The primary purpose of this study is to model the water vapor flow produced by a comet
impact on the Moon using the Direct Simulation Monte Carlo (DSMC) method. Toward that end,
our DSMC solver was modified in order to model the cometary water from the time of impact
until it is either destroyed due to escape or photodestruction processes or captured inside one of
the lunar polar cold traps.
In order to model the complex flow induced by a comet impact, a 3D spherical parallel
version of the DSMC method was implemented. The DSMC solver was also modified to take as
input the solution from the SOVA hydrocode for the impact event at a fixed interface. An
unsteady multi-domain approach and a collision limiting scheme were also added to the previous
implementation in order to follow the water from the continuum regions near the point of impact
to the much later rarefied atmospheric flow around the Moon.
The present implementation was tested on a simple unsteady hemispherical expansion
flow into a vacuum. For these simulations, the data at the interface were provided by a 1D
analytical model instead of the SOVA solution. Good results were obtained downstream of the
interface for density, temperature and radial velocity. Freezing of the vibrational modes was also
observed in the transitional regime as the flow became collisionless.
The 45° oblique impact of a 1 km radius ice sphere at 30 km/s was simulated up to
several months after impact. Most of the water crosses the interface under 5 s moving mostly
directly downstream of the interface. Most of the water escapes the gravity well of the Moon
within the first few hours after impact. For such a comet impact, only ~3% of the comet mass
remains on the Moon after impact. As the Moon rotates, the molecules begin to migrate until they
are destroyed or captured in a cold trap. Of the 3% of the water remaining on the Moon after
impact, only a small fraction, ~0.14% of the comet mass, actually reaches the cold traps; nearly
all of the rest is photo-destroyed. Based on the surface area of the cold traps used in the present
simulations, ~1 mm of ice would have accumulated in the polar cold traps after such an impact.
Estimates for the total mass of water accumulated in the polar cold traps over one billion years
are consistent with recent observations. / text
| Identifer | oai:union.ndltd.org:UTEXAS/oai:repositories.lib.utexas.edu:2152/ETD-UT-2010-05-1255 |
| Date | 11 October 2010 |
| Creators | Stewart, Bénédicte |
| Source Sets | University of Texas |
| Language | English |
| Detected Language | English |
| Type | thesis |
| Format | application/pdf |
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