GRSG 34th Conference 2023

Title: Challenges and opportunities of inter-mission change detection between magellan and the future VERITAS and EnVision missions to Venus

Author: Gerard Gallardo i Peres


There is only one demonstrated instance of ongoing volcanism on Venus, as argued in Herrick & Hensley (2023), yet there are many indirect observations of volcanism: variations of atmospheric sulphur concentration, NIR emissivity hotspots detected by VIRTIS and VEM which correlate with volcanic features, and radiometry signatures consistent with ongoing lava flows.

The detection and quantification of volcanic activity in the Venusian surface is a long-held objective, as it plays a central role in deciphering the planet’s origin, geological history, and the driving factors behind its runaway greenhouse atmospheric effect. The Synthetic Aperture Radar (SAR) onboard NASA’s Magellan space mission imaged 98% of the planet’s surface and unveiled, with the finest resolution to date, a dominantly volcanic landscape with a wide array of volcanic and tectonic features. With EnVision and VERITAS missions both carrying radar systems as main payload instruments, image comparison aimed at surface change detection provides a unique opportunity since it could enable the global gathering and quantification of ongoing eruptive activity on the planet. Furthermore, the right methodology could also lead to the detection of other natural surface processes such as landslides, aeolian transportation and local tectonism.

Thus, the distribution of tectonic activity and of accumulated sediment could be investigated across Venus’ surface. Gaining a better understanding of these three groups of process (volcanism, tectonism and sedimentation) will make vital contributions to a more defined model of the planet’s surface and interior dynamics. Nevertheless, inter-mission SAR image comparison is made very challenging by the likely differing spatial resolution, viewing geometry and carrier frequencies of the images to be compared. The SAR backscatter greatly depends on the wavelength and the geometry of the incident radiation, which will be different due to both the orbital characteristics of each mission and their payloads’ design.

The sources and levels of noise may differ, and a pixel-to-pixel comparison will only be meaningful if the resolutions of the images are comparable, which hinders detection of small changes and creates difficulties in the mathematical framework typically used for incoherent SAR change detection. Other factors, such as precise orbit position and a detailed topography, are also critical to ensure meaningful change detection, and their accuracy and availability on Venus is limited. The success of a robust, automatic change detection of the surface of Venus therefore greatly depends on the capacity to model the impact of these ‘system’ changes between the SAR missions.

Both EnVision and VERITAS SAR systems are configured for consistency of observations, in a way that Magellan was not, and this will facilitate change detection; however, the differences between these 2 missions and Magellan are vast. Therefore, perhaps the most fundamental point of an inter-mission change detection methodology on Venus will be its capacity to discern between the image changes that are in fact associated to physical changes on the surface, and those that are just induced by the different characteristics of the SAR sensors. Here we present an overview of a developing methodology to robustly address change detection on Venus; one that can differentiate between system-induced changes and ‘real’ changes.