  Water is the main constituent of interstellar ices, and it plays a key role
in the evolution of many regions of the interstellar medium, from molecular
clouds to planet-forming disks. In cold regions of the ISM, water is expected
to be completely frozen out onto the dust grains. Nonetheless, observations
indicate the presence of cold water vapor, implying that non-thermal desorption
mechanisms are at play. Photodesorption by UV photons has been proposed to
explain these observations, with the support of extensive experimental and
theoretical work on ice analogues. In contrast, photodesorption by X-rays,
another viable mechanism, has been little studied. The potential of this
process to desorb key molecules, such as water, intact rather than fragmented
or ionised, remains unexplored. We experimentally investigated X-ray
photodesorption from water ice, monitoring all desorbing species. We find that
desorption of neutral water is efficient, while ion desorption is minor. We
derive for the first time yields that can be implemented in astrochemical
models. These results open up the possibility of taking into account the X-ray
photodesorption process in the modelling of protoplanetary disks or X-ray
dominated regions.
