The Radio-Science Experiment onboard EnVision

The Radio-Science experiment of EnVision comprises two sub-experiments (the Gravity experiment and the Radio-occultation experiment) that both relies on the Telemetry, Tracking and Command (TT&C) system of the spacecraft but that have different scientific goals and different operation modes.

The Gravity experiment

The internal structure of Venus is still uncertain (size and state of the core, mantle viscosity, average lithospheric and crustal thicknesses, as well as their lateral variations). These are key parameters to constrain the mantle composition, thermal evolution and deep interior of the planet. Without the availability of seismic data and Venus having no internal magnetic field to constrain the core state, the gravity field and the moment of inertia can be used to determine the radial structure of the planet. Thanks to its 6 cycles mission and to a 3 to 7 hours of tracking per day, the EnVision gravity field solution will improve the Magellan solution [1] by providing a better global degree strength (l=95 or 204 km at 3-σ) as well as areas of higher resolution and accuracy (<154 km over 40% of the planet, <2 mGal). The location of these areas depends on the pericenter position of the science orbit. With the current baseline orbit, the regions of lowest resolution (~200 km and 6 mGal accuracy), are located in the southern hemisphere (mainly at about 60°). Having a better resolved gravity field will allow to study gravity/topography ratios over structures of interest to understand their crustal and lithospheric morphologies [e.g.,2]. The k₂ tidal potential Love number will be determined with a precision better than 1% (compared with the 22% for the Magellan solution [3]), which will result in an improved constraint of the state and the size of the core [e.g. 4]. The rotation state (spin, pole position variability) will also be improved, thanks to the tracking data and its coupling with the radar tie points.

The Radio-occultation experiment

Radio-occultations will be performed to derive the atmospheric structure (temperature, pressure, number density) and the electron density profile of the ionosphere of Venus. Thanks to the addition of a Master Reference Oscillator (developed by BHE, Hungary) that includes an Ultra-Stable Oscillator provided by CNES, the experiment will be performed in a one-way mode with two coherent downlinks (X and Ka bands), allowing to probe the atmosphere down to 35 km both at ingress and egress. Although a lot of radio-occultation experiments have already been performed by Venus Express [5,6] and Akatsuki [7], the short orbital period of EnVision (1.5 hours instead of 24 hours for Venus Epress and ~9 days for Akatsuki) and its near polar orbit will allow to cover a wide range of latitudes, longitudes and local times as well as to observe short-term temporal variations of the temperature and pressure profiles caused by atmospheric waves. Furthermore, the high expected signal-to-noise ratio and the dual X and Ka band communication will allow to study amplitude scintillations, giving access to convective velocities in the clouds [8]. Moreover, the use of a dual X and Ka band communication link will enable for the first time to estimate both liquid and gaseous phases of H₂SO₄ at the base of the clouds. SO₂ profiles at 45-55 km will also be derived, allowing a synergy with the measurements of VenSpec-H. H₂SO₄ and SO₂ estimates are of a great importance to better understand the sulfur cycle and, in consequence, its potential link with the planet volcanic activity.

[1] Konopliv A.S., Banerdt W.B. and Sjogren W.L., Icarus, 2-18, 139, 1999. [2] Maia, J. S., & Wieczorek, M. A., JGR Planets, https://doi.org/10.1029/2021JE007004, 2022 [3] Konopliv A.S. and Yoder C.F., GRL, vol.23 (14), 1857-1860, 1996. [4] Dumoulin C., Tobie G., Verhoeven O., Rosenblatt P. and Rambaux N., JGR Planets, doi:10.1002/2016JE005249, 2017 [5] Tellmann S., Pätzold M., Häulser B., Bird M.K. and Tyler G.L., JGR, doi:10.1029/2008JE003204, 2009. [6] Oschlisniok J., Häusler B., Pätzold M., Tellmann S., Bird M.K., Peter K. and Andert T.P., Icarus, doi:10.1016/j.icarus.2021.114405, 2021. [7] Ando, H., Noguchi, K., Imamura, T., Takagi, M., Sugimoto, N., Matsuda, Y., et al. JGR Planets, https://doi.org/10.1029/2025JE009165, 2025 [8] Oschlisniok, J., Pätzold, M., Tellmann, S., Dumoulin, C., and Rosenblatt, P. EPSC-DPS2025-642, https://doi.org/10.5194/epsc-dps2025-642, 2025.