A key instrument on the Perseverance Rover is PIXL (Planetary Instrument for X-ray Lithochemistry), a microfocus X-ray fluorescence instrument that can analyze the elemental chemistry. Customised polycapillary X-ray focusing optics applied to an X-ray beam (from a rhodium anode, grounded-cathode design X-ray tube) provides for micro-X-ray fluorescence analysis of rocks or soil at a spatial resolution of some 120 microns (suggestive of the use of photonic fibres). The induced X-ray fluorescence information is returned to earth via telemetry. The effort is supported by SHERLOC, an instrument on the end of the rover's robotic arm that can hunt for sand-grain-sized clues in the rocks, working in tandem with WATSON, a camera that is intended to take close-up pictures of rock textures. The high X-ray flux of PIXL gives rise to high sensitivity and hence fast acquisition times, allowing rapid scanning of most of the detectable elements, 26+ in all, now detectable at lower concentrations than possible on previous landed payloads; several new elements can be detected that were not previously detectable on these missions. In particular, PIXL can measure a large number of major and minor elements at 0.5 wt% in 5 seconds, detecting important trace elements at the 10’s of ppm level.

The Perseverance Rover has its antecedents, most recently the Curiosity Rover (functioning for more than 3000 days). Just as is now true of the Perseverance Rover, the Curiosity Rover (which landed on Aeolis Palus inside the Gale crater), was equipped with a radioisotope power system, electricity being generated by the heat given off via the radioactive decay, in these particular cases by plutonium (presumed to be 238Pu, t1/2 87.7 years), such a system generally being referred to as a radioisotope thermoelectric generator (RTG) although for these space missions the power source is referred to as a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG). Power production is suggested to be at the level of some few hundred watt. Although one is not privy to the particular inner workings, in principle the nuclear battery makes use of thermocouples, converting the decay heat into electricity via the Seebeck effect. Importantly, this type of generator has no moving parts.  For the Curiosity mission, instead of use of an x-ray tube for fluorescence analysis, use was made of an Alpha Particle X-ray Spectrometer (APXS), an updated version of the spectrometers used on the Mars Exploration Rover (MER) and Mars Pathfinder missions. In these, the samples are bombarded with X-rays and alpha particles from a curium source, commonly 244Cu, emitting alpha particles with an energy of 5.8 MeV, while x-rays of energy 14 and 18 keV are emitted via the decay of 240PPu. The APXS power has been provided by a thermoelectric generator, of a form previously discussed.

It may not be too well known but with colleagues, the majority of whom were based in Malaysia, this author played what came to be a pivotal role in developing the science that has since been applied in the elemental analysis techniques discussed above, although perhaps now superseded [1]. At Sunway University we too continue to play important roles in impactful research and education in applied and radiation physics. In that very same spirit of perseverance that has this month lead to the very first flight taking place on another planet, we continue to involve ourselves in exploration of the sub-atomic world, joining the efforts of the many others who have contributed to society in such ways, not always within the limelight. For its part, CAPRT, the Centre for Applied Physics and Radiation Technologies at the School of Engineering and Technology, in unison with the Physics Department of the University of Melbourne, co-hosted the 3rd International Forum on Advances in Radiation Physics (IFARP-3, 24-25th February 2020), a virtual platform meeting with contributions from some 20 countries. Later this year we will also hold the 15th meeting of the prestigious International Symposium on Radiation Physics (ISRP-15, 6-10 December; do visit the website isrp15.com to see the very many world leading nuclear and radiation scientists who will be presenting at the meeting). Standing back from the many radiation physics aspects surrounding the functioning of the Perseverance Rover, in the many months since the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) came to so overwhelmingly control our lives, such activities all point to the power of perseverance and of curiosity, even when confronted by difficulties and complexity. We press on regardless.

[1]. The use of coherent gamma-ray scattering for the characterisation of materials. J.R.Mossop, S.A.Kerr, D.A.Bradley, C.S.Chong and A.M.Ghose, Volume 255, Issues 1–2, 15 March 1987, Pages 419-422. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

D A Bradley
Head, CAPRT
@email