Iron rusts. On Earth, this common chemical reaction often signals the presence of something far more interesting than just corroding metal for example, living microorganisms that make their living by manipulating iron atoms. Now researchers argue these microbial rust makers could provide some of the most promising biosignatures for detecting life on Mars and the icy moons of the outer Solar System.
The familiar sign of flakey rust, or iron oxide as it's more properly known may well be something we can use to identify biological processes on other worlds (Credit : Laitr Keiows)
Laura Tenelanda-Osorio and colleagues from the University of Tübingen in Germany have compiled a comprehensive review of how iron metabolising bacteria leave distinctive fingerprints in rocks and minerals, and why these signatures matter for astrobiology. The research, published in Earth-Science Reviews, bridges decades of terrestrial microbiology with the practical challenges of searching for life beyond Earth.
Iron ranks among the most abundant elements in the Solar System, and Earth's microorganisms have evolved remarkably diverse ways to exploit it. Some bacteria oxidise ferrous iron to generate energy, essentially breathing iron the way humans breathe oxygen. Others reduce ferric iron, using it as the final electron acceptor in their metabolism. These processes don't happen in isolation. Iron metabolising microbes link their element of choice to the carbon and nitrogen cycles, coupling iron transformations to carbon dioxide fixation, organic matter degradation, and even photosynthesis.
The byproducts of these microbial reactions create what researchers call biogenic iron oxyhydroxide minerals. These aren't subtle traces. Organisms that thrive in neutral pH environments and oxidise iron produce distinctive structures such as twisted stalks, tubular sheaths, and filamentous networks of iron minerals mixed with organic compounds. The minerals precipitate as the bacteria work, forming rusty deposits that can persist in the geological record for billions of years. This durability makes iron biosignatures particularly attractive for planetary exploration. Unlike fragile organic molecules that degrade under radiation and harsh chemistry, mineralised iron structures can survive. Researchers have identified these biosignatures in environments ranging from hydrothermal vents on the ocean floor to terrestrial soils, from acidic mine drainage to neutral freshwater springs. Wherever liquid water contacts iron bearing rocks, iron metabolising bacteria typically establish themselves.
The red colour of Mars comes from the dusty iron oxide all over its surface (Credit : Kevin Gill)
Mars presents an obvious target. The planet's distinctive red colour comes from oxidised iron in surface dust and rocks. Ancient Mars hosted liquid water, and spacecraft have documented iron rich minerals throughout the geological record. If microbial life ever evolved on Mars, iron metabolism would have provided an accessible energy source. The minerals these hypothetical organisms produced could still exist, locked in ancient sediments awaiting discovery by rovers equipped with the right instruments.
The icy moons Europa and Enceladus offer different but equally compelling possibilities. Both harbor subsurface oceans beneath frozen shells. Europa's ocean likely contacts a rocky seafloor, where water and rock interactions would release dissolved iron. Enceladus actively vents ocean material through ice geysers at its south pole. Mission concepts propose sampling these plumes or landing near the vents, analyzing ejected particles for iron minerals that might betray biological origins.
The review emphasises that recognising biogenic iron minerals requires understanding how they form, what textures they create, and how they differ from abiotic iron precipitates. Mission planners must equip spacecraft with instruments capable of detecting not just iron minerals generally, but the specific morphological and chemical signatures that distinguish biology from geology.
The stakes are high. Finding iron biosignatures on another world wouldn't just confirm life exists elsewhere, it would reveal that the same fundamental chemistry supporting Earth's deep biosphere operates throughout the Solar System.
Source : Terrestrial iron biosignatures and their potential in solar system exploration for astrobiology
