 GRAIN-SURFACE TEXTURAL INDICATORS OF VOLATILES IN TERRESTRIAL MARS-REGOLITH ANALOGS: IMPLICATIONS FOR INTERPRETING SAND AND SILT IMAGED BY THE PHOENIX OPTICAL MICROSCOPE AT THE PHOENIX MARS LANDER LANDING SITE - II.  M.A. Velbel1, L.S. Slajus1, B.D. Wade2, P.R. Conrad3, Q.J. Costello3, M.G. Harju3, K.A. Hunnewell3, L.S. Jackson3, P.R. Kurtz3, J.R. Marcero3, A.M. Mason3, A.C. Muethel3, J. Mukhtar3, M.P. O'Connor3, L.D. Peterson3, J.A. Quinn3, L.J. Shehi3, and I.C. Walker3. 1Michigan State University, Department of Geological Sciences, East Lansing, MI, USA 48824-1115 (velbel@msu.edu), 2MSU, Department of Plant, Soil & Microbial Sciences, 3MSU, 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 [1-7]. 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 a variety of scientific instruments, including an Optical Microscope (OM). The OM was equipped with a fixed-focus, fixed-magnification optical system, and LEDs in red, blue, green and ultraviolet for simulating color imaging. OM image spatial resolution was determined by the pixel dimension of 4 μm/pixel [8]. A variety of substrates were distributed on a rotating wheel the movement of which enabled the OM to focus and photograph each sample individually [8]. Previous research has classified grain types by color (black and brown) [9], quantified grain form by measuring the long and short dimensions of individual grains directly from OM images [10], and measured particle sizes and size distributions [11]. This presentation describes early results of an investigation to re-interpret grain shapes of the coarsest grains imaged by the Phoenix OM by comparison with grains from the same size fractions of well-studied terrestrial (basaltic) analogs of Mars' surface materials.       Materials and Methods: Six terrestrial materials widely regarded by the community as being analogous to fine-grained Mars regolith ("soil") were examined. Among these, NASA JSC Mars-1 Mars Soil Simulant consists of glassy phyllosilicate-poor palagonitic volcanic ash from the late Pleistocene Pu'u Nene cinder cone at 1850 m elevation above mean sea level (AMSL) on the south flank of Mauna Kea volcano on Hawai'i [12]. HWMK600 consists of phyllosilicate-poor palagonitic volcanic tephra collected at 3730 m elevation AMSL along the side road to the Very Long Baseline Array (VLBA) Telescope on Mauna Kea [13,14].   Grains from each sample were mounted on aluminum stubs using carbon adhesive tabs, coated with carbon, and imaged using a JEOL 6610LV scanning electron microscope (SEM) in secondary electron imaging mode, with energy dispersive spectroscopy (EDS). Context images of each entire mount were acquired at the same resolution as the OM (4 μm/pixel), and 150 grains from each sample were imaged at one grain per frame to survey grain-surface textures. Grain surface-feature inventories [15,16] were compiled from >100 grains per sample. Features inventoried included vesicles, glassy melt-textured surfaces, planar fracturing, adhering fine particles, and cracks.     Figure 1.  Cracks on the surface of a volcanic ash particle; JSC Mars-1.  SEM secondary electron image.  Results: JSC Mars-1. Vesicles ranging in size up to lengths comparable to grain size occurred on >50% of the grains examined. No grains showed glassy melt-textured surfaces. Fine adhering particles resolvable in whole-frame images of individual grains occurred on all grains. Networks of intersecting and randomly oriented shallow cracks that appear to be confined to grain surfaces (Fig. 1) occurred on more than half of the grains.  HWMK600. Vesicles ranging in size up to lengths comparable to grain dimensions (Fig. 2) occurred on ~50% of the grains. Up to ~5% of the grains exhibited glassy melt-textured surfaces. Fine adhering particles resolvable in whole-frame images of individual grains (Fig. 3) occurred on nearly all grains.  Cracks occurred on approximately one-tenth of the grains. Cracks apparently confined to the surface material on a grain were more abundant than cracks extended deeper into the grain itself.   file:///C:/Users/Michael/Documents/MV/Teaching/Ug%20Service-Elective/HRS/HRS14-15%20Sands%20of%20Mars%20IV/Project/LPSC15/velbel@msu.edu  Figure 2.  Vesicles exposed at the surface of a volcanic ash particle from HWMK600.  SEM secondary electron image.   Figure 3.  Adhering particles on the surface of a volcanic ash particle from HWMK600. SEM secondary electron image.  Discussion: The vesicles in JSC Mars-1 and HWMK600 indicate volcanic eruptions of volatilebearing magma. Such volatiles contributed to global atmospheric inventories at the time of eruption, but may or may not have been locally abundant in the local environment at the time of post-depositional tephra alteration or weathering. The persistence of primary rockforming minerals (e.g., plagioclase), and the absence of any instances of common primary-mineral chemicalweathering textures [17-20], in all three analogs examined for the previous study [21] suggested limited duration and/or intensity of water-driven chemical weathering in the three analog environments.   Previous work [21] suggested that the difference in the abundance of grains with cracked surfaces between Mauna Kea palagonitic tephras JSC Mars-1 (>50%) and HWMK600 (~10%) might be associated with the climate histories, moisture regimes, and consequent weathering processes of the two sample localities. However, all textural features cataloged in this study have been documented to occur on un-weathered volcanic ash and tephra [22-24]. Adhering fine particles and cracks could be caused by volatile-driven weathering of volcanic ash and tephra after deposition [21], but microscopically similar materials are attributed to processes of welding and adhesion, in some instances under the influence of water vapor, in the ash clouds [22-24]. Vesicles in the two Mauna Kea analog materials (e.g., Fig. 2), indicative of the presence of volatiles in the magma during eruption, are in many instances large enough to have been visible at OM resolution. Smallerscale features (e.g., cracks, adhering fine particles) on grain surfaces from Mars regolith analog materials (Fig. 1,3), that might record the presence of volatiles either during eruption or during post-depositional alteration or weathering, are too small to have been imaged at OMachievable resolution. Consequently, the significance of the OM imagery for assessing habitability remains under-constrained.   References: [1] Arvidson R. E. et al. (2008) JGR, 113, E00A03, doi:10.1029/2007JE003021. [2] Mellon M. T. et al. (2008) JGR, 113, E00A23, doi:10.1029/2007JE00303. [3] Seelos K. D. et al. (2008) JGR, 113, E00A13, doi:10.1029/2008JE003088. [4] Heet T. L. et al. (2009) JGR, 113, E00E04, doi:10.1029/2009JE003416. [5] Mellon M. T. et al. (2009) JGR, 114, E00E06, doi:10.1029/2009JE003418. [6] Levy J. S. et al. (2010) Icarus, 206, 229-252. [7] Gallagher C. et al. (2011) Icarus, 201, 458-471. [8] Hecht  M. H. et al. (2008) JGR, 113, E00A22, doi:10.1029/2008JE003077. [9] Goetz W. et al. (2010) JGR, 115, E00E22. [10] Goetz W. et al. (2010) LPSC XLI, Abstract #2738. [11] Pike W. T. et al. (2011) GRL, 38, L24201. [12] Allen C. C. et al. (1998) EOS, Transact. AGU, 79, 129-136. [13] Morris R V. et al. (2000) JGR, 105, E1757-E1817. [14] Morris R. V. et al. (2001) JGR, 106, E5057-E5083. [15] Higgs (1979) J. Sed. Pet., 49, 599-610. [16] Goudie et al. (1984) ESPL, 9, 289299. [17] Velbel M. A. (2007) Developments in Sedimentology 58, 113-150. [18] Velbel M. A. and Barker W. W. (2008) Clays & Clay Minerals 56, 111-126.  [19] Velbel M. A. and Losiak A. I. (2010) J. Sed. Res., 80, 771-780. [20] Velbel M. A. (2009) GCA, 73, 60986113. [21] Velbel et al. (2014), LPS XLIV, #2264. [22] Heiken G. and Wohletz K. (1991) SEPM SP, 45,1926. [23] Clarke et al. (2009) JVGR, 180, 225-245. [24] Lautze et al. (2012) Phys. Chem. Earth, 45-46, 13-127. 
