Life Detection with the Enceladus Orbiting Sequencer Christopher E. Carr & Maria T. Zuber Gary Ruvkun Massachusetts Institute of Technology, Department of Massachusetts General Hospital, Dept. of Mol. Biology Earth, Atmospheric and Planetary Sciences Harvard Medical School, Dept. of Genetics Cambridge, MA 02139 Boston, MA 02114 [email protected], [email protected] [email protected] Abstract— Widespread organic synthesis in the early solar RNA or DNA on Mars due to the significant meteoritic nebula led to delivery of similar complex organics, probably transfer between Earth and Mars [1, 9-11]. However, the including nucleobases or their precursors, to many potentially ability to detect and sequence other nucleic acids and more habitable locations such as Mars, Europa, and Enceladus. If generic IPs would provide additional sensitivity. We review life evolved beyond Earth, the presence of these organics could prospects for extant life on Enceladus and describe an have biased life towards utilization of informational polymers (IPs) like RNA or DNA. Given this, searching for and instrument concept specifically geared to searching for life sequencing any such IPs offers a definitive, information rich, on this remarkable icy moon. approach to life detection that complements existing methods. Saturn’s icy moon Enceladus offers possibly the best conditions in the solar system to find extant life beyond Earth. 2. IS ENCELADUS HABITABLE? Recent discovery of a salt-water plume likely derived from sub-surface liquid reservoirs provides direct access to this A Brief History of Enceladus potentially habitable environment. We describe an instrument Enceladus is a volatile-rich (density 1.6 g/cm3) moon concept, the Enceladus Orbiting Sequencer (EOS), specifically geared to search for life on Enceladus. As a payload on board located in Saturn’s E-ring (Fig. 1A). Enceladus likely an Enceladus flyby or orbiter mission, EOS would capture ice formed from icy planetesimals produced in the solar nebula grains from the plume, then concentrate and characterize any that were then partially devolatilized during their long charged polymers using nanopore or semiconductor consolidation in the Saturn subnebula [12]. Its icy shell may sequencing. Searching for life on Enceladus could give us our cover a subsurface ocean, which may be in direct contact first glimpse of a second genesis and test whether biochemistry with its rocky core (Fig. 1B). Plume ice from Enceladus is varied or universal. feeds the Saturn E-ring, which has been observed to be stable since the 1960s. As Enceladus sweeps counter- TABLE OF CONTENTS clockwise around Saturn and through the ring, it collects irradiated ring particles and leaves a shadow (Fig. 1C). 1. INTRODUCTION ................................................. 1 2. IS ENCELADUS HABITABLE? ............................ 1 3. MISSION ARCHITECTURE ................................. 4 4. EOS INSTRUMENT CONCEPT ........................... 5 5. ICE GRAIN CAPTURE ........................................ 6 6. INFORMATIONAL POLYMER SEQUENCING ...... 6 7. SUMMARY AND CONCLUSIONS ......................... 8 ACKNOWLEDGEMENTS ......................................... 8 REFERENCES ......................................................... 8 BIOGRAPHY ........................................................ 10 1. INTRODUCTION Recent findings of nucleobases or their precursors on meteorites [1, 5] and in interstellar space suggest that life Fig. 1. Saturn's icy moon Enceladus. (A) Ice from beyond Earth could be based on nucleic acid-like polymers Enceladus forms Saturn's outermost ring. (B) Enceladus has such as RNA or DNA. Furthermore, complex organic a rocky core of radius 150-160 km, covered by 90-100 km synthesis is widespread in stellar nebulae [3, 6], likely of H2O. (C) Enceladus both creates and sweeps through the around all stars. As a result, common sources of organic E-ring. The bright dot is the plume, whereas the moon is a material were delivered to multiple potentially habitable tiny black region above the bright dot. (D) The surface of zones in our solar system, such as Mars, Europa, and Enceladus has geologically young, fractured (blue) regions Enceladus. This delivery could have biased the development and older, cratered regions. (E) Cassini identified the of life towards similar biochemical solutions such as Enceladus plume during a 2005 flyby. All images from utilization of informational polymers (IPs) like RNA and NASA Cassini mission (CICLOPS); panel A is contrast DNA. An even stronger case can be made to search for enhanced, B is artist's impression, D and E are false color. 978-1-4673-1813-6/13/$31.00 ©2013 IEEE 1 The pattern of surface modifications, from the heavily- cratered north to the fractured almost un-cratered south polar terrain (SPT, Fig. 1D, lower right), implies episodic geologic activity over a 4 billion year period [13]. Also indicative of geological activity is the moon’s limited topographic relief [14], which is unusual for small, low gravity (0.1 m/s2) satellites. While originally considered too small to be active, Cassini flybys identified the geologically young (<1 million years) SPT, associated with elevated temperatures, tectonic rifts, and ice jets [13] (Fig. 1E). Plume Characterization Fig. 2. Plume sources and likely structure. (A) The four Imaging of the SPT revealed four main subparallel furrows major sulci are associated with eight plume sources (yellow and ridges (sulci), each about 130 km in length with central points) and large heat anomalies (overlay). (B) Damascus two km wide depressions. These sulci are associated with Sulcus, relief 10x exaggerated. (C) Plume source model: I, heat anomalies and eight known plume sources (Fig. 2A-B). II, III indicate predominant ice grain type. Image credits NASA/JPL/GSFC/SwRI/SSI for A and B. Ground-based spectroscopic analysis of the plume, including gaseous components, argued for low sodium levels consistent with a deep ocean, a freshwater reservoir, or ice [15]. However, the Cassini Cosmic Dust Analyzer related to life on Earth, unless life on Earth and Enceladus (CDA) revealed a population of sodium-rich (0.5-2%) both derived from another (outer planets or beyond) source grains in the plume, consistent with a subsurface ocean [16] that had significant meteoritic transfer to both bodies. and an explanation for the lack of salt in the plume vapor: sodium salts would remain in the liquid phase as the water Liquid water?—Despite the generally frigid surface freezes out as part of the ice crust, with sublimation of this temperature (-200˚C) of Enceladus, several plume and crust yielding plume gas. Later flybys and analysis by the surface characteristics argue for a liquid water source for at CDA strengthened the case for a subsurface salt-water least part of the plume. First, the substantial heat output of reservoir [17], consistent with models of a subsurface ocean the SPT, at 16 GW [19], is most consistent with an elevated in contact with the moon’s rocky core (Fig. 2C). temperature source. Second, modeling of plume ice particle growth suggests source temperatures of at least 190K [18]. To date, observations and models argue for three general Third, the presence of sodium ice grains requires rapid types of ice grains in the plume [17]: Type I grains are freezing, consistent with quickly cooled liquid water. small, salt poor, and likely formed from nucleation of gas Fourth, the fracture patterns in the south polar terrain are phase water vapor, leading to high ejection speeds and best explained by a global liquid ocean [20]. Of note, explaining their dominant contribution to the E-ring. Type II methanogens in permafrost can stay metabolically active at - grains contain organic and/or siliceous material, might 20˚C by utilizing tightly bound unfrozen water [21]. indicate surface/ocean communication, and, speculatively, could be derived from solid clathrates [18]. More organics Essential nutrients?—Enceladus acquired organic material are observed in the center of the plume [17]. Type III grains as a consequence of its primordial origin, and by delivery of are salt-rich larger particles found in higher densities in the organics by comets and meteors. Some organic material has lower part of the plume, and presumably formed from a been definitively detected in the plume [17]. The organics rapidly frozen spray of salt water. They constitute 70% of available to any origin of life on Enceladus may have even the grains above 0.2 μm in size, have lower ejection speeds, included the building blocks of life as we know it (see Why and dominate the mass flux (99%). search for nucleic acids and related polymers? below). Extant Life? Energy source? — Any life at Enceladus is unlikely to use photosynthesis due to low light levels and the inability of If extant life exists on Enceladus, it either evolved there (a light to penetrate into the subsurface. Similarly, oxidative second genesis) or was carried there from somewhere else phosphorylation appears unlikely. The most likely model for (panspermia). Either way, life as we know it requires liquid any life on Enceladus is chemolithotrophy. water, essential nutrients (mainly made of the CHNOPS elements), and a source of energy (chemical or redox Ecosystems that apply to Enceladus (depending neither on gradient). If we are to posit a second genesis on Enceladus, photosynthesis nor on oxygen) include methanogens that it must also have had time to evolve. We address these consume H2 produced by serpentinization