Rosetta’s mission finished last September, but astronomers are still finding incredible stuff in the images and data we’ve received. The latest discovery is an unexpected sight for a comet: dunes. If there are dunes, there also must be wind. But how is this possible when comets don't have an atmosphere that allows them to produce dense enough winds?

To investigate this, researchers from the Pierre-and-Marie-Curie University (UPMC) in France studied photos of Comet 67P/Churyumov-Gerasimenko to try and model how these dunes formed.

They found that while comets lack dense enough winds to produce dunes, they do expel a lot of gas due to being heated up by the Sun. The researchers delved into this further and came up with a creative idea: They modelled the outgassing of the comet.

The study, published in the Proceedings of the National Academy of Sciences, estimates that the outgassing on the Sun-lit side of the comet creates a tenuous atmosphere and a pressure difference between the day side and the night side of 67P. The pressure is at least 100,000 times weaker than Earth’s, but since the gravity of the comet is so low, the slow wind can form dunes.

The computer model designed by the researchers included how the grains of dust that make up the surface of the comet stick to each other and also how they might interact with the atmosphere. Their predictions match well with the observations from Rosetta.

The researchers think this approach could help astronomers understand how the smallest bodies in the Solar System are eroded by both external and internal processes.

Rosetta was a mission by the European Space Agency that studied the comet for over two years. The spacecraft even released a lander in November 2014, which crashlanded on the comet in September 2016.

^{67P (left) and a close-up picture of the comet (right). The dunes are highlighted in the red ellipse. ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA}