  Uranus and Neptune are the last unexplored planets of the Solar System. I
show that they hold crucial keys to understand the atmospheric dynamics and
structure of planets with hydrogen atmospheres. Their atmospheres are active
and storms are believed to be fueled by methane condensation which is both
extremely abundant and occurs at low optical depth. This means that mapping
temperature and methane abundance as a function of position and depth will
inform us on how convection organizes in an atmosphere with no surface and
condensates that are heavier than the surrounding air, a general feature of gas
giants. Using this information will be essential to constrain the interior
structure of Uranus and Neptune themselves, but also of Jupiter, Saturn and
numerous exoplanets with hydrogen atmospheres. Owing to the spatial and
temporal variability of these atmospheres, an orbiter is required. A probe
would provide a reference profile to lift ambiguities inherent to remote
observations. It would also measure abundances of noble gases which can be used
to reconstruct the history of planet formation in the Solar System. Finally,
mapping the planets' gravity and magnetic fields will be essential to constrain
their global composition, structure and evolution.
