  We present Hubble Space Telescope (HST) transit observations of the
Hot-Jupiter WASP-79b acquired with the Space Telescope Imaging Spectrograph
(STIS) in the near ultraviolet (NUV). Two transit observations, part of the
PanCET program, are used to obtain the transmission spectra of the planet
between 2280 and 3070{\AA}. We correct for systematic effects in the raw data
using the jitter engineering parameters and polynomial modelling to fit the
white light curves of the two transits. We observe an increase in the
planet-to-star radius ratio at short wavelengths, but no spectrally resolved
absorption lines. The difference between the radius ratios at 2400 and
3000{\AA} reaches $0.0191\pm0.0042$ ($\sim$4.5$-\sigma$). Although the NUV
transmission spectrum does not show evidence of hydrodynamical escape, the
strong atmospheric features are likely due to species at very high altitudes.
We performed a 1D simulation of the temperature and composition of WASP-79b
using Exo-REM. The temperature pressure profile crosses condensation curves of
radiatively active clouds, particularly MnS, Mg$_2$SiO$_4$, Fe, and
Al$_2$O$_3$. Still, none of these species produces the level of observed
absorption at short wavelengths and can explain the observed increase in the
planet's radius. WASP-79b's transit depth reaches 23 scale height, making it
one of the largest spectral features observed in an exoplanet at this
temperature ($\sim$1700 K). The comparison of WASP-79b's transmission spectrum
with three warmer hot Jupiters shows a similar level of absorption to WASP-178b
and WASP-121b between 0.2 and 0.3$\mu$m, while HAT-P-41b's spectrum is flat.
The features could be explained by SiO absorption.
