Biosignatures from circular spectropolarimetry: key science for ELTs? K. G. Strassmeier, T. A. Carroll & M. Mallonn (Leibniz-Institute for Astrophysics Potsdam (AIP), Germany; kstrassmeier@aip.de)

Abstract: Vegetation has a fivefold higher reflectivity in the NIR than in the optical domain. The steep gradient at around 700 nm, the Vegetation Red Edge (VRE), can in principle be used to discriminate exo-planets with and without vegetation. Transiting exo-planets pose the possibility to isolate their (absorption-line) spectrum from that of its host star without the need for spatially resolving the planet. While the expected difference of the Stokes-I amplitude between a Super- Earth with and without an atmosphere in front of a G2V star would be just ≈3×10-5, its differential Stokes-V signal could be larger by up to an order of magnitude. Thus, a possible way out is to observe in polarized light and use the known albedo-polarization relation for planetary surfaces and/or atmospheres. Polarization degrees of up to 20% are expected from planets in short-period orbits. Already the isolation of the VRE from a transmission spectrum is beyond current instrumentation, the aim to detect a differential circular polarization (CP) signal from wavelengths blueward-minus-redward of the VRE in transmission spectra of exoplanets is clearly left to the Extremely Large Telescopes (ELT). While it is tempting to interpret such a simple CP detection as a sign of chirality in the atmosphere of the exoplanet, e.g. due to chlorophyll, its signal will be deeply buried in the photon noise and systematics, even for ELTs. We present our current status of signal-reconstruction algorithms used for magnetic- field mapping of stellar surfaces and how these could be employed for the isolation of biosignatures. We also present the current spectro-polarimetric instrumentation for the LBT and the ESO E-ELT.

Towards Polarimetric Exoplanet Imaging with ELTs Christoph U. Keller (Leiden Observatory, keller@strw.leidenuniv.nl), Visa Korkiakoski (Leiden Observatory), Michiel Rodenhuis (Leiden Observatory), Frans Snik (Leiden Observatory)

Abstract: A prime science goal of Extremely Large Telescopes (ELTs) is the detection and characterization of exoplanets to answer the question: are we alone? ELTs will obtain the first direct images of rocky exoplanets in the habitable zone and search for atmospheric biomarkers. However, the required instrumental technologies are not yet at a level where an instrument could be built that would achieve this goal. Polarimetry will be an important ingredient in future high-contrast instruments as it will provide a major contrast improvement for planets located within the first two Airy rings and offers unique diagnostic capabilities for liquid water (ocean glint, water clouds and their rainbows), hazes and dust in exoplanetary atmospheres. We will describe novel instrumental approaches to improving subsystems, in particular polarimetry, wavefront sensing and adaptive optics control. To reach contrasts of 10-9 and beyond to image rocky exoplanets from the ground, a series of individually optimized subsystems cannot succeed; rather, entire combinations of subsystems must be optimized together. We will describe our efforts at measuring and controlling wavefronts with 40’000 degrees of freedom, reaching the photon-noise limit in high-contrast polarimetric imaging at telescopes and our plans to reach a contrast of at least 10-9 in broadband light under realistic, simulated ground-based conditions in the laboratory and to test new approaches at telescopes, in particular achromatic aperture and focal-plane coronagraphs, focal-plane wavefront-sensing and speckle suppression, integral-field polarimetry and high-contrast data reduction algorithms.

Fig 1. Expected distribution of exoplanets according to mass (in Earth masses) and radiative-equilibrium temperature (in K) that can be detected by the EPICS/EPOL polarimetric exoplanet imager for the E-ELT. Several rocky exoplanets in the habitable zone will be within reach of this instrument with an expected inner working angle of 0.027 arcsec. With advanced polarimetry concepts, we expect to reduce that to 0.01 arcsec.

Remotely sensing homochirality, a powerful generic biosignature William B. Sparks (Space Telescope Science Institute, sparks@stsci.edu), James H. Hough (University of Hertfordshire, UK), Ludmilla Kolokolova (University of Maryland, College Park), Thomas Germer (National Institute of Standards and Technology), Frank Robb (University of Maryland, School of Medicine)

Abstract: A high quality biosignature arises uniquely from biological processes. If a biosignature can additionally be used in remote sensing, then it can be useful for future telescopic studies of extrasolar planets where remote sensing is a necessity. The remarkable phenomenon of homochirality may be such a biosignature. The optical activity of biological molecules, together with their handedness, can yield a unique signature in circular polarization. Photosynthesis, a surface phenomenon relying on strong polarization-sensitive transitions in the visible, where light from the host star is abundant, is a natural remote sensing target for this approach. Both microbial photosynthesis, which has dominated terrestrial life for much of the history of Earth, and macroscopic vegetation, may in principle be observed. Precision polarimetry from space is likely to be needed, and we describe a promising, innovative approach to acquire sensitive full Stokes polarimetry with a compact, robust configuration well-suited to space application. The homochirality phenomenon is likely to be generic to all biochemical life, and pure in that abiotic processes do not result in homochirality nor do abiotic processes produce circular polarization features with similar character to the biological ones. This uniquely powerful biosignature is amenable to remote sensing, in principle, through circular polarization spectroscopy.

Laboratory insights into the detection of surface biosignatures by remote-sensing techniques

Olivier Poch1, Antoine Pommerol2, Bernhard Jost2, Isabel Roditi3, Joachim Frey4, and Nicolas Thomas2 (1 Center for Space and Habitability, University of Bern, 2 Physikalisches Institut, University of Bern, 3 Institut für Zellbiologie, University of Bern, 4 Institut für Veterinär-Bakteriologie, University of Bern)

Abstract:

With the progress of direct imaging techniques, it will be possible in the short or long-term future to retrieve more efficiently the information on the physical properties of the light reflected by rocky exoplanets (Traub et al., 2010). The search for visible-infrared absorption bands of peculiar gases (O2, CH4 etc.) in this light could give clues for the presence of life (Kaltenegger and Selsis, 2007). Even more uplifting would be the direct detection of life itself, on the surface of an exoplanet. Considering this latter possibility, what is the potential of optical remote-sensing methods to detect surface biosignatures?

Reflected light from the surface of the Earth exhibits a strong surface biosignature in the form of an abrupt change of reflectance between the visible and infrared range of the spectrum (Seager et al., 2005). This spectral feature called "vegetation red-edge" is possibly the consequence of biological evolution selecting the right chemical structures enabling the plants to absorb the visible energy, while preventing them from overheating by reflecting more efficiently the infrared. Such red-edge is also found in primitive photosynthetic bacteria, cyanobacteria, that colonized the surface of the Earth ocean and continents billions of years before multicellular plants (Knacke, 2003). If life ever arose on an Earth-like exoplanet, one could hypothesize that some form of its surface-life evolves into similar photo-active organisms, also exhibiting a red-edge.

In this paper, we will present our plan and preliminary results of a laboratory study aiming at precising the potentiality of remote sensing techniques in detecting such surface biosignatures. Using equipment that has been developed in our team for surface photometry studies (Pommerol 2011, Jost 2013, Pommerol 2013), we will investigate the reflectance spectra and bidirectional reflectance function of soils containing bacteria such as cyanobacteria, in various environmental conditions. We will also present our plan to incorporate polarization measurements, and particularly circular polarization, because it can be a marker of homochirality, which is supposed to be a universal property of life. Finally, the analyses of both biotic and abiotic materials will help to assess if (or in which peculiar conditions) remote sensing techniques can discriminate between false positive and strong biomarkers.

Ultimately, these laboratory data can serve as reference data to guide and interpret future observations, paving the way for the detection of life on distant exoplanets.

References:

Jost et al. 2013 Icarus 225, 352

Kaltenegger and Selsis 2007 in Extrasolar Planets: Formation, Detection and Dynamics

Knacke 2003 Astrobiology 3, 531

Pommerol et al. 2011 Planetary and Space Science 59, 1601

Pommerol et al. 2013 Journal of Geophysical Research: Planets 118, 2045

Seager et al. 2005 Astrobiology 5, 372

Traub and Oppenheimer 2010 in Exoplanets, 111