Does Europa have a subsurface ocean?

1. Does Europa have a subsurface ocean? Pappalardo et. al Nicole Bonneau

2. What is Europa like? • Density is 3.01 g/cm • Just smaller than the Moon, it is the 15th largest object in the Solar System • Europa is differentiated, primarily silicate rock • There is an induced magnetic field, which is produced by the satellite in response to a changing plasma environment • This leads us to believe that there is an ocean on Europa

3. What is Europa like?

4. How do we know this? • In the 1970s there were a few flybys, most notably the two Voyager missions • In 1995, Galileo began close orbits of the Jovian system. This is where most of what we know about Europa comes from

5. Why is liquid water on Europa important to us? • Life! • Life on Earth began in the oceans so if other satellites have life they will most likely be in or near liquid water, which is why a subsurface ocean is the subject of much study and debate

6. What are the conditions for life? • Presence of organic compounds • Adequate heat to support biomass • Water

7. Does Europa meet these conditions? • Europa is one of the exceedingly few objects that may meet these conditions • We are still unclear if there is enough heat to support life • The possibility of the subsurface ocean makes Europa a priority for exobiological exploration

8. Tidal Heating • Similar to what we see with the Moon-Earth system, Jupiter is what heats Europa through tidal flexing, which is what we see on Io as well. • It is believed that the amount of tidal heating has varied through time • Modeling depends on the rheology of ice as well as the thickness of both the ice and water layer • Tidal heating depends on too many features of the ice cap that we are uncertain about

9. Goals of the paper • Present and analyze the evidence of geological and geophysical properties of Europa in relation to a subsurface ocean • This includes the age of the surface as well as surface traits Artist rendition of ice fracturing on Europa

10. Age of the surface • There have been two models proposed by Neukum (model 1) and Shoemaker and Wolfe (model 2) • These models are ways to determine the age of the Europan surface

11. Model 1 as proposed by Neukum • Uses craters to determine the surface age • Neukum insinuates that the craters from the youngest impact basins on the Moon and on the Jovian planets, specifically the Gilgamesh basin on Ganymede, are the same age • This means that the surface dates back to the Late Bombardment, 3.8 Ga, and that the cratering flux in the Jovian system declined at the same rate as we see with the lunar plains.

12. Model 2 as proposed by Shoemaker and Wolfe • The craters in the Jovian system were not from the Bombardment era like the craters in the Inner Solar System • It would take a million kilometer sized asteroids for just one to hit Europa and there are not enough asteroids in the asteroid belt for this to be plausible • The Jovian system regularly gets hit by Jupiter family comets thought to originate in the Kuiper belt • This results in a surface age of about 54 Myr

13. Which model is more accurate? • Model 2 is more accurate because the magnetic field suggests that Europa has undergone significant tidal heating in the billion years, which supports a young surface • There have been four close passes of three comets, Brooks 2, Gehrels 3, and two times by Shoemaker-Levy/9 in the last 150 years, which also supports model 2

14. Geology of the Satellite • Relaxed structures of impact morphologies • Multiringed structures of impact morphologies • Lenticulae • Cryovolcanism • Pull apart bands • Lithosphere • Chaos • Ridges • Surface Frosts • Topography • Tectonics

15. Relaxed Structures of impact morphologies • Many Europan crates have unusually low depth-to-diameter ratios • This suggests a relaxation of the edges of craters, and most relaxation usually occurs right after the formation of the crater due to a low viscosity lithosphere over a liquid layer

16. Multiringed Structures of impact morphologies • An ice shell between 3-6 kilometers thick would produce a multiringed structure because these formations suggest that the ice is overlaying a low viscosity lithosphere at a depth of 6-15 km • This low viscous material does not need to be liquid water but could be a warm ice slurry instead

17. Lenticulae • Circular to elliptical pits, domes, and dark spots, which are widespread, that modify and disrupt the surrounding ridged plains • These features were formed by an upward deformation process believed to be a diapiric intrusion because the possibility that they are dikes are very unlikely since it would be very hard to maintain a narrow plume through 100km of water or another low viscous material

18. Lenticulae • A diapiric model for Europa shows that diapirs will rise through liquid water in about 100,000 years, which is remarkably less time than the surface age of 54 Myr according to model 2, and explains why there are so many of these freckles all over the moon.

19. Cryovolcanism • The presence of cryovolcanism implies that there is some liquid water below the surface, and, as we see in the picture below, there are some water-rich melt on the surface which obviously supports water under the surface • Cryovolcanism depends on a volatile-bearing liquid, but this could come from a heating event of the surroundings. Therefore, it does not require a liquid ocean but supports past heating and melting events in the subsurface

20. Pull apart bands • Very similar to mid-oceanic spreading centers on Earth • These tell us that Europa’s surface reacted in a brittle manner, separating and allows for the area to be filled in by a dark material • Pull apart bands support a warm and mobile material in the subsurface that could be warm ductile ice instead of liquid water

21. Chaos • Typically composed of mismatched blocks of older ridged plains that have been translated and rotated within a melted material • The blocks of older ridged plains suggests that chaos is stratigraphically young • In a model, these blocks are like icebergs , buoyant in liquid water then freezing in place. However, to have such a widespread area of chaos there would have been a large heating event that we cannot yet explain • Arrows denote tilted plates

22. Chaos • Chaos and lenticulae look to have formed around the same time indicating that they may be related • If we know that the lenticulae were formed by diapiric upwelling then chaos may illustrate areas of more intense diapiric upwelling • The rough texture of chaos presents a complication until we understand how water would behave in a vacuum, which is thought that it would become turbulent a perhaps even form an outer ice shell, or what we see on Europa

23. Ridges • Long and narrow, they are only a few kilometers wide but can be several kilometers long • Most of the surface is covered in overlapping double ridges • 5 models: volcanism, tidal squeezing, diapirism, compression, and wedging Double ridges

24. How did the ridges form?

25. Ridges • Each of the models place liquid water in different places on Europa and also in the ridge formation process • The volcanism and wedging models require liquid water at the surface, whereas the tidal squeezing model places the water several kilometers down, and the last two models just require the subsurface to deform ductily • Overall, ridge formation does not tell us where and if there is liquid water on Europa

26. Surface Frosts • Voyager data suggests that there us a regular deposition of frost on the Europan surface from a liquid interior through vents, much like geysers • This obviously supports a liquid ocean but Galileo found no evidence of these geysers

27. Topography • The highest standing point on Europa is only 2 kilometers tall. The inability to support a taller feature suggests a warm weak lithosphere, but does not require a liquid ocean

28. Tectonics • If the rigid subsurface layer is detached from the silicate interior, than the surface layers would rotate faster than the core becoming nonsynchronous • The silicate mantle is most likely synchronous, and this detaching or decoupling of the outer shell is necessary for nonsynchronous rotation, which is easily attained when liquid water facilitates the decoupling

29. Current Activity • With such a young surface the search for any changes has been the subject of comparison between the Voyager and Galileo missions data, since there was a large enough span of time between the missions that the surface could have changed • These changes could include: tectonic activity, which would form new lineaments; changes in the lenticulae or chaos terrain; or venting of water through plumes

30. What does this all mean? • When we look at all the pieces individually a subsurface ocean is not definitive • However, when we combine all of the geologic features they prove to be compelling evidence for a warm, low-viscosity material just below the ice cap. This material, be it liquid water, warm ice, or an ice slurry, played a key role in forming the geologic features we see on the surface

31. Important Outstanding Questions • Many questions have been answered thanks to the Galileo data, however we still have more to be answered: • What is the age of the surface? • How thick is the ice cap and the ocean underneath? • How has Europa’s heat sources changed over time? • How were geological processes actually forms?

32. http://apod.nasa.gov/apod/image/1101/europa_galileo_900.jpg

33. References • Khurana, K. K., M. G. Kivelson, D. J. Stevenson, G. Schubert, C. T. Russel, R. J. Walker, and C. Polanskey, Induced magnetic fields as evidence for subsurface oceans in Europa and Callisto, Nature, 395, 777-780, 1998. • NeukumG, ., Bombardment history of the Jovian system, in The Three Galileos: The Man, the Spacecraft, the Telescope, edited by C. Barbieri et al., pp. 201-212, Kluwer Acad., Norwell, Mass., 1997 • Rathbun J, . A., G. S. Musser Jr., and S. W. Squyres, Ice diapirs on Europa: Implications for liquid water, Geophys.Res. Lett., 25, 4157-4160, 1998. • Shoemaker, E. M., R. F. Wolfe, and C.S. Shoemaker, Asteroid and comet flux in the neighborhood of Earth, in Global Catastrophesin Earth History, edited by V. L. Sharpton and P. D. Ward, Spec. Pap. Geol. Soc. Am., 247, 155-170, 1990.