A Planetary Hit-and-Run?

Title: The possibility of a giant impact on Venus

Authors: M. Bussmann, C. Reinhardt, C. Gillmann, T. Meier, J. Stadel, P. Tackley, and R. Helled

First Author’s Institution: University of Zürich, Zürich, Switzerland

Status: Published in A&A [open access]

Venus is commonly regarded as Earth’s twin due to its similar size, mass, rocky composition, and location in the inner solar system. These similarities might lead you to expect a familiar atmosphere and landscape. Instead, where Earth harbors lush forests and shimmering blue oceans, Venus offers only scorched plains, volcanos, and sulfuric skies. Further, unlike Earth, Venus has a retrograde rotation at a very slow speed and has no moon. These stark differences call into question how Venus diverged so dramatically from its twin. The authors of today’s paper propose an answer as violent as the planet itself: a car crash of planetary proportions.

Setting the Scene (of the Crime)

Since we cannot travel back 4.5 billion years to witness the solar system’s formation, we must work backward from present conditions to reconstruct the past. Simulations of the early solar system reveal that giant collisions were very common, making Venus a likely target during its youth. But what kind of collision could explain Venus’s peculiar state? Giant impacts can dramatically alter a planet’s angular momentum and form moons, yet Venus rotates unusually slowly and has no moon at all. The authors investigated whether a giant impact could be consistent with both of these observations, and if so, what initial conditions would be required.

To test different scenarios, the authors employed a suite of 3D impact simulations using the smoothed particle hydrodynamics method, a commonly used computational method for simulating the mechanics of solid body interactions. With these models, they varied the parameters that would set the initial conditions of Venus and the impactor before the collision. Those parameters are the impactor mass, the impact velocity, the impact parameter (i.e., the sine angle at which the impactor strikes the planet, where a head-on collision with an impact angle of 0 corresponds to an impact parameter of 0), and the pre-impact thermal profile of Venus (i.e., the temperature as a function of planetary radius). They also include models where Venus has an initial rotation speed, and others where Venus starts with no rotation. They run a total of 81 simulations with varying initial conditions and categorize the outcomes into scenarios: merger, graze-and-merge, and hit-and-run (see Figure 1). In a merger, at least 95% of the total colliding mass remains gravitationally bound to Venus, essentially creating a single combined body. A graze-and-merge represents a two-stage process where the impactor grazes Venus during an initial encounter, enters a temporary orbit, and then collides again, merging permanently. Finally, in a hit-and-run, the impactor skims Venus but escapes its gravitational grip entirely, leaving two separate intact bodies that drift apart after the collision. The authors then compare these post-impact Venus models to its current day properties to see which ones could have led to the Venus we observe today.

Finding the Suspects

With these simulations, the authors observe some key trends. They find that all collisions involving a low mass (0.01 Earth masses) impactor lead to mergers, but for collisions involving a high mass (0.1 Earth masses) impactor, the outcome depends on both the impact parameter and velocity. High-mass impactors with a low-impact parameter result in mergers, regardless of initial velocity. However, for high-mass objects with high-impact parameters (i.e., collisions that are not head on, but at an angle), they find that the impact velocity determines the type of collision, with velocities of 10 km/s resulting in graze-and-merge collisions and higher velocities of 15 km/s leading to hit-and-run events. Each of these scenarios leads to different transfers of angular momentum, resulting in very different post-impact rotation rates.

To match Venus’s present-day rotation, the impact scenario must leave the planet spinning with a post-impact rotation rate exceeding 48 hours. The transfer of angular momentum during the collision, as well as the initial rotation of Venus, will determine which impact scenario is consistent with the rotation we measure today. The authors find that 17 of the 91 collisions result in a post-impact rotation in agreement with this constraint. Out of those, they find that two collision scenarios lead to the slightly retrograde motion similar to that of Venus today. To further analyze the list of suspects, the authors examined the debris disk generated by each collision. Since Venus does not have a moon, scenarios producing massive debris discs are disfavored because such discs may contain enough material to coalesce into orbiting satellites. They discover a positive correlation between disc mass and post-impact angular momentum, and find that the collisions that are compatible with Venus’s present rotation also tend to produce lower disc masses. This paints a consistent picture with Venus’s lack of a moon. 

While these scenarios leave the authors with many combinations of initial conditions that create post-collision rotation speeds and debris disc masses consistent with Venus’ present-day properties, the authors suggest studying the effect of post-impact temperature on Venus’s thermal evolution as an additional avenue for differentiating between these impact scenarios. They find that their collision models produce diverse thermal structures and melting profiles of Venus, and they plan to further study these in their future work.

This study demonstrates that a high speed collision could explain Venus’s slow, retrograde rotation and lack of a moon. However, because the suspect fled the scene billions of years ago, we must work backwards to narrow down what occurred. Future work examining these post-impact temperature profiles may help determine more likely collision scenarios and reveal how a planet similar to Earth became an uninhabitable and formidable world. Understanding how Venus became so radically different from Earth is crucial for explaining the diversity of terrestrial planets beyond our solar system and may hold keys to understanding planetary habitability.

Astrobite edited by Bill Smith

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