Mercury, the Sun's innermost planet, is one of the darkest major bodies in the solar system. It reflects less than 10 percent of the sunlight that strikes its surface. A recent study suggests that passing comets could coat Mercury with carbon-rich molecules, darkening the surface. This 2015 image of Mercury from the Messenger spacecraft features a wide, deep basin known as Rembrandt. [NASA/JHUAPL/CIW]