Production of nitrogen and carbon species by thunderstorms on Venus

The effects of the newly discovered thunderstorms on Venus upon the nitrogen and carbon species in its atmosphere were calculated. An Earth-like lightning frequency of 100 sec^?1 was used for Venus, in accord with recent optical measurements by Pioneer-Venus (W. J. Borucki, J. W. Dyer, G. Z. Thomas, J. C. Jordon, and D. A. Comstock, submitted for publication). The rate of NO production by thunder shock waves, 2.5 × 10¹¹ g year^?1, is about an order of magnitude smaller than on the Earth. But on Venus, in the absence of precipitation, which is the major removal mechanism of odd nitrogen from the Earth's atmosphere, the mixing ratios of odd nitrogen species might be considerably higher. The global CO production is governed by CO₂ photolysis rather than by CO₂ pyrolysis by lightning. However, thunderstorms produce about 2.5 × 10¹¹ g year^?1 of CO in the cloud layer, far from the high altitude CO₂ photolysis region.     

The rate of small impacts on Earth

Abstract— Asteroids tens to hundreds of meters in diameter constitute the most immediate impact hazard to human populations, yet the rate at which they arrive at Earth's surface is poorly known. Astronomic observations are still incomplete in this size range; impactors are subjected to disruption in Earth's atmosphere, and unlike the Moon, small craters on Earth are rapidly eroded. In this paper, we first model the atmospheric behavior of iron and stony bodies over the mass range 1–10¹² kg (size range 6 cm‐1 km) taking into account deceleration, ablation, and fragmentation. Previous models in meteoritics deal with rather small masses (<10⁵–10⁶ kg) with the aim of interpreting registered fireballs in atmosphere, or with substantially larger objects without taking into account asteroid disruption to model cratering processes. A few earlier attempts to model terrestrial crater strewn fields did not take into account possible cascade fragmentation. We have performed large numbers of simulations in a wide mass range, using both the earlier “pancake” models and also the separated fragments model to develop a statistical picture of atmosphere‐bolide interaction for both iron and stony impactors with initial diameters up to ?1 km. Second, using a compilation of data for the flux at the upper atmosphere, we have derived a cumulative size‐frequency distribution (SFD) for upper atmosphere impactors. This curve is a close fit to virtually all of the upper atmosphere data over 16 orders of magnitude. Third, we have applied our model results to scale the upper atmosphere curve to a flux at the Earth's surface, elucidating the impact rate of objects <1 km diameter on Earth. We find that iron meteorites >5 times 10⁴ kg (2.5 m) arrive at the Earth's surface approximately once every 50 years. Iron bodies a few meters in diameter (10⁵–10⁶ kg), which form craters ?100 m in diameter, will strike the Earth's land area every 500 years. Larger bodies will form craters 0.5 km in diameter every 20,000 years, and craters 1 km in diameter will be formed on the Earth's land area every 50,000 years. Tunguska events (low‐level atmospheric disruption of stony bolides >10⁸ kg) may occur every 500 years. Bodies capable of producing hazardous tsunami (?200 m diameter projectiles) should strike the Earth's surface every ?100,000 years. This data also allows us to assess the completeness of the terrestrial crater record for a given area over a given time interval.     

Radiocarbon on Titan

Abstract— We explore the likely production and fate of ¹⁴C in the thick nitrogen atmosphere of Saturn's moon Titan and investigate the constraints that measurements of ¹⁴C might place on Titan's photochemical, atmospheric transport and surface‐atmosphere interaction processes. Titan's atmosphere is thick enough that cosmic‐ray flux limits the production of ¹⁴C: absence of a strong magnetic field and the increased distance from the Sun suggest production rates of ?9 atom/cm²/s, ?4x higher than Earth. The fate and detectability of ¹⁴C depends on the chemical species into which it is incorporated: as methane it would be hopelessly diluted even in only the atmosphere. However, in the more likely case that the ¹⁴C attaches to the haze that rains out onto the surface (as tholin, HCN or acetylene and their polymers), haze in the atmosphere or recently deposited on the surface would be quite radioactive. Such radioactivity may lead to a significant enhancement in the electrical conductivity of the atmosphere which will be measured by the Huygens probe. Measurements with simple detectors on future missions could place useful constraints on the mass deposition rates of photochemical material on the surface and identify locations where surface deposits of such material are “freshest”.     

The structure and microphysical properties of the Venus clouds: Venera 9, 10, and 11 data

Data processing and interpretation of the nephelometer measurements made in the Venus atmosphere aboard the Venera 9, 10 and 11 landers in the sunlit hemisphere near the equator are discussed. These results were used to obtain the aerosol distribution and its microphysical properties from 62 km to the surface. The main aerosol content is found in the altitude range between 62 km (where measurements began) and 48 km, the location of the cloud region. Three prominent layers labeled as I (between 62 and 57 km), II (between 57 and 51 km) and III (between 51 and 48 km), each with different particle characteristics are discovered within the clouds. The measured light-scattering patterns can be intrepreted as having been produced by particles with effective radii from 1 to 2 μm depending on height and indices of refractivity from 1.45 in layer I to 1.42 in layer III. These values do not contradict the idea that the droplets are made of sulfuric acid. In layers II and III the particle size distribution is at least bimodal rather than uni-modal. The index of refraction is found to decrease to 1.33 in the lower part of layer II, suggesting a predominant abundance of larger particles of different chemical origin, and chlorine compounds are assumed to be relevant to this effect. In the entire heightrange of the Venera 9–11 craft descents, the clouds are rather rarefied and are characterized by a mean volume scattering coefficient σ ～ 2 × 10^?5 cm^?1 that corresponds to the mean meteorological range of visibility of about 2 km. The average mass content of condensate is estimated to be equal to 4 × 10^?9 g/cm³, and the total optical depth of clouds to τ ～ 35. Near the bottom of layer III clouds are strongly variable. In the subcloud atmosphere a haze was observed between 48 and 32 km; that haze is mainly made of submicron particles, r_eff ～ 0.1μm. The atmosphere below that is totally transparent but separate (sometimes possibly disappearing) layers may be present up to a height of 8 km above the surface. A model of this region with a very low particle density (N ? 2–3 cm^?3) strongly refractive large particles (r_eff ? 2.5 μm; 1.7 < n < 2.0) provided satisfactory agreement. The optical depth of aerosol in the atmosphere below the subcloud haze does not exceed 2.5.     

Does Io have an ammonia atmosphere?

An atmosphere containing 0.5 cm atm of ammonia is assumed on Io. Such an atmosphere will be frozen at the unilluminated pole during the solstices, but will evaporate at the equinoctial seasons. The ammonia atmosphere will explain: (1) the posteclipse brightenings and their observed times of occurrence and nonocurrence; (2) the observed departure from a two-layer model beating curve upon emergence from eclipse; (3) the discordant temperatures obtained at 10 and 20 μm; and (4) discordant temperatures obtained at 10 and 20 μm during the total phase of an eclipse by Jupiter.In order to explain items 3 and 4 above, a proton flux in Jupiter's magnetosphere of 1.1 × 10⁹ cm^?2s^? at an energy of 0.5MeV at io's distance from Jupiter is assumed. This flux is 40 times the flux in Divine's (1972) “upper-limit” model of the Jovian radiation belts, while the proton energy is eight times less. The proton flux, plus the solar ultraviolet and infrared flux absorbed by the ammonia, will heat the atmosphere to 245 ± 10°K. At this temperature the occultation atmospheric upper limit allows the addition of 4 cmatm of nitrogen.     

Solar-wind interactions with the moon: Nature and composition of nitrogen compounds

The lunar atmosphere and magnetic field are very tenuous. The solar wind, therefore, interacts directly with the lunar surface material and the dominant nature of interaction is essentially complete absorption of solar-wind particles by the surface material resulting in no upstream bowshock, but a cavity downstream. The solar-wind nitrogen ion species induce and undergo a complex set of reactions with the elements of lunar material and the solar-wind-derived trapped elements. The nitrogen concentration indigeneous to the lunar surface material is practically nil. Therefore any nitrogen and nitrogen compounds found in the lunar surface material are due to the solar-wind implantation of nitrogen ions. The flux of the solar-wind nitrogen ion species is about 6×10³ cm^–2 s^–1. Since there is no evidence for accumulation of nitrogen species in the lunar surface material, the outflux of nitrogen species from the lunar material to the atmosphere is the same as the solar-wind nitrogen ion flux. The species of the outflux are primarily NO and NH₃, and their respective concentrations in the near surface lunar atmosphere are found by calculation to be 327 and 295 cm^–3. The calculated concentration of NH₃ seems to be consistent with the sunrise concentration results of the mass spectrometer implanted on the lunar surface. This is not the case for the concentration of NO. According to the presently calculated concentration value of NO, the mass spectrometer should have detected NO at sunrise, but no report was made for its detection. There is also discrepancy about the concentration of N₂ which is explained in this paper. The concentrations of nitrogen species in the lunar material at the time of sample collection on the Moon remained about the same when the samples were analyzed on the Earth. However, no specific experiment was planned to detect the nitrogen species in the lunar material samples.     

Lightnings and nitric oxide on Venus

In an updating of energy characteristics of lightnings on Venus obtained from Venera-9 and -10 optical observations, the flash energy is given as 8 × 10⁸ J and the mean energy release of lightnings is 1 erg cm^?2 s which is 25 times as high as that on the Earth. Lightnings were observed in the cloud layer. The stroke rate in the near-surface atmosphere is less than 5 s^?1 over the entire planet if the light energy of the stroke exceeds 4 × 10⁵ J and less than 15 s^?1 for (1–4) × 10⁵ J.The average NO production due to lightnings equals 5 × 10⁸ cm^?2 s^?1, the atomic nitrogen production is equal to 7 × 10⁹ cm^?2s^?1,the N flux toward the nightside is 3.2 × 10⁹ cm^?2s^?1, the number densities [N] = 3 × 10⁷cm^?3 and [NO] = 1.8 × 10⁶cm^?3 at 135 km. Almost all NO molecules in the upper atmosphere vanish interacting with N and the resulting NO flux at 90-80 km equals 5 × 10⁵cm^?2s^?1, which is negligibly small as compared with lightning production. If the predissociation at 80–90 km is regarded as the single sink of NO, its mixing ratio, f_NO, is 4 × 10^?8, for the case of a surface sink f_NO = 0.8 × 10^?9 at 50 km. Excess amounts, $\text{f}{}_{\mathrm{<mtext>NO</mtext>}}\text{? 4 × 10}{}^{\mathrm{?8}}$, may exist in the thunderstorm region.     

Thermal structure of the Martian atmosphere during the dissipation of the dust storm of 1971

The secular variation of the thermal structure of the Martian atmosphere during the dissipation phase of the 1971 dust storm is examined, using temperatures obtained by the infrared spectroscopy investigation on Mariner 9. For the latitude range ?20° to ?30°, the mean temperature at the 2mbar level is found to decrease from approximately 220 K in mid-December 1971 to about 190 K by June 1972 while for the 0.3mbar level a decrease from 203 K to 160 K is observed. Over the same period, the amplitude of the diurnal temperature wave also decreased. Assuming a simplified radiative heating model, the dust optical depth is found to decrease approximately exponentially with an e-folding time of about 60 days at both the 0.3 and 2mbar levels. Stokes-Cunningham settling alone cannot account for this behavior. Sedimentation models which include both gravitational settling and vertical mixing are developed in an effort to explain the time evolution of the dust. Within the framework of a model which assumes an effective vertical diffusivity K independent of height, a mean dust particle diameter of ～2 μm is inferred. To provide the necessary vertical mixing, K ? 10⁷ cm²sec^?1 is required in the lower atmosphere.     

Impact processing of nitrogen on early Mars

An intense impact flux upon a planet having a CO₂ + N₂ atmosphere, such as Mars, provides energy to synthesize nitric oxide, NO, which is likely converted into nitrate minerals. The same impact flux can decompose nitrate minerals if present in the crust. We build a numerical model to study the effects of early impact processes on the evolution of nitrogen in a dominantly CO₂ atmosphere. We model the period of intense post-accretionary bombardment, the roughly 500 Myr period after crustal stabilization that locks in previously accreted volatiles. A best-guess, “fiducial” set of parameters is chosen, with a fixed “veneer” of post-accretionary impactors (δR=950 m thick), assumed to contain carbon at 1 wt% (fg=0.01), with a molar C/N ratio of 18, an initial atmospheric pressure of 1 bar (with CO₂/N₂ = 36), and a power law impactor mass distribution slope b=0.75. This model produces a nitrate reservoir RNO₃?0.5×¹⁰¹⁹ moles, equivalent to ∼30 mbars of N₂, during the intense impact phase. Starting with 1 bar, the atmosphere grows to 2.75 bars. Results of models with variations of parameter values show that RNO₃ responds sluggishly to changes in parameter values. To significantly limit the size of this reservoir, one is required to limit the initial total atmospheric pressure be less than about 0.5 bars, and the impactor volatile content fg to be less than 0.003. The value of fg substantially determines whether the atmosphere grows or not; when fg=0.01, the atmosphere gains about 1.7 bars, while for fg=0.003, the atmosphere gains less than 200 mbars, and for fg=0.001, it loses about 400 mbars. Impact erosion is a minor sink of N, constituting generally less than 10% of the total supply. The loss of impactor volatile plumes can take almost 50% of incoming N and C under fiducial parameters, when atmospheric pressures are low. This nitrogen does not significantly interact with Mars, and hence is not properly delivered. When the initial N is greater than the delivered N, most of the nitrogen ends up as nitrates; when delivered N is larger, most nitrogen ends up in the atmosphere. The reason for this dichotomy seems to be that initial nitrogen is present during the whole bombardment, while delivered N, on average, only experiences half the bombardment. The operating caveat here is that the above results are all conditioned on the assumption that impact processes dominate this period of Mars atmospheric evolution.     

The role of photochemical processes in evolution of the isotopic composition of the atmosphere of Titan

Molecular nitrogen, the main component of the modern atmosphere of Titan, may have formed without significant changes in the nitrogen and hydrogen isotopic composition from the clathrate hydrate of ammonia NH₃ · H₂O_SLD, which is the main accreted form of nitrogen. The most preferable transformation mechanism of NH₃ · H₂O_SLD into atmospheric N₂ is its thermal decomposition in the interior of Titan rather than the photochemical decomposition of ammonia in the upper atmosphere of early Titan. The photolysis of ammonia does not lead to a change in the isotopic composition of nitrogen, as all the nitrogen remains in Titan’s atmosphere. The photolysis of NH does not lead to a change in the isotopic composition of nitrogen in Titan’s atmosphere. Fractionation of hydrogen and nitrogen isotopes during the impacts of comets with Titan does not seem to be significant either. It will be possible to determine the dissociative fractionation factor, the original ratio ¹⁴N/¹⁵N, and the mass of Titan’s original atmosphere when fractionation of nitrogen isotopes in Titan’s atmosphere is examined in additional theoretical and experimental studies that take into account processes occurring during the formation of a system of Saturn’s satellites.     

A two-dimensional hydrostatically equilibrium atmosphere of a neutron star with given differential rotation

An analytic solution has been found in the Roche approximation for the axially symmetric structure of a hydrostatically equilibrium atmosphere of a neutron star produced by collapse. A hydrodynamic (quasione-dimensional) model for the collapse of a rotating iron core in a massive star gives rise to a heterogeneous rotating protoneutron star with an extended atmosphere composed of matter from the outer part of the iron core with differential rotation (Imshennik and Nadyozhin, 1992). The equation of state of a completely degenerate iron gas with an arbitrary degree of relativity is taken for the atmospheric matter. We construct a family of toroidal model atmospheres with total masses M≈ 0.1?2M _⊙ and total angular momenta J≈(1?5.5)×⁴⁹ erg s, which are acceptable for the outer part of the collapsed iron core, in accordance with the hydrodynamic model, as a function of constant parameters ω₀ and r ₀ of the specified differential rotation law Ω=ω₀exp[?(rsinθ)²/r ₀ ² ] in spherical coordinates. The assumed rotation law is also qualitatively consistent with the hydrodynamic model for the collapse of an iron core.     

Vertical distribution of water vapor and mars model lower and middle atmosphere

Observations and model calculations of water vapor diffusion suggest that about half the amount of water vapor is distributed with constant mixing ratio in the Martian atmosphere, the other half is the excess water vapor in the lower troposphere. During 24 hr the total content of water vapor may vary by a factor of two. The eddy diffusion coefficient providing agreement between calculations and observations is K = (3–10) × 10⁶ cm² sec^?1 in the troposphere. An analytical expression is derived for condensate density in the stratosphere in terms of the temperature profile, the particle radius r, and K. The calculations agree with the Mars 5 measurements for r = 1.5 μm, condensate density 5 × 10^?12 g/cm³ in the layer maximum at 30 to 35 km, condensate column density 7 × 10^?6 cm^?2, K = (1?3) × 10⁶ cm² sec^?1, and the temperature profile T = 185 ? 0.05z ? 0.01z² at 20 to 40 km. Condensation conditions yield a temperature of 160°K at 60 km in the evening; the scale height for scattered radiation yields T = 110°k at 80 to 90 km. The Mars model atmosphere has been developed up to 125 km.     

Ozone and photochemistry of the Martian lower atmosphere

The calculation of number densities of CO₂, H₂O and N₂ photolysis products was carried out for the Martian atmosphere at heights up to 60 km. The ozone distributed in the atmosphere as a layer of 10 km width with [O₃] max = 2.5 × 10⁹ cm³ at height of 35 km which agree well with the results of u.v. observations on the evening terminator from the Mars-5 satellite. The calculated densities of O₂, CO and H₂O are also in good agreement with the measured data. The eddy diffusion coefficient is equal to 3 × 10⁶ in the troposphere (h ? 30 km) and 10⁸ cm² s^?1 above 40 km. The dependence of the total ozone content on water vapour amount in the atmosphere is considered; the hypothesis about the influence of water ice aerosol on the ozone formation is proposed to explain the high concentrations of ozone in the morning.     

On the production of nitric oxide by cosmic rays in the mesosphere and stratosphere

Nitric oxide is formed in the atmosphere through the ionization and dissociation of molecular nitrogen by galactic cosmic rays. One NO molecule is formed for each ion pair produced by cosmic ray ionization.The height-integrated input (day and night) to the lower stratosphere is of the order of 6 × 10⁷ NO molecules cm^?2/sec in the auroral zone (geomagnetic latitude Φ ? 60°) during the minimum of the sunspot cycle and 4 × 10⁷ NO molecules cm^?2/sec in the subauroral belt and auroral region (Φ? 45°) at the maximum of solar activity. The tropical production is less than 10^?7 NO molecules cm^?2/sec above 17 km and at the equator the production is only 3 × 10⁶NO molecules cm^?2/sec.     

Methane abundance in the atmosphere of Uranus

Modeling of the geometric albedo of Uranus in and near prominent methane absorption bands between 0.5 and 0.9 μm indicates that the visible atmosphere probably consists of a thin aerosol haze layer (τ_scat ? 0.3?0.5; ω_H ? 0.95) above an optically thick, semi-infinite Rayleigh scattering atmosphere. A significant depletion of methane gas above the haze layer is indicated. The mixing ratio of methane in the lower atmosphere is consistent with a value of CH₄/H₂ ? 3 × 10^?3, comparable to those derived for Jupiter and Saturn.     

The evolution of an impact-generated atmosphere

Shock wave and thermodynamic data for rock-forming and volatile-bearing minerals are used to determine minimum impact velocities (v_cr) and minimum impact pressures (p_cr) required to form a primary H₂O atmosphere during planetary accretion from chondritelike planetesimals. The escape of initially released water from an accreting planet is controlled by the dehydration efficiency. Since different planetary surface porosities will result from formation of a regolith, v_cr and p_cr can vary from 1.5 to 5.8 km/sec and from 90 to 600 kbar, respectively, for target porosities between 0 and ～45%. On the basis of experimental data, hydration rates for forsterite and enstatite are derived. For a global regolith layer on the Earth's surface, the maximum hydration rate equals 6 × 10¹⁰ g H₂O sec^?1 during accretion of the Earth. Attenuation of impact-induced shock pressure is modeled to the extent that the amount of released water as a function of projectile radius, impact velocity, weight fraction of water in the target, target porosity, and dehydration efficiency can be estimated. The two primary processes considered are the impact release of water bound in hydrous minerals (e.g., serpentine) and the subsequent reincorporation of free water by hydration of forsterite and enstatite. These processes are described in terms of model calculations for the accretion of the Earth. Parameters which lead to a primary atmosphere/hydrosphere are: an accretion time of ? 1.6 × 10⁸years, the use of an accretion model defined by Weidenschilling (1974, 1976), a mean planetesimal radius of 0.5 km, a hydration rate of 6 × 10¹⁰ g H₂O sec^?1 inferred from a mean porosity of ～ 10% for the upper 1 km of the accreting Earth, and values for the dehydration efficiency, DE, of 0.55 and 0.07 for the maximum and minimum pressure decay model, respectively. Conditions which prohibit the formation of a primary atmosphere include an accretion time much longer than 1.6 × 10⁸ years, a hydration rate for forsterite and enstatite well in excess of 6 × 10¹⁰ g H₂O sec^?1, and a dehydration efficiency DE < 0.07. We conclude that the concept of dehydration efficiency is of dominant importance in determining the degree to which an accreting planet acquires an atmosphere during its formation.     

On the Evolutionary State of High-Latitude Variable V534 Lyr

Based on the high spectral resolution monitoring conducted at the 6-m BTA telescope, we study the optical spectrum of the high-latitude variable V534 Lyr. Heliocentric radial velocities V_r corresponding to the positions of all metal absorption components, as well as the Na I D and Hα lines were measured during all the observational sets. The analysis of the velocity field examining the lines of various nature revealed a low-amplitude variability of V_r based on the lines with a high excitation potential, which are formed in deep layers of the stellar atmosphere, and allowed to estimate the systemic velocity of V_sys ≈ ?125 kms^?1 (V_lsr ≈ ?105 kms^?1). The distance estimate of d ≈ 6 kpc for the star leads to its absolute magnitude of \(M_V \approx - 5_ \cdot ^m 3\), what corresponds to the spectral classification. The previously undetected spectral phenomenon was revealed for this star: at certain times a splitting of the profiles of low-excited absorptions is observed, reaching ΔV_r = 20–50 kms^?1. A combination of the parameters: reduced metallicity [Met/H]_⊙ = ?0.28, high nitrogen abundance [N/Fe] = +1.10, large spatial velocity, high luminosity, a strong variability of the emission-absorption profiles of HI lines, splitting of metal absorptions at different times of observations and the variability of the velocity field in the atmosphere allow to classify V534 Lyr as a pulsating star in the thick disk of our Galaxy.     

High Resolution Optical Spectroscopy of Hot Post-AGB Star Candidates LS IV-04 1 and LB3116

We present LTE analysis of high resolution optical spectra for B-type hot PAGB stars LS IV-04 1 and LB3116 (LSE 237). The spectra of these high Galactic latitude stars were obtained with the 3.9-m Anglo-Australian Telescope (AAT) and the UCLES spectrograph. The standard 1D LTE analysis with line-blanketed LTE model atmospheres and spectral synthesis provided fundamental atmospheric parameters of T_eff= 15 000±1000 K, log g= 2.5±0.2, ξ = 5.0±1.0 km s^?1, [M/H] = ?1.81 dex, and v sin i= 5 km s^?1 for LSIV-04 1 and T_eff= 16 000±1000 K, log g= 2.5±0.1, v sin i= 25 km s^?1, and [Fe/H] = ?0.93 dex for LB 3116. Chemical abundances of ten different elements were obtained. For LS IV-04 1, its derived model temperature contradicts with previous analysis results. The upper limits for its nitrogen and oxygen abundances were reported for the first time. The magnesium, silicon and calcium were overabundant (i.e. [Mg/Fe] = 0.8 dex, [Si/Fe] = 0.5 dex, [Ca/Fe] = 0.9 dex). With its metal-poor photosphere and V_LSR ≈ 96 km s^?1, LSIV-04 1 is likely a population II star and most probably a PAGB star. LTE abundances of LB 3116 were reported for the first time. The spectrum of this helium rich star shows 0.9 dex enhancement in the nitrogen. The photosphere of the star is slightly deficient in Mg, Si, and S. (i.e. [Mg/Fe] = ?0.2 dex, [Si/Fe] = ?0.4 dex, [S/Fe] = ?0.2 dex). The Al is slightly enhanced. The phosphorus is overabundant, i.e. [P/Fe] ≈ 1.7 ± 0.47 dex, hence LB3116 may be the first example of a PAGB star which is rich in phosphorus. With its high radial velocity (i.e.V_LSR = 73 km s^?1), and the deficiencies observed in C, Mg, Si, and S indicate that LB 3116 is likely a hot PAGB star at high galactic latitude.     

Stellar occultation spikes as probes of atmospheric structure and composition

The characteristics of spikes observed in the occultation light curves of β Scorpii by Jupiter are reviewed and discussed. Using a model in which the refractivity (density) gradients in the Jovian atmosphere are parallel to the local gravitational field, the spikes are shown to yield information about (i) the [He]/-[H₂] ratio in the atmosphere, (ii) the fine scale density structure of the atmosphere and (iii) high-resolution images of the occulted stars. The spikes also serve as indicators for ray crossing. Observational limits are placed on the magnitude of horizontal refractivity gradients; these appear to be absent on scales of a few kilometers at altitudes corresponding to number densities less than 2 × 10¹⁴ cm^?3. Spikes are produced by atmospheric density variations, perhaps due to atmospheric layers, density waves or turbulence. To discriminate among these possibilities, future occultation observations should be made from a number of observation sites at two or more wavelengths simultaneously with high time resolution techniques. Given a large telescope and suitable observing techniques, useful information about Jupiter's atmosphere can be obtained from future occultations of early-type stars as faint as V ～ + 6–7.     

Gamma-ray bursts and the production of cosmogenic radionuclides in the Earth’s atmosphere

An explanation is offered for the impulsive increase in the concentration of cosmogenic radiocarbon in annual tree rings (Δ¹⁴C ～ 12‰) from AD ?775. A possible cause of such an increase could be the high-energy emission from a Galactic gamma-ray burst. It is shown that such an event should not lead to an increase in the total production of ¹⁰Be in the atmosphere, as distinct from the effect of cosmic-ray fluxes on the atmosphere. At the same time, the production of an appreciable amount of ³⁶Cl, which can be detected in Greenland and Antarctica ice samples of the corresponding age, should be expected. This allows the effects caused by a gamma-ray burst and anomalously powerful proton events to be distinguished.