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普通公众的极大兴趣。拟议的方案之一涉及陨石撞击地球表面，将氨基酸等生物重要化合物输送到原始地球上。90年代初，S的几部著作报道了在陨石中发现了70多种氨基酸，其中大部分是已知的陆地上没有的氨基酸。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。实验室对高能过程的实验，如紫外线光子照射或宇宙射线轰击星际冰模拟物，确实发现这些过程在生产大有机分子方面非常有效，特别是在氨基酸的形成方面。尽管在过去20年中取得了这一进展，但直接检测星际介质中的氨基酸仍然是难以捉摸的。之前的工作集中在银河系中形成大质量恒星的区域寻找氨基酸。这些区域相对较热，众所周知，这些区域显示出一种活跃的化学物质，导致大量复杂有机物的产生。向这些区域观测到的光谱被毫米和亚毫米波长的分子发射线高度占据，就像是一片线条森林。在这个高度密集的光谱中，最亮的谱线特征对应于星际介质中更丰富的物种。这使得对氨基酸等含量较低的物种的鉴定具有挑战性。因此，到目前为止，还没有关于在星际介质中确实检测到氨基酸的报道。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，太阳类型系统形成的初始阶段以非常低的温度(绝对零度以上约10度)为特征，更适合于检测氨基酸。在第一步中，甘氨酸和丙氨酸的化学特性将在理论上被描述，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模拟。这将使我们能够确定光谱中检测到甘氨酸和丙氨酸的概率较高的部分。最后，我们将利用位于智利查南托高原的阿塔卡马大毫米/亚毫米阵列(ALMA)前所未有的能力对这些物种进行深入观察。这一设施灵敏度的提高(与以前的仪器相比提高了10倍以上)首次提供了完成对空间中氨基酸的检测的机会。这项研究不仅将使我们能够充分描述氨基酸的生物前化学，而且将使我们能够直接将这些复杂有机物在星际介质中的形成与它们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对氨基酸的探测将是我们理解地球上生命出现的一个里程碑。

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.