Planetary Surface Exploration

Faculty member Ralf Gellert is currently the lead scientist for the Alpha-Particle X-ray Spectrometer (APXS) on board the Mars Exploration Rovers (MER). Together with a team of scientists from the Max-Planck Institute in Mainz, the Jet Propulsion Lab and NASA we are performing the daily operations of the instruments on Mars and analyzing the returned data. The APXS is one of the analytical instruments mounted on the rover arm. It measures the chemical composition of rocks and soils with x-ray spectroscopy. Since the successful landing on Mars in January 2004 the rovers Spirit and Opportunity returned more than 100 APXS measurements from each site along their traverse of nearly 5 kilometers. Results of the APXS on both landing sites contributed to the findings of MER that water played a major role during the formation of the encountered rocks and soils. In collaboration with the group of Prof. Iain Campbell we are improving the theoretical model for the data analysis. The APXS uses alpha particles and x-rays from radioactive sources to excite characteristic elemental radiation. The GUPIX package for PIXE (Proton induced X-ray Emission) will be extended to include both of these excitation modes.


John L. (Iain) Campbell
Ralf Gellert
Joanne M. O'Meara

Planetary Surface Exploration

Written by Michael Stuck

The University of Guelph is playing an important role in the exploration of Mars. One of NASA’s current missions on Mars, the Mars Science Laboratory (MSL), involves the rover known as Curiosity, which features a scientific instrument developed by University of Guelph Professor Ralf Gellert

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.

 Picture of the APXS instrument on the robotic arm of Curiosity.
1: Picture of the APXS instrument on the robotic arm of 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 team in August 2012 at NASA’s Jet Propulsion Lab in Pasadena, just before Curiosity’s landing on Mars.
2: The APXS team in August 2012 at NASA’s Jet Propulsion Lab in Pasadena, just before Curiosity’s landing on Mars. 

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.

 Diagram of the mechanism involved during spectroscopy at the atomic level.
3: Diagram of the mechanism involved during spectroscopy at the atomic level.

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.”

You can visit Professor Gellert’s website and more details about the APXS are provided by the Canadian Space Agency:

NASA Science Mars Exploration Program. APXS. Retrieved from: 
Bruker. Handheld XRF: How it works. Retrieved from: