  Enceladus is a prime target in the search for life in our solar system,
having an active plume likely connected to a large liquid water subsurface
ocean. Using the sensitive NIRSpec instrument onboard JWST, we searched for
organic compounds and characterized the plume's composition and structure. The
observations directly sample the fluorescence emissions of H2O and reveal an
extraordinarily extensive plume (up to 10,000 km or 40 Enceladus radii) at
cryogenic temperatures (25 K) embedded in a large bath of emission originating
from Enceladus' torus. Intriguingly, the observed outgassing rate (300 kg/s) is
similar to that derived from close-up observations with Cassini 15 years ago,
and the torus density is consistent with previous spatially unresolved
measurements with Herschel 13 years ago, suggesting that the vigor of gas
eruption from Enceladus has been relatively stable over decadal timescales.
This level of activity is sufficient to maintain a derived column density of
4.5x1017 m-2 for the embedding equatorial torus, and establishes Enceladus as
the prime source of water across the Saturnian system. We performed searches
for several non-water gases (CO2, CO, CH4, C2H6, CH3OH), but none were
identified in the spectra. On the surface of the trailing hemisphere, we
observe strong H2O ice features, including its crystalline form, yet we do not
recover CO2, CO nor NH3 ice signatures from these observations. As we prepare
to send new spacecraft into the outer solar system, these observations
demonstrate the unique ability of JWST in providing critical support to the
exploration of distant icy bodies and cryovolcanic plumes.
