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We investigated the relaxation states of 11 large impact basins on Mercury based on an updated crustal thickness map, finding that the pre-Tolstojan basins have comparable instead of varied relaxation states, suggesting that Moho temperature (Temp Moho) did not decrease substantially from ∼4.2 to 3.8 Ga. At the same time, mantle uplift beneath the Caloris basin is the least degraded, therefore implying a sharp decrease of Temp Moho ∼ 3.8 Ga. These findings contrast with our thermal evolution models that predict a fast decrease of Temp Moho between ∼4.2 and 3.8 Ga. Therefore, the discrepancies in the cooling rate suggest that the relatively elevated bombardment history between ∼4.2 and 3.8 Ga might have input additional energy to Mercury and substantially decreased the cooling rate. Plain Language Summary Formation of large impact basins on Mercury, a terrestrial rocky planet, was accompanied with a rebound of the floor, which resulted in an uplift of the mantle plug beneath it. These structures are likely to subside or relax, a process influenced by the thermal structure. The crustal thickness model of Mercury reveals the geometric shape of mantle plugs under large impact basins, indicating variation in the degree of relaxation, thus differences in the temperatures on Mercury lithosphere when the basins formed. Therefore, the present-day shape of mantle plugs under the age-determined basins provides information about the thermal history of Mercury. The discrepancy between the thermal history as inferred from age-determined basin relaxation measurements and an analytical model of planetary cooling suggests that impact bombardments might have input additional energy to Mercury during the period of basin formation.

Context. Chinese lunar missions have grown in number over the last ten years, with an increasing focus on radio science investigations. In previous work, we estimated two lunar gravity field models, CEGM01 and CEGM02. The recently lunar mission, Chang’e 5T1, which had an orbital inclination between 18 and 68 degrees, and collected orbital tracking data continually for two years, made an improved gravity field model possible. Aims. Our aim was to estimate a new lunar gravity field model up to degree and order 100, CEGM03, and a new tidal Love number based on the Chang’e 5T1 tracking data combined with the historical tracking data used in the solution of CEGM02. The new model makes use of tracking data with this particular inclination, which has not been used in previous gravity field modeling. Methods. The solution for this new model was based on our in-house software, LUGREAS. The gravity spectrum power, post-fit residuals after precision orbit determination (POD), lunar surface gravity anomalies, correlations between parameters, admittance and coherence with topography model, and accuracy of POD were analyzed to validate the new CEGM03 model. Results. We analyzed the tracking data of the Chang’e 5T1 mission and estimated the CEGM03 lunar gravity field model. We found that the two-way Doppler measurement accuracy reached 0.2 mm s−1 with 10 s integration time. The error spectrum shows that the formal error for CEGM03 was at least reduced by about 2 times below the harmonic degree of 20, when compared to the CEGM02 model. The admittance and correlation of gravity and topography was also improved when compared to the correlations for the CEGM02 model. The lunar potential Love number k2 was estimated to be 0.02430±0.0001 (ten times the formal error). Conclusions. From the model analysis and comparison of the various models, we identified improvements in the CEGM03 model after introducing Chang’e 5T1 tracking data. Moreover, this study illustrates how the low and middle inclination orbits could contribute better accuracy for a low degree of lunar gravity field.

Our results show that the existence of Venus's inner core cannot be constrained by the present-day observed tidal Love number k2. No phase transition from perovskite to post-perovskite occurs at the bottom of Venus' mantle if the mantle's FeO content is less than 8.1wt%. The expected precision of k2 from the candidate EnVision mission will be sufficient to reduce the size of the model space.