The giant Martian sandstorm of 2018 wasn’t just a wild ride — it also gave us a previously undetected gas in the planet’s atmosphere. For the first time, the ExoMars orbiter sampled traces of hydrogen chloride, composed of a hydrogen and a chlorine atom.

This gas presents Mars scientists with a new mystery to solve: how it got there.

“We’ve discovered hydrogen chloride for the first time on Mars,” said physicist Kevin Olsen of the University of Oxford in the UK.