Though Mars has the largest volcanoes in the solar system, many scientists have claimed they are long-extinct. No one has ever seen smoke or lava erupting from their towering summits.

But new evidence from a European spacecraft may force researchers to rethink that assumption: Maybe these volcanoes are not dead after all — just dormant.

The shakeup comes from new research led by Alexander Trokhimovskiy, based on data from the European Space Agency's ExoMars Trace Gas Orbiter (TGO), which studies gases in the Red Planet's air from space . The orbiter detected hydrogen chloride near Alba Mons , which sits in the northern hemisphere. The chemical is a foul-smelling gas that often vents from volcanoes.…