Spin rate periodic variations and polar drift determination from the EnVision radio-science experiment.

Abstract:

The EnVision Radio-Science Experiment (RSE) will improve the determination of the gravity field of Venus using the Doppler radio-tracking data of the spacecraft during the science phase covering 6 Venusian days (or 4 Earth’s years) (Rosenblatt et al., 2021). The Venus rotation model (spin rate and pole position) is also determined with the gravity field since it provides the reference frame attached to the planet in which the gravity field is expanded in spherical harmonics. Both gravity field and Venus’s rotation model are estimated in the Precise Orbit Determination (POD) process and so their solutions are correlated. The monitoring of tie-points identified on planetary surface images is helpful to decrease this correlation. Here, we study the ability of the EnVision radio-science experiment to determine the expected spin rate periodic variations and polar drift using future EnVision Doppler data and tie-Points data determined from the radar images.

 

The determination of Venus spin rate and pole position

The spin rate of Venus has been obtained using a number of different methods. These include Doppler tracking data from the Pioneer Venus Orbiter and Magellan spacecraft (Konopliv et al., 1999), as well as numerous radar data from Earth and space-borne sources collected over the last 50 years (e.g. Shafer & Hensley, 1991; Davies et al., 1992; Campbell et al., 2019; Margot et al., 2021). Other methods include thermal data from the surface and Magellan altimetry data (Mueller et al., 2012). However, the published solutions differ by up to about 10 minutes, depending on the kind of data and its time span, used to estimate this rotation rate. A recent analysis of 8 years of Venus Express tracking data yields to a rotation value consistent with the previous values and reduces their widespread down to about 3 minutes (Levesque et al., 2026).

Nevertheless, it was not possible to estimate the periodic variations of the spin rate due to the inaccuracy of the reconstructed Venus Express orbit from the Doppler data (Levesque et al., 2026). Margot et al., (2021) reported variations of spin rate but no periodicities. The precession rate could not be detected using Doppler data of either Magellan or Venus Express, while the radar speckle approach developed by Margot et al. (2021) measured a precession rate providing a 7% error on the Moment of Inertia (MoI) of the planet. This error is however too large to further constrain the interior structure of Venus (Dumoulin et al., 2017). The performance analysis of the EnVision Radio-Science Experiment (RSE) has shown that the spin rate could be retrieved with an error of 0.03 minute and the MoI with an error of 1.9%, allowing for better constraining the internal structure of Venus (Rosenblatt et al., 2024).

 

The determination of polar drift and spin rate periodic variations

Venus's expected Chandler Wobble is a slow motion of the spin pole with respect to the surface of the planet, which on a timescale of a few years should correspond to a linear drift of 22 m/yr across the surface (Phan &, Rambaux, 2025). This polar drift is sensitive to the state and size of the Venus’s core, so it could be used to further constrain the interior of the planet (Phan & Rambaux, 2025).

The spin rate periodic variations are expected to have three different sources of variations induced by the triaxiality of Venus’ figure, by the exchange of angular momentum between the atmosphere and the solid planet and by the core-mantle coupling. The amplitude of these three sources are respectively 2 minutes, 25-50 seconds and 3.5-20.4 seconds (Cottereau et al., 2011).

 

We perform the simulations of the Doppler tracking data using the GINS software, and of the tie-points determined from the radar images in order to assess the accuracy of the retrieved spin rate periodic variations and polar drift from EnVision.

 

References:

Campbell B. A. et al. (2019), Icarus 332, 19–23 ; Cottereau L. et al. (2011), Astronomy and Astrophysics - A&A 531, 45 ; Davies M. E. et al. (1992), Celestial Mechanics and Dynamical Astronomy 53(4), 377–397; Konopliv A., et al. (1999), Icarus 139(1), 3–18 ; Lévesque M. et al. (2026), Planetary Science Journal, 7:21 (11 pp) ; Margot J.-L. et al. (2021) Nature Astronomy 5(7), 676–683 ; Mueller N. et al. (2012), Icarus 217(2), 474–483 ; Phan P.-L. & Rambaux N. (2025). Astronomy & Astrophysics 699, A65 ; Rosenblatt P. et al. (2021), Remote sensing, vol. 13, 1624; Rosenblatt P. et al.  (2024), EPSC2024–410 ; Shaffer, S. & S. Hensley (1991), Interoffice Memo, Pasadena, California, USA n°3346-91-112.