Continuous Mapping of Surface Dielectric Permittivity and Rock Density in Jezero Crater with the RIMFAX Radar

The RIMFAX Ground Penetrating Radar on the Perseverance Rover has sounded the full traverse of Perseverance with a 0.1 m sounding interval [1, 2, 3]. Due to the air-lifted nature of the antenna 0.75 m above ground, the first strong reflection after the antenna feedpoint is the ground reflection. The strength of this reflection depends on the radar system, as well as surface roughness effects and most importantly the dielectric permittivity of the Martian surface. We operate a measurement mode on Mars which has been ground tested over a metal plate at JPL before launch in order to retrieve the real part of the permittivity.

Fig. 1 shows the permittivity distribution along the traverse of sol 340. The reflectivity calculation is based on the mean amplitude in a 3 ns window around the surface reflection time. Spikes in permittivity can be related to a local change in surface roughness as well as material changes such as a transition from loose regolith to solid rock. Towards the end of the drive, sol 340 is dominated by smooth regolith of slightly higher permittivity. The increased smoothness is in accordance with the largescale terrain smoothness in the HiRISE image. Using the lunar regolith derived formula by [4], the density rises by 0.2 g/cc along traverse.

We address roughness effects by applying a scattering model based on image-retrieved local scattering length. Different frequency bands are tested to minimize roughness effects as well as methods to quantify the surface reflection amplitude. As a result, we present the first continuous permittivity and density distribution along the Perseverance traverse, mapping out formation changes and the statistical surface rock property distribution as seen with radar waves.

[1] Hamran, S-E., et al. (2022), Ground penetrating radar observations of subsurface structures in the floor of Jezero Crater, Science Advances, in press

[2] Casademont, T. M., et al. (2022), Dielectric Permittivity and Density of the Shallow Martian Subsurface in Jezero Crater, LPSC 2022

[3] Hamran, S-E., et al. (2020), Radar Imager for Mars' Subsurface Experiment—RIMFAX, Space Sci. Rev., Vol 216, No. 8

[4] Olhoeft, G. R. and Strangeway, D. W., (1975), Dielectric properties of the first 100 meters of the Moon, Earth Planet. Sci. Lett., Vol. 24, No. 3