Seismic Exploration of the Solar System's Icy Moons. S. D. Vance*, S. Kedar, B. G. Bills, D. Wenkert; Jet Propul- Sion Laborator

Planetary Science Vision 2050 Workshop 2017 (LPI Contrib. No. 1989) 8136.pdf

Seismic Exploration of the Solar System's Icy Moons. S. D. Vance*, S. Kedar, B. G. Bills, D. Wenkert; Jet Propul- sion Laboratory, California Institute of Technology, CA (*[email protected]).

Introduction: Seismic investigations offer the to evaluate geophysical measurements. The calculation most comprehensive view into the deep interiors of [3] uses available geophysical constraints to propagate planetary bodies and thus hold the potential for enabling profiles in density, sound speed, temperature, and elec- detailed exploration and resource utilization on icy sat- trical conductivity, for specified heat flux configura- ellites in the coming decades. Missions under way (In- tions and ocean compositions. A sample output for Eu- Sight) and in development (e.g., a Europa lander con- ropa (Fig. 1) illustrates the unique signatures of these cept [1]) have identified seismology as a critical meas- key measures that might be used to distinguish between urement to constrain interior structure and thermal state an ocean dominated by MgSO4 and one with a seawater of astrobiological targets. By pinpointing the radial composition identical to Earth’s. depth of compositional interfaces, seismic investigations can complement otherwise non-unique composition and density structures inferred from gravity and magnetometry studies, such as those planned for NASA’s Europa mission and ESA’s JUICE mission. Seismic investigations also offer information about fluid motions within or beneath ice, which complements magnetic studies, and they can record the dynamics of the shell, providing new information on how cracks form and propagate. Seismology fits well with other geophysical investigation of oceanic icy moons, as demonstrated here using physically consistent interior models. Planning for the coming decades will require more detailed modeling and laboratory studies of geo- Fig. 1. Modeled density structure of Europa vs pressure physical data related to habitability, comparative plane- (left) for pure water (solid lines), 10 wt% MgSO tology making use of past and pending mission data, and 4 (dashes), and seawater (dot-dash; 35 g/kg solution) for development of innovative technology. 5 km and 30 km thick ice. Grey lines indicating density The View to 2050: Currently known ocean worlds along the melting curves illustrate that the Gibbs Sea- pose an intriguing challenge for human and robotic ex- water package is unstable above 100 MPa for high sa- ploration: to access vast reserves of liquid water and to look for extant life. With strong evidence for oceans in linities. Depth-dependent curves (right) of temperature Jupiter’s moons Europa, Ganymede, Callisto, and Sat- (top), sound speed (middle), and electrical conductivity urn’s moons Enceladus and Titan, the coming decades (bottom) illustrate the distinct signatures that may be of further data analyses reconnaissance will set the stage observed by future investigations. for detailed in situ measurements. Interior of Callisto: Models for Callisto point to an ocean intermediate to that of Europa and larger Gan- Any landed mission should carry with it a seismic ymede [4] (Fig. 2). The warmer pure water models do experiment to constrain present-day activity, deep sub- not have high-pressure ice because the high inferred surface chemistry, and accessibility of liquids. This in- gravitational moment of inertia (assuming a hydrostatic formation will be critical for any drilling activities, and body) requires a low-pressure ocean. Ice V is present in for constraining the nature of any subsurface ocean and the cooler cases, but not ice VI. The lowest temperature its suitability for extant life. An ancillary outcome of case produces buoyant high-pressure ice III in the lower such an investigation would be to provide reconnaissance for robotic or crewed outposts for ongoing scien- part of the ocean. Sound speeds for different phases are tific exploration and in situ resource utilization. Callisto distinct. Recent progress in understanding the heat in particular is a relatively accessible target that has transport and convection in such high-pressure layers been considered for such activities [2]. points to the key role of fluids within the ice [5]. The presence of fluids would strongly influence the sound Relation to Other Geophysical Measurements: speed profile and associated seismic attenuation. Planned missions to the Galilean satellites would provide powerful prior constraints for seismic investigations that could enable characterization of the deeper interior: ice shell and ocean thickness, radial density structure, and ocean electrical conductivity. Interior and Habitability of Europa: We are constructing interior models for icy moons that can be used Planetary Science Vision 2050 Workshop 2017 (LPI Contrib. No. 1989) 8136.pdf

Combining the two capabilities may offer a cost effec- 1350 0 tive and relatively simple deployment approach that will

100 1300 bypass the challenges of traditional seismic networks.

200 Depth (km) A distributed network of seismometers will diverge 1250

245 250 255 260 265 270 275 from traditional networks in one more key aspect. It Temperature (K) 1200 0 0 0 will require autonomous smart on-board data processing ) -3 capabilities, and tolerance to node failures. Even with 100 100 100 1150 increased power and communication capabilities, the re-

Depth (km) 200 200 200 1100 Density (kg m turn from multiple triaxial seismometers recording 24- 1.61.8 2 2.2 2 2.1 2.2 3.8 4 4.2 Sound Speed (km s-1 ) bit data at standard 100Hz implies an exhorbitant data 1050 0 volume. Future planetary seismologists will have to

1000 100 forgo the natural desire to process every bit of data on

950 Depth (km) 200 Earth, and rather, design and implement seismic data

0 100 200 300 400 500 1 1.5 2 2.5 processing software that can adapt to the seismic signals Pressure (MPa) Electrical Conductivity (S m -1 ) it records,and that enables interaction between seismic Fig. 2. Model for Callisto’s interior, displayed as in Fig. nodes, which may be invisible to network operators. 1, for pure water (solid) and 10 wt% MgSO4 (dashes) This will be particularly challenging if the network is to for ice thicknesses ranging from 60 km to 130 km . be deployed onto the surface of a never before explored Planning: Implementing seismic investigations planetary body, highlighting the importance of compar- will be dictated by technical constraints on needed ative planetology and detailed physically consistent measurement sensitivity imposed by the physical envi- models encompassing the full range of possible activity ronment. Precedents for planetary seismology ap- [9]. proaches exist in the Apollo seismic network [6] and detailed studies for a Lunar Geophysical Network [7] and References: [1] Pappalardo, R., et al. (2013). Astrobi- documentation for the planned Mars InSight mission ology, 13(8):740–773. [2] Troutman, P.A., et al. (2003). [8]. Icy moons of the outer planets differ from Earth, the In AIP Conference Proceedings, 654, 821. [3] Vance, Moon and Mars, and from one another, so that their pos- S.D., (2016) http://github.com/vancesteven/PlanetPro- sible seismic characteristics much be evaluated care- file. [4] Vance, S. et al. (2014). Plan. Space Sci., 96:62– fully and on an individual basis. 70. [5] Choblet et al. (2017). Icarus, in press. [6] Goins, Long Term Vision for Icy-Moon Seismic Net- N. R. et al. (1981). JGR: Solid Earth 86(B1),378–388. works: Sensitive seismometers are critical for detecting [7] Shearer, C. and Tahu, G. (2010). Lunar Geophysical faint motions deep within the planetary bodies that can Network (LGN). Planetary Science Decadal Survey, be used to reconstruct their interior workings while Mission Concept Study Report. [8] Banerdt, W. et al. shedding light on fundamental processes such as tecton- (2013). LPSC Proceedings, 44, 1915. [9] Vance, S.D. et ics, and where relevant, volcanism, ocean noise, ice al. (2016). arXiv:1610.10067. flow, and geysering. To detect the minimum number of events required for constructing a model of the interior, a geophysical investigation has two options: (1) In- creased sensitivity enabling the detection of low magni- tude quakes; (2) Deploying a network of seismometers whose sensitivity requirements may be relaxed some- what. The challenge of the first option (chosen by the InSight mission) is the consequent instrument and lander size and complexities. Option 2 is prohibitively expensive using current lander technology. There is a dire need for a solution that circumvents both complexities by enabling simple deployments of future seismic networks to icy moons. The obvious solution calls for the capability to land multiple broad-band, high-dynamic-range, sensitive MEMS seismometers. MEMS technology is on the verge of meeting the sensitivity requirements for study- ing the interior of most of the Solar Systems Icy Moons. In parallel, 3D printing technologies are emerging that are capable of seamless integration of power, communication, data-processing and shock absorption systems.