  Oxygen is the most common element after hydrogen and helium in Jupiter's
atmosphere, and may have been the primary condensable (as water ice) in the
protoplanetary disk. Prior to the Juno mission, in situ measurements of
Jupiter's water abundance were obtained from the Galileo Probe, which dropped
into a meteorologically anomalous site. The findings of the Galileo Probe were
inconclusive because the concentration of water was still increasing when the
probe died. Here, we initially report on the water abundance in the equatorial
region, from 0 to 4 degrees north latitude, based on 1.25 to 22 GHz data from
Juno Microwave radiometer probing approximately 0.7 to 30 bars pressure.
Because Juno discovered the deep atmosphere to be surprisingly variable as a
function of latitude, it remains to confirm whether the equatorial abundance
represents Jupiter's global water abundance. The water abundance at the
equatorial region is inferred to be $2.5_{-1.6}^{+2.2}\times10^3$ ppm, or
$2.7_{-1.7}^{+2.4}$ times the protosolar oxygen elemental ratio to H (1$\sigma$
uncertainties). If reflective of the global water abundance, the result
suggests that the planetesimals formed Jupiter are unlikely to be water-rich
clathrate hydrates.
