The search for evidence of past habitability addresses whether life is unique to Earth or if the conditions necessary for its emergence existed on other worlds. Astrobiologists define “past habitability” as the environmental state of a planetary body that, at some point, possessed the foundational requirements for life to arise and thrive, even if those conditions were subsequently lost. This inquiry spans the nearest planets and moons in our solar system to distant exoplanets, seeking environments that once offered the right mix of resources and stability to support a biosphere.
The Fundamental Indicators of Past Habitability
Scientists assess the historical potential for life by searching for physical and chemical indicators related to three core requirements common to all known terrestrial life. The first requirement is the presence of persistent liquid water, which acts as a solvent to facilitate the chemical reactions necessary for metabolism. This focuses the search on planets within the circumstellar habitable zone, where a planet can maintain liquid water on its surface given a suitable atmosphere.
A stable energy source is also required to sustain metabolism, derived either from the heat of a host star or from internal planetary processes. On Earth, life uses sunlight for photosynthesis, but geothermal or chemical energy can also support ecosystems, as seen in deep-sea hydrothermal vents. Finally, the environment must contain the necessary chemical building blocks, commonly referred to as CHNOPS: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur.
Remote Detection: Searching for Atmospheric Biosignatures
For distant exoplanets, the search for past life relies on analyzing their atmospheres through remote sensing. The most common method, transit spectroscopy, involves observing a planet as it passes in front of its host star. Astronomers collect starlight filtered through the atmosphere, which contains absorption features that reveal the types and concentrations of gases present.
A strong indicator of a potential biosphere is chemical disequilibrium, which identifies gas combinations that would quickly react and neutralize each other unless continuously replenished by a large-scale active process. For instance, the simultaneous presence of molecular oxygen (\(O_2\)) and methane (\(CH_4\)) is highly suggestive of life, as these gases are chemically incompatible. Their coexistence on modern Earth is maintained by a global biosphere, where biological activity produces methane and photosynthesis produces oxygen, creating an imbalance unlikely to be sustained geologically.
Scientists also consider biosignatures from oxygen-poor environments, analogous to early Earth. The co-detection of abundant methane and carbon dioxide (\(CO_2\)), coupled with an absence of carbon monoxide (\(CO\)), could signal microbial life. This combination suggests a metabolism that consumes \(CO\) while producing \(CH_4\) and \(CO_2\). The search also includes trace gases like nitrous oxide (\(N_2O\)) and sulfur compounds such as dimethyl sulfide (\(DMS\)), which are biogenic on Earth.
Direct Detection: Geological Evidence and Fossilized Traces
In the solar system, the hunt for past habitability focuses on the physical material of planetary bodies, particularly Mars and the icy moons. On Mars, geological evidence points to an ancient past with flowing water, including massive river valleys and ancient lakebeds like Jezero Crater. Analysis of Martian rocks reveals mineral deposits—such as salts, clays, and calcium sulfate—that only form in the presence of liquid water, suggesting a warmer, wetter period.
The search for fossilized traces of ancient life is guided by Earth’s earliest life record, which includes microfossils, layered structures called stromatolites, and molecular fossils. Molecular fossils are durable organic compounds, such as lipids, that survive in rock for billions of years and possess a structure uniquely produced by biological systems. The Perseverance rover has found a potential biosignature in a rock called “Cheyava Falls,” containing organic compounds alongside minerals like vivianite and greigite, which are common biogenic byproducts of microbial activity on Earth.
Chemical analysis also focuses on isotopic fractionation, a subtle chemical signature created when life preferentially uses lighter isotopes of an element. Living organisms favor carbon-12 over carbon-13 during metabolism, resulting in a distinct ratio preserved in rock. This isotopic fingerprint, alongside specific mineral assemblages and organic compounds, helps distinguish between ancient biological activity and purely abiotic processes.
Current and Future Telescopes Driving the Search
The James Webb Space Telescope (JWST) is transforming the exoplanet search by using its infrared capabilities to characterize the atmospheres of small, rocky planets, particularly those orbiting cool M-dwarf stars. JWST uses transit spectroscopy to detect the chemical fingerprints of biosignature gases, helping identify the most promising targets and determining whether temperate planets retain substantial atmospheres.
Closer to home, the Mars Perseverance Rover is executing a core objective of the Mars Sample Return campaign by collecting and sealing rock and soil core samples. The rover analyzes samples in situ using instruments like SHERLOC and PIXL before caching them for a future mission to retrieve and bring back to Earth. Returning these samples allows analysis with advanced laboratory instruments too large and complex to send to Mars.
Focused on the outer solar system, the Europa Clipper mission will conduct multiple flybys of Jupiter’s icy moon, Europa, to assess its potential habitability. The spacecraft will characterize the moon’s subsurface ocean, ice shell, and composition, including searching for carbon-containing compounds. Looking further ahead, new ground-based facilities, such as the Extremely Large Telescope (ELT), will use immense light-gathering power to directly image and spectroscopically characterize the atmospheres of Earth-size planets in the habitable zones of Sun-like stars.