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Europa Quad Echo (EQE) Europa Quad Echo (EQE) Sub-cryospheric investigation into the habitability of Europa Editors: - Kolja Hanke - Micha Nebel - Benjamin Heikki Redmond Roche December 2017 Contents Page Chapter 1 - Introduction To The Exploration Of Europa (Page 2) Chapter 2 - The Structure Of Europa (Page 3) Chapter 3 - The Required Parameters For Life (Page 5) Chapter 4 - The Zones Of Europa (Page 8) Chapter 5 - Indicators For Life (Page 10) 5.1 - Indicators of Astrobiology & Instrumentation (Page 10) 5.2 - Sensitivity of Instruments (Page 12) 5.3 - Amino-Acids as potential Indicator of Life (Page 13) 5.4 - Sterilisation (Page 15) Chapter 6 - Target Zones For Exploration (Page 17) 6.1 - Cryogenic Morphology Of Europa (Page 17) 6.2 - Target Zones Of Europa (Page 24) Chapter 7 - Discussion & Mission Options (Page 29) Chapter 8 - Reference List (Page 33) 1 December 2017 1. Introduction On the 8th of January 1610, Galileo Galilei pointed his telescope at Jupiter and made a ground- breaking discovery; changing humanity’s perspective of the Earth, the universe and its own place within it. He had discovered the four moons of Jupiter: Io, Ganymede, Callisto and Europa. In the following few centuries Europa, the smallest Galilean moon, was disregarded until exploration occurred in the 1970s with the Pioneer (10 & 11) and Voyager (1 & 2) flybys; and subsequently the Galileo Mission (1995-2003), which undertook detailed analyses of the Galilean network for the first time. The data collected by these missions revealed the Europan surface consists of water ice and seems to be very young, according to the small number of visible craters (Carlson et al., 2009). Moreover, the Europan magnetic field, measured during the Galileo Mission, yielded captivating data that indicated the existence of a saline ocean beneath the icy crust (Carlson et al., 2009). In 2013 the Hubble Space Telescope observed plumes, rising from the surface of Europa supporting the theory of a subsurface ocean (NASA, 2017). The high probability of a subcryospheric ocean existing on Europa results in this being the most likely place in the Solar System (that we currently know of) to harbour life, with a potential environment analogous to the Archaean and Proterozoic oceans (NASA, 2017). The main objective of this paper is to develop a theoretical mission, which could be able to detect possible traces of these extra-terrestrial life, that may have developed in a subsurface ocean of Europa. Therefore, we first look at the structure of Europa, define the requirements life needs to develop and flourish and identify the zones in which possible life could exist. After that we focus on which indicators life leaves behind, how and with what instrumentation to measure these traces. The final part of this paper proposes suitable target zones on Europa, as well as the three different mission options ranging from what is possible now (Technology Readiness Level = TRL 9) to what will be possible in the near future (TRL 3), to prove the existence of life on the Jovian moon. 2 December 2017 2. The Structure of Europa During the eight years of the Galileo mission (1995-2003), the spacecraft explored the Jupiter system including the moon Europa (NASA, 2017). Thanks to its findings, today we know a great deal more about the gravitational force, electromagnetic characteristics and the shape of Europa (Schubert et al., 2009). With the collected data, it is possible to create simple models for the inner structure of Europa. These models indicate that Europa consists of a metallic core, surrounded by a silicate mantle which is covered with an outer shell of liquid water/ice (Fig.4A). The concept that there may be an ocean beneath Europa's icy surface first arose after measurements of the electromagnetic induction signals of Europa by the magnetometer of Galileo (Schubert et al., 2009). The collected data lead to the conclusion that only a salty ocean could explain the disturbances in Jupiter's magnetic field, which could be observed around Europa (NASA, 2017). According to the models of Europa's inner structure the thickness of the ice is estimated to be ~25 kilometres and protect the underneath laying ocean from incoming radiation (Paranicas et al., 2007). The temperatures in the ice vary between -183°C (top layer) and -3.15°C (bottom layer) (Kargel et al., 2000). The huge differences in salinity (Table 2.1) within the ice is due to convection movements within the ice: relatively warm ice rises from the bottom layers to the cooler ones above in a diapiric manner (see Chapter 6). Below the ice, the supposed ocean reaches depths of potentially up to ~100 kilometres. The temperatures in the ocean increases from top to bottom (0-100°C). This increase in temperature is assumed to be generated from hydrothermal vents in the on the seafloor. These hydrothermal vents may heat the surrounding area up to ~250°C similar to deep sea vents on earth (Table 2.1). The plumes spotted by the Hubble Space Telescope in 2013 may possibly be formed by, and therefore, indicative for the existence of these hydrothermal vents at the seafloor (NASA, 2017). The geological explanation for the formation of these vents is attributed to plate tectonics generated by tidal heating caused by Jupiter's gravity. This tidal energy, which stretches the Europan surface, most likely turns into mechanical energy, resulting in friction and heat and as such providing the mechanism required for temperatures conducive to subcryospheric liquid ocean (Europa Clipper, 2017). The boundary layer, estimated to be around 25 kilometres deep, is followed by the silicate mantle, in which the temperatures further increase from 250°C - 850°C by a depth of 250 kilometres (Table 2.1). 3 December 2017 Table 2.1: The structure of Europa Zone Temp. (°C) pH Pressure Radiation Chemical Conditions Surface (Top Ice) -183.15 - 0.1 µPa 7.0 to 8.1 H, C, N, O, Na, Mg, Upto 10 meters depth =10-12 bar a MeV/cm^2 s Al, Si, S, Cl, K, Ca, → highest Fe (average mass fluxes → exogenic) values at equator b Salinity: 240 - 400 g/kg d Mid. Ice -93.15 - - Between 1 - Salinity: 240 - 400 (10 m to 10 km) 10 meters g/kg depth no radiation left b Bottom Ice (up to 25 km -3.15 - - - Salinity: 240 - 400 depth) g/kg Ocean (0km depth - 0 - 100 Alkaline 0 MPa - Salinity: For a beneath 25km ice layer e pH 8 - 12 d e MgSO4-rich ocean to 125 km) ** (Eutectic → estimated to be temperature in range of 1 - 16 238 K d) g/kg f Boundary layer (Plume 100 – 250 - 0 - 80 MPa - - height/Fluid peroclation Depth into mantle) → 100 - 125km depth Silicate Mantle (125 - 250 - 850 - 81 - 500 - - 250 km) MPa a(McGrath, 2000) b(Paranicas et al., 2009 c(Cooper et al., 2001) d(Kargel et al., 2000) e(Vance et al, 2009) f(Hand et al., 2007) 4 December 2017 3. Required parameters for life While searching for life in space it is fundamental to initially define the parameters required for life to exist and copulate. Therefore, all of the important parameters regarding forms of life have been investigated and listed in Table 3.1, so that the ranges in which life as we know it can exist are clear. In combination with parameters of Europa´s interior (Chapter 2) Zones of a habitable environment on Europa have been defined (Chapter 4). As stated in Table 3.1 there are two relevant radiation parameters, ultraviolet (UV) radiation and ionizing radiation. Whereby life can stand a UV radiation up to 5,000 J/m2 and a ionizing radiation of up to ~6000 Grays (Pappalardo et al., 2009). The six most abundant elements (Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorous and Sulphur) in biomolecules as we know them must be present to form life. For -3 Nitrogen, e.g. Aerobic microorganisms need at least 1–5 × 10 atmospheres N2, for fixation (Pappalardo et al., 2009) (Life, 2017) (McKay, 2014). A certain environment is habitable for life so long as the salinity values are no higher than 25 to 33%. The range of pH-values is wide (0 to 12.5) and covers almost the whole spectrum (0 to 14) (Pappalardo et al., 2009). Therefore, the pH-values are probably not a significant restricting factor for life forms. Similarly, pressure is going to be of limited significance as the upper limit is 1680 MPa and Earth´s surface has a pressure value of 0.1 MPa. For liquid water, as in the case of the Europan ocean, temperature is more likely to be a limiting parameter before pressure (Pappalardo et al., 2009) (Life, 2017). Temperatures in which forms of life can exist ranging from - 20 to 122 °C (McKay, 2014; Table 3.1). With regard to these values and the conditions which can be observed on earth, it seems likely that many different environments are habitable for life. However, the probability of finding forms of life in space is much more remote as conditions are significantly more challenging (see Chapter 2). The zones of Europa which are the most conducive to life are discussed in more detail, particularly regarding the possibility of finding forms of life, in Chapter 4. Table 3.1: Requirements for known carbon based lifeforms PARAMETER RANGE REMARK TEMPERATURE -20 - 122°C; 253.15 - 395.15 K Upper limit solubility of lipids in water/protein stability, Psychrophiles live up to -20 °C but can survive in a ‘dormant’ mode at much lower temperatures ~196 °C. c pH 0 - 12.5 Life known to survive at pH: (0) Cyanidium, Archaea - 5 December 2017 Natronobacterium, Protists – know to survive in >10.5.
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