Calcretes of Olduvai Gorge and the Ndolanya Beds of northern Tanzania

Pedogenic calcretes are closely associated with Pliocene to Holocene wind-worked deposits of volcanic ash in the Olduvai and Ndolanya Beds of northern Tanzania. The typical profile with calcrete consists of an unconsolidated sediment layer, an underlying laminar calcrete, and a lowermost massive calcrete. The laminar calcrete is a relatively pure limestone, whereas massive calcrete is aeolian tuff cemented and replaced by calcite. An Olduvai calcrete profile can develop to a mature stage in only a few thousand years. Carbonatite ash was the dominant source for most of the calcite in the calcretes. Replacement was a major process in formation of the massive calcretes, and oolitic textures have resulted from micrite replacing pelletoid clay coatings around sand grains. Phillipsite and possible other zeolites were extensively replaced in the massive calcretes. Replacement of clay by micrite in the Olduvai calcretes is accompanied by dissolution or leaching of phengitic illite and the formation of clay approaching the composition of halloysite or kaolinite. In the upper calcrete of the Ndolanya Beds, montmorillonite was altered to a kaolinite-type mineral and to dioctahedral chlorite. Authigenic dolomite, zeolite, and dawsonite in the Olduvai calcretes probably received at least some of their components from replaced materials.     

Ice clathrate as a possible source of the atmospheres of the terrestrial planets

The presence and compositions of atmospheres on the terrestrial planets do not follow directly from condensation models which would have Earth accreting near 500°K. No single mechanism yet proposed adequately accounts for the abundances of noble gases and carbon and nitrogen in the atmospheres. We show that the composition of clathrates forming at low temperatures in cold regions of the nebula can be predicted. Addition of about 1 ppm clathrate material to the Earth can explain observed abundances of Ar, Kr, and Xe. Condensation and adsorption processes occuring at 400–500°K are necessary to explain the observed abundances of Ne, H₂O, C, and N. Possible sources of clathrates could be cometary bodies formed in the outer solar system.     

Analysis of stellar occultation data: Effects of photon noise and initial conditions

A new inversion technique for obtaining temperature, pressure, and number density profiles of a planetary atmosphere from an occultation light curve is described. This technique employs an improved boundary condition to begin the numerical inversion and permits the computation of errors in the profiles caused by photon noise in the light curve. We present our assumptions about the atmosphere, optics, and noise and develop the equations for temperature, pressure, and number density and their associated errors. By inverting in equal increments of altitude, Δh, rather than in equal increments of time, Δt, the inversion need not be halted at the first negative point on the light curve as required by previous methods. The importance of the boundary condition is stressed, and a new initial condition is given. Numerical results are presented for the special case of inversion of a noisy isothermal light curve. From these results, simple relations are developed which can be used to predict the noise quality of an occultation. It is found that fractional errors in temperature profiles are comparable to those of pressure and number density profiles. An example of the inversion method is shown. Finally, we discuss the validity of our assumptions. In an appendix we demonstrate that minimum fractional errors in scale height determined from the inversion are comparable to those from an isothermal fit to a noisy isothermal light curve.     

Absolute spectrophotomery of comets 1973XII and 1975IX: II. Profiles of the Swan bands

Absolute spectrophotometric measurements of the Swan bands of two comets have been compared with computed synthetic spectra using modern Franck-Condon and Hönl-London factors, and varying rotational, vibrational, and electronic excitation temperatures. Rotational and vibrational temperatures were obtained for the comets. Although the electronic excitation temperature and the molecular column density cannot be separated, a relationship is found between these two quantities. A review is made of recent determinations of column densities for CN in comets.     

Direct imaging of extra-solar planets with stationary occultations viewed by a space telescope

The feasibility of detecting planets outside the solar system through imaging at optical wavelengths by a telescope in space is considered. The “black” limb of the moon can be used as an occulting edge to greatly reduce the background light from the planet's star. With this technique and if certain other technical requirements can be realized, a hypothetical Jupiter-Sun system could be detected at a distance of 10 pc. For this system, a signal-to-noise ratio of 9 could be achieved in less than 20 min with a 2.4-m telescope in space. For diffraction limited optics, the required integration time is inversely proportional to the fifth power of the telescope aperture. An orbit for the telescope is described that could achieve a stationary lunar occultation of any star that would last nearly two hours, providing six times more integration time than required by the hypothetical Jupiter-Sun example.     

How to compare the surface of Io to laboratory samples

Since one does not know the photometric functions of various parts of Io, one cannot convert the observed geometric albedo of the satellite to a parameter more directly measurable in the laboratory. One must therefore convert laboratory reflectances to geometric albedos before quantitative comparisons between Io's surface and a laboratory sample are made. This procedure involves determining the wavelength dependence of the sample's photometric function. For substances such as sulfur, whose reflectance varies strongly with wavelength, it is incorrect to assume that the photometric function, and hence the ratio (laboratory reflectance/geometric albedo) is independent of wavelength. To illustrate this point, measurements of the color dependence of this ratio for sulfur are presented for the specific case in which the measured laboratory reflectance is the sample's normal reflectance. In general, unless the laboratory reflectance is precisely the geometric albedo, a wavelength-dependent correction factor must be determined before the laboratory sample can be compared quantitatively with Io's surface.     

The occultation of ϵ Gem by Mars as observed from Agassiz Station

Observations of the April 8, 1976 occultation of ? Gem by Mars made at the Agassiz Station of the Harvard College Observatory have been analyzed to yield temperature profiles of the Martian atmosphere for number densities between 10¹³ and 10¹⁵ cm^?3. Pronounced wavelike structure is evident in both immersion and emersion profiles, with a peak-tto-peak variation of up to 50°K and a vertical scale of 20 km.     

Reservoirs of atmospheric chlorine: Prospects for HOCl revisted

We have re-examined the prospects of HOCl as an inert reservoir for atmospheric chlorine in the light of new theoretical calculations and available experimental measurements of its photodissociation cross-sections. The theoretical calculations and most recent laboratory studies imply that the broad maxima 3200 Å observed in two other experimental spectra may not belong to HOCl. On the basis of this implication HOCl could have a long lifetime against photodissociation in the stratosphere, and could, thereby, become a reservoir for atmospheric chlorine comparable to ClONO₂ or even HCl. In this capacity HOCl could reduce the predicted ozone destruction due to any given level of total chlorine burden. We have also examined the difficulties in laboratory measurements of the HOCl absorption spectrum with particular emphasis on identifying the impurities which may be present in the experimental system. It appears that specialized new experiments are needed to clearly establish the nature and strength of HOCl absorption in the neighbourhood of 3200 Å. Some refinements in the theoretical calculations also seem desirable. In view of the difficulties involved in the laboratory determination of HOCl photodissociation cross-sections, it is suggested that a search for possible stratospheric HOCl by atmospheric spectroscopists would be worthwhile.     

On the relationship of 6300 Å airglow to parameters of the F-region of the ionosphere

It is shown that variations in 6300 Å airglow intensities can, under certain assumptions, be simply related to $\text{?}{}_0\text{F2}$ and its time derivative. In deriving the relationship it is not necessary to assume that the concentration of the neutral atmosphere remains constant and so the relationship is useful on occasions when changes in the neutral atmosphere do occur making it difficult to obtain agreement between observed and calculated 6300 Å intensities; An example is given of a night in which a post-midnight enhancement occurred in the airglow and for which the observations could not be reproduced using a neutral atmosphere constant with time. It is shown that the airglow variations can be explained in terms of the variations of f₀F2, implying that the airglow is due to recombination and that, during the night, changes occurred in the concentrations of the constituents of the neutral atmosphere.     

A study of the magnetic and velocity fields in an active region

A time sequence of magnetograms and velocity-grams in the H and Fe i 6569 Å lines has been made at a rate of 12 h^–1 of McMath Region 10385 from 26 to 29 October, 1969. The 14 flares observed during this period have been studied in relation to the configuration and changes in the magnetic and velocity fields. There was little correlation between flare position and the evolutionary changes in the photospheric magnetic and velocity field, except at large central meridian distances where the velocity observations suggested shearing taking place at flare locations. At central meridian distances > 30° we found that flares are located in areas of low line-of-sight photospheric velocity surrounded by higher velocity hills. The one exception to this was the only flare which produced a surge. Blue-shifted velocity changes in the photosphere of 0.3 to 1 km s^–1 were observed in localized areas at the times of 8 of 14 flares studied.Visiting Astronomer, Kitt Peak National Observatory.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.