49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 1793.pdf

CONSTRUCTION OF MARSIS 3D RADAR MAPS OF THE MARTIAN POLAR REGIONS. Y. Gim1, D. Bellutta2, and J. Plaut1. 1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 ([email protected]), 2Dept. of Computer Science, Dartmouth College, Hanover, NH.

Introduction: The polar regions of Mars are sites of the surface can be considered smooth, considering of deposition of volatile-rich deposits, including signif- the long wavelengths, 60-150 m, used by MARSIS. icant amounts of water ice in the polar plateau “layered However, reflections from off-nadir surface features deposits” (North/South Polar Layered Deposits: such as ridges and craters can also be detected; we call NPLD/SPLD). The thickness of these deposits reaches such signals “clutter.” 3700 m, based on radar sounding measurements by To reduce data rate with little science impact, MARSIS (Mars Advanced Radar for Subsurface and MARSIS uses an onboard Synthetic Aperture Radar Ionospheric Sounding). Over the past 13 years, (SAR) processing whose resolution is very close to the MARSIS has collected almost 2000 sounding meas- first FZ size [2]. The onboard SAR uses a short- urements over both the NPLD and SPLD. Although aperture Doppler filtering algorithm that separates sur- each measurement reveals vertical profiles of the lay- face and subsurface returns into three different look ered deposits over a long horizontal track of 1000s km directions whose horizontal separation on the surface in the direction of the MARSIS platform movement, its corresponds to the FZ: zero Doppler for nadir returns cross-track swath is limited to several km, mostly de- directly below MEX S/C, a positive Doppler from the termined by surface/subsurface scattering properties at forward looking surface, and a negative Doppler for MARSIS’ low frequencies. This makes it challenging the backward looking surface. To compensate fre- to understand the full picture of the layered deposits. quency-dependent ionospheric delays and dispersions Recently, we have constructed 3D radar maps of the within each band’s 1 MHz bandwidth, SAR-processed NPLD/SPLD by projecting the MARSIS measure- MARSIS data contain frequency-dependent amplitude ments into a polar sterographic volumetric map cen- and phase information at a frequency step of 1.9 kHz tered at each pole. The 3D maps allow us to investi- or 512 steps over the 1 MHz bandwidth. gate the deposits horizontally or vertically and provide Ionospheric correction: Accurate and high- a more complete perspective on the NPLD/SPLD. fidelity ionospheric correction is crucial in interpreting MARSIS: The instrument is a radar sounder MARSIS data as well as constructing 3D projected aboard ESA’s Mars Express (MEX) spacecraft (S/C) maps. Over- or under-estimated ionospheric correc- and has been operational since 2005 [1]. It is a multi- tions could place the surface return above or below the band radar with four center frequencies at 1.8, 3, 4, and physical Mars surface, or reduce overall signal-to- 5 MHz; each band has a 1 MHz bandwidth. The low noise (SNR) levels; such effects will introduce sur- frequencies were selected for maximum penetration in face/subsurface registration errors or smearing when Mars’ subsurface while avoiding significant ionospher- multiple measurements taken years apart are com- ic absorptions by staying slightly above peak plasma bined. We have developed a sophisticated ionospheric frequencies of 1-3 MHz. Being so close to the plasma correction algorithm [3] by searching for best-fit TEC frequencies, however, introduces significant iono- values so that ionospheric-corrected MARSIS surface spheric delays and dispersions that are proportional to returns match what would be expected in the absence the integrated electron density or Total Electron Con- of Mars ionosphere, based on surface echo simulations. tent (TEC) between MEX S/C and the surface. After MARSIS data are corrected for frequency- MARSIS has collected surface and subsurface ra- dependent ionospheric delays and dispersions, they dar echoes and their round-trip time when MEX S/C is become reflected power vs time via a standard range- at altitudes below 1000 km and both the peak plasma compression process. To increase SNR and reduce frequency and TEC values are relatively low. Radar radar speckle noise, the three-look SAR data are com- reflections occur when transmitted waves from bined in power. MARSIS encounter dielectric discontinuities in the Projection: We first set up a polar stereographic medium such as vacuum to ice, ice to bedrock or varia- projection map centered at the pole. The map has 1.5 tions in dust content in ice; the larger the dielectric km horizontal and 50 m vertical pixel sizes and con- discontinuity, the stronger the reflection. tains 70 deg. and higher latitudes. The horizontal pixel At MARSIS wavelengths, most of radar reflections is chosen as half of the first FZ size and the vertical result from smooth dielectric interfaces whose horizon- pixel is approximately half of MARSIS vertical resolu- tal extents are on the order of 2-3 km, called the first tion in ice. For each pixel position, we first find the Fresnel Zone (FZ). Surface smoothness is relatively radar measurement when S/C is directly overhead, and defined with respect to the wavelength. On Mars, most then project the reflected power vs. time data down- 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 1793.pdf

ward when the round-trip time between S/C and the pixel location falls within the radar time window. We find that this is a straight projection process without requiring any orbit-to-orbit adjustments although two orbits filling the same pixel may take place months to years apart. Unlike a migration-based projection meth- od used for SHARAD (SHAllow RADar) data collect- ed at 20 MHz [4], ours limits the horizontal projection to within the FZ due to more specular nature of scatter- ing at MASIS’ low frequencies. Once the radar echo record passes the expected surface position, we adjust the wave velocity to that of pure water ice, to approxi- mate the true depth position of subsurface reflectors within the PLD. We have used 1965 and 2077 orbit data to construct four 3D radar maps of NPLD and SPLD, respectively: 3 highest frequency bands and all of the data combined. Interpolation and averaging: Due to non-uniform MARSIS track distributions over the poles, not all the pixels are filled up via the projection process. On the

other hand, near the poles, there is a high track density corresponding multiple orbits covering the same pix- els. When there is an empty pixel, mostly along the map’s outer edges, we use a nearest neighbor interpo- lation in the horizontal direction only; this is justifiable due to the lateral continuity of the layered deposits. If there is more than one orbit covering the same pixel, we average them in power, improving the signal-to- noise ratio of the final product. Image mosaic: Figure 1 shows MARSIS 5 MHz 2D sliced map-views of the 3D radar maps of NPLD and SPLD at 9 different depths each separated by 500 meters in the vertical (or depth) direction. Darker re- gions within each 2D mosaic indicate very little radar echo while bright features correspond to detectable radar echoes. Cross-sectional views of NPLD (not shown here) indicate that MARSIS penetrates all the way to the basal boundary of the deposits, evidenced by two strong radar reflection lines, one from the sur- face and the other from the crust. SPLD cross-section

views show more internal layered structures than does Figure 1: Radar maps of NPLD (top) and SPLD (bot- the NPLD at MARSIS frequencies. More detailed tom). Each mosaic is a 2D cross-section of the 3D analysis is provided in [5]. map at 9 different depths, each 500 m apart. The hori- References: [1] Plaut et al., Science (2007) Science zontal and vertical extents of each 2D mosaic is ap- 316, 92-94 [2] Jordan et al., (2009) Planetary and proximately 2410 km. Images are the result from Space,1975-1986 [3] McMichael et al., (2017) Radar MARSIS’ 5 MHz band. Conference, 2017 IEEE, 0873-0878 [4] Putzig et al., (2017) Icarus 000, 1-10 [5] Plaut et al., (2018) this conference