  We present 1-5um spectroscopy of the coldest known brown dwarf, WISE
J085510.83-071442.5 (WISE 0855), performed with the Near-Infrared Spectrograph
(NIRSpec) on board the James Webb Space Telescope (JWST). NIRSpec has
dramatically improved the measurement of spectral energy distribution of WISE
0855 in terms of wavelength coverage, signal-to-noise ratios, and spectral
resolution. We have performed preliminary modeling of the NIRSpec data using
the ATMO 2020 models of cloudless atmospheres, arriving at a best fitting model
that has T_eff=285 K. That temperature is ~20 K higher than the value derived
by combining our luminosity estimate with evolutionary models (i.e., the radius
in the model fit to the SED is somewhat smaller than expected from evolutionary
models). Through comparisons to the model spectra, we detect absorption in the
fundamental band of CO, which is consistent with an earlier detection in a
ground-based spectrum and indicates the presence of vertical mixing. Although
PH_3 is expected in Y dwarfs that experience vertical mixing, it is not
detected in WISE 0855. Previous ground-based M-band spectroscopy of WISE 0855
has been cited for evidence of H_2O ice clouds, but we find that the NIRSpec
data in that wavelength range are matched well by our cloudless model. Thus,
clear evidence of H_2O ice clouds in WISE 0855 has not been identified yet, but
it may still be present in the NIRSpec data. The physical properties of WISE
0855, including the presence of H_2O clouds, can be better constrained by more
detailed fitting with both cloudless and cloudy models and the incorporation of
unpublished 5-28um data from the Mid-infrared Instrument on JWST.
