As I'm sure you're aware, geologists love rocks, and they especially love the rocks on Mars. The first thing they all want to know about a rock is what's it made of. For that, it's good to just take a look at the color of the rock surface. When everything is being done on the alien landscape of another world, it's easy enough to electronically get the color wrong, or not quite right. To that end, artists, photographers, and a few scientists have noticed that by looking at the color of a shadow on a neutral white or gray background, you can infer the color contributed to the scene by the sky.
NASA / JPL / MSSS / Gordan Ugarkovic
Curiosity Marsdial on Mars!
Four Mastcam images of the calibration target -- the Marsdial -- were taken on Curiosity's sol 3 (August 9, 2012) over a period of about 8 minutes. In that time, the shadow of the gnomon moved slightly, marking time on Mars with a sundial.
On Earth, shadows take on a sky blue tinge (what I like to call "cerulescence"). On Mars, it's a salmon color (what I like to call "arangidescence"). And so, the MarsDials bear a small metal post that casts a shadow onto some white and gray rings of known value or grayness. In the coming weeks, we will electronically create some sundial-style hour lines on Curiosity's MarsDial. Along with good geology, we'll do some timekeeping with the Sun's apparent motion across an alien sky. Stay tuned for our upcoming parallel EarthDial project. We want people everywhere to do some sundialing to help know and appreciate our place in space.