Light Spots on Pluto and Dark Spots on Mercury Explained

They sit at opposite ends of the solar system. One is the smallest planet, one was demoted from planet to dwarf planet. Old pictures of Mercury show strange dark spots on its surface. New photos of Pluto show odd light spots floating above it. Both have finally been explained.

Dark spots were seen on the surface of Mercury in 1974 during the Mariner 10 flyby and more recently as the Messenger spacecraft orbited the planet from 2011 to 2015 before its planned crash ending in April 2015. At the time, the spots, also called “low-reflectance material,” were assumed to be carbon deposits left in and around impact craters from comets.

However, new analysis of data from Messenger’s Gamma-Ray and Neutron Spectrometer (GRNS) showed the spots have high concentrations of graphite carbon which is actually left over from when Mercury was young (4.6 billion years ago) and covered with molten magma. Being light, graphite would have floated to the surface of the magma where it solidified and formed the original planetary crust, which the spots are remnants of. Models using the data have confirmed this analysis.

Meanwhile, in the outer reaches of the solar system, New Horizons is slowly sending back all of the data it collected on its flyby of Pluto, including some new photos of the sub-planet’s atmosphere. That atmosphere – composed primarily of nitrogen – appeared in previous photos as a haze but John Spencer of the Southwest Research Institute in Boulder, Colorado, saw solid light spots above and in the haze that are consistent with what could be clouds on Earth and other planets.

The new data will be presented by Spencer and others in an upcoming paper in Science. While they’re pretty confident in their theory, a definitive “Yep, they’re clouds” probably won’t be made until the rest of the data returns from New Horizons over the upcoming year.

Mercury doesn’t get much attention and Pluto hasn’t gotten much respect so it’s nice to see them getting a little more press, even if it’s new speculation on old information. The data doesn’t change but the eyes and tools for analysis do, which is why it’s so important to continue to study what has been collected from these planets in the past as new missions are planned for the future.