VARIATIONS IN METER-SCALE SURFACE TEXTURES WITH LATITUDE IN DEUTERONILUS MENSAE, MARS. R. M. E.. Williams, CEPS/NASM MRC 315, Smithsonian Institute, Washington, DC 20013- 7012, USA, [email protected] Observations: A complex physiographic province Discussion: The geographical distribution of pitted called “fretted terrain” is characterized by rectilinear surface textures suggests a latitude-dependent degrada- valleys on the heavily cratered uplands and isolated tion process, such as devolatilization, affecting the mesas and knobs on the sparsely cratered lowland upper few meters of the martian surface. The increase plains at mid-latitudes (25° bands centered at ~40° N in pits with latitude may reflect variations in initial and ~45° S) [1]. Prior studies based on Viking images volatile content or efficiency in volatile removal. A interpreted certain landforms within the fretted terrain combination of geological processes may have con- (e.g. lobate aprons and lineated valley floor material) tributed to the development of the observed surface as evidence for ice-enabled flow of material, presuma- textures, including ice sublimation, ice melt and ae- bly mass wasted debris from valley walls and escarp- olian activity. The age of these landforms in Deu- ments [1-2]. In this investigation, we examined high teronilus Mensae is poorly constrained, but the resolution (1.4-12 m/pix) Mars Orbiter Camera (MOC) scarcely cratered surface as documented by Mangold images of these landforms to map the spatial distribu- [3] gives an age in the Late Amazonian period. This tion of meter-scale surface textures within one region study agrees with several recent works [4; 5 and refer- of the fretted terrain: Deuteronilus Mensae (30-50°N, ences therein] that propose a latitude dependent near- 330-350°W). surface ground-ice cemented dust mantle that has un- Four hundred nineteen MOC images acquired be- dergone degradation coinciding with the last major tween May 1999 and September 2004 of lobate aprons obliquity excursion a few hundred thousand years ago. or valley floors within the study region were exam- ined. Four surface texture types classes were defined This work was supported by grant NASW-02034 from that represent a continnum from small to large void the Mars Data Analysis Program of NASA. spaces, as illustrated in Figure 1. The spatial distribu- tion of four meter-scale surface textures exhibit a lati- References: [1] Squyres S. W. (1979) JGR, 84, 8087-8096. [2] tude dependence with knobby terrain more prevalent at Luccitta B. K. (1984) JGR, 89, B409-B418. [3] Mangold N. (2003) lower latitudes and pitted terrain more common at JGR, 108¸ 2-1 to 2-13. [4] Mustard J. F. et al., (2001) Nature, 412, higher latitudes (Figure 2). 411-414. [5] Head J. W. III et al. (2003) Nature, 426, 797-802. 70 4 19 62 69 61 86 75 41 60 50 Knobs-Coral 40 Knobs-Bumpy Pits-Wavy Pits-Irregular 30 Frequency 20 10 0 32-34 34-36 36-38 38-40 40-42 42-44 44-46 46-48 Latitude Figure 2: Frequency histogram of meter-scale surface tex- ture classes in 2 degree latitude bins observed on lobate aprons and valley floors in Deuteronilus Mensae. Multiple Figure 1: Examples of meter-scale surface texture surface textures could occur in the same image. Numbers classes in Deuteronilus Mensae (from upper left to at top refer to the total number of images in each latitude lower right): Knobs—Brain coral, Knobs—Bumpy, bin. Knobby surface textures are more common at lower Pits—Wavy, Pits—Irregular. Illumination is from latitudes while pitted terrains dominate at higher latitude. the lower left. .