Ice Coverage of Dust Grains in Cold Astrophysical Environments

Surface processes on cosmic solids in cold astrophysical environments lead to gas-phase depletion and molecular complexity. Most astrophysical models assume that the molecular ice forms a thick multilayer substrate, not interacting with the dust surface. In contrast, we present experimental results demonstrating the importance of the surface for porous grains. We show that cosmic dust grains may be covered by a few monolayers of ice only. This implies that the role of dust surface structure, composition, and reactivity in models describing surface processes in cold interstellar, protostellar, and protoplanetary environments has to be reevaluated.

Figure 1

Electron microscopy images of amorphous carbon grains produced by gas-phase condensation and subsequent deposition onto a substrate. The left-hand, high-resolution transmission electron microscopy image shows the porous structure of the condensed carbonaceous grains. The agglomerate of nanometer-sized primary grains, which are attached to the Lacey carbon support film, is visible in the left upper corner. The right-hand image is a field-emission scanning electron microscopy image of the porous grain layer. Visible individual grains are still agglomerates of smaller grains.

Figure 2

TPD curves for CO ices of different nominal thicknesses deposited onto a pure KBr substrate (left) and onto a 60 nm layer of carbon grains (right). The dashed lines show constant peak temperatures indicating the monolayer ice coverage.

Figure 3

The dependence of the nominal ice thickness corresponding to the monolayer-multilayer transition on the thickness of the grain layer.

Figure 4

Schematic showing dust grains (gray) mixed with ice molecules (blue) and the main sources of their processing in astrophysical environments.