What if the clues to finding life on Mars are hiding in our own backyard? In Earth’s most extreme environments, from freezing tundras to boiling, acidic pools, microbial life thrives where almost nothing else can. Some of these environments don’t just support life, they also help preserve its traces. Among them, hot springs stand out for producing unique silica-rich rocks called silica sinters, which can trap evidence of ancient microbes.
Silica sinters form when silica-rich water rises from hot springs, cools, and evaporates, leaving behind hardened silica that can trap and fossilize microbes that live in or around the water. NASA’s Spirit rover has discovered similar silica sinters in the Gusev Crater on Mars, sparking interest in whether ancient Martian hot springs might also preserve signs of past life.
An international team of researchers explored whether fat-like molecules from cells, known as lipids, could survive in these silica sinters and be detected using instruments similar to those on Mars rovers. Lipids can survive for millions of years, acting as chemical fossils, or biomarkers, in the fossil record. These molecules provide clues into what kind of life once lived in these environments and help scientists reconstruct ancient ecosystems.
The scientists collected silica sinter samples from 6 hot springs in the Taupō Volcanic Zone of New Zealand. These rocks originally formed in waters with a wide temperature and pH range, from 77°F to 203°F (25°C to 95°C), and very acidic to basic waters. First, they chemically extracted lipids from the sinters. Then, they characterized them using an instrument that breaks down molecules into smaller pieces and identifies them based on their mass, called a gas chromatograph-mass spectrometer or GC-MS.
The team used the GC-MS to identify a wide range of lipid molecules, including fatty acids, alcohols, sterols, and n-alkanes from the sinters. Most of these molecules likely originated from bacteria that use sunlight or sulfate for energy, and these types of microbes are well adapted to live in extreme conditions. Some lipids were also from other sources, like algae and plants. The researchers interpreted these diverse lipids as signs of both recent and ancient microbial life, as they included a mix of heat-altered and fresh compounds, pointing to fossilized older communities preserved alongside newer ones in the silica.
The scientists also found that the shape and texture of the sinter rocks influenced how well the lipid biomarkers were preserved. Fine, spiky-textured sinters, or spicular sinters, retained more lipids than knobby or crusty ones. These spiky textures form at the edges of hot spring pools where microbes interact with rapidly cooling, silica-rich water, creating delicate silica structures that grow upward like tiny fingers. The researchers suggested that these fine textures could help shield microbes from erosion and radiation. Scientists think finger-like silica structures could be especially promising for detecting past life on Mars because the Spirit rover has seen similar structures there.
To test whether current rover instruments could detect ancient lipids like these, the researchers analyzed 2 of the silica sinter samples they collected using a method similar to the system used by NASA’s Curiosity rover, called pyrolysis–GC-MS. In this process, the lipids don’t need to be chemically extracted first, as the entire sample is heated until its molecules break into gases, which are then analyzed.
In one sinter sample, the instrument was able to detect simple lipids that often come from living organisms, like n-alkanes, pristanes, and phytanes. In another, sulfur-rich sample, the system mostly detected sulfur-based molecules called thiophenes, which have also been found on Mars. However, it could not detect more complex biomarkers like hopanes or sterols. The researchers found that these molecules were either destroyed by the heat or were present in amounts too small for the pyrolysis-GC-MS to detect.
Based on these results, the researchers concluded that current rover instruments can detect simple, robust lipids, but they may miss more fragile or complex ones. To improve the chances of finding ancient biosignatures, the team recommended that future Mars missions include less destructive detection methods. Even with these challenges, they suggested that silica-rich rocks like those found in Gusev Crater are prime targets in the search for ancient Martian life. By identifying which rock textures are best at preserving lipids that can be detected with existing rover tools, scientists are moving one step closer to uncovering signs of past life on Mars.
