Gellert Group

Planetary Surface Exploration

The instrument is called the Alpha Particle X-Ray Spectrometer (APXS) and is one of 11 scientific instruments onboard the Curiosity rover. Ralf Gellert is the Principal Investigator for APXS and leads a team of scientists from the University of Guelph, NASA’s Jet Propulsion Laboratory, and other institutions.

Professor Gellert’s involvement with APXS precedes the MSL Mission, dating back to 2001 and his work on the earlier version of the instrument. At the Max-Planck Institute for Chemistry in Mainz, Germany, he worked as the lead engineer for APXS, which was selected for deployment on the Mars Exploration Rovers (MER), named Spirit and Opportunity. Gellert and a small team built the device and developed the calibration and analysis software. Gellert led the MER APXS operations and data analysis from 2005 until Opportunity, the longer standing of the two MER, stopped working in 2017. He also led a successful proposal to NASA for a new APXS for the MSL. In 2005, after the department that Gellert worked in closed, he came to the University of Guelph where he continued his work with the ongoing MER missions and built a lab to develop and test the new version of APXS for Curiosity.

Curiosity launched in November 2011, equipped with the new APXS model, and successfully landed on the surface of Mars in August 2012. Since Curiosity’s landing, the Guelph APXS team has been responsible for the daily operation of the device, collaborating with the scientists operating the other 10 instruments onboard Curiosity to determine what to do next. Gellert explains, “Each day, we are discussing with the whole science team and the rover planners, what to do next. This involves ‘That rock looks interesting, let’s drive over there’, then there is a discussion of ‘ok, is it worth the resources or could we drive somewhere else?’”. Once a decision is reached, measurements are taken and data are collected, which are then used to inform future decisions. 

The APXS is used to measure the chemical composition of Martian rocks and soils. It is located on the robotic arm of Curiosity so that it can be placed near, or in contact with, samples of interest. The APXS works by bombarding the sample with radiation (alpha particles and X-rays), which can knock tightly bound electrons from the atoms within the sample, creating a vacancy. When the vacancy is then filled by another electron, an X-ray is emitted in the process. These X-rays are unique signatures for each element in the periodic table, allowing the APXS to determine the types and abundance of elements in the sample.

The APXS team is focused on giving precise concentration values to geological experts for interpretation.  The team is constantly working to improve their ability to produce more reliable and accurate values. Gellert explains, “our scientific goal for the APXS team consists of trying to extract all possible information from data we get from Mars. This means if we know we can measure 16 elements, can we measure even more? We see trace elements that are not that abundant in terrestrial samples that we calibrated with. Can we use the data in ways that we didn’t even think about at launch?”

There is still a lot to explore on the surface of Mars. The experts working on the MSL mission are investigating Mars in a way which has never been done before, which is a learning process. Opportunities for scientific inquiry have led to many fascinating student projects at the University of Guelph, many of which have been supervised by Professor Gellert. The projects involve trying to analyze and interpret publicly available data from Mars to gain new understandings. According to Gellert, “all these 4th year projects were very interesting. Sometimes you will see stuff that experts have overlooked over the years because there is so much data.”

More details about the APXS are provided by the Canadian Space Agency: https://www.asc-csa.gc.ca/eng/astronomy/mars/apxs.asp

References

https://link.springer.com/article/10.1007/s11214-012-9892-2#Sec26 
https://link.springer.com/article/10.1007/s11214-012-9873-5?no 
NASA Science Mars Exploration Program. APXS. Retrieved from: https://mars.nasa.gov/msl/spacecraft/instruments/apxs/ 
Bruker. Handheld XRF: How it works. Retrieved from: https://www.bruker.com/products/x-ray-diffraction-and-elemental-analysis/handheld-xrf/how-xrf-works.html 
 

Research Overview

Curiosity 10 years later

• The Curiosity Rover has covered a straight-line distance of 10517.1m, and 
• climbed 627.3m up Mt Sharp.
• During that time, APXS has analyzed 1263 scientifically interesting targets, 
• downlinked in 2770 discrete EDRs (Experiment Data Records).
• To acquire these measurements, APXS was solar powered on for 255 days, 7:53:20.

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