Héctor Martínez Rodríguez EUROPA’S ICY SURFACE Héctor Martínez Rodríguez Sistema Solar y Exoplanetas Mayo 2014

Overview Discovered in 1610 by G. Galilei and S. Marius (JupII). Synchronously locked to Jupiter, Io and Callisto (Laplace resonance) Completely covered by ice. One of the smoothest objects in the Solar System (∼30−70 Ma) Salty ocean beneath its surface (tidal energy dissipation). No mission has ever landed on Europa Tenuous atmosphere ( 𝑂 2 ). Induced magnetic field through interaction with Jupiter's (periodic variation in its direction) Composed of silicate rock. Layered internal structure

Overview: Europa’s surface Outer layer of water around 100 km thick with an iced upper crust (geological evidences show a low viscosity layer beneath the surface). High albedo (0.64) Spectroscopic data are consistent with hydrated minerals, suggesting the presence of water on the surface in the past (NIMS) Few craters, tectonically active and young (tidal effects due to the resonances with Io and Jupiter) 200 km high water vapor plumes (high heat flow, HST, December 2013)

Overview: Europa’s surface Surface chemistry ruled by salts, sulfur and water. Dark streaks crisscrossing the entire globe (salts deposited by evaporating water that emerged from within) and lenticulae (upwelling of hot material) Stresses that fracture the ice shell arise due to gravity anomalies driven from convection (tidal torque too weak). Complex tectonic structures (e.g. “chaotic terrain”) Debate still persists 𝑇 𝑒𝑞𝑢𝑎𝑡𝑜𝑟 ≅110 K 𝑇 𝑝𝑜𝑙𝑒𝑠 ≅50 K

Overview: Europa’s surface

Europa’s salty surface The less ice there is, the darker the surface is. Pure ice is shown in dark blue

Europa’s salty surface Asymmetric water-related absorption bands between 1.0 and 2.5 microns. Spectra returned by NIMS (0.7-5.0 microns): heavily hidrated minerals Trailing hemisphere of Europa is constantly bombarded by Jupiter's co-rotating magnetospheric particles: broad band centered at 2800 Å (UVS) due to magnetospheric sulfur and oxygen ions interacting within the ice lattice

Europa’s salty surface

Europa’s surface: models The thickness of the shell is important to understand Europan geological features and processes: Chaos features Impact crater morphology Formation of ridges, bands and faults Convection

Europa’s surface: models Thick-shell: several tens of km thick, lower convective layer, upper cold cover that dissipates heat by conduction. Under the brittle lithosphere, there would be a thick layer of ductile ice with a convective sublayer Thin-shell: few km thick, heat only transmitted by conduction. The ocean would be exposed locally on the surface Magnetic data (Galileo, 1998) support the existence of a salty ocean, as well as geological studies The largest impact structures suggest a thick shell of about ∼19−25 𝑘𝑚

Europa’s surface: models

Europa’s surface: thick-shell model Brittle–ductile transition (BDT) in the ice shell at 2 km deep: the temperature is high enough to permit ductile creep dominate over brittle failure as a deformation mechanism (end of the rigid crust) Tidal heating in the warm ice of the convective layer would explain high heat flows Heat flow essential to understand the evolution and present state of Europa, as well as to constrain geodynamic models Internal ocean beneath the icy shell which top is ∼20 𝑘𝑚 below the surface

Europa’s surface: thick-shell model There is still much uncertainty about the flow mechanism dominating the convective layer, which largely depends on the grain size of the ice and on the water ice viscosity Various methods to determine its thickness: mechanical, thermodynamical, impact cratering Estimated heat flow of ∼70−200 mW m −2 Biological activation in the ice caused by water plumes

Europa’s ocean Salty liquid water ocean about 100 km deep and 2-3 times the volume of all the liquid water on Earth. Primarily composed of 𝑀𝑔𝑆 𝑂 4 and 𝑁 𝑎 2 𝑆 𝑂 4 𝑀 𝑔 2+ and 𝑆𝑂 4 2− are the dominant ions (consistent with Galileo NIMS observations of the nIR absorptions bands) The temperature of the water may be suppressed to about -20°C by the dissolution of salts, but it ultimately hovers near the freezing point. Tidal heating and antifreeze substances (ammonia, salts) decrease ice’s melting point Pressures are not high (𝑔=1.31 ms −2 ≲1/7 𝑔 ⊕ ). The ocean is driven by gravity forces and thermal convection

Europa’s ocean Tidal heating essential to explain its existence Measurements of the librations of Europa characterise the ocean. The icy shell does not librate independently from the interior Potential for extraterrestrial life

Europa’s surface: resume

Europa’s surface: resume

References Billings, S. E., & Kattenhorn, S. A. 2005, Icarus 177 397–412 Goldreich, P. M., & Mitchell, J. L. 2010, arXiv:0910.0032v3 Hand, K. P., & Chyba, C.F. 2007, Icarus 189 424–438 Kargel, J. S. et al. 2000, Icarus 148, 226 –265 Priscu, J. C., & Hand, K.P. 2012, Microbe / Volume 7, Number 4

References Ruiz, J. 2012, Earth, Moon and Planets 109:117–125 Ruiz, J., & Fairén, A.G. 2005, Earth, Moon and Planets 97: 79–90 Ruiz, J., & Tejero, R. 2003, Icarus 162 362–373 Ruiz, J., & Tejero, R. 2005, Icarus 177 438–446

References http://science.nasa.gov/ http://hubblesite.org/ http://nineplanets.org/ http://solarviews.com/ http://www.planetary.org/