 MICROSCOPY OF MARS GLOBAL SIMULANT MGS-1.  M. A. Velbel1, B. D. Wade2, M. B. Widener3, D. A. Wakefield3, B. S. Sollenberger3, X. Shan3, R. Raghunath3, E. E. McLaren3, C. E. MacNee3, D. A. Leen3, A. R. Dubinski3, T. L. Dalrymple3, M. Collins3, and D. E. Abbott3, 1Department of Earth and Environmental Sciences, Michigan State University, East Lansing, MI 48824 USA (velbel@msu.edu), 2Michigan State University, Department of Plant, Soil & Microbial Sciences, 3Michigan State University, Honors College.   Introduction:  The Phoenix Mars Lander (PHX) 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). Samples for delivery to the OM were passed through a 200 µm sieve [2]. The OM was equipped with a fixed-focus, fixed-magnification optical system, two lenses, 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 [2]. 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], measured particle sizes and size distributions [4], and compared grain form among different PHX grain types [5] and with several terrestrial analogs [6-11]. This presentation describes preliminary results of our microscopic investigation of Mars Global Simulant MGS-1 [12].  Methods and Materials:  Grains were characterized by optical reflected light microscopy and backscattered scanning electron microscopy (BSEM). Four-hundred eighty (480) of the largest grains were selected. Each was described and measured manually from the optical images, examined by SEM, and described and measured manually from the SEM images.  SEM. 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 in secondary electron imaging mode (SSEM), with energy dispersive spectroscopy (EDS). Forty (40) grains from each of twelve (1) sample mounts were imaged at one grain per frame to survey grain-surface textures.  Grain characterization. Characteristics examined included grain vs. aggregate; color and transparencytranslucency-opacity (only in the optical images); and textural properties - grain size and shape (equantcy and roundness). Results:  The grains selected for imaging were mostly medium sand size, ~300 ± 150 µm. Nearly 100% were individual grains, not aggregates. Brown and black grains constituted most of the grains imaged. Brown grains were most abundant, approximately twice as abundant as black grains. Grey or white grains  constituted <10% of the grains examined. Sub-angular to angular grains were the most abundant; very few well-rounded grains were observed.  Discussion: The grains selected for imaging and EDS analysis represent only the coarsest fraction of MGS-1 [12]. However, this is only slightly larger than the dimensions of the sieve through which PHX samples were passed to the OM sample wheel to acquire the PHX images [2] with which these results will be compared.  Ongoing work: Future work includes but is not limited to (1) comparison of MGS-1 optical images with PHX OM images, (2) relating grain color and transparency as determined from the color optical images to compositional data from EDS, and (2) quantitative image analysis of grains sizes and shape metrics.  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] Pike W. T. et al. (2011) GRL, 38, L24201. [5] Goetz W. et al. (2010) LPS XLI, Abstract #2738. [6] Brugman B. L. et al. (2014) LPSC XLV, Abstract #2626. [7] Velbel M. A. et al. (2015) LPSC XLVI, Abstract #2264. [8] Velbel M.A. et al. (2017) LPSC XLVIII, Abstract #2490. [9] Peterson L.D. and Velbel M.A. (2018) LPSC XLIX, Abstract #2541. [10] Velbel M. A. et al. (2018) LPSC XLIX, Abstract #2768.  [11] Velbel M. A. et al. (2019) LPSC L, Abstract #2938. [12]  Cannon K. M. et al. (2019) Icarus, 317, 470-478. 
