  Remote sensing observations suffer significant limitations when used to study
the bulk atmospheric composition of the giant planets of our solar system. This
impacts our knowledge of the formation of these planets and the physics of
their atmospheres. A remarkable example of the superiority of in situ probe
measurements was illustrated by the exploration of Jupiter, where key
measurements such as the determination of the noble gases' abundances and the
precise measurement of the helium mixing ratio were only made available through
in situ measurements by the Galileo probe. Here we describe the main scientific
goals to be addressed by the future in situ exploration of Saturn, Uranus, and
Neptune, placing the Galileo probe exploration of Jupiter in a broader context.
An atmospheric entry probe targeting the 10-bar level would yield insight into
two broad themes: i) the formation history of the giant planets and 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. An atmospheric probe could represent a significant ESA contribution to
a future NASA New Frontiers or flagship mission to be launched toward Saturn,
Uranus, and/or Neptune.
