Venus: A Thick Basal Magma Ocean May Exist Today

Abstract:

Magma oceans are ubiquitous during the formation of rocky bodies. In larger planets such as Earth and Venus, crystallization of the mantle may proceed from the middle outwards because solid bridgmanite is neutrally buoyant at mid-mantle pressures. Surficial magma oceans rapidly cool to space and fully solidify within ~10–100 Myr. In contrast, a basal magma ocean (BMO) may survive for billions of years. A long-lived BMO in Earth has been proposed to resolve many geochemical conundrums. Models predict that Earth’s BMO solidified over a billion years ago—seismology confirms there is no global melt layer in the lower mantle. However, the interior structure of Venus may resemble that of early Earth. Detecting a BMO in Venus would provide a unique window into the formation and interior evolution of both planets.

The predicted lifespan of a BMO in Venus may extend to today. Numerical models of mantle dynamics suggest that the heat flow from the mantle to the surface of Venus is roughly half Earth’s total (i.e., ~20 vs. 44 TW) in the absence of plate tectonics. I built parameterized models for the BMO and core to track their thermochemical evolution. My benchmark models for Earth feature a BMO with an initial thickness of 750 km, which solidifies ~1.5 Gyr ago. In this model, the inner core of Earth reaches the correct size and the core hosts a dynamo at all times. For Venus, the heat flow out of the BMO was halved at all times. The present-day thickness of the BMO is then >200 km. While absolute temperature sets the interior structure, thermal gradients control interior dynamics. Venus (correctly) is not predicted to have a dynamo today. Because liquid silicates are electrically conductive at extreme pressures, the BMO could have hosted a dynamo until recently.

A BMO in Venus has many implications for future missions. New geophysical orbiters such as VERITAS (NASA) and EnVision (ESA) could directly detect a BMO by measuring the tidal Love number and phase lag. Measurements of noble gases in the atmosphere (NASA DAVINCI+) would be interpreted differently if a large internal reservoir still exists. For example, perhaps the BMO hides the “missing” argon-40. Finally, aerial platforms and/or orbiters could detect crustal remanent magnetism that may preserve signatures of a BMO-hosted dynamo.

O’Rourke, J. G. (2020). GRL. doi:10.1029/2019GL086126