischarge models to specific sinuous ridges.  The results of this research were presented in a manuscript published in the Journal of Geophysical Research--Planets. 

More recent analysis has focused on understanding meter to decameter undulations in the longitudinal profiles of some sinuous ridges in the AZP.  Rather than the anticipated monotonic decline, the measured slopes from CTX and HiRISE DEMs show modest increases and decreases along course, as reported at the American Geophysical Union’s fall meeting (Lefort et al., 2010).  Potential explanations for these observations include post-formation modification of the landform by localized erosion or differential settling.  To assess the effects of structural deformation, we have examined a terrestrial analog site, inverted channels of the Cedar Mountain Formation near Green River, Utah, as an example that has experienced stress and strain associated with burial and uplift.  In this case, the largest structures were associated with a regional fault zone with offsets of ~10 m.  Other fault and fracture features (offsets of <5 m) were also noted.  In all cases, these features were not readily apparent in aerial photographs, suggesting that absent ground-based images it may be difficult to identify or corroborate the presence of potential faults or fractures affecting AZP longitudinal profiles.

Williams was awarded a grant to ensure data is available to adequately characterize Melas Basin for a potential future landing site through the Critical Data Products program.  As part of this work, she has identified data needs and requested data from existing spacecraft resources (MRO and  Odyssey).  She is working on constructing a geomorphic map of the proposed landing ellipse and identifying key subregions within the ellipse that can address key scientific questions during a future lander mission. 

Williams is helping to unravel the relative roles of various cohesion mechanisms in forming meandering streams in collaboration with PI Alan Howard (University of Virginia).  In minimally vegetated settings on Earth, bank cohesion is caused by clay, ice, or chemical cementation.  In June of this year, she conducted field work on the Quinn River in the Black Rock Desert, Nevada.  Mud drapes are an important factor stabilizing banks at this location, but the relative role of carbonate and salt cements is still being determined.  Research results were presented at the American Geophysical Union’s fall meeting (Matsurbara et al., 2010).  These observations will form of a basis for comparison with perennial ice in stream banks investigated in the coming year.

Principal Investigator Williams continues to investigate inverted paleochannels on Earth to characterize their diagnostic morphology and evaluate the applicability of paleohydrologic models through her Mars Fundamental Research Program (MFRP) grant.  A field guide to inverted paleochannels sites near Green River, Utah and their relevance in studying martian landforms was accepted for the volume Analogs for Planetary Exploration to be published by the Geological Society of America in early 2011.     In addition, this fall she led a reconnaissance field expedition to several inverted channels located in the Atacama Desert, Chile. 

Papers: 

 Burr, D. M., R. M. E. Williams, M. Chojnacki, K. D. Wendell, and J. P. Emery, 2010. Inverted fluvial features in the Aeolis/Zephyria Plana region, Mars: formation mechanism and initial paleodischarge estimates. Journal of Geophysical Research, 115, E07011, doi:10.1029/2009JE003496.

Weitz, C. M., Milliken, R. E., Grant, J. A., McEwen, A. S., Williams, R. M. E., Bishop, J. L., and Thomson, B. J., 2010.  Mars Reconnaissance Orbiter observations of light-toned layered deposits and associated fluvial landforms on the plains adjacent to Valles Mariners, Icarus. 205, 73-102. 

Abstracts:

Matsubara, Y., Howard, A. D., Burr, D. M., Williams, R. M. and Moore, J. M., 2010, Highly sinuous terrestrial mud meanders as martian analogs, Eos Trans., AGU Fall Meeting, EP51C-0565, Eos. Trans., 91 (34), p. 297.

Williams, R.M.E., Rogers, A.D., Chojnacki, M., Hardgrove, C., and Boyce, J., 2010, Episodic alluvial fan formation in western Terra Tyrrhena, Mars, Lunar and Planetary Science Conference XLI: Houston, TX.  Abstract #2185 (CD-ROM).

Figure 1: Harris crater is located north of Hellas Planitia (A). Harris crater superposes an earlier impact crater (B).  The alluvial fan in this study is located on an isolated crater rim septa (two white arrows B) in the northern portion of the crater.  Three fan units and separate source regions are mapped in (C).  (D) Subscene of CTX image P14_006528_1583_XN_21S292W showing the morphology of the alluvial fan complex in Harris crater. The two alluvial fans have distinct apexes (black arrows) and source regions, separated by _350 m ridgeline (white arrow).