Summary of the Keck Institute for Space Studies workshop entitled "Tidal Heating: Lessons from Io and the Jovian System," held on October 15-19, 2018

Bierson, C. J.

Davies, A. G.

de Kleer, K.

DellaGiustina, D.

Ermakov, A. I.

Fuller, J.

Hamilton, C.

Harris, C.

Hay, H.

Jacobson, R. A.

Keane, J.

Kestay, L.

Khurana, K.

Kirby, K.

McEwen, A.

Park, R. S.

Stevenson, D.

Tsai, V. C.

Turtle, E.

Caltech Authors - Main

TIDAL HEATING: LESSONS FROM IO AND THE JOVIAN SYSTEM (REPORT FROM THE KISS 
WORKSHOP).  R.S. Park1, K. de Kleer2, A. McEwen3, C.J. Bierson4, A.G. Davies1, D. DellaGiustina3, A.I. 
Ermakov1, J. Fuller2, C. Hamilton3, C. Harris5, H. Hay3, R.A. Jacobson1, J. Keane2, L. Kestay6, K. Khurana7, K. Kirby8, 
V. Lainey1, I. Matsuyama3, C. McCarthy9, F. Nimmo4, M. Panning1, A. Pommier10, J. Rathbun11, G. Steinbrügge12, 
D. Stevenson2, V.C. Tsai2, and E. Turtle8, 1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, 
CA 91109 (Ryan.S.Park@jpl.caltech.edu); 2California Institute of Technology, Pasadena, CA 91125; 3University of 
Arizona, Tucson, AZ, 58721; 4University of California – Santa Cruz, Santa Cruz, CA 95064; 5University of Michigan, 
Ann Arbor, MI 48109; 6US Geological Survey, Flagstaff, AZ, 86001; 7University of California – Los Angeles, CA 
90095; 8Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723; 9Columbia University, Palisades, 
NY 10964; 10University of California – San Diego, La Jolla, CA 92093; 11Planetary Science Institute; 12University of 
Texas at Austin, Austin, TX 78705,  
 
Introduction: The evolution of the interior of plan-
ets and satellites, as well as their potential habitability, 
are central questions in Planetary Sciences.  Recent dis-
coveries from spacecraft missions and telescopic pro-
grams have illuminated the central role that tidal heating 
plays in the evolution of many worlds across our Solar 
System and beyond.  This fundamental process drives 
the orbital evolution of star–planet or planet–satellite 
systems as a whole, and shapes the surface environ-
ments and geological activity of planetary bodies, im-
pacting the habitability of ocean worlds such as Europa 
and Enceladus.  However, despite its broad ranging im-
portance, there remain fundamental gaps in our under-
standing of tidal heating and coupled orbital evolution.  
For example: Where and how is tidal heat actually dis-
sipated?  How do we link physical measurements in the 
laboratory on crust and mantle analogues to planetary-
scale observations of deformation or heat flow?  How 
are subsurface oceans created and maintained?   
While there are well-tested tools in place to study a 
variety of individual planetary processes, there is no es-
tablished path forward for unraveling the interconnected 
roles of tidal heating and orbital evolution.  Such inves-
tigations will require innovative approaches and new 
technologies that can pin down the orbital evolution and 
deformation of these worlds by integrating laboratory 
work, spacecraft and ground-based observations, and 
numerical models.  To address pathways to these ques-
tions the Keck Institute for Space Studies (KISS) work-
shop entitled “Tidal Heating: Lessons from Io and the 
Jovian System” was held October 15–19, 2018. The 
central objective of this workshop was to integrate the 
numerous recent advances across the relevant fields, in-
cluding laboratory studies, telescopic/spacecraft data, 
and instrumentation under development, to construct a 
coherent path forward for understanding tidal heating 
and its influence on the evolution of planetary systems.  
The combination of recent scientific advances as well as 
the forthcoming Europa Clipper and JUpiter ICy moons 
Explorer (JUICE) missions, the formation of the NASA 
Ocean Worlds program, potential New Frontiers-class 
missions to tidally heated worlds (e.g., Io, Enceladus, 
and Titan) and potential ocean world landers and 
penetrators make it clear that now is a critical time for 
integrating what we know, identifying what we do not 
know, and creating a clear roadmap for the future sci-
ence investigations/technologies that will be needed to 
optimize efforts in the coming decades.  
Five Key Questions about Tidal Heating: We 
identified five key questions to drive future research and 
exploration: 
Q1: What do volcanic eruptions tell us about the in-
teriors of tidally heated bodies?  Volcanism provides in-
formation about interiors that are otherwise inaccessi-
ble, as well as evidence that there is sufficient internal 
energy to melt the interior (see Fig. 1).  When combined 
with laboratory experiments under controlled pressure 
and temperature, eruptive products can constrain tem-
perature and pressure with depth.  Measurements of 
eruptive temperature, heat flow, and eruptive style are 
greatly desired to distinguish between compositions. 
Q2: How is tidal dissipation partitioned between 
solid and liquid materials?  Io’s intense volcanism is a 
clear sign of tidal dissipation but where the dissipation 
occurs is still up for debate.  If dissipation concentrates 
in the deep mantle, tidal heating would preferentially 
occur at the poles.  If dissipation instead concentrates in 
the asthenosphere, tidal heating would occur at low lat-
itudes, with minima at the poles.  Laboratory and mod-
eling efforts are needed to better interpret the observa-
tions, particularly in consideration of possible end-
Figure 1: Thermal emission from Io’s volcanoes.  This Mollweide 
equal-area plot centered on 180 W longitude shows 250 active or re-
cently active volcanoes that show measurable thermal emission, with 
the size and color of the symbol reflecting the magnitude of their heat 
flow contributions.  The plot includes the locations of outburst erup-
tions at Surt, Tvashtar, Amirani, Pillan, Gish Bar Patera, Heno Patera, 
Rarog Patera and 201308C.  Because of their transient nature out-
bursts are shown as black or red squares.  From Davies et al. (2015) 
[1], using data from Veeder et al. (2012; 2015) [2-3]. 
1925.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)
member tidal dissipation scenarios within a solid inte-
rior vs. dissipation affected by a global melt layer or in-
terconnected melt phase (see Fig. 2).  
Q3: Does Io have a melt-rich layer, or “magma 
ocean”, that mechanically decouples the lithosphere 
from the deep interior?  Images of Io from Voyager and 
Galileo clearly show distributed and extensive volcan-
ism that is likely driven by tidal heating.  However, to 
understand the dynamics, it is important to know if the 
melt is interconnected or if it is presented as isolated 
pockets.  A tool to probe melt in the interior would be 
via magnetic induction. A magma ocean beneath Io’s 
surface can be modeled as a conducting layer.  The nat-
ural time variation in Jupiter’s magnetic field induces 
eddy currents in this layer, producing an observable sec-
ondary magnetic field.  The depth, thickness, and elec-
trical conductivity of Io’s magma ocean can be con-
strained by inverting the secondary magnetic field  
measured by satellite magnetometers [5]. Electrical la-
boratory measurements can be used to interpret the field 
signals in terms of melt fraction [6]. 
Q4: Is the Jupiter/Laplace System in equilibrium?  
The Io–Europa–Ganymede system is a complex and 
delicately built tidal engine that powers Io’s extreme 
volcanism and warms water oceans in Europa.  Io’s 
gravity generates a tidal bulge within Jupiter, whose dis-
sipation transfers some of Jupiter’s rotational energy 
into Io’s orbit, moving it outwards and deeper into a 2:1 
eccentricity resonance with Europa.  This increases Io’s 
eccentricity, resulting in enhanced tidal heating.  Ulti-
mately, Jupiter’s rotational energy is converted into a 
combination of gravitational potential energy and heat 
via dissipation in both Jupiter and the satellites. Two 
possible resonance scenarios have been proposed: one 
in which total heat produced is constant and one with 
heat production varying around a long-term equilib-
rium, with eccentricities also varying.  These questions 
can be answered by measuring how orbits are changing, 
measuring heat flow, and understanding isotope geo-
chemistry for long-term evolution. 
Q5: Can stable isotopes inform long-term evolu-
tion?  A major obstacle to our understanding of tidal 
heating is our almost complete lack of knowledge about 
the long-term evolution of tidally-heated systems in the 
Solar System.  The questions of whether these satellites 
are currently in equilibrium, what their orbital and tidal 
energy dissipation histories have been over their life-
times, and how long they have been geologically active 
are all central to interpreting current signatures, yet are 
areas of major uncertainties and debate [7].  An intrinsic 
difficulty in determining the long-term history of tid-
ally-heated worlds is that their tidally-powered geolog-
ical activity rapidly resurfaces them and alters visible 
signatures.  Isotope ratios, which are insensitive to many 
of these alteration processes and hence preserve long-
term records of processes, provide a potential window 
into these otherwise-inaccessible periods of these ob-
jects’ histories. 
Avenues	for	Progress:	The most promising ave-
nues to address these questions include a new spacecraft 
mission making close flybys of Io, missions orbiting and 
landing on ocean worlds such as Europa and Enceladus, 
technology developments to enable advanced tech-
niques, closer coupling between laboratory experiments 
and tidal heating theory, and advances in Earth-based 
telescopic observations, especially for exoplanets.  An 
Io mission should: characterize volcanic processes to 
measure passive heat flow and/or high-spatial-resolu-
tion broad-wavelength-coverage by infrared cameras on 
spacecraft (Q1); test interior models via a set of geo-
physical measurements coupled with laboratory experi-
ments and theory (Q2 and Q3); measure the total heat 
flow from Io as well as the rate of Io’s orbital migration 
(i.e., to complement similar measurements expected at 
Europa and Ganymede) to measure the total energy pro-
duction of the Jovian system (Q4); and measure stable 
isotopes in Io’s atmosphere and plumes (Q5).  No new 
technologies are required for such an Io mission follow-
ing advances in radiation design and solar power real-
ized for Europa Clipper and JUICE. In the longer-term, 
seismology is a promising avenue for future explora-
tion, and interferometric synthetic aperture radar could 
be revolutionary, but requires advanced power systems 
and lower mass for future Solar System operations.   
References: [1] Davies et al., Icarus, 262, 2015; [2] 
Veeder et al., Icarus, 219, 2012; [3] Veeder et al., Ica-
rus, 245, 2015; [4] Hamilton et al., GSA abstract, 67-
14, 2018; [5] Khurana et al., Science, 6034, 2011; [6] 
Pommier, A., Surveys in Geophysics, 35, 2014; [7] Cuk 
et al., Astron. J., 820,  2016. 
Acknowledgement: This work is supported in part by 
the W.M. Keck Institute for Space Studies and carried 
out in part at the Jet Propulsion Laboratory, California 
Institute of Technology, under contract with the Na-
tional Aeronautics and Space Administration. 
Figure 2: Illustration of the structure of Io’s interior, with arbitrary 
layer thicknesses, considering end-member tidal dissipation scenarios 
within a solid interior (left panels) versus how dissipation processes 
would be affected by either a magma ocean, or globally extensive 
high-partial melt layer (i.e., a magmatic sponge).  Adapted from Ham-
ilton et al. (2018) [4]. 
1925.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)


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Tidal Heating: Lessons from Io and the Jovian System (Report from the KISS Workshop)

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