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We report measurements of the rate of hydrous alteration of amorphous Mg‐SiO smokes as a function of temperature as inferred by observing changes in the infrared spectra of these materials. We show that under the conditions reported for the nucleus of Comet Halley based on measurements made by the Vega and Giotto missions, amorphous, anhydrous Mg‐SiO smokes would become hydrated within several weeks in the dusty regolith observed on the surface. However, if such grains were released in “jets” or from loose ice fragments, then previously amorphous grains would retain their anhydrous nature. Similarly, brief periods of aqueous activity on meteorite parent bodies would convert amorphous, fine‐grained material to hydrated phyllosilictes much more rapidly than coarse mineral grains. A kinetic model might therefore be developed to explain the observed textural relationships in the matrices of carbonaceous chondrites such as Mokoia, where amorphous phyllosilicates are intimately associated with coarse anhydrous grains.

Abstract-Samples of silica-rich and MgO-rich condensed, amorphous magnesiosilica smokes were hydrated to monitor systematic mineralogical and chemical changes as a function of time and temperature controlled by their unique metastable eutectic compositions, their porous texture, and the ultrafine, nanometer grain size of all entities. At water supersaturated conditions, proto-phyllosilicates formed by spinodal-type homogeneous nucleation. Their formation and subsequent growth was entirely determined by the availability of water via pore spaces inherited from the original smokes and the textural continuity of magnesiosilica material with a mostly smectite-dehydroxylate composition. The results may have implications for the hydration of proto-CI material, the presence of rare periclase and brucite in primitive solar system bodies, and the pervasiveness of hydrated amorphous magnesiosilica dust and saponite proto-phyllosilicates in icy-protoplanets, such as comet nuclei.

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An apparatus has been constructed to provide thermodynamic data for models of planetary atmospheres. Often these data are needed at low temperatures, especially for the outer planets and their satellites, but are not readily available in the literature. The vapor pressure of propane was measured from 85 to 240 K to demonstrate one application of the apparatus for the acquisition of these types of data and to assess the performance of the system. This molecule was chosen because it is available in high purity, it has a well-established vapor-pressure curve, and it exhibits only one phase change over this temperature range. Our results compare favorably with the values available in the literature. The major components of the system include several types of pressure measuring instruments (1000 and 1 Torr capacitance manometers, spinning-rotor gauge), a residual gas analyzer to monitor sample purity in situ, and a helium closed-cycle refrigerator for cooling. The gas-handling manifold was constructed using materials and techniques adapted from the semiconductor production industry to minimize sample impurities which constitute a significant source of error in these types of measurements. Several unique design features were also incorporated in the construction of the sample cell to facilitate proper correction for thermal transpiration—an important factor for pressure measurements at low temperatures—and to ensure that the temperature sensor accurately reflected the sample temperature. The operational temperature and pressure limits are 62–240 K and 3×10−6–103 Torr, respectively. The lowest achievable temperature is governed by the no-load temperature of the first stage of the refrigerator and vertical thermal gradients along the sample cell walls, while the minimum obtainable pressure is set by the base pressure of the manifold and a slight outgassing rate.