prise only about 1% of the mass. Under these conditions, sorbed gases and 

 reactive intermediates produced on a grain would remain on that grain (or 

 desorb into the gas phase where other processes would govern their fate) until 

 the grain is accreted with others into larger objects; consequently, the composi- 

 tion and abundance of the products and the rates at which they could form may 

 well be strongly constrained and different from those observed in terrestrial 

 laboratories. Gas-grain interactions that are independent of a bulk solid phase 

 should be amenable to study under microgravity conditions. 



Other hypothetical gas-grain processes of nebular or interstellar relevance that 

 merit study include the hydration of silicate grains to phyllosilicates by gaseous 

 water, the photoirradiation of icy mantles of grains by starlight, and the thermal 

 evolution of interstellar condensates in the solar nebula. 



Technology is being developed on Spacelab to levitate individual small 

 neutral particles (<1 -cm-diameter) in acoustic levitation chambers. (Charged 

 particles can be levitated electrostatically.) These levitation devices allow full 

 control of the dynamics of the particle, including translation about the levitation 

 chamber, spin angular momentum and orientation, and shape for liquid droplets. 

 This technology could be used to suspend grains in gas chambers to study the 

 physical and chemical interactions between gases and grains. The goal of such 

 experiments would be to understand gas-grain processes in interstellar clouds and 

 in the primordial solar nebula. An additional experiment might be to examine 

 the collective response of a large number of small suspended particles with 

 various initial conditions, including a central force, to simulated nebula conden- 

 sation processes. 



5.2 Reactions of Neutral Atomic Oxygen 



The continuous source of oxygen atoms flowing with a narrow range of high 

 velocities in low Earth orbit provides a unique opportunity to study oxygen 

 atom chemistry as it relates to biological and biogenic molecules. The glow 

 observed on the windward surfaces of the Space Shuttle shows that the differ- 

 ential velocity of about 8 km/sec between the Shuttle and ambient oxygen 

 atoms is sufficient to overcome the activation energy for reaction between 

 ground-state oxygen atoms and certain, as yet unspecified, large molecules at 

 rates sufficient to permit detection. Oxygen atom reaction rates and activation 

 energies couid be studied by injecting various reactants into a cell through which 

 the oxygen atoms flow and monitoring the products downstream by either 

 absorption or emission spectroscopy in the ultraviolet, visible, and perhaps 

 infrared. Impact velocity can be varied from an upper limit of 8 km/sec to a few 

 meters per second by having the incoming stream of oxygen bounce off several 

 deflection plates or by injecting the reactants with a velocity vector either paral- 

 lel, antiparallel, or perpendicular to the incoming beam. Interesting reaction 

 partners to choose from are plentiful with, for example, bacteria and biologically 



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