Wednesday, January 15, 2014

Biosignatures of water, amino acids and long chain aliphatic compounds in thermal infrared (4000 - 700 cm-1).



Most living organisms have several chemical compounds in common as water, long chain aliphatic compounds, and amino acids. These compounds are part of thin layers like: membranes, cuticles, cell walls, tubules, fibers, etc...that have characteristic spectral emissivity features in the thermal infrared (TIR: 4000 - 700 cm-1) similar to transmission features, although almost always different from reflectance measurements made in the lab. For example, reflectance measurements of very thin layers of alkanes show very weak features (Clark et al., 2009) that are almost never shown in transmission measurements, because even thin layers prepared in the lab lacks the structure of natural layers, enhancing volume scattering as opposed to surface scattering.
Three samples were measured using directional hemispherical reflectance (DHR), and then calculated to emissivity (e = 1-DHR): 1) a very thin layer of water on aluminum foil, 2) Fagus grandifolia (Beech) fresh leaf; and 3) a sample of dried bacteria Oenococcus oeni. Water features of a very thin layer of water is used as a proxy of surface films, or interstitial spaces or  cytoplasm water: broad troughs centered at approximately 2648 and 1866 cm-1 and a broad peak between them at 2112 cm-1 (Fig.1 blue); leaf surfaces are almost always covered with a very thin layer of waxes of mostly long chain aliphatic compounds as hydrocarbons, fatty acids, esters, alcohols, etc..and it is used here as a proxy for this kind of compounds that can be found in several membranes of living organisms. CH2 the main molecule of aliphatic long chain organic compounds has very strong features at 2916 and 2846 cm-1 with one peak between them at 2863 cm-1 (Fig 1 green); dried colony of Oenococcus oeni  is a proxy for a thin layer of protein and its amine groups that have typical double troughs at 1626 and 1518 cm-1 with a peak between both at 1576 cm-1 (Fig.1 red). Ratio of these troughs and peaks in a hyperspectral infrared image would probably detect living organisms features either on Earth or in other planets when the surveyed area is at relatively warm temperatures.



Figure 1. Emissivity spectra.  1) Water spectrum (blue) features of a very thin layer water show two broad troughs centered at approximately 2648 and 1866 cm-1 and a broad peak between them at 2112 cm-1; 2) Fagus grandifolia (Beech) fresh leaf spectrum(green) with thin layer of waxes on its surface shows strong CH2 features at 2916 and 2846 cm-1 with one peak between them at 2863 cm-1 (note water features between 2732 and 1743 slightly modified by CH2 strong features); 3) dried Oenococcus oeni colony is a proxy for a thin layer of protein and its amine groups that have typical double troughs at 1626 and 1518 cm-1 and a peak between both of them at 1576 cm-1 .

Interestingly, although the search for life is one of the main reasons of recent Planet exploration, there are no sensors capable of detecting most of these features present in living organisms, either on Earth or in other planets or moons.

Clark, R.N., Churchin, J.M., Hoefen, T.M. and Swayze, G.A. 2009 Reflectance spectroscopy of organic compounds: 1. Alkanes. Journal of Geophysical Research: 114, E03001 1-19.