![]() Internally, the composition-dependent density profile results in a ''basalt barrier'' at the mantle transition zone, which strongly affects Venus' mantle evolution. Also, a combination of plagioclase crustal rheology and dislocation creep can weaken the lithosphere sufficiently to facilitate lithospheric overturns without applying plastic yielding. with near-zero mobility over the entire model time. with mobility that is high during overturns and near zero between overturns, or stagnant-lid tectonics, i.e. On the other hand, olivine-crustal-rheology models exhibit either standard episodic-lid tectonics, i.e. With a ''weak'' plagioclase-rheology crust, models exhibit episodic overturns but with continuously high surface mobility and high distributed surface strain rates between overturns, leading to a new tectonic regime that we name ''deformable episodic lid''. We find that surface tectonics is strongly affected by crustal rheology. Here, we use global 2-D thermochemical convection models with realistic parameters, including rheology (dislocation creep, diffusion creep, and plastic yielding), an experiment-based plagioclase (An$_$) crustal rheology, and intrusive magmatism, to investigate the tectonics and mantle evolution of Venus. To explain Venus' young surface age and lack of plate tectonics, Venus' tectonic regime has often been proposed to be either an episodic-lid regime with global lithospheric overturns, or an equilibrium resurfacing regime with numerous volcanic and tectonic activities. The latter two will be heavily influenced by data returned from the DAVINCI mission. The most pressing issues we determined for safely landing in tessera terrain are the development of an improved hazard divert system (e.g., fans capable of diverting the lander several hundred meters), development of reliable hazard detection and avoidance, and improved characterization of the winds to constrain lander trajectories/landing. Due to mass (and therefore power) constraints, a divert maneuver of only a few tens of meters is possible during the final few kilometers of descent. For propulsion during divert maneuvers, a system of fans can effectively push against the atmosphere. ![]() The high pressure and temperature of the atmosphere near the surface (∼92 bars and 735 K at the surface and ∼76 bars and 714 K at the chosen landing site of western Ovda Regio) preclude the use of conventional rocketry and thrusters. Alternatively, the lander requires a site-agnostic design (i.e., it must be capable of safely landing and operating on any surface in any orientation), though this carries a greater risk of landing in a location of low scientific value if it is without some means to characterize the surface and alter its trajectory (i.e., without an HDA system). Upcoming missions (VERITAS, DAVINCI, and EnVision) will further constrain these hazards, but not to the degree necessary to identify safe landing locations at the scale of the lander prior to its launch, requiring some form of autonomous hazard detection and avoidance (HDA) to identify and pilot towards high-value science targets and simultaneously away from high risk (i.e., high hazard) locations as the lander descends to the surface. The greatest hazards we identified are: the degrading effects of the atmosphere on imagery and LIDAR data of the surface, unknown and variable wind speeds, and unknown slopes and surface roughness. ![]() We evaluated the potential dangers associated with landing in the tesserae on the dayside and methods by which to mitigate these for an example lander largely based on the 2020 Venus Flagship Mission Concept Study. ![]()
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