The odor of hydrogen chloride in Mars’s atmosphere indicates that some form of volcanic activity is still going on.

This image shows the northern flanks of the Alba Mons volcano on Mars.

ESA / TGO / CaSSIS

Mars boasts some of the largest volcanoes on the solar system, and in the past, the planet has experienced massive eruptions that reshaped most of its surface. But whether the source of that volcanism is completely dead or the oven is still baking is a matter of intense debate.

A new study published in Science Advances adds another piece to the puzzle: A team of scientists have found localized emissions of hydrogen chloride (HCl) gas, which on Earth is linked to forms of dormant volcanic activity such as fumaroles or hydrothermal systems.

The ExoMars Trace Gas Orbiter (TGO) studies molecules (including water molecules, as shown here, as well as hydrogen chloride, as discussed in the story.…