Decomposition of the Venus time-variable gravity due to the atmospheric mass motion as inferred from global circulation models

The state of the art Institut Pierre-Simon Laplace general circulation model (GCM) shows that the dynamic atmosphere of Venus creates a time-variable gravity (TVG) signal only a factor of 3 below the sensitivity of the existing Magellan gravity data. Therefore, the next mission to Venus will most likely be sensitive to Venus' TVG due to the atmospheric mass motion. In this paper, we conduct a principal component analysis (PCA) of TVG as inferred from the GCM in order to isolate distinct time-dependent terms (or patterns) that can be solved for once the measurements become available.

Venus' atmospheric TVG is dominated by the thermal tide at a period of one Venus day (≈116 Earth days, or Ed). The first 4 principal components associated with the thermal tide explain 84% of the variance in TVG. The thermal tide expanded in spherical harmonics has the highest contribution from the degree 1 terms, which correspond to the center-of-mass motion on the order of less than 1 cm and, therefore, is unmeasurable from the orbit. The degree 2 terms in the thermal tide provide the torque on Venus' atmosphere and, indirectly, on its solid body. This torque is of the opposite sign than the torque due to gravitational solid body tide. Thus, measuring the degree 2 thermal tide in addition to the gravitational tide will shed light on the Venus torque balance. The second largest time-variable signal revealed by PCA (5^th and 6^th patterns) is attributed to a retrograde wave with a period of ≈7 Ed. This wave is dominated by a spherical harmonic of degree 3 and order 1. This pattern explains 3% of the variance in TVG. Based on the spatial pattern and period, this wave could be identified as a Kelvin-type wave (Covey and Schubert, 1982; Lebonnois et al., 2016).

In summary, we have decomposed Venus' TVG into 6 distinct components that combined account for 87% of the variance in Venus' TVG. Venus' atmospheric TVG, if measured, would be highly complementary to lander measurements and remote sensing orbital data as it would provide a global insight into deep Venus' atmosphere. Our analysis will be helpful in connecting the GCM results and future gravity observations.