  Jupiter's deep abundances help to constrain the formation history of the
planet and the environment of the protoplanetary nebula. Juno recently measured
Jupiter's deep oxygen abundance near the equator to be 2.2$_{-2.1}^{+3.9}$
times the protosolar value (2$\sigma$ uncertainties). Even if the nominal value
is supersolar, subsolar abundances cannot be ruled out. Here we use a
state-of-the-art one-dimensional thermochemical and diffusion model with
updated chemistry to constrain the deep oxygen abundance with upper
tropospheric CO observations. We find a value of 0.3$_{-0.2}^{+0.5}$ times the
protosolar value. This result suggests that Jupiter could have a carbon-rich
envelope that accreted in a region where the protosolar nebula was depleted in
water. However, our model can also reproduce a solar/supersolar water abundance
if vertical mixing is reduced in a radiative layer where the deep oxygen
abundance is obtained. More precise measurements of the deep water abundance
are needed to discriminate between these two scenarios and understand Jupiter's
internal structure and evolution.
