  In response to ESA Voyage 2050 announcement of opportunity, we propose an
ambitious L-class mission to explore one of the most exciting bodies in the
Solar System, Saturn largest moon Titan. Titan, a "world with two oceans", is
an organic-rich body with interior-surface-atmosphere interactions that are
comparable in complexity to the Earth. Titan is also one of the few places in
the Solar System with habitability potential. Titan remarkable nature was only
partly revealed by the Cassini-Huygens mission and still holds mysteries
requiring a complete exploration using a variety of vehicles and instruments.
The proposed mission concept POSEIDON (Titan POlar Scout/orbitEr and In situ
lake lander DrONe explorer) would perform joint orbital and in situ
investigations of Titan. It is designed to build on and exceed the scope and
scientific/technological accomplishments of Cassini-Huygens, exploring Titan in
ways that were not previously possible, in particular through full close-up and
in situ coverage over long periods of time. In the proposed mission
architecture, POSEIDON consists of two major elements: a spacecraft with a
large set of instruments that would orbit Titan, preferably in a
low-eccentricity polar orbit, and a suite of in situ investigation components,
i.e. a lake lander, a "heavy" drone (possibly amphibious) and/or a fleet of
mini-drones, dedicated to the exploration of the polar regions. The ideal
arrival time at Titan would be slightly before the next northern Spring equinox
(2039), as equinoxes are the most active periods to monitor still largely
unknown atmospheric and surface seasonal changes. The exploration of Titan
northern latitudes with an orbiter and in situ element(s) would be highly
complementary with the upcoming NASA New Frontiers Dragonfly mission that will
provide in situ exploration of Titan equatorial regions in the mid-2030s.

Earth and Planetary Astrophysics;Instrumentation and Methods for Astrophysics