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The dramatic discovery of volcanic hotspots in Venus Express data suggest that the next stage of Venus exploration must focus on its surface: the geological interface between its dense, hostile atmosphere and its Earth-like but puzzling interior. Magellan data revealed an incredible number of volcanoes, as well as rift systems, mountain belts, and a range of features still poorly understood, on a world with a crater count indicative of mean surface age of only 500 Ma, as young as Europe. To understand this complex world, EnVision will measure the rate and nature of geological activity and its influence on atmospheric chemistry. Discovering why Venus is so similar to Earth and yet also so different reaches to the core of the Cosmic Vision questions:

The three main science themes covered by the mission are:

Global Resurfacing and tectonic activity

Volcanic Activity

Weathering and Surface Processes

Figure 3.1 : Images of the Venus surface as recorded by Venera landers
Venera 9 landed on a talus slope of about 30˚; Veneras 10, 13 and 14 landed on rolling plains containing varying degrees of loose sediment on a flat, plate-like, bedrock. Lander image data reprocessed by and © Don P. Aldiss, used with permission.

The bedrock recorded at the Venera 10, 13 and 14 sites consists of laminated or thinly bedded sheets with varying degrees of coarse sediment or regolith. The 5 cm distance between notches on the lander rings of Venera 13 and 14 give an indication of scale (Marov and Grinspoon, 1998). Although chemically similar to basalts, there is no evidence for any features typical of basaltic lava flows such as pillows, rubble, and levées or other channel features. Florensky et al. (1983) describe the Venera 14 layers as sedimentary bedding, or possibly of basaltic pyroclastic deposits, formed by cycles of air fall or ground flow. Based on load carrying capacities derived from the penetrometer and dynamic loads during lander impact (Marov and Grinspoon, 1998; Surkov et al., 1984), the strength of the surface at the Venera 13 site is similar to that of a dense sand or weak rock. At the Venera 14 and Vega 2 sites the recorded strengths are higher but similar to that of a sedimentary sandstone and less than half that of an average basalt.
A major problem is the low resolution of Magellan SAR image data, which although sampled at 75 m pixel⁻¹, is only 110 m along track (north-south) and varies between 101 m and 250 m along track (east-west). Thus almost the entire area imaged by each Venera lander is below the resolution of a single Magellan image cell. New high resolution data are needed in order to identify the location of each lander and so provide context that will help reveal the true nature of the surface environment.

The surface scattering properties (Figure 3.2) reveal that nearly half the surface has properties consistent with a pyroclastic or sedimentary origin, while Doppler centroid data (Figure 3.3) indicate equator-facing microdunes not visible in Magellan image data.

Figure 3.2 : Scattering properties of the Venus surface
Exponential surfaces are consistent with a depositional or pyroclastic origin, Gaussian surfaces may represent erosional or mass wasted surfaces, while Hagfors surfaces are probably loose regolith. Derived from Magellan altimeter backscatter data, processed by Stanford University and published in the Global Vector Data Record.

Figure 3.3 : Doppler centroid of the Venus surface
Doppler shifts in the altimeter backscatter data imply sub-wavelength surface asymmetries consistent with microdunes converging on the major equatorial rift arcs and driven by Hadley cell circulation. Derived from Magellan altimeter backscatter data, processed by Stanford University and published in the Global Vector Data Record.

Together with the lander data, these observations demonstrate an inadequate understanding of the Venus surface and an underestimation of the importance of weathering, transport, deposition and, perhaps, rapid lithification in the hot, dense, supercritical surface conditions.

See also the Frequently Asked Question: What world-wide firsts will EnVision accomplish during its mission?