Cranfield University Develops Replica Soil for Mars

Soil experts from Cranfield University have developed a replica soil for Mars in a project for the European Space Agency (ESA) to help maximise the performance of the next robotic rovers to be sent to the planet.

The 'soil surrogate' is one of a number of soil types characterized by the current NASA Mars Exploration Rovers (MER), Spirit and Opportunity, which were sent to Mars in 2003.

The replica soils will be used to help recreate the terrain of the Martian surface and conduct a number of experimental tests on earth prior to sending the ESA rover (ExoMars) to Mars in 2018.

NASA performed similar experiments to enable characterisation of the physical environment on Mars, and to maximise the performance of their vehicle - and minimise the possibility of the rover getting stuck in the soil. These experiments were repeated on Mars and involved wheel trenching and slipping, creating ruts which were used to reliably infer the soil physical properties.

A number of calibration experiments were carried out on earth, using systems similar to those used on Spirit and Opportunity, and four different soil types were identified. Earth based simulants of each of these were labelled Engineering Soil (ES)1, ES2, ES3 and ES4.

Astrium, Europe's leading space company, working with ESA, approached Cranfield in late 2010 to source new materials that could replicate the most complex of these soil types - ES2 and ES4. While ES2 was discovered to be fairly easy to simulate using existing materials, ES4 proved much more complicated.

The complex structure of the soil meant that there were a number of strict criteria around grain size (greater than 50% of the sample had to be within a very narrow size range), relative density, cohesion and angle of internal friction. With Cranfield's state-of-the-art soil laboratories and testing facilities, the team were able to analyse a number of materials, all tested in dry environments with less than 50% humidity.

The resulting replica soil, based on a pure silica flour, will now be used by Astrium in the development of the ExoMars rover. Based on the experience developed over the course of this activity, Cranfield have also been awarded a further contract by Astrium to test a number of potential new wheel designs as part of the ESA Sample Fetching Rover study. Using the ES4 replica soil, wheels of varying stiffness have been tested and new test data produced to further improve our understanding of Martian terramechanics.

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