Fig. 2. A simplified model for enriching D/H on dust grains in the context of the dust cycle observed on comet 67P/C-G.

Creating enriched layers of water on dust grains begins with (A) water ice from the nucleus sublimating to expose dust surfaces. (B) HDO, hypervolatiles, and potentially H₂¹⁷O, preferentially adsorbs to the exposed surfaces creating a highly enriched near surface layer. (C) Additional slightly enriched water ice layers accumulate. (D) The dust cycle for 67P (30) begins during perihelion, or phase 1, when increasing activity releases new dust grains into the coma. These grains release hypervolatiles and the top layers of enriched water (31). (E) During both phases 1 and 2, about 20% of the dm-size dust grains are redeposited onto the surface in colder regions of the nucleus (30, 31) to be (F) reactivated during the next pre-perihelion, or phase 3. At this time, the ice on the dust grains is depleted in hypervolatiles and the most enriched layers of water ice are released.