1. Does Europa Have a Subsurface Ocean?
R.T. Pappalardo et al

2. What’s the Point? Does Europa have a Subsurface Ocean?
Potential for Life!
It’s been proposed that Europa has a subsurface ocean, which would be really cool! We know that there is a band about 100km thick of water, but we’re not sure if it’s liquid or solid or something in between. If it is liquid, there would be more water than on earth! The main point of this paper is trying to use geological evidence to determine if there is an ocean. One cool potential impact of this paper is that if it is liquid water, the potential for life is really high!

3. Galileo Mission Launched 10/18/1989, reached Jupiter in December 1995
Looked at objects in the Jovian system, including Europa!
Solid-state imaging camera, near-infrared mapping spectrometer, ultraviolet spectrometer, photopolarimeter radiometer, magnetometer, energetic particles detector, plasma investigation, plasma wave subsystem, dust detector, heavy ion counter.
NASA

4. A Differentiated Europa
The Galileo spacecraft gave us Europa’s gravitational field
From this information we can extrapolate that Europa is differentiated!
The Galileo mission determined the axial moment of inertia, which was much less than that of a uniformly dense sphere. Therefore we can see that Galileo must be differentiated. Anderson et al determined that the most likely interior structure is that of a Fe or Fe-FeS core. (iron sulfide) surrounded by an anhydrous rocky layer, and then a thick layer of H2O.

5. Thermal Modeling of Europa
Thermal models of Europa suggest that accretional and radiogenic heat probably were enough for liquid oceans in the past, the question basically boils down to whether there is enough heat to keep a liquid underbelly. If you only account for radiogenic heat, there isn't enough for liquid water. This figure is a model of what Europa’s interior structure looks like.

6. The Interplay of Tidal Heating and Subsolidus Ice Freezing
Radiogenic heat isn’t enough, so tidal heating comes into play.
Tidal heating can change throughout time
Rheology of ice layer throughout time can as well
We really don’t have an answer for this question. The amount to which tidal heating can actually heat ice depends on the magnitude of tidal deformation, which can vary, and the rheology of the ice at a given period of tidal heating. Most notably, tidal dissipation is the greatest at the base of the ice, where it would be the warmest and most deformable. In addition, other properties such as thermal conductivity of the ice, physical state (cracks/density/etc.) matter when we’re trying to determine liquidity.

7. How Old is Europa’s Surface?
Two basic models-resurfacing or low impact
Cratering on Europa is similar in size-frequency distribution to that of terrestrial planets
Using impact frequency as our guide we know that Europa has surface indicative of age from gyr. The other option is using what we know about current distribution of objects in the solar system in order to estimate the age of Europa. There is a school of thought that the two main asteroid belts do not contribute much to cratering in the Jovian system, and that instead it is Jupiter family comets making the craters. This gives us a much younger age, more consistent with resurfacing due to tidal heating.

8. How Old is Europa’s Surface:Fin
We are most interested in determining the youngest surface
Chaos regions seem to be the youngest.
The youthful age is a bit of an enigma, because it seems as though ridge forming and chaos forming processes are at odds, so it may be that they form episodically, and now is a chaos episode.

9. Geological Evidence
Do the surface features we see necessitate liquid water?

10. Impact Morphology
These are examples of impact morphology on the surface, the arrows are secondary craters. Craters on Europa are fairly shallow, indicative of isostatic relaxation. This is important because significant relaxation means a weak (although not definitely liquid) lithosphere. The multitiered structures on Europa were probably formed early on by large impactors punching through a brittle lithosphere to viscous under material. Basically, impact craters suggest 6-15km thick shell over a low viscosity underbelly.

11. Lenticulae
The most recent features on the surface are lenticulae, which are circular/elliptical pits/domes/dark spots. They disrupt the orderly ridged plains that dominate Europa's surface. Lenticulae are very similar in size and spacing, suggesting that they are related. The theory of their formation is that diapiric intrusion into a thin rigid surface layer caused warping. This can be triggered by compositional layers, in which density inversions lead to warping, but most likely thermal explanation.

12. Cryovolcanic Features
There isn't much evidence for widespread volcanic flooding of Europa’s surface. However there is evidence for cryovolcanic features, which is basically liquid water instead of liquid rock. Patches of smooth dark material may represent flows. A-volatile bearing water body can force it’s way through the surface, or simply to the tip of an upwards propagating crack. Moreover, an isolated reservoir can develop excess pressure due to volume change from ice crystallization or regional stresses. It’s very hard to model these types of features because we cant really constrain them compositionally, but these features do offer evidence for some amount of subsurface heating and melting, albeit potentially from isolated reservoirs instead of an ocean.

13. Pull-Apart Faults We see evidence for pull-apart faults on Europa
Brittle outer layer
Analogous to terrestrial spreading centers!
We see pull apart faults that can be restored with very few gaps. This is evidence of the surface behaving brittlely, pulling apart over a low viscosity sub surface material, and then that material upwelling to create new surface. We also see ridges around the troughs, much like midocean ridges.

14. Europa’s Lithosphere
The big question-could ductile subsurface material decouple the rigid shell of Europa? Using superplastic flow models, we can derive ductile strength curves. Above thermal gradient lines is a hard lithosphere, below is when you begin to see melting/ductile behavior. This paper assumes that the ice is polycrystalline and has tensile strength of 2MPA, which is the dashed line. The transition depth seems to be 2km. However it could be thicker if tidal heating is dispersed nonuniformly. This also raises the potential for an asthenosphere, in the sense of a convecting lower ice lithosphere below a conducting upper lithosphere.

15. Chaos Be Thy Name
These are chaos regions. Composed of polygonal blocks of the ridged plains that have been translated and rotated. It is almost all young! These have been interpreted to be areas of local amplified heat flow, and perhaps total melting! However, the heat necessary to do this is much above the models generated from radiogenic and tidal heating. You can also think that solid state ice has risen from below diapirically, much like the process of lenticulae. As this occurred, the localized melting/warming would have intensified tidal heating, creating a positive feedback loop. The most likely scenario is one that mixes the two-solid state ice rising through a slurry. Either way, chaos is good evidence for warm material at shallow depths.

16. Ridges Volcanism Tidal Squeezing Diapirism Compression
We see a lot of ridges on the surface of Europa, most commonly doublet ridges. These are the main possible causes. The ridges could be the debris from explosive volcanism, which would explain the dark edges of the ridges. However, the uniformity of ridges on Europa would require very consistent volcanism. The tidal squeezing model suggests that diurnal variations in tidal stresses squeezed icy debris to the surface through fractures. However, the quick orbital period makes it tough to model a conduit structure for this. Very similarily, ridges could rise dispirismly. This would leave surface features intact on the ridge slopes, but this is hard to determine from the pictures. Compression, much like two plates on earth, could be form ridges as ice plates deform.

17. Global Tectonics Nonsynchronous Rotation Diurnal Stressing
If there is decoupling, then the surface may rotate faster than synchronous, meaning that it’s face would change towards jupiter. However, while there is significant evidence of nonsynchronous orbit, this may not mean there is a subsurface ocean, as warm ice may be enough. Small fractures on Europa’s surface have opposite orientations, indicating that there most likely is diurnal stressing, so that tidal stresses pull in opposite directions depending on the day. diurnal stressing and nonsynchronous rotation are both facilitated by a liquid interior, providing good evidence for a liquid ocean.

18. What does it all mean? No one piece is definite evidence of an ocean
Taken together, they are indicative of warm, low-viscosity interior.