Cassini Bistatic Radar Campaign during the Prime and Equinox Missions: Heterogeneous Reflections from Titan's Solid Surfaces

During the Cassini Prime (2004-2008) and Equinox (2008-2010) missions, the Radio Science Subsystem onboard the Cassini spacecraft conducted seven bistatic radar (BSR) observations of Titan, Saturn’s largest moon. A variety of terrains, including unique geological features such as plains and dunes identified by the Cassini RADAR, were observed across equatorial, mid-latitude, and south-polar regions.

In this radio science experiment, Cassini’s High-Gain Antenna (HGA) transmitted unmodulated, right-hand circularly polarized signals at three frequencies—S-band (λ=13 cm), X-band (λ=3.6 cm) and Ka-band (λ=0.94 cm)—toward Titan’s surface. The antenna was pointed to enable quasi-specular reflections from the illuminated portion of the moon’s surface to be received by NASA’s Deep Space Network (DSN) antennas on Earth, which have the capability to receive both left-hand and right-hand circularly polarized components of the reflected signals. The investigation of quasi-specular echoes, when detectable, can provide constraints on surface roughness and near-surface effective dielectric constant, which is connected to the structural and compositional properties of Titan’s terrains.

Analysis of the BSR data from these seven experiments reveals highly heterogeneous scattering behavior across Titan’s surface. Reflections range from barely detectable signals, characterized by broad, diffuse echoes just above the noise floor, to narrower and more powerful reflections suggesting the presence of very smooth and isolated patches of land. Such heterogeneity in surface scattering was also noticed during ground-based observations of Titan by means of the Green Bank Telescope and Arecibo Observatory.

In this work, we present a detailed analysis of this largely unexploited dataset, highlighting regional variations in forward scattering and providing preliminary findings about surface roughness and near-surface dielectric constant of various regions on Titan. Whenever possible, we compare the BSR findings with Cassini SAR maps of Titan’s surface and discuss correlations between scattering variations observed by the two instruments.