Chemical Pathways to Life: Amino Acids and their Precursors in the ISM

几十年来，关于地球上生命出现的长期问题引起了研究人员和公众的极大兴趣。其中一个设想方案涉及陨石撞击地球表面，将氨基酸等生物学上重要的化合物送到原始地球。在90年代早期，一些研究报告在陨石中发现了70多种氨基酸，其中大多数在地球上没有发现。这一发现支持了生命起源的外生假说。因此，目前认为，氨基酸可能是在星际介质中形成的，在那里发现了复杂的有机物，即大的碳基分子。实验室实验的高能过程，如紫外光子照射或宇宙射线轰击星际冰类似物，确实发现这些过程在产生大有机分子方面非常有效，特别是，尽管在过去的二十年里取得了这一进展，但在星际介质中直接检测氨基酸仍然是难以捉摸的。以前的工作集中在寻找大质量恒星形成的银河系区域中的氨基酸。这些区域相对较热，并且已知显示出诱导产生大量复杂有机物的活性化学。朝向这些区域观察到的光谱被毫米和亚毫米波长的分子发射线高度填充，类似于线的森林。在这个高度密集的光谱中，最亮的线特征对应于星际介质中更丰富的物种。这使得鉴定不太丰富的物质如氨基酸具有挑战性。因此，迄今为止还没有关于星际介质中氨基酸的确切检测的报道。在这个研究项目中，我们将使用一种新的理论和观测方法来检测星际介质中最简单的氨基酸，甘氨酸和丙氨酸。这种方法认为，太阳能型系统形成的初始阶段，其特征是非常冷的温度（绝对零度以上约10度），更适合于检测氨基酸。在第一步中，甘氨酸和丙氨酸的化学性质将在理论上被表征，假设非常冷的条件类似于年轻的太阳系。这将有助于我们推断在低温下甘氨酸和丙氨酸的破坏和形成所涉及的主要化学途径。在第二步中，我们将对这些物种的发射光谱进行理论建模。这将使我们能够确定光谱中检测甘氨酸和丙氨酸的概率较高的部分。最后，我们将使用位于智利Chajnantor高原的阿塔卡马大型毫米/亚毫米阵列（阿尔马）的前所未有的能力对这些物种进行深入观测。这一设施的灵敏度提高了（相对于以前的仪器，灵敏度提高了10倍以上），这是第一次有机会在太空中完成氨基酸的检测。这项研究不仅将使我们能够充分表征氨基酸的前生物化学，而且将使我们能够直接将星际介质中这些复杂有机物的形成与随后向行星系统的输送联系起来。在太阳系形成的早期阶段发现氨基酸，将是我们理解地球生命起源的一个里程碑。

The long-standing question about the emergence of life on Earth has attracted great interest among researchers and the general public for decades. One of the proposed scenarios involves the delivery of biologically important compounds such as amino acids on the primordial Earth by the impact of meteorites on the Earth's surface. In the early 90's, several works reported the discovery of more than seventy amino acids in meteorites, the majority of which have no known terrestrial occurrence. This finding supports the exogenous hypothesis for the origin of life. It is thus currently believed that amino acids may have formed in the interstellar medium, where complex organics, i.e. large carbon-based molecules, have been found. Laboratory experiments of highly energetic processes such as illumination by ultraviolet photons or bombardment by cosmic rays on interstellar ice analogs, have indeed found that these processes are very efficient in the production of large organic molecules and, in particular, in the formation of amino acids.Despite this progress in the past twenty years, the direct detection of amino acids in the interstellar medium remains elusive. Previous works focused on the search of amino acids in regions across the Galaxy where massive stars form. These regions are relatively hot, and are known to show an active chemistry which induces the production of large amounts of complex organics. The spectrum of light observed toward these regions is highly populated by molecular emission lines at millimeter and sub-millimeter wavelengths, resembling a forest of lines. In this highly populated spectra, the brightest line features correspond to the more abundant species in the interstellar medium. This makes the identification of less abundant species such as amino acids challenging. As a consequence, no firm detection of amino acids in the interstellar medium has been reported to date. In this research project, we will use a novel theoretical and observational approach to detect the simplest amino acids, glycine and alanine, in the interstellar medium. This approach considers that the initial stages in the formation of Solar-type systems, characterized by very cold temperatures (about 10 degrees above absolute zero), are better suited for the detection of amino acids. In a first step, the chemistry of glycine and alanine will be characterized theoretically assuming very cold conditions resembling those of young Solar-type systems. This will help us to infer the main chemical routes involved in the destruction and formation of glycine and alanine at cold temperatures. In a second step, we will perform theoretical modelling of the emission spectrum of these species. This will allow us to establish the parts of the spectrum of light where the probability to detect glycine and alanine is higher. Finally, we will perform deep observations of these species by using the unprecedented capabilities of the Atacama Large Millimeter/Sub-millimeter Array (ALMA) located on the Chajnantor plateau in Chile. The increased sensitivity of this facility (by more than a factor of 10 with respect to previous instrumentation) offers the opportunity, for the first time, to accomplish the detection of amino acids in space. This research not only will allow us to fully characterize the pre-biotic chemistry of amino acids, but will allow us to directly link the formation of these complex organics in the interstellar medium with their subsequent delivery onto planetary systems. The detection of amino acids at the early stages in the formation of Solar-type systems will represent a milestone in our understanding of the emergence of life on Earth.