Balme, M. R.

Crown, D. A.

Welty, C. B.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsSmall-scale morphologic properties of martian gullies:insights from analysis of HiRISE imagesConference or Workshop ItemHow to cite:Welty, C. B.; Crown, D. A. and Balme, M. R. (2008). Small-scale morphologic properties of martian gullies:insights from analysis of HiRISE images. In: 39th Lunar and Planetary Science Conference (Lunar and PlanetaryScience XXXIX), 10-14 Mar 2008, League City, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2008/Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukSMALL-SCALE MORPHOLOGIC PROPERTIES OF MARTIAN GULLIES: INSIGHTS FROM ANALYSIS OF HIRISE IMAGES.  C. B. Welty1, D. A. Crown2, and M. R. Balme2, 1Department of Geosciences, University of Arizona, Tucson, AZ 85721, 2Planetary Science Institute, 1700 E. Ft. Lowell Rd., Suite 106, Tucson, AZ 85719, carrieb2@email.arizona.edu.  Introduction: The discovery of abundant, geologi-cally recent gullies on Mars in Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) images [1] has led to numerous studies examining potential mecha-nisms for recent fluid or fluid-rich surface processes [e.g., 1-2]. The process or processes involved in Mar-tian gully formation provide important insights into Martian geologic and volatile history.  New gully de-posits have been found in comparison of MOC images acquired over the extent of the MGS mission [3], lead-ing to consideration of contemporary fluid activity.  Gullies appear to be the youngest features in most lo-cations [4], serving as evidence of geologically recent surface flow and erosive processes [5]. Detailed analyses of Mars Reconnaissance Orbiter HiRISE images provide new insights into gully charac-teristics and formation processes.  The purpose of this project is to document the small-scale morphologies of Martian gully systems in order to explore the potential diversity of gully formation processes and to determine if small-scale gully morphology can be used to distin-guish between the various mechanisms proposed on the basis of larger-scale characteristics.  The initial focus of this work has been on examination of boulder distributions and upslope initiation points.   Background: As described by Gulick et al. [6], based on HiRISE image analysis, gullies exhibit a wide range of morphologies, sizes, and geologic set-tings.  Multiple episodes of fluid flow causing erosion and the formation of gullies have taken place, explain-ing the broad range of features observed [7].  Martian gullies have a characteristic set of components.  Gul-lies can be divided into an alcove at the top of the slope, one or more channels extending downslope, and debris fan at the base [1, 8].  Gullies tend to be concen-trated in groups on the walls of craters, pits, and can-yons.  Although a cluster can differ greatly from an adjacent cluster [6], intra-cluster gullies tend to have similar morphologies.   Methods: Geomorphologic analyses were facili-tated by use of Geographic Information Systems (GIS) to correlate and analyze datasets.  Over 40 HiRISE images (~1 meter/pixel) containing at least one group-ing of gullies were imported into a GIS database with a THEMIS IR image mosaic (230 meters/pixel) and MOLA DEM (~128 pixels/degree) for geographic ref-erence and topographic analyses.  Gully features were qualitatively characterized and descriptions compiled, including information on boulder distributions, sinuos-ity ratio, dimensions of gully system components (length and widths of alcoves, channels, and debris aprons), types of initiation points, evidence for multi-ple episodes of activity, and other general qualitative observations. Results: Multiple stages of gully formation are clearly present in the population examined upon de-tailed analysis of over 40 HiRISE images.  Gully sys-tems exhibit different stages of development, with both simple, immature and complex, well-developed mor-phologies evident.  Mature gullies tend to be well-developed with deeply incised and often sinuous chan-nels, frequently terminating in heavily braided fans.  In contrast, immature gullies are small, subtle features, visible only as poorly developed alcoves and/or chan-nels.  Different stages of maturity are intermixed within individual, isolated groupings of gullies (Figure 1).    Figure 1. HiRISE subimage PSP_2514_1420 contains a gully cluster with different levels of maturity. Image centered at 37.9oS, 217.9oE; 0.25 meters/pixel. Image credit: NASA/JPL/University of Arizona  The resolution of HiRISE images allows detailed characterization of the upper reaches of gully systems and provides information on potential gully initiation processes.  There are two primary features interpreted here as gully initiation morphologies: small hollows at the tops of the slopes with debris deposited immedi-Lunar and Planetary Science XXXIX (2008) 2295.pdfately below (Figure 2) and discrete “pinholes” with a single channel or streak extending down the slope be-low (Figure 3).  Based on multiple exposures of these morphologies, the pinhole morphology can evolve into an alcove with maturity, or the alcove can form just below the pinhole.  These two distinct styles of gully formation suggest a diversity in early stage gully mor-phology, which may be still recognizable in the more complex morphologies of mature gully systems.    Figure 2. HiRISE subimage PSP_003287_1115 shows gully initiation as small hollows that may enlarge to form well-developed alcoves with maturity. Image centered at 68.3oS, 1.2oE; 0.50 meters/pixel.  Image credit: NASA/JPL/University of Arizona   Figure 3. HiRISE subimage PSP_004176_1405 shows “pinhole” initiation, with a simple, narrow channel extending from a discrete pinhole.  Image centered at 39.4oS, 202.7oE; 0.25 meters/pixel.  Image credit: NASA/JPL/University of Arizona  HiRISE images allow the occurrence of boulders associated with gully systems to be examined.  Boul-ders tend to be concentrated in alcoves compared to the rest of the gully and the surrounding hillslope.  Although boulder abundance in gullies and on the sur-rounding slopes differs, concentrations tend to de-crease downslope (heaviest concentration generally in alcove, then channel, then hillslope, then least on the apron).  More mature gullies tend to contain a greater concentration of boulders in the alcove than less ma-ture gullies.  Boulder concentrations range from hill-slopes that are heavily covered with boulders to those with few boulders, but the overall downslope trend is consistent.   Discussion: The distribution of boulders in asso-ciation with gullies offers insights into transport proc-esses.  Several processes could account for the de-creasing downslope distribution.  Boulders could be moved downslope in early stages of formation by rockfalls or debris flows and preferentially buried downslope as gullies mature and deposit finer grained sediment.  Because multiple stages of gully formation are evident in the images, widespread burial of large boulders in the distal regions of gully systems seems unlikely for the population of features observed thus far.  Observations of gully alcoves and the associated hillslopes show evidence for rockfalls and the move-ment of individual boulders downslope. The concen-trations of boulders decreasing down-gully and the greater boulder concentration in the gullies than on the surrounding hillslopes [9] are consistent with rockfalls originating in the alcoves as the primary emplacement mechanims for the boulders. Alternatively, the boulder distribution could be a result of debris flows with a progressive decrease in the ability to support clasts downslope.  Erosion within the alcove could be fluvial in nature, facilitated by freeze-thaw, and/or the result of other erosive processes.     HiRISE images have allowed new observations and interpretations of Martian gully systems. Multiple stages of gully formation and the corresponding mor-phologic diversity are clearly evident. The presence of different early-stage gully morphologies indicates that different initiation mechanisms may operate on Mars. The boulder distributions indicate that alcoves erode actively and contribute rocks in a predictable manner to the adjacent hillslopes.  Future research will extend these analyses and further explore the small-scale mor-phologies of Martian gullies.   References: [1] Malin M.C. and Edgett K.S. (2000) Sci-ence, 288, 2330–2335. [2] Costard F. et al. (2002) Science, 295, 110-113. [3] Malin M.C. et al. (2006) Science, 314, 1573–1577. [4] Dickson J.L. et al. (2007) Icarus, 188, 315–323. [5] Mest S.C. et al. (1998) LPS XXIX, Abstract 1334. [6] Gulick, V.C. et al. (2007) Eos Trans. AGU, 88(52), Ab-stract P31B-0439. [7] Berman D.C. et al. (2005) Icarus 178, 465–486. [8] Hartmann W.K. et al. (2003) Icarus, 162, 259–277. [9] McEwen A.S. et al. (2007) Science, 317, 1706-1709.        Lunar and Planetary Science XXXIX (2008) 2295.pdf

Small-scale morphologic properties of martian gullies: insights from analysis of HiRISE images

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