Geological Modeling of Subsurface Structures from RIMFAX Ground Penetrating Radar Observations in Jezero Crater, Mars.

Introduction:  NASA's Perseverance Rover is exploring the Jezero Crater carrying the RIMFAX ground penetrating radar. RIMFAX has acquired a continuous subsurface radar image at 10 cm intervals along the rover’s > 25 km long ground track across the crater floor and fan, probing depths of >30 m below the rover, Figure 1. RIMFAX provides subsurface context for better understanding the depositional environments of the geological units that the rover has examined on the crater floor and the fan thus far. The Mars 2020 mission objectives are to seek signs of ancient life on Mars and cache a set of samples for possible return to Earth by a follow-on mission, [1]. An important part of the mission is to document the geological settings the cached samples are collected from. The Radar Imager of the Mars’ Subsurface Experiment is a gated FMCW radar operating in the frequency band of 150 – 1200 MHz. The antenna is a slotted bow tie antenna 70 cm above the surface. The typical center frequency of the reflected signal at 10–15-meter depth is 400 Mhz. RIMFAX collects soundings in three operating modes (surface, shallow and deep) every 10 cm along the rover’s traverse path, [2].

 

 

Figure 1 Orbital context maps of the RIMFAX observations in Jezero Crater. Orbital High Resolution Imaging Science Experiment (HiRISE) color base map showing the path (pale white lines) of the Perseverance rover.

RIMFAX Radar Data: In the Margin Unit RIMFAX images several strong dipping layers that get horizontal at depth resembling clinoforms, Figure 2. The location of these reflecting layers is picked from the RIMFAX radargrams giving interface points in depth and location of the different layers.  

 

Figure 2 Radargrams give the reflected signal where white is high reflecting amplitude and black low amplitude. The depth axis for this radargram is 35 meters.

 

Geological Modeling: The RIMFAX data can be used to make 3D models of the subsurface. The depth and locations of selected layers are picked from the RIMFAX Marginal Unit radargrams. From these layers, a geological model is made using GemPy, see [4] which employs a universal cokriging interpolation method, [4]. GemPy allows the user to generate complex 3D structural geological models through the interpolation of layer interfaces and orientation measurements and topography.  

Figure 3 shows a 2D cross section through the GemPy model. Figure 4 shows a map view of the model, indicating locations of outcropping layer boundaries. Figure 5 shows a 3D view of the subsurface layers in relation to surface topography.

Figure 3. 2D cross section through the GemPy model from West to East. Non-shaded layers are truncated by surface topography.

Figure 4. GemPy model reconstructed geologic map based on the extrapolation of picked layer depths and locations in the RIMFAX radargrams in the Margin Unit (dots).   

Figure 5. 3D geological model reconstructed from RIMFAX data showing the subsurface dipping layers and where they crop out on the surface.

 

Discussion: The large-scale subsurface structures revealed by the RIMFAX in the Marginal Unit are reminiscent of deltaic clinoforms. Figure 4 shows that Marginal Unit layers are cropping out in complicated patterns that are not apparent in orbital images or topographic maps. Deeper eroded layers are predicted to be cropping out in the wall of Neretva Vallis. Using RIMFAX radar data in conjunction with geological modelling is a powerful tool for studying subsurface structures on Mars.

 

Acknowledgments: The data used in this work are available at the NASA PDS Geosciences Node (https://doi.org/10.17189/1522644). This work was supported by the Research Council of Norway, grant no. 309835.

References:

[1] Farley et. al. (2021) Space Science Review, [2] Hamran et. al. (2021) Space Science Review, [3] Hamran et. al. (2022) Science Advances, [4] de la Varga et. al. (2019) GemPy, Geosci. Model Dev., 12, 1–32