IO AS an EXTREME EXOPLANET ANALOGUE. J. Radebaugh1 , A. S. Mcewen2, D

Exoplanets in our Backyard 2020 (LPI Contrib. No. 2195) 3050.pdf

IO AS AN EXTREME EXOPLANET ANALOGUE. J. Radebaugh1 , A. S. McEwen2, D. Ragozzine3, J. T. Keane4, A.G. Davies5, K. de Kleer4, C. W. Hamilton2, F. Nimmo6, A. Pommier7, P. Wurz8 and the IVO Mission Science Team. 1Department of Geological Sciences, Brigham Young University, S-389 ESC, Provo, UT 84602, [email protected]. 2Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ. 3Department of Physics and Astronomy, Brigham Young University, Provo, UT. 4California Institute of Technology, Pasadena, CA. 5NASA Jet Propulsion Laboratory, Pasadena, CA. 6University of California Santa Cruz, CA. 7University of California San Diego, CA. 8Uni- versity of Bern, Switzerland.

Introduction: Jupiter’s moon Io is the most tidally Heat Generation: Io’s tidal heat is maintained by a heated world in the solar system. Its surface (Fig. 1) is 1:2:4 mean motion Laplace resonance [2] between Io, mottled by hundreds of continually erupting silicate vol- Europa, and Ganymede. This forces eccentricity in the canoes, producing towering plumes and polluting the orbits of each moon, resulting in tidal deformation and rest of the Jovian system with sulfur and other volatiles. dissipation. Every 42 hours, Io’s shape is deformed by The tidal energy is so great that it may produce a global, as much as hundreds of meters, generating significant subsurface magma ocean—akin to the early Earth. frictional heating of the interior silicate rocks and a sur- face heat flux 40 times that of Earth. Exoplanet obser- vations have revealed mean motion resonances, or near- commensurabilities, in many well-characterized multi- planet systems [3, 4, 5, 6]. TRAPPIST-1 has multiple terrestrial planets in Laplace-like resonances. Tidal heating models suggest that the innermost world (TRAPPIST-1b) may have a heat flow as high as Io’s [7, 8], potentially producing a magma ocean [9]. Fur- thermore, ancient resonances are likely [10]. Observa- tions of exoplanets like those in the TRAPPIST-1 sys- tem may reveal how orbital resonances form and evolve, and observations of Io will inform how these worlds re- act to this extreme tidal forcing. Heat Release: Heat generated inside Io is trans- ferred to the surface through conduits and fractures to emerge as vast lava flows. These volcanic eruptions re- lease a significant amount of sulfur [11], powering gi- gantic volcanic plumes that coat the surface in SO2 frost and creating an atmosphere that changes with day/night and volcanic activity [12]. Much of this material is Fig. 1. Io in false color as seen by Galileo. All dark spots ejected into the Jovian magnetosphere, producing a to- are recent or active volcanic eruptions. rus the scale of Io’s orbit. A Na cloud is also present.

Exoplanet analogues to the Io plasma torus and Na and Io is an important analogue to a key group of super- K clouds should be detectable in the visible or UV [13]. heated exoplanets—including worlds powered by tidal Io surface is covered with discrete, long-lived vol- heating, extreme insolation, abundant heat-producing canic source regions [14]. This, along with tectonic ev- radionuclides, or left-over accretional energy. Such ob- idence for a thick, cool crust suggests that the dominant jects have been termed “lava worlds”, “highly volcanic style of heat release for Io is through volcanic heat pipes planets” and “magma ocean worlds” [1]. This makes Io rather than conduction or convection [15, 16]. In this a high-value target for understanding the generation and heat loss mode, magma is advected vertically to the sur- transfer of large amounts of heat, the longevity of bodies face, where it is deposited in flows that radiate the heat in resonance orbits, and the evolution of planetary inte- to space (Fig. 2). This efficiently transfers heat out of Io riors and surfaces with high volcanic output. Io has been while resurfacing and burying the colder outer layers. studied extensively from Earth and space-based assets, This process also affects Io’s global tectonics—produc- and there are notable similarities between Io and several ing isolated mountain blocks, up-thrust from global exoplanets. In this work, we outline potential synergies crustal shortening [15, 17]. Heat pipe volcanism was between Io and exoplanets, and some key observations likely prevalent on the early Earth, Moon, and other ter- that may address outstanding science problems for both restrial planets, and may also be important on tidally classes of worlds. heated exoplanets [16]. Exoplanets in our Backyard 2020 (LPI Contrib. No. 2195) 3050.pdf

Some eruptive centers (e.g., Loki) are active on close to the star that the surface is expected to be 2000– timescales (~480 days) that arise from the Laplace res- 2800°C, exceeding the melting temperature of mafic sil- onance but are longer than the orbital period [18, 19]. icates (1100–1400°C or less, depending on volatile con- Tidally heated exoplanets may also show longer-period tent). Since Kepler-78b is likely tidally locked, the near- modulations to thermal output, observable in the IR. side could be dominated by a magma ocean (Fig. 3) [27]. Similar “exo-Ios” may be common in the cosmos, and observations of these worlds have the potential to address fundamental questions in the evolution of all terrestrial bodies.

Fig. 2. Heat Pipe mechanism for heat transfer in Io.

Observations of an exoplanetary surface with excess thermal emissions and abundant sulfur and silicate mag-

matic minerals (e.g., pyroxene and olivine; [20]) might Figure 3. Artist rendition of a magma ocean world. indicate the planetary surface is currently dominated by volcanic processes, similar to Io. Silicate composition Exoplanet in our backyard: Io is a promising sci- can be determined by NIR and transit spectroscopy, and entific destination that represents endmember states of temperatures can be measured by multispectral IR ob- many different aspects: tidal heating, orbital resonances, servations [14, 21]. Observations of heterogeneous or heat generation and output, volcanic activity, plumes, distributed heat sources, potentially detectable with ob- atmosphere-magnetosphere interactions, rapid spatial servations of secondary eclipses, phase curves or planet- and temporal changes, a melt-bearing interior and inte- planet occultations [22, 23], may reveal extreme rior structure. All of these characteristics have been ob- amounts of internal heating. served or postulated for exoplanets, and future observa- Io’s Magma Ocean: Galileo magnetometer obser- tions may be able to address key questions relevant to vations revealed that Io contains a significant conduc- exoplanets, Io, terrestrial planets, and icy satellites. The tive layer responding to Jupiter’s magnetic field. Given proposed Io Volcano Observer (IVO) Discovery mis- the high heat output and Io’s bulk composition, this is sion [28] would address many of the questions around best explained by a global magma ocean [24]. Magma the thermal and orbital evolution of Io, which will better oceans are likely a ubiquitous stage of terrestrial planet prepare us to explore these other unique objects outside formation. The best example is the Moon, as evidenced our solar system. by the plagioclase-rich lunar highland crust, formed by flotation in a global magma surface layer. While magma References: [1] Henning et al. (2018). [2] Peale et oceans may have been common in the past, Io is the only al. (1979). [3] Steffen & Lissauer (2018). [4] Fabrycky solar system world that may have an active interior sili- et al. (2014). [5] Trevor (2019). [6] Quinn (2019). [7] cate magma ocean. The nature of this ocean, its depth Miguel et al. (2019). [8] Luger et al. (2017). [9] Mac- and fraction of solids [24], is still under discussion. Donald et al. (2016). [10] Barr et al. (2018). [11] Con- Magma Ocean Worlds: As many exoplanets orbit solmagno (1979). [12] Geissler et al. (2001). [13] Oza extremely close to their parent stars, their equilibrium et al. (2019). [14] Veeder et al. (1994). [15] O’Reilly & temperatures are high enough to prevent rocky material Davies (1981). [16] Moore (2017). [17] Schenk et al. from cooling. Even without dynamical interactions, (2001) JGR. [18] Rathbun et al. (2002). [19] de Kleer et these bodies have magma oceans on their surfaces. One al. (2019). [20] Geissler et al. (1999). [21] Davies et al. example is Kepler-78b: a 1.7 Earth-mass, 1.2 Earth-ra- (2015). [22] Luger et al. (2018). [23] Demory et al. dius planet orbiting its sun-like star at 2 stellar radii, (2016). [24] Khurana et al. (2011). [25] Pepe et al. every 8 hours [25]. Its size and mass led to the predic- (2013). [26] Price & Rogers (2019). [27] Kite et al. tion that it is rocky and probably iron-rich [26]. It is so (2016). [28] McEwen et al. LPSC (2019).