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Other Evidence for Water on Europa
By Cynthia Phillips
from the SETI Institutes Center for the Study of Life in the Universe
posted: 07:00 am ET
15 May 2003
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Thermal models of Europas subsurface suggest that it is possible, but not definite, that liquid water could be present In three previous articles, we considered the Galilean satellites and the fact that tidal flexing, due to their resonant orbits, provides heat for volcanism on Io and could result in the presence of liquid water beneath Europas icy surface. We also summarized the evidence for liquid water at Europa based on geological evidence from images of Europa taken by the Voyager and Galileo spacecraft. The geological evidence is tantalizing, but incomplete it suggests that liquid water could be present, but also allows for the possibility that the strange features we see on Europas surface could all have formed through the motion of soft ice, without any liquid water at all.able -->  | | SETI THURSDAY | | | | | |  |  | Images |
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 | Images of Europa's surface, taken by the Galileo spacecraft, show features that could have formed in the presence of liquid water. These include chaotic terrain (top left); an enigmatic dark spot nicknamed "The puddle" (bottom left); cycloidal ridges (right); and a shallow impact crater (bottom right). Photo Credit: NASA / Caltech / Cynthia Phillips
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|  | TODAY'S DISCUSSION | | | | | | | Fortunately, there are other methods available, in addition to geological techniques, which can provide information about the presence, or absence, of water at Europa. Thermal models of Europas subsurface are one theoretical way to study what lies beneath Europas surface, and we will consider them in this article. Models of Europas gravitational field show that Europa possesses a surface layer about 100 km thick of material with the density of water, on top of a rocky interior and a metallic core. The surface layer is most likely H2O, but since the densities of solid ice and liquid water are very close, gravity models cannot distinguish between the two. So we know that there is about 100 km of some combination of water and/or ice at Europas surface, but other than knowing that the very top of this water layer is frozen solid, we do not know how thick the surface ice layer is, or if there is liquid water under it at some depth. Thermal models of Europas subsurface suggest that it is possible, but not definite, that liquid water could be present. Thermal models include sources of heat and methods of cooling, and attempt to determine the thermal gradient and state (solid or liquid) of subsurface materials. In the case of Europa, these models include heating from tidal dissipation and radiogenic sources, and cooling due to conduction and convection of heat. Tidal heating, as discussed in a previous article comes from the flexing of Europa by Jupiters gravity, as a result of Europas non-circular orbit due to its resonance with Io and Ganymede. As Europas distance from Jupiter changes over the course of its orbit, Jupiters gravitational attraction changes (since gravitational force is dependent on distance), and thus the tidal bulge raised by Jupiter goes up and down. This flexing causes heating of Europas subsurface. Radiogenic heating is caused by the decay of long-lived radioactive isotopes that were incorporated in Europa when it formed, or brought to it after formation. Conduction is the direct transfer of heat from warmer to cooler regions, while convection is heat transfer due to motion of the materials warmer materials move upwards, and cooler materials move downwards. Both conduction and convection result in the transfer of heat from Europas warmer interior to its frigid surface, and the net cooling of Europa as a result. For Europa, thermal models that include all of these effects have been inconclusive. Some models have predicted that convection would remove all the heat from a liquid layer, resulting in Europa being frozen solid rather quickly. Other models have predicted that it would be difficult to produce enough heat to melt a solid ice layer into water, but that if a water layer existed there would be enough heat to maintain it as liquid indefinitely, due to a balance of cooling and heating sources. There are still a number of unknown quantities in these models. For example, tidal heating is the most important heat source at Europa, but also the most poorly known. It is strongly affected by the rheology of ice, which is its behavior when pushed or pulled or squeezed. However, the rheology of ice is difficult to study under conditions similar to Europa, since Europas surface temperature is a chilly 100 K! It is also hard to study ice being stretched over the long periods associated with Europas tidal cycle laboratory measurements are easier to make over time periods of seconds, not days. We also dont know enough about the composition of the ice on Europa most models assume that it is pure water ice, but a small amount of other material present in the ice, especially other volatiles such as ammonia or salts, could dramatically alter the rheology of Europas ice. Most models also assume that the ice layer is solid, but the rheology could be changed if the ice layer is broken or fractured, or if the grain size is different than assumed in the models. So in addition to the geological evidence, thermal models provide one more tantalizing, yet insufficient, clue to Europas subsurface structure. Water could be present, but is not required. Fortunately, however, we have some more definite evidence for subsurface water on Europa. It comes from an unlikely source magnetic field measurements. Well consider those in a future article.
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;%20an%20enigmatic%20dark%20spot%20nicknamed%20%20The%20puddle%20%20(bottom%20left);%20cycloidal%20ridges%20(right);%20and%20a%20shallow%20impact%20crater%20(bottom%20right).%20Photo%20Credit:%20NASA%20/%20Caltech%20/%20Cynthia%20Phillips)
