Jahnavi Shah

PhD Student, Planetary Science

Spring Research Update

Happy Spring all!

Today, I want to talk a little bit about a new project I started working on related to impact modelling to simulate the formation of impact craters on Titan and assess their degradation. I use iSALE (impact-Simple Arbitrarty Lagragian Eurlerian), a multi-material, multi-rheology shock physics code (Amsden et al., 1980; Collins et al., 2004; W√ľnnemann et al., 2006). The hydrocode has been updated extensively over time, for example, the original code was capable of simulating only single-material, Newtonian-fluid flow. I’m not going to explain all the technical and mathematical here, partly because I’m still learning as I go. But as brief summary, the hydrocode solves fluid flow (of continuous media) problems using the Lagrangian and/or Eulerian method and can make predictions about the media’s response to deformation.

Here is an example of a 1.6 km diameter impactor striking a uniform target of the same granite-like material at a velocity of 6.5 km/s.

Let’s take a closer look at some of the input parameters and mesh geometry as I have just recently started to set up my own runs. The model intakes parameters such as impact angle, impactor, and target properties (e.g. density) and outputs size and shape of craters, features associated with crater formation process such as impact melt and ejecta, along with properties such as temperature, density, and pressure changes. Here is a peek at the input file for the example shown above:

Let me briefly walk through the different groups of parameters. The more straightforward parameters are the global setup parameters (includes planet radius, gravity, and surface temperature) and time parameters (includes timestep and end time). The parameters that I often change for different runs are mesh geometry (which I will go into further detail), projectile parameters (includes resolution), and target parameters (includes material and position of the target layers). The outputs can be plotted using a special python package (pySALEPlot). The outputs include temperature, density, pressure, and damage.

Mesh geometry
The boundary conditions are pretty much standard for impact modelling. No slip: zero velocity in both coordinates, free slip: zero velocity normal to boundary, outflow: material allowed to flow across boundary. This is the part which requires a trace of simple arithmetic to calculate number of cells needed.

Let’s look at an example of a 10 km diameter, spheroid, ice impactor striking a target of 2 layers (50 km water ice above a water ocean) on Titan. These are new results so I haven’t quite interpreted them yet. I’m hoping to have a few runs completed soon, and then I can explain it a bit more.

10 km diameter impactor, 50 km water-ice later above a water ocean

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