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Soil water contents from both smooth and rough bare soil were estimated from remotely sensed surface soil and air temperatures. We found an inverse relationship between two thermal parameters and gravimetric soil water content for Avondale loam when its water content was between air‐dry and field capacity. These parameters, daily maximum minus minimum surface soil temperature and daily maximum soil minus air temperature, appear to describe the relationship reasonably well. These two parameters also describe relative soil water evaporation (actual/potential). Surface soil temperatures showed good agreement between three measurement techniques: in situ thermocouples, ground‐based infrared radiation thermometer, and the thermal infrared band of an airborne mutispectral scanner.

Artificial ablation studies have been performed on iron and nickel‐iron samples by using an arc‐heated plasma of ionized air. Experiment conditions simulated a meteoroid traveling about 12 km/s at an altitude of 70 km. The artificially produced fusion crusts and ablation debris show features very similar to natural fusion crusts of the iron meteorites Boguslavka, Norfork, and N'Kandhla and to magnetic spherules recovered from deep‐sea Mn nodules. X ray diffraction, electron microprobe, optical, and scanning electron microscope analyses revealed that important mineralogical, elemental, and textural changes occur during ablation. Some metal is melted and ablated. The outer margin of the melted rind is oxidized and recrystallizes as a discontinuous crust of magnetite and wüstite. Adjacent to the oxidized metallic ablation zone is an unoxidized metallic zone in which structures such as Widmannstatten bands are obliterated as the metal is transformed to unequilibrated α2 nickel‐iron. Volatile elements such as S and P are vaporized in the ablation zone. Less volatile elements such as Si and Mn undergo fractionation and are concentrated in the oxide phases, while Ni is concentrated in the metal phases. Dissimilar phases form an intimate intergrowth that persists to a submicroscopic level. Identification of meteor ablation debris in particle collections cannot be based on a single parameter; elemental and mineralogical composition as well as morphological and textural features must be considered.

Artificial meteor ablation was performed on natural minerals, composed predominately of magnetite and hematite, using an arc heated plasma stream of air. Analysis of the ablated debris indicates most was composed of two or more minerals. Wustite, a metastable mineral, was found to occur as a common product. The "magnetite" model, whose content was 80% magnetite, 14% hematite, 4% apatite, and 2% quartz, yielded ablated products consisting of over 12 different minerals. Magnetite occurred in 91% of all specimens examined, hematite in 16%, and wustite in 39%. The "hematite" model, whose content was 96% hematite and 3% quartz, yielded ablated products consisting of over 13

A successful flight of a recoverable payload designed to collect particles from a noctilucent cloud was made on August 1, 1968, at 0410 local daylight time (0910 UT), from Fort Churchill, Canada. Photographs obtained by project observers at Thompson, Canada, 400 km southwest of Churchill, showed that the noctilucent cloud was over the launch site at the time of flight. Electron microscope examinations of collecting surfaces exposed above 60 km showed about 6000 particles/cm • of a certain type on surfaces facing the flight direction. Few of these particles were found on nonflight control surfaces. Most of the particles range in size from 0.1-0.2 • and do not exhibit features suggesting that any liquid was associated with them. The shape of these particles is generally equant, often rod-like, subrounded to rounded; the particles are moderately dense to electrons, show internal structure, and have occasional protuberances. Considerable variation of particle concentration was observed from one flight surface to the next. Flight surfaces show some damage from aerodynamic heating. Thus, caution is necessary in applying these findings pending confirmatory sounding rocket flights through noctilucent clouds. Evidence from this flight, from previous rocket and satellite flights, and theoretical studies suggests that the noctilucent cloud particles are uplifted from lower altitudes and could have a terrestrial as well as cosmic origin. Intensive investigations of noctilucent clouds have been made since their first recorded discovery as an unusual phenomenon by Backhouse [1885]. These tenuous clouds form at an altitude of 80 km during the summer seasons above latitudes of 60 ø in both hemispheres. Fogle and Haurwitz [1966] have summarized the historical and observational data that they and numerous other investigators have obtained. Despite extensive study, the origin and composition of noctilucent clouds remain in doubt. Helmholtz [1889] and later StSrmer [1933] observed and confirmed that the clouds were composed of particles made visible by scattered sunlight. Visual observations by Vestine [1934], Astapovich [1939], Ludlam [1957], and Witt [1957] led these investigators to predict a particle size less than i /•. Spectra obtained by Grishin [1956] were later interpreted by Deirmendjian and Vestine [1959], who concluded that dielectric spheres smaller than 0.8 /• could cause the observed scattering. Polarization measurements were interpreted by Witt [1960a, b] and Villmann [1962] using Mie scattering