1. EART193 Planetary Capstone
Francis Nimmo

2. 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

3. 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 m/s (vapour)
Europa plumes seen with two different techniques, operate only occasionally
Exit velocities ~700 m/s (?)

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

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

6. Jets and heat sources

7. 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

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

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

10. 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

11. 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

12. Nimmo et al.
(2014)

13. 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

14. 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

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

16. 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

17. 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

18. 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