New Publication: Streaming Instability in Protolunar Disks

The Limited Role of the Streaming Instability in Protolunar Disks

Miki Nakajima, Jeremy L. Atkins, Jacob B. Simon, Alice C. Quillen
The Planetary Science Journal, Volume 5, Number 6 (2024)

I’m excited to share that my colleagues and I have published our work on the role of the streaming instability in the formation of the moon in the latest issue of The Planetary Science Journal!

When a Mars-sized object collided with the early Earth billions of years ago, it created a disk of vaporized rock that would eventually coalesce into our Moon. But the exact mechanism by which tiny particles in this disk grew into a massive satellite has remained unclear. A variety of impact models have been proposed, varying the size, energy, number of impactors, etc., but no model has been able to fully explain the Moon’s properties.

One differing prediction between models is the resulting impact disk’s level of gas content: some impact models produce lots of vapor, and some don’t. Vapor-rich disks can have trouble forming Moon-sized objects due to drag: any clumps inspiral and crash into the Earth before they can get large enough to keep themselves in orbit. If Moons can’t form in vapor-rich disks, we can rule out models that produce them, which would help narrow down the possible impact scenarios.

One effect that has allowed planetesimals to survive and grow in vapor-rich protoplanetary disks is the streaming instability: essentially, the vapor orbits more slowly than the particles, creating a headwind that ends up quickly creating clumps, allowing solid matter to accrete into a moonlet faster than the inspiral timescale. The streaming instability has been successful in explaining how kilometer-sized planetesimals form in protoplanetary disks around young stars. We explored whether the streaming instability also played a significant role in forming our Moon.

Using detailed hydrodynamic simulations, we found that the conditions in protolunar disks are actually quite hostile to the streaming instability. The key issue is that these disks are much hotter and more compact than protoplanetary disks, with temperatures exceeding 3000 K and very high pressure variations. While the streaming instability is able to form 100 km moonlets, under these extreme conditions, it simply isn’t strong enough to overcome the vapor drag, and moonlets all inspiral within within a few months.

Our results suggest that the less-energetic collision models, i.e. those that produce vapor-poor disks, are thus much more likely to create disks that successfully spawn large Moons. This has important implications for understanding not just our own Moon, but also the diverse satellite systems we observe around gas giants and the potential moons of exoplanets: since impacts with planets much larger than Earth are likely to produce vapor-rich disks, it’s unlikely that we will find large moons around such planets. In other words, we’re more likely to find large Moons around “small” planets, like Earth.

Read the paper on PSJ, or download the PDF!