Suspected ongoing hydrothermal activity within Saturn’s second satellite, Enceladus, is attracting attention as a potential site for future exploration for life in the solar system. A joint research team has recently conducted a 147-day experiment simulating Enceladus’ hydrothermal environment and their results confirmed that a total of 28 different peptides^*1 were synthesised from six amino acids under these conditions. This finding from the icy moon is expected to provide new insights into the polymer synthesis in Enceladus’ subsurface ocean, constraints on the types of organic molecules that are considered abiotic, and may also provide a basis for future life detection missions.

The joint research group consists of Wataru Takahagi (Master student of Keio University at the time of the research), Takazo Shibuya (senior scientist), Yoshinori Takano (senior scientist), and Ken Takai (director) from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC); Kosuke Fujishima (specially appointed associate professor) from the Earth-Life Science Institute (ELSI); as well as students and researchers from Keio University, the University of Lausanne, and the Atmosphere and Ocean Research Institute of the University of Tokyo. The group experimentally proved the existence of a process that synthesises peptides – one of the building blocks of life – from simple amino acids. This mechanism was demonstrated using a high-pressure autoclave that simulated the subsurface ocean environment of Saturn’s icy moon, Enceladus.

Despite being a small celestial body of approximately 500 km in diameter, previous investigations and observations have revealed that Saturn’s second satellite, Enceladus, likely contains a global subsurface ocean, and that plumes of gases and frozen seawater are released into space from the south polar region (Image 1). Data from NASA’s Cassini spacecraft^*2 showed that the plumes contained solid materials comprised of salt and ice, inorganic substances such as ammonia and carbon dioxide, as well as simple organic materials. In recent years, nanosilica and hydrogen molecules found in the plumes have revealed that Enceladus has an ongoing hydrothermal system.

In this work, the joint research team assumed that amino acids found in carbonaceous meteorites could also exist in Enceladus’ subsurface ocean, therefore experimentally validating the possibility of the polymerisation of amino acids under simulated hydrothermal reactions within the rocky core of Enceladus.

The main component of the rocky core of Enceladus is presumed to be similar to that of Earth’s mantle, which is composed of olivine. The joint research team reproduced the reaction of olivine and hydrothermal water on Enceladus in the lab for more than 140 days (Image 2). As a result of thermal cycling (30-100℃) under a high-pressure alkaline hydrothermal environment (>pH 10), they discovered that a total of 28 dipeptides were synthesised at different concentrations from six amino acids (glycine, alanine, glutamic acid, aspartic acid, serine, and valine) (Image 3). They also found that the synthesis of peptides was remarkably suppressed in the control experiment performed without rocks. Therefore, olivine, the primary component of Enceladus’ rocky core, can support polymerisation of amino acids to make abiotic peptides.

Does life exist beyond Earth? This is one of the key questions in astrobiology. Research institutes around the world are currently preparing for future exploration of life in the solar system.  Enceladus is a leading candidate for such exploration because of the active heat source as well as possible ongoing chemical evolution that may lead to life. This study experimentally proved that simple organic substances could be efficiently polymerised under the presence of olivine rocks to make a minimal polymer. Hence, the group’s results will stimulate future discussions related to how and what we look for as a biosignature on Enceladus and other ocean worlds.

*1) Peptide: An organic substance formed by combining two or more amino acids. Two amino acids joined together is called a dipeptide. When more than 50 amino acids are connected, it is considered to be a protein. All of these are critical building blocks of life.

*2) Cassini spacecraft: A spacecraft that completed its mission to Saturn in 2017. It significantly contributed to the understanding of Saturn’s satellite system, including Enceladus and Titan. It was a spacecraft that achieved pioneering research results, which led to the subsequent exploration of life in the solar system. Its accomplishments, in particular, include the confirmation of eruptions of plumes, which is direct evidence of Enceladus’ underground ocean.