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:
- How important is geology and atmosphere to sustaining life?
- Are the initial conditions of planetary formation of key importance, or its later evolution?
The three main science themes covered by the mission are:
Without question, the distribution of volcanoes (Figure 2.1) is far from random, with a concentration two to four times greater than the global mean in the so-called "BAT" region between Beta, Atla and Themis.
As well as a large number, there is also a wide variety of volcanic features on Venus. Some, such as large shield volcanoes, are relatively well understood but others, such as Farra (pancake domes), coronae and canali, are more enigmatic. Many of the most important details are at the resolution limit of Magellan data; high resolution topographic and image data are essential to properly understand the morphology and relationships between flows, canali, fractures and vents. Determining how many and what types of volcanic features are active at the present day will also help constrain the resurfacing history and thermal balance of the planet. Recent and perhaps continuing volcanic activity may have been observed by Venus Express (Shalygin et al., 2014; Smrekar et al., 2010b). Detection and mapping of any eruptions that occur, together with their associated flows and outgassing, as well as monitoring of surface deformation caused by magmatic activity, are essential to achieving this goal.
The notion that the Venus interior is volatile-free has been challenged by the discovery of a Tambora-scale pyroclastic deposit, Scathach Fluctus (Ghail and Wilson, 2013), and the inference that such deposits might be more widespread than previously realised. Surface images (Figure 3.1) captured by Soviet Venera landers reveal a landscape more consistent with pyroclastic or sedimentary deposits, not the basaltic lava flows widely assumed to cover the plains. Pyroclastic eruptions on Venus require a significant volatile component to disrupt the magma at surface pressures; even a small number of explosive eruptions would represent a significant contribution to the atmosphere, particularly if the volcanic gases are water vapour or sulphur dioxide. While these gases are known to vary considerably in the upper atmosphere (Esposito, 1984; Marcq et al., 2011), it is far from clear whether variability there is correlated with changes in the lower atmosphere, which have yet to be quantified. New high resolution infrared spectra from below the clouds and especially near the surface are required to address this fundamental lack of knowledge.
See also the Frequently Asked Question: What world-wide firsts will EnVision accomplish during its mission?