CHirality Analyzer In-Situ (CHAIS) - A Novel Approach to Planetary Surface Characterisation

The exploration of planetary surfaces is fundamental to our understanding of planetary evolution, habitability, and the origin of life. The chemical composition is typically amongst the main scientific goals of planetary exploration missions. Current orbital space instruments for probing chemical composition of planetary surfaces such as VIRTIS on Rosetta and NIRS3 on Hayabusa-II rely on classical infrared spectroscopy to measure minerals and organic matter.  However, spectrometers for space exploration are more limited in resolution and bandwidth due to constraints in mass, size, power consumption, thermal stability, and space-graded material availability than what can be achieved in a laboratory environment with prepared samples. Beyond these engineering constraints, classical spectroscopy is also intrinsically limited in measuring chemical bonds and not the molecular structure of a given compound. 

Alternatively, mass spectrometers such as COSAC on the Philae lander [5], SAM on the Curiosity rover [3], and MOMA on the Exomars rover [1] can deliver outstanding sensitivity, but at the expense of payload mass, instrument complexity, and the destruction of the sample during measurement. As a result of these constraints, these instruments are primarily suited for a select few mission profiles, notably flagships or large-class mission architectures. Furthermore, COSAC, SAM, and MOMA are currently the only space instruments capable of measuring chirality - the proportion of left vs right handed enantiomers in a sample. While studying these enantiomeric ratios provides key data in the context of prebiotic chemistry and understanding the emergence of homochirality on Earth, these instruments can only make a handful of measurements of this kind over their lifetime. Additionally to the limited number of uses of the chiral columns in the gas-chromatographs (GC), this technique is suitable for volatile compounds excluding chiral targets of interest such as amino acids.

In response to these limitations, we present a novel approach to planetary surface spectroscopy, leveraging polarization as an additional observable to complement traditional spectroscopic techniques and mass spectroscopy chirality measurements. Introducing the Chirality Analyzer In-Situ (CHAIS), a near-infrared optical bench (1.5 - 4.0 um) specifically designed to measure Vibrational Circular Dichroism (VCD) for planetary exploration—a technique well-established in terrestrial chemistry and pharmaceutical laboratories. The objective of CHAIS is to validate this approach through measurements in a true-to-life configuration using planetary analogs, with the aim of assessing the sensitivity thresholds necessary for the development of a space-optimized version of the instrument. By analyzing linear and circular polarization signatures as a function of wavelength, CHAIS is anticipated to enable the non-destructive characterisation of the typically featureless felsic primordial crust minerals present on planetary bodies like the Moon and Mars. The addition of polarimetry to CHAIS not only enhances the capability to discern certain molecular structures while measuring chirality, but also takes advantage of the instrument's relative simplicity and the absence of required sample preparation, enabling a higher number of measurements.

Beyond composition analysis, CHAIS, as the name suggests, is built to measure chirality. Our approach would allow for the measurement of enantiomeric excess and homochirality (100\% enantiomeric excess) which paves the way towards a robust biosignature detection tool. Additionally, other existing spectropolarimeters, such as TreePol [4] and LSDpol [2], operate in the visible spectrum using different methodologies, yet similarly demonstrate the use of optics for detecting chirality and biosignatures. Ultimately, this integration of polarimetry into classical infrared spectroscopy will enable the non-destructive characterisation of a wide variety of planetary surfaces and the development of a new type of space instrument optimized for exploring and understanding the origin and formation of biotic matter. 

Racemic abiotic samples like plastic films as well as Mars analogs such as gypsum have been measured in transmission as part of the first test cases to validate the bench. Preliminary spectroscopy data is shown in Figure 1. In this work we present initial findings with CHAIS including first light results, first spectroscopy and spectro-polarimetry data from a variety of biotic and abiotic samples, as well as the next steps towards a point-and-shoot space instrument of adequate sensitivity to characterise the chemical composition and chirality of surface minerals.

Acknowledgments
This research is co-funded by the Centre National d’ ́Etudes Spatiales (CNES) and the Aix-Marseille Origines Institute
through the A*MIDEX Program.

References

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