It’s been over a year since NASA’s New Horizons flew by Pluto, snapping high-resolution photos of the dwarf planet and its largest moon Charon. In addition to showing the erstwhile planet in all its glory, the images also revealed several intriguing features on the surface of its largest moon, which, at half the diameter of Pluto, is the largest satellite relative to its parent in the Solar System.

In the photos taken by the spacecraft during its flyby last July, two features stood out on Charon’s surface — a canyon system that stretches more than 1,000 miles and a large, reddish blotch on the north pole that is unlike anything seen anywhere else in our solar system.

Photo: NASA/JHUAPL/SwRI

After poring over images and other data gathered by New Horizons for over a year, a team of researchers now believe they have solved the mystery of how the weird red spot came to be. As scientists explain in a new study published in the journal Nature Wednesday, the polar coloration comes from Pluto itself.

As the paper explains, Pluto is continuously leaking methane from its atmosphere in all directions, some of which gets trapped by Charon’s gravity.

“The methane molecules bounce around on Charon’s surface until they either escape back into space or land on the cold pole, where they freeze solid, forming a thin coating of methane ice that lasts until sunlight comes back in the spring,” lead author Will Grundy, a New Horizons co-investigator from Lowell Observatory in Flagstaff, Arizona, said in a statement.

Once this happens, methane ice quickly sublimates away but heavier hydrocarbons remain, which, stuck to Charon’s surface, are converted by sunlight into a thick, sticky, red-colored residue known as tholin. This process, repeated over millions of years, is what gives Charon’s poles its unique red spot.

“Who would have thought that Pluto is a graffiti artist, spray-painting its companion with a reddish stain that covers an area the size of New Mexico?” Grundy, said. “Every time we explore, we find surprises. Nature is amazingly inventive in using the basic laws of physics and chemistry to create spectacular landscapes.”

And this is far from the only surprise Pluto has in store for us.

In a separate study recently published in the journal Icarus, a team of researchers describe the detection of low-energy X-rays from dwarf planet.

The discovery, made using NASA’s Chandra X-ray Observatory, marks the first time X-rays coming from an object in the Kuiper belt — a cold, icy region of the solar system beyond the orbit of Neptune — have been detected.

Photo: X-ray: NASA/CXC/JHUAPL/R. McNutt et al; Optical: NASA/JHUAPL

“Before our observations, scientists thought it was highly unlikely that we’d detect X-rays from Pluto, causing a strong debate as to whether Chandra should observe it at all,” co-author Scott Wolk of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, said in a statement. “Prior to Pluto, the most distant solar system body with detected X-ray emission was Saturn's rings and disk.”

According the researchers, the intensity of X-rays being emitted by Pluto cannot be explained solely by the interaction of solar wind with Pluto’s escaping atmosphere, which is less “comet-like” than previously imagined. Perhaps interplanetary magnetic fields are somehow directing more solar wind particles than expected into the region around Pluto, or maybe Pluto has a much wider and longer tail of gases trailing it than New Horizons detected.

“It likely means that Pluto significantly perturbed the solar wind, and causes it to wrap around itself and focus into a long downstream tail, where it mixes with Pluto’s escaping atmosphere and makes X-rays,” the researchers said.