Aldehydes are ubiquitous in star-forming regions and carbonaceous chondrites, serving as essential intermediates in metabolic pathways and molecular mass growth processes to vital biomolecules necessary for the origins of life. However, their interstellar formation mechanisms have remained largely elusive. Here, we unveil the formation of lactaldehyde (CH₃CH(OH)CHO) by barrierless recombination of formyl (HĊO) and 1-hydroxyethyl (CH₃ĊHOH) radicals in interstellar ice analogs composed of carbon monoxide (CO) and ethanol (CH₃CH₂OH). Lactaldehyde and its isomers 3-hydroxypropanal (HOCH₂CH₂CHO), ethyl formate (CH₃CH₂OCHO), and 1,3-propenediol (HOCH₂CHCHOH) are identified in the gas phase utilizing isomer-selective photoionization reflectron time-of-flight mass spectrometry and isotopic substitution studies. These findings reveal fundamental formation pathways for complex, biologically relevant aldehydes through non-equilibrium reactions in interstellar environments. Once synthesized, lactaldehyde can act as a key precursor to critical biomolecules such as sugars, sugar acids, and amino acids in deep space.

醛在恒星形成区域和碳质球粒陨石中无处不在，是生命起源所必需的重要生物分子的代谢途径和分子量生长过程的必需中间体。然而，它们的星际形成机制在很大程度上仍然难以捉摸。在这里，我们揭示了甲酰 （HĊO） 和 1-羟乙基 （CH₃ĊHOH） 自由基在由一氧化碳 （CO） 和乙醇 （CH₃CH₂OH） 组成的星际冰类似物中通过无屏障重组形成乳醛 （CH₃CH（OH）CHO）。利用异构体选择性光电离反射器飞行时间质谱和同位素取代研究，在气相中鉴定乳醛及其异构体 3-羟基丙醛 （HOCH₂CH₂CHO）、甲酸乙酯 （CH₃CH₂OCHO） 和 1,3-propenediol （HOCH₂CHCHOH）。这些发现揭示了复杂的、具有生物相关性的醛通过星际环境中的非平衡反应的基本形成途径。合成后，乳醛可以作为深空关键生物分子（如糖、糖酸和氨基酸）的关键前体。