Evidence of Water Plumes on Europa?

             Europa, one of Jupiter's four largest moons, has an ocean lurking underneath its ice shell. The ocean's presence has been speculated based on Europa's surface features including rift zones, ice ridges, and chaotic terrain. These features are caused by tidal interactions between Europa, Jupiter and two other moons; Ganymede and Io. Much like tides on the Earth, Europa's surface is pulled and pushed causing the ice to crack (rift zones), pile up (ridges), or a mix of both (chaotic terrain) (Fig. 1). The tidal forces are much stronger than Earth's because Jupiter is much more massive than the Earth, while Europa is about the same size of our moon.  The discovery of plumes on Europa is significant because of Europa's potential to harbor life in its ocean. Europa's ice shell is potentially tens of kilometers thick, making drilling to its ocean a difficult and expensive task. Eruptions from plumes may deposit material from Europa's interior onto its surface where it can be more readily studied by the currently planned fly-by mission or a future lander mission. 

Figure 1. Possible cross section of Europa's ice crust and subsurface ocean. The crust may be tens of kilometers thick while the ocean may be hundreds of kilometers deep. In the background are Jupiter, and Io, one of the moons that controls Europa's tides. Artist's depiction of plumes, rift zones and chaotic terrain are shown from left to right. Image Credit: NASA/ CalTech

          Thanks to the Hubble Telescope, Europan plumes have now been imaged. In 2012, scientists at the Southwest Research Institute discovered water vapor signatures near Europa's surface, possibly from plume activity (Fig. 2). More recently, Hubble was used again to directly image plumes erupting from Europa. Out of ten observations, the plumes were spotted three times. Enceladus, a moon of Saturn, exhibits similar surface features to Europa and regularly erupts icy plumes captured by the Cassini spacecraft. These plumes erupt water like Yellowstone's Old Faithful geyser at semi-regular intervals during certain points in its orbit. One hypothesis is that Europa would experience a similar eruption cycle, initial investigations suggest this may not be the case.

Figure 2. Plumes containing water, erupting from Europa taken by the Hubble Space Telescope. A higher resolution image of Europa has been superimposed for clarity. Image Credit: NASA/ ESA 

            The locations of Enceladus' plumes tend to cluster in the southern hemisphere where its tides with Saturn are the strongest.  Europa's plumes originally clustered near the southern pole, but the recent observations showed the plumes may originate near the equator. The timing of the eruptions was compared to Europa's tidal cycle, and indicated that the eruptions do not always occur at the predicted times. This has lead several studies to suggest that plumes may not be controlled by tides, but may be caused by internal convection in the subsurface ocean. In theory Europa's tides heat the ocean, causing warm water to rise to the icy shell where it can break through the surface (Fig. 3). Until more observations can be made, the eruptions on Europa remain unpredictable. To learn more about NASA's plans for Europa click here.

Figure 3. Artist's depiction of what plumes may look like from the surface of Europa. Jupiter is pictured in the background. Image Credit: NASA/ CalTech