Quantitative Planetary Protection for Sample Return from Ocean Worlds
Abstract
Volcanism on ocean worlds [1,2] facilitates ocean sample return missions, enabling uniquely flexible, sensitive, and specific laboratory analyses on Earth to study how far chemistry has evolved in presumably habitable oceans [3,4]. Such mission concepts have yet to quantitatively address planetary protection (PP) for ocean worlds [3,4]. These harbor liquid water [5,6], metabolically useful energy [7], and organic matter to support life [8]. Ocean temperatures may not exceed the limit for life as we know it [9,10], they are shielded from exogenic radiation by kilometers of ice, and their material has likely not been naturally exchanged with Earth [11]. The above factors would place sample return missions in Cat. V - Restricted Earth Return [12,13]. Forward PP requirements for Europa [13] and other ocean worlds [14] require that the probability of "introduction of a single viable terrestrial microorganism into a liquid-water environment" be lower than 10 ^{-4}. This probability should be estimated from (F1) "bioburden at launch," (F2) "cruise survival for contaminating organisms," (F3) "organism survival in the radiation environment adjacent to the target," (F4) "the probability of encountering […] the target," (F5) "the probability of surviving landing/impact on the target," (F6) "mechanisms and timescales of transport to the subsurface," and (F7) "survival […] after subsurface transfer" [13,14]. The compliance of specific designs of known cost could be evaluated from measurements of molecular contaminants as robust and universal proxies for microbial particulates [15] (F1); known microbial radiation tolerance [16] and planetary radiation budgets [17] (F2-F3); trajectory design (F4); projected impact velocities [18] (F5); ice transport timescales [19] (F6), and biomass growth rates in ice [20] (F7). In contrast, current backward PP requirements are only qualitative. Current policy [13,15] prohibits "destructive impact upon return," and requires that (B1) "unless the sample [undergoes] sterilization […], the sample container must be sealed [via] fail-safe containment with a method for verification of its operation before Earth return;" (B2) "for unsterilized samples, […] containment […] shall be maintained until [sample transfer to] an appropriate receiving facility;" (B3) "a method to 'break the chain of contact' with the target" be specified; (B4) "no uncontained hardware that contacted the target […] shall be returned to Earth;" (B5) "reviews and approval of the continuation of the flight mission shall be required [prior to] launch from Earth; leaving the target for return to Earth; and commitment to Earth reentry;" and (B6) "life detection and biohazard testing, or a proven sterilization process, shall be [a] precondition for the controlled distribution of […] the sample." These provisions and their means of evaluation could be quantified. A maximum leakage rate could be specified for particles above 10 nm (the size of prions, the smallest known pathogens [21]) (B1-B2), even for terminal velocity impact, whether unintended or otherwise (minimizing the risk of failure of reentry system elements, but requiring preservation of the samples and of sensors monitoring their thermal and structural integrity). For leak detection, He is commonly used [22], but its van-der-Waals radius of 0.14 nm could place too stringent a constraint for containment of pathogens over 70 times larger. To meet (B3)-(B4), uncontained parts in contact with ocean world material could be jettisoned prior to reentry with maximum probabilities of Earth/Moon impact, or of microbial survival upon reentry. (B6) could require either life detection prior to or after opening the sealed container [16,23-25], or sterilization [26]. Further guidance on (1) evaluating the 10 ^{-4} probability of forward contamination, (2) possibly extending this probability to ocean worlds other than Europa, (3) quantifying backward PP requirements, and (4) assessing the compliance of specific designs, would be a crucial step towards ocean world sample return missions, which are uniquely poised to inform us about the likelihood of life on other worlds. A possible path to setting policy could involve tasking experts (including COSPAR PP Panel members) to recommend quantitative means of evaluating provisions (F1)-(F7) and (B1)-(B6), and whether to include (B3)-(B5) in the COSPAR PP Policy [13]. The ongoing European Planetary Protection of Outer Solar System effort, seeking to "provide science and policy recommendations for the definition, improvement, and implementation of an adequate [PP] policy for outer solar system bodies" [27], may achieve this task. References: [1] Porco et al. (2006) Science 311, 1393. [2] Roth et al. (2014) Science 343, 171. [3] Tsou et al. (2012) Astrobiol 12, 730. [4] McKay et al. (2014) Astrobiol 14, 352. [5] Kivelson et al. (2000) Science 289, 1340. [6] Thomas et al. (2016) Icarus 264, 37. [7] Hsu et al. (2015) Nature 519, 207. [8] Waite et al. (2009) Nature 460, 487. [9] Takai et al. (2008) PNAS 105, 10949. [10] Sekine et al. (2015) Nature Comm 6, 8604. [11] Worth et al. (2013) Astrobiol 13, 1155. [12] Space Studies Board (1998) doi: 10.17226/6281. [13] Kminek and Rummel (2015) Space Res Today 193, 7. [14] NASA NPR 8020.12D (2011). [15] Summons et al. (2014) Astrobiol 14, 969. [16] Mileikowsky et al. (2000) Icarus 145, 391. [17] Hand et al. (2007) Astrobiol 7, 1006. [18] Nicholson (2009) Trends Microbiol 17, 243. [19] Showman and Han (2005) Icarus 177, 425. [20] Price and Sowers (2004) PNAS 101, 4631. [21] Silveira et al. (2005) Nature 437, 257. [22] Younse et al. (2012) doi: 10.1109/AERO.2012.6187048. [23,24] Yano et al. (2016a,b) 41 ^{st COSPAR Sci. Assy.} (this volume). [25] Takano et al. (2014) Adv Space Res 53, 1135. [26] Daspit et al. (1975) Acta Astro 2, 649. [27] INAF (retrieved 2/19/2016) PPOSS. http://www.inaf.it/it/sedi/sede-centrale-nuova/direzione-scientifica/relazioni-internazionali/pposs.
- Publication:
-
41st COSPAR Scientific Assembly
- Pub Date:
- July 2016
- Bibcode:
- 2016cosp...41E1420N