In approximately three months, NASA and ESA will pick the destination of the next flagship mission. The teams preparing the proposals have turned in their reports (PowerPoint presentations at http://www.lpi.usra.edu/opag/nov2008Meeting/agenda.html ). Now it is up to the technical reviewers and agency administrators.
Much of the information that is likely to drive the decision we are not privy to. How mature is the technology? How well defined is the science? Does the proposed budget have adequate resources?
Some of the issues, though, the informed public can speculate on and discuss. What follows is my personal assessment. Your comments and views are very much welcomed. If you have something long, please e-mail me (vkane56@hotmail[dot]com) and I'll post it directly. Otherwise, feel free to use the comments link that appears at the bottom of the post.
Both missions are similar in that their core focus is on moons with ice covered oceans. The Titan Saturn System Mission (TSSM) orbiter has an advantage in that it would provide in situ sampling of two moons by passing through the geysers of Enceladus and dipping into the atmosphere of Titan. The Europa Jupiter System Mission (EJSM) will provide some sampling of the tenuous atmosphere of Europa and a plume of Io, but clearly TSSM has the advantage here. If the Europeans contribute a lander and balloon, then the in situ advantage of TSSM is multiplied many times over. (ESA will not make that decision for another two years after decision to fly to Saturn or Jupiter is made.)
However, the advantage is not all to TSSM in regards to studying large icy worlds. Titan is shrouded by a thick, murky atmosphere that makes it difficult to study the surface. Maps will be made in two colors at moderate resolutions (50 m), but detailed geologic studies will be hampered both by the resolution and the lack of shadows to define topography. Jupiter's moons lack meaningful atmospheres, which means that their surfaces can be studied remotely in great detail both in spatial and spectral resolution. Remember the frustrations of trying to study Mars when all we had were Viking images of moderate resolution and limited spectral coverage? TSSM, as I understand the proposal, is limited to similar resolution at Titan while EJSM will have the spatial and spectral resolution of the Mars Reconnaissance Orbiter. If the balloon portion of TSSM flies, its high resolution images will help, but we don't get to chose where the winds will carry the balloon so we don't get to chose what will be imaged and the number of images will be limited by available relay bandwidth.
EJSM has another advantage. Jupiter possesses three large icy moons that allow comparative studies of the formation and evolution of these worlds. Saturn possesses just one large moon, and much of its geologic history will be difficult to impossible to study because of the extensive surface modifications caused by wind and rain.
In the end, I think it is a wash as whether we learn more about ice-ocean worlds (likely a common type of world that we will be increasingly able to study as we get better at studying planets and moons around other stars) from TSSM or EJSM. Both would revolutionize our understanding, but in different ways.
The crux of the decision based on science return in my mind comes from the nature of the objects. Titan rivals Mars as the most Earth-like body in the solar system. Both Mars and Titan have meaningful atmospheres and substantial reservoirs of liquids (in some epoch less frozen than in others). If we our priority is to understand Earth-like bodies, then TSSM is the mission of choice.
Jupiter's system, on the other hand, provides a nearby example of a gas giant with multiple planet-sized moons. We know that gas giants are common around other stars and can reasonably assume that many have large moons. We still lack an adequate survey of the Jovian system. Galileo's 1970s vintage instruments and tiny bandwidth greatly limited what we could learn. Cassini has provided that survey for the Saturn system, and there's little mention of Saturn system science in TSSM presentations. EJSM would bring our understanding of the Jovian system up to par with that of the Saturn system after Cassini.
So, for me, it comes down to whether to initiate the in-depth exploration of an Earth analog with TSSM or complete the in-depth survey of a gas giant system and do comparative studies of icy moons with EJSM. Both are compelling. I will be happy with either choice.
So I did I vote in the poll on this website? I voted for EJSM. I think that wrapping up the survey of the Jovian system makes sense, and the technology investments of the past two decades are ready to be used.
I want to see Titan explored, and believe that NASA should abandon its Mars focus at the end of the coming decade to refocus those funds on a series of Titan missions. (I don't think the study of Mars will end -- too many nations now have or soon will have the technical capability to take the baton.) For Titan, I think that we should think in terms of a sequence of missions for this world. We first need a long lived orbiter that can act for a decade or more as a relay craft and map the surface of the moon. We need at least three follow up in-situ missions (although they could be combined into one or several launches): a very capable balloon platform to perform atmspheric chemistry and subsurface sounding, a lake lander to sample the disolved organics, and a long-lived lander (or preferably 3) that would provide the network science to study the climate and interior. The key, though, is to have that orbiter in place. Without it, the bandwidth for the balloon and seismic studies is simply too small. Right now, the TSSM orbiter is very large, very capable (if you offer $3B, scientists and engineers will find ways to use it). Perhaps two smaller orbiters that split up the tasks is what we should be aiming for.
My preference for EJSM is that it finishes a task. My problem with TSSM is that it doesn't go far enough, and we should be thinking more long term.