Q: How should the search for ET be conducted?

Chyba: The exploration of Europa requires a series of missions. We still need the Orbiter --now slipped to 2008 for programmatic reasons -- and we need a lander or landers. We should also think about sample return [probably only possible in the near term via an impact mission] and, eventually, getting down through the ice. But we can learn a lot about the ocean from orbiting and landing on the surface.

Farmer: Clearly humans have a role to play in the long run. But we have so much to learn about doing science in both environments (Mars and Europa) that we need to do a lot more reconnaissance before embarking on such a cost-intensive and risky venture. That is what the Mars robotic program is trying to do -- lay that basic groundwork.

Deep drilling from robotic platforms appears to be beyond our grasp, and for Mars this could mean that humans will need to be involved. For Europa there are even more challenging technology problems involved with developing landing systems that could survive the extreme conditions there and have enough longevity to allow deep penetration through the ice.

A really basic requirement for both types of missions is some sort of long-lived radioactive heat source. Even if we had the plutonium (present supplies are severely limited), there is the question of public concern about the safety of launching these materials which will need to be addressed.

But if all of these problems could be solved, I would go to Mars and look for deep subsurface groundwater (that emphasis is in fact a part of the new Mars Program) and go to Europa and try and test the hypothesis about a subsurface ocean using a combination of orbital and landed mission elements. Once we have built up a good case for liquid water on those bodies, then we could progress to the next step and go after the water environments with the goal of life detection.

There are probably useful intermediate steps that could and should be taken using robotic platforms. For Mars, we could place lander/rover packages at sites where there is good evidence that subsurface water has come to the surface recently and then look for bio-signatures in the residual ground ice at those places. Note that bio-signatures survive quite well in similar environments on Earth (e.g. polar ice, Siberian permafrost, etc.).

For Europa, we could place landers on one of the ridge features where the ice crust appears to have recently pulled apart allowing water or warmer ice-brine mixtures to flow up from below. In both cases, we could look for organic signatures in the ice to see if a promising subsurface signature is found. That could be the basis for more involved missions to deep drill.

Of the two, I would give Mars priority because it is the most easily attainable. Both Mars and Europa are equally interesting, although for different reasons.

Jakosky: If cost is no object, it doesn't matter whether we do it with humans or robotically. The end result is the same in terms of places that we can explore.

Next Page: If we find ET, what do we do with it?