The Mars Sample Return Goldilocks?

Scientists have demonstrated a powerful new method for detecting signs of life in rocks similar to those expected from future Mars sample return missions – a crucial step in developing protocols to handle extraterrestrial materials safely and effectively. The study, led by Yohey Suzuki from the University of Tokyo, Japan, and published in the International Journal of Astrobiology, focuses on optical-photothermal infrared (O-PTIR) spectroscopy as a sensitive and minimally destructive technique for biosignature detection in Mars-analogue basalt.

The researchers tested O-PTIR on 100-million-year-old basaltic rock cores drilled from the South Pacific seafloor – considered a good analogue for Mars's basaltic terrain — and successfully detected spectral signatures consistent with microbial cells embedded within clay-filled fractures. These results highlight the potential of O-PTIR to identify submicron-scale microbial structures alongside mineralogical contexts, essential for interpreting possible life in Martian samples.

“To prepare for Mars Sample Return (MSR), analytical instruments of high sensitivity need to be tested on effective Mars-analogue materials,” the authors wrote. Identifying and characterizing potential biosignatures in returned Martian rocks is a major priority for space agencies.

The researchers previously demonstrated that nanoscale secondary ion mass spectrometry (NanoSIMS), transmission electron microscopy (TEM), and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) could be used to detect microbial life in Earth’s deep basalt, but these approaches are time-consuming, destructive, and require extensive sample preparation. By contrast, O-PTIR – developed by Ji-Xin Cheng and first commercialized by Anasys Instruments in 2017 – provides high spatial resolution (down to 0.5 microns) and can analyze thick rock sections (100 microns) without the need for pulverization or ultra-thin slicing, preserving samples for future tests.

Suzuki and colleagues compared O-PTIR with more established techniques such as Fourier-transform infrared (FT-IR) microscopy and μ-Raman spectroscopy. Conventional FT-IR, limited by spatial resolution around 10 microns, failed to detect microbial signals in these dense basalt samples. μ-Raman spectroscopy, while widely used for mineral and organic analysis, was hampered by autofluorescence, masking potential biosignatures.

In contrast, O-PTIR succeeded in identifying both microbial markers – such as amide I and II bands indicative of peptides – and smectite clays, particularly the iron-rich nontronite known to form in water-altered basalt. Co-localization of these signatures points to habitable conditions and past microbial activity. “We demonstrated our new method can detect microbes from 100-million-year-old basalt rock,” said Suzuki in a press release from the University of Tokyo. “But we need to extend the validity of the instrument to older basalt rock, around 2 billion years old, similar to those the Perseverance rover on Mars has already sampled.”

Unlike FT-IR, O-PTIR requires no sample preparation beyond embedding and polishing rock fragments – crucial for Martian material, where preserving limited and precious samples is essential. Additionally, by allowing mapping of organic and mineral signals together, O-PTIR helps distinguish biological from abiotic structures, a central challenge in life detection.

The study emphasizes that while O-PTIR is not a substitute for high-resolution techniques such as TEM or NanoSIMS, it offers a powerful complementary tool that is less invasive and can guide more targeted follow-up analyses. "We came up with optical photothermal infrared (O-PTIR) spectroscopy, which succeeded where other techniques either lacked precision or required too much destruction of the samples," Suzuki added in the release.

Looking ahead, the team aims to apply O-PTIR to other Mars-relevant rocks, such as carbonates, which on Earth often host microbial life and may exist on Mars. “It might only be a matter of years before we can finally answer one of the greatest questions ever asked,” said Suzuki.