Being the closest to the sun, you'd think Mercury would shine the brightest among the inner planets of the solar system. In truth, however, it reflects even less light than our own moon. Scientists have long speculated on why that is so and what leads to Mercury's very dark facade. Thanks to data gathered by the dearly departed MESSENGER probe, they confirm that carbon is indeed to blame. But interestingly enough, the carbon didn't come from outside forces but right from the planet's crust itself.