Numerical Simulation of Primary Impact Distribution on Phobos

The craters on the Martian moons, Phobos and Deimos, are in a state close to geometric saturation, indicating that they are geologically old [1]. Secondary impacts from Phobos are thought to contribute to an increased impact flux beyond expectations [2,3]. Previous crater counting also showed that the trailing hemisphere of Phobos has a higher crater density than the leading hemisphere, obscuring evidence of synchronous orbit with Mars [4]. Additionally, craters of a significant size, identified as secondary in the crater catalog [5], exhibit a distribution showing an asymmetry between the leading and the trailing hemispheres on Phobos, although this asymmetry is less pronounced than predicted by theoretical models.

To investigate this discrepancy, we performed detailed impact simulations by randomly distributing impactors around a virtual sphere modeled 200,000 km from Phobos. The simulations accounted for the gravitational fields and sizes of Mars and Phobos, with varying distances between Mars and Phobos. Our findings indicate that increasing the Mars-Phobos distance decreases the asymmetry between the leading and trailing hemispheres; however, the predicted asymmetry remains larger than observed values. This suggests that additional mechanisms contributing to a higher impact flux may be at play on Phobos, beyond impacts originating from outside the Martian system.

References:

[1]Hirata, N., 2017: Spatial distribution of impact craters on Deimos, Icarus, 288, 69–77.

[2]Ramsley, K. R., and Head III, J. W., 2013: Mars impact ejecta in the regolith of Phobos: bulk concentration and distribution, Planetary and Space Science, 87, 115–129.

[3]Nayak, M., et al., 2016: Effects of mass transfer between Martian satellites on surface geology, Icarus, 267, 220–231.

[4]Schmedemann, N., et al., 2014: The age of Phobos and its largest crater, Stickney, Planetary and Space Science, 102, 152–163.

[5]Salamunićcar, G., et al., 2014: Integrated method for crater detection from topography and optical images and the new PH9224GT catalogue of Phobos impact craters, Advances in Space Research, 53.12, 1798–1809.