As part of the SETI Institutes astrobiology proposal, Dr. Christopher Chyba and I will be investigating potential environments for life on Europa. Our investigation will have two components. First, we will study the dataset of Galileo images of Europas surface, as taken over a five and a half-year period, to search for any changes that could be due to geological activity. Second, we will study various models of geological activity on Europa to determine their implications for the formation of transport of biologically useful material from the surface to the ocean, or vice versa.

The first aspect of our work, a search for changes on Europas surface due to current geological activity, will compare images of Europas surface using systematic image processing techniques. It will follow the methods I used in a previous search that compared images of Europas surface taken by the Voyager spacecraft with those taken 20 years later by the Galileo spacecraft. This comparison allowed a long baseline in time over which changes could occur, but was hampered by the inherently low resolution of the Voyager images of the surface of Europa. In the research we will conduct as part of the NASA Astrobiology Institute investigation, we will instead compare images of Europas surface taken during Galileos five and a half years of observations in a search for current geological activity. We will have a shorter baseline in time than for the Voyager/Galileo comparisons, but we will be able to compare images at much higher resolution to allow the detection of smaller changes on Europa.

Any changes on Europas surface due to current geological activity would indicate the location and style of that activity, and would have important implications for the presence of liquid water beneath Europas icy surface. A null result, in which no changes attributable to surface geological activity are detected, would allow us to place a lower limit on the surface age of Europa, and an upper limit on the global resurfacing rate. These limits will depend on the details of the images compared, their resolution, and the percentage of Europas surface that they cover.

Coupled with this search for changes on Europas surface, we propose to evaluate the myriad models now in the literature for the formation and evolution of various geological surface features on Europa to examine their astrobiological consequences. The questions we wish to consider for each mechanism include: (1) Does the process itself form or bury compounds of astrobiological interest? (2) Does it result in the transfer of material, formed by this or other processes, from the surface or near-surface to a subsurface ocean? (perhaps providing food or fuel for a possible subsurface biosphere) (3) Does the process result in the transfer of material from the subsurface/ocean layer to the surface? (perhaps making material with biological signatures available to near-surface exploration) (4) If material is transferred from the ocean to the surface, or the surface to the ocean, by any of these mechanisms, can we estimate the physical and chemical processing done on that material during the trip? (perhaps allowing biological signatures to arrive intact.)

We will examine these questions for the various mechanisms proposed for the formation of surface features on Europa. We will then consider two classes of broad results from this work: (1) What feature type, given all the different formation mechanisms, is of greatest astrobiological interest for a future lander? Which has the best overall chances of having material brought from the subsurface up to the surface intact? (2) What is the overall budget of materials available for a possible subsurface biosphere? We should be able to put upper and lower bounds on the total amount of material transferred from the surface to the ocean by considering the highest and lowest cases for each feature type.

The combination of these two lines of inquiry will allow a characterization of the possible environments for photosynthesis-independent life on Europa. If we are successful in finding surface changes due to geological activity, we will be able to combine them with the analysis of the second part of our study to examine the consequences of this activity for the presence of a biosphere. Even if we are unsuccessful, we will be able to establish important limits on the rate and style of geological activity on Europa, and the potential amounts of material available to fuel a biosphere.