  Clouds are prevalent in many of the exoplanet atmospheres that have been
observed to date. For transiting exoplanets, we know if clouds are present
because they mute spectral features and cause wavelength-dependent scattering.
While the exact composition of these clouds is largely unknown, this
information is vital to understanding the chemistry and energy budget of
planetary atmospheres. In this work, we observe one transit of the hot Jupiter
WASP-17b with JWST's MIRI LRS and generate a transmission spectrum from 5-12
$\rm{\mu}$m. These wavelengths allow us to probe absorption due to the
vibrational modes of various predicted cloud species. Our transmission spectrum
shows additional opacity centered at 8.6 $\rm{\mu}$m, and detailed atmospheric
modeling and retrievals identify this feature as SiO$_2$(s) (quartz) clouds.
The SiO$_2$(s) clouds model is preferred at 3.5-4.2$\sigma$ versus a cloud-free
model and at 2.6$\sigma$ versus a generic aerosol prescription. We find the
SiO$_2$(s) clouds are comprised of small ${\sim}0.01$ $\rm{\mu}$m particles,
which extend to high altitudes in the atmosphere. The atmosphere also shows a
depletion of H$_2$O, a finding consistent with the formation of
high-temperature aerosols from oxygen-rich species. This work is part of a
series of studies by our JWST Telescope Scientist Team (JWST-TST), in which we
will use Guaranteed Time Observations to perform Deep Reconnaissance of
Exoplanet Atmospheres through Multi-instrument Spectroscopy (DREAMS).
