Reinvestigation of the irregularities in the 3H decay

Having in mind the potential impact of the results presented by Veprev and Muromtsev (2012) [13] on our knowledge of the universe, we reinvestigated the liquid scintillation measurement of the count rate variations of ³H. Making use of the sophisticated Quantulus liquid scintillation spectrometer, we found that the measurement of the high-energy tail of ³H spectrum may be significantly influenced by instrumental instability. Thus, the possible explanation for the relatively high count rate variations of Veprev and Muromtsev (2012) [13] can be attributed mainly to the walk of the cut-off in the integrated spectrum, although weak variations of different origin could be masked by such cut-off drifts. In our experiment we have also registered the oscillatory behavior of measured high-energy tail of ³H spectrum, but with very small amplitude (less than 0.5%), which cannot be easily explained only by instrumental instability. When the total ³H spectrum was measured, no significant variations in the count rate were found.     

The 3–5 MHz global reflectivity map of Mars by MARSIS/Mars Express: Implications for the current inventory of subsurface H2O

We extracted the surface echo power from 2 years of MARSIS measurements. The retrieved values are calibrated to compensate for changes in the distance of the spacecraft to the surface and for the attenuation of the signal by the ionosphere. The results are used to build the first global map of surface echo power at 3–5 MHz. The surface echo power variations are primarily caused by kilometer-scale surface roughness. Then, we derive the values of dielectric constant of the shallow subsurface materials by normalizing the surface echo power map using a simulation of MARSIS signal from the MOLA topography. As a result, we obtain a map that characterizes the dielectric properties of the materials down to a few decameters below the surface. Dielectric properties vary with latitude, with high values in mid-latitudes belts (20–40°) and lower values at both equatorial and high latitudes. From the comparison of MARSIS reflectivity map to GRS observations, we conclude that the reflectivity decrease observed poleward of 50–60° corresponds to the onset of water-ice occurrence within the regolith. Assuming homogenous ground composition and texture at the scale of the MARSIS resolution cell, our inferred volume of ground water ice is of 10⁶ km³, equivalent to a polar cap. Low reflectivity areas are also observed in equatorial regions. From radar studies alone, equatorial low dielectric constant values could have different interpretations but the correlation with GRS hydrogen distribution rather points toward a water-related explanation.     

Cosmological implications of BL Lac objects

It has become clear in recent years that relativistic beaming is a good explanation for the BL Lac phenomenon. Of studies based on the relativistic beaming model of BL Lac objects, we note that the orientation of jet's axis to the line-of-sight is very small and, therefore, the observed flux emitted from a rapidly moving source is orders of magnitude higher than the flux in its rest-frame:F _obs = ^{3 +} F _intr, where is the bulk relativistic Doppler factor. Then the observed apparent magnitudem _v must be corrected for this effect. For our 39 samples, the corrected apparent magnitudem _v ^corr and logZ have a good correlation.     

CASSINI CIRS OBSERVATIONS OF A ROLL-OFF IN SATURN RING SPECTRA AT SUBMILLIMETER WAVELENGTHS

The Cassini Composite Infrared Spectrometer (CIRS) spatially resolved Saturn’s main rings in the far-infrared, measuring the spectrum from 20 to 400 wavenumbers (cm⁻¹) (tens of microns to submillimeter wavelengths). We find a spectral roll-off below 50 cm⁻¹ (200 μm) for each of the A, B and C rings. From these data we derive temperatures and emissivities for each ring. Mie calculations of individual water ice particles show a natural variation in the optical properties of the rings similar to the roll-off we observe in our data. A simple radiative transfer model placing a distribution of water ice particles randomly in a layer provides a good fit to the data and illustrates one possible interpretation of the results. This is most likely only part of the explanation for the roll-off effect as the impact of shape, surface, and composition variations have been left for future analysis.     

γ-ray bursts from QSOs?

One of the most amazing phenomena in astronomy, during the last twenty years, have been cosmic gamma-ray bursts (GRBs). The duration of these events vary from a few milliseconds to hundreds of seconds. We have never been able to identify the source of these bursts in other wavelengths. These objects have also never been seen in-rays after the initial bursts although there is some very weak statistical evidence that some of the bursts will repeat (Quashnock and Lamb 1993). The standard explanation for these bursts has been that they are somehow related to neutron stars in our own Galaxy. The latest results from the Burst and Transient Source Experiment aboard the Compton Gamma-Ray Observatory (Fishmanet al. 1994) show clearly that there is no excess concentration of these events (743 bursts) in the Galactic plane. After this, a more promising explanation is that the bursts are related to the Galactic halo or that the origin is extragalactic. In this letter we show that it is very probable that the origin of these events is the QSOs and that the radiation comes from the same synchrotron source as in the other observed wavelengths.     

Fluctuations of energetic particle flux during solar cycle based on measurements in the solar wind,in the magnetosphere,and at Earth

We present the results of our studies of the cosmic-ray fluctuations in the frequency range 10⁻⁴−1.67 × 10⁻³ Hz based on energetic particle flux measurements on spacecraft in the solar wind, in the magnetosphere, and at Earth in the 11-year solar cycle. The cosmic-ray fluctuation spectrum is shown to have an 11-year modulation related to the solar cycle. A different behavior of the level of energetic particle fluctuations measured in different regions of space is observed for cosmic rays of different origins. We conclude that the new, previously unknown phenomenon of 11-year modulation of the cosmic-ray fluctuation spectrum has been established. A possible explanation of this phenomenon is given.     

On the detectivity of advanced galactic civilizations

Even with slow rates of technological advance, extraterrestrial civilizations substantially in our future will have technologies and laws of nature currently inaccessible to us, and will probably have minimal interest in communicating with us. If this communication horizon is ～10³ years in our future, other crude estimates previously published imply that only ～10^?4 of the technical civilizations in the Galaxy are accessible to us. The mean distance to the nearest such society is then ～10⁴ light years. Radio detection of extraterrestrial intelligence seems to imply either (1) much larger telescopes or antenna arrays for the detection of civilizations within our Galaxy than now exist; or (2) attention to the nearer extragalactic systems, with smaller radio telescopes, to detect the very small fraction of very advanced societies which may choose to make their presence known to emerging civilizations via antique communication modes.     

Solar system isotopic anomalies: Supernova neighbor or presolar carriers?

I evaluate several nuclear and chemical problems related both to the recent scenario suggesting that the known isotopic anomalies in the solar system have resulted from a supernova near the protosolar nebula and to the model of extinct presolar carriers. Major features include: (1) Large quantities of extinct ²⁴⁸Cm and ³⁶Cl are predicted from the Cameron-Truran model of a minor injection about 10⁶ yr before condensation; (2) an extinct-carrier model of ²⁶Mg is set forth in detail with a solid chemistry picture of the early solar system; (3) a major thermonuclear supernova responsible for ²⁶Al, ²⁴⁴Pu, and ⁴⁰K would have to have occurred several million years (3 m.y.) before condensation and contributed a large fraction of the major stable chemical elements; (4) carbon isotope families are to be expected if the oxygen isotope families are due to a late injection of ¹⁶O; (5) the Earth and E meteorites may have condensed primarily in a carbon-rich nebula existing before admixtures of a major late ¹⁶O-rich mixture; (6) the extinct-presolar-carrier model is the single best explanation of all anomalies.     

Shocked meteorites: Argon-40-argon-39 evidence for multiple impacts

Abstract— To contribute to the understanding of the impact history of asteroids, we performed a high-resolution ⁴⁰Ar-³⁹Ar study of ten moderately to highly shocked chondrites, which we selected according to the shock classification given by Stöffler et al. (1991). Two recent shocked chondrite falls and two highly shocked eucrites completed our sample suite. When possible, we separated impact melt from host rock for separate analysis. In total, we studied 28 samples from 14 meteorites. In some cases, atmospheric Ar that we associate with terrestrial weathering was identified and corrected for. The ages we obtained range between ～100 Ma and ～4.1 Ga and are clearly distinct from primordial ages that correspond to solar system formation. We reproduced the previously reported cluster of L-chondrite ages, ～500 Ma. The most prominent result of our study is that, in the case of chondrites, melts generally are older than host rocks or melt-embedded unmolten rocks. To solve this apparent paradox, we propose that the melt-forming event, which was the most severe shock episode in the history of these meteorites, has not been the only occasion affecting their K-Ar systems. At least one later impact metamorphism must have occured. The response of the K-Ar clock to this second event was more severe in the host rock than in the previously (in the first event) generated melt veins and pockets because of different Ar retention rates. Hence, impact metamorphism on meteorite parent bodies indeed was a multistage process extending in time over billions of years.     

Population i pulsating stars

On the basis of our age estimations of Population I pulsating stars in our Galaxy (Tsvetkov, 1986a), the mean ages of 6 open star clusters containing 21 Delta Scuti-variables and of 8 star clusters and associations containing 13 classical cepheids, have been evaluated. These mean cluster age estimations weighted according to the probabilities for different evolutionary phases of the pulsating stars, are obtained in the evolutionary track systems of Iben (1967) and Paczyski (1970); the cluster ages are larger in the former system. Our results are compared with those obtained from various methods by other authors. Clusters with classical cepheids and with Delta Scuti-stars have ages, respectively, in the ranges 10⁷–10⁸ years and 10⁶–10⁹ years. It is shown that the use of simple period-age(-colour) relations for Population I pulsating stars gives sufficiently accurate cluster age estimations. By use of our period-age relations for classical cepheids (Tsvetkov, 1986a), the mean ages of 56 other star clusters and associations in our Galaxy, the Magellanic Clouds, and M 31 galaxy have been estimated in both systems of tracks. The results are generally in agreement with those obtained from various methods by other authors. The use of Population I pulsating stars in star clusters and associations is one of the simplest and most easily applied methods for determining cluster ages; but there are some limitations in its application.     

Galactic civilizations: Population dynamics and interstellar diffusion

The interstellar diffusion of galactic civilizations is reexamined by potential theory; both numerical and analytical solutions are derived for the nonlinear partial differential and difference equations which specify a range of relevant models, drawn from blast wave physics, soil science, and, especially, population biology. An essential feature of these models is that, for all civilizations, population growth must be limited by the carrying capacity of the planetary environments. Dispersal is fundamentally a diffusion process; a directed density-dependent diffusivity describes interstellar emigration. We concentrate on two models, the first describing zero population growth (ZPG) and the second which also includes local growth and saturation of a planetary population, and for which we find an asymptotic travelling wave solution. For both models the colonization wavefront expands slowly and uniformly, but only the frontier worlds are sources of further expansion. For nonlinear diffusion with growth and saturation, the colonization wavefront from the nearest independently arisen galactic civilization can have reached the Earth only if its lifetime exceeds 2.6 × 10⁶ years. If discretization can be neglected, the critical lifetime is 2.0 × 10⁷ years. For ZPG the corresponding number is 1.3 × 10¹⁰ years. These numerical results depend on our choices for the specific emigration rate, the distribution of colonizable worlds, and, in the second model, the population growth rate; but the dependence on these parameters is entrancingly weak. We conclude that the Earth is uncolonized not because interstellar spacefaring societies are rare, but because there are too many worlds to be colonized in the plausible lifetime of the colonization phase of nearby galactic civilizations. This phase is, we contend, eventually outgrown. We also conclude that, except possibly early in the history of the Galaxy, there are no very old galactic civilizations with a consistent policy of conquest of inhabited worlds; there is no Galactic Empire. There may, however, be abundant groups of ～10⁵ to 10⁶ worlds linked by a common colonial heritage. The radar and television announcement of an emerging technical society on Earth may induce a rapid response by nearby civilizations, thus newly motivated to reach our system directly rather than by diffusion.     

Butterfly diagrams of strongly spotted late type stars

At present, long-term (over 30 years) multicolor photometric observations give the possibility to determine general properties of spotted areas on late-type stars. Star-spot modeling from broadband photometric data has been carried out by Alekseev and Gershberg since 1996 under the assumption that spots are situated in two latitudinal zones. Here we propose a new analysis of their results for several G and K dwarf stars with high irregular activity. On these stars, EK Dra, VY Ari, V775 Her, and V833 Tau, two spot belts exist separately and do not merge into a single equatorial active region, as occurs on cooler red-dwarf stars. The zonal spottedness models allow us to fit simultaneously both rotational modulation and long-term variability of stellar brightness. These models give evidence for an equatorward drift of the lower latitude boundary of the spotted region, φ₀, during the rising phase of activity, beyond any possible errors concerned with our methodology. In order to evaluate the drift rate we introduce the concept of `effective' spot belt, whose width is independent of longitude. This permits us to construct butterfly diagrams for stellar spots. The equatorward drift rates of the lower boundary of the spotted region D=dφ_low/dt are (− 1)–(− 2) deg year⁻¹ in the years of increasing spottedness. These values are less than the analogous solar one D≈−4 deg year⁻¹ for the rising phase of the cycle. Thus, cyclic activity can be revealed from butterfly diagrams and derived drifts of starspots prior to a possible detection from the spectral analysis of photometric variability. Finally, we briefly discuss a possible explanation of high-latitude activity and surface drifts of starspots in the framework of the current state of dynamo theory.     

New model calculations for the production rates of cosmogenic nuclides in iron meteorites

Abstract— Here we present the first purely physical model for cosmogenic production rates in iron meteorites with radii from 5 cm to 120 cm and for the outermost 1.3 m of an object having a radius of 10 m. The calculations are based on our current best knowledge of the particle spectra and the cross sections for the relevant nuclear reactions. The model usually describes the production rates for cosmogenic radionuclides within their uncertainties; exceptions are ⁵³Mn and ⁶⁰Fe, possibly due to normalization problems. When an average S content of about 1 ± 0.5% is assumed for Grant and Carbo samples, which is consistent with our earlier study, the model predictions for ³He, ²¹Ne, and ³⁸Ar are in agreement. For ⁴He the model has to be adjusted by 24%, possibly a result of our rather crude approximation for the primary galactic α particles. For reasons not yet understood the modeled ³⁶Ar/³⁸Ar ratio is about 30–40% higher than the ratio typically measured in iron meteorites. Currently, the only reasonable explanation for this discrepancy is the lack of experimentally determined neutron induced cross sections and therefore the uncertainties of the model itself. However, the new model predictions, though not yet perfect, enable determining the radius of the meteoroid, the exposure age, the sulphur content of the studied sample as well as the terrestrial residence time. The determination of exposure ages is of special interest because of the still open question whether the GCR was constant over long time scales. Therefore we will discuss in detail the differences between exposure ages determined with different cosmogenic nuclides. With the new model we can calculate exposure ages that are based on the production rates (cm³STP/(gMa)) of noble gases only. These exposure ages, referred to as noble gas exposure ages or simply ^3,4He, ²¹Ne, or ^36,38Ar ages, are calculated assuming the current GCR flux. Besides calculating noble gas ages we were also able to improve the ⁴¹K‐⁴⁰K‐and the ³⁶Cl‐³⁶Ar dating methods with the new model. Note that we distinguish between ³⁶Ar ages (calculated via ³⁶Ar production rates only) and ³⁶Cl‐³⁶Ar ages. Exposure ages for Grant and Carbo, calculated with the revised ⁴¹K‐⁴⁰K method, are 628 ± 30 Ma and 841 ± 19 Ma, respectively. For Grant this is equal to the ages obtained using ³He, ²¹Ne, and ³⁸Ar but higher than the 3⁶Ar‐ and ³⁶Cl‐³⁶Ar ages by ?30%. For Carbo the ⁴¹K‐⁴⁰K age is ?40% lower than the ages obtained using ³He, ²¹Ne, and ³⁸Ar but equal to the ³⁶Ar age. These differences can either be explained by our still insufficient knowledge of the neutron‐induced cross sections or by a long‐term variation of the GCR.     

Uranium isotopic composition and absolute ages of Allende chondrules

A handful of events, such as the condensation of refractory inclusions and the formation of chondrules, represent important stages in the formation and evolution of the early solar system and thus are critical to understanding its development. Compared to the refractory inclusions, chondrules appear to have a protracted period of formation that spans millions of years. As such, understanding chondrule formation requires a catalog of reliable ages, free from as many assumptions as possible. The Pb‐Pb chronometer has this potential; however, because common individual chondrules have extremely low uranium contents, obtaining U‐corrected Pb‐Pb ages of individual chondrules is unrealistic in the vast majority of cases at this time. Thus, in order to obtain the most accurate ²³⁸U/²³⁵U ratio possible for chondrules, we separated and pooled thousands of individual chondrules from the Allende meteorite. In this work, we demonstrate that no discernible differences exist in the ²³⁸U/²³⁵U compositions between chondrule groups when separated by size and magnetic susceptibility, suggesting that no systematic U‐isotope variation exists between groups of chondrules. Consequently, chondrules are likely to have a common ²³⁸U/²³⁵U ratio for any given meteorite. A weighted average of the six groups of chondrule separates from Allende results in a ²³⁸U/²³⁵U ratio of 137.786 ± 0.004 (±0.016 including propagated uncertainty on the U standard [Richter et al. 2010]). Although it is still possible that individual chondrules have significant U isotope variation within a given meteorite, this value represents our best estimate of the ²³⁸U/²³⁵U ratio for Allende chondrules and should be used for absolute dating of these objects, unless such chondrules can be measured individually.     

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.     

Reseeding of early earth by impacts of returning ejecta during the late heavy bombardment

Mounting attention has focused on interplanetary transfer of microorganisms (panspermia), particularly in reference to exchange between Mars and Earth. In most cases, however, such exchange requires millions of years, over which time the transported microorganisms must remain viable. During a large impact on Earth, however, previous work (J.C. Armstrong et al., 2002, Icarus 160, 183-196) has shown that substantial amounts of material return to the planet of origin over a much shorter period of time (< 5000 years), considerably mitigating the challenges to the survival of a living organism. Conservatively evaluating experiments performed [by others] on Bacillus subtilis and Deinococcus radiodurans to constrain biological survival under impact conditions, we estimate that if the Earth were hit by a sterilizing impactor ∼ 300 km in diameter, with a relative velocity of 30 km s⁻¹ (such as may have occurred during the Late Heavy Bombardment), an initial cell population in the ejecta of order 10³-10⁵ cells kg⁻¹ would in most cases be sufficient for a single modern organism to survive and return to an again-clement planet 3000-5000 years later. Although little can be said about the characteristics or distribution of ancient life, our calculations suggest that impact reseeding is a possible means by which life, if present, could have survived the Late Heavy Bombardment.     

SULPHUR ISOTOPE RATIOS IN THE CANYON DIABLO METALLIC SPHEROIDS*

Sulphur contents and δ³⁴S values have been determined for metallic spheroids collected from the vicinity of Meteor Crater, Arizona. Large spheroids with diameters ～ 1.3 mm contain about 20% more sulphur than do small spheroids with diameters ～ 0.5 mm. The mean δ³⁴S value for large spheroids is + 0.23°/∞, while that for small spheroids is + 0.4°/∞, both measured relative to Canyon Diablo troilite. Abrasion experiments on large spheroids show that δ³⁴S varies radially within them, with values of ～ 1.9δ/∞ at their surfaces, falling to zero at their centres. The favoured explanation for the observed sulphur content and δ³⁴S patterns is low temperature oxidation during post-formation weathering.     

Sputtering and high altitude meteors

Conventional ablation theory assumes that a meteoroid undergoes intensive heating during atmospheric flight and surface atoms are liberated through thermal processes. Our research has indicated that physical sputtering could play a significant role in meteoroid mass loss. Using a 4th order Runge-Kutta numerical integration technique, we tabulated the mass loss due to the two ablation mechanisms and computed the fraction of total mass lost due to sputtering. We modeled cometary structure meteoroids with masses ranging from 10⁻¹³ to 10⁻³ kg and velocities ranging from 11.2 to 71 km s⁻¹. Our results indicate that a significant fraction of the mass loss for small, fast meteors is due to sputtering, particularly in the early portion of the light curve. In the past 6 years evidence has emerged for meteor luminosity at heights greater than can be explained by conventional ablation theory. We have applied our sputtering model and find excellent agreement with these observations, and therefore suggest that sputtered material accounts for the new type of radiation found at great heights.     

Tests for spatial isotropy of three thousand gamma-ray bursts found in BATSE archival data

We apply isotropy tests to our new uniform catalog of cosmic gamma-ray bursts (GRBs) (Stern and Tikhomirova 1999). The catalog contains trigger and nontrigger bursts found in 1024-ms BATSE records over seven years. Based on this catalog, we confirm isotropy of the GRB spatial distribution for a sample that surpasses previous samples in size (2934 bursts) and in achieved threshold (fluxes down to 0.1 phot. cm^?2 s^?1, which is a factor of ～2 lower than the BATSE trigger threshold). We also confirm that there is no excess of bursts toward the galaxy M 31.     

Space distribution and birth-rate of pulsars

Using the data of the 321 pulsars so far known the Galactic distribution and the luminosity function of pulsars have been investigated. The total number of pulsars in our Galaxy is found to be 9 × 10⁴. If the mean age of pulsars is 1.8 × 10⁶ years, the birth-rate of pulsars in the Galaxy will be one every 20 years. This rate is not in contradiction with the birth-rate of supernovae.