The Internal Structure Of Venus Studied By Using The Geodetic Data

Venus was once considered to be Earth's sister planet because of the similarity in mass and radius.However,there are many differences between the surface of Venus and Earth.Understanding the internal structure of Venus,especially the deep structure of Venus which is closely related to the thermal state and dynamics of the mantle,is of great significance for finding the reason of such difference between Venus and Earth.In addition,the internal structure of Venus,especially the structure of the core-mantle boundary,holds the clues about the origin and evolution of Venus.Due to the extremely harsh environment on the surface of Venus,it is difficult for the lander to work properly.At present,there is almost no in situ seismic data for Venus.With the development of space technology over the past few decades,more and more Venus orbiter have been successfully launched.The accumulated observations of geodetic data make it possible to study the internal structure of Venus by using the geodetic data.In this study,we used the currently observed geodetic data of Venus to study the internal structure of Venus.The main contents and results of this study include:?1?The interior of Venus is not completely elastic,and a rheological model is needed to approximate the viscoelastic response of the tidal force of the Sun.We compared the Maxwell model and Andrade model.The results show that the Andrade model has a wider attenuation band than Maxwell model.However,the attenuation rate is slower than the Maxwell model.The attenuation of the Maxwell model mainly occurs within the period larger than the effective Maxwell time?_m.The dissipation effect of the Andrade model is 5 times stronger than that of Maxwell model in the period of M₂tide.However,the difference will be decreased with the increasing of the period.In the period larger than the effective Maxwell time,the dissipation effects of two models are consistent.Compared with the Maxwell model,the Andrade model is more suitable for studying short tidal period.?2?Most researches in the past addressing the internal structure of Venus have used the forward modeling method,but none of the existing research considered the inversion method.MCMC algorithm has been used in the research of geophysical inversion.The MCMC algorithm also widely used in the inversion of the internal structure of the Moon,Mars,and Mercury by using geodetic data.Therefore,we run simulation inversions of the 3-layer Venus model by using the MCMC algorithm.For the assumption of 3-layer model with k₂=0.295±0.033,Mo I=0.033±0.0165 and?=5242.7±2.6 kg/m³,the liquid iron-rich core of Venus has a radius of 3294_-261⁺²¹⁵ km,suggesting a larger core than previous research has indicated.the average density of mantle and core are 4101_-375⁺³²⁵ and 11885⁺⁹⁹⁵_-1242 kg/m³,respectively.P-wave and S-wave velocity of the mantle of Venus are 6720_-731⁺⁶⁷¹ and 9707_-650⁺⁹⁰² m/s,respectively.?3?In addition,we analyzed the effect of different noise level of observations on the parameter estimation.The results show that about 1%uncertainty of the geodetic data was sufficient to estimate the interior parameters,and cause about 2%uncertainty of the parameters and an offset of 0.1%from the“True value”.With the increasing of noise,the uncertainties and the mean value's deviation of the parameters are both increased.?4?In this study,we built an adaptive model of Venus's internal structure with the parameters that varying with depth to fill in the deficiencies of the previous Venus models.In the core region,we introduced the 3^rd Birch-Murnaghan Equation of State to calculate the parameters of core from the core composition;at the bottom of the mantle,our model considered the viscosity change caused by the temperature gradient.In addition,our model coupled the temperature of mantle,temperature of core,and the state of core.The Andrade rheological model was applied to these models.We calculated the synthetically value of the tidal Love number k₂ of models and compared with the observed value.For acceptable models with an entirely liquid core,we find that the core radius ranges from 3061 to 3249 km,with light elements content from 1to 10 wt%.For acceptable models with liquid outer core and a solid inner core,the outer core radius ranges from 3064 to 3214 km,with an inner core radius of 510 to 3035km and light elements contents from 4 to 10 wt%.In this case,the composition of TBL?Thermal Boundary Layer?at the bottom of Venus mantle is different from the D''layer of Earth.This layer may be composed of perovskite instead of post-perovskite.In addition,the D”layer was commonly linked to the mantle plume of Earth,which may indicate that there is a different between Venus's mantle plue and that of Earth.Through this study,we found that currently geodetic data of Venus is not sufficient for uniquely determining the internal structure of Venus.More accurate observations of k₂ obtained by the future En Vision mission is expected to reduce the uncertainty of the internal parameter estimation.In the future,if the tidal Love number h₂ or the tidal dissipation factor Q of Venus can be obtained from the altimetry data,it would help for the estimation of the viscousity structure and thermal state of Venus mantle.It will provide the possibility to estimate the temperature of the core-mantle boundary and new evidence for determining the score state of Venus.