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:
Global Resurfacing and tectonic activity
The key question for Venus surface science is how geologically active the planet is today? Since Magellan, our understanding has been predicated by the near random distribution of its ~940 impact craters (Figure 1.1) which imply a globally uniform surface age (McKinnon et al., 1997; Phillips et al., 1992; Strom et al., 1994) of ~750 Ma. This observation led first to the hypothesis of episodic global resurfacing (Fowler and O'Brien, 1996; Noack et al., 2012; Papuc and Davies, 2012; Turcotte, 1993; Turcotte et al., 1999).
Whether or not this happened, Venus is considered to now be in a stagnant lid regime (Armann and Tackley, 2012; Reese et al., 1998, 1999; Solomatov and Moresi, 1996), in which the absence of an asthenosphere means that the lithospheric lid is coupled to a high viscosity mantle that convects slowly enough for conductive cooling to be the dominant heat transport mechanism.
This self-consistent view has been challenged by many authors on a number of counts, primarily in that it does not explain the observed geological complexity. Geological observations of Magellan data imply a variety of age relationships and long-term activity (Chetty et al., 2010; DeShon et al., 2000), with at least some into the recent past (Ghail, 2002; Price et al., 1996; Smrekar et al., 2010a). Even a cursory inspection of Figure 1 shows that there is a non-random distribution of topography and that there is an association between geology and elevation, such that the uplands are consistently more deformed than the lowlands, but it is not clear that the distribution of impact craters is strictly random either (Campbell, 1999; Hauck et al., 1998; Price et al., 1996). New observations about the degree of crater alteration (Herrick and Rumpf, 2011) permit a wider range of recent geological activity (Campbell, 1999; Guest and Stofan, 1999; Hansen and Young, 2007; Johnson, 2003; Stofan et al., 2005). A striking aspect of the tectonic features of Venus is their organisation with respect to the geoid (Jurdy and Stefanick, 1999), particularly the great circle rift arcs (Figure 1.2), which are similar in extent to Earth's mid-ocean ridge system but which apparently have much more limited extension.
The reasons for these major differences between Venus and Earth are not understood: neither stagnant lid convection nor episodic global resurfacing can explain the range and complexity of features observed on Venus. Neither can account for its likely internal heat production. Better quality data — particularly a globally uniform map of the gravity field — will help to resolve some of these questions. The key to understanding the processes that link together these observations is the current rate of surface deformation and the history of geological activity.
See also the Frequently Asked Question: What world-wide firsts will EnVision accomplish during its mission?