DUST SIZE AND SHAPE FROM MSL ENGINEERING CAMERAS Hao Chen-Chen ([email protected]), Santiago Pérez-Hoyos, Agustín Sánchez-Lavega Dpt. Fisica Aplicada I, ETS Ingenieros Bilbao, University of the Basque Country (UPV/EHU). Bilbao, Spain

Abstract Although not designed for this specific purpose, images taken by the Science Laboratory (MSL) rover Engineering Cameras ( and Hazcam) can be used for retrieving the dust aerosol physical properties at Crater by evaluating the sky brightness as a function of the scattering angle. A retrieval scheme based on a radiative transfer model using discrete ordinates is proposed in this poster. Results obtained for dust particle size distribution effective radius values (mostly within 1.0 and 1.9 microns range region) and particle shape (cylindrical, aspect ratios around 1.5) agrees with previous studies Grupo de Ciencias Planetarias [1][2][3][4]

1. INTRODUCTION 2. MSL ENGINEERING CAMERAS - Dust plays a critical role in the behaviour and dynamics of Martian The MSL rover is equipped with 12 engineering cameras, built atmosphere (opacity, heating rates), providing information in mesoscale, under the same design as the Mars Exploration Rovers (MER) [5]. synoptic and planetary scales (dust devils, local/regional/global dust The objective of these cameras is to support surface operations of the storms, vortex, etc.). rover providing surrounding terrain views; detect and avoid hazards and - Martian dust radiative properties (single scattering albedo, extinction characterize the rover’s position and orientation [6] efficiency, phase function) mainly depends on the particle: size, shape and HAZCAM(HAZARD AVOIDANCE CAMERA) optical refraction index NAVCAM (NAVIGATION CAMERA) - Aerosol size and shape can be constrained by evaluating the sky brightness as a function of the angle away from the Sun (scattering angle) [4]

MSL ENGINEERING CAMERA OBSERVATIONS Navcam and Hazcam observations were calibrated and Front Hazcam navigated following procedure derived by Soderblom et al. (2008) [7] and particularised for MSL [8]: positive Rear Hazcams azimuth direction (Left) Navcam Sun pointing observations. (Centre) Navcam 360º

Sky survey. Image credits: NASA/JPL (Right) Hazcam Source: “The Engineering Cameras. Maki et al. 2012 backward scattering region. Technical specifications can be found in [6]; optics performance in [5].

4. RESULTS 3. RETRIEVAL METHODOLOGY SIZE Dust aerosol properties are characterised by comparing observations with H. Chen-Chen et al., (2018); manuscript under review radiative transfer model computations. Structure: • Plane-parallel atmosphere • Pressure, Temperature, density and abundance from MCD [9],[10] Aerosol: • Dust aerosol single scattering albedo and phase function: T-Matrix [11] • Dust refraction index from Wolff et al. 2009 [12] • Dust column optical depth: Navcam, Mastcam [8][13] RT solver: • Python implementation of discrete ordinates method, DISORT [14][15]

Left-top: MSL Navcam (blue) derived daily behaviour of dust column optical depth for 2.5 Martian Years compared with MSL Mastcam retrievals (gray)[13]. Left-bottom: Navcam observations-model comparison outputs for dust particle size distribution effective radius compared with Chemcam [1]. Right: Correlation study between aerosol optical depth and dust effective radius; R2 ~ 0.50, Left: Chi-squared goodness-of-fit mapping for effective radius and optical depth retrieval from Navcam suggesting a medium-moderate trend to find Sun-pointing observation. larger dust particles when atmospheric dust Center: Observation (Navcam sky-survey) and DISORT model sky radiance simulation comparison. loading is higher, in accordance with [1], [2], [4]. Right: Sky brightness as a function of the scattering angle curve fitting comparison between observation (cyan) and modelled curves with with spherical(yellow) and cylindrical (red) dust aerosol shape.

SHAPE 5. CONCLUSIONS - Radiative transfer results validate the scientific use of MSL Engineering Cameras for dust aerosol particle studies.

- Navcam Sun-pointing images retrieved dust particle size distribution effective radii agrees with previous studies using different instruments at Gale Crater, observing seasonal differences for the aerosol size(smooth during low dust season, steep increase and peaks during dust season Ls > 150º) and dependent on the atmospheric dust loading. [1][2]

- Navcam and Hazcam observations of the backward scattering region (up Top-Left: MSL Navcam and Hazcam observations. Seasonal variation of the sky brightness as a function to 160º) of the sky intensity constrained the single scattering phase of the scattering angle. Sky radiance data (colorbar) were retrieved along the principal plane, calibrated function of particles. Comparison with RT models for different aerosol and navigated following [8]. Hor. axis: Ls(deg); ver. axis: Scattering angle (deg, 0º = solar disc centre) Top-Right: Results from the comparison of observation and modelled data. Aerosol single scattering shapes generated with T-matrix [11] showed that best fitting single phase functions for cylindrical (red), spheroid (blue) and chebyshev (yellow) particle shapes were scattering phase functions corresponded to irregular particles (spheroids evaluated. On the vertical axis the diameter-to-length aspect ratio is given. For chebyshev particles the integer part indicates the degree of the polynominal. and cylinders) with Diameter-Length aspect ratios around 1.5. [3][4]

Aerosol single scattering phase functions generated with T- Matrix [11] for different shape References and diameter-length aspect [1] McConnochie, T., et al. (2017). Icarus, 2017, doi: 10.1016/j.icarus.2017.10.043. ratios . [2] Vicente-Retortillo, A., et al. (2017). Geophys. Res. Lett., 44, 3502-3508, doi:10.1002/2017GL072589. Left: Complete phase function [3] Tomasko, M., et al. (1999). J. Geophys. Res., 104, 8987-9007, doi:10.1029/1998JE900016. th Right: Zoom within the 60º to [4] Smith, M.D., and M.J. Wolff (2014). 5 MAMO, Abstract #2101. [5] Maki, J.N., et al. (2003). J. Geophys. Res., 108 (E12), 8071, doi:10.1029/2003JE002077. 180º scattering angle region. [6] Maki, J.N., et al. (2012). Space Sci. Rev., 170, 77-93, doi:10.1007/s11214-012-9882-4. Values in log-scale. [7] Soderblom, J. M., et al. (2008). J. Geophys. Res., 113, E06S19, doi:10.1029/2007JE003003 [8] Chen-Chen, H., et al. (2018). Manuscript under review. [9] Forget, F., et al. (1999). J. Geophys. Res., 104 (E10), 24155-24176, doi:10.1029/1999JE001025 [10] Millour, E., et al. (2015). EPSC Abstracts, Vol. 10, EPSC2015-438. [11] Mishchenko, M., et al. (1998). J.Quant.Spec.Rad.Tra., Vol, 160, 309-324, doi: 10.1016/S0022-4073(98)00008-9 Acknowledgements [12] Wolff, M.J., et al. (2009), J. Geophys. Res., 114, E00D04, doi:10.1029/2009JE003350 This work was supported by the Spanish project AYA2015-65041-P with FEDER support, Grupos Gobierno Vasco IT-765-13, [13] Lemmon, M. T., (2014). Eighth International Conference on Mars (2014). Abstract #1338. 2014LPICo1791.1338L Universidad del Paías Vasco UPV/EHU programme UFI11/55, and Diputación Foral de Bizkaia-Aula EspaZio Gela. Thanks to [14] Adamkovics, M., et al. (2016). Icarus, 270, 376-388, doi: 10.1016/j.icarus.2015.05.023 Dr Mark Lemmon for the MSL Mastcam optical depth data. [15] Stamnes, K., et al. (1988). App. Opt., 27, 2502-2509, doi:10.1364/AO.27.002502.

H. Chen-Chen, S. Pérez-Hoyos and A. Sánchez-Lavega – Universidad del País Vasco (UPV/EHU) – Sesión de Pósters: Poster CP4 - XIII REUNIÓN CIENTÍFICA DE LA SOCIEDAD ESPAÑOLA DE ASTRONOMIA. Salamanca, 16-20 Julio 2018.