If a geologist has ever shown you a set of their photos you have probably seen that they always include objects such as pens, lens caps, rock hammers or coins in the image. Then you probably asked why, to which they replied that it is for scale. What does this all mean and why is it relevant to the Curiosity Rover and the penny that NASA sent to Mars?
We use an object of known size for scale so that when we go back to look at object we know roughly how big it is, this can be very important as some geologic features such as folds look very similar over very large size differences (from the microscopic scale right up to tens of kilometres). The objects we use are ones that carry into the field such as lens caps, coins, pens and hammers, as well as our colleagues as it is easy to look at the object once we return home and recognise the scale.
One of the instruments on NASA’s Mars Curiosity Rover is the Mars Hand Lens Imager (MAHLI) , which is designed to take high resolution photos, allowing geologists to examine the surface of Mars in the same way as if they were looking at the rocks using their own hand lens. In order to get the best results out of this instrument, it is necessary to calibrate it for scale, colour and depth. So how did NASA do this? They simply included a calibration target consisting of a metric scale bar (good to see that not all Americans are glued to the imperial system), a variety of colour references, a 3D stair pattern and a 1909 Lincoln Penny on the outside of the rover. This target will be used to check the calibration and assist in scaling images. The coin was included as a direct reference to geologists and the use of coins for scale, as well as a way for the general public to get an idea of the scale of the images sent back. After all scale objects work best if you know their size instantaneously.