Radioactivity backgrounds in ZEPLIN-III

We examine electron and nuclear recoil backgrounds from radioactivity in the ZEPLIN-III dark matter experiment at Boulby. The rate of low-energy electron recoils in the liquid xenon WIMP target is 0.75 ± 0.05 events/kg/day/keV, which represents a 20-fold improvement over the rate observed during the first science run. Energy and spatial distributions agree with those predicted by component-level Monte Carlo simulations propagating the effects of the radiological contamination measured for materials employed in the experiment. Neutron elastic scattering is predicted to yield 3.05 ± 0.5 nuclear recoils with energy 5-50 keV per year, which translates to an expectation of 0.4 events in a 1 yr dataset in anti-coincidence with the veto detector for realistic signal acceptance. Less obvious background sources are discussed, especially in the context of future experiments. These include contamination of scintillation pulses with Cherenkov light from Compton electrons and from β activity internal to photomultipliers, which can increase the size and lower the apparent time constant of the scintillation response. Another challenge is posed by multiple-scatter γ-rays with one or more vertices in regions that yield no ionisation. If the discrimination power achieved in the first run can be replicated, ZEPLIN-III should reach a sensitivity of ∼1 × 10⁻⁸pb · yr to the scalar WIMP-nucleon elastic cross-section, as originally conceived.     

GEM-based TPC with CCD imaging for directional dark matter detection

The most mature directional dark matter experiments at present all utilize low-pressure gas Time Projection Chamber (TPC) technologies. We discuss some of the challenges for this technology, for which balancing the goal of achieving the best sensitivity with that of cost effective scale-up requires optimization over a large parameter space. Critical for this are the precision measurements of the fundamental properties of both electron and nuclear recoil tracks down to the lowest detectable energies. Such measurements are necessary to provide a benchmark for background discrimination and directional sensitivity that could be used for future optimization studies for directional dark matter experiments. In this paper we describe a small, high resolution, high signal-to-noise GEM-based TPC with a 2D CCD readout designed for this goal. The performance of the detector was characterized using alpha particles, X-rays, gamma-rays, and neutrons, enabling detailed measurements of electron and nuclear recoil tracks. Stable effective gas gains of greater than 1 × 10⁵ were obtained in 100 Torr of pure CF₄ by a cascade of three standard CERN GEMs each with a 140 µm pitch. The high signal-to-noise and sub-millimeter spatial resolution of the GEM amplification and CCD readout, together with low diffusion, allow for excellent background discrimination between electron and nuclear recoils down below ∼10 keVee (∼23 keVr fluorine recoil). Even lower thresholds, necessary for the detection of low mass WIMPs for example, might be achieved by lowering the pressure and utilizing full 3D track reconstruction. These and other paths for improvements are discussed, as are possible fundamental limitations imposed by the physics of energy loss.     

Measurement of the scintillation time spectra and pulse-shape discrimination of low-energy β and nuclear recoils in liquid argon with DEAP-1

The DEAP-1 low-background liquid argon detector was used to measure scintillation pulse shapes of electron and nuclear recoil events and to demonstrate the feasibility of pulse-shape discrimination down to an electron-equivalent energy of 20 keV_ee.In the surface dataset using a triple-coincidence tag we found the fraction of β events that are misidentified as nuclear recoils to be $<1.4\times {10}^{-7}$ (90% C.L.) for energies between 43–86 keV_ee and for a nuclear recoil acceptance of at least 90%, with 4% systematic uncertainty on the absolute energy scale. The discrimination measurement on surface was limited by nuclear recoils induced by cosmic-ray generated neutrons. This was improved by moving the detector to the SNOLAB underground laboratory, where the reduced background rate allowed the same measurement to be done with only a double-coincidence tag.The combined data set contains 1.23 × 10⁸ events. One of those, in the underground data set, is in the nuclear-recoil region of interest. Taking into account the expected background of 0.48 events coming from random pileup, the resulting upper limit on the level of electronic recoil contamination is $<2.7\times {10}^{-8}$ (90% C.L.) between 44–89 keV_ee and for a nuclear recoil acceptance of at least 90%, with 6% systematic uncertainty on the absolute energy scale.We developed a general mathematical framework to describe pulse-shape-discrimination parameter distributions and used it to build an analytical model of the distributions observed in DEAP-1. Using this model, we project a misidentification fraction of approximately ${10}^{-10}$ for an electron-equivalent energy threshold of 15 keV_ee for a detector with 8 PE/keV_ee light yield. This reduction enables a search for spin-independent scattering of WIMPs from 1000 kg of liquid argon with a WIMP-nucleon cross-section sensitivity of ${10}^{-46}$ cm², assuming negligible contribution from nuclear recoil backgrounds.     

FT-IR measurements of cold C3H8 cross sections at 7–15 μm for Titan atmosphere

We present absorption cross sections of propane (C₃H₈) at temperatures from 145 K to 297 K in the 690–1550 cm⁻¹ region. Pure and N₂-broadened spectra were measured at pressures from 3 Torr to 742 Torr using a Bruker IFS125 FT-IR spectrometer at JPL. The gas absorption cell, developed at Connecticut College, was cooled by a closed-cycle helium refrigerator. The cross sections were measured and compiled for individual spectra recorded at various experimental conditions covering the planetary atmosphere and Titan. In addition to the cross sections, a propane pseudoline list with a frequency grid of 0.005 cm⁻¹, was fitted to the 34 laboratory spectra. Line intensities and lower state energies were retrieved for each line, assuming a constant width. Validation tests showed that the pseudoline list reproduces discrete absorption features and continuum, the latter contributed by numerous weak and hot band features, in most of the observed spectra within 3%. Based on the pseudoline list, the total intensity in the 690–1550 cm⁻¹ region was determined to be 52.93 (±3%) × 10⁻¹⁹ cm⁻¹/(molecule cm⁻²) at 296 K; this value is within 3% of the average from four earlier studies. Finally, the merit of the pseudoline approach is addressed for heavy polyatomic molecules in support of spectroscopic observation of atmospheres of Titan and other planets. The cold cross sections will be submitted to the HITRAN database (hitran.harvard.edu), and the list of C₃H₈ pseudolines will be available from a MK-IV website of JPL (http://mark4sun.jpl.nasa.gov/data/spec/Pseudo).     

Eclipse reappearances of Io: Time-resolved spectroscopy (1.9-4.2 μm)

We obtained time-resolved, near-infrared spectra of Io during the 60-90 min following its reappearance from eclipse by Jupiter on five occasions in 2004. The purpose was to search for spectral changes, particularly in the well-known SO₂ frost absorption bands, that would indicate surface-atmosphere exchange of gaseous SO₂ induced by temperature changes during eclipse. These observations were a follow-on to eclipse spectroscopy observations in which Bellucci et al. [Bellucci et al., 2004. Icarus 172, 141-148] reported significant changes in the strengths of two strong SO₂ bands in data acquired with the VIMS instrument aboard the Cassini spacecraft. One of the bands (4.07 μm [ν₁ + ν₃]) observed by Bellucci et al. is visible from ground-based observatories and is included in our data. We detected no changes in Io’s spectrum at any of the five observed events during the approximately 60-90 min during which spectra were obtained following Io’s emergence from Jupiter’s shadow. The areas of the three strongest SO₂ bands in the region 3.5-4.15 μm were measured for each spectrum; the variation of the band areas with time does not exceed that which can be explained by the Io’s few degrees of axial rotation during the intervals of observation, and in no case does the change in band strength approach that seen in the Cassini VIMS data. Our data are of sufficient quality and resolution to show the weak 2.198 μm (4549.6 cm⁻¹) 4ν₁ band of SO₂ frost on Io for what we believe is the first time. At one of the events (June 22, 2004), we began the acquisition of spectra ∼6 min before Io reappeared from Jupiter’s shadow, during which time it was detected through its own thermal emission. No SO₂ bands were superimposed on the purely thermal spectrum on this occasion, suggesting that the upper limit to condensed SO₂ in the vertical column above Io’s surface was ∼4 × 10⁻⁵ g cm⁻².     

Background studies for acoustic neutrino detection at the South Pole

The detection of acoustic signals from ultra-high energy neutrino interactions is a promising method to measure the flux of cosmogenic neutrinos expected on Earth. The energy threshold for this process depends strongly on the absolute noise level in the target material. The South Pole Acoustic Test Setup (SPATS), deployed in the upper part of four boreholes of the IceCube Neutrino Observatory, has monitored the noise in Antarctic ice at the geographic South Pole for more than two years down to 500 m depth. The noise is very stable and Gaussian distributed. Lacking an in situ calibration up to now, laboratory measurements have been used to estimate the absolute noise level in the 10-50 kHz frequency range to be smaller than 20 mPa. Using a threshold trigger, sensors of the South Pole Acoustic Test Setup registered acoustic events in the IceCube detector volume and its vicinity. Acoustic signals from refreezing IceCube holes and from anthropogenic sources have been used to test the localization of acoustic events. An upper limit on the neutrino flux at energies E_ν > 10¹¹ GeV is derived from acoustic data taken over eight months.     

Carbon dioxide on the satellites of Saturn: Results from the Cassini VIMS investigation and revisions to the VIMS wavelength scale

Several of the icy satellites of Saturn show the spectroscopic signature of the asymmetric stretching mode of C-O in carbon dioxide (CO₂) at or near the nominal solid-phase laboratory wavelength of 4.2675 μm (2343.3 cm⁻¹), discovered with the Visible-Infrared Mapping Spectrometer (VIMS) on the Cassini spacecraft. We report here on an analysis of the variation in wavelength and width of the CO₂ absorption band in the spectra of Phoebe, Iapetus, Hyperion, and Dione. Comparisons are made to laboratory spectra of pure CO₂, CO₂ clathrates, ternary mixtures of CO₂ with other volatiles, implanted and adsorbed CO₂ in non-volatile materials, and ab initio theoretical calculations of CO₂ * nH₂O. At the wavelength resolution of VIMS, the CO₂ on Phoebe is indistinguishable from pure CO₂ ice (each molecule’s nearby neighbors are also CO₂) or type II clathrate of CO₂ in H₂O. In contrast, the CO₂ band on Iapetus, Hyperion, and Dione is shifted to shorter wavelengths (typically ∼4.255 μm (∼2350.2 cm⁻¹)) and broadened. These wavelengths are characteristic of complexes of CO₂ with different near-neighbor molecules that are encountered in other volatile mixtures such as with H₂O and CH₃OH, and non-volatile host materials like silicates, some clays, and zeolites. We suggest that Phoebe’s CO₂ is native to the body as part of the initial inventory of condensates and now exposed on the surface, while CO₂ on the other three satellites results at least in part from particle or UV irradiation of native H₂O plus a source of C, implantation or accretion from external sources, or redistribution of native CO₂ from the interior.The analysis presented here depends on an accurate VIMS wavelength scale. In preparation for this work, the baseline wavelength calibration for the Cassini VIMS was found to be distorted around 4.3 μm, apparently as a consequence of telluric CO₂ gas absorption in the pre-launch calibration. The effect can be reproduced by convolving a sequence of model detector response profiles with a deep atmospheric CO₂ absorption profile, producing distorted detector profile shapes and shifted central positions. In a laboratory blackbody spectrum used for radiance calibration, close examination of the CO₂ absorption profile shows a similar deviation from that expected from a model. These modeled effects appear to be sufficient to explain the distortion in the existing wavelength calibration now in use. A modification to the wavelength calibration for 13 adjacent bands is provided. The affected channels span about 0.2 μm centered on 4.28 μm. The maximum wavelength change is about 10 nm toward longer wavelength. This adjustment has implications for interpretation of some of the spectral features observed in the affected wavelength interval, such as from CO₂, as discussed in this paper.     

Mass composition of the escaping plasma at Mars

Data from the Ion Mass Analyzer (IMA) sensor of the ASPERA-3 instrument suite on Mars Express have been analyzed to determine the mass composition of the escaping ion species at Mars. We have examined 77 different ion-beam events and we present the results in terms of flux ratios between the following ion species: CO⁺₂/O⁺ and O⁺₂/O⁺. The following ratios averaged over all events and energies were identified: CO⁺₂/O⁺ = 0.2 and O⁺₂/O⁺ = 0.9. The values measured are significantly higher, by a factor of 10 for O⁺₂/O⁺, than a contemporary modeled ratio for the maximum fluxes which the martian ionosphere can supply. The most abundant ion species was found to be O⁺, followed by O⁺₂ and CO⁺₂. We estimate the loss of CO⁺₂ to be by using the previous measurements of Phobos-2 in our calculations. The dependence of the ion ratios in relation to their energy ranges we studied, 0.3-3.0 keV, indicated that no clear correlation was found.     

Dust extinction and X-ray emission from the starburst galaxy NGC 1482

We present the results based on multiwavelength imaging observations of the prominent dust lane starburst galaxy NGC 1482 aimed to investigate the extinction properties of dust existing in the extreme environment. (B-V) colour-index map derived for the starburst galaxy NGC 1482 confirms two prominent dust lanes running along its optical major axis and are found to extend up to ∼11 kpc. In addition to the main lanes, several filamentary structures of dust originating from the central starburst are also evident. Though, the dust is surrounded by exotic environment, the average extinction curve derived for this target galaxy is compatible with the Galactic curve, with R_V = 3.05, and imply that the dust grains responsible for the optical extinction in the target galaxy are not really different than the canonical grains in the Milky Way. Our estimate of total dust content of NGC 1482 assuming screening effect of dust is ∼2.7 × 10⁵ M_⊙, and provide lower limit due to the fact that our method is not sensitive to the intermix component of dust. Comparison of the observed dust in the galaxy with that supplied by the SNe to the ISM, imply that this supply is not sufficient to account for the observed dust and hence point towards the origin of dust in this galaxy through a merger like event.Our multiband imaging analysis reveals a qualitative physical correspondence between the morphologies of the dust and Hα emission lines as well as diffuse X-ray emission in this galaxy. Spatially resolved spectral analysis of the hot gas along outflows exhibit a gradient in the temperature. Similar gradient was also noticed in the measured values of metallicity, indicating that the gas in the halo is not yet enriched. High resolution, 2-8 keV Chandra image reveals a pair of point sources in the nuclear region with their luminosities equal to 2.27 × 10³⁹ erg s⁻¹ and 9.34 × 10³⁹ erg s⁻¹, and are in excess of the Eddington-limit of 1.5 M_⊙ accreting source. Spectral analysis of these sources exhibit an absorbed-power law with the hydrogen column density higher than that derived from the optical measurements.     

Venus night airglow: Ground-based detection of OH, observations of O2 emissions, and photochemical model

Venus nightglow was observed at NASA IRTF using a high-resolution long-slit spectrograph CSHELL at LT = 21:30 and 4:00 on Venus. Variations of the O₂ airglow at 1.27 μm and its rotational temperature are extracted from the observed spectra. The mean O₂ nightglow is 0.57 MR at 21:30 at 35°S-35°N, and the temperature increases from 171 K near the equator to ∼200 K at ±35°. We have found a narrow window that covers the OH (1-0) P1(4.5) and (2-1) Q1(1.5) airglow lines. The detected line intensities are converted into the (1-0) and (2-1) band intensities of 7.2 ± 1.8 kR and <1.4 kR at 21:30 and 15.5 ± 2 kR and 4.7 ± 1 kR at 4:00. The f-component of the (1-0) P1(4.5) line has not been detected in either observation, possibly because of resonance quenching in CO₂. The observed Earth’s OH (1-0) and (2-1) bands were 400 and 90 kR at 19:30 and 250 and 65 kR at 9:40, respectively. A photochemical model for the nighttime atmosphere at 80-130 km has been made. The model involves 61 reactions of 24 species, including odd hydrogen and chlorine chemistries, with fluxes of O, N, and H at 130 km as input parameters. To fit the OH vibrational distribution observed by VEX, quenching of OH (v > 3) in CO₂ only to v ? 2 is assumed. According to the model, the nightside-mean O₂ emission of 0.52 MR from the VEX and our observations requires an O flux of 2.9 × 10¹² cm⁻² s⁻¹ which is 45% of the dayside production above 80 km. This makes questionable the nightside-mean O₂ intensities of ∼1 MR from some observations. Bright nightglow patches are not ruled out; however, the mean nightglow is ∼0.5 MR as observed by VEX and supported by the model. The NO nightglow of 425 R needs an N flux of 1.2 × 10⁹ cm⁻² s⁻¹, which is close to that from VTGCM at solar minimum. However, the dayside supply of N at solar maximum is half that required to explain the NO nightglow in the PV observations. The limited data on the OH nightglow variations from the VEX and our observations are in reasonable agreement with the model. The calculated intensities and peak altitudes of the O₂, NO, and OH nightglow agree with the observations. Relationships for the nightglow intensities as functions of the O, N, and H fluxes are derived.     

Methane evolution from UV-irradiated spacecraft materials under simulated martian conditions: Implications for the Mars Science Laboratory (MSL) mission

Fifteen organic and three inorganic compounds were tested for methane (CH₄) evolution under simulated martian conditions of 6.9 mbar; UVC (200-280 nm) flux of 4 W m⁻²; 20 °C; simulated optical depth of 0.1; and a Mars gas composition of CO₂ (95.3%), N₂ (2.7%), Ar (1.7%), O₂ (0.13%), and water vapor (0.03%). All three inorganic compounds (i.e., NaCl, CaCO₃, graphite) failed to evolve methane at the minimum detection level 0.5 ppm, or above. In contrast, all organic compounds evolved methane when exposed to UV irradiation under simulated martian conditions. The polycyclic aromatic hydrocarbon, pyrene, released the most methane per unit of time at 0.175 nmol CH₄ g⁻¹ h⁻¹, and a spectral reflectance target material used for the MER rovers and Phoenix lander released the least methane at 0.00065 nmol CH₄ cm⁻² h⁻¹. Methane was also released from UV-killed bacterial endospores of Bacillus subtilis. Although all organic compounds evolved methane when irradiated with UV photons under martian conditions, the concentrations of residual organics, biogenic signature molecules, and dead microbial cells should be relatively low on the exterior surfaces of the MSL rover, and, thus, not significant sources of methane contamination. In contrast, kapton tape was found to evolve methane at the rate of 0.00165 nmol CH₄ cm⁻² h⁻¹ (16.5 nmol m⁻² h⁻¹) under the UV and martian conditions tested. Although the evolution of methane from kapton tape was found to decline over time, the large amount of kapton tape used on the MSL rover (lower bound estimated at 3 m²) is likely to create a significant source of terrestrial methane contamination during the early part of the mission.     

Core-collapse supernovae and the Younger Dryas/terminal Rancholabrean extinctions

Early predictions that some supernovae release large quantities of prompt high energy photons are now corroborated by optical identification of core-collapse supernovae associated with extragalactic GRBS (beamed γ-ray bursts) and XRFS (beamed or un-beamed X-ray flashes). Given the in-galaxy supernova frequency and GRB and XRF recurrence statistics, significant Earth-incident events during the past several million years very likely occurred and nearby events should have affected the Earth and other planetary atmospheres, including terrestrial surface solar UV, the Earth’s climate, and its ecology. The Younger Dryas Stadial (∼12,900 to 11,550 calendar yr BP) began with sharply cooler temperatures in the Earth’s northern hemisphere, regional drought, paleoecological evidence compatible with increased UV, and abrupt increases in cosmogenic ¹⁴C and ¹⁰Be in ice and marine cores and tree rings. In North America, stratigraphic and faunal sequences indicate that a major pulse of mammalian extinctions (at least 23-31 genera) began very close to 12,830 calendar yr BP and was sudden: deposits one century younger are devoid of diverse extinct fauna remains. A 10 s beamed GRB within 2 kpc of the Earth delivers 100 kJ m⁻² fluence to the Earth’s atmosphere, where it causes spallation and catalytic reactions depleting 35-50% O³, and producing excess NO_x species (which favor cooling, drought, and surface fertility), ¹⁴C, and ¹⁰Be. An un-beamed, 10⁵⁰ erg hard photon impulse at ∼250 pc produces similar terrestrial atmospheric effects. A well-characterized massive star supernova, the unusually close Vela event (d = 250 ± 30 pc; total energy of 1-2 × 10⁵¹ erg; age constrained from remnant nebula shock velocities considerations at 13,000-16,000 yr and from the pulsar characteristic age at ∼11,400 yr) may have initiated the Younger Dryas climate change, and caused the extinction of the terminal Rancholabrean fauna.     

Archival HST study of novae. I. The Seyfert galaxy NGC 3627

A V-band nova search was carried out in NGC 3627 with archival Hubble Space Telescope WFPC2 data which was obtained in the period between November 1997 and January 1998. A total of four novae candidates were discovered which corresponds to a global nova rate of R = 83.65 ± 7.58 yr⁻¹. Taking into account the K-band luminosity obtained from 2MASS (Jarrett et al., 2003) yielded a luminosity specific nova rate (LSNR) of ν_K = 9.60 ± 1.64 novae per year per 10¹⁰L_⊙,K. Excluding one of the candidates which may be a long-period variable leads to a LSNR of ν_K = 7.20 ± 1.23 novae per year per 10¹⁰L_⊙,K. These values are higher than other known nova rates for external galaxies except the Magellanic Clouds.     

Adsorptive fractionation of HDO on JSC MARS-1 during sublimation with implications for the regolith of Mars

A chamber was constructed to simulate the boundary between the ice table, regolith and atmosphere of Mars and to examine fractionation between H₂O and HDO during sublimation under realistic martian conditions of temperature and pressure. Thirteen experimental runs were conducted with regolith overlying the ice. The thickness and characteristic grain size of the regolith layer as well as the temperature of the underlying ice was varied. From these runs, values for the effective diffusivity, taking into account the effects of adsorption, of the regolith were derived. These effective diffusivities ranged from 1.8 × 10⁻⁴ m² s⁻¹ to 2.2 × 10⁻³ m² s⁻¹ for bare ice and from 2.4 × 10⁻¹¹ m² s⁻¹ to 2.0 × 10⁻⁹ m² s⁻¹ with an adsorptive layer present. From these, latent heats of adsorption of 8.6 ± 2.6 kJ mol⁻¹ and 9.3 ± 2.8 kJ mol⁻¹ were derived at ice-surface temperatures above 223 ± 8 K and 96 ± 28 kJ mol⁻¹ and 104 ± 31 kJ mol⁻¹ respectively for H₂O and HDO were derived at colder temperatures. For temperatures below 223 K, the effective diffusivity of HDO was found to be lower than the diffusivity of H₂O by 40% on average, suggesting that the regolith was adsorptively fractionating the sublimating gas with a fractionation factor of 1.96 ± 0.74. Applying these values to Mars predicts that adsorbed water on the regolith is enriched in HDO compared to the atmosphere, particularly where the regolith is colder. Based on current observations, the D/H ratio of the regolith may be as high as 21 ± 8 times VSMOW at 12°S and L_S = 357° if the regolith is hydrated primarily by the atmosphere, neglecting any hydration from subsurface ice.     

Vertical profile of H2SO4 vapor at 70-110 km on Venus and some related problems

The vertical profile of H₂SO₄ vapor is calculated using current atmospheric and thermodynamic data. The atmospheric data include the H₂O profiles observed at 70-112 km by the SOIR solar occultations, the SPICAV-UV profiles of the haze extinction at 220 nm, the VeRa temperature profiles, and a typical profile of eddy diffusion. The thermodynamic data are the saturated vapor pressures of H₂O and H₂SO₄ and chemical potentials of these species in sulfuric acid solutions. The calculated concentration of sulfuric acid in the cloud droplets varies from 85% at 70 km to a minimum of 70% at 90 km and then gradually increasing to 90-100% at 110 km. The H₂SO₄ vapor mixing ratio is ∼10⁻¹² at 70 and 110 km with a deep minimum of 3 × 10⁻¹⁸ at 88 km. The H₂O-H₂SO₄ system matches the local thermodynamic equilibrium conditions up to 87 km. The column photolysis rate of H₂SO₄ is 1.6 × 10⁵ cm⁻² s⁻¹ at 70 km and 23 cm⁻² s⁻¹ at 90 km. The calculated abundance of H₂SO₄ vapor at 90-110 km and its photolysis rate are smaller than those presented in the recent model by Zhang et al. (Zhang, X., Liang, M.C., Montmessin, F., Bertaux, J.L., Parkinson, C., Yung, Y.L. [2010]. Nat. Geosci. 3, 834-837) by factors of 10⁶ and 10⁹, respectively. Assumptions of 100% sulfuric acid, local thermodynamic equilibrium, too warm atmosphere, supersaturation of H₂SO₄ (impossible for a source of SO_X), and cross sections for H₂SO₄·H₂O (impossible above the pure H₂SO₄) are the main reasons of this huge difference. Significant differences and contradictions between the SPICAV-UV, SOIR, and ground-based submillimeter observations of SO_X at 70-110 km are briefly discussed and some weaknesses are outlined. The possible source of high altitude SO_X on Venus remains unclear and probably does not exist.     

Minimum estimates of the amount and timing of gases released into the martian atmosphere from volcanic eruptions

Volcanism has been a major process during most of the geologic history of Mars. Based on data collected from terrestrial basaltic eruptions, we assume that the volatile content of martian lavas was typically ∼0.5 wt.% water, ∼0.7 wt.% carbon dioxide, ∼0.14 wt.% sulfur dioxide, and contained several other important volatile constituents. From the geologic record of volcanism on Mars we find that during the late Noachian and through the Amazonian volcanic degassing contributed ∼0.8 bar to the martian atmosphere. Because most of the outgassing consisted of greenhouse gases (i.e., CO₂ and SO₂) warmer surface temperatures resulting from volcanic eruptions may have been possible. Our estimates suggest that ∼1.1 × 10²¹ g (∼8 ± 1 m m⁻²) of juvenile water were released by volcanism; slightly more than half the amount contained in the north polar cap and atmosphere. Estimates for released CO₂ (1.6 × 10²¹ g) suggests that a large reservoir of carbon dioxide is adsorbed in the martian regolith or alternatively ∼300 cm cm⁻² of carbonates may have formed, although these materials would not occur readily in the presence of excess SO₂. Up to ∼120 cm cm⁻² (2.2 × 10²⁰ g) of acid rain (H₂SO₄) may have precipitated onto the martian surface as the result of SO₂ degassing. The hydrogen flux resulting from volcanic outgassing may help explain the martian atmospheric D/H ratio. The amount of outgassed nitrogen (∼1.3 mbar) may also be capable of explaining the martian atmospheric ¹⁵N/¹⁴N ratio. Minor gas constituents (HF, HCl, and H₂S) could have formed hydroxyl salts on the surface resulting in the physical weathering of geologic materials. The amount of hydrogen fluoride emitted (1.82 × 10¹⁸ g) could be capable of dissolving a global layer of quartz sand ∼5 mm thick, possibly explaining why this mineral has not been positively identified in spectral observations. The estimates of volcanic outgassing presented here will be useful in understanding how the martian atmosphere evolved over time.     

Calculations of the thermochemistry of six reactions leading to ammonia formation in Titan's atmosphere

The thermochemical properties of the six reactions: (1) N₂+hν (solar EUV) → N⁺ + N(⁴S) + e⁻, (2) N⁺ + H₂ → NH⁺ + H, (3) NH⁺ + H₂ → NH⁺₂ + H, (4) NH⁺₂ + H₂ → NH⁺₃ + H, (5) NH⁺₃ + H₂ → NH⁺₄ + H, and (6) NH⁺₄ + e⁻ → NH₃ + H, were theoretically proposed by Atreya in 1986 and were cited in 2003 by Bernard who assumed that this chain reaction would lead to ammonia formation in Titan's atmosphere. The thermochemical properties of these six reactions have been calculated by means of the coupled cluster singles and doubles (CCSD) at the CCSD/cc-pvdz level, and the CCSD/6-311++g(3df,3pd) level, and G2 method. The geometries of the reactants and products of reactions have been optimized, the energies of reactions have been computed. The analysis of the results shows that: (I) The free energies of four reactions among these six reactions are negative. It means that these reactions, namely reactions (1)-(6) except reaction (2), can react spontaneously in Titan's low temperature environment. The converted temperatures of reactions (3) and (5) are 11881.7 and 4596.9 K, respectively. (II) Reaction (2) is an endothermic reaction, its converted temperature is 1797.6 K. When T<1797.6 K, reaction (2) cannot react forward spontaneously. The barrier of reaction (2) is 26.154 kcal mol⁻¹, which is probably too high to allow it to occur in the atmosphere of Titan. The rate for this reaction at 300 K has been calculated, and the value is k=4.16×¹⁰⁻⁷ s⁻¹. (III) The results of the three methods are more or less the same. So it is concluded that this chain reaction cannot be a pathway to lead to ammonia (gas phase) formation in Titan's atmosphere.     

Impact-induced N2 production from ammonium sulfate: Implications for the origin and evolution of N2 in Titan’s atmosphere

Chemical reactions and volatile supply through hypervelocity impacts may have played a key role for the origin and evolution of both planetary and satellite atmospheres. In this study, we evaluate the role of impact-induced N₂ production from reduced nitrogen-bearing solids proposed to be contained in Titan’s crust, ammonium sulfate ((NH₄)₂SO₄), for the replenishment of N₂ to the atmosphere in Titan’s history. To investigate the conversion of (NH₄)₂SO₄ into N₂ by hypervelocity impacts, we measured gases released from (NH₄)₂SO₄ that was exposed to hypervelocity impacts created by a laser gun. The sensitivity and accuracy of the measurements were enhanced by using an isotope labeling technique for the target. We obtained the efficiency of N₂ production from (NH₄)₂SO₄ as a function of peak shock pressure ranging from ∼8 to ∼45 GPa. Our results indicate that the initial and complete shock pressures for N₂ degassing from (NH₄)₂SO₄ are ∼10 and ∼25 GPa, respectively. These results suggest that cometary impacts on Titan (i.e., impact velocity v_i > ∼8 km/s) produce N₂ efficiently; whereas satellitesimal impacts during the accretion (i.e., v_i < 4 km/s) produce N₂ only inefficiently. Even when using the proposed small amount of (NH₄)₂SO₄ content in the crust (∼4 wt.%) (Fortes, A.D. et al., 2007. Icarus 188, 139-153), the total amount of N₂ provided through cometary impacts over 4.5 Ga reaches ∼2-6 times the present atmospheric N₂ (i.e., ∼7 × 10²⁰-2 × 10²¹ [mol]) based on the measured production efficiency and results of a hydrodynamic simulation of cometary impacts onto Titan. This implies that cometary impacts onto Titan’s crust have the potential to account for a large part of the present N₂ through the atmospheric replenishment after the accretion.     

Transport of ionizing radiation in terrestrial-like exoplanet atmospheres

The propagation of ionizing radiation through model atmospheres of terrestrial-like exoplanets is studied for a large range of column densities and incident photon energies using a Monte Carlo code we have developed to treat Compton scattering and photoabsorption. Incident spectra from parent star flares, supernovae, and gamma-ray bursts are modeled and compared to energetic particles in importance. Large irradiation events with fluences of 10⁶-10⁹ erg cm⁻² at the conventional habitable zone can occur at a rate from many per day (flares from young low-mass parent stars) to ∼100 per Gyr (supernovae and gamma-ray bursts). We find that terrestrial-like exoplanets with atmospheres thinner than about 100 g cm⁻² block nearly all X-rays, but transmit and reprocess a significant fraction of incident γ-rays, producing a characteristic, flat surficial spectrum. Thick atmospheres (?100 g cm⁻²) efficiently block even γ-rays, but nearly all the incident energy is redistributed into diffuse UV and visible aurora-like emission, increasing the effective atmospheric transmission by many orders of magnitude. Depending on the presence of molecular UV absorbers and atmospheric thickness, up to 10% of the incident energy can reach the surface as UV reemission. For the Earth, between 2×10⁻³ and 4×10⁻² of the incident flux reaches the ground in the biologically effective 200-320 nm range, depending on O₂/O₃ shielding. For atmospheres thicker than ∼50 g cm⁻² in the case of pure Rayleigh scattering and ∼100 g cm⁻² in the case of O₂/O₃ absorption, the UV reemission exceeds the surficial transmitted ionizing radiation. We also discuss the effects of angle of incidence and derive a modified two-stream approximation solution for the UV transfer. Finally, we suggest that transient atmospheric ionization layers can be frequently created at altitudes lower than the equilibrium layers that result from steady irradiation and winds from the parent star. We suggest that these events can produce frequent fluctuations in atmospheric ionization levels and surficial UV fluxes on terrestrial-like planets.     

Background discrimination capabilities of a heat and ionization germanium cryogenic detector

The discrimination capabilities of a 70 g heat and ionization Ge bolometer are studied. This first prototype has been used by the EDELWEISS dark matter experiment, installed in the Laboratoire Souterrain de Modane, for direct detection of WIMPs. Gamma and neutron calibrations demonstrate that this type of detector is able to reject more than 99.6% of the background while retaining 95% of the signal, provided that the background events distribution is not biased towards the surface of the Ge crystal. However, the 1.17 kg day of data taken in a relatively important radioactive environment show an extra population slightly overlapping the signal. This background is likely due to interactions of low energy photons or electrons near the surface of the crystal, and is somewhat reduced by applying a higher charge-collecting inverse bias voltage (−6 V instead of −2 V) to the Ge diode. Despite this contamination, more than 98% of the background can be rejected while retaining 50% of the signal. This yields a conservative upper limit of 0.7 event day⁻¹ kg⁻¹ keV_recoil⁻¹ at 90% confidence level in the 15–45 keV recoil energy interval; the present sensitivity appears to be limited by the fast ambient neutrons. Upgrades in progress on the installation are summarized.