The gradual accretion of material within a protoplanetary disk should, in conventional models, allow us to go all the way from dust grains to planetesimals to planets. But a new way of examining the latter parts of this process has emerged at the University of Arizona Lunar and Planetary Laboratory in Tucson. There, in a research effort led by Erik Asphaug, a revised model of planetary accretion has been developed that looks at collisions between large objects and distinguishes between ‘hit-and-run’ events and accretionary mergers.

The issue is germane not just for planet formation, but also for the appearance of our Moon, which the researchers treat in a separate paper to extend the model for early Earth and Venus interactions that appears in the first. In the Earth/Venus analysis, an impact might be a glancing blow that, given the gravitational well produced by the Sun, could cause a surviving large part of an Earth-impactor (the authors call this a ‘runner’) to move inward and subsequently collide with Venus. So we’re not talking about impacts alone, but about impact ‘chains.’ The implications of this multi-impact theory on planet composition may be profound.

Alexandre Emsenhuber (now at Ludwig Maximilian University, Munich) is lead author of the paper on Earth/Venus interactions, pointing to the different impact scenarios for Earth and Venus:

“The prevailing idea has been that it doesn’t really matter if planets collide and don’t merge right away, because they are going to run into each other again at some point and merge then. But that is not what we find. We find they end up more frequently becoming part of Venus, instead of returning back to Earth. It’s easier to go from Earth to Venus than the other way around.”

Image: The terrestrial planets of the inner solar system, shown to scale. According to ‘late stage accretion’ theory, Mars and Mercury (front left and right) are what’s left of an original population of colliding embryos, and Venus and Earth grew in a series of giant impacts. New research focuses on the preponderance of hit-and-run collisions in giant impacts, and shows that proto-Earth would have served as a ‘vanguard’, slowing down planet-sized bodies in hit-and-runs. But it is proto-Venus, more often than not, that ultimately accretes them, meaning it was easier for Venus to acquire bodies from the outer solar system. Credit: Lsmpascal – Wikimedia Commons.

This work draws on a 2019 analysis by the same authors that first examined hit-and-run collisions and subsequent mergers of the two bodies. The authors point out that most simulations of this stage of planetary evolution assume perfect mergers for all impacts that are not completely catastrophic. Reflecting on this, they write:

Emsenhuber & Asphaug (2019a, hereafter Paper I) showed that this is not generally the case. They studied the fate of the runner following hit-and-runs into proto-Earths at 1 au, for thousands of geometries, and found that, contrary to expectation, only about half the time (depending on the runner’s egress velocity, which depends on the impact velocity and angle) do they return to collide again with proto-Earth. When they do, the return collision happens on a timescale of thousands to millions of years.

That work — fully treated in the first of the papers cited below — also revealed that the majority of the runners that did not return to the forming Earth would be likely to collide with Venus, given the assumption of their current masses and orbits. Those runners that did return would show an impact velocity in the second collision similar to the egress velocity after the first hit and run, thus slower than the original impact because of momentum loss. Follow-on collisions, then, are likely to be slow.

So we have a scenario in which the Earth takes repeated hits and spins off many impactors toward the inner system as they fall deeper into the Sun’s gravity well rather than eventually assimilating them itself. It’s an interesting notion given that, while Earth and Venus (so-called ‘sister planets’) have similar mass and density, Venus is nonetheless in a distinctly different state, its rotation retrograde compared to other planets, with a single rotation taking 243 days. There are also no moons at Venus. Do impacts during formation account for the differences?

To put the thesis to the test, the scientists built predictive models from 3D simulations of such impacts, drawing on machine learning techniques. They simulated terrestrial planet evolution over the course of 100 million years, calculating both hit-and-run collisions and those in which the impactor merged with the object struck.

The simulations explore the dynamical evolutions of remnants of hit-and-run collisions until the impactor is finally accreted or ejected.The different scenarios, says Asphaug, portray a sharply different formation history for the two worlds:

“In our view, Earth would have accreted most of its material from collisions that were head-on hits, or else slower than those experienced by Venus. Collisions into the Earth that were more oblique and higher velocity would have preferentially ended up on Venus…. We find that most giant impacts, even relatively ‘slow’ ones, are hit-and-runs. This means that for two planets to merge, you usually first have to slow them down in a hit-and-run collision. To think of giant impacts, for instance the formation of the moon, as a singular event is probably wrong. More likely it took two collisions in a row.”

Image: The Moon is thought to be the aftermath of a giant impact. According to a new theory, there were two giant impacts in a row, separated by about 1 million years, involving a Mars-sized ‘Theia’ and proto-Earth. In this image, the proposed hit-and-run collision is simulated in 3D, shown about an hour after impact. A cut-away view shows the iron cores. Theia (or most of it) barely escapes, so a follow-on collision is likely. Credit: A. Emsenhuber/University of Bern/University of Munich.

Earth’s impact history thus has a telling influence on planetary composition. From the paper:

…if the terrestrial planets formed in multiple giant impacts, then Venus is significantly more likely than Earth to have accreted a massive outer solar system body during the late stage of planet formation. Earth, by contrast, has no terrestrial planet beyond its orbit to act as a vanguard. Mars is about the same mass as the late-stage projectiles…, 0.1 M?, and thus relatively inconsequential in terms of slowing them down through hit-and-run, so Earth has to do it on its own.

The late stage of terrestrial planet evolution in our own Solar System thus may hinge on how each world dealt with these impact runners. One thing that emphatically emerges from the work is that, according to these simulations, the terrestrial planets were hardly isolated during this period. Hit-and-run objects strike one planet, then the other, the probability of the impacts factored into the simulation via relative velocity and orbital configuration choices in the analysis.

In this study, Earth slows down projectiles, but accretes no more than half of them itself. Venus becomes a sink for these objects, retaining the majority of them in all simulations after their encounter with Earth as the slowed velocity of the runner allows for subsequent accretion. This would naturally lead to differences in composition between Venus and Earth and would account for differences in everything from Venus’ spin state, its formation (or lack of it) of moons, to its core-mantle dynamics. The authors promise a follow-up paper exploring these issues.

The papers are Emsenhuber et al., “Collision Chains among the Terrestrial Planets. II. An Asymmetry between Earth and Venus,” Planetary Science Journal Vol. 2, No. 5 (23 September, 2021), 199 (full text). The second paper is Asphaug et al., “Collision Chains among the Terrestrial Planets. III. Formation of the Moon,” Planetary Science Journal Vol. 2, No. 5 (23 September, 2021), 200 (full text)