Near-Infrared Detection of Potential Evidence for Microscopic Organisms on Europa

Abstract

The possibility of an ocean within the icy shell of Jupiter's moon Europa has established that world as a primary candidate in the search for extraterrestrial life within our Solar System. This paper evaluates the potential to detect evidence for microbial life by comparing laboratory studies of terrestrial microorganisms with measurements from the Galileo Near Infrared Imaging Spectrometer (NIMS). If the interior of Europa at one time harbored life, some evidence may remain in the surface materials. Examination of laboratory spectra of terrestrial extremophiles measured at cryogenic temperatures reveals distorted, asymmetric nearinfrared absorption features due to water of hydration. The band centers, widths, and shapes of these features closely match those observed in the Europa spectra. These features are strongest in reddish-brown, disrupted terrains such as linea and chaos regions. Narrow spectral features due to amide bonds in the microbe proteins provide a means of constraining the abundances of such materials using the NIMS data. The NIMS data of disrupted terrains exhibit distorted, asymmetric near-infrared absorption features consistent with the presence of water ice, sulfuric acid octahydrate, hydrated salts, and possibly as much as 0.2 mg cm(-3) of carbonaceous material that could be of biological origin. However, inherent noise in the observations and limitations of spectral sampling must be taken into account when discussing these findings.

Surface biosignatures of exo-Earths: Remote detection of extraterrestrial life

“…These features in the infrared look quite similar throughout our spectral library with minor variations seen only in the relative strengths and depths of individual absorption features, which may be due to differences in cell composition and constituent concentrations. Our results agree with Dalton et al (10), who, based on their research on the spectral characteristics of Escherichia coli and Deinococcus radiodurans, suggest that all carbon-based organisms irrespective of their functionality or hardiness will look quite similar in the infrared portion of the reflectance spectrum. This feature in the infrared up to 2.5 μm may be a unique biosignature of life on potentially habitable planets.…”

“…This excess energy causes the development of pigments that the organism uses for screening UV radiation, photosynthesis, and oxidative damage prevention. In the SWIR region of the spectrum, the spectral features are from weak absorptions by functional groups present in cellular proteins, nucleic acids, lipids, and carbohydrates and strong absorptions from water in its free and bound states (water of hydration) (10). The absorption features of microorganisms owing to their water of hydration are seen to occur near 0.95, 1.15, 1.45, and 1.92 μm.…”

“…In addition, recent studies show that planet characterizability (3,14) is usually excellent in the visible waveband, which is useful in distinguishing different surface features. Finally, reflection spectra of microbes in the near-infrared (up to 2.5 μm) portion are thought to have absorption features unique to life owing to their water of hydration and biomolecular components (10).…”

Spectroscopic and spectrometric differentiation between abiotic and biogenic material on icy worlds

“…Methane has a stronger line at 7.67 µm and since methane is a by product of many chemosynthetic ecosystems on Earth, detection of methane features serves as a useful signpost. Finally, lipids and proteins display C-H absorption features near 2.3 µm (Dalton et al 2003).…”

“…Dalton et al (2003) froze samples of the microbes Sulfolobus shibatae, Deinococcus radiodurans, and Escherichia coli to characterize spectral signatures under Europan conditions of vacuum and low temperature (100 K), funding that the 2.05 and 2.17 µm C-N bands of proteins were subtle but distinguishable in the laboratory.…”