The possibility of a giant impact on Venus

TL;DR

The paper investigates whether a late giant impact could account for Venus' slow retrograde rotation and lack of a moon. Using 3D SPH simulations of differentiated Venus and differentiated impactors across a range of masses, velocities, angles, thermal states, and pre-impact spins, the study finds that many collision scenarios can reproduce Venus' present rotation while producing only small, likely-degenerate circumplanetary discs. The results show a strong dependence of angular-momentum transfer on collision parameters and reveal a broad spectrum of post-impact thermal states, from shallow surface melting to deep magma oceans, shaping Venus' early thermal evolution. This supports giant impacts as a viable pathway for Venus' divergent evolution and provides a framework to interpret future constraints from Venus missions on its formation history.

Abstract

Giant impacts were common in the early evolution of the Solar System, and it is possible that Venus also experienced an impact. A giant impact on Venus could have affected its rotation rate and possibly its thermal evolution. In this work, we explore a range of possible impacts using smoothed particle hydrodynamics (SPH). We consider the final major collision, assuming that differentiation already occurred and that Venus consists of an iron core (30% of Venus' mass) and a forsterite mantle (70% of Venus' mass). We use differentiated impactors with masses ranging from 0.01 to 0.1 Earth masses, impact velocities between 10 and 15 km/s, various impact geometries (head-on and oblique), different primordial thermal profiles, and a range of pre-impact rotation rates of Venus. We analyse the post-impact rotation periods and debris disc masses to identify scenarios that can reproduce Venus' present-day characteristics. Our findings show that a wide range of impact scenarios are consistent with Venus' current rotation. These include head-on collisions on a non-rotating Venus and oblique, hit-and-run impacts by Mars-sized bodies on a rotating Venus. Importantly, collisions that match Venus' present-day rotation rate typically produce minimal debris discs residing within Venus' synchronous orbit. This suggests that the material would likely reaccrete onto the planet, preventing the formation of long-lasting satellites - consistent with Venus' lack of a moon. We conclude that a giant impact can be consistent with both Venus' unusual rotation and lack of a moon, potentially setting the stage for its subsequent thermal evolution.

Figures (9)

• Figure 1: Pre-impact temperature profiles of Venus. The blue and orange curves correspond to the cold model and hot model with surface temperatures of 1000K and 1500K, respectively. Both models assume a differentiated planet with an iron core (30% by mass) and a rocky mantle (70% by mass) and include a temperature jump of 1500K at the core-mantle boundary.
• Figure 2: Post-impact rotation period of Venus of the 'cold models' inferred from our simulations. The upper panel shows collisions with a 0.01 impactor, while the lower panel shows collisions with a 0.1 impactor. The marker colour describes the pre-impact rotation period, the edge colour the impact velocity, and the marker symbol the collision outcome according to the legend. The different pre-impact rotation periods are also displayed with the respective coloured vertical lines. The grey area corresponds to periods that are consistent with Venus' present-day period.
• Figure 3: Disc mass generated for different collisions and plotted according to the total post-impact bound angular momentum (planet + disc) in units of the angular momentum of the system Earth-Moon, $L_{em}$. The marker colour describes the pre-impact rotation period and the marker size the impactor mass according to the legend. The marker symbol describes the collision outcome. The greyed area corresponds to the post-impact angular momentum that is consistent with Venus' present rotation period.
• Figure 4: Snapshots of a cross-sectional slice of a head-on collision between a non-rotating Venus and a 0.1 impactor at 10kms, shown at multiple time steps. Initial energy deposition at the impact site generates pressure waves that converge at the antipode, causing significant heating and deformation. The outcome of this collisions is a merger.
• Figure 5: Snapshots of a cross-sectional slice of an oblique collision (impact parameter -0.7) between a rotating Venus (6-hour period) and a 0.1 impactor at 10kms, shown at multiple time steps. The impactor grazes the planet and escapes with minimal disruption, classifying this as a hit-and-run event.