Exploration of Enceladus^|^apos; Water-Rich Plumes toward Understanding of Chemistry and Biology of the Interior Ocean

Abstract

Enceladus is the only icy satellite known to exhibits on-going geological activity of water-rich plumes derived from the interior ocean. Here, we propose a sample return and in-situ measurement mission for Enceladus' plume materials. Depending on the cost, mission duration, and propulsion system, we propose three types of missions to Enceladus; type 1: free-return trajectory, type 2: trajectory orbiting Saturn, and type 3: trajectory orbiting Enceladus. Type 2 and 3 are preferable to type 1 in order to achieve lower encountering velocity to the plumes (> 4 km/s and 0.2 km/s for type 2 and 3, respectively) and, thus, to collect multiple and intact samples. High resolution mass spectroscopy of the gas components will provide essential information to understand the physical and chemical conditions of both the interior ocean and the solar nebula. Furthermore, detailed onboard and onshore analyses of returned samples could provide geochemical, preboplogical, and, potentially, biological context in the interior ocean of Enceladus.

Returning Samples From Enceladus for Life Detection

“…Notably, these measurements can address more than the driving question of life detection. For example, isotopic measurements could ascertain the origin of Enceladus or the materials it accreted (Sekine et al, 2014). More generally, these measurements would establish how far prebiotic chemistry has progressed inside Enceladus and quantify its habitability, thereby ensuring meaningful results even in the case of negative life detection.…”

“…Samples can be collected in one or repeated passages through the plume. In order of complexity, architectures comprise a single fly-through on a Sun-orbiting ("free return") trajectory, a Saturn orbiter with Cassini-like fly-throughs steered by Titan gravity assists, and an Enceladus orbiter (Sekine et al, 2014). The traded quantities are mission duration, v (fuel mass, cost), and sampling velocity, whereas sampling altitude is set by navigation uncertainty in all cases.…”

“…To search for life in the plume, mission concepts ranging from in situ analyses (Lunine et al, 2015;MacKenzie et al, 2016;Eigenbrode et al, 2018), to orbiting or landing (MacKenzie et al, 2020), to sample return (Tsou et al, 2012;Sekine et al, 2014) have been proposed or discussed, as steps on a path (Sherwood, 2016) toward in situ characterization of subsurface liquid (Konstantinidis et al, 2015). In addition to scientific interest, recent political will in the United States has spurred roadmapping of the exploration of "ocean worlds" (Enceladus is a prime example) to search for life (Hendrix et al, 2019).…”

“…In this paper, we focus on the return of a plume sample, following up on Sekine et al (2014) and Takano et al (2014). First, we briefly review the evidence on Enceladus' habitability and rationalize the value of sample return for life detection.…”

Key Technologies and Instrumentation for Subsurface Exploration of Ocean Worlds

“…Plume-sample return mission concepts like LIFE by Tsou et al (2012) and several concepts studied by Sekine et al (2014) mention the importance of the flyby velocity for the intact collection of plume materials. They propose to minimize the Enceladus flyby velocity for a Saturn orbiter to about 3-4 km/s.…”