Interesting this comes up now, there was a talk at my lab a couple of days ago by Jim Bell of ASU - he is/was responsible for the Pancam payload on MER, i.e. th

Interesting this comes up now, there was a talk at my lab a couple of days ago by Jim Bell of ASU - he is/was responsible for the Pancam payload on MER, i.e. the main stereo cameras. The talk was about colour calibration used on these rovers. Essentially as Spirit and Opportunity were only designed for a 90 day jaunt, the calibration target wasn't designed to be in use for very long. Over 3000 days later it's covered in dust. However, there's also a white/grey patch that has a ring magnet underneath it. The centre of the magnet is totally clear of dust (which is largely magnetic, lots of Iron on Mars), and is pretty much the cleanest part of the rover as a result. Turns out it's quite useful because as more dust accumulates, we can better simulate the Martian atmosphere. You can see the magnet in the top right, note the centre is still white! http://www.hour25online.com/pix/Spirit_dust-comparison_feb05a_02a.jpg http://www.hour25online.com/pix/Spirit_dust-comparison_feb05... MSL Curiosity uses the flight spare from the MER project. It's the thing that looks a bit like a joystick. https://planetary.s3.amazonaws.com/assets/images/z_changeover/marsdial_msl_sol3_animation_ugordan.gif https://planetary.s3.amazonaws.com/assets/images/z_changeove... The six rings on the surface are where the new 'sweep' magnets are, you can see some small debris has gotten stuck to them. It looks like they upgraded it a bit and put them on all the colour targets. This approach is also used for other sensors onboard (UV photodiodes in this case): http://www.spaceflight101.com/uploads/6/4/0/6/6406961/9777658_orig.gif http://www.spaceflight101.com/uploads/6/4/0/6/6406961/977765...