Abstract EANA2024-109

The detection and measurement of biosignatures are essential for identifying potential traces of life from a distance, given the highly restricted possibilities for direct sampling and return of biologically relevant samples to Earth. A universal characteristic of life is homochirality, the accumulation of a single enantiomer of chiral molecules (Blackmond, 2010). Homochirality ensures the correct spatial arrangement of molecules in functional macromolecular structures such as DNA and proteins. One notable property of homochiral macromolecules is their interaction with light: biogenic matter exhibits optical activity, rotating the plane of linear polarization upon scattering (Pasteur, 1848). Additionally, scattering of unpolarized light, such as that from stars, can induce characteristic patterns of circular polarization at specific wavelengths (Sparks et al., 2009; Patty et al., 2018). This phenomenon, unique to highly asymmetric macromolecular structures built from homochiral building blocks, may serve as an agnostic biosignature.

Circular polarization can be measured using a Stokes polarimeter, an instrument that uses polarizers and wave retarders to determine the Stokes parameters I, Q, U, and V, which describe the polarization state of electromagnetic waves. The SenseLife project aims to develop full-Stokes spectropolarimetric methods for remote sensing operations and telescopic observations capable of detecting circular polarization signals indicative of possible life on icy ocean worlds such as Europa and Enceladus. A feasibility study has demonstrated the effectiveness of airborne circular polarization measurements, paving the way for the development of advanced remote sensing operations (Patty et al., 2021).

Modeling, combined with extensive laboratory and instrumentation work, will help establish the sensitivity benchmarks required for our polarimeter. While various modeling scenarios address the feasibility of fly-by remote sensing operations, one near-future scenario under investigation involves light passing through the plumes ejected from Enceladus' south pole. This light could encounter ejected biological material, and the resulting scattering events could induce partial circular polarization which may be detectable by ground-based telescopes using our highly sensitive FlyPol spectropolarimeter (Patty et al., 2021). Potential light sources include stars behind the plumes when Enceladus is in Saturn's shadow to minimize ice particle albedo or reflected sunlight from Saturn during Enceladus' transit. Additionally, Saturn's E-ring is a target of interest for our method. All scenarios demand high instrument sensitivity and systematic corrections, which we consider technically feasible, as we can already detect circular polarization fractions as small as 10^-6.

The potential signals depend significantly on the metabolic pathways and macromolecules within cells, as well as the scattering angles and substrates surrounding cells. These parameters are explored in our laboratory work, where we culture Earth analogs and expose them to various environmental conditions to construct a signal library. This library will aid in predicting potential signals in our modeling scenarios and interpreting actual detected signals.

 

References

Blackmond, D. G. (2019). The Origin of Biological Homochirality. Cold Spring Harbor Perspectives in Biology, 11(3), a032540. https://doi.org/10.1101/cshperspect.a032540

Pasteur, L. (1848). Memoires sur la relation qui peut exister entre la forme crystalline et al composition chimique, et sur la cause de la polarization rotatoire. Compt. Rend., 26, 535–538.

Patty, C. H. L., ten Kate, I. L., Sparks, W. B., & Snik, F. (2018). Remote sensing of homochirality: A proxy for the detection of extraterrestrial life. In Chiral Analysis: Advances in Spectroscopy, Chromatography and Emerging Methods: Second Edition (pp. 29–69). Elsevier Inc. https://doi.org/10.1016/B978-0-444-64027-7.00002-1

Patty, C. H. L., Kühn, J. G., Lambrev, P. H., Spadaccia, S., Jens Hoeijmakers, H., Keller, C., Mulder, W., Pallichadath, V., Poch, O., Snik, F., Stam, D. M., Pommerol, A., & Demory, B. O. (2021). Biosignatures of the Earth: I. Airborne spectropolarimetric detection of photosynthetic life. Astronomy and Astrophysics, 651. https://doi.org/10.1051/0004-6361/202140845

Sparks, W. B., Hough, J. H., Kolokolova, L., Germer, T. A., Chen, F., DasSarma, S., DasSarma, P., Robb, F. T., Manset, N., Reid, I. N., Macchetto, F. D., & Martin, W. (2009). Circular polarization in scattered light as a possible biomarker. Journal of Quantitative Spectroscopy and Radiative Transfer, 110(14–16), 1771–1779. https://doi.org/10.1016/j.jqsrt.2009.02.028