I’ve seen them referred to as “eyeshadow”, “paint” and “whatever those are” but in truth, they are an important part of the Mars Science Laboratory’s science mission: calibration targets.
In the shake, rattle and roll of liftoff, flight and, in the case of Curiosity, the Mars Science Laboratory, seven minutes of terror as it falls to the planet’s surface, things can shake loose, degrade over time or stop working altogether. Calibration activities remain the best way to assure data collected are correct.
When conducting science remotely, it’s important that scientists are sure of what they are looking at and of various findings they make. In other words it’s one the ways scientists can say “how we know what we know”. The best way to achieve this is by including a well-known and characterized source to check your data against. These are referred to as calibration targets or cal targets.
There are many types of cal targets and infinite uses for them from calibrating the white balance on your digital SLR camera to telling a spacecraft light-years away that indeed, what it is looking at is white or, it is blue not white or that the chemical signature we are seeing is cadmium, which shows up as a particular color in the spectrum.
Whatever the composition of the target, the process is the same; a reading is taken from the cal target, from the subject and often from a third source. The readings from each of the sources are compared against each other and if they are in agreement, the instrument is correctly calibrated. If there is disagreement, data from each source is analysed to determine the location of the error.