Hogenboom, M.

Rossi, L.

Snik, F.

Stam, D.

Leiden University Scholary Publications

Geophysical Research AbstractsVol. 18, EGU2016-4721-1, 2016EGU General Assembly 2016© Author(s) 2016. CC Attribution 3.0 License.Modelling the circular polarisation of Earth-like exoplanets: constraintson detecting homochiralityMichael Hogenboom (1), Daphne Stam (1), Loic Rossi (1), and Frans Snik (2)(1) Delft University of Technology, Netherlands (m.r.hogenboom@student.tudelft.nl), (2) Leiden Observatory, LeidenUniversity, Leiden, the NetherlandsThe circular polarisation of light is a property of electromagnetic radiation from which extensive information canbe extracted. It is oft-neglected due to its small signal relative to linear polarisation and the need for advancedinstrumentation in measuring it. Additionally, numerical modelling is complex as the full Stokes vector must al-ways be computed. Circular polarisation is commonly induced through the multiple scattering of light by aerosols[?] and multiple reflections of light by rough surfaces [?]. Most interestingly, distinctive spectral circular polari-metric behaviour is exhibited by light reflected by organisms due to the homochiral molecular structure of allknown organisms [?]. Especially fascinating is the unique circular polarimetric behaviour of light reflected byphotosynthesising organisms at the absorption wavelength of the chlorophyll pigment [?]. This presents the previ-ously unexplored possibility of circular polarimetry as a method for identifying and characterising the presence oforganisms, a method which could be applied in the hunt for extraterrestrial life.To date, few telescopes exist that measure circular polarisation and none that have been deployed in space. Obser-vations of the circular polarisation reflected by other planets in the solar system have been made with ground-basedtelescopes, with significant results [?]. However, none of these observations have been made at the phase angles atwhich exoplanets will be observed. Also, none have been made of the Earth, which is the logical starting point forthe study of biologically induced circular polarisation signals. This introduces the need for numerical modelling todetermine the extent to which circular polarisation is present in light reflected by exoplanets or the Earth.In this study, we model the multiple scattering and reflection of light using the doubling-adding method [?]. Wewill present circular polarisation signals for both spatially resolved and spatially unresolved planets, using variousatmospheric and surface properties and across a range of phase angles. As a test, the calculated degree of circularpolarisation resulting from the multiple scattering of light in an atmosphere with varying properties was comparedwith results presented by Kawata [?] and was found to be in agreement.Initial modelling of the atmospheric scattering of light by a planetary disk has shown a presence of degree ofcircular polarisation in the order of 10−4. This represents a static case with one cloudy hemisphere, one cloudlesshemisphere and a Lambertian surface. Results containing varied patchy cloud patterns shall also be presented ina bid to reflect the random nature of planetary cloud cover. We will also present the calculated degree of circularpolarisation of planets with various cloud coverage and a circularly polarising surface in order to discover theinfluence of organisms on the numerical results.References[1] J. E. Hansen and L. D. Travis. Light scattering in planetary atmospheres. Space Science Reviews, 16:527–610,October 1974.[2] J. C. Kemp and R. D. Wolstencroft. Circular Polarization: Jupiter and Other Planets. Nature, 232:165–168,July 1971.[3] J. Bailey. Circular Polarization and the Origin of Biomolecular Homochirality. In G. Lemarchand andK. Meech, editors, Bioastronomy 99, volume 213 of Astronomical Society of the Pacific Conference Series,2000.[4] L. Nagdimunov, L. Kolokolova, and D. Mackowski. Characterization and remote sensing of biological par-ticles using circular polarization. Journal of Quantitative Spectroscopy and Radiative Transfer, 131:59–65,December 2013.[5] J. F. de Haan, P. B. Bosma, and J. W. Hovenier. The adding method for multiple scattering calculations ofpolarized light. Astronomy and Astrophysics, 183:371–391, September 1987.[6] Y. Kawata. Circular polarization of sunlight reflected by planetary atmospheres. Icarus, 33:217–232, January1978.

Modelling the circular polarisation of Earth-like exoplanets: constraints on detecting homochirality

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Modelling the circular polarisation of Earth-like exoplanets: constraints on detecting homochirality

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