  The population of strongly irradiated Jupiter-sized planets has no equivalent
in the Solar System. It is characterised by strongly bloated atmospheres and
atmospheric large-scale heights. Recent space-based observations of SO2
photochemistry demonstrated the knowledge that can be gained from detailed
atmospheric studies of these unusual planets about Earth's uniqueness. Aims.
Here we explore the atmosphere of WASP-172b a similar planet in temperature and
bloating to the recently studied HD~149026~b. In this work, we characterise the
atmospheric composition and subsequently the atmospheric dynamics of this prime
target. Methods. We observed a particular transit of WASP-172b in front of its
host star with ESO's ESPRESSO spectrograph and analysed the spectra obtained
before during and after transit. Results. We detect the absorption of starlight
by WASP-172b's atmosphere by sodium (5.6sigma), hydrogen (19.5sigma) and
obtained a tentative detection of iron (4.1sigma). We detect strong - yet
varying - blue shifts, relative to the planetary rest frame, of all of these
absorption features. This allows for a preliminary study of the atmospheric
dynamics of WASP-172b. Conclusions. With only one transit, we were able to
detect a wide variety of species, clearly tracking different atmospheric layers
with possible jets. WASP-172b is a prime follow-up target for a more in-depth
characterisation both for ground and space-based observatories. If the
detection of Fe is confirmed, this may suggest that radius inflation is an
important determinant for the detectability of Fe in hot Jupiters, as several
non-detections of Fe have been published for planets that are hotter but less
inflated than WASP-172b.
