The more thoroughly we inspect Pluto, the weirder the dwarf planet gets.

NASA’s New Horizons sent back a handful of new images this week, which reveal our faraway neighbor is more complex than previously believed. In addition to icy mountains, odd patterns and even the outline of a heart, scientists have discovered Pluto might also feature dunes, suggesting Pluto once had a much thicker atmosphere.

“We have detected vast fields of features that look like dunes,” said S. Alan Stern, the principal investigator of New Horizons. “Now we are being careful to say they look like dunes. They may or may not actually be dunes. Their origin is under debate.”

Dunes are piled up particles—sand, for example—from wind. Pluto’s atmosphere is believed to be way too thin and weak for wind to sculpt the alleged dunes observed by New Horizons. So that begs the question: How did the dunes get there?

Scientists don’t even know what the alleged dunes are made of. Stern said it could be ice particles, or it could be sandlike bits of rock. “We just don’t know.”

What Stern does know, however, is that the data being sent back by New Horizons is incredibly valuable. Most of the stuff scientists are seeing is nothing like they predicted, which is what makes Pluto such a mystery.

Although New Horizons flew by Pluto in July, it’s going to take a year or more to send all that information across the vastness of space. The dwarf planet is 4.67 billion miles away after all. Below is the description NASA provided with the image you see above.

This synthetic perspective view of Pluto, based on the latest high-resolution images to be downlinked from NASA’s New Horizons spacecraft, shows what you would see if you were approximately 1,100 miles (1,800 kilometers) above Pluto’s equatorial area, looking northeast over the dark, cratered, informally named Cthulhu Regio toward the bright, smooth, expanse of icy plains informally called Sputnik Planum. The entire expanse of terrain seen in this image is 1,100 miles (1,800 kilometers) across.