EART193 Planetary Capstone Francis Nimmo

Cryovolcanic eruptions Inferring processes from ongoing eruptions (Enceladus, maybe Triton & Europa) is possible Inferring processes from deposits alone is very hard Little convincing evidence for cryolavas

Plumes Enceladus, Cassini image (Porco et al. 2006) Europa, Hubble Images (Roth et al. 2014, Sparks et al. 2016) Enceladus plumes operate continuously, modulated at orbital frequency Mixture of water vapour & salty ice grains (from subsurface ocean) Exit velocity 300-500 m/s (vapour) Europa plumes seen with two different techniques, operate only occasionally Exit velocities ~700 m/s (?)

Enceladus – plume & jets R=252 km Geologically active! (geysers) 500 km South Pole Porco et al., Science 2006

“Tiger Stripe” Region No impact craters (young) Correlate with jet locations Centred at South Pole 30 km South Pole

Jets and heat sources

Enceladus thermodynamics H0=400 kJ/kg, H=0 kJ/kg so u=900 m/s, vapour mass fraction ~0.2 These both agree pretty well with observations So we can constrain the source properties of the Enceladus jets

Time-varying stresses 1/6 orbit later Pericentre Apocentre Looking down on the orbit plane Tension Normal stress Enceladus Orbital period 33 hours Compression

Prediction Fractures are closed at pericentre, open at apocentre . Show unaltered model here….. Pericentre Apocentre

Cassini ISS (visible) image . Imaging campaigns to investigate temporal variability of the entire, cap-integrated plume (as opposed to individual jets) have been ongoing since early in the orbital tour. Typical image is above: bright limb is placed off the pic to avoid too much scattered light from Enc bright limb. Resolution: from 2 – 6 km/pix . We deliberately gathered light in a 400-km thick `box’ cutting across the plume with a center at 300-km altitude to avoid steep gradients in the vertical brightness profile near the surface  ~ 40 minutes travel time up to box’s center altitude . Images were taken over a large range of phase angle (150 – 165 deg)  brightness variations due to particles’ phase function needed to be corrected for in order to measure plume’s inherent brightness variation . We extrapolated the particle size distributions given for these altitudes by Hedman et al. (2009) down to ~ 0.1 microns (ie, through the visible range). Assumed Mie scattering to compute phase function. Resulting function agreed with Ingersoll and Ewald also. . The integrated plume brightness as a function of mean anomaly is shown in next slide. Cassini ISS (visible) image

6 years of observations Non-zero “background” Pericentre Apocentre . Data collected over 6+ years  repeatable signal . Error bars include photon noise and systematic effects (like the uncertainty in the throughput of the camera filters) Non-zero “background” Pericentre Apocentre

Nimmo et al. (2014)

Collimated dust Expanding vapour Thermal emission Vapour condenses in near-surface Cracks open and close ~1 km ~0.3m Boiling front ~10 km ~1m Tidal pumping “halo” of warm ice

Cryolavas? Predicted on the basis of Voyager images to occur on icy satellites, but it appears rare and hard to prove Eruption of water (or water-ice slurry) is difficult due to low density of ice This image shows one of the few examples of potential cryovolcanism on Ganymede. The caldera may have been formed by subsidence following eruption of volcanic material, part of which forms the lobate flow within the caldera. The relatively steep sides of the flow suggest a high viscosity substance, possibly an ice-water slurry. Caldera rim Lobate flow(?) Schenk et al. Nature 2001

Cautionary Tales Ganymede (Voyager) Ganymede (Galileo) Enceladus (Voyager vs. Cassini) Moore & Pappalardo 2011 Scale bars are 50 km

Titan Cryovolcanism one possible explanation for the atmospheric CH4 (destroyed by photolysis) and 40Ar (outgassed) Images are radar, not visible light Usual problem of inferring process from images Lopes et al. 2013

Pluto and Ariel Moore et al. 2016 1200km Close-up view of Ariel showing flat-floored graben. It has been suggested the flat floors are due to cryovolcanic flooding. Wright Mons on Pluto It can’t be made of nitrogen ice (too soft) Usual density problem if erupting water

Triton Captured Kuiper Belt object Thin N2 atmosphere 250 km Captured Kuiper Belt object Thin N2 atmosphere Active geysers at the surface, driven by solar heating? Possible low-viscosity cryolava flows