Venus’ dense atmosphere prevents optical surface observation, making radar the only geological analysis method. Without ground truth on Venus, terrestrial analogues provide the only means to validate radar interpretation techniques for planetary volcanic terrains. This study presents a multi-parameter radar analysis of the Askja volcanic area, Iceland, serving as a Venus analogue for advancing planetary radar interpretation.

Venus’ surface is dominated by volcanic terrain [2] and understanding what changes the new missions may ‘see’ requires analysis of radar signatures at barren, volcanic terrains on Earth. The Askja volcanic area provides an ideal natural laboratory [1] basaltic composition, diverse flows (historical to >6100 years), minimal vegetation cover, and accessibility for validation [5,6]. Key questions for this study include: (1) Can we differentiate volcanic and sedimentary surfaces using radar backscatter? (2) Can we date flows based on radar characteristics? (3) Can we identify flow emplacement and modification processes?

Methodology and Data Acquisition
This research analyses ten lava flow units from the 1961 Vikrahraun eruption to >6100-year-old flows using multi-scale radar data: high-resolution F-SAR (2m), which was collected by the DLR for the VERITAS mission in Aug 2023, at X-band (3.1 cm), S-band (9.4 cm), and L-band (23.8 cm) with full polarimetry [3,4], and Sentinel-1 C-band (30m) [7]. Flow units are mapped using S-band HH imagery, stratigraphic relationships, and field data.

Key Findings
Backscatter curves as a function of incidence angle for individual lava flows show a strong decrease in backscatter with increasing incidence angle (10-15 dB decrease from 10° to 80°), consistent with typical rough surfaces scattering behaviour.

S-band HH analysis reveals systematic changes in backscatter with increasing age: youngest flows (1961) show highest mean backscatter (-7.9 dB), and the oldest flows (>6100 years) show lowest (-16.8 dB); ca 10 dB backscatter decrease over 6000 years. This marked decrease with flow age is caused by post-emplacement weathering, smoothing and mantling. The backscatter-age relationship is non-linear and indicates rapid decrease over the first 2000 years, then gradual surface homogenization.

The age-backscatter correlations across X, C, S, and L bands reflect systematic changes in lava flow surface characteristics over time. Young flows with original emplacement textures produce high backscatter, while older flows develop smoother surfaces through weathering, resulting in lower backscatter. Surface roughness and backscatter decrease with age, enabling relative dating using multi-parameter radar data.

L-band provides the highest dynamic range, making it optimal for lava flow differentiation. Co-polarized channels (HH, VV) yield similar values, while cross-polarized (HV) shows ca 10 dB lower returns and exhibits the highest dynamic range across units. HV polarization shows the greatest brightness and texture variations and is most suited to flow discrimination; values indicate surface scattering dominance with multiple scattering contributions.

Polarimetric analysis successfully differentiates scattering mechanisms and flow morphologies. High HH and HH/HV ratios indicate smooth pāhoehoe flows and mantled surfaces, while high HV backscatter indicates rough a’ā flows and steep terrain slopes. This aligns with scattering theory: smooth pāhoehoe surfaces produce specular reflection while rough a’ā surfaces produce diffuse scattering.

Implications
The study demonstrates that multi-parameter radar data enables discrimination of flow morphologies and ages through wavelength-scale roughness analysis, with HV polarization providing optimal discrimination capabilities. The systematic backscatter decrease with age indicates post-emplacement modification processes including weathering, aeolian mantling, and surface smoothing. These findings provide ground truth for interpreting radar signatures of volcanic terrains on Venus and other planetary bodies. The established relationships between surface age, morphology, and radar characteristics offer a framework for relative dating of volcanic surfaces and identifying flow types using airborne and spaceborne radars. Other analogues, which are more arid and topographically complex, could be analysed next for further validation.

Conclusions
Backscatter statistics from multi-parameter radar allows effective differentiation of lava flow ages and types. Age-backscatter correlations provide quantitative dating frameworks, while polarimetric analysis enables morphological and textural discrimination. These results will directly support upcoming Venus missions, providing validated frameworks for analysing volcanic terrains across planetary surfaces.

References: [1] Adeli et al., 2023; [2] Brossier et al., 2020; [3] Horn et al., 2017; [4] Keller et al., 2024; [5] Mason et al., 2024; [6] Raguso et al., 2025; [7] Torres et al., 2012.