Ph.D. Defense: Final Oral Examination for the Degree of Doctor of Philosophy of Scott Van Bommel

Event Details

  • Date:
  • Time: 1:30 PM
  • Location: MacN 415

Thesis Title:

Expanding the Capability of the Alpha Particle X-ray Spectrometer Including Quantification of Fine-Scale Chemistry and Atmospheric Monitoring


The chemical composition of the surface of Mars has been investigated with the Alpha Particle X-Ray Spectrometer (APXS) dating back to the first Martian mobile explorer in 1997. The Alpha Proton X-Ray Spectrometer on the Sojourner rover predated the significantly improved Mars Exploration Rover (MER) APXS which flew on twin missions, each landing in 2004. The most recent and Canadian-built APXS, part of the Mars Science Laboratory (MSL) mission that landed in 2012, built on the experience of its MER predecessors and provides further design enhancements benefiting the mission's science objectives. The APXS is perfectly suited for spaceflight through its use of 244Cm sources to generate the alpha particles and X-rays required for X-ray spectroscopy. The complementary use of both particle-induced X-ray emission and X-ray fluorescence excitation methods provides the necessary level of sensitivity across the geologically relevant elements. The work presented herein focuses on three manuscripts and the associated unpublished work in their support that investigates the APXS method in detail to enhance the scientific return.

The longevity of the overall science mission offers a wealth of data for analysis across multiple landing sites on Mars. Though primarily used for solid samples, the APXS on the MER rover Opportunity has monitored the change in atmospheric argon density over six Mars years. The observed argon mixing ratio is consistent with previous published global climate models. Two short-lived spikes in argon density during the equatorial migration of the argon-enriched polar air mass are observed for the first time.

A novel analytical method is presented to deconvolve the endmember chemistry of surface features smaller than the dime-sized APXS field of view through a combined analysis of images and APXS data. The use of images in a new way facilitates the generation of a three-dimensional operational environment for the APXS. The analysis method improves the interpretation of important APXS results in a constrained operational environment. Specific to the MSL mission primary objective, the method resolved chemistry at the sub-cm scale showing clear evidence of either multiple fluid events or an evolving fluid at some point on the surface of Mars.

Examination Committee:

  • Dr. Robert Wickham, Chair
  • Dr. Ralf Gellert, Advisor
  • Dr. J. L. (Iain) Campbell
  • Dr. Dennis Mücher
  • Dr. David Chettle, External Examiner (McMaster University)