What happens when worlds collide? The question recalls the novel by Philip Wylie and Edwin Balmer, which appeared as a serial in Blue Book magazine beginning in 1932 and concluded the following year. The book version of When Worlds Collide appeared in 1933, and the movie, directed by Rudolph Maté, came out in 1951 in a George Pal production. I would wager that most Centauri Dreams readers have seen it.

Let’s hope we never share such a fate, but it’s likely that collisions are commonplace in the late stages of planet formation, and many researchers believe that Earth’s Moon was the result of the collision of our planet with a Mars-sized planet about 4.5 billion years ago. Scientists at Durham University and the University of Glasgow have recently developed computer simulations tracking atmosphere loss during such collisions using the COSMA supercomputer, which is part of the DiRAC High-Performance Computing facility in Durham.

The work involves smoothed particle hydrodynamics (SPH) simulations to model giant impacts. The planets are modeled as digital ‘particles’ and run through a process of evolution under different conditions of gravity and pressure. This work appears to be the first time the erosion of thin atmospheres has been studied with full 3D, high-resolution simulations of giant impacts at various impact angles and speeds, injecting a range of atmospheric masses for the proto-Earth. The work takes advantage of a program called SWIFT to model hydrodynamics and gravity.

We can get a look at the results in a new paper in the Astrophysical Journal. The authors have created cross-section animations of the early stages of such collisions, modeling not only head-on strikes but grazing impacts like the one thought to have produced our Moon. They likewise model fast as well as slow giant impacts using 100 million particles. As you can see in the image below, these are color-coded for material as well as internal energy.

Image: Still image cross-section showing the impact (inset) and aftermath (main picture) of a 3D simulation of a giant planetary impact using 100 million particles, coloured by their internal energy, similar to their temperature. Credit: Dr Jacob Kegerreis, Durham University.

Jacob Kegerreis (Durham University) places the work in context:

“We know that planetary collisions can have a dramatic effect on a planet’s atmosphere, but this is the first time we’ve been able to study the wide varieties of these violent events in detail. In spite of the remarkably diverse consequences that can come from different impact angles and speeds, we’ve found a simple way to predict how much atmosphere would be lost. This lays the groundwork to be able to predict the atmospheric erosion from any giant impact, which would feed into models of planet formation as a whole. This in turn will help us to understand both the Earth’s history as a habitable planet and the evolution of exoplanets around other stars.”

We learn that the type of impact tells the tale in terms of how much atmosphere is lost. Higher-speed and/or head-on impacts create the greatest atmospheric loss, sometimes completely destroying not just the atmosphere but parts of the planet’s mantle layer beneath the crust. Earth’s likely grazing impact is the kind that disrupts but does not destroy the atmosphere. What actually happened on Earth during this period is not known, but the researchers believe our planet lost somewhere between 10 and 50 percent of the atmosphere.

Atmospheric survival is not assured in the early planet-forming environment, a time when planetary embryos collide after their accretion from a proto-planetary disk. To acquire an atmosphere, planets accrete gases from surrounding materials as well as from impacting volatiles, eventually outgassing volatiles from within. That means an infant rocky world must deal with not only the radiation pressure of its star but small and large impacts. In the case of Earth, we have Moon-formation scenarios but little information about the proto-Earth’s crust, ocean or atmosphere. Consider the evidence from early Solar System materials:

Focusing on the atmosphere, the Earth’s volatile abundances are remarkably different from those of chondrites (Halliday 2013), which act as a record of the condensable components of the early Solar System. Specifically, nitrogen and carbon are depleted compared with hydrogen, which could be explained by the loss of N2 and CO2 with an eroded atmosphere while retaining H2O in an ocean (Sakuraba et al. 2019). Unlike the abundances, the isotope ratios match those of primordial chondrites. Hydrodynamic escape – driven by XUV radiation from the star or heat from the planet below – preferentially removes lighter isotopes, while impacts remove bulk volumes of atmosphere. This suggests that impacts (not necessarily giant ones) are the primary loss mechanism, driving fractionation by removing more atmosphere than ocean while preserving isotope ratios…

Interestingly, there is at least one paper positing two occasions where Earth lost its atmosphere based on the relative abundances of helium and neon in mantle reservoirs of different ages. If giant impacts are relatively common, this would ensure that exoplanets in mass ranges between Earth and Neptune would show wide variation in atmospheric masses. Impacts, in other words, take precedence over irradiation and photoevaporation, according to the authors.

In Earth’s case, the ‘big whack’ that created the Moon may have been only part of the story:

…only around 10% of the atmosphere would have been lost from the immediate effects of the collision. This suggests that the canonical impact itself cannot single-handedly explain the discrepancies between the volatile abundances of the Earth and chondrites by eroding the early atmosphere, compared with alternative, more-violent Moon-forming scenarios. However, the caveat of ‘immediate’ erosion is important, because we have here only considered the direct, dynamical consequences of a giant impact.

The paper goes on to point to the thermal effects of such an impact, producing significant atmosphere erosion. Interestingly, the presence of an ocean can enhance atmosphere loss, creating the higher figure of 50 percent loss mentioned earlier. This work makes it clear how little we know about atmospheric loss, but it does produce a scaling law for total atmospheric erosion for a variety of scenarios involving Mars-sized impactors.

Promising targets for future study include: investigations of different impactor and target masses; extensions to both more massive and even thinner atmospheres; the inclusion of an atmosphere on the impactor as well as the target; and testing the dependence on the planets’ materials, internal structures, and rotation rates. This way, robust scaling laws could be built up to cover the full range of relevant scenarios in both our solar system and exoplanet systems for the loss and delivery of volatiles by giant impacts.

The paper is Kegerreis et al., “Atmospheric Erosion by Giant Impacts onto Terrestrial Planets,” Astrophysical Journal Vol. 897, No. 2 (15 July 2020). Abstract / preprint.