It turns out that I wrote my previous post about the process to plan for possible planetary partnerships between NASA and ESA a few hours too early. Since I wrote that, two articles have appeared that give additional information, especially from the European perspective.
Amy Svitak (whose stories I have come to look forward to) at Space News writes a good summary of the issues focusing on joint Mars and Jupiter system missions. She reports that industry bids for building ESA's ExoMars rover have come back higher than expected, giving ESA a possible additional incentive to continue a partnership with NASA to share costs. (ESA originally sought a partnership because its previous cost estimates for the entire mission exceeded its budget.)
On the journal Science's website is a story that focuses on a new redefinition of ESA's three candidate missions for its next large science mission, one of which is the Jupiter Ganymede Orbiter (JGO). All three candidate proposals depended on a partnership with NASA (and one on a partnership with Japan). However, none of these missions were top-ranked candidates in the U.S. astronomy or planetary Decadal Surveys. Given budget issues in both these programs at NASA, ESA has concluded that NASA partnerships are unlikely as previously envisioned. (For JGO, the previous plan had been for NASA to fly its own complimentary Jupiter Europa Orbiter and contribute instruments to JGO. NASA can no longer do the former, but still plans to offer to do the latter if JGO is eventually selected as ESA's next large mission.) ESA has asked the three proposing teams to reexamine their proposals and rescope them as ESA-only missions. The new mission proposals are due in a year.