Earlier this month, planetary scientists made an unexpected observation: Pluto might be home to a series of dunes.

“We have detected vast fields of features that look like dunes,” S. Alan Stern, the principal investigator for NASA’s New Horizons mission, told the New York Times. “Now we are being careful to say they look like dunes. They may or may not actually be dunes.”

The goal of the New Horizons mission is to help humans better understand the planets (or, in the case of Pluto, dwarf planet) at the edge of our solar system. NASA launched its nuclear-powered reconnaissance spacecraft in 2006. The spacecraft just passed Pluto, becoming the first spaceship to do so, this past July.

As it flew by, the spacecraft took a series of photographs of the dwarf planet. Some were sent down to Earth immediately; the rest will be downlinked to Earth over the course of a year.

The reason for the lengthy process is that the images are “lossless” – they are uncompressed, producing high-quality images without any pixelation. Due to their high-resolution, lossless images require large amounts of data, and as a result, a longer time to acquire.

It is in these lossless photos that scientists spotted the dune-like structures.

Dunes form when particles, carried through the air by wind, come to a rest to form a pile.  Over time, the wind will continue to deposit particles on this pile, forming a dune.

Pluto’s current atmosphere is too thin and weak to generate enough wind to create the kilometers of dunes seen in the photos. It is possible then, that the dwarf planet once had a thicker atmosphere.

It is also possible that some force other than wind created these landforms, in which case they would not be dunes.

“Their origin is under debate,” Dr. Stern told the New York Times.

The composition of these potential dunes is also unclear. On Earth, dunes are largely composed of sand grains. Some of the landforms on Pluto are bright and reflective, suggesting the possibility of ice particles; others are very dark, as sand-like bits of rock would be.

“The dunes may all be identical in composition, but some may have a veneer of dark stuff on them,” Dr. Stern told the New York Times. “Or they may be different. We just don’t know.”

Dunes aren’t the only possible landform that the New Horizon photos are revealing. The images also reveal mountains, possible nitrogen ice flows, and networks of valleys that may have been the result of material flowing over Pluto’s surface.

“The surface of Pluto is every bit as complex as that of Mars,” said the leader of the New Horizons Geology, Geophysics and Imaging team.

“Pluto is showing us a diversity of landforms and complexity of processes that rival anything we’ve seen in the solar system,” Dr. Stern said in a statement.“If an artist had painted this Pluto before our flyby, I probably would have called it over the top – but that’s what is actually there.”