PENGUINS-ADELiE: A Mission to Investigate Europa s Subsurface Ocean

Transcription

1 1 PENGUINS-ADELiE: A Mission to Investigate Europa s Subsurface Ocean Portable EuropaN organism-detection and Under Ice Navigation Systems and Autonomous Deployable Europan Life Explorers Melissa Harnois Taylor Bobowski Laci Brock ASTRONOMY 475/575 Planetary Astrobiology Mission Design Proposal April 19, 2016

2 2 Introduction Though humanity s desire to search for non-terrestrial life within our Solar System has always been great, our ability to conduct such a search has not been possible until recently. Numerous potential harbors of primitive life exist in the Solar System, providing a wealth of possibility for future astrobiological explorations. Investigations of extreme environments on these planetary bodies are constrained by current technological capabilities and feasibility. To begin any investigation, rigorous criteria must be put in place in order to focus and explore the most viable locations. A variety of potential criteria for habitability have been proposed over the past few decades; the most common constraints include the need for an energy source, liquid solvent (e.g., water), and the elements necessary to build organic compounds. These constraints, in part, help to define the requirements for life as we know it. Though the most likely location for life is argued to be within the Circumstellar Habitable Zone (CHZ) (i.e., a Sun-like star with an Earthlike planet), the ingredients for life as we know it have been discovered outside of this region and within our own Solar System. Icy moons of the Jovian planets seem like unlikely locations for life. The majority of these moons have little-to-no atmosphere, are constantly bombarded by high levels of radiation, and are over five times farther from the Sun than Earth. However, there is evidence that Jupiter s moon, Europa, contains a vast liquid water ocean beneath a thick layer of ice and may host a primitive ecosystem. Thus, the potential discovery of non-terrestrial life on Europa is of particular interest to the scientific community. We propose the PENGUINS-ADELiE mission an in situ lander designed to penetrate Europa s icy shell and test for biotic signatures in the moon s subsurface ocean. Scientific Motivation Of the four Galilean moons of Jupiter, Europa is of particular interest for astrobiological exploration and may possess conditions favorable for life. A number of flyby missions have collected data suggesting Europa harbors a subsurface ocean beneath a debated thickness of ice. The earliest observations were conducted by Pioneers 10 and 11 in 1973 and 1974, respectively, but were low-resolution and provided little information of the moon s active surface. Voyagers 1 and 2 flew by later in 1979 and were able to capture higher quality images that hinted at an icy, young surface. Europa s surface appeared smooth, lacked impact craters, and exhibited signs of tectonic deformation driven by on-going geological activity (Pappalardo et al., 1999). Europa s unusual surface morphologies were unlike those of any other icy satellite. These patterns in the Europan crust could not be explained by tidal interactions with Jupiter and orbital resonances with Io and Ganymede alone. After subsequent theoretical calculations and more recent observations from the Galileo spacecraft in 1995, gravity data suggest a differentiated layer of some combination of liquid and/or solid water-ice up to 100 km in depth, which would be twice the size of Earth s oceans (Pappalardo et al., 1999), and is believed to contribute to the observed surface features. Although evidence strongly supports a Europan subsurface ocean, the thickness of the upper ice layer and lower liquid layer are poorly constrained. Models of Europa s interior structure have suggested an outer layer of brittle ice, an underlying ductile layer of convecting ice, and a lower layer of liquid water (Billings & Kattenhorn, 2005). Thickness estimates of the upper ice layers have ranged from a thick-ice model with a km layer of ice, whereas other estimates support a thin-ice model with <10 km of ice (Billings & Kattenhorn, 2005; Pappalardo

3 3 et al., 1999). A thin-ice layer would more easily support a Europan ecosystem by allowing surface-to-ocean mixing and supply enough free energy for potential organisms (Chyba & Phillips, 2001). Mixing could be induced by tidal interactions with Jupiter. For example, Gaidos and Nimmo (2000) argued that tidal stresses from Jupiter could crack Europa s surface to allow material from the interior to reach the surface, but this model required a thickness of ice of 1 km or less. A thicker ice shell seems more likely according to models regarding Europa s interior convection, thermal equilibrium, and temperature gradients (Billings & Kattenhorn, 2005). However, a thick ice shell does not rule out the possibility for life on Europa. Subduction via cryotectonic processes is one potential pathway for surface-to-interior interactions of water (Kattenhorn & Prockter, 2014). Perhaps the most promising prospect, though, is the presence of shallow subglacial lakes analogous to Earth s Antarctic environments. Schmidt, Blankenship, Patterson, and Schnek, (2011) argue subglacial lakes exist separately from the subsurface ocean and could explain the formation of Europa s unique surface features called chaotic terrains. Their model is thermodynamically probable and suggestive of a thin-ice layer of only 3 km in these regions. Exploration of the Europan surface and subsurface ocean would provide key insights into the moon s composition, chemistry, and geophysical processes that may be conducive to life. A Europan lander could constrain the available ingredients and further evaluate the habitable nature of this icy world. Mission Specifications We propose a mission for in situ exploration of the Europan surface and sub-surface ocean to help constrain potential habitability of icy moons falling outside the realm of the Circumstellar Habitable Zone (CHZ). To accomplish this objective, a set of two coupled systems containing a cryobot and hydrobot analogous to deep sea and Antarctic exploration (e.g., ARGO/JASON (Ballard, Yoerger, Stewart, & Bowen, 1991), VALKYRIE/ARTEMIS (Stone, Hogan, Siegel, Lelievre, & Flesher, 2014) will be utilized. PENGUINS 1 and 2 (Portable EuropaN organism-detection and Under Ice Navigation Systems) are cryogenically-based, heated submersible vehicles that will penetrate Europa s icy shell to reach liquid water and deploy ADELiE 1 and 2 (Autonomous Deployable Europan Life Explorers), autonomous hydrodynamic ocean explorers. Each unit system will travel to different locations (and potentially differing depths) of the Europan subsurface in order to reach the liquid ocean below and test for evidence of life. Landing Site Selection of a landing site is of utmost importance in order to successfully proceed with the scientific objectives of this mission. After reconnaissance from several low-orbit flybys, we conclude the most probable location for astrobiological investigation is where the ice appears less thick and liquid-to-surface interactions are likely occurring. These regions may host environments conducive to life analogous to Earth s Antarctic and Siberian subglacial ecosystems (see e.g., Murry et al., 2012). Target locations are in geologically active regions of chaotic terrain asymmetrical cracks and ridges that scar the Europan surface in which previous studies have suggested active cryotectonics (Kattenhorn & Procker, 2014) and convective upwelling of the subsurface ocean (Schmidt, Blankenship, Patterson, & Schnek, 2011). Target landing sites must also consider the intense radiation environment on Europa s surface due to interactions with Jupiter s magnetic field. Patterson, Paranicas, and Prockter

4 4 (2012) analyzed electron bombardment on the Europan surface, and their data suggest landing sites for both spacecraft viability and potential habitability are on Europa s trailing hemisphere near the polar regions. Therefore, our top candidates for landing sites are two regions of chaotic terrain: Thera Macula (50 S, 180 W) and Thrace Macula (45 S, 171 W). Mission Payload and Instrumentation Each PENGUINS-ADELiE cryobot/hydrobot unit will include a radiation-shielded suite of advanced scientific instrumentation to accomplish its primary mission objective: to isolate DNA, detect nucleic acids, and perform basic tests to determine the presence of active photosynthetic organisms. Table 1 provides a breakdown of instrumentation and payload mass. The mission will be accomplished in a two-part procedure by first investigating and analyzing the moon s surface for microorganisms, and second by penetrating the ice layer to detect microorganisms in the subsurface ocean. Each unit system is powered by a radioisotope thermoelectric generator (RTG) similar to that of the successful Mars Curiosity rover. Solar power is an inefficient choice for a submersible vehicle designed to travel beneath the ice. RTGs will supply power to PENGUINS-ADELiE via radioactive decay of Plutonium-238 (t1/2=87.7 years), which are estimated to produce ~110 W of continuous power and have the added ability to charge lithium-ion batteries for stored energy (Holgate et al., 2015). The cryobots, PENGUINS 1 and 2, are each equipped with an autonomous laboratory unit (ALU) and will utilize robotic arms to collect a sample of the Europan surface to test for evidence of metabolic byproducts. After surface testing concludes, PENGUINS will begin descent into the icy shell to reach and investigate the subsurface ocean. Vertical descent is driven by thermal energy in order to melt through the ice and will inherently be gravity-assisted from Europa. Upon arriving at Europa s liquid ocean layer, PENGUINS terminates its descent; hydrobots, ADELiE 1 and 2, are deployed from their respective cryobot. Each ADELiE is small enough to fit within PENGUINS but large enough to contain sufficient instrumentation to carry out the mission objective. ADELiE units each possess their own ALU to obtain and test a sample of water for the presence of photosynthetic organisms. ADELiE units are tethered to their respective PENGUINS units for communications and power via by high-energy lasers and fiber optic cables. Such cryobot technology is successfully being tested in Antarctica and has been suggested for exploration of icy moons like Europa (Stone et al., 2014). Table 1. PENGUINS-ADELiE Instrumentation and Payload Mass Instrumentation Mass (kg) Radioisotope Thermoelectric Generator 2 x 60 Plutonium Pellets 2 x 30 Submersible Vehicle 2 x 250 Autonomous Laboratory Unit 4 x 200 Robotic Arm 2 x 150 Pump 2 x 100 Total Mass: 1980

5 5 Mission Objectives Search For Photosynthetic Life The main objective will be to obtain and use two liquid samples from Europa s ocean to determine the presence of nucleic acid and photosynthetic life. ADELiE 1 and 2 are fully equipped, each with an ALU, to perform multiple biochemical assays as described in detail below. DNA Isolation The first test requires collection of 25 ml of water from the surrounding environment. The 25 ml will be evenly separated into twenty 1 ml samples (25 ml to allow for 5 ml overage in case of pipetting errors). Any cells present in the samples will be broken open by adding ß- mercaptoethanol and lysis buffer. The samples will then be shaken gently at 24ºC for 5 minutes. Next, a Co-Precipitant Linear Polyacrylamide, known as Pink, will be added and each sample will then be treated with a series of elution buffers. After each buffer, the samples will be filtered through a fiberglass membrane by centrifugation. Pink will bind very small amounts of nucleic acid which will help minimize loss of any isolated nucleic acid through the membrane pores (Kappel et al., 2012). Once the sample has been collected on the membranes, 100 µl of nuclease-free water will be added to each membrane surface and all samples will be centrifuged again. Finally, all samples will be recombined into one tube. Experiment I The total sample will be used to determine the presence of nucleic acid which will begin with continual heating (to above 95ºC). As the sample is heated, spectroscopy will be used to take multiple readings at 260 nm. There are three possible outcomes. First, since nucleic acids are comprised of bases (A, T, G, C, *U in RNA) which absorb light at 260 nm, we will know that double-stranded DNA is present in the sample if the absorbance at 260 nm increases with temperature. This is because increasing the temperature will eventually denature the duplex form of DNA, exposing the bases, and therefore show increasing absorbance levels (Karl, 2009). Another possible outcome is that we observe no absorbance in our sample at 260 nm, which will indicate that there is no nucleic acid present. The third and final possibility is that we observe absorbance at 260 nm, but do not observe an increase in maximum absorbance as the sample is heated. This scenario would indicate that there is RNA present in the sample. RNA is also a nucleic acid comprised of bases which absorb light in the ultraviolet range. However, since RNA is single-stranded, the bases will already be fully exposed to the 260 nm when heating begins (Karl, 2009). Thus, heating the sample will not increase the absorbance. The presence of RNA as the primary form of nucleic acid is a very likely possibility especially if we are assuming that life on Europa is capable of evolving in a way that resembles the way we believe life on Earth has evolved over time. The RNA-world hypothesis, which is based on the fact that RNA, in simplest terms, is less complex than DNA, states that RNA was most likely the original form of nucleic acid utilized by early organisms. Experiment II Another 25 ml of liquid from the surrounding environment will need to be collected and this time maintained at the same temperature as the exterior environment for the entirety of the experiment. This second sample will be used to determine the presence of photosynthetic

6 6 organisms. At the core of photosynthesis is uptake of carbon and release of a specific byproduct which can differ from organism to organism. Most organisms on Earth which are capable of photosynthesizing utilize Carbon in the form of Carbon Dioxide and release Oxygen gas as waste product. Since we do not know if organisms on Europa photosynthesize the way that most Earthdwelling photosynthetic organisms do, the remaining water sample will be tested for photosynthetic organisms by using radio-labeled Carbon (C14). Once the C14 has been in the sample for 72 hours, the same nucleic acid isolation protocol will be used to isolate any DNA present. Once the nucleic acid has been isolated, we will then use a simple IR Spectroscopy method to determine if there is any C14 present in the DNA. If the readings show C14 present in the sample, we will know that there are organisms capable of metabolizing carbon since it is present in their nucleic acid. Revealing a carbon-based life form on a terrestrial body other than Earth will indicate important links between life on Earth and the potential for similar life-forms elsewhere in the galaxy. Alternative Investigation If there are no findings from Experiments I and II, ADELiE will use radar data and echolocation to search for chemoautotrophic activity near potential hydrothermal vent systems. Once these vent systems are located if any exist the two experiments can be repeated to test for any life forms which are situated in these warmer and potentially more nutrient-rich environments. Contamination For this particular mission, the issue of nucleic acid contamination within the ALUs are a major concern since any contamination of materials within the lab unit could lead to false positives. Thus, before embarking on this mission, PENGUINS-ADELiE will need to be thoroughly clear of all Earth organisms and their byproducts. The spacecraft will undergo proper sterilization procedures prior to launch in an effort to reduce the likelihood of contamination. Conclusion We propose a mission to search for life in the Europan subsurface ocean through the deployment of two PENGUINS cryobot units, each equipped with a submersible ADELiE hydrobot. PENGUINS-ADELiE will land at a target location chosen due to accessibility and likelihood of harboring a primitive ecosystem. The units will conduct two experiments to determine the presence and nature of biomolecules (DNA and RNA) and test for active photosynthesis, respectively. The data, once collected and transmitted, will help provide valuable insight into the conditions present on the Europan surface and within the subsurface ocean. Such a mission will greatly expand our knowledge of life and habitability within our Solar System.

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