  The ice giants Uranus and Neptune are the least understood class of planets
in our solar system but the most frequently observed type of exoplanets.
Presumed to have a small rocky core, a deep interior comprising ~70% heavy
elements surrounded by a more dilute outer envelope of H2 and He, Uranus and
Neptune are fundamentally different from the better-explored gas giants Jupiter
and Saturn. Because of the lack of dedicated exploration missions, our
knowledge of the composition and atmospheric processes of these distant worlds
is primarily derived from remote sensing from Earth-based observatories and
space telescopes. As a result, Uranus's and Neptune's physical and atmospheric
properties remain poorly constrained and their roles in the evolution of the
Solar System not well understood. Exploration of an ice giant system is
therefore a high-priority science objective as these systems (including the
magnetosphere, satellites, rings, atmosphere, and interior) challenge our
understanding of planetary formation and evolution. Here we describe the main
scientific goals to be addressed by a future in situ exploration of an ice
giant. An atmospheric entry probe targeting the 10-bar level, about 5 scale
heights beneath the tropopause, would yield insight into two broad themes: i)
the formation history of the ice giants and, in a broader extent, that of the
Solar System, and ii) the processes at play in planetary atmospheres. The probe
would descend under parachute to measure composition, structure, and dynamics,
with data returned to Earth using a Carrier Relay Spacecraft as a relay
station. In addition, possible mission concepts and partnerships are presented,
and a strawman ice-giant probe payload is described. An ice-giant atmospheric
probe could represent a significant ESA contribution to a future NASA ice-giant
flagship mission.
