Ludmilla Kolokolova, Lev Nagdimunov, Alex Poonai (University of Maryland), William Sparks (Space Telescope Science Institute)

Transcription

1 Ludmilla Kolokolova, Lev Nagdimunov, Alex Poonai (University of Maryland), William Sparks (Space Telescope Science Institute) From Stars to Life, Gainesville FL, April 6, 2013

2 Linearly polarized light: can be formed by a reflection from a surface or scattering by a particles Circularly polarized light: is formed at scattering of light by mirror- asymmetric scatterers

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4 All amino acids of biological origin are left-handed All sugars of biological origin are right-handed

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6 L- purine D- purine

7 David Cline (Department of Physics & Astronomy, University of California, Los Angeles, CA) On the physical origin of the homochirality of life European Review, Vol. 13, Supp. No. 2, (2005) 1. DNA: Self-replication would not work with racemic (equal number of L and D molecules) systems 2. Errors in DNA Replication: Without a pure chiral structure, the error rate in replication would be unacceptable to long-lived systems. 3. In a prebiotic medium: Homochirality must have been either (A) Established in a very short time on earth (100 Myr), or, (B) Existed in ISM organic materials near the solar system

8 High circular polarization (red and white regions, white corresponds to ~17%) in near-infrared light from reflection nebulae in the star forming regions Orion OMC1 (a region in the Orion nebula M42), Bailey et al. (1998) The origin of circular polarization in star forming regions: alignment of dust particles in magnetic field of the star

9 Homochirality seen in living organisms on Earth may have an astrophysical origin. Circular polarised light could imprint chiral asymmetry in interstellar molecules before or during the formation of the Solar System due to asymmetric photosynthesis asymmetric photolysis

10 Several carbonaceous meteorites (Murchison, Murrey, Tagish Lake) contained a wide variety of organic compounds including many amino acids (more than 100 have now been identified). Engel et al. (1982, 1990, 1997) find large enantiomeric excesses (up to 50%) in common protein amino acids such as alanine, while Cronin and Pizzarello (1997) find enantiomeric excesses (of 2-15%) in several a-methyl amino acids.

11 P c = % Hale-Bopp [Rosenbush et al. 1997]; S4 (LINEAR) [Rosenbush et al. 2007b]; P c = % Q4 (NEAT) [Rosenbush et al. 2007a]. P c = % P c = % The brightness (top), linear (middle) and circular (bottom) polarization for comet SW3-B obtained with FORS by ESO SW3 team [Tozzi et al. 2006] on June 17, CP for comet Tempel 1 measured on July 8, 2005 with instrument FORS at VLT ESO [Boehnhardt et al. 2005].

12 William Sparks Thomas A. Germer Feng Chen Shiladitya DasSarma Priya DasSarma Frank T. Robb Neill Reid Measures the whole Stokes vector Wavelength range: 400 nm 2 micron Achieve a polarization measurement of 1:10 4 for circular polarization level (i.e. 0.01%)

13 Ancient fossil record, possibly to >3.5 billion yr Hardy, adaptable and diverse Abundant: marine, fresh water, on rock, in rock, soil Chlorophyll-based photosynthetic prokaryotes Unicellular; colonial Cyanobacteria are thought to be responsible for the rise of oxygen in the Earth s atmosphere about 2.5 billion yr ago. Credit to:

14 Sharp change in circular polarization magnitude around peak absorption for Chlorophyll. Circular polarization often changes sign entirely at peak absorption.

15 Biological Objects Non-Biological Objects

16 Structure Cyanobacteria Vegetation Optical Properties dominated by Chlorophyll

17 Insert absorption into this plot. Know number of particles. Modeled circular polarization reproduces correctly the trend measured in the lab

18 Spectral and phase angle dependence of circular polarization for chlorophyll aggregates of different porosity but the same number and size of monomers. More compact aggregates produce larger circular polarization.

19 Increasing the number of particles and/or the size of the particles forming the aggregate results in increased circular polarization over most wavelengths and phase angles. The best conditions for measuring circular polarization are at medium phase angles, when exoplanets are in the most favorable observational conditions

20 The quick change in circular polarization showed above is known as Cotton effect. It is related to the change in circular dichroism of the optically active molecules. In the case of chlorophyll, this has been attributed to the optical activity of the asymmetric substituents of three carbons, highlighted in red (Houssier, Sauer 1969).

21 λ,nm Amino acids also exhibit characteristic circular dichroism particularly due to the optical activity of their carboxyl group which exhibits a n π* transition near 220 nm. This activity has been shown to be a result of the presence of the carboxyl group and varies only slightly with changing R-groups (Meierhenrich et al. 2010)

22 CONCLUSIONS Circular polarization from biological molecules experiences a sharp change inside absorption bands that can serve as a biosignature. This change can be observed near the wavelength of the star maximum radiation for photosynthetic chromophores and around 200 nm for amino acids. Circular polarization of an optically active ( biological ) particle increases with size and compactness of the particle. Circular polarization of biological origin increases with phase angle, reaching its maximum around º; these angles are fortuitously convenient for observations of exoplanets and, thus, search for life there.