52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548) 2536.pdf

ANALYSIS OF RADAR ATTENUATION IN THE POLE LAYERED DEPOSITS OF WITH 3D RADAR IMAGING. A. T. Russell1, N. E. Putzig1, N. Abu Hashmeh2, and J. L. Whitten2, 1Planetary Science Institute, Lakewood, CO, 80401, USA. 2Tulane University, New Orleans, LA, 70118, USA. Contact: arus- [email protected]

Introduction: The variability of the Late tain few internal reflectors. We mapped the base of all climate is recorded in the north and south LRZ appearing in the PA3D data (Fig. 2). polar layered deposits (PLD) [1-3]. The PLD contain mixtures of ice and dust, and radar sounding results have shown them to be predominately composed of ice [4]. Relative to the NPLD, the SPLD is much more attenuating of the Mars Reconnaissance Orbiter Shallow Radar (SHARAD) signal, including in some of its thinnest and thickest regions [5]. We investigate this radar behavior to better understand the structure and composition of the PLD and to characterize the radar properties of the SPLD. Methods: We used the Planum Australe 3D (PA3D) volume data, produced using 2093 SHARAD tracks in the south polar region [6], to map the basal reflector of the SPLD everywhere that the signal pene- tration allows (Fig. 1). Our procedure involves tracing subsurface reflectors that either originate at the edge of the SPLD or can be correlated to ones that do.

Fig. 2. Thickness map of the LRZs

The sample values of the PA3D are of de-biased re- flection strength. We found the minimum sample value within the 3D volume and subtracted that value from all the frames in the volume. We then squared the samples in the resulting frames in order to recover sample values that were suitable for radiometric use. We used these values to produce a return signal power- value map of the basal reflector (Fig. 3).

Results: The thicker poleward portions of Australe Lingula and Promethei Lingula contain few SPLD and LRZ basal reflections in the PA3D data. Even in some of the thinnest (< 1 km thick) regions of the SPLD (near 155°E, 190°E, and 270°E), the PA3D data show no reflectors and reveal very little detail about Fig. 1. Thickness map of the SPLD. internal structure. The SPLD basal reflector has power values < 0.005 Prior studies have identified areas of very low re- dB. Lower power values (< 0.002 dB) coincide with turned radar power within the SPLD that are known as the thicker parts of the SPLD. The highest power val- low reflectivity zones (LRZ) [5],[7-9]. These facies are ues occur in some of the thinnest regions of the SPLD located at the top of the stratigraphic column and con- (130°E, 240°E, and the periphery).

52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548) 2536.pdf

Ongoing Work: We are currently mapping the deepest visible reflectors throughout the SPLD to char- acterize the extent of the SHARAD signal attenuation in the PA3D data. We are comparing these results with a similar anal- ysis of 2D SHARAD data [10]. These maps will also be compared to maps produced using MARSIS data to determine the effects of frequency.

Fig. 3. Power-value map of the basal reflector of the SPLD

Acknowledgments: We are grateful to the MRO Project and the SHARAD Team for supporting the 3-D projects and to SeisWare, Inc. for access to the inter- pretation software used for the analysis and displays of radar data and mapping results presented here. We would like to thank Fritz Foss for his guidance in our use of the PA3D data. References: [1] Herkenhoff K. E. and Plaut J. J. (2000) Icarus, 14, 243-253. [2] Koutnik M. et al. (2002) JGR, 105, E11, 5100. [3] Laskar J. et al. (2004) Icarus, 170, 343-364. [4] Zuber M. T. et al. (2007) Science, 317, 1718-1719. [5] Whitten J. L. and Camp- bell B. A. (2018) JGR: Planets, 123, 1541-1554. [6] Foss II F. J. et al. (2017) The Leading Edge, 36, 43-57. [7] Bierson C. J. et al. (2016) Geophys. Res. Lett., 43, 4172-4179. [8] Phillips R. J. et al. (2011) Science, 332, 838-841 [9] Putzig N. E. et al. (2018) Icarus, 308, 138- 147. [10] Abu Hashmeh N. et al. (2021) LPSC this conference.