  We present ALMA maps of the starless molecular cloud core Ophiuchus/H-MM1 in
the lines of deuterated ammonia (ortho-NH2D), methanol (CH3OH), and sulphur
monoxide (SO). The dense core is seen in NH2D emission, whereas the CH3OH and
SO distributions form a halo surrounding the core. Because methanol is formed
on grain surfaces, its emission highlights regions where desorption from grains
is particularly efficient. Methanol and sulphur monoxide are most abundant in a
narrow zone that follows the eastern side of the core. This side is sheltered
from the stronger external radiation field coming from the west. We show that
photodissociation on the illuminated side can give rise to an asymmetric
methanol distribution, but that the stark contrast observed in H-MM1 is hard to
explain without assuming enhanced desorption on the shaded side. The region of
the brightest emission has a wavy structure that rolls up at one end. This is
the signature of Kelvin-Helmholtz instability occurring in sheared flows. We
suggest that in this zone, methanol and sulphur are released as a result of
grain-grain collisions induced by shear vorticity.
