Venus surface compositions suggest upper mantle temperatures like Earth, so why is there no magnetic field?

The interior of Venus remains a mystery and, it is challenging to reconcile the available meager observations. The leading theory for the absence of a Venusian magnetic field is that heat within Venus remains trapped beneath the stagnant lid, raising the mantle temperature and limiting flow through the core-mantle boundary. While there are only three surface compositions from the Venera and Vega landers, they are consistent with Mid-Ocean Ridge Basalts (MORB) or Ocean Island Basalts (OIB) implying that in the melting region the mantle of Venus is nothotter than the mantle of Earth. This is a surprising result because stagnant (or squishy) lid planets have hotter interiors than mobile lid planets implying that Venus has not been in the stagnant (or squishy) lid mode of convection for much of its evolution or, another heat transport mechanism—such as heat piping—has played a critical role in the flux of heat through the lithosphere of Venus. We see little change in the geoid or topography power spectra between the calculations suggesting that the presence or absence of lithospheric mobility has only a modest impact on the large-scale geoid or topography. While the patterns of the geoid or topography are not likely to be matched by any convection calculation, the power spectrum is independent of coordinate system and thus, a more robust comparison between calculation and planet. The cases we have found predict a significant heat flux from the core to the mantle—as long as 1000 Myr after an overturn event—inconsistent with the absence of a present-day magnetic field and the estimated age of the surface from cratering, the next step will be to consider a Basal Magma Ocean (BMO) to sequester heat within the core.