Department of Chemistry, University of Wisconsin-Parkside, Kenosha, WI53141-2000, USA
Abstract Silicates are abundant on both Earth and Mars, and hold great potential for harboring biosignatures. Biosignatures are signs of past or present life and may be either organic or inorganic in nature. Our most recent work, which we review here, is a survey of how different classes of organic compounds interact with highly basic sodium silicate solution to model the formation of biosignatures in nature. Our work focuses on using IR (infra-red) spectroscopy as a way to determine the mechanisms by which organics are preserved within silicates. Throughout the chapter, we cite relevant studies by others, while still maintaining the focus on the review of our own work. We ultimately summarize how various classes of organics interact with sodium silicate in terms of both physical and spectral properties and describe their astrobiological significance.
One of the goals listed in NASA's Astrobiology Roadmap is to determine how to recognize biosignatures, which are signatures of life on early Earth and potentially on other worlds. A central requirement for this goal is that biosignatures must be defined in terms that can be measured and quantified (Morison, 2001; NASA, 2007a). A recent report on the astrobiology strategy for the exploration of Mars discusses various aspects of biosignatures (National Academies Report, 2007). It lists various recommendations and goals, such as to develop a catalog of biosignatures that reflect fundamental and universal characteristics of life that are not limited to an Earth-centric perspective, to determine which characteristics of Martian materials result from non-biological processes and which result from biological processes, and to follow the carbon to get to the biosignatures, among others. The report also points out that biosignatures may be mineralogical and inorganic in their nature. An example is the formation of biosignatures by the rapid mineralization that can protect microorganisms and organic molecules against degradation. This process is known as entombment. The database of terrestrial biosignatures needs to be expanded to facilitate determination of the mineralogical biosignatures of the Martian minerals (National Academies Report, 2007).
Various aspects of silicon biomineralization have recently been reviewed (Mann, 2001; Perry et al., 2003; Vrieling et al., 2003). While building on the background of others, we address several new questions that are associated with the silicification of the organic materials. The first question is if the silicified organic material can be identified in situ by IR (infra-red) spectroscopy. Such an approach is of interest to astrobiology, since the IR instrument has been miniaturized and made robotic, and it has been already used on extraterrestrial missions. The second question addresses silicification of the metal complexes of some bio-organic materials and their in situ identification by the IR. The in situ identification of the organic materials within their silicate and silicate-metal complexes is important for the upcoming missions to Mars, which are not of the return-sample type. Our ultimate goal is to identify the silicified organic materials by their characteristic IR bands. The final question addresses the changes that the organic material causes in the structure and appearance of the silica gel that it helps form. We do address this question and bring up a possibility that such changes in the silica gel need to be considered as inorganic signatures of various groups of organic compounds. We cite the relevant studies by others, while still remaining the focus on the review of our own work.
We are investigating the role of organic compounds in silicification. Silicate fossils may contain organic material preserved within the silica matrix. Alternatively, the organic material may induce morphological changes of the silica, but may no longer be present in the silica fossil. We have created model systems in the laboratory that provide insight into the nature of silicate fossils. These models have also been evaluated for the presence of organic materials via IR (infra-red) spectroscopy, and have been assessed for astrobio-logical relevance (Kolb et al., 2004; Perry et al., 2005; Kolb and Liesch, 2006, 2007; Liesch and Kolb, 2007a, b).
We are particularly interested in exploring various roles of silicate fossils in astrobiology, such as in the preservation of organic material on meteorites, during interplanetary transport, and possibly on Mars, and also as indicators of past life. Our model is comprised of a basic aqueous solution of sodium silicate, which we have treated with various classes of organic molecules, primarily those that are present on meteorites. Examples include amino acids, sugar-like compounds, Maillard products (which are formed from the reaction between the amino acids and sugars), alcohols and amino alcohols (Kolb and Liesch, 2006; Liesch and Kolb, 2007a). We have also initiated study of some phosphorylated biomolecules that are associated with the life on Earth, such as ATP, AMP, and DNA (Kolb and Liesch, 2007).
The key part of any silicification process is the formation of silica gel from silicic acid or its salts, such as sodium silicate (Iler, 1979). Figure 1 shows the polymerization
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