International Journal of Astrobiology cambridge.org/ija Research Article Cite this article: Guzman M et al (2019). Collecting amino acids in the Enceladus plume. International Journal of Astrobiology 18, 47–59. https://doi.org/10.1017/ S1473550417000544 Received: 22 April 2017 Revised: 11 December 2017 Accepted: 11 December 2017 First published online: 28 February 2018 Key words: Amino acids; biomarkers; bubbles; Enceladus; plume Author for correspondence: Melissa Guzman, E-mail: melissa.guzman@ community.isunet.edu © Cambridge University Press 2018. This is a work of the U.S. Government and is not subject to copyright protection in the United States. Collecting amino acids in the Enceladus plume Melissa Guzman 1 , Ralph Lorenz 2 , Dana Hurley 2 , William Farrell 3 , John Spencer 4 , Candice Hansen 5 , Terry Hurford 3 , Jassmine Ibea 6 , Patrick Carlson 7 and Christopher P. McKay 1 1 NASA Ames Research Center, Moffett Field, CA 94035, USA; 2 Johns Hopkins Applied Physics Lab, Laurel, MD 20723, USA; 3 NASA Goddard Spaceflight Center, Greenbelt, MD 20771, USA; 4 Southwest Research Institute, Boulder, CO 80302, USA; 5 Planetary Science Institute, Tucson, AZ 85719, USA; 6 Evergreen Valley College, San Jose, CA 95135, USA and 7 University of California, Berkeley, CA 94720, USA Abstract We numerically model the dynamics of the Enceladus plume ice grains and define our nom- inal plume model as having a particle size distribution n(R) ∼ R -q with q = 4 and a total par- ticulate mass rate of 16 kg s -1 . This mass rate is based on average plume brightness observed by Cassini across a range of orbital positions. The model predicts sample volumes of ∼1600 μg for a 1 m 2 collector on a spacecraft making flybys at 20–60 km altitudes above the Enceladus surface. We develop two scenarios to predict the concentration of amino acids in the plume based on these assumed sample volumes. We specifically consider Glycine, Serine, α-Alanine, α-Aminoisobutyric acid and Isovaline. The first ‘abiotic’ model assumes that Enceladus has the composition of a comet and finds abundances between 2 × 10 -6 to 0.003 μg for dissolved free amino acids and 2 × 10 -5 to 0.3 μg for particulate amino acids. The second ‘biotic’ model assumes that the water of Enceladus’s ocean has the same amino acid composition as the deep ocean water on Earth. We compute the expected captured mass of amino acids such as Glycine, Serine, and α-Alanine in the ‘biotic’ model to be between 1 × 10 -5 to 2 × 10 -5 μg for dissolved free amino acids and dissolved combined amino acids and about 0.0002 μg for particulate amino acids. Both models consider enhancements due to bubble bursting. Expected captured mass of amino acids is calculated for a 1 m 2 collector on a spacecraft mak- ing flybys with a closest approach of 20 km during mean plume activity for the given nominal particle size distribution. Introduction The Cassini Mission discovered jets of fine icy particles lofted by water vapour and venting from rifts in the ice cover of Enceladus (Hansen et al. 2006; Porco et al. 2006; Spencer et al. 2006; Waite et al. 2006). This plume has been investigated extensively as the Cassini spacecraft passed through the plume multiple times over the course of a decade at elevations ranging from 50 to hundreds of kilometers and at relative velocities from ∼6 to 18 km s -1 . The data from these flybys indicate that the plume originates from a global ocean below an ice crust approximately 10–40 km thick (Iess et al. 2014; Thomas et al. 2016) and maybe even <5 km thick at the south pole region (Čadek et al. 2016). The plume contains organic compounds detected up to C6, the limit of the mass analysis instrument (Waite et al. 2006). Nitrogen is present in the form of ammonia (Waite et al. 2009) and amines (Postberg et al. 2015) and sulphur in the form of hydrogen sulfide (Waite et al. 2009). Sodium detected in the particles indicate that the ocean salinity is about 0.5 to 2% dominated by NaCl, with lower levels of K present – indicating a water activity suitable for life. Nanometer-sized silica particles detected in the E ring of Saturn, and which are derived from Enceladus, indicate that the ocean contains hydrothermal vents. Enceladus’s ocean is in contact with the rocky core at temperatures of ∼100°C and the pH of the ocean is estimated between 8.5 and 10.5 (Hsu et al. 2015; Sekine et al. 2015; Waite et al. 2017). All the elements needed for life (C,H,N,O,P,S) except P have been detected. Phosphorus has been reported in comets (Altwegg et al. 2016) by mass spectrometry detection of the element as m/z 31. Phosphorus is expected to be present in the Enceladus ocean due to rock–water interactions. Although the Cassini Ion Neutral Mass Spectrometer (INMS) was sensitive to mass 31, the mass resolution was not adequate to separate P from other materials with m/z 31. Redox energy sources essential for life below the thick ice have not yet been fully elucidated but CO 2 and H 2 have been detected (Waite et al. 2009, 2017) which form a suitable redox couple for methanogens. Thus, there is every indication that the ocean on Enceladus is habitable and that the icy particles in the plume are samples of that habitable water. Dynamical and compositional models of the micron-sized icy particles in Saturn’s E ring and the Enceladus plume have been developed from Cassini’s Cosmic Dust Analyzer https://doi.org/10.1017/S1473550417000544 Downloaded from https://www.cambridge.org/core. IP address: 3.226.242.225, on 24 Nov 2021 at 22:17:06, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms.