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Astrobiology eXploration at Enceladus: Looking for Life on an Icy Moon

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photograph of water ice plumes on Enceladus
Plumes of salty water ice emerge from Enceladus's cracked ice shell. [NASA/JPL/Space Science Institute]

Saturn’s moon Enceladus is a promising place to look for life in our solar system. A recent research article introduces the Astrobiology eXploration at Enceladus mission concept, which aims to examine the icy moon’s habitability and geological history.

A Promising Target

visible-light and infrared images of Enceladus

Composite optical and infrared images of Enceladus taken at various angles. The “tiger stripes” from which the plumes originate overlap with a warm region at the moon’s south pole. [NASA/JPL-Caltech/University of Arizona/LPG/CNRS/University of Nantes/Space Science Institute]

When the Cassini spacecraft examined the Saturn system from 2004 to 2017, it made surprising discoveries about Enceladus, a tiny, frosty moon that holds the title of most reflective object in the solar system. Enceladus’s sparkling ice shell is marred by long fissures, from which jets of water ice spray. That in and of itself is already exciting, but Cassini’s instruments picked up on something even more thrilling: the plumes are salty and contain organic molecules. Some of these molecules appear to be chunks of larger, more complex molecules that broke apart as they entered Cassini’s instruments.

Further data suggested that Enceladus’s plumes emanate from a global ocean that sloshes beneath the moon’s icy surface, warmed by tidal stresses from Saturn’s gravitational pull. The combination of water, organic molecules, and heat makes Enceladus an enticing target in the search for life beyond Earth. But the Cassini mission is far in our rear-view mirror — where do we go from here?

Mission to an Icy Moon

Long before spacecraft drop into orbit around planets or pass by moons, long before launch or loading or assembly, teams of scientists craft detailed mission plans that outline what the mission will accomplish and how. Today’s research article introduces one such plan developed during a session of the Jet Propulsion Laboratory’s Planetary Science Summer School, which invites teams to put their heads together to devise missions to explore the solar system. (Astrophysicists and solar physicists have their own versions of this program, too!)

illustrations of Enceladus's plumes and surrounding regions

Illustration of Enceladus’s plume environment and the specific regions targeted by AXE. Click to enlarge. [Adapted from Seaton et al. 2023]

Marshall Seaton (Jet Propulsion Laboratory) and collaborators concocted an Enceladus-exploration mission called Astrobiology eXploration at Enceladus (AXE): a New Frontiers–class mission (i.e., costing about a billion dollars) that would take our understanding of Enceladus to a whole new level.

From Idea to Implementation

AXE has some big goals, like figuring out if there’s life in Enceladus’s subsurface ocean. For a mission proposal to be successful, it has to outline exactly how that mission will achieve its goals. This means laying out the physical quantities the instruments must measure, and how precisely the measurements must be made. It even has to wrestle with possible complications, like instrument degradation or failure.

plots showing how biotic and abiotic processes have different molecular abundance distributions

Demonstration of how the abundance of molecules differs between biotic processes (i.e., life) and abiotic processes. Click to enlarge. [Seaton et al. 2023]

As an example, in order for AXE to achieve its objective of determining whether the organic molecules sprayed out in Enceladus’s plumes are the result of life in the oceans, it must carry a mass spectrometer capable of detecting chemical compounds with masses between 2 and 600 atomic mass units down to a concentration of just ten parts per billion. Despite these exacting requirements, Seaton and collaborators have determined that AXE should be able to achieve its lofty science goals using just 30 flybys worth of data. Choosing multiple flybys rather than an orbiter or lander (what’s known as “mission architecture”) simplifies certain aspects of the mission.

As you can imagine, there’s far more involved in crafting a mission plan than can be included in this short summary — be sure to check out the full article linked below to learn more!

Citation

“Astrobiology eXploration at Enceladus (AXE): A New Frontiers Mission Concept Study,” K. Marshall Seaton et al 2023 Planet. Sci. J. 4 116. doi:10.3847/PSJ/acd119