SAMUEL CABOT'S RESEARCH SITE

ANCIENT VENUSIAN METEORITES ON THE MOON

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Illustration of a comet impacting a terrestrial planet. Venus, which has a semi-major axis, radius, and mass comparable to Earth's, may have retained a thin atmosphere billions of years ago, and have appeared similar to the planet in this image. Image by S. Cabot.

Venus' modern atmospheric D/H indicates that the planet once hosted oceans of water. One fascinating possibility is that Venus' atmosphere was at some point thin, and that its water was largely condensed as it is on Earth. This scenario may have been possible during an epoch where the Sun was once dimmer, billions of years ago. However, a runaway greenhouse eventually ensued, and the water photo-dissociated. Hydrogen escaped hydrodynamically, whereas Oxygen was depleted through a combination of non-thermal escape, reactions with magma ocean, and solar wind pickup. A recent, cataclysmic resurfacing event (established by cratering ages) has buried any traces of Venus' past state. In short, Venus' evolution remains unconstrained.

We recently developed an end-to-end model for the transport of Venusian surface material to the lunar regolith (Cabot & Laughlin 2020). If Venus' atmosphere was once comparable to Earth's (~1 bar at the surface), then asteroid impacts would have transferred detectable quantities of surface rocks to the Moon. Obtaining just one piece of Ancient Venus would confirm its former thin atmosphere, and reveal key characteristics of its ancient surface conditions. We used analytic spall theory (developed in papers by Jay Melosh) which predicts the mass of lightly-shocked target material ejected in an impact. Next, we compared analytic, secular trajectories to N-body integrations of meteorites originating from Venus, and determined the fraction accreted by the Moon. Finally, we identified petrological signatures (oxygen fractionation ratios, noble gas ratios, and metal abundance ratios) that would lead to identification.

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The physical processes and zones involved during an asteroid impact, and peak shock pressures experienced by the target. Image from Cabot & Laughlin (2020), adapted from Melosh (1984)

A nominal 8 km wide asteroid impacting at 30 km/s ejects about 40,000 billion kg of lightly shocked material. Peak shock pressures may reach tens of GPa, but not enough to induce widespread melting, which would otherwise reset key chemical ratios. Furthermore, lying deeper within the Sun's gravitational well and having a slightly lower escape velocity, Venus experiences more asteroid collisions that eject lightly shocked material than Earth does.

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Evolution of massless test particles starting at Venus. This figure, from Cabot & Laughlin (2020) compares secular evolution to that of a REBOUND N-body simulation. Each track represents a single test particle.

We modeled the trajectories of ejected surface rocks through the Solar System. Numerical and secular evolution tracks exhibit qualitatively similar trends. Importantly, the numerical simulations capture close encounters and mean motion resonances, both of which excite the test particles to Earth-crossing orbits. In total, approximately 60% of ejecta reach 1 AU. We resolved 0.07% of test particles colliding with the Moon after 1 Myr.

Integrated over the cratering history of the Solar System, we predict about 0.2 ppm of Ancient Venusian material in the lunar regolith today. Venusian rocks or grains may be identified in samples obtained by the upcoming Artemis missions that will characterize the lunar regolith. Triple oxygen isotope fractionation may be a distinguishing feature from a Venusian rock, as it would lie on a separate fractionation line than other differentiated Solar System bodies. Alternatively, noble gas ratios could help identify a Venusian origin, specifically following precise abundance constraints in Venus' current atmosphere via the upcoming DAVINCI+ mission.

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Illustration of the transport of Venusian surface material to the Moon. A comet or asteroid strikes Venus, and ejects particles into the Solar System. The combination of Venus' orbital speed and residual velocity from ejection, plus resonances and close-encounters, efficiently moves ejecta to orbits that cross 1 AU. A small fraction collide with the Moon. Figure by S. Cabot.