Cassini
revealed that Saturn's Moon Enceladus hosts a subsurface ocean that meets the accepted criteria for habitability with bio-essential elements and compounds, liquid water, and energy sources available in the environment. Whether these conditions are sufficiently abundant and collocated to support life remains unknown and cannot be determined from
Cassini
data. However, thanks to the plume of oceanic material emanating from Enceladus’ south pole, a new mission to Enceladus could search for evidence of life without having to descend through kilometers of ice. In this article, we outline the science motivations for such a successor to
Cassini
, choosing the primary science goal to be determining whether Enceladus is inhabited and assuming a resource level equivalent to NASA's Flagship-class missions. We selected a set of potential biosignature measurements that are complementary and orthogonal to build a robust case for any life detection result. This result would be further informed by quantifications of the habitability of the environment through geochemical and geophysical investigations into the ocean and ice shell crust. This study demonstrates that Enceladus’ plume offers an unparalleled opportunity for
in situ
exploration of an Ocean World and that the planetary science and astrobiology community is well equipped to take full advantage of it in the coming decades.
We recommend the establishment of a dedicated Ocean Worlds Exploration Program within NASA to provide sustained funding support for the science, engineering, research, development, and mission planning needed to implement a multi-decadal, multi-mission program to explore Ocean Worlds for life and understand the conditions for habitability. The two new critical flagship missions within this program would 1) land on Europa or Enceladus in the decade 2023-2032 to investigate geophysical and geochemical environments while searching for biosignatures, and 2) access a planetary ocean to directly search for life in the decade 2033-2042. The technological solutions for a landed mission are already in-hand, evidenced by the successful delta-Mission Concept Review of the Europa Lander pre-flight project in the fall of 2018. Following an initial landed mission, an ocean access mission will require substantial research, development, and analog testing this decade to enable the initiation of a pre-flight project at the start of the following decade. This ambitious goal could not have been put forward with comparable, requisite credibility in any preceding decade. Both science and technology have now matured so that we may prioritize the direct search for signs of life, applying lessons learned from planetary and Earth exploration to in situ investigations of worlds where water and life are most likely to exist today. This new era of planetary investigation will require significant support for cross-disciplinary research and development, bringing together planetary scientists and engineers with those that study the Earth. To chart these new waters, a diversity of disciplines must be accompanied by a diversity of experience, skill, and perspective. All facets of this Ocean Worlds Exploration Program must actively ensure equitable access and inclusion across the spectrum of contributors in Planetary Science, Exploration, and Astrobiology. Hence, we recommend an Ocean Worlds Exploration Program that includes specific initiatives designed to entrain and retain scientists and technologists from historically under-represented groups and to support a healthy work-life balance that enables equitable participation from across the entire community.
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