Venus may harbor active volcanoes that produce the high amounts of sulfur dioxide in its atmosphere.

Scientists argue whether the sulfur dioxide found by the European Space Agency’s Venus Express comes from volcanic eruptions not long ago, or simply lingers on from eruptions that happened as far back as 10 million years ago.

“Volcanoes are a key part of a climate system,” said Fred Taylor, a Venus Express scientist from Oxford University.

Sulfur compounds don’t stay long in Earth’s atmosphere because they react with the Earth surface in the end, but they may take longer to react with surface rocks on Venus.

Venus Express analyzed how the Venusian atmosphere absorbs starlight and sunlight, which indicates the type of atoms and molecules in the atmosphere. The spacecraft watched as the sulfur dioxide in the upper atmosphere dropped by two-thirds over several days.

“I doubt the volcanic hypothesis,” said Jean-Loup Bertaux, a Venus Express chief investigator from the French Aeronomy Service. “However, I must admit that we don’t understand yet why there is so much SO2 (sulfur dioxide) at high altitudes, where it should be destroyed rapidly by solar light, and why it changes so quickly.”

That change in sulfur dioxide levels was smaller in the lower atmosphere, where Venus Express examined sulfur dioxide levels by how much infrared radiation that they absorbed. A stronger infrared signature means more sulfur dioxide.

Scientists hope they can prove or disprove active volcanoes on Venus, either by looking for local plumes of gas rising from volcanoes or finding hot spots on the surface that suggest fresh lava flows. That adds up to more work ahead for Venus Express.