University of Tokyo seeks submicron life in Martian-style rocks

11 Mar 2025

Optical-photothermal infrared technique improves upon existing fluorescence methods in search for microbes.

Samples of rock collected from the surface of Mars could be delivered to Earth for study within the next decade, if the current plans of international space agencies come to fruition.

Once those rocks are here the search for evidence of living organisms within them will begin in earnest, in accordance with safety protocols drawn up by the international Committee on Space Research (COSPAR).

A project at the University of Tokyo has now applied a photothermal optical spectroscopy technique to the task, investigating whether it could detect evidence of living microbes while satisfying these safety protocols when the time comes, and published the results in International Journal of Astrobiology.

The method, termed optical-photothermal infrared (O-PTIR) spectroscopy, has advantages over existing approaches such as Fourier transform infrared microscopy and secondary-ion mass spectroscopy, commented the project. O-PTIR works by irradiating samples with a tunable IR laser, and measuring the photothermal effect duly induced in the sample with a separate shorter wavelength probe pulse.

Localized IR absorption affects the intensity and refraction of the returned probe light, and variations in the response due to extrinsic materials can be mapped with high spatial resolution - higher than other methods or direct laser imaging can achieve, according to the project.

The Tokyo team applied O-PTIR to basalt samples known to be good models for prominent rock types on the Martian surface, and compared it with existing analytical techniques applied to the same samples.

Answers to the greatest questions

"We first tested conventional analytical instruments, but none could detect microbial cells in the 100-million-year-old basalt rock we use as the Martian analogue," commented Yohey Suzuki from the Department of Earth and Planetary Science at the University of Tokyo.

"So we had to find an instrument sensitive enough to detect microbial cells, and ideally in a nondestructive way, given the rarity of the samples we may soon see."

For this initial test the basalt rocks had their outer layers removed and were cut into slices. Although this is slightly destructive, it leaves plenty of material intact for other kinds of analyses, noted the project, and follows the same preservation philosophy developed for samples of Moon rock collected by lunar missions.

The O-PTIR platform proved able to image details as small as 0.5 microns in 100-micron-thick sections of basalt rock core, and could successfully indicate the presence of microbial cells. Compared to alternatives such as deep UV fluorescence, O-PTIR combines high spatial resolution with negligible interferences, commented the team in its paper.

"We demonstrated our new method can detect microbes from 100-million-year-old basalt rock; but we need to extend the validity of the instrument to older basalt rock similar to those the Perseverance rover on Mars has already sampled, and test other rock types common on Mars and here on Earth which often contain life as well." said Yohey Suzuki.

"It's an exciting time to work in this field. It might only be a matter of years before we can finally answer one of the greatest questions ever asked."