Distribution of lesions in fetal brains following experimental infection of pregnant sheep with Toxoplasma gondii.

Six ovine fetal brains were harvested 33 to 35 days postchallenge from 5 ewes, each of which was given 3000 Toxoplasma gondii oocysts on day 90 of pregnancy. Histopathologic examination of transverse sections taken at 13 levels in the fetal brains revealed the presence of toxoplasmosis-related lesions in all 6 brains. However, lesions were not randomly distributed (P = .007); they were most numerous at the level of the optic tract, the rostral margin of the pons, and 4 mm caudal to the ansate sulcus and were absent in all sections at the level of the caudal cerebellum. Lesion distribution may be due to hemodynamic factors, differences in the expression of endothelial surface receptor molecules at the level of the blood-brain barrier, or the presence of localized permissive/inhibitory factors within the brain. The results have implications for the selection of areas of brain from aborted ovine fetuses to be examined histopathologically for laboratory diagnosis.

Number of portal tract macrophages correlates with the modified hepatic activity index in chronic hepatitis C infection.

The Ishak modified hepatic activity index (mHAI) is widely used to score disease activity in chronic hepatitis C infection. However, the scoring of the mHAI components is subjective and prone to interobserver variation. Liver injury results in increased numbers of portal tract macrophages, which are easily identified via periodic acid-Schiff with diastase digestion stain. Evaluation of 30 liver biopsies from patients with chronic hepatitis C revealed increasing numbers of portal tract macrophages as scores of liver inflammation increased. Specifically, the number of PASD-positive portal tract macrophages per centimeter of biopsy length correlated with the level of portal inflammation and total mHAI score, and these correlations were statistically significant (P = .039 and .029, respectively). Although the portal macrophage count appeared to correlate with the interface activity and lobular necroinflammatory score, this did not meet statistical significance (P = .073 and .079, respectively). Interobserver agreement by κ analysis was greater for the portal macrophage count than for any individual component of the mHAI score. In summary, the number of periportal ceroid-laden macrophages correlates with liver inflammation as measured using the mHAI, with better interobserver agreement. This technique may serve as a useful adjunct to the mHAI in the assessment of liver injury in hepatitis C.

Mars: an evolving atmosphere.

Photochemical reactions in the martian exosphere produce fast atoms of oxygen, carbon, and nitrogen and provide large escape fluxes of these elements. They appear to play a crucial role in the evolution of the martian atmosphere. The relative outgassing rates of H(2)O and CO(2) on Mars are comparable with terrestrial values, although absolute rates for Mars are lower by a factor of 10(3). Nitrogen is a trace constituent, less than 1 percent, of the present martian atmosphere.

Mars: is nitrogen present?

If the atmosphere is uniformly mixed, a mixing ratio of nitrogen to carbon dioxide of 5 percent is consistent with the observational data on the ultraviolet dayglow of Mars. If the magnitude of the eddy coefficient in the atmosphere is similar to that for the earth, this limit is reduced to less than 0.5 percent.

Isotopic composition of the martian atmosphere.

Results from the neutral mass spectrometer carried on the aeroshell of Viking 1 show evidence for NO in the upper atmosphere of Mars and indicate that the isotopic composition of carbon and oxygen is similar to that of Earth. Mars is enriched in (15)N relative to Earth by about 75 percent, a consequence of escape that implies an initial abundance of nitrogen equivalent to a partial pressure of at least 2 millibars. The initial abundance of oxygen present either as CO(2) or H(2)O must be equivalent to an exchangeable atmospheric pressure of at least 2 bars in order to inhibit escape-related enrichment of (18)O.

Structure of the neutral upper atmosphere of Mars: results from viking 1 and viking 2.

Neutral mass spectrometers carried on the aeroshells of Viking 1 and Viking 2 indicate that carbon dioxide is the major constituent of the martian atmosphere over the height range 120 to 200 kilometers. The atmosphere contains detectable concentrations of nitrogen, argon, carbon monoxide, molecular oxygen, atomic oxygen, and nitric oxide. The upper atmosphere exhibits a complex and variable thermal structure and is well mixed to heights in excess of 120 kilometers.

Fixation of nitrogen in the prebiotic atmosphere.

Reactions between nitrogen and water in the air surrounding lightning discharges can provide an important source of nitric oxide even under conditions where oxygen is a minor atmospheric constituent. Estimates are given for the associated source of soluble nitrite and nitrate. It is shown that lightning and subsequent atmospheric chemistry can provide a source of nitrate for the primitive ocean as large as 106 tons of nitrogen per year, sufficient to fill the ocean to its present level of nitrate in less than 10(6) years.

Stability of the martian atmosphere.

A detailed chemical dynamic model is presented for a moist martian atmosphere. Recombination of carbon dioxide is catalyzed by trace amounts of water. The abundances of carbon monoxide and molecular oxygen should vary in response to changes in atmospheric water and atmospheric mixing.

Changing composition of the global stratosphere.

The current understanding of stratospheric chemistry is reviewed with particular attention to the influence of human activity. Models are in good agreement with measurements for a variety of species in the mid-latitude stratosphere, with the possible exception of ozone (O(3)) at high altitude. Rates calculated for loss of O(3) exceed rates for production by about 40 percent at 40 kilometers, indicating a possible but as yet unidentified source of high-altitude O(3). The rapid loss of O(3) beginning in the mid-1970s at low altitudes over Antarctica in the spring is due primarily to catalytic cycles involving halogen radicals. Reactions on surfaces of polar stratospheric clouds play an important role in regulating the abundance of these radicals. Similar effects could occur in northern polar regions and in cold regions of the tropics. It is argued that the Antarctic phenomenon is likely to persist: prompt drastic reduction in the emission of industrial halocarbons is required if the damage to stratospheric O(3) is to be reversed.

Helium on venus: implications for uranium and thorium.

Helium is removed at an average rate of 10(6) atoms per square centimeter per second from Venus's atmosphere by the solar wind following ionization above the plasmapause. The surface source of helium-4 on Venus is similar to that on Earth, suggesting comparable abundances of crustal uranium and thorium.

Isotopic composition of nitrogen: implications for the past history of mars' atmosphere.

Models are presented for the past history of nitrogen on Mars based on Viking measurements showing that the atmosphere is enriched in (15)N. The enrichment is attributed to selective escape, with fast atoms formed in the exosphere by electron impact dissociation of N(2) and by dissociative recombination of N(2)(+). The initial partial pressure of N(2) should have been at least as large as several millibars and could have been as large as 30 millibars if surface processes were to represent an important sink for atmospheric HNO(2) and HNO(3).

Jupiter's Ionosphere: Prospects for Pioneer 10.

Model Jovian ionospheres are constructed for comparison with Pioneer 10 results. Electron density maxima are predicted at a level approximately 220 kilometers above an assumed reference height where the hydrogen density is 10(16) molecules per cubic centimeter. It may be possible to use observations of the electron density to locate the turbopause. Attention is drawn to a possible strong source of ionized sodium from lo which might lead to large electron densities at low altitudes.

Transformations of nitrogen in a polluted estuary: nonlinearities in the demand for oxygen at low flow.

Oxidation of sewage ammonium in the Potomac River is described in terms of a simple kinetic model, with growth of nitrifying bacteria limited by the supply of ammonium ion. The oxidation rate varies inversely with freshwater inflow, and the associated demand for oxygen varies as the inverse square of the freshwater inflow rate. Similar behavior is observed for the Delaware River. The model accounts for the observed concentrations of ammonium and nitrous oxide.

Escape of hydrogen from venus.

Recombination of O(2)(+) represents a source of fast oxygen atoms in Venus' exosphere, and subsequent collisions of oxygen atoms with hydrogen atoms lead to escape of about 10(7) hydrogen atoms per square centimeter per second. Escape of deuterium atoms is negligible, and the ratio of deuterium to hydrogen should increase with time. It is suggested that the mass-2 ion observed by Pioneer Venus is D(+), which implies a ratio of deuterium to hydrogen in the contemporary atmosphere of about 10(-2), an initial ratio of 5 x 10(-5) and an original H(2)O abundance not less than 800 grams per square centimeter.

Composition and structure of the martian upper atmosphere: analysis of results from viking.

Densities for carbon dioxide measured by the upper atmospheric mass spectrometers on Viking 1 and Viking 2 are analyzed to yield height profiles for the temperature of the martian atmosphere between 120 and 200 kilometers. Densities for nitrogen and argon are used to derive vertical profiles for the eddy diffusion coefficient over the same height range. The upper atmosphere of Mars is surprisingly cold with average temperatures for both Viking 1 and Viking 2 of less than 200 degrees K, and there is significant vertical structure. Model calculations are presented and shown to be in good agreement with measured concentrations of carbon monoxide, oxygen, and nitric oxide.

Ultraviolet observations of venus from mariner 10: preliminary results.

An objective grating spectrometer on Mariner 10 has measured air-glow in the wavelength range 200 to 1700 angstroms. The data reveal the presence of significant concentrations of hydrogen, helium, carbon, and oxygen atoms in the upper atmosphere of Venus. A preliminary analysis of the hydrogen data indicates an exospheric temperature of 400 degrees K. There is evidence for intense air-glow emission at wavelengths longward of 1350 angstroms; the nature of this emission is unclear, but the radiation is spatially extensive and detectable on both day and night sides of the planet.

Mercury's Atmosphere from Mariner 10: Preliminary Results.

Analysis of data obtained by the ultraviolet experiment on Mariner 10 indicates that Mercury is surrounded by a thin atmosphere consisting in part of helium. The partial pressure of helium at the terminator is about 5 x 10(-12) millibar. The total surface pressure of the atmosphere is less than about 2 x 10(-9) millibar. Upper limits are set for the abundance of various gases, including hydrogen, oxygen, carbon, argon, neon, and xenon. The wavelength dependence of Mercury's surface albedo is similar to that of the moon over a broad range of wavelengths from 500 to 1600 angstroms. Strong signals were recorded by the airglow instrument as Mariner 10 passed through the cavity behind Mercury. They are as yet unexplained but may provide information on the properties of the local plasma.

Composition and structure of the venus atmosphere: results from pioneer venus.

Results from the Pioneer Venus sounder probe neutral mass spectrometer indicate that there is no difference in the isotopic ratios of carbon and oxygen between Venus and Earth to within +/- 5 percent. The mixing ratio of nitrogen is 3.5(+3)(-2) percent with an isotopic ratio within 20 percent of that of Earth. The ratio of argon-36 to argon-40 is 85 percent, and the ratio of argon-38 to argon-36 is 20 percent. The mixing ratios of argon-36 and argon-40 are approximately 40 and 50 parts per million, respectively, with an error of about a factor of 2 (mainly toward a lesser amount) resulting from uncertainty in the response of the ion pump to rare gases. Hydrogen chloride cannot account for more than a few percent of the 36 mass peak, and therefore the large excess of primordial argon is a reasonable conclusion. The ratio of neon-20 to argon-36 of 0.5 +/- 0.3 is definitely terrestrial in character rather than solar. These results indicate that there is a large excess of all primordial noble gases on Venus relative to Earth. There appears to be a considerably higher abundance of sulfur compounds below 20 kilometers than in or above the main cloud layer. The 32 and 60 mass peaks show a sharp increase below 22 kilometers, indicating the possible production of sulfur and carbon oxysulfide (COS) at the expense of sulfur dioxide.

Venus lower atmospheric composition: preliminary results from pioneer venus.

Initial examination of data from the neutral mass spectrometer on the Pioneer Venus sounder probe indicates that the abundances of argon-36, argon-38, and neon-20 in the Venus atmosphere are much higher than those of the corresponding gases in Earth's atmosphere, although the abundance of radiogenic argon-40 is apparently similar for both planets. The lower atmosphere of Venus includes significant concentrations of various gaseous sulfur compounds. The inlet leak to the mass spectrometer was temporarily blocked by an apparently liquid component of the Venus clouds during passage through the dense cloud layer. Analysis of gases released during the evaporation of the droplets shows the presence of water vapor to some compound or compounds of sulfur.

Size distribution of fine particles from coal combustion.

Measurements of the particle size distribution at the outlets of six coal-fired utility boilers showed a peak at a particle diameter near 0.1 micrometer. This submicrometer mode appears to be a general feature of coal combustion that results from a volatilization-condensation process in the boiler. At the boilers tested, the submicrometer mode contained 0.2 to 2.2 percent of the total fly ash mass. The importance of this mode is greater than its small quantity suggests because particles in the submicrometer size range are often much more difficult to collect with conventional particulate control devices than larger particles. Thus, the submicrometer mode may significantly influence the design and selection of future power plant emission controls. The particle mass in the submicrometer mode was correlated with the nitric oxide concentration in the flue gas. This correlation suggests that control of nitric oxide by modification of the combustion conditions may reduce the generation and emission of submicrometer particles.