  Uranus and Neptune are the archetypes of "ice giants", a class of planets
that may be among the most common in the Galaxy. They hold the keys to
understand the atmospheric dynamics and structure of planets with hydrogen
atmospheres inside and outside the solar system; however, they are also the
last unexplored planets of the Solar System. 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
giant planets. Owing to the spatial and temporal variability of these
atmospheres, an orbiter is required. A probe would provide a reference
atmospheric profile to lift ambiguities inherent to remote observations. It
would also measure the 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, atmospheric dynamics, structure and evolution. An
exploration of Uranus or Neptune will be essential to understand these planets
and will also be key to constrain and analyze data obtained at Jupiter, Saturn,
and for numerous exoplanets with hydrogen atmospheres.
