Raman spectroscopy searches for signs of life on alien worlds

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The rover Perseverance’s Sherloc instrument is equipped with the first UV Raman Spectrometer to be flown to the surface of Mars. (Credit: NASA/JPL-Caltech)

Raman spectroscopy enables the non-destructive chemical analysis of materials based on the interaction of light with the chemical bonds in the material. The technique is currently playing an important role onboard the Perseverance rover, which landed on Mars in February to begin a two-year mission hunting for signs of ancient life on the Martian surface.

The deep UV (DUV) resonance Raman and fluorescence spectrometer aboard Perseverance’s arm-mounted Sherloc instrument (scanning habitable environments with Raman and luminescence for organics and chemicals) is the first UV Raman spectrometer to be on Mars. It will use a 248.6nm DUV laser and a sub-100µm spot size, scanning over a 7 x 7mm area, to perform non-contact, spatially resolved detection and characterisation of organics and minerals on the Martian surface.

Sherloc’s goals are to assess past aqueous history, detect the presence and preservation of potential biosignatures – organic molecules created by biological processes – and to support the selection of the samples that could eventually be returned to Earth. It will measure mineralogy containing carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur – six chemical elements whose covalent combinations make up most biological molecules – and measure the distribution and type of organics preserved at the surface, while correlating them to textural features.

To achieve this, Sherloc also comprises an auto-focusing camera that shoots black-and-white images that are then used by its colour camera, aptly named Watson (wide angle topographic sensor for operations and engineering), to zero in on rock textures. The DUV laser will then aim for the centre of rock surfaces depicted in Watson’s images to perform the Raman spectroscopy and detect minerals in microscopic rock features. This data will then be superimposed on Watson’s images to create mineral maps that help scientists determine which rock samples Perseverance should drill and seal in metal tubes to leave on the Martian surface, for returning to Earth in a future mission.

Highly sensitive instrumentation

Sherloc’s investigation will make use of two spectral phenomena, native fluorescence and pre-resonance/resonance Raman scattering. These events occur when a high-radiance, narrow linewidth laser source illuminates a sample.

Organics that fluoresce absorb an incident photon and re-emit at a higher wavelength, with the difference between the excitation wavelength and emission wavelength indicating the number of electronic transitions. This phenomenon enables a powerful means to find trace organics in the Martian surface.

A close-up view of an engineering model of Sherloc showing its auto-focusing camera. (Credit: NASA/JPL-Caltech)

DUV-induced native fluorescence is very sensitive to condensed carbon and aromatic organics, enabling detection at or below 10-6 w/w (weight for weight, 1 part per million) at sub-100µm spatial scales. Sherloc’s deep UV resonance Raman enables detection and classification of aromatic and aliphatic organics with sensitivities of 10-2 to below 10-4 w/w at sub-100µm spatial scales. In addition to organics, the deep UV Raman enables detection and classification of minerals relevant to aqueous chemistry with grain sizes below 20µm.

The native fluorescence emission of organics extends from approximately 270nm into the visible. This is especially useful, according to Nasa, because it ‘creates’ a fluorescence-free region (from 250 to 270nm) where Raman scattering can occur. With Sherloc’s narrow linewidth 248.6nm DUV laser, additional characterisation by Raman scattering from aromatics and aliphatic organics and minerals can be observed. Furthermore, excitation with a DUV wavelength enables pre-resonance and resonance signal enhancements of organic/mineral vibrational bonds by the coupling of the incident photon energy to the vibrational energy. This results in highly sensitive measurements with low backgrounds, while avoiding damage or modification of organics.

From Mars to moons

In addition to searching for signs of life on Mars, Raman spectroscopy could also one day be used to hunt for microbes on other worlds in our solar system.

‘The technology behind Sherloc that will look for past life in Martian rocks is highly adaptive and can also be used to seek out living microbes and the chemical building blocks for life in the deep ice of the moons of Saturn and Jupiter,’ explained Luther Beegle, principal investigator for Sherloc.
In particular, Enceladus, Europa and Titan are each thought to have vast oceans of liquid water containing chemical compounds associated with biological processes below their thick icy exteriors. If microbial life exists in those waters, it may be possible to find evidence of it in the ice as well.

At Nasa’s Jet Propulsion Laboratory (JPL) in Southern California, scientists are therefore developing a ‘wireline analysis tool for the subsurface observation of northern ice sheets’, also dubbed Watson. This 1.2m long tube-like prototype has already been coupled to a planetary deep drill and successfully tested in the ice of Greenland.

According to JPL, a smaller version of Watson could be used in a future robotic mission to explore whether life could survive on one of the three moons. The instrument would scan into the ice in search of biosignatures. If successful, the ice could then be collected from the borehole wall to gather samples for further study.

However, DUV laser Raman spectroscopy would also enable Watson to analyse the materials exactly where they are found, before ice samples are retrieved for studying on the moon’s surface. This would provide scientists additional information about these samples by studying where they are in the context of their environment.

‘It would be great if we first studied what these samples actually looked like in their natural environment before scooping and blending them up into a slurry for testing,’ said Mike Malaska, an astrobiologist at JPL and the lead scientist for Watson. ‘That’s why we’re developing this non-invasive instrument for use in icy environments: to get a deep look into the ice and identify clusters of organic compounds – maybe even microbes – so they can be studied before we analyse them further and lose their native context or modify their structure.’

Terrestrial testing

Just as Sherloc underwent extensive testing on Earth before going to Mars, so must Watson before it is sent to the outer solar system. To see how the instrument might perform in the icy crust of Enceladus and the moon’s extremely low temperatures, the Watson team chose Greenland as an ‘Earth analogue’ for field tests of the prototype in 2019. The results of the field test were published in Astrobiology late last year, and presented at the American Geophysical Union autumn meeting in December.

Watson was tested in Greenland to search for signs of life in the ice 110m down a borehole. (Image: NASA/JPL-Caltech)

Earth analogues share similar characteristics with other locations in our solar system. In the case of Greenland, the environment near the middle of the island’s ice sheet and away from the coast approximates the surface of Enceladus, where ocean materials erupt from the small moon’s prolific vents and rain down. The mangled ice at the edge of Greenland’s glaciers near the coast, meanwhile, can serve as an analogue for Europa’s buckled deep icy crust.

During the testing, Watson descended 110m into a borehole and used its UV laser to illuminate the walls of the ice, causing some molecules to fluoresce. The spectrometer then measured the fluorescence to give the team insight into their structure and composition.

As expected with the tests being on Earth, the team detected biosignatures in Greenland’s ice. Mapping their distribution along the walls of the deep borehole, however, raised new questions about how these features originated. The scientists discovered that microbes deep in the ice tend to clump together in blobs, not in layers as expected.

Watson produced this fluorescence map of a borehole in Greenland’s ice. Left: nebulous blobs of biosignatures; right: a colourised version, grouping together similar organic chemicals. (Image: NASA/JPL-Caltech)

‘We created maps as Watson scanned the sides of the borehole and the clustering hotspots of blues, greens and reds – all representing different kinds of organic material,’ said Malaska. ‘And what was interesting was that the distribution of these hotspots was pretty much the same everywhere we looked: no matter if the map was created at 10m or 100m in depth, these compact little blobs were there.’

By measuring the spectral signatures of these hotspots, the team identified colours consistent with aromatic hydrocarbons (some that may originate from air pollution), lignins (compounds that help build cell walls in plants), and other biologically-produced materials (such as complex organic acids also found in soils). In addition, the instrument recorded signatures similar to the fluorescence produced by clusters of microbes.

The team was encouraged by how sensitive Watson was to such a wide variety of biosignatures. According to Rohit Bhartia, principal investigator for Watson and deputy principal investigator for Sherloc, such high sensitivity would be useful on missions to ocean worlds, where the distribution and density of any potential biosignatures are unknown. ‘If we were to collect a random sample, we are likely to miss something very interesting, but through our first field tests, we’re able to better understand the distribution of organics and microbes in terrestrial ice that could help us when drilling into the crust of Enceladus,’ he said.

The team seeks to perform more testing, ideally in other Earth analogues that approximate the conditions of other icy moons.

Paper in Astrobiology: Malaska et al. ‘Subsurface In Situ Detection of Microbes and Diverse Organic Matter Hotspots in the Greenland Ice Sheet’

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