1. The Origin of the Elongated Crater Population on Mars

 

ESA supervisor: Elliot Sefton-Nash
Collaborator(s): Olivier Witasse

Site: ESTEC

The Martian Moons eXploration mission (MMX) aims to resolve whether Phobos and Deimos are captured asteroids, or the remnants of an equatorial debris disk formed around Mars following a giant impact. If such a debris disk existed, ‘moonlets’ may have had slowly decaying orbits, leading to formation of elongated craters due the low impact angles of bolides (< 5°). 

This ‘decaying moonlet’ hypothesis may be addressed with available remote sensing data and numerical modelling of atmospheric entry. Previous work has collected size, location and orientation statistics for several hundred elongated craters on Mars. The absence of crater morphologies consistent with very low impact angles has been used to argue against the spiraling moonlet hypothesis for the formation of elongated craters, but the absence of comprehensive atmospheric entry and impact modelling for decaying moonlets leaves open the possibility that impact angles for spiraling moonlets may have been increased by drag in a thicker ancient atmosphere.

The distinguishing morphological properties of elongated craters and their ejecta become more pronounced with decreasing impact angle, which allows easy retrieval of impact direction. However, for elliptical craters with less grazing impact angles, or degraded morphological indicators, impact direction may be ±180° ambiguous. In these cases, the presence of any crater floor asymmetry may allow retrieval of impact sense.

Furthermore, retrieved crater azimuths are sensitive to the spatial sampling of crater shape, which is determined by mapping crater rims in a Geographic Information System (GIS). Comparison with databases compiled by other research groups is needed to understand uncertainty on retrieved azimuths.

Finally, to investigate the decaying moonlet hypothesis, true polar wander of Mars’ rotation axis is expected to be the predominant factor determining whether crater azimuths align with paleo-equators, above which moonlets in a quasi-stable debris disk could gradually decay. While obliquity cycles would indeed modify the relationship between latitude, azimuth and orbit plane inclination, a transient debris disk that lingered for several million years would be most dynamically stable in an equatorial orbit, even throughout obliquity variations. Exploration of crater azimuth alignment with paleo-equators could help identify groups of craters with origins from the same orbit plane. Several preliminary candidates have been identified, but refinement of the analysis and detailed study of candidate craters is needed to further test the hypothesis.

Several objectives are defined for this project:

  • Analysis of crater morphology to retrieve impact sense, using topographic data from stereo-imagery or laser altimeter datasets.
  • Comparison of retrieved crater azimuths with those from other databases.
  • Characterisation with remote sensing data of elongated crater candidates to build a database and compare traits (geomorphology, geologic age, inferred orbit of impactor).

Project duration: 6 months.

Desirable expertise or programming language:

Suitable candidates for this position would benefit from being familiar with Mars or other planetary surface datasets, planetary geology/geomorphology, geospatial mapping and analysis in Geographic Information Systems (e.g. QGIS, ArcGIS), and programming in Matlab/GNU Octave/Python.

 

To apply for this project please fill in an online application form through the following link.