Lunar Exploration as a Probe of Ancient Venus

TL;DR

This paper explores the possibility of detecting ancient Venusian surface material on the Moon, which could reveal Venus's past climate and water history through analysis of lunar samples.

Contribution

It demonstrates that asteroid impacts can transfer Venusian surface material to the Moon, providing a method to study Venus's ancient environment via lunar regolith analysis.

Findings

Over 0.07% of Venusian ejecta may land on the Moon.

Venusian material could be present at up to 0.2 ppm in lunar regolith if water was lost in the last 3.5 Gyr.

Ejecta impacts at low velocities preserve original surface signatures.

Abstract

An ancient Venusian rock could constrain that planet's history, and reveal the past existence of oceans. Such samples may persist on the Moon, which lacks an atmosphere and significant geological activity. We demonstrate that if Venus' atmosphere was at any point thin and similar to Earth's, then asteroid impacts transferred potentially detectable amounts of Venusian surface material to the Lunar regolith. Venus experiences an enhanced flux relative to Earth of asteroid collisions that eject lightly-shocked ($\lesssim 40$ GPa) surface material. Initial launch conditions plus close-encounters and resonances with Venus evolve ejecta trajectories into Earth-crossing orbits. Using analytic models for crater ejecta and \textit{N}-body simulations, we find more than $0.07\%$ of the ejecta lands on the Moon. The Lunar regolith will contain up to 0.2 ppm Venusian material if Venus lost its water in the last 3.5 Gyr. If water was lost more than 4 Gyr ago, 0.3 ppm of the deep megaregolith is of Venusian origin. About half of collisions between ejecta and the Moon occur at $\lesssim6$ km s$^{-1}$, which hydrodynamical simulations have indicated is sufficient to avoid significant shock alteration. Therefore, recovery and isotopic analyses of Venusian surface samples would determine with high confidence both whether and when Venus harbored liquid oceans and/or a lower-mass atmosphere. Tests on brecciated clasts in existing Lunar samples from Apollo missions may provide an immediate resolution. Alternatively, regolith characterization by upcoming Lunar missions may provide answers to these fundamental questions surrounding Venus' evolution.