When NASA’s Opportunity rover landed on Mars in 2004, it found something unexpected. Embedded in the rocks of a plain called Meridiani Planum, scientists detected a mineral called jarosite. On Earth, jarosite forms in one very specific type of place — highly acidic, iron-rich water, where sulfur-loving microorganisms drive the chemistry. That discovery became a turning point for extremophiles astrobiology — the study of how organisms that thrive in extreme conditions inform our search for life beyond Earth. Scientists knew exactly where to find a matching environment on Earth. It was a river in southwestern Spain, running red through the hills of Huelva, called the Río Tinto.
That discovery changed things for astrobiology — the field studying where and how life can exist in the universe. Suddenly, a river that Spanish microbiologists had studied for decades became one of the most important planetary reference sites on Earth. In addition, the microorganisms living in it became models for thinking about what life on Mars might once have looked like.
This is one of the reasons why the EXPLORA project chose the Río Tinto and the Antarctic region as its two core study sites. Both environments mirror conditions found elsewhere in the solar system. Moreover, both host microorganisms with remarkable survival strategies. Together, they offer clues about where to look for life beyond Earth — and what to look for when we get there.
Why does Río Tinto look like Mars?
The answer lies in the minerals. The iron oxide and sulfate deposits in the Río Tinto closely match those detected on Mars. This gave the river its formal status as a geochemical and mineralogical Mars reference site. The three key minerals are jarosite, goethite, and hematite. In the Río Tinto, iron-oxidising bacteria break down iron sulfide minerals. As a result, they produce sulfuric acid and create these compounds as part of their metabolism. The river’s red colour, its extreme acidity, and its unusual mineral content all result from microbial work running for millions of years.
The same minerals appear on Mars
On Mars, the same minerals appear in the same geological settings. NASA’s Opportunity rover detected jarosite and hematite at Meridiani Planum. If those minerals formed on Mars the same way they form in the Río Tinto — through the activity of acid-loving, iron-oxidising microorganisms — then Mars may once have hosted life. That remains an open question. However, it is no longer a purely theoretical one.
To explore this further, researchers carried out two major drilling projects at the Río Tinto. The MARTE project ran from 2003 to 2006. Then the IPBSL project followed from 2011 to 2015. Both aimed to find evidence of underground microbial life and study the chemistry that supports it. Consequently, both treated the river as a test bed for Mars exploration — practising the drilling methods, contamination controls, and detection tools that future planetary missions would need. The microorganisms living in the Río Tinto river are, therefore, far more than subjects of local scientific curiosity.
What traces do Río Tinto microbes leave behind?
A biosignature is any trace left behind by a living organism. For example, it can be a chemical signal, a mineral texture, or a physical pattern preserved in rock. Importantly, these traces can survive long after the organism itself has died. Scientists use biosignatures from the Río Tinto as a template for designing detection tools for Mars missions.
Microbial activity in the Río Tinto leaves its mark in several ways. It creates coatings on rock surfaces shaped by biofilm growth. It also traps gas bubbles in sediment layers and leaves microscopic fossil structures in jarosite. As a result, researchers now know which patterns and chemical signals to look for on Mars. To understand the full chemistry behind the Río Tinto’s extreme conditions, our dedicated article covers the science in depth.
Antarctica and the icy moons
The link between Antarctica and the outer solar system works differently. In contrast to the Río Tinto connection, it is not chemical — it is structural. More than 400 subglacial lakes sit beneath the Antarctic ice sheet. Each one holds liquid water sealed under kilometres of ice. Moreover, each one is cut off from sunlight and fed by chemical energy rather than the sun.
Why Europa and Enceladus matter
Jupiter’s moon Europa has a global ocean beneath a thick ice shell. Similarly, Saturn’s moon Enceladus harbours a subsurface ocean. Water vapour plumes escaping from cracks in Enceladus’ surface contain molecular hydrogen. Scientists believe that hydrogen comes from chemical reactions between water and iron-rich rock far below. On Earth, exactly that kind of chemistry sustains microbial communities in some of Antarctica’s deepest subglacial lakes — and it does so entirely without sunlight.
Lake Vostok sits 4 kilometres below the East Antarctic surface. Researchers estimate that its water has been isolated for between 15 and 25 million years. For that reason, evolution there may have produced organisms found nowhere else on Earth. These conditions closely parallel what scientists think exists beneath the ice of Europa and Enceladus. For a broader overview of the microorganisms that live in Antarctica, our dedicated article covers all habitats in full.
What Antarctic lakes tell us about icy moons
Antarctic subglacial lakes show that microbial communities can survive for vast timescales in sealed, dark, and cold conditions. They do so through chemosynthesis — using chemical reactions rather than sunlight as their energy source. Scientists caution against drawing exact comparisons between any one Antarctic lake and a specific moon. Nevertheless, the broader principle holds: sealed liquid water under ice can support life. Moreover, direct evidence from Earth now backs that principle strongly.
What are the limits of the analogy?
Honest science means being clear about what we do not yet know. The Río Tinto and Antarctica are powerful reference points. However, they are not perfect copies of Mars or icy moons.
Mars today is a very different place
Mars today is almost entirely dry. Its atmosphere reaches less than 1% of Earth’s thickness. Whatever acidic, iron-rich lakes existed on Mars billions of years ago, they operated under conditions with no exact match on Earth. Similarly, the water beneath Europa’s ice may be far saltier, colder, and under far greater pressure than anything found in Antarctica.
Detecting biosignatures on Mars is also much harder than detecting life in the Río Tinto. In Spain, researchers collect samples directly, culture organisms in the lab, and sequence their DNA. On Mars, however, detection must happen remotely. Instruments must be small enough to fit on a rover. Furthermore, organic molecules on the Martian surface degrade quickly under UV radiation and oxidative chemistry. Therefore, the Río Tinto teaches scientists what to look for — but the search itself will be far more demanding.
What the analogy does provide
Diverse microorganisms thrive in acidic and strongly oxidising conditions broadly similar to those in Mars iron-rich regions. Consequently, the Río Tinto has helped scientists build realistic biological expectations for Mars exploration. Without that biological framework, researchers would search Mars with no model for what life in those conditions might consume, produce, or leave behind.
What does EXPLORA contribute to this research?
EXPLORA is not an astrobiology mission. Its primary goals focus on Earth: finding novel enzymes, antioxidants, antimicrobial compounds, and other biomolecules for pharmaceutical, cosmetic, nutraceutical, and plastic-recycling uses. Nevertheless, the research it carries out at the Río Tinto and in Antarctica contributes directly to the scientific base that astrobiology depends on.
Building the biological picture
Every new microorganism that EXPLORA characterises from the Río Tinto adds to our understanding of what life looks like in iron-rich, acidic conditions. In addition, every compound isolated from a cold-adapted Antarctic microbe expands our knowledge of how biology functions in dark, nutrient-poor environments. As we explored in our article on polyextremophiles and their industrial potential, the organisms that thrive under the harshest conditions on Earth are precisely the ones that reshape our ideas about where life can exist.
Fieldwork as practice for planetary missions
EXPLORA’s work also covers the legal frameworks that govern biological sample collection — specifically the Nagoya Protocol and the Antarctic Treaty. These rules matter for astrobiology too. For example, any future mission that returns samples from another planetary body will face similar questions about access, handling, and responsibility. Consequently, the careful, protocol-driven fieldwork happening at the Río Tinto and in Antarctica today is, in a quiet way, practice for the missions that come next.
Frequently asked questions
What makes Río Tinto a Mars reference site? The Río Tinto shares key minerals with Mars — jarosite, hematite, and goethite — as well as high iron content, strong acidity, and sulfate chemistry. NASA’s Opportunity rover detected these same minerals on the Martian surface. Because microorganisms in the Río Tinto produce these minerals as part of their metabolism, the river helps scientists understand what biological evidence in those conditions might look like on Mars.
What is a biosignature? A biosignature is any physical, chemical, or structural trace left by a living organism that survives long after the organism itself is gone. For example, it can include mineral textures shaped by microbial activity, preserved organic molecules, or rock patterns caused by biofilms. Studying biosignatures in the Río Tinto helps researchers design better detection tools for Mars exploration.
Why do scientists compare Antarctica to Europa and Enceladus? Antarctica hosts more than 400 subglacial lakes — bodies of liquid water sealed beneath kilometres of ice, in total darkness, and sustained by chemical rather than solar energy. Europa and Enceladus are thought to have similar subsurface oceans. As a result, microbial life in Antarctic subglacial lakes shows that sealed, dark, cold liquid water is not incompatible with life — which makes those moons credible targets in the search for extraterrestrial biology.
Does this mean there is life on Mars? No confirmed evidence of life on Mars exists today. However, Río Tinto research shows that the chemical and mineral conditions on ancient Mars were not hostile to microbial life. In addition, scientists now have a biological model for what life in those conditions might produce. Whether life actually arose on Mars, therefore, remains an open question.
What does EXPLORA contribute to astrobiology? EXPLORA’s primary mission focuses on biotechnology. Nevertheless, by characterising the microorganisms of the Río Tinto and Antarctica in depth, the project adds to the scientific foundation that astrobiology depends on. Moreover, a more complete picture of what life looks like under multiple extreme conditions directly informs how we search for life beyond Earth.
