The dense clouds in the interstellar medium consist primarily of
Hydrogen and Helium gas, remaining from the formation of the Universe.
Other atomic and molecular gases such as NH3, HCN, CO and CO2
are also present, having formed in the cloud itself, or having been ejected
from a nearby star. The solid-state species only constitute around 1% of
the total matter!
Dust grains in the interstellar medium are so cold (10 K) that most
debris accretes icy mantles, either from freeze-out of gas-phase reaction
products or by retention of atomic species in collision with the grains.
Two types of icy mantle are thought to exist:
-
polar ices (in regions where there is more atomic Hydrogen than molecular
Hydrogen (H2))
-
a-polar ices (in regions where there is less atomic Hydrogen than molecular
Hydrogen (H2))
The grains are only a few microns diameter, so the ice mantle
thickness does not exceed 10 nm. Both amorphous and crystalline ices have
been identified in the interstellar medium, from a number of different
molecular species.
Why is it important to understand gas-dust interactions?
|
astronomy viewpoint
|
-
to model the role of icy dust in the evolution of dense molecular clouds
-
to study star formation
|
chemistry viewpoint
|
-
to explain the existence of complex molecular species in the interstellar
medium
-
to understand the nature of the icy mantles and dust cores
-
to learn about the bonding and interactions at the gas-ice interface
-
to study molecular species adsorbing, desorbing and scattering at ice surfaces
-
to ecludiate the rates and mechanisms of chemical reactions on ice surfaces
|
Almost nothing is known about how the dust grains form, or the role
they play in the chemical evolution of the interstellar medium. However,
when atmospheric chemists started to study the interactions between gases
and ices in the upper atmosphere they found a feasible way to explain why
the Ozone hole existed. Similarly, heterogeneous chemistry will help astronomers
to explain the production rates for molecular hydrogen and explain the
diversity of molecular species that exist in the interstellar medium. By
understanding gas-dust interactions we can start to solve some of the key
questions facing 21st century astronomers...
The Surface Astrophysics experiment is part of 
Nottingham Astronomy Research Forum
and is based in the Surface
Science Group in the School
of Chemistry at the University
of Nottingham
These pages were last updated 14/10/01
