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Life on the Icy Moons and Ocean Worlds of the Outer Solar System
The following essay was contributed by space systems engineer Kostas Konstantinidis
A reading of this essay is featured on the Metaculus Journal Podcast here.
The search for life elsewhere in the universe is one of the driving forces behind space exploration. Its discovery would be one of the great scientific discoveries of the 21st century.
Some of the strongest candidates for hosting life in the present day that are closest to us are the so-called icy moons (also known as the “ocean worlds”) orbiting around the gas giant planets in the outer solar system. By closely investigating these moons we could discover a second genesis of life, or at least constrain the conditions under which life could emerge and thrive.
This essay is grounded by the following forecast questions asking:
Geology and habitability
The icy moons are bodies varying in size from small, such as Enceladus, to Titan, which is larger than the Earth’s moon. They generally comprise a rocky core covered by a layer of water ice. For many of them, the tidal pull of the main planet generates enough heat to maintain an ocean of liquid water under that ice cover. In cases where this ocean is assumed to be in direct contact with the rocky core, such as Jupiter’s moon Europa and Saturn’s moon Enceladus, the basic habitability requirements for supporting microbial life are seemingly fulfilled: liquid water, energy sources to sustain metabolism, and chemical compounds that can be used as nutrients over a period of time long enough to allow the development of life.
Emergence of life and how it looks
Given that these basic conditions are fulfilled, the most plausible mechanism for the occurrence of life on the icy moons is that it could emerge around hypothesized hydrothermal vents at the bottom of their oceans. (A similar hypothesis has been made about the emergence of life on Earth.) Such life would eschew our more familiar photosynthesis as an energy source for a chemosynthetic process. Life discovered on the icy moons would very likely represent a second genesis, as it would be practically impossible for life to migrate there from the inner solar system as the panspermia hypothesis suggests. Any life there would most likely be microbial, although there is some debate as to whether macrofauna could be sustained in these environments.
The search for extraterrestrial life will not be straightforward, as life itself is hard to define and we are moreover constrained by “life as we know it.” The search for that life will necessarily take a reductive approach, searching for signs of the basic characteristics that such life must possess, such as evidence of metabolism and replication and cellular structures.
Where to look for it
Once life is created near the vents, it can then migrate and populate other hospitable niches near the vents in the ocean body and on the ocean-ice shell interface. From there, life and its signatures can be transferred closer to the surface by various mechanisms, such as via channels ejecting plumes of ocean materials to space, as observed on Enceladus, or via less dramatic glaciological processes where material from the ocean is transported to the surface over geologic time scales.
Mission concepts to look for life
Several early space mission designs (also known as mission concepts) exist to search for life and its signatures in these environments. Due to their relatively high potential, these missions tend to focus on Europa and Enceladus.
A first mission concept dedicated to the direct search for biosignatures would fly through the plumes of ocean materials to analyze them on-the-spot—such as ELF, or Enceladus Life Finder. A similar but more ambitious concept would collect a sample from the plumes and return it to Earth for analysis in dedicated laboratories, e.g., LIFE, or Life Investigation For Enceladus.
Sampling the surface and the near subsurface can allow the detection of biosignatures deposited by the plumes or by glaciological processes. Planetary landers such as the Europa Lander concept currently planned by NASA would allow for such sampling.
Water pockets and chambers under the ice and along the plume channels are particularly interesting targets, as they might host better quality biosignatures—or even active microbial ecosystems. Missions to probe them would consist of a landing element and a secondary probe to access the water pockets, either by melting a few hundred meters through the ice or by rappelling down the plume channel to reach liquid water.
The subglacial ocean is the potential source and host of currently existing microbial life on the icy moons. To gain access to the ocean, a subglacial probe would have to go through the entire ice shell, which is at least a few kilometers thick. It could then deploy probes to explore the ice-ocean interface or deploy a submersible to navigate several kilometers to the bottom where it could locate a vent.
Technologies and challenges
For these increasingly ambitious mission concepts a host of technology needs and challenges appear: New life detection instruments should be developed for the detection of life signs that go beyond the state of the art.
Also necessary would be advanced landing systems beyond what have already been used for landing on the Moon and Mars, which would allow safe and accurate landing on very challenging terrain on icy moons.
Subglacial mobility comes with its own set of challenges. Mobility can be provided by melting the ice via heat, as mechanical drilling alone is inefficient on the ultra-cold ice presented by these moons. Nuclear energy sources are critical for providing the energy needed for melting the ice. Nuclear sources on-board melting probes can be used directly to melt the ice in an efficient manner. Alternatively, nuclear energy sources can remain on the surface and can provide electrical power to the probe via a tether, making the melting probe more maneuverable.
Submersibles to access the ocean must also be developed. The main challenge here is arguably the very high level of autonomy needed for submersible operations: A submersible must autonomously navigate the ocean, search for life and its signatures, and communicate its findings back to Earth. Communicating wirelessly through kilometers of ice also poses significant difficulties.
Planetary protection regulations designed to protect any target alien life and the Earth itself present another hurdle. Microbial life from the Earth can remain on a spacecraft after launch and withstand the radiation and vacuum of space for years. Significant effort goes toward identifying and limiting the bioload that remains on a spacecraft or lander that is to be sent to a sensitive area. The cleanliness requirements for any mission to a potential icy moon habitat will be much stricter than any past mission. Such regulations might also constrain the use of nuclear power sources in specific areas of the icy moons. Conversely, any sample returned to the Earth must also fulfill strict isolation requirements during atmospheric entry and subsequent handling.
Timelines and schedules
A consistent approach looking for life in the icy moons would first establish the habitability conditions of the icy moons, and would then look for indications of biosignatures, before eventually investigating any alien life, in its own habitat, in the icy moon’s ocean.
Past missions such as Cassini orbiting Saturn and Galileo’s study of Jupiter have already established the icy moons as a key target in the search for life in our solar system. NASA’s Europa Clipper and ESA’s JUICE—scheduled to be launched this decade and to arrive in the Jupiter system in the early 2030s—will further establish the habitability of icy moons, and in particular Europa.
The progressively ambitious concepts presented above are in the early definition phases and do not yet have a set launch date. Existing strategic planning studies foresee two guiding approaches: On the one hand, a “stepwise” approach would require waiting for the scientific results from one mission before moving on to the implementation of the next concept in the mission chain. This would require most of this century to execute fully. The “adventurous” approach would instead create a dedicated Ocean Worlds Exploration Program similar in philosophy to the Mars Exploration Program, which would implement missions in parallel paths and support the systematic development of necessary technologies. This approach might take only half as long as the former, but would depend on the willingness of agencies to implement it.
The mechanism by which life emerges is still unknown and estimates for its probability are therefore difficult to make. The search for extant life in the solar system—and the parallel endeavours searching for extinct life and for exoplanet life—will ultimately help us constrain our estimates.
I predict that life will indeed eventually be found where conditions are hospitable; further, I forecast that these conditions will be present on Europa and Enceladus at minimum. Finally, I tend toward an optimistic view that the search for life has the potential to become a guiding goal behind solar system exploration, and thus that a dedicated and systematic effort will be made by space agencies.
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