APXS: Canada's contribution to the Mars Science Laboratory
On November 26, 2011, the National Aeronautics and Space Administration (NASA) launched the Mars Science Laboratory (MSL) mission and its rover named "Curiosity" to investigate whether Mars ever was, or is still today, an environment able to support microbial life. The six-wheeled rover is equipped with the largest, most advanced suite of science instruments ever sent to the Martian surface. A mobile robotic laboratory, Curiosity will be able to analyze samples on board to determine Mars' habitability, characterize its climate and radiation and pave the way for human exploration.
Curiosity is expected to land on Mars on August 6, 2012, near the foot of a layered mountain inside Gale crater, a location most likely to have retained clues to the presence of liquid water (a condition favourable to life) that is also safe for landing. Layers of this mountain contain minerals that form in water. The portion of the crater floor where Curiosity will land has an alluvial fan likely formed by water-carried sediments.
Weighing in at 900 kilograms, Curiosity is roughly the size of a small car (twice as long and five times as heavy as the Mars Exploration Rovers, Spirit and Opportunity). The rover's motorized wheels and robust suspension system will allow it to roll over obstacles as high as 65 centimetres. Whereas previous rovers were powered by solar energy, Curiosity will carry a radioisotope power system that generates electricity from the heat of radioactive decay, which will make operations more independent of Martian seasons. The rover will travel about 100 metres per day for the 5-20 kilometres traverse it may take to reach the lower portion of Gale mountain. Curiosity will investigate a diverse and scientifically rich region that possibly includes several distinct environmental phases of Mars's early history. The mission has an operating lifespan of a full Mars year (687 Earth days) or more.
Curiosity has the capacity to acquire and assess dozens of samples of rock and soil during its planned mission. Located within the rover's belly, the Sample Analysis at Mars (SAM) and the Chemistry and Mineralogy (CheMin) instruments will analyse the powdered samples in unprecedented detail. Remote and contact instruments, like Canada's Alpha Particle X-Ray Spectrometer (APXS), will perform on-site analysis of the area surrounding the rover and select promising samples for further study on board.
The Maple Leaf on Mars: Canada's Contribution
The Mars Science Laboratory will mark the first time Canada investigates the surface of Mars, and the second time the Canadian Space Agency (CSA) lands on the Red Planet (the first being the Phoenix Mars Lander mission in 2008, in which the Canadian weather station discovered snow falling in the atmosphere of Mars).
The CSA is providing the APXS instrument to the MSL mission. An improved version of a similar instrument on the previous Mars rovers Pathfinder, Spirit, and Opportunity, APXS will determine the chemical composition of rock and soil samples.
Roughly the size and shape of a Rubik's cube, the APXS sensor head will be mounted at the end of the rover's robotic arm. It will be used regularly during the mission by being placed against the surface of a sample, for example a 2 centimetre circular patch of soil or a rock (about the size of a two-dollar coin). The instrument's built-in Curium source irradiates the sample with alpha particles and x-rays. Since each element in the sample is stimulated to emit a well-defined energy signature, APXS then measures the characteristic x-ray radiation to determine the sample's composition.
The longer the instrument measures the x-rays, the better it can detect and quantify the composition of samples—even tiny amounts of important geological trace elements. Most measurements will take two to three hours for a complete analysis of all elements (including trace elements). APXS is also able to analyze samples quickly (the instrument can take stock of major and minor elements in about 5-10 minutes). Together with complementary information from color microscopic (MAHLI) images and millimetre-scaled elemental profiles from the laser spectrometer ChemCam, this will provide a rapid assessment of the sample. These in-situ measurements along the traverse will be used to establish the geological context of the surrounding and help to decide which samples warrant further, extensive investigations by Curiosity's laboratory instruments.
On Earth, scientists study the elemental composition of rocks and soil to reveal information about the history of the rock, how it was formed, and if it was altered by water. Studying the chemical composition of rocks on Mars will give researchers a greater understanding of the formation and evolution of the geology near MSL's landing site. APXS will search for evidence of rocks that were formed in the presence of liquid water, or that were altered by water, wind or ice.
APXS can detect some elements that are the building blocks of life (like phosphorus and sulphur) and can estimate if the sample contains significant amounts of bound water. The capability to precisely detect very small amounts of salts, typically left after water interacted with the rock, or major changes in elemental chemistry produced by interaction with water in the past, are among APXS's strengths.
The APXS science team is lead by Dr. Ralf Gellert of the University of Guelph, who is also the lead scientist for the earlier versions of APXS on the Mars Pathfinder (Sojourner) and the Mars Exploration Rovers (Spirit and Opportunity), and provided the scientific design of APXS based on its predecessors. Before joining the University of Guelph, he worked at the Max Planck Institute for Chemistry in Germany, where the Spirit and Opportunity's APXS instruments were developed, built and calibrated by a small team lead by the Dr. Rudolf Rieder. During the ongoing Mars Exploration Rover (MER) mission, APXS has provided key evidence that water once played a major role in Mars's geological history. An important aspect of the MSL APXS investigation is to tie the MSL mission results to the MER mission findings.
The APXS science team is composed of members from the following institutions: University of Guelph, University of New Brunswick, University of Western Ontario, NASA's Jet Propulsion Lab (a division of Caltech), University of California, San Diego, Cornell University and the Rensselaer Polytechnic Institute. Funded by the CSA, the Operations Centre for APXS at the University of Guelph will be the first facility in Canada to command a science instrument on another planet.
The CSA is investing approximately $17.8 million in the design, building, scientific support, and primary operations of the APXS instrument. The CSA managed the development and building of the instrument with MacDonald, Dettwiler and Associates Ltd. (MDA) the prime contractor for APXS. The University of Guelph provided the scientific direction for the design and engineering support during the development, calibrated the APXS instrument and will lead the science operations for the instrument. Components of APXS were tested in Brampton, Montreal, Vancouver, Ottawa, Toronto and Guelph.
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