EnVision, a low-altitude Venus polar orbiter, is the M5 mission candidate in the ESA Science Program. It will carry 5 instruments and 1 experiment (a S-band Synthetic Aperture Radar, a Subsurface Radar, 3 spectrometers and a radio science experiment). The mission is studied in close collaboration with NASA and still in the concept phase. EnVision will investigate Venus from its inner core to its atmosphere at an unprecedented scale of resolution, characterising in particular, core and mantle structure, signs of active and past geologic processes and looking for evidence of the past existence of oceans. EnVision wil l help understanding why the most Earth-like planet in the solar system has turned out so differently, opening a new era in the exploration of our closest neighbour.
Venus accounts for 40% of the mass of terrestrial planets in our solar system, yet even fundamental parameters such as the relative size of its core to mantle are unknown. As we expand the scope of planetary science to include those planets around other stars, the lack of measurements for basic planetary properties such as moment of inertia, whether parts of its current surface was formed in the presence of water, core-size and density variations with depth, and thermal profile for Venus hinders our ability to compare the potential uniqueness of the Earth and our solar System to other planetary systems.
Given its similar size and bulk composition as Earth, Venus is expected to be volcanically and tectonically active today (unlike Mars, which, being only a tenth of Earth’s mass, has lost more of its internal heat). Steep slopes and landslides are common on Venus’ surface geology, implying active uplift, but existing data provide no constraint on current rates of tectonic activity. Comparaison of gravity mapping and topography have revealed some regions which appear to be at higher elevation than would be the case if the mantle were stagnant, implying that they lie atop magmatic upwelling. There are regions where volcanic activity is particularly likely to occur. Sites of potential volcanism identified in Venus Express data were in these locations. Therefore the observation and characterisation of these regions are of high priority.
EnVision : Science investigation
EnVision will use a number of different techniques to search for active geological processes, measure changes in surface temperature associated with active volcanism, characterise regional and local geological features, determine crustal support mechanisms and constrain mantle and core properties :
Densely fractured plains (image right, cut by NNW set of faults) embaying tessera (bottom, cut by multiple sets of faults) indicating that emplacement of the material of the densely fractured plains postdated formation of tessera terrain. Both densely fractured plains and tessera are embayed by even younger regional plains (dark areas). A steep-sided volcanic dome is seen in the upper left. Its age relations with the surrounding regional plains are not known. Portion of Magellan image centered at 46N, 360E. After Basilevsky and McGill (2007), Surface evolution of Venus, in Exploring Venus as a Terrestrial Planet, Geophysical Monograph Series 176, edited by L. W. Esposito, E. R. Stofan and T. E. Cravens, pp. 23-43, American Geophysical Union, Washington, DC.
- Use differential interferometric SAR (DiffInSAR) to look for cm-scale surface changes over large areas. This can detect volcanic changes such as new lava flows, or for possible inflation or deflation of underground magma chambers;
- Search for changes in repeated radar imagery of regions of interest. Change detection can be achieved either by looking for changes in reflection properties, or from changes in unit boundaries;
- Look for thermal signature of volcanic activity. This can be done through repeated observations either in near-IR spectral window regions and in microwave wavelengths;
- Spectrometry of different surface units is critical for understanding their composition. Although the optically thick Venus atmosphere precludes this at most wavelengths, there are five spectral windows between 0.8 and 1.2 μm at which thermal emission from the surface escapes to space. Surface emissivity at these wavelengths encodes crucial information about mineralogical characteristics like Fe content. Emissivity mapping helps to test theories of surface composition, for example about whether the tessera highlands are composed of felsic materials akin to Earth’s granitic continental crust, and therefore were formed in a volatile-rich environment;
- Search for plumes of possibly volcanogenic gases such as H2O, SO2, CO or OCS, reflecting current and active volcanic activity;
- Sensitivity to unit boundaries beneath the surface, and ability to detect buried structures, would significantly enhance the reconstruction of stratigraphy and thus in the reconstruction of the geological record. Subsurface sounding can enable measurement of the depth of volcanic flows and therefore the volume of volcanic effusions; of buried unit boundaries beneath volcanic plains, such as mapping of the edges of tessera regions; and could reveal unexpected features such as buried impact craters;
- The internal properties of Venus. Venus is less dense than expected if it had similar Earth’s bulk composition. In order to assess information on its interior structure, it is crucial to determine the k2 Love number and the moment of inertia. The tidal Love number, estimated from Doppler tracking of Magellan and Pioneer Venus Orbiter spacecraft data, is not accurate enough to constrain the state and size of the core, the mantle composition and viscosity. EnVision shall map the gravity field for at least 50% of the surface with spatial resolution of better than 200 km, that is equivalent to the knowledge of spherical harmonics to at least degree of 90 and accuracy of at least 10 mGal/km. EnVision will also measure the k2 Love number of the planet with an accuracy better than ±0.01 –compared to its current knowledge to ±0.066 uncertainty – allowing to constrain the size and state of Venus’ core and the composition of Venus’ mantle.
More information on the EnVision mission website