  Water and ammonia vapors are known to be the major sources of spectral
absorption at pressure levels observed by the microwave radiometer (MWR) on
Juno. However, the brightness temperatures and limb darkening observed by the
MWR at its longest wavelength channel of 50 cm (600 MHz) in the first 9
perijove passes indicate the existence of an additional source of opacity in
the deep atmosphere of Jupiter (pressures beyond 100 bar). The absorption
properties of ammonia and water vapor, and their relative abundances in
Jupiter's atmosphere do not provide sufficient opacity in deep atmosphere to
explain the 600 MHz channel observation. Here we show that free electrons due
to the ionization of alkali metals, i.e. sodium, and potassium, with sub-solar
metallicity [M/H] (log based 10 relative concentration to solar) in the range
of [M/H] = -2 to [M/H] = -5 can provide the missing source of opacity in the
deep atmosphere. If the alkali metals are not the source of additional opacity
in the MWR data, then their metallicity at 1000 bars can only be even lower.
The upper bound of -2 on the metallicity of the alkali metals contrasts with
the other heavy elements -- C, N, S, Ar, Kr, and Xe -- which are all enriched
relative to their solar abundances having a metallicity of approximately +0.5.
