  Didymos is a binary near-Earth asteroid. It is the target of the DART and
HERA space missions. The primary body, Didymos, rotates close to the spin at
which it is expected to shed mass. The secondary body, Dimorphos, is a 140
meters moon that orbits the primary body in about 12 hours. Here we investigate
the possible origin of Dimorphos. Using 1D models of ring/satellite
interactions, we study the evolution of material lost from Didymos' surface and
deposited as a ring at its equator. We find that due to viscous spreading, the
ring spreads outside the Didymos' Roche limit forming moonlets. A fraction of
the mass will form Dimorphos and a set of objects near the Roche limit, while
most of the ring's mass falls back on Didymos. To match the properties of
today's Dimorphos, the total mass that must be deposited in the ring is about
25% of Didymos' mass. It is possible that a fraction of the material travelled
several times between the ring and the surface of Didymos. The models produce
an orbit similar to that observed for a Didymos tidal parameter k2/Q<1e-5. If
the ring deposition timescale is long (>100 yr) (so the material flux is small)
Dimorphos could be irregularly shaped as it forms from the collision of
similar-sized satellitesimals. However, the top-shape of Didymos is expected to
be achieved due to a fast spin-up of the asteroid, which would result in a
short deposition timescale (<yr). In that case, the satellite would form from
progressively accreting material at the Roche Limit, resulting in an
ellipsoidal Dimorphos constructed of small pieces with sizes of the order of
meters, which is apparently in agreement with the recent images of Dimorphos
obtained by DART mission.
