1 Introduction

More than 60 years ago, S. L. Miller revealed a pathway through which life's building blocks can form from mixtures of simple reduced gases [Miller, 1953]. Subsequent research demonstrated the formation of amino acids, nucleobases, and even sugars from such reduced gases using discharge, heating, or UV irradiation [Bahadur, 1954; Harada and Fox, 1964; Ponnamperma et al., 1969]. However, a reducing atmosphere used in these experiments have been argued by prior researchers due to the metal‐depleted magma; an atmosphere predominantly composed of H 2 O, CO 2 , and N 2 has been suggested [Abelson, 1966; Holland, 1984; Kasting, 1993; Zahnle et al., 2010].

To provide alternative models for Miller‐Urey type synthesis, researchers have investigated reactions associated with meteorite impacts, comet impacts, or reactions around seafloor hydrothermal vents to produce amino acids [Huber and Wächtershäuser, 2006; Nakazawa, 2008; Furukawa et al., 2009; Goldman et al., 2010; Martins et al., 2013]. In the meteorite impact model, metals or carbon in extraterrestrial objects impacting on the early Earth work as reductants and catalysts [Nakazawa et al., 2005; Furukawa et al., 2014]. The formation of hydrogen, carbon monoxide, ammonia, hydrogen cyanide, and hydrocarbons, as well as amines, carboxylic acids, and even glycine, have been suggested by experimental simulations of iron or carbon‐bearing meteorite impacts [Mukhin et al., 1989; Gerasimov et al., 2002; Sugita and Schultz, 2003; Nakazawa et al., 2005; Sekine et al., 2006; Furukawa et al., 2009; Kurosawa et al., 2013; Furukawa et al., 2014]. In the hydrothermal model, oxidation of ferric iron in seafloor ultramafic rocks is the key reaction to reduce carbon and nitrogen and form hydrocarbons and ammonia [Summers, 1999; McCollom and Seewald, 2007; Smirnov et al., 2008]. Among these products, soluble compounds are partitioned into the ocean while nonpolar volatiles are released into the atmosphere. Among the soluble compounds, yields of reduced compounds with low mass, such as ammonia, methylamine, or acetic acid, are relatively high compared with the yields of amino acids in these experimental demonstrations [Huber and Wächtershäuser, 2006; Furukawa et al., 2009]. Furthermore, these reduced compounds have been found in carbonaceous chondrites [Shimoyama et al., 1989; Martins et al., 2006; Pizzarello et al., 2011]. Therefore, the composition of the prebiotic ocean seems to have been gradually enriched in reduced compounds of low mass, in addition to original solutes such as bicarbonate and chlorine.

Considering both the simple flux decay model and the late heavy bombardment model, lunar crater records suggest that the impact flux to the early Earth was significantly higher than today [Culler et al., 2000; Hartmann et al., 2000]. When meteorites hit the ocean at hypervelocity, a shock wave is generated and propagates through oceanic water. This physical process might have provided dissolved organics in the ocean that can chemically react with each other. Shock‐induced chemical reactions in solution have rarely been investigated in the context of prebiotic chemistry. Blank and coworkers have shown that shock waves in solution could have been a driving force for chemical evolution by demonstrating the formation of simple peptides from amino acids using shock waves in solution [Blank et al., 2001]. This study investigates a more elementary step in the chemical evolution, from reduced compounds of low mass to simple building blocks of life such as amino acids.