Titan's system is dynamic and continuously evolving so that observations during the 2025-2035 timeframe would be highly complementary to Cassini's observations in terms of seasonal phase. The list of questions pertaining to Titan as a system is lengthy as befits a world perhaps second only to Earth in its level of atmospheric and geologic activity. We are only beginning to discover the small cryogenic world of Enceladus, and hence there also remain many questions on non-understood processes. Several analyses by scientific panels (e. g. a 2007 report by the US National Research Council) have recognized Titan and Enceladus as key targets for the search for life. Questions raised by Cassini-Huygens drive the principal objectives of the mission TANDEM:
1. Titan space environment (more details): Investigate thoroughly Titan's atmosphere, its ionosphere and exosphere as well as its plasma and magnetic field environment. Titan's upper atmosphere (> 400km) plays a key role for the chemistry and physics of the entire atmosphere. The altitude range from ~400km to 950 km is below the reach of Cassini in-situ measurements but above the region of intense remote sensing and most Huygens measurements, and can justifiably be called the ÒagnostosphereÓ. Understanding this broad transition region is a key component in understanding the entire atmosphere together with its relevance for astrobiology..
2. Titan's neutral atmosphere (more details): obtain data on temperature, pressure, CH4 and C2H6 humidity; wind fields; internal structure and evolution of clouds; haze characteristics; evaporation rates and temperature over lakes; surface composition and thermal properties; volumetric changes in lakes and river systems; solar partitioning.
3. Titan's surface (more details): Obtain infrared stereo and radar surface mapping on Titan with resolutions <100 m; highest-resolution (< 1 m) compositional context and infrared imaging from a near-surface platform (also required for selecting sampling sites for surface chemistry); high-resolution in-situ measurements of surface material from a variety of locations; global compositional mapping with resolutions < 1 km; ground penetrating radar to determine the depth and vertical structure of near sub-surface deposits.
4. Enceladus (more details): Perform equivalent measurements for Enceladus' jets and plumes and surface.
5. Titan & Enceladus interiors & origins (more details): Explore the interiors of Titan and Enceladus by quantifying spatial and temporal variations of topography, gravity field and magnetic field (if any); seismicity, near-surface thermal gradient and thermophysical properties; depth and composition of the interior liquid water layer (if any); size and state of the rocky core ; structure of the crust and depth of the "methanifer", role of other fluids ; composition of surface materials and of cryovolcanic plumes and cryomagma ; sources of atmospheric methane, noble gas proportions; major element isotopic ratios in principal compounds in atmospheric and surface materials. For constraints on Titan and Enceladus formation and evolution models we need to measure various isotopic ratios such as 36Ar/38Ar (for escape processes), D/H in surface H2O (for the origin of methane in Titan's atmosphere), 16O, 17O, 18O in water ice, Xe isotopes, 15N/14N in NH3 on the surface (NH3 amount available during the satellite's formation); also noble gases ratios: 36Ar/Kr/Xe (origin of the atmosphere); and data on the densities and gravity fields.
6. The astrobiological potential (more details) of these two objects is enormous and begs to be investigated. TANDEM will measure elemental, isotopic and molecular composition, and chirality of Titan's aerosols and of its various surface materials; chemical analysis of the ejecta produced by cryovolcanism; molecular and isotopic composition of Enceladus' plumes; seasonal and long-term variations for Titan global climate models. Titan's peculiar stratospheric polar hood has many analogies with Earth's ozone hole and its methane hydrological cycle with probable sparse but violent downpours is a greenhouse weather pattern taken to extremes. The climate/weather/hazard links to our own planet are thus obvious.