Lecture 6: Enceladus

A. P. Ingersoll

api@gps.caltech.edu

1. Enceladus is a small moon (diameter = 500 km) in the outer solar system(average surface temperature = 75 K). The surface is water ice, which is hard as
granite at these temperatures. The vapor pressure at these temperatures isessentially zero. You would expect it to be geologically dead – covered with craters
and no other features.
2. Instead Voyager found evidence that the craters were being covered up. Either
flow of a liquid or fallout of particles from a volcanic plume. Cassini has been in orbit
around Saturn since 2004, and every few months there is a close approach toEnceladus. They have found a lot of geological activity. I would like to tell you about
the observations and some of the modeling. The big questions are still unanswered:
How is the energy generated? Is there liquid water in the interior? Is the liquidwater in contact with warm rock? The last question is important for life, because life
needs chemical energy and nutrients, not just water.
3. The first flyby did not have any images, but it did show an anomaly in themagnetic field. The interpretation is that the spacecraft had flown through a cloud ofgas that was being ionized by UV radiation. Moreover, the gas was concentrated
around the south pole.
4. The next flyby revealed plumes of solid particles, many with velocities greaterthan the escape velocity, which is 235 m/s. For comparison, the thermal velocity ofwater vapor molecules (kT/m)1/2 is 300 m/s at T = 200 K.
5. As the spacecraft flew past, it took pictures of the plumes from different angles,
and it was possible to located the sources of the plumes on the surface.
6. The sources turned out to be these five cracks, called tiger stripes. The south pole
is in the middle of the tiger stripes, near the bottom of the image.
7. The plume sources are in red. The yellow places are hot spots measured by theinfrared radiometer on the Cassini spacecraft.
8. Here are the first infrared scans. The upper box (a) is the long range view. It
shows that the south pole is anomalously warm. This view allows one to estimate
the total power from the south pole, which must be coming from the interior ofEnceladus, because it is much greater than the absorbed sunlight. The lower box (b)shows the south pole in greater detail. Panels (c) – (f) show attempts to fit thespectrum of the infrared radiation. You can’t do it with a single blackbody. You needa mixture of temperatures. The best fit to this spectrum has a high temperaturecomponent (T = 133 K) covering a few % of each pixel. This high temperaturecomponent is too small to be resolved.

9. More infrared scans. One pixel right over the tiger stripe has an average
brightness temperature of 91 K, but the spectrum implies a component at 145 K
covering ~4% of the field of view.
10. This is a thermal map of the south polar region. The tiger stripes are the warm
places. Every point inside the box was mapped.
11. Here is one of the tiger stripes up close. The width is 1‐2 km.
12. This is the closest image. It reveals boulders in the 20‐50 m size range. No one
understands where the boulders come from.
13. The camera went back and took another picture of the flow feature in thevoyager image.
14. It’s not clear if the craters were covered by a flowing liquid or fallout from theplumes.
15. The near infrared instrument took these images, which reveal mostly water ice
but also organics in the vicinity of the tiger stripes. 3.44 microns is the wavelengthof the vibration of a C‐H molecular bond.
16. The UV spectrometer followed two stars as they passed behind Enceladus. One
star passed through the plum.
17. There was dip in the starlight before the star passed behind the solid surface
18. Here’s the spectrum of the light compared to a spectrum of water vapor. Use thisto measure the density of the vapor in the plume.
19. Besides the remote sensing instruments, Cassini carried instruments thatsampled the gases and particles directly.
20. The mass spectrometer detected peaks at mass 18 (water vapor), mass 44(carbon dioxide), mass 16 (methane) and mass 28 (either CO or N2). Further
analysis reveals acetylene, propane, and maybe benzene (C6H6).
21. Here’s a summary of what we know.
22. Here’s a summary of what we would like to know. The big question is whether
there is liquid water close to the surface. Actually, there are many big questions, likehow is the heat generated, but I won’t address them today. It has to be tidal heating,but so far, the models have failed to find the power by factors of 3 or 4.
23. Here’s a model that produces the power, but it is inconsistent with the long‐term
evolution of Enceladus in its orbit. It is also too cold, in my opinion, because they

overestimated the thermal conductivity of the porous ice. The heat flux is known, sothe temperature is inversely proportional to the thermal conductivity.
24. The problem is that the vapor in the pores is saturated at the temperature of theice. At cold temperatures the vapor pressure is very low, and the porous ice can’t
carry the observed heat flux. It has to warm up. The question is, does it melt?
25. Besides diffusion in porous ice, the other way to get the heat out ishydrodynamic flow of vapor in cracks. So I have been looking at this problem, usingideas from rocket nozzles and high speed flow in pipes. The difference is that thefluid is exchanging mass, momentum, and energy with the walls.
26. A few results. There is a supersonic solution – the smooth curve, and many
subsonic solutions – the bent curves, but they are unstable. By adjusting the power
and the width of the cracks, you can match the speed of the gas at the vents and thesize of the particles.
27. The main point is that the crack has to be wider than 10 cm to get the heat out,
and it helps to have several cracks per tiger stripe.
28. Summary of results. Heat generated away from the cracks has to get out bydiffusion in a porous medium, but the pores have to be large – particle size > 1 mm,
and that is unlikely. Instead, the ice will flow and the pores will close up due tohydrostatic pressure on the grains. Heat generated on the sides of the crack can getout by hydrodynamic flow provided the crack is wide enough (width > 10 cm). If theheat can’t get out as vapor, the ice will melt, and it will come out as liquid. The basic
result is that we don’t know the conditions below the surface.
29. Let’s be optimists and assume the ice does melt. What are the possibilities forlife?
30. Liquid water is not enough. Life requires nutrients, not just C, H, O, N, but also Fe,P, Mn, and many other elements. It would help if the liquid water were in contact
with rock.
31. Life also requires energy. Without sunlight, life can get energy from theoxidation state of the rocks. This is life at the bottom of the ocean.
32. This is life in sedimentary rocks at 1 km depth. There’s more we can learn about
Enceladus with Cassini. Stay tuned.
33.. The end.