Dive Europa: a Search-For-Life Initiative

Dive Europa: A Search-for-Life Initiative

Takeshi Naganuma1,2 and Hirohiko Uematsu3

1 Faculty of Applied Biological Science, Hiroshima University 1-4-4 Kagamiyama, Higashi-hiroshima, 739-8528 Japan E-mail: [email protected] 2 Deep Sea Research Department, Japan Marine Science & Technology Center 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan 3 Office of Space Utilization Systems, National Space Development Agency of Japan 2-1-1 Sengen, Tsukuba, 305-8505 Japan

ABSTRACT Liquid water, underwater volcanoes and possibly life forms have been suggested to be present beneath the estimated 10 km-thick ice shell of Europa, the Jovian satellite J2. Europa’s possible ocean is estimated to be 100-200 km deep. Despite the great depth of the Europa’s ocean, hydrostatic pressure at the seafloor would be 130-260 MPa, corresponding to 13-26 km depth of a theoretical Earth’s ocean. The hydrostatic pressure is not beyond the edge of existing deep-sea technology. Here we propose exploration of Europa’s deep-sea by the use of current technologies, taking a symbolic example of a deep submergence vehicle Shinkai 6500 which dives to a depth of 6.5 km deep (50 km depth of Europa’s ocean). Shinkai 6500 is embarkable in the payload bay of the Space Shuttles in terms of size and weight for the transportation to a Low Earth Orbit (LEO). Secondary boost is needed for interplanetary flight from the LEO.On-orbit assembly of the secondary booster is a technological challenge. The International Space Station (ISS) and ISS-related technologies will facilitate the secondary boost. Also, ice shell drilling is a challenge and is needed before the dive into Europa’s ocean. These challenges should be overcome during a certain leading time for matured experience in the ISS operation. KEYWORDS: Europa, Hydrothermal vent, Deep-sea diving, Submersible, Drilling

LIFE ON EUROPA? 7. Outer margins of the fractures are diffuse, suggesting liquid water venting from the fractures6, 8. Subsurface liquid Why Europa? is also suggested by non-synchronous rotation of Europa9. Europa, the Jovian satellite J2, is thought to have both The ice shell and possible water layer is estimated to be an ocean and hydrothermal vents. It has been more and total 100-200 km thick10. The lower part of the shell is more convincingly postulated that liquid water is present presumed to be geothermally heated, which could result in beneath the Europa’s ice shell1, 2. Occurrence of melting the lower ice shell. Europa’s volcanism is strongly hydrothermal vents, which is very suggestive of the suggested by the active volcanism of Io, the Jovian satellite presence of life3, is also eagerly discussed4, 5. Arthur C. J14, 11. Jupiter’s tidal force influences the Io’s magmatism, Clarke is one of the advocators for the life on Europa and and the same is assumed for the case of Europa. As a result, he included this idea in his popular novel 2010: Odyssey Europa is presumed to have a liquid water layer between Two as early as in 1982. Other than Clarke, many believe the outer ice shell and the rocky interior. that it would be in Europa’s ocean that extraterrestrial life The ice shell and water layer are estimated to be a total is first discovered. of 100-200 km thick (Table 1)10. Even the lowest estimation Europa’s surface is covered with an ice shell which of 100 km thick is 10 times larger than the maximum depth shows characteristic band-like fractures and iceberg drifts6, of the Earth’s ocean. However, hydrostatic pressures will not be proportionally high, as Europa’s gravity (1.3 m s-2) is 13% of the Earth’s gravity (9.8 m s-2). Hydrostatic pressure at 100-200 km deep in Europa’s ocean will be ca. Received 6 September, 1998 130-260 MPa, which is equivalent to 13-26 km deep in the Earth’s ocean. Higher values are possible, assuming higher *Address correspondence to: Takeshi Naganuma density of Europa’s seawater, although the chemical [email protected] composition is not yet known.

－ 126 － Naganuma & Uematsu

Table 1. Earth and Europa. Comparison of physical parameters.

Earth Europa

Distance from the Sun 1 AU (1.5 x 1011 m) 5.2 AU (7.8 x 1011 m) Mass 6.0 x 1024 kg 4.8 x 1022 kg Radius 6.4 x 106 m 1.6 x 106 m Gravity 9.8 m s-2 1.3 m s-2 Depth of Ocean Average 3.8 km 100-200 km? (Existing/Presumed) Maximum 11 km (Ice shell 5-10 km?) Maximum hydrostatic pressure 110 MPa 130-260 MPa?

Being optimistic, diving to the Europa’s seafloor is Photolytic O2-supply from H2O is possible at least to worth considering. Existing deep-sea technology such as the distance of the giant planet, Jupiter. Occurrence of deep submergence vehicles (DSVs) and remotely operated atmospheric O2, though tenuous, has been reported from vehicles (ROVs) may overcome the high hydrostatic three outer Galilean satellites: Europa, Ganymede and pressure of Europa’s ocean. For example, the manned DSV Callisto13, 14. This does not lead directly to the presence of 2- Shinkai 6500 is able to dive down to a depth of 6.5 km in dissolved O2 in the Europa’s ocean, however, SO4 and - - the Earth’s ocean, equivalent to about 50 km depth in NO3 are probably present and may serve as O-donors (e - Europa’s ocean. There is a safety margin for the maximum acceptors). On the other hand, H2-supply, either volcanic diving depth. From a purely technical point of view, Shinkai or photosynthetic, seems to be rather limited, and only a 6500’s maximum diving depth, at least for the pressure few planets and satellites are thought to have or to have hull, is >10 km on Earth12 and >76 km on Europa. Other had volcanic activities. Therefore the key to the occurrence subsystems of Shinkai 6500 may not necessarily hold such of life lies in the occurrence of volcanism on water-bearing pressure resistance. The deeper part of the Europa’s ocean planets and satellites such as Earth and Europa. could be surveyed by ROVs such as Kaiko, which dives to Mars is thought to have had its plate tectonics and ocean a depth of 11 km of the Earth’s ocean (83 km on Europa) in the past, and the possibility of early life forms on the during normal operations. It should be noted that the DSV past Mars has been a focus of discussion. It is not the Shinkai 6500 and the ROV Kaiko are not necessarily meant intention of this manuscript to argue about past Martian to be delivered to Europa; they are only taken as symbolic life. Instead, it should be pointed out that the possibility of examples to demonstrate that existing technology seems present Martian life is rather low, because Mars is too small to be sufficiently applicable for the exploration of Europa’s to retain a substantial atmosphere with liquid water and to seafloor. This exploration will be not merely an exapnsion develop its plate/plume tectonics (crustal magamatism has of human being’s reach but also a search-for-life on other played an important role in the Martian volcanism)4, 15. planets and satelites, as Europa is likely to have an ocean, Thus, Mars lacks a substantial source of H2-supply. Venus seafloor hydrothermal vents, and life forms. is also known to have mantle plumes and volcanisms4, 16. However, Venus is thought to have developed neither plate Water and volcanoes: Sources of e--donors and e-- tectonics nor the associated hydrothermal vents due to the acceptors absence of liquid water. Venus’ orbit is inside of the If a life form is a build-up of CHON compounds, life is habitable zone17, and the atmospheric temperature is too regarded as the transient between the oxidized end (CO2, high to keep H O liquid. Europa, in contrast, is likely to + + 2 NO3 ) and the reduced end (CH4, NH4 ) of CHON have both liquid water and continued volcanism. transformation. For this, as well as the presence of liquid

H2O, sources for oxidation and reduction of CHON- compounds are essential for life. In the Earth’s biosphere, Habitability sources of CHON-oxidation (e--acceptors) are 1) Distance from the Sun, size (mass) and presence of magnetosphere are the factors to determine the habitability photolytical and photosynthetic release of O2 from H2O, 2- - of planets and satellites18. Distance determines the heat 2) O-atoms in SO4 (via sulfate reduction), NO3 (via nitrate flux from the Sun, and the Earth’s orbit is within the reduction) and CO2 (methanogenesis), and 3) organics (e.g., humics) and metal atoms such as Fe and Mn. On the other habitable zone of 0.95-1.15 AU where liquid water could hand, sources of CHON-reduction (e--donors) are 1) be present17. Size is the factor controlling that a planet or + satellite is able to retain both volatiles and volcanoes for photosynthetic release of H 2 (in the form of NADPH ) billions of years. A planet or satellite having >0.07 times from H2O and 2) geo-/hydrothermal (i.e., volcanic) supply the mass of the Earth could retain nitrogen and oxygen for of H2 from the Earth’s interior.

－ 127 － Biological Sciences in Space, Vol.12 No.2(1998)

over 4.5 Gyr 18. A planet or satellite having >0.23 times water. The ice shell has remarkable fractures (called lineae) mass of the Earth is capable of sustaining plate tectonics with dark and diffuse margins, suggesting that subsurface and the associated volcanism18. Magnetosphere contributes liquid containing inorganics/organics has been vented6. to habitability, as it protects atmospheric N2, O2 and CO2 Vented material may be exposed to photolysis or from sputtering by charged particles. The Earth is photooxidation under Europa’s oxygen atmosphere. accommodated with habitable distance, size and suitable Subsurface chemistry is little known. However, vented magnetosphere. sulfur and ultramafic (Mg-rich) silicate lava are reported Europa is 5.2 AU (7.8 x 1011 m) distant from the Sun, from Io’s volcanism11, and the same may hold true for outside of the habitable zone, and solar radiation is not Europa’s volcanism and may influence Europa’s seawater enough to melt Europa’s ice shell. However, Europa has chemistry. massive heat flux from its interior, i.e., tidal heating Diving with existing technology influenced by Jupiter’s tidal force. This probably results in the presence of volcanism and liquid water beneath the ice Submersibles. Well-known deep submergence vehicles shell. Tidal heating thus gives Europa both volcanism and (DSVs) for depths greater than 3.8 km (average depth of liquid water, in spite of its small mass (0.008 of the Earth’s the Earth’s ocean) are; Alvin (maximum diving depth, 4 mass). The ice shell may serve as a protect for the possible km; USA), Nautile (6 km, France), Mir I and II (6 km, ocean biosphere against the loss of volatiles and the Russia) and Shinkai 6500 (6.5 km, Japan). Shinkai 6500 sputtering of charged particles. Thus, although Europa’s reaches the greatest depth among the existing DSVs, and atmosphere and magnetosphere13, 19 could be too tenuous this article focuses on Shinkai 6500, a product of the best for protection, the ice shell may contribute to securing deep-sea technology today. Europa’s habitability. Shinkai 6500 has been operated by the Japan Marine Science and Technology Center since 1990 for scientific research of the deep-sea (= shinkai in Japanese). Record DIVE INTO EUROPA’S OCEAN maximum diving depth is 6.527 km in the Japan Trench. There is a safety margin of 3.5 km depth for the operational Europa’s ocean maximum of 6.5 km, which yields the theoretical maximum As more data and interpretations are accumulated, the of 10 km, at least for the pressure hull12. This does not occurrence of Europa’s ocean is more and more necessarily mean Shinkai 6500 is capable of diving to a 1, 2 convincingly supported . Although the depth fractions depth of 10 km on Earth; subsystems other than the pressure of the ice shell and the liquid layer are not known, total hull may not function at such great depths. Dimensions of thickness of <200 km limits the maximum hydrostatic Shinkai 6500 are 9.7 m long, 2.7 m wide and 3.2 m high, pressure estimation; it will be approximately <260 MPa. and weight is 26 tons in air (Table 2)20. The pressure- The most widely cited thickness is 150 km, which would resistant hull, 2.0 m across, is made of the titanium alloy, correspond to about 200 MPa. There would be shallower Ti-6Al-4V ELI. Life-support time with respect to O -supply seafloor raised by seafloor spreading and/or hot spot 2 and CO2-removal for crew of 3 (usually 2 pilots and 1 volcanism. For example, the Earth’s mid-ocean ridges are scientist) is 129 hours for emergency mode. Acoustic mostly 2-3 km deep compared with the average ocean depth navigation is computer-assisted, and voices and images of 4 km. (one image every 10 seconds) are transmitted acoustically. Little is known of the chemical composition of the ice During the Europa expedition, images and data will be and liquid water. However, postulation of hydrothermal transmitted acoustically through the water column, received venting may suggest an acidic tendency for the ice and by sub-ice stations and radio-transmitted to the Earth.

Table 2. Shinkai 6500 and Space Shuttle’s payload bay. Comparison for embarkability.

Shinkai 6500 Space Shuttle’s payload bay

Dimension Length 9.5 m Length 18.3 m Breadth 2.7 m Diameter 4.6 m Height 3.2 m

Weight 26.0 ton 29 ton (LEO*) 4 ton (GSO**)

* Low Earth Orbit ** Geostatic Orbit

－ 128 － Naganuma & Uematsu

Carriers. The DSV Shinkai 6500 is embarkable in the thickness and to locate thinner sites suitable for drilling. payload bay of the Space Shuttles which is 18.3 m long Drilling technology has been continually developed and 4.6 m across, capable of carrying a maximum of 29 largely owing to the petroleum industry. Scientific drilling tons to Low Earth Orbit (LEO) (Table 2). Thus, all the has been developed accordingly. The deepest record of Shinkai 6500’s dimensions are below the limits of the research drilling is >12 km below land surface in Kola payload bay in size and weight, when transported to LEO. Peninsula, Russia. For deep-sea drilling, >7.7 km However, for interplanetary flight to Europa, additional penetration below seafloor has been drilled for petroleum measures are needed to give Shinkai 6500 escape velocity exploration in the Gulf of Mexico. As to scientific deep- or to boost directly to interplanetary space. sea drilling, >2 km below the seafloor has been drilled by Although the technical feasibility needs to be the Ocean Drilling Program22. More advanced scientific investigated further, one idea is to launch a secondary ocean drilling is planned23, 24. Ice drilling has been booster separately to the LEO where Shinkai 6500 is already conducted in Antarctica and Greenland, and the record is stationed. The secondary booster shall then be installed to >3 km through the Greenland ice bed25. Thus, the current Shinkai 6500 to give the interplanetary flight capability. drilling technology all together would be capable of boring The most feasible place and scenario for this on-orbit Europa’s ice shell to 10 km depth. Possible technological assembly to take place may be the International Space challenges are to bore holes of a few to 10 m across and to Station (ISS) with the assist of crew Extra-Vehicular increase anti-freezing ability of drilling fluids. Mechanical Activity (EVA). The first element of ISS is to be launched and chemical developments are needed. in 1998, and the entire assembly sequence shall be Drilling the ice shell would also require measures to completed by 2004. The ISS has a robot arm called a prevent blow-out of underlying liquid through the Remote Manipulator System (RMS), which is used to boreholes. Blow-out prevention technology developed by grapple a module during the assembly sequence. Shinkai the petroleum industry could be available to suppress liquid 6500 will be installed with the Flight Releasable Grapple venting from Europa’s ocean. Fixture for readily grappling by the RMS. Being thus A power supply for the construction and operation of grappled on the ISS, Shinkai 6500 will be on the ISS for the Europa’s ice shell drilling system is definitely needed, installation of the secondary booster. Once the secondary and current technology would be insufficient. Solar booster is brought up to the ISS and the assembly is radiation would be too weak to generate sufficient completed, Shinkai 6500 with the booster will be released electricity at 5.2 AU from the Sun, yielding only <4% of away from the ISS and the space launch to the deep space the radiation reaching to the Earth’s upper atmosphere. shall take place. We all know this is a challenging scenario Development of high-performance fuel batteries or high- and it involves new technologies such as the on-orbit safety nuclear reactors is strongly required. assembly of the secondary booster and the space launch. Overall feasibility The on-orbit assembly may involve the EVA activities around the ISS, where we have to resolve safety issues in Technological challenges such as on-orbit assembly and addition to many technical issues, and the space launch ice shell drilling should be overcome during a certain may involve the precise attitude and timing control at the leading time (5-10 years?) for matured experience in the ignition of the booster. Even with all those challenges, the ISS operation. In addition to technological issues, costs human interest will keep driving the technology to evolve for the program should be considered, although the cost and our search-for-life will continue to get closer to the estimation is beyond the scope of this article. There is too reality. much uncertainty in the cost estimation. For example, the costs for ice shell drilling, which is ever progressing per Drilling Europa’s ice shell. Although this communication se, is highly variable according to the types of drilling, focuses on diving into the Europa’s ocean, it should be e.g., with or without borehole measurement and ice-core appropriate to mention drilling Europa’s ice shell to make recovery. Although uncertainty in the cost estimation is holes for the dive. Since the ice shell is made of water-ice, left, we have to minimize the costs by maximum utilization density and hardness are within the feasible ranges of of existing technologies. In this context, the core of the existing technology. Drilling on the Earth has already feasibility assessment lies in the point that most of the sub- overcome heavier and harder substrates to many km deep. programs such as space-transportation, deep-sea diving and Instead, a potential problem would be thickness. The ice shell drilling are based on existing and not-too-far- thickness of Europa’s ice shell is not clearly known, advanced technologies. Organizing sub-programs into one however, it has been estimated to be 10-20 km thick10, 21. major program is needed and the most important factor, The ice shell could be thinner at dark spots 60-140 km but the least costly. Only existing technology, a relatively across (called maculae) where asteroid or comet impacts small budget, and enthusiasm are the necessary and took place. Moreover, the ice shell might have been <6 km sufficient conditions for the search-for-life on Europa. thick when ice fractures were formed21, and some such areas may still be thin. Seismic (may be called icemic) surveys will be needed to estimate precisely the ice shell

－ 129 － Biological Sciences in Space, Vol.12 No.2(1998)

ACKNOWLEDGMENTS Head, P. Geissier, S. Fagents, A.G. Davies, M.H. Carr, H.H. Part of this article was prepared during the deep-sea Breneman and M.J.S. Belton (1998), High-temperature silicate volcanism on Jupiter’s satellite Io, Science, 281, 87-90. diving expedition to the hydrothermal vents in the Mid- Atlantic Ridge. We thank Prof. Peter A. Rona, Rutgers 12. Takagawa, S., D. Kikuchi, K. Takahashi, Y. Yamaguchi, K. University, and Dr. B. E. Tucholke, Woods Hole Inoue and T. Nishimura (1990), Design and construction of spherical pressure hull of SHINKAI 6500, Report of Japan Oceanographic Institution, for reading portions of the Marine Science and Technology Center, 23, 329-343 (in manuscript during the cruise. Many thanks to the Shinkai Japanese with English abstract). 6500 Operation Team for detailed discussion and to Dr. 13. Hall, D.T., D.F. Stroble, P.D. Feldman, M.A. McGrath and Takeo Tanaka, Japan Marine Science and Technology H.A. Weaver (1995), Detection of an oxygen atmosphere on Center, for providing the drilling information. Special Jupiter’s satellite Europa. Nature, 373, 677-679. thanks to Yuki Murakami, Hiroshima University, for her 14. Hunten, D.M., Europa’s oxygen atmosphere, Nature 373, 654 help in preparing the manuscript. (1995). 15. Bock, G.R. and J.A. Goode (1996), Evolution of Hydrothermal REFERENCES Ecosystems on Earth (and Mars?), John Wiley & Sons, Inc., 1. McKinnon, W.B. (1997), Galileo at Jupiter: meetings with pp.334. remarkable satellites, Nature, 390, 23-26. 16. Phillips, R.J. and V.L. Hansen, Geological evolution of Venus: 2. Carr, M.H., M.J.S. Belton, C.R. Chapman, M.E. Davies, P. rises, plains, plumes, and plateaus, Science, 279, 1492-1497. Geissier, R. Greenberg, A.S. McEwen, B.R. Tufts, R. Greeley, 17. Kasting, J.F., D.P. Whitmire and R.T. Reynolds (1993), R. Sullivan, J.W. Head, R.T. Pappalardo, K.P. Klaasen, T.V. Habitable zones around main sequence stars, Icarus, 101, 108- Johnson, J. Kaufman, D. Senske, J. Moore, G. Neukum, G. 128. Schubert, J.A. Burns, P. Thomas and J. Veverka (1998), 18. Williams, D.M., J.F. Kasting and R.A. Wade (1997), Habitable Evidence for a subsurface ocean on Europa, Nature, 391, 363- satellites around extrasolar giant planets, Nature, 385, 234- 365. 236. 3. Kerr, R.A. (1996), Does Europa’s ice hide an ocean? Science 19. Kivelson, M.G., K.K. Khurana, S. Joy, C.T. Russell, D.J. 274, 2015. Southwood, R.J. Walker and C. Polanskey (1997), Europa’s 4. Lewis, J.S. (1995), Physics and Chemistry of the Solar System, magnetic signature: report from Galileo’s pass on 19 December Academic Press, San Diego, pp.591. 1996, Science, 276, 1239-1241. 5. Pappalardo, R.T., J.W. Head, R. Greeley, R.J. Sullivan, C. 20. http://jamstec.go.jp/jamstec/6kside.gif Pilcher, G. Schubert, W.B. Moore, M.N. Carr, J.M. Moore, 21. Kerr, R.A. (1997), Once, maybe still, an ocean on Europa, M.J.S. Belton and D.L. Goldsby (1998), Geological evidence Science, 277, 764-765. for solid-state convection in Europa’s ice shell, Nature, 391, 22. Storms, M.A. (1990), Ocean Drilling Program (ODP) deep 365-368. sea coring techniques, Mar. Geophys. Res. 12, 109-130. 6. Belton, M.J.S., J.W. Head III, A.P. Ingersoll, R. Greeley, A.S. 23. CONCORD (1997), Reports of the Conference on Cooperative McEwen, K.P. Klaasen, D. Senske, R. Pappalardo, G. Collins, Ocean Rise Drilling, July 22-24, 1997, Tokyo, 116. A.R. Vasavada, R. Sullivan, D. Simonelli, P. Geissier, M.H. Carr, M.E. Davies, J. Veverka, P.J. Gierasch, D. Banfield, M. 24. Austin, J.A. and N.G. Pisias (1998), A New Vision for Bell, C.R. Chapman, C. Anger, R. Greenberg, G. Neukum, Scientific Ocean Drilling, A report from ComPost II: The US C.B. Pilcher, R.F. Beebe, J.A. Burns, F. Fanale, W. Ip, T.V. Committee for Post-2003 Scientific Ocean Drilling, Joint Johnson, D. Mrrison, J. Moore, G.S. Orton, P. Thomas and Oceanographic Institutions, Washington, DC, 30.. R.A. West (1996), Galileo’s first image of Jupiter and the 25. Dansgaard, W., S.J. Johnson, H.B. Clausen, D. D-J.N.S. Galilean satellites, Science, 274, 377-385. Gundesstrup, C.U. Hammer, C.S. Hvidberg, J.P.Steffensen, 7. Kerr, R.A. (1997), An ocean emerges on Europa, Science, 276, A.E. Sveinbjornsdottir, J. Jouzel and G. Bond (1993), Evidence 355. for general instability of past climate from a 250 kyr ice-core record, Nature, 364, 218-220. 8. Sullivan, R., R. Greeley, K. Homan, J. Klemaszewski, M.J.S. Belton, M.H. Carr, C.R. Chapman, R. Tufts, J.W. Head III, R. Pappalardo, J. Moore, P. Thomas and the Galileo Imaging Team (1998), Episodic plate separation and fracture infill on the surface of Europa, Nature, 391, 371-373. 9. Geissier, P.E., R. Greenberg, G. Hoppa, P. Helfenstein, A. McEwen, R. Pappalardo, R. Tufts, M. Ockert-Bell, R. Sullivan, R. Greeley, M.J.S. Belton, T. Denk, B. Clark, J. Burns, J. Veverka and the Galileo Imaging Team (1998), Evidence for non-synchronous rotation of Europa, Nature, 391, 368-370. 10. Anderson, J.D., E.L. Lau, W.L. Sjogren, G. Schubert and W.B. Moore, Europa’s differentiated internal structure: inferences from two Galileo encounters, Science, 276, 1236-1239. 11. McEwen, A.S., L. Keszthelyi, J.R. Spencer, G. Schubert, D.L. Matson, R. Lopes-Gautier, K.P. Klaasen, T.V. Johnson, J.W.

－ 130 －