Lithospheric Thickness: A Lens on Understanding the Evolution of Earth’s Twin

Abstract:

Venus is the ultimate laboratory for understanding the evolution of rocky planets. Magellan was the last dedicated mission to study Venus’s surface and interior. Researchers interpreted this data to indicate a lack of plate boundaries, a geologically inactive planet that ‘resurfaced’ roughly 500 m.y. ago, and a thick lithosphere. Recently, new insights on processes though out the solar system, along with limited but crucial surface emissivity data from VIRTIS, have begun to overturn our understanding of Venus that had stagnated for decades. Lithospheric thickness is a key factor in planetary activity: Volcanism and deformation occur more readily on thin lithosphere. Lithosphere thickness is debated on Venus, and evidence of thin lithosphere has largely been interpreted as anomalous due to the (past?) paradigm that Venus is geologically dead.

Here we present 75 new elastic thickness (Te) estimates derived from modeling topographic flexure at coronae, plus 30 previously studied coronae. We compare these results, along with 14 published values, to Te values derived from analysis of admittance, the spectral representation of the ratio of gravity to topography. Due to poor gravity resolution, Te values from admittance have large error bars and cannot be obtained everywhere. Despite these challenges, there is clear agreement between the majority of Te estimates from topographic modeling of coronae and estimates for admittance, which represent the regional value of Te.

This agreement supports the admittance analysis showing ~50% of Venus has thin (<20 km) lithosphere, and argues against past interpretation of thin elastic lithosphere at coronae as due only to high localized plume heat flow, and regionally thin lithosphere from admittance as better interpreted as crustal isostasy rather than flexural compensation by thin lithosphere. This new understanding of Venus' lithosphere is consistent with a geologically active planet. Global reconnaissance with high resolution topography, radar imaging, NIR spectroscopy, and interferometry is needed to investigate which process are active today, the nature of the geodynamic system driving activity, and to understand the conditions that do and do not allow the initiation of plate tectonics.