 GRAIN SHAPE AND SIZE ANALYSIS OF SAND- AND SILT-SIZE SEDIMENT IN A TERRESTRIAL PERIGLACIAL LANDSCAPE: A POSSIBLE PROCESS ANALOG FOR SAND AND SILT IMAGED BY THE PHOENIX OPTICAL MICROSCOPE AT THE PHOENIX MARS LANDER LANDING SITE.  B.L. Brugman1, L. Culp2, J. Gibson2, L. Hicks2, K. Lecos2, A. Wayne2, and M.A. Velbel1, 1Michigan State University, Department of Geological Sciences, 2Michigan State University, Honors College. Introduction:  The Phoenix Mars Lander landed in Vastitas  Borealis,  near  Mars'  northern  polar  cap,  on May 25 2008, and operated until November 2, 2008. The landing site is in a valley dominated by periglacial polygonal  patterned  ground with 3  to  6  meter  polygons, with a thin layer of basaltic sand overlying permafrost [1].  Depth to ice was 2-6 cm.  A Robotic Arm (RA) dug trenches  and  acquired  samples  of  dry soil and sublimation residues from water ice.  The RA delivered  samples  to  several  instrument  packages  containing a variety of scientific instruments, including an Optical  Microscope  (OM).   The  OM  imager  was equipped with a fixed-focus, fixed-magnification optical system, two lenses, and LEDs in red, blue, green and ultraviolet for simulating color imaging.  A variety of substrates  were  distributed  on  a  rotating  wheel  the movement of which enabled the OM to focus and photograph  each  sample  individually  [2].   Previous  research has classified grain types by color  (black and brown)  [3],  quantified  grain  form by measuring  the long and short dimensions of individual grains directly from OM images [4], and measured particle sizes and size distributions [5]. This presentation describes preliminary  results and next steps in our effort to describe and interpret differences between grain samples imaged by the OM, based on a terrestrial, periglacial analog of those collected by Phoenix Mars Lander.  Methods: Analog grain samples were studied for shape variation between two different depositional subenvironments (ice wedge and polygon interior).   The most abundant  coarse  grains  imaged by the OM are coarse silt (32 µm < ϕ < 62.5 µm; >50%) and very fine sand (62.5 µm < ϕ < 125 µm ; ~33%) [3-5].  All grains that passed a 200  µm sieve were imaged by OM [3-5]. Analog samples were split & passed through a 177 µm sieve, to examine grains most similar to OM samples. Grains from each sample were imaged under a scanning electron microscope (SEM).  A method similar to Riley-sphericity and grain elongation [4] was used to measure equancy, defined here as the ratio of characteristic width to maximum length of the grain under observation.  Results: No significant difference in equancy was observed in our study.   However,  statistical  analyses revealed  variation  in  grain-size  between  the  periglacially modified glacial till in a polygon interior (Fig. 1) and the sandy ice-wedge fill (Fig. 2).  Although grain sizes from both samples are within the same size-fraction  limits  for  sand  grains,  the  observed  variation between these limits within the two samples is statistically significant, and may reflect differences in depositional and weathering processes unique to the respective environments, but undetectable under current measurement protocols. Summary: The next phase of this study will examine whether size-variation across depositional sub-environments is also represented in sand grains imaged directly by the OM on the Phoenix Mars lander.  Measurements of width and length will be repeated on images of Mars grains at similar resolution to those conducted on the periglacial analog used in the first phase of the study.  These measurements will then be statistically  analyzed  for  significant  size  variation.   These results may help constrain similarities and differences in the sorting, abrasion, and weathering processes and conditions that occur during cryogenic landform evolution. References: [1] Arvidson R. E. et al. (2008) JGR, 113,  E00A03,  doi:10.1029/2007JE003021.  [2]  Hecht M.  H.  et  al.  (2008)  JGR,  113, E00A22, doi:10.1029/2008JE003077. [3] Goetz W. et al. (2010) JGR,  115,  E00E22.  [4] Goetz  W. et  al.  (2010)  LPS XLI, Abstract #2738. [5] Pike W. T. et al. (2011) GRL, 38, L24201. Figure 1. SEM image of <177 µm fraction of periglacially modified glacial till.  Image acquired at Phoenix OM-relevant 4 µm/pixel resolution.   Figure 2. SEM image of <177 µm fraction of sandy ice-wedge fill.  Image acquired at Phoenix OM-relevant 4 µm/pixel resolution.   
