Monte Carlo modeling of neutral gas and dust in the coma of Comet 1P/Halley

The neutral gas environment of a comet is largely influenced by dissociation of parent molecules created at the surface of the comet and collisions of all the involved species. We compare the results from a kinetic model of the neutral cometary environment with measurements from the Neutral Mass Spectrometer and the Dust Impact Detection System onboard the Giotto spacecraft taken during the fly-by at Comet 1P/Halley in 1986. We also show that our model is in good agreement with contemporaneous measurements obtained by the International Ultraviolet Explorer, sounding rocket experiments, and various ground based observations.The model solves the Boltzmann equation with a Direct Simulation Monte Carlo technique (Tenishev, V., Combi, M., Davidsson, B. [2008]. Astrophys. J. 685, 659-677) by tracking trajectories of gas molecules and dust grains under the influence of the comet’s weak gravity field with momentum exchange among particles modeled in a probabilistic manner. The cometary nucleus is considered to be the source of dust and the parent species (in our model: H₂O, CO, H₂CO, CO₂, CH₃OH, C₂H₆, C₂H₄, C₂H₂, HCN, NH₃, and CH₄) in the coma. Subsequently our model also tracks the corresponding dissociation products (H, H₂, O, OH, C, CH, CH₂, CH₃, N, NH, NH₂, C₂, C₂H, C₂H₅, CN, and HCO) from the comet’s surface all the way out to 10⁶ km.As a result we are able to further constrain cometary the gas production rates of CO (13%), CO₂ (2.5%), and H₂CO (1.5%) relative to water without invoking unknown extended sources.     

Hydrated minerals on Europa’s surface: An improved look from the Galileo NIMS investigation

The surface composition of Europa is of great importance for understanding both the internal evolution of Europa and its putative ocean. The Near Infrared Mapping Spectrometer (NIMS) investigation on Galileo observed Europa and the other Galilean satellites from 0.7 to 5.2 μm with spatial resolution down to a few kilometers during flybys by the spacecraft as it orbited Jupiter. These data have been analyzed and results published over the life of the Galileo mission and afterward. One result was the discovery of hydrated minerals at some locations on Europa and Ganymede. The data are noisy, especially for Europa, due to radiation affecting the NIMS electronics and detectors, and other artifacts are also present. The NIMS data are now being reprocessed using the accumulated knowledge gained over the entire missions to remove noise spikes and compensate for some other defects in the data. We are analyzing these reprocessed data in an attempt to defined better the nature of the hydrate spectral features and improve their interpretation. We report here on analyses of two NIMS reprocessed observations for the 0.7-3-μm region. A revised hydrate spectrum is calculated and mapped in detail across two lineaments. The spectrum shows the expected distorted water features but little or no spectral structure in these features. A narrow, weak spectral feature appears at 1.344 μm, which is weakly correlated with lower albedo. Several other weak features may be present but are difficult to confirm in these limited data sets. The hydrate signature shows the greatest strength within and toward the center of the lineaments, confirming and strengthening the association of the hydrate with these endogenic features. This trend may indicate that the material in the lineaments is youngest toward the center and has more water frost coverage toward the edge. A small, visually dark, circular feature has a spectrum that shows both hydrate and crystalline water ice features and perhaps contains a hydrate different in spectral characteristics and perhaps composition than found in the lineament.     

Variability and trends in daily minimum and maximum temperatures and in the diurnal temperature range in Lithuania,Latvia and Estonia in 1951–2010

Spatial distribution and trends in mean and absolute maximum and minimum temperatures and in the diurnal temperature range were analysed at 47 stations in the eastern Baltic region (Lithuania, Latvia and Estonia) during 1951–2010. Dependence of the studied variables on geographical factors (latitude, the Baltic Sea, land elevation) is discussed. Statistically significant increasing trends in maximum and minimum temperatures were detected for March, April, July, August and annual values. At the majority of stations, the increase was detected also in February and May in case of maximum temperature and in January and May in case of minimum temperature. Warming was slightly higher in the northern part of the study area, i.e. in Estonia. Trends in the diurnal temperature range differ seasonally. The highest increasing trend revealed in April and, at some stations, also in May, July and August. Negative and mostly insignificant changes have occurred in January, February, March and June. The annual temperature range has not changed.     

Tertiary alkaline Roztoky Intrusive Complex, České středohoří Mts., Czech Republic: petrogenetic characteristics

The ?eské st?edoho?í Mts. is the dominant volcanic center of the Oh?e (Eger) rift zone. It hosts the Roztoky Intrusive Complex (RIC), which is made up of a caldera vent and intrusions of 33–28-Ma-old hypabyssal bodies of essexite–monzodiorite–sodalite syenite series accompanied by a radially oriented 30–25-Ma-old dike swarm comprising about 1,000 dikes. The hypabyssal rocks are mildly alkaline mostly foid-bearing types of mafic to intermediate compositions. The dike swarm consists of chemically mildly alkaline and rare strongly alkaline rocks (tinguaites). The geochemical signatures of the mildly alkaline hypabyssal and associated dike rocks of the RIC are consistent with HIMU mantle sources and contributions from lithospheric mantle. The compositional variations of essexite and monzodiorite can be best explained by fractional crystallization of parent magma without significant contributions of crustal material. On the other hand, the composition of monzosyenite, leuco-monzodiorite and sodalite syenite reflects fractional crystallization coupled with variable degrees of crustal assimilation. It is suggested that the parent magmas in the Oh?e rift were produced by an adiabatic decompression melting of ambient upper mantle in response to lithospheric extension associated with the Alpine Orogeny.     

Bayesian foreground analysis with CMB data

The quality of CMB observations has improved dramatically in the last few years, and will continue to do so in the coming decade. Over a wide range of angular scales, the uncertainty due to instrumental noise is now small compared to the cosmic variance. One may claim with some justification that we have entered the era of precision CMB cosmology. However, some caution is still warranted: The errors due to residual foreground contamination in the CMB power spectrum and cosmological parameters remain largely unquantified, and the effect of these errors on important cosmological parameters such as the optical depth τ and spectral index n_s is not obvious. A major goal for current CMB analysis efforts must therefore be to develop methods that allow us to propagate such uncertainties from the raw data through to the final products. Here we review a recently proposed method that may be a first step towards that goal.     

Neutron star retention and millisecond pulsar production in globular clusters

We investigate the conditions by which neutron star retention in globular clusters is favoured. We find that neutron stars formed in massive binaries are far more likely to be retained. Such binaries are likely to then evolve into contact before encountering other stars, possibly producing a single neutron star after a common envelope phase. A large fraction of the single neutron stars in globular clusters are then likely to exchange into binaries containing moderate-mass main-sequence stars, replacing the lower-mass components of the original systems. These binaries will become intermediate-mass X-ray binaries (IMXBs), once the moderate-mass star evolves off the main sequence, as mass is transferred on to the neutron star, possibly spinning it up in the process. Such systems may be responsible for the population of millisecond pulsars (MSPs) that has been observed in globular clusters. Additionally, the period of mass-transfer (and thus X-ray visibility) in the vast majority of such systems will have occurred 5–10 Gyr ago, thus explaining the observed relative paucity of X-ray binaries today, given the MSP population.     

Relative Height Estimation by Cross-Correlating Ground-Range Synthetic Aperture Sonar Images

The relative height of the seafloor can be estimated by using two vertically displaced receivers. In this paper, we propose techniques to improve the accuracy of the estimated height. Our results are based on the use of synthetic aperture sonar (SAS) imaging, which implies coherent addition of complex images acquired from a moving platform. The SAS processing improves the along-track (or azimuth) resolution, as well as the signal-to-noise ratio (SNR), which in turn improves the estimated height accuracy. We show that the shift of the effective center frequency induced by coherent, frequency-dependent scattering affect the time-delay estimates from complex cross correlations, and we propose a correction technique for broadband signals with uneven magnitude spectra. To reduce the effect of geometrical decorrelation and increase the coherence between the images, we beamform the sonar images onto an a priori estimate of the seafloor height before correlating. We develop a mathematical model for the imaging geometry. Finally, we demonstrate our proposed estimators by providing relative seafloor height estimates from real aperture and SAS images, obtained during the InSAS-2000 experiment at Elba Island in Italy. In particular, we demonstrate that the SAS image quality is significantly improved by inclusion of the height estimates as a priori information.     

Surface current measurements by HF radar in freshwater lakes

HF radar has become an increasingly important tool for mapping surface currents in the coastal ocean. However, the limited range, due to much higher propagation loss and smaller wave heights (relative to the saltwater ocean), has discouraged HF radar use over fresh water, Nevertheless, the potential usefulness of HF radar in measuring circulation patterns in freshwater lakes has stimulated pilot experiments to explore HF radar capabilities over fresh water. The Episodic Events Great Lakes Experiment (EEGLE), which studied the impact of intermittent strong wind events on the resuspension of pollutants from lake-bottom sediments, provided an excellent venue for a pilot experiment. A Multifrequency Coastal HF Radar (MCR) was deployed for 10 days at two sites on the shore of Lake Michigan near St. Joseph, MI. Similarly, a single-frequency CODAR SeaSonde instrument was deployed on the California shore of Lake Tahoe. These two experiments showed that when sufficiently strong surface winds (2 about 7 m/s) exist for an hour or more, a single HE radar can be effective in measuring the radial component of surface currents out to ranges of 10-15 km. We also show the effectiveness of using HF radar in concert with acoustic Doppler current profilers (ADCPs) for measuring a radial component of the current profile to depths as shallow as 50 cm and thus potentially extending the vertical coverage of an ADCP array