Mars 2020 PIXL Perseverance Rover: Differentiating X-Ray Spectroscopy

Presenter(s)

Kenzie Mounir

Abstract or Description

The Perseverance Mars rover landed on Mars on February 18th, 2021. The mission’s primary objective is to seek out signs of ancient life and collect samples of rock and regolith (broken rock and soil) for return to Earth. Perseverance landed in Jezero crater, chosen because billions of years ago Jezero held a 40 km in diameter, and few hundred meters deep lake with both inflow and outflow. Additionally, a prominent delta, fine grained lacustrine sediments, and carbonate-bearing rocks offer attractive targets for past habitability and biosignature preservation potential.  

Perseverance is equipped with a suite of scientific instruments to investigate the biological potential of Mars and to select scientifically interesting samples for returning to Earth to tell us about Mars’ geological history. Our work focuses on the Planetary Instrument for X-Ray Lithochemistry (PIXL). PIXL is an X-ray fluorescence (XRF) spectrometer that scans the surface of Mars and is capable of raster-scanning and thus generating composition maps with sub-mm spatial resolution. Incident X-rays generated by PIXL’s X-ray tube irradiate the sample and elements present therein emit fluorescent X-ray radiation with discrete energies that are diagnostic of specific elements like iron and rubidium. PIXL can directly detect elements of atomic number >10. By measuring the intensities of the emitted energies, it is possible to determine how much of each element is in the sample.  

When an atom emits a fluorescence X-ray, it is tabulated into an energy dispersive spectrum through signal processing electronics. Complications arise when two X-rays interact with the detector simultaneously, prompting the instrument to perceive the energy as the sum of the two X-rays, as opposed to two individual quanta. These “pile-up peaks” are logically most common among the largest magnitude peaks within a spectrum. Primary characteristic lines for Mars samples include silicon (1.74 keV), calcium (3.69 keV), and iron (6.40 keV and 7.06 keV). In instances where iron Kα (6.40 keV) and Kβ (7.06 keV) impinge the detector simultaneously, the resultant signal is processed as if it was a single 13.46 keV X-ray quanta. The primary X-ray line for rubidium is 13.40 keV. As the typical gain for PIXL is around 8 eV/channel, this means the peak centroids of the Fe Kα-Kβ pile-up and Rb Kα are only separated by around 7 channels. Thus, the ability to quantify Rb precisely is dependent on the Fe concentration in the sample and a well-constrained understanding of pile-up behavior on the PIXL flight instrument.  

It is important to differentiate between the pile-up peak and rubidium because rubidium can tell us a lot about Mars’ geological history. It has a slight stimulatory effect on metabolism, due to it being chemically similar to potassium. Understanding rubidium is important for understanding Mar’s potential for habitability and geologic history. Potassium and rubidium are usually found together in minerals and soils. Plants tend to adsorb rubidium quite quickly. When stressed by deficiency of potassium some plants, such as sugar beets, will respond to the addition of rubidium. Since rubidium and potassium are usually found together it is also important to understand how potassium maintains osmotic pressure between cells and interstitial fluid. Putting all of this together allows us to make justified assumptions of what Mars was like millions of years ago. 

We will model PIXL data from Mars with a MATLAB analytical routine. We will characterize both Fe lines as well as the Fe Kα-Kβ pile-up and Rb Kα lines. Through the analysis of tens of thousands of data points, we will be able to constrain the intensity of the Fe pile-up with respect to the primary Fe line. This will improve our ability to precisely quantify Rb and thus make more accurate biological assessments that are dependent on precise trace element chemistry.