The Origin of the Moon

The leading theory for the formation of the Moon suggests that a Mars-sized impactor struck the proto-Earth in a violent collision shortly after the formation of the solar system. The impact produces vapor, liquid, and solid debris. The debris either escapes the system, falls back to the Earth, or enters circumterrestrial orbits forming a “protolunar disk“. It is from this disk that the Moon is thought to have formed.

Planetary Scale Giant Impacts
Snapshot from a 3-D numerical simulation of a magnetized, Moon-forming giant impact.  The snapshot is taken ~1 hr after initial impact. Color denotes density; black lines denote magnetic field lines.

We cannot simply go into a laboratory and smash two planets together. To study the evolution of a planetary scale giant impact, we must apply numerical simulations. These simulations are complicated; they evolve the collision of bodies subject to self-gravity, a non-trivial equation of state, and potentially even magnetic fields — all in 3D.

On a day-to-day basis, I crash planets into one another using the astrophysical magnetohydrodynamics framework Athena++. I specialize in investigating the role of magnetic fields in the Moon-forming giant impact. Magnetic fields are amplified as they are stretched and wound by turbulence generated by the planetary collision.

The Evolution of the Protolunar Disk
Density slice through the protolunar disk midplane ~220 hr post-impact. Overpotted are streamlines corresponding to the x- and y-components of the magnetic field. Vigorous MHD turbulence drives disk evolution.

The birthplace of our Moon was a complex environment. Following the Moon-forming giant impact, a debris disk settled into orbit about the Earth. This protolunar disk was comprised of liquid and vapor lunar rock. It is likely that hot vapor in the protolunar disk interacted with a magnetic field that was amplified by the giant impact. In particular, we anticipate that the protolunar disk was unstable to the magnetorotational instability (MRI). The MRI yields vigorous magnetic turbulence and grows magnetic field strengths exponentially in time. It is possible that the protolunar disk could have exhibited magnetic field strengths comparable to those inside a medical magnetic resonance imaging machine! The MRI enables angular momentum transport. The protolunar disk is forced to spread and material is accreted onto the proto-Earth making Moon formation less efficient. Material accreted onto the proto-Earth must pass through the boundary layer, i.e., the transition layer between the proto-Earth and the innermost regions of the protolunar disk. This boundary layer may be unstable to supersonic shear instabilities that source spiral waves which propagate into the protolunar disk, hence mixing material between the molten Earth and debris disk. Understanding the dynamics of the protolunar disk is essential to understand when, where, and how our Moon formed. Furthermore, numerical simulations of the early evolution of protolunar disk may help reconcile the anomalous similarities between the isotopic ratios of lunar and terrestrial samples.

Numerical Magnetohydrodynamics

I am interested in the development and implementation of numerical algorithms for computational magnetohydrodynamics. I am a contributor to the Athena++ magnetohydrodynamics framework.

Athena++ Logo courtesy of Chris White (@c-white) and Kengo Tomida (@tomidakn) via the Athena++ Website.