One of the readers of this blog, John Rehling, sent me the following argument in favor of selection the Europa-Jupiter mission over the Titan-Saturn mission.
(I don't want the headaches of being a discussion board administrator, which is why this is a blog. I will post -- always with permission and anonymously if requested -- thoughtful commentaries sent to me at vkane56[at]hotmail[dot]com.)
I will self-identify, as you know, as a jovian proponent. Both my heart and brain are there, and I haven't felt myself budging.
This is despite the fact that I think that Titan is easily the most interesting world of the bunch in discussion. I really enjoyed reading the two proposals, but in reading them, I found my preference coming out in another way. I first read the EJSM proposal. Perfect? No. Io gets shortchanged, Ganymede gets overemphasized, and as Jason points out, the astrobiological motive is overplayed. But I did come away quite certain of what the combo mission is intended to do. It will leapfrog quite radically over the accomplishments of Galileo. I remember jovianists contemplating with envy what Cassini might have done at Jupiter. This combo proposal would radically surpass even that.
Having read it first, I found the TSSM proposal failed to connect with me, even given that, again, I think that world clearly exceeds the rest of the field in interest. But my gut reaction has a strong basis in logic -- Titan is a target-rich environment for science, to be sure. But the questions are, relative to the jovian system, comparatively formless. I'm sure we would see a fascinating world emerge from the haze, with a methanifer system laid out before our eyes, maybe itself the most interesting thing in the solar system. But it's all exploration, if you will, and little science. This would be Titan's Mariner 9 mission (Cassini is more of a Mariner 6+7), whereas the EJSM combo is more of a Viking orbiters mission to Jupiter. And this recapitulates what is obvious: the Jovian system is one step further along in its exploration and yet it is also "due", with its last dedicated [partial-success] mission having launched in 1989. It was built when Ronald Reagan was president! Titan is receiving fresh flybys from Cassini at present.
The jovian system has to win. The papers have yet to be written on what Cassini WILL HAVE found, and on which questions merit focus in the exploration architecture.
EJSM may effectively close the door on jovian system science in this epoch (with the exception of Io, which could merit its own dedicated missions), if Europa ends up, against appearances, to be a dead end for studies of the ocean-surface interface.
Titan, on the other hand, is not yet well enough known for the architecture to be optimized for its next mission. As rich a bounty as the current proposal would undoubtedly reap, there's just no excuse for leaping ahead with a possible gap between Titan as we see it now and Titan as we may see it before TSSM would arrive.