Chapter 13 - Salty ocean and submarine hydrothermal vents on Saturn’s Moon Enceladus—Tall plume of gas, jets of water vapor & organic-enriched ice particles spewing from its south pole

Abstract

Saturn’s icy moon Enceladus—one of the most mysterious bodies in the solar system—is a high-priority target for astrobiology as it harbors a subsurface ocean that erupts into space and is likely habitable. Enceladus’ geysers, for example, are blasting out salty water and other material from an ocean that sloshes beneath its icy shell. The subsurface ocean of Enceladus is likely in contact with its rocky core, making possible a variety of complex chemical reactions. Enceladus is unique amongst Ocean Worlds in our solar system because the contents of its internal ocean are continuously emitted to space by its present-day activity, and some of these materials are redeposited on the surface. This tiny moon of Saturn thus presents an opportunity to directly measure the composition of the ocean and seek evidence for habitability (including past or extant life), either by collecting and analyzing plume particles, or via more ambitious mission concepts that involve landing, surface sampling and analysis, and potential deployment of subsurface probes to reach the ocean itself. Searching for life at Enceladus is a top priority, near-term goal of the “NASA Roadmap to Ocean Worlds” exploration program. Enceladus is such a scientifically compelling target that provides a great opportunity to understand organic chemistry in an ocean beyond Earth’s, with great potential to provide at least some new clues about the origin of life and the potential for life elsewhere in the Universe. Because life on Earth exists whenever liquid water, organics, and energy coexist, understanding the chemical components of the emanating ice particles could indicate whether life is potentially present on Enceladus. One of the spectacular discoveries of the Cassini spacecraft mission (June 30, 2004–September 15, 2017) was the plume of water vapor and icy particles (dust) originating near the south pole of Enceladus. Gaining insight into Enceladus is particularly important in view of the presence of a subsurface organic-rich ocean hiding beneath its icy crust. The chapter begins with an introduction on glaring similarities and dissimilarities between the solar system’s planets Earth and Saturn. Importance of Cassini Spacecraft Mission in understanding Saturn and its two major moons (Enceladus and Titan) are discussed next. Role of tidal heating in maintaining liquid oceans inside cold planets and their moons has been given due importance. The curious finding of jets of water vapor and sodium-salt-rich ice particles spewing from Enceladus’ South Pole has been addressed in adequate detail. The Discovery of the presence of complex organic compounds in the subglacial water ocean of Enceladus—an important discovery in terms of the possibility of life on this moon—has been discussed. Mechanisms responsible for driving the cryovolcanic plume emission from the warm fractures in Enceladus have also been examined. Another curious finding addressed in this chapter is the possible presence of hydrothermal vents on the seafloor of Enceladus. Resemblance of Enceladus’s organics-rich ocean to Earth’s primitive prebiotic ocean has been briefly discussed in the background of its importance as a favorable scenario for life’s emergence on this distant icy moon. This chapter addresses the science goals and mission concept for future exploration of Enceladus. The chapter winds up with a discussion on an implementable mission concept, known as Enceladus Life Finder (ELF) Mission, to further explore Enceladus in the near future, and the End-of-Life Services by Astroscale (ELSA) mission concept for removal of orbital debris.