Mars has consistently captured humanity’s imagination as a potential abode for extraterrestrial life. Its relative proximity and early observations suggested conditions that might once have supported living organisms. The pursuit of understanding whether life ever existed or currently exists on Mars remains a central goal in astrobiology, driving decades of scientific inquiry and robotic exploration.
Mars’s Past Potential for Life
Early Mars was a dramatically different world from the cold, arid planet we observe today. Geological evidence indicates a warmer, wetter past, where liquid water flowed freely across its surface. Networks of ancient river valleys and vast lakebeds, such as those found in Gale Crater and Jezero Crater, suggest sustained aqueous activity billions of years ago. These features point to a time when Mars possessed a thicker atmosphere, capable of trapping heat and supporting a water cycle.
Mineralogical findings further support this, with the detection of clays and sulfates that typically form in the presence of water. For instance, the Mars Reconnaissance Orbiter has identified phyllosilicates, a type of clay mineral, in ancient Martian terrains, which form through water interaction with volcanic rocks. These conditions align with the fundamental requirements for life as we know it: liquid water, stable energy sources, and essential chemical elements. Early Mars appears to have offered an environment where these ingredients could have converged.
Evidence from Martian Exploration
Decades of robotic missions have provided evidence regarding Mars’s past and present habitability. The Viking landers in the 1970s conducted experiments designed to detect microbial life, yielding ambiguous results. One experiment, the Labeled Release experiment, showed a temporary increase in gas release upon nutrient addition, which initially suggested metabolism but was later attributed to chemical reactions within the soil.
Later missions, like the Spirit and Opportunity rovers, provided extensive evidence of past water activity. Opportunity discovered hematite spherules, dubbed “blueberries,” and concentrations of sulfate salts, both indicative of aqueous environments. The Curiosity rover, exploring Gale Crater since 2012, found direct evidence of organic molecules preserved in ancient mudstones. These complex carbon-containing molecules, detected within three-billion-year-old rock samples, include thiophenes, benzene, toluene, and small carbon chains. While organic molecules are building blocks of life, they can also form through non-biological processes, such as geological or atmospheric chemistry, making their interpretation challenging.
Curiosity also detected methane in Mars’s atmosphere, with concentrations fluctuating seasonally and daily. Methane on Earth is largely produced by biological processes, but it can also arise from geological activity, like water-rock interactions. The fluctuating nature of Martian methane plumes complicates their origin. The Perseverance rover, currently exploring Jezero Crater, is building on these discoveries, searching for more definitive biosignatures within ancient lakebed sediments and former river deltas.
The Search Continues
The ongoing exploration of Mars focuses on uncovering definitive signs of past microbial life, or even present-day life if it exists in subsurface refugia. The Perseverance rover, which landed in Jezero Crater in 2021, is equipped with advanced instruments specifically designed for this purpose. Its SHERLOC and PIXL instruments can map organic molecules and minerals at a fine scale, helping to distinguish between biological and non-biological origins.
Perseverance is also collecting and caching samples of Martian rock and regolith for a future return to Earth. This Mars Sample Return mission, a joint effort between NASA and the European Space Agency, aims to bring these samples to terrestrial laboratories for detailed analysis. Scientists on Earth will be able to use a much wider array of sophisticated instruments to search for subtle biosignatures, such as specific isotopic ratios, complex molecular structures, or cellular remains. This approach is considered the most promising way to definitively determine if life ever existed on Mars.
Imagining Martian Life
If life were to be discovered on Mars, it would most likely take the form of microbial organisms, similar to Earth’s extremophiles. Given the planet’s current harsh surface conditions—intense radiation, extremely cold temperatures, and a thin, dry atmosphere—any extant life would likely reside underground, shielded from radiation and potentially accessing subsurface water or geochemical energy sources. Such life might resemble Earth’s chemoautotrophs, which derive energy from chemical reactions rather than sunlight.
The discovery of extraterrestrial life, even microscopic, would profoundly impact our understanding of the universe. It would suggest that life is not unique to Earth and might be a common phenomenon throughout the cosmos, arising wherever conditions permit. This finding would reshape our perspectives on the origins of life itself, indicating that the processes leading to abiogenesis might be more universal than currently understood. It would also provide a second data point for studying life’s evolution, allowing comparisons between two independent origins of life and revealing common principles or unique adaptations.