Scientists Reveal Mars Once Stayed Warm With Stinky Volcano Gases, Not Just CO₂

Imagine the Red Planet billions of years ago. Instead of a cold desert, Mars had rivers, lakes, and maybe even shallow seas. The big mystery has always been how the planet stayed warm enough for liquid water when the Sun was weaker.

A new study published in Science Advances in September 2025 provides an unexpected answer. Scientists found that Mars was likely warmed by stinky volcanic gases such as hydrogen sulfide, the gas that smells like rotten eggs. These gases, released during powerful eruptions, could have created a hazy atmosphere that trapped heat and allowed water to flow.

The discovery is not just theory. In May 2024, NASA’s Curiosity rover crushed a rock and revealed elemental sulfur crystals inside. This was the first time sulfur was found in pure form on Mars, unbound to oxygen. The rover’s finding gave strong evidence that supports the new research led by scientists at the University of Texas at Austin.

“We were very excited to see the news from NASA and a large outcrop of elemental sulfur. One of the key takeaways from our research is that as S₂ was emitted, it would precipitate as elemental sulfur. When we started working on this project, there were no such known observations.” — Chenguang Sun, co-author and assistant professor at UT Austin

What Did Scientists Discover About Sulfur on Mars?

For decades, researchers thought that carbon dioxide and sulfur dioxide were the main gases that influenced the Martian climate. But those gases alone could not explain all the evidence. For example, Mars has widespread sulfate deposits and signs of long-lasting water, yet previous models did not fully match what rovers and orbiters observed.

The new research used Martian meteorites to estimate volcanic chemistry and ran more than 40 computer simulations with different temperatures, pressures, and chemical conditions. Unlike earlier studies that only looked at gases released at the surface, this study tracked sulfur as it separated from magma deep below the crust and rose toward the surface. This gave scientists a more realistic view of what gases actually reached the atmosphere.

To better understand this process, scientists created a model showing how Martian magmas released sulfur gases, how those gases condensed, and how they later transformed into the sulfate deposits observed today.

The results showed that reduced sulfur gases, mainly hydrogen sulfide (H₂S) and disulfur (S₂), were far more common than sulfur dioxide. These gases not only made the air smell terrible but also reacted with other chemicals in the atmosphere. Together, they could create thick hazes and even sulfur hexafluoride, a greenhouse gas more than 22,000 times stronger than carbon dioxide.

This process helps explain why Mars once had a climate warm enough for rivers and lakes despite weaker sunlight. It also fits with what NASA’s Curiosity rover has seen on the ground, including deposits of sulfates and elemental sulfur.

Why Does This Matter for Life on Mars?

The idea of stinky gases shaping a planet’s climate might sound odd, but it is very important. On Earth, hydrogen sulfide plays a key role in deep-sea hydrothermal vents. These vents, which also smell like rotten eggs, are home to rich communities of microbes and other life.

By showing that Mars had similar sulfur-rich conditions, the study adds weight to the idea that the Red Planet could have supported life in its ancient past. If microbes can thrive near sulfur vents in Earth’s oceans, maybe life had a chance on Mars in lakes, rivers, or underground water systems.

The research also highlights how sulfur frequently changed forms on Mars. Meteorites show high concentrations of reduced sulfur, while the surface shows oxidized sulfur bonded to oxygen. This means sulfur was cycling through multiple stages, a process that shaped both the atmosphere and geology.

“This indicates that sulfur cycling, the transition of sulfur to different forms, may have been a dominant process occurring on early Mars.” — Lucia G. Bellino, lead author and doctoral student at UT Austin

The new findings also help explain a puzzle that earlier climate models could not solve. Sulfur gases that condensed into sulfide and elemental sulfur could later oxidize, forming the sulfate minerals found across the Martian surface today.

Who Will Benefit From This Mars Discovery?

This discovery affects more than just scientists who study Mars.

• Planetary scientists now have new tools to build more accurate models of Mars’ ancient climate.
• NASA missions can refine their search for sulfur compounds as signs of habitability.
• Astrobiologists can connect sulfur-rich conditions on Mars with similar environments that support life on Earth.
• Exoplanet researchers can use this knowledge to predict which distant worlds might also host liquid water.
• And for the public, it makes Mars’ history more relatable. The very gases that smell bad on Earth may have made another planet more Earth-like billions of years ago.

“Bellino hopes that climate modeling experts can use her team’s research to predict how warm the early Mars climate might have been, and, if microbes were present, how long they could have existed in a warmer atmosphere.” — Lucia G. Bellino

Could Rotten-Egg Gases Have Made Mars More Earth-Like?

It is a strange thought. The gases that smell the worst to us may have created some of the most favorable conditions for life elsewhere. Instead of relying on carbon dioxide alone, Mars likely kept warm through a mix of rotten-egg hydrogen sulfide and other sulfur gases.

These volcanic gases may even have sparked a cycle of climate change, shifting from haze to warmth to sulfate deposits over millions of years. They provided not only warmth but also chemical ingredients that life could use.

The research team plans to go further. They will test whether volcanic activity could also have been a major source of water on early Mars and whether reduced sulfur gases could have served as food for microbes. These questions link Mars directly to Earth’s hydrothermal systems, where sulfur supports entire ecosystems.

Today, Mars looks dry and barren. Average temperatures sit around –80°F. But long ago, stinky volcanoes may have filled its skies with protective hazes and made rivers possible. That makes the planet’s story even more fascinating and keeps the hope of finding traces of life alive.

What’s the Final Takeaway From This Study?

The new study from researchers at the University of Texas at Austin reminds us that science often advances with surprises. Mars’ ancient warmth may not have come from the gases we expected. Instead, it came from sulfur-rich volcanoes releasing clouds of rotten-egg smells into the sky. Those same gases might have made Mars habitable, even if only for a short time.

The research was supported by the University of Texas at Austin’s Center for Planetary Systems Habitability, the National Science Foundation, and the Heising-Simons Foundation.

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