Laboratory study of amino acids on amorphous Mg-silicate using infrared spectroscopy and X-ray diffraction – implications for the survival and delivery of interstellar organics to the solar nebula and early Earth

What is it about?

In this work we examine how amino acids deposit and survive thermal processing on amorphous silicate powders as a means to understanding how amino acids formed in space could be delivered to the early Earth and therefore contribute to the organic inventory from which life arose. The amino acids glycine, alanine, glutamic acid, and aspartic acid represent the most abundent extraterrestrial amino acids found in meteorites and were deposited directly onto amorphous MgSiO₃ particles by sizzling droplets of amino acid solutions onto the silicate particles held at elevated temperature. The amorphous silicate was produced by a sol-gel process and prepared in hydrogenated and dehydrogenated forms. The deposited amino acids were characterised using IR spectroscopy and synchrotron X-ray powder diffraction. Only glycine and alanine deposited on the silicate surface. These silicates were then heated and the in situ temperature-dependent loss of the amino acids observed by X-ray diffraction. Glycine was lost from silicates at temperatures below its pure decomposition point, with a ~15°C difference between hydrogenated and dehydrogenated silicates. Alanine survived well above its melting point, and notable differences between L- and D- forms, both in terms of desorption temperatures and secondary phases were observed. Glycine formed methylamine, serine, magnesium oxalate, and diketopiperazine, most likely during the deposition process, while alanine formed dimethyl ether (DME) during heating, a molecule of astrobiological interest. L-alanine was lost at lower temperatures than D-alanine, again with differences between hydrogenated and deydrogenated silicate.

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Photo by NASA Hubble Space Telescope on Unsplash

Photo by NASA Hubble Space Telescope on Unsplash

Why is it important?

Life began early on in Earth's history and the necessary organic building block molecules such as amino acids would have already been present on the planet. While a proportion will have been formed on the Earth itself, it is likely that the original organic inventory would have also been supplemented by the delivery of extraterrestrially formed molecules, delivered by asteriod and comet impacts (with possible formation also occurring during the impacts themselves), or by the infall of dust particles. The infall rate of micormetorite particles some 4.2–3.9 billion years ago was several orders of magnitude greater than it is today, making this the likely dominant source of organic carbon on the early Earth. A wide range of extraterrestrial amino acids have been found in carbonaceous chondrite meteorites, which preserve a mixture of both pre-solar and early solar nebula material. Glycine, the simplest amino acid, has also been observed in comet tails. While some amino acids likely form within meteorite/asteriod parent bodies, some are also believed to form in interstellar space. However, amino acids such as glyine cannot survive the harsh radiation environment of space, such that their formation must take place within the coating of ice that surrounds interstellar dust particles, where they are shielded from harmfull radiation. Pre-solar grains would have fed directly into the Sun's protoplanetary disk as well as being incorporated into comets and other planetesimal bodies. Eitherway, on close approach to the Sun, the ice coating from dust grains (either free -floating or released from comets) is lost to space, along with the amino acids. Some mechanism of transfering cosmically-formed amino acids to the bare surface of the dust grain is therefore needed. The fact that ancient micrometeorites recovered from Antarctica contain significant quantities of amino acids bears witness to this. In our experiments, the amino acids are deposited via liquid phase and we discuss possible scenarios where highly localised and transient melting events can result in liquid water existing within dust grain mantles and comets allowing for amino acid molecules to bind to silicate dust grain surfaces.

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