Raman spectroscopy able to detect biomolecules below surface of Mars
- Evaluation of 469-day long-term experiment with biomolecules on outer wall of International Space Station completed.
- Biomolecules placed in Mars-analogous regolith can be identified by Raman spectroscopy. Biomolecules are protected against destructive ultraviolet (UV) radiation in Martian soil
- Research method opens up improved perspectives in the search for life on Mars and in Solar System.
- Focus: Space, exploration, astrobiology
Chlorophyllin, beta-carotene, melanin, chitin, cellulose, naringenin and quercetin – such exotic-sounding biological compounds are important components of terrestrial organisms that can withstand extreme environmental conditions. Between October 2014 and February 2016, these seven molecules were subjected to a long-term stress test in space. Can these substances survive the harsh radiation conditions? To what extent do the extreme temperature differences in space affect them? How do they change? And could they also be identified on Mars with remotely controlled measuring instruments? For 469 days, the biomolecules on the outer wall of the International Space Station ISS were exposed to intense radiation and a 90-minute day-night cycle. The results of the experiment led by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) reveal that the biomolecules would survive almost unchanged in Martian soil and that they could be identified using Raman spectroscopy on Mars.
"Our results are the first systematically measured Raman signatures, virtual fingerprints of isolated biomolecules exposed to space in the low-Earth orbit," explains Mickael Baqué from the DLR Institute of Planetary Research. "They confirm that we can use Raman spectroscopy, a fast and non-destructive measurement technique, to search for traces of life on Mars – particularly below the surface, which is shielded from UV radiation." Mickael Baqué is the first author of a study now published in the journal Science Advances summarising the measurements and results of the BIOlogy and Mars EXperiment (BIOMEX). BIOMEX was one of four experiments combined under the EXPOSE-R2 umbrella. The European Space Agency (ESA) and the Russian agency Roskosmos on the ISS carried out the EXPOSE-R2 experiments jointly. On 18 June 2016, the samples, protected from light and environmental influences after the experiment, returned to Earth with ESA astronaut Tim Peake in a Soyuz capsule. DLR was one of the entities that then conducted their evaluation.
Has there been life on Mars?
The search for fossil or living organisms on other celestial bodies is one of the great driving forces of current planetary research. Life has only been found on Earth so far, but it is conceivable that life once developed on Mars, Earth’s outer neighbouring planet, or perhaps even exists there today. Three to four billion years ago, there was water on Mars, the atmosphere was denser than today, and temperatures were higher. Mobile Mars rovers such as the Curiosity rover, which landed in the Gale crater ten years ago, have demonstrated that the most important chemical elements for the prerequisites of life, such as carbon, hydrogen, oxygen, nitrogen, sulphur and phosphorus are present in sedimentary rocks. However, traces of life, so-called biosignatures, have not yet been discovered. "The biomolecules exposed and subsequently investigated in BIOMEX play a key role in the current and future search for biosignatures," explains the former head of the BIOMEX experiment, Jean-Pierre Paul de Vera from the Microgravity User Support Center (MUSC) at the DLR Space Operations and Astronaut Training facility. "This is because, in order to be able to detect traces of life at all, we need to know what the harsh environmental conditions do to potential organisms and their molecular components on Mars, how stable they are, how they change as a result of UV radiation, if at all, and how the signal measured as a result varies."
Some organisms like the extremes – like on Mars
The much stronger UV radiation on Mars and presence of ionising radiation, along with the oxidising environment and extreme temperature differences between day and night, are particularly harmful to fossil or existing organisms. These conditions are felt not only on the ground but also centimetres to metres below the surface. Consequently, seven types of biomolecules were selected for the several hundred samples that made up BIOMEX, such as archaea, single-celled organisms without a cell nucleus similar to those on Earth at the very beginning of the development of life and whose existence billions of years ago is also thought to be possible on Mars. The biomolecules selected for BIOMEX are part of terrestrial organisms that can survive under the most extreme conditions – drought, cold, heat, UV radiation – and are known as extremophilic organisms.
Such biomolecules have already been shown to be identifiable by Raman spectroscopy (see section below) in laboratory investigations on Earth. As part of the BIOMEX research, the biomolecules were deposited onto two different Mars analogue materials developed at the Berlin Museum for Natural History or mixed with the regolith. One of the simulated martian soils is a regolith, which consists of primarily phyllosilicates and corresponds to early Mars, while the other is a sulphur-containing substrate, which is more similar to a regolith created during the martian Theiikian period. The samples were then surrounded or vacuum sealed in three layers under highly transparent glass by an 'air' comparable to the martian atmosphere, so that only the uppermost layer was directly exposed to the space conditions and the biomolecules in the two layers below were to a certain extent protected and represented samples below the surface of Mars. BIOMEX was brought to the ISS on 24 July 2014 with the Progress 56P supply mission and exposed to space conditions on 22 October 2014 by cosmonauts Maxim Surayev and Alexandr Samokutyaev by removing the protective cover on the Zvezda Service Module of the Space Station.
ISS was the ideal platform for BIOMEX
"The ISS orbits Earth at an altitude of around 400 kilometres, and UV radiation there is often stronger than on Earth," de Vera explains. "The ISS offered ideal conditions for this experiment because the space conditions are much closer to the situation on the surface of Mars, whose atmospheric protection is much weaker than Earth's and therefore also receives a lot of UV radiation." Part of BIOMEX was an accompanying experiment led by DLR researcher Elke Rabbow from the DLR Institute of Aerospace Medicine in Cologne, in which the same biomolecules were exposed to near-martian radiation conditions and temperature differences in a space simulation chamber. After the return of BIOMEX, the samples from space and those from the terrestrial laboratory were compared.
"The evaluation of the data was very complex and had to be carried out very carefully," says Baqué, looking back on the years that followed the experiment. "It became particularly difficult when the signatures of the biomolecules were joined in the Raman spectra by diagnostic lines of inorganic substances such as the iron-containing mineral haematite or inorganic carbon, and we had to keep them apart. But in the end, we now have a solid result that can really improve the search for former or extant life on Mars." As expected, the ultraviolet radiation strongly altered the signals of the Raman spectrum in all samples that were on the surface layer of the experimental setup and were directly exposed to UV radiation. But only minor changes in the spectra were observed in the two series of samples below, which were shielded from the UV light.
"This finding is fundamental for Mars missions looking for biosignatures beneath the Martian surface," de Vera is pleased to say. "However, biosignatures directly on the surface are more difficult to identify using Raman spectroscopy. But other methods are even better suited for this today." Raman spectroscopy is currently being carried out by NASA's Mars 2020 mission, which has been operating in the Jezero crater since 2021, and by the SuperCam and SHERLOC experiments on its Perseverance rover. It will also be used on the Rosalind Franklin rover of European ExoMars mission. DLR researchers are involved in both missions.
BIOMEX demonstrated the possibility of detecting biomolecules exposed to space conditions or a Mars-analogous environment using Raman spectroscopy. This also provides a basis for a consolidated, space-proven database of spectroscopic biosignatures in extraterrestrial environments.
In addition to the DLR Institutes for Planetary Research, Optical Sensor Systems and Aerospace Medicine and the Space Experiments User Center (MUSC) at the DLR Space Operations and Astronaut Training Centre, the following German institutions were also involved: the Robert Koch Institute, the Museum of Natural History and the Technical University of Berlin, TH Wildau, the Fraunhofer Institute for Cell Therapy and Immunology, the GFZ Potsdam, the University of Potsdam and the Heinrich Heine University of Düsseldorf.
Raman spectroscopy
How do you determine which substances something is made of? In our everyday lives, the eye is usually enough. What looks like wood is mostly made of wood, we recognise plastic easily, and stones look just like stones. However, these qualitative assessments are not sufficient in science and technology. For example, are those stones limestone, sandstone or volcanic rock? Geologists and mineralogists first look at the stone, hit it with a hammer, identify different minerals using a magnifying glass and determine whether it is an igneous granite, metamorphic schist or sedimentary sandstone, and so forth. But what if the mineral and elemental abundances need to be determined? In that case, a piece of the stone is sent to the laboratory for analysis. It can make a great difference, for example, in the search for deposits, whether the rock contains one percent iron or 17 percent.
Of course, this is not so easy on Mars.
One method of analysis from a distance is spectroscopy, which is used in various ways. The reflective behaviour (reflection spectroscopy) of substances in different wavelengths of the electromagnetic spectrum (UV light, visible light, infrared light, etc.) can be measured and examined in the form of a continuous spectrum – in other words, its ‘colour’ is quantified as a function of wavelength. Alternatively, the spectrum emitted naturally by the rock or when it is excited by a laser can measured (emission spectroscopy; measurements of infrared or, more rarely, fluorescence and radioactivity). Some methods are possible both from orbit and on the ground; some are limited to the proximity to the object.
With Raman spectroscopy, the examined object is irradiated with light of a specific, discrete wavelength, such as with a laser, and a specific part of the scattered laser light at the target point is measured as a spectrum. This part is also called Rayleigh scattering or rotational Raman scattering. The resulting Raman spectrum, with its characteristic Stokes lines, is like the fingerprint of the investigated material and can be used to derive the material properties and compositions.
The reason for these special scattering effects is the Raman effect. The Raman effect is based on an interaction between incident light and matter. It can come from the transfer of energy to matter ('Stokes side' of the spectrum) or from matter to light ('anti-Stokes side'). The colour, i.e. the wavelength, of the light is a function of its energy. Thus, UV light is radiation with higher energy than visible light, which we sometimes experience painfully when we suffer from sunburn. The Raman shifts of the wavelength of the irradiated light are thus an expression of energy transfers – which can be measured and used diagnostically.
The Raman effect is named after Chandrasekhara Venkata Raman (1888-1970), an Indian physicist who made these discoveries at Calcutta University and was awarded the Nobel Prize for physics in 1930.
One major advantage of Raman spectroscopy in the robotic exploration of a planetary surface is that it works reliably, non-destructively, quickly, without sample preparation and at distances of up to a few metres. Both rocks and organic targets can be investigated. For this reason, Raman spectroscopy is increasingly becoming an important method of analysis on missions, looking for early, fossil, or extant life on other celestial bodies. The DLR Institutes of Planetary Research and Optical Sensor Systems have spent many years working on the development of Raman spectrometers for use in missions to explore the Solar System.