Missions to the moons of Jupiter and Saturn are being considered by the European Space Agency. Image: NASA/JPL.

Two missions to the outer Solar System, Laplace (Europa and Jupiter Mission) and Tandem (Titan and Enceladus Mission), are competing against each other for selection later this year; Emily Baldwin summarises the key goals of each ambitious mission as described by the European Space Agency's director David Southwood and the Open University's Professor John Zarnecki in today's lectures at the National Astronomy Meeting.

Laplace and Tandem fall under the European Space Agency's "Cosmic Visions" umbrella, missions that are designed to tackle some of the big scientific questions concerning the evolution of the Solar System and our place in it. The Laplace mission proposal outlines an orbiter that will probe the Jovian environment, with particular emphasis on the Galilean satellite Europa, which may harbour an ocean underneath its icy crust. The key aim, therefore, is to determine whether this tiny world could be inhabitable and to what extent its possible habitability is related to the initial conditions and formation scenario of the Jovian satellites. The mission may even include an impactor that would penetrate into the surface and perform analyses of the icy surface and possible subsurface ocean. "The Europa mission could teach us about the existence of an ocean and its habitability, the conditions of the sea-floor and even the interaction of this proposed ocean with the surface," says Zarnecki.

Laplace is in direct competition with Tandem, a mission designed to follow in the footsteps of Cassini-Huygens in the Saturnian system, with the ultimate destination of Titan, via Enceladus. Enceladus, just 500 kilometres in diameter, is full of big surprises and has already revealed dramatic organic-laden jets of water vapour emanating from the infamous 'tiger-stripes' that feature heavily in the satellite's southern hemisphere. Part of the mission proposal includes plans for a lander or a penetrator, both of which would permit in-situ measurements of the local surface environment. At Titan, an orbiter, aerial platform (a giant balloon that would traverse Titan's hazy, methane-rich atmosphere) and a number of mini surface landers are envisaged. "The Huygens lander mission was a huge success, but we only have three and a half hours of data sent back from the surface," says Zarnecki. "The Tandem mission package would greatly increase what we learnt in that relatively short time."

Both missions have clear aims: to study the potential habitability of outer Solar System satellites that have already proven themselves as viable candidates for conditions favourable to life. As well as the obvious astrobiological advances that would be made, the missions would also provide insight into the interiors and evolution of these outer Solar System worlds, their potential heat sources and interior-surface interaction.

The tough decision as to whether we go back to either Jupiter or Saturn will be made in the autumn of this year, with the selected mission eventually launching in 2018.