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      Quantitative evaluation of the feasibility of sampling the ice plumes at Enceladus for biomarkers of extraterrestrial life

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          Significance

          The search for organic biosignatures indicative of life elsewhere in our solar system is an exciting quest that, if successful, will have a profound impact on our biological uniqueness. Saturn’s icy moon Enceladus is a promising location for a second occurrence of life due to its salty subsurface ocean. Plumes that jet out through the ice surface vents provide an enticing opportunity to sample the underlying ocean for biomarkers. The experiments reported here provide accurate modeling of our ability to fly through these plumes to efficiently and nondestructively gather ice particles for biomolecular analysis. Our measured efficiencies demonstrate that Saturn and/or Enceladus orbital missions will gather sufficient ice to make meaningful measurement of biosignatures in the Enceladus plumes.

          Abstract

          Enceladus, an icy moon of Saturn, is a compelling destination for a probe seeking biosignatures of extraterrestrial life because its subsurface ocean exhibits significant organic chemistry that is directly accessible by sampling cryovolcanic plumes. State-of-the-art organic chemical analysis instruments can perform valuable science measurements at Enceladus provided they receive sufficient plume material in a fly-by or orbiter plume transit. To explore the feasibility of plume sampling, we performed light gas gun experiments impacting micrometer-sized ice particles containing a fluorescent dye biosignature simulant into a variety of soft metal capture surfaces at velocities from 800 m ⋅ s −1 up to 3 km ⋅ s −1. Quantitative fluorescence microscopy of the capture surfaces demonstrates organic capture efficiencies of up to 80 to 90% for isolated impact craters and of at least 17% on average on indium and aluminum capture surfaces at velocities up to 2.2 km ⋅ s −1. Our results reveal the relationships between impact velocity, particle size, capture surface, and capture efficiency for a variety of possible plume transit scenarios. Combined with sensitive microfluidic chemical analysis instruments, we predict that our capture system can be used to detect organic molecules in Enceladus plume ice at the 1 nM level—a sensitivity thought to be meaningful and informative for probing habitability and biosignatures.

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          Most cited references55

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          Cassini observes the active south pole of Enceladus.

          Cassini has identified a geologically active province at the south pole of Saturn's moon Enceladus. In images acquired by the Imaging Science Subsystem (ISS), this region is circumscribed by a chain of folded ridges and troughs at approximately 55 degrees S latitude. The terrain southward of this boundary is distinguished by its albedo and color contrasts, elevated temperatures, extreme geologic youth, and narrow tectonic rifts that exhibit coarse-grained ice and coincide with the hottest temperatures measured in the region. Jets of fine icy particles that supply Saturn's E ring emanate from this province, carried aloft by water vapor probably venting from subsurface reservoirs of liquid water. The shape of Enceladus suggests a possible intense heating epoch in the past by capture into a 1:4 secondary spin/orbit resonance.
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            Cassini ion and neutral mass spectrometer: Enceladus plume composition and structure.

            The Cassini spacecraft passed within 168.2 kilometers of the surface above the southern hemisphere at 19:55:22 universal time coordinated on 14 July 2005 during its closest approach to Enceladus. Before and after this time, a substantial atmospheric plume and coma were observed, detectable in the Ion and Neutral Mass Spectrometer (INMS) data set out to a distance of over 4000 kilometers from Enceladus. INMS data indicate that the atmospheric plume and coma are dominated by water, with significant amounts of carbon dioxide, an unidentified species with a mass-to-charge ratio of 28 daltons (either carbon monoxide or molecular nitrogen), and methane. Trace quantities (<1%) of acetylene and propane also appear to be present. Ammonia is present at a level that does not exceed 0.5%. The radial and angular distributions of the gas density near the closest approach, as well as other independent evidence, suggest a significant contribution to the plume from a source centered near the south polar cap, as distinct from a separately measured more uniform and possibly global source observed on the outbound leg of the flyby.
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              Liquid water on Enceladus from observations of ammonia and 40Ar in the plume

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                Author and article information

                Journal
                Proc Natl Acad Sci U S A
                Proc Natl Acad Sci U S A
                pnas
                PNAS
                Proceedings of the National Academy of Sciences of the United States of America
                National Academy of Sciences
                0027-8424
                1091-6490
                14 September 2021
                07 September 2021
                07 September 2021
                : 118
                : 37
                Affiliations
                [1] aSpace Sciences Laboratory, University of California, Berkeley , CA 94720;
                [2] bSchool of Physical Sciences, University of Kent , Kent CT2 7NH, United Kingdom;
                [3] cDepartment of Chemistry, University of California, Berkeley , CA 94720
                Author notes
                1To whom correspondence may be addressed. Email: butterworth@ 123456berkeley.edu .

                Edited by Jonathan I. Lunine, Cornell University, Ithaca, NY, and approved August 2, 2021 (received for review March 31, 2021)

                Author contributions: J.S.N., R.A.M., and A.L.B. designed research; J.S.N., B.K., and V.S. performed research; J.S.N., B.K., R.A.M., and A.L.B. analyzed data; J.S.N., R.A.M., and A.L.B. wrote the paper; and M.C.P. supervised light gas gun experiments and provided supporting modeling.

                Article
                202106197
                10.1073/pnas.2106197118
                8449353
                34493668
                1093a05f-6b76-4daf-a4cc-ccc954ee4926
                Copyright © 2021 the Author(s). Published by PNAS.

                This open access article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).

                Page count
                Pages: 6
                Product
                Funding
                Funded by: National Aeronautics and Space Administration (NASA) 100000104
                Award ID: 80NSSC17K0600
                Award Recipient : James S New Award Recipient : Bahar Kazemi Award Recipient : Richard A. Mathies Award Recipient : Anna L Butterworth
                Funded by: RCUK | Science and Technology Facilities Council (STFC) 501100000271
                Award ID: ST/S000348/1
                Award Recipient : Vassilia Spathis Award Recipient : Mark C Price
                Categories
                413
                Physical Sciences
                Earth, Atmospheric, and Planetary Sciences

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