Line broadening in the neutral and ionized mercury spectra

The neutral, singly, doubly and triply ionized mercury (Hg I–IV, respectively) spectral line shapes and line center positions have been investigated in the laboratory helium plasma at electron densities ranging between 9.3 × 10²² m^?3 and 1.93 × 10²³ m^?3 and electron temperatures around 19,500 K, both interesting for astrophysics. The mercury (natural isotope composition) atoms were sputtered from the cylindrical amalgamated gold plates located in the homogenous part of the pulsed helium discharge operating at a pressure of 665 Pa in a flowing regime. The mercury spectral line profiles were recorded using the McPherson model 209 spectrograph and the Andor ICCD camera as the detection system. This research presents Stark broadening parameters, the width (W) and the shift (d), of one Hg I, 19 Hg II, 6 Hg III and 4 Hg IV lines, not investigated so far. Our experimental W values were compared with the data calculated applying various approaches. The shape and intensity of astrophysically important 398.4 nm Hg II spectral line was discussed taking into account the isotope shift, hyperfine structure and Penning effects. At the mentioned plasma parameters the Stark broadening is found to be a main line broadening mechanism of the lines (λ > 200 nm) in the Hg I–IV spectra.     

The surface energy balance at the Huygens landing site and the moist surface conditions on Titan

The Huygens Probe provided a wealth of data concerning the atmosphere of Titan. It also provided tantalizing evidence of a small amount of surface liquid. We have developed a detailed surface energy balance for the Probe landing site. We find that the daily averaged non-radiative fluxes at the surface are 0.7 W m^?2, much larger than the global average value predicted by McKay et al. (1991) of 0.037 W m^?2. Considering the moist surface, the methane and ethane detected by the Probe from the surface is consistent with a ternary liquid of ethane, methane, and nitrogen present on the surface with mole fractions of methane, ethane, and nitrogen of 0.44, 0.34, and 0.22, respectively, and a total mass load of ～0.05 kg m^?2. If this liquid is included in the surface energy balance, only a small fraction of the non-radiative energy is due to latent heat release (～10^?3 W m^?2). If the amount of atmospheric ethane is less than 0.6×10^?5, the surface liquid is most likely evaporating over timescales of 5 Titan days, and the moist surface is probably a remnant of a recent precipitation event. If the surface liquid mass loading is increased to 0.5 kg m^?2, then the liquid lifetime increases to ～56 Titan days. Our modeling results indicate a dew cycle is unlikely, given that even when the diurnal variation of liquid is in equilibrium, the diurnal mass variation is only 3% of the total liquid. If we assume a high atmospheric mixing ratio of ethane (>0.6×10^?5), the precipitation of liquid is large (38 cm/Titan year for an ethane mixing ratio of 2×10^?5). Such a flux is many orders of magnitude in excess of the photochemical production rate of ethane.     

The role of sputtering and radiolysis in the generation of Europa exosphere

The exosphere of an atmosphereless icy moon is the result of different surface release processes and subsequent modification of the released particles. At Europa icy moon, water molecules are directly released, but photolysis and radiolysis due to solar UV and Jupiter’s magnetospheric plasma, respectively, can result in OH, H, O and (possibly) H₂ production. These molecules can recombine to reform water and/or new chemical species. As a consequence, Europa’s neutral environment becomes a mixture of different molecules, among which, H₂O dominates in the highest altitudes and O₂, formed mainly by radiolysis of ice and subsequent release of the produced molecules, prevails at lower altitudes. In this work, starting from a previously developed Monte Carlo model for the generation of Europa’s exosphere, where the only considered species was water, we make a first attempt to simulate also the H₂ and O₂ components of the neutral environment around Europa, already observed by the Hubble Space Telescope and the Ultraviolet Imaging Spectrograph on board Cassini, during its flyby of Jupiter. Considering a specific configuration where the leading hemisphere coincides with the sunlit hemisphere, we estimate along the Europa–Sun line an O₂ column density of about 1.5 × 10¹⁹ m^?2 at the dayside and 3 × 10¹⁸ m^?2 at the nightside. In this work we also improve our previous estimation of the sputtered H₂O exosphere of this moon, taking into consideration the trailing–leading asymmetry in the magnetospheric ion bombardment and the energy and temperature dependences of the process yields. We find that a density of 1.5 × 10¹² H₂O/m³ is expected at altitudes ～0.1R_E above the surface of the trailing hemisphere. Additionally, we calculate the escape of H₂O, O₂ and H₂. The total number of neutral atoms in Europa’s neutral torus, is estimated to be in the range 7.8 × 10³²–3.3 × 10³³.     

Statistical analysis of the observations of the MEX/ASPERA-3 NPI in the shadow

The analyser of space plasma and energetic atoms (ASPERA-3) neutral particle imager (NPI) on board Mars Express (MEX) is devoted to energetic neutral atom (ENA) detection within the Martian environment. These ENAs originate from the interaction between the energetic ions flowing inside the Martian environment and the exospheric neutral gas, thus providing crucial information about the dynamics of this interaction. NPI records the instantaneous angular distribution of the energy-integrated ENA signal. In order to identify recurrent ENA signals in the Martian environment, we have performed a statistical analysis of the NPI data. Count rates have been averaged using different methods in order to be able to discriminate signals coming from the planet, from a selected direction, or from specific planetographic regions at the planetary surface. Possible recurrent ENA signals (about 5×10⁶ (cm² sr s)^?1) are found coming from the terminator direction and above the atmosphere toward nightside when the spacecraft was inside the planetary shadow, mainly close to the shadow edge. Some significant signal was found from the anti-Mars directions in 2005. No statistically significant signal related to pick-up ions from the atmosphere or related to magnetic anomalies above the sensor intrinsic error (estimated as 3×10⁶ (cm² sr s)^?1) was observed. Our analysis shows that particular attention should be given to the use of NPI data when performing statistical studies; in fact, the sensor has some intrinsic limitations due to inadequate UV suppression, difficulties in sector inter-calibrations, and variations in the sector response versus time.     

Identification of Zirconium Oxide molecular lines in the sunspot umbral spectra

The ZrO molecule has been detected in sunspot umbrae through the identification of following laboratory molecular transitions: ¹Σ⁺ ? X¹Σ⁺ (0, 0), A³Φ₂ ? X²Δ₁ (0, 0), A³Φ₃ ? X²Δ₂ (0, 0), A³Φ₄ ? X²Δ₃ (0, 0), B³Π₂ ? X³Δ₃ (0, 0), B³Π₁ ? X³Δ₂ (0, 0) and B³Π₀ ? X³Δ₁ (0, 0) in red – infrared region using high resolution, visible range Fourier Transform Spectrum of sunspot umbra observed at the National Solar Observatory in Kitt Peak (NSO/KP). Much new identification has been made in the searched spectral wavenumber region from 16650 cm^?1 to 18007 cm^?1 of sunspot spectrum. Equivalent widths of well resolved lines, versus rotational quantum number J have been used to determine the effective rotational temperature for seven bands of the ZrO molecule. This result agrees well with the temperatures derived for other molecules’ presence in sunspot umbrae. It is evident that ZrO molecular lines are formed in higher layers of the atmosphere of relatively “cold” sunspots.     

Charting thermal emission variability at Pele,Janus Patera and Kanehekili Fluctus with the Galileo NIMS Io Thermal Emission Database (NITED)

Using the NIMS Io Thermal Emission Database (NITED), a collection of over 1000 measurements of radiant flux from Io’s volcanoes (Davies, A.G. et al. [2012]. Geophys. Res. Lett. 39, L01201. doi:10.1029/2011GL049999), we have examined the variability of thermal emission from three of Io’s volcanoes: Pele, Janus Patera and Kanehekili Fluctus. At Pele, the 5-μm thermal emission as derived from 28 night time observations is remarkably steady at 37 ± 10 GW μm^?1, re-affirming previous analyses that suggested that Pele an active, rapidly overturning silicate lava lake. Janus Patera also exhibits relatively steady 5-μm thermal emission (≈20 ± 3 GW μm^?1) in the four observations where Janus is resolved from nearby Kanehekili Fluctus. Janus Patera might contain a Pele-like lava lake with an effusion rate (Q_F) of ≈40–70 m³ s^?1. It should be a prime target for a future mission to Io in order to obtain data to determine lava eruption temperature. Kanehekili Fluctus has a thermal emission spectrum that is indicative of the emplacement of lava flows with insulated crusts. Effusion rate at Kanehekili Fluctus dropped by an order of magnitude from ≈95 m³ s^?1 in mid-1997 to ≈4 m³ s^?1 in late 2001.     

The distribution of Titan's high-altitude (out to ∼50,000 km) exosphere from energetic neutral atom (ENA) measurements by Cassini/INCA

We report observations of Titan's high-altitude exosphere detected out to about 50,000 km altitude. The observations were made by the Ion Neutral Camera (INCA) on board the Cassini spacecraft. INCA detects energetic neutral atoms (ENA) that are formed when the ambient magnetospheric ions charge exchange with Titan's neutral atmosphere and exosphere. We find that Titan's exospheric H₂ distribution follows closely a full Chamberlain distribution including ballistic, escaping and satellite distributions. As expected, neutral densities are dominated by a satellite distribution above about 10,000 km. The maximum detectable extent of the exosphere (～50,000 km) coincides with the radius of the Hill sphere of gravitational influence from Saturn. While we find no direct indications of a neutral Titan torus with densities greater than about 1000 cm^?3, we observe interesting asymmetries in the distribution that warrants further investigation. Based on these findings we compute the average precipitating ENA flux to be about 5×10⁶ keV/(cm² s), or 8×10^?3 erg/(cm² s), which is directly comparable to that of precipitating energetic ions (Sittler, et al., 2009) and slightly higher than that of solar EUV (Tobiska, 2004). Thus, the energy deposited by precipitating ENAs must also be taken into consideration when studying the energy balance of Titan's thermosphere.     

Atomic oxygen on the Venus nightside: Global distribution deduced from airglow mapping

The Visible and Infra-Red Thermal Imaging Spectrometer (VIRTIS) instrument on board the Venus Express spacecraft has measured the O₂(a¹Δ) nightglow distribution at 1.27 μm in the Venus mesosphere for more than two years. Nadir observations have been used to create a statistical map of the emission on Venus nightside. It appears that the statistical 1.6 MR maximum of the emission is located around the antisolar point. Limb observations provide information on the altitude and on the shape of the emission layer. We combine nadir observations essentially covering the southern hemisphere, corrected for the thermal emission of the lower atmosphere, with limb profiles of the northern hemisphere to generate a global map of the Venus nightside emission at 1.27 μm. Given all the O₂(a¹Δ) intensity profiles, O₂(a¹Δ) and O density profiles have been calculated and three-dimensional maps of metastable molecular and atomic oxygen densities have been generated. This global O density nightside distribution improves that available from the VTS3 model, which was based on measurements made above 145 km. The O₂(a¹Δ) hemispheric average density is 2.1 × 10⁹ cm^?3, with a maximum value of 6.5 × 10⁹ cm^?3 at 99.2 km. The O density profiles have been derived from the nightglow data using CO₂ profiles from the empirical VTS3 model or from SPICAV stellar occultations. The O hemispheric average density is 1.9 × 10¹¹ cm^?3 in both cases, with a mean altitude of the peak located at 106.1 km and 103.4 km, respectively. These results tend to confirm the modeled values of 2.8 × 10¹¹ cm^?3 at 104 km and 2.0 × 10¹¹ cm^?3 at 110 km obtained by Brecht et al. [Brecht, A., Bougher, S.W., Gérard, J.-C., Parkinson, C.D., Rafkin, S., Foster, B., 2011a. J. Geophys. Res., in press] and Krasnopolsky [Krasnopolsky, V.A., 2010. Icarus 207, 17–27], respectively. Comparing the oxygen density map derived from the O₂(a¹Δ) nightglow observations, it appears that the morphology is very different and that the densities obtained in this study are about three times higher than those predicted by the VTS3 model.     

Penetration of nearby supernova dust in the inner solar system

We investigate the method by which nearby supernovae – within a few tens of pc of the solar system – can penetrate the solar system and deposit live radioactivities on earth. The radioactive isotopic signatures that could potentially leave an observable geological imprint are in the form of refractory metals; consequently, it is likely they would arrive in the form of supernova-produced dust grains. Such grains can penetrate into the solar system more easily than the bulk supernova plasma, which gets stalled and deflected near the solar system due to the solar wind plasma pressure. We therefore examine the motion of charged grains as they decouple from the supernova plasma and are influenced by the solar magnetic, radiation, and gravitational fields. We characterize the dust trajectories with analytical approximations which display the roles of grain size, initial velocity, and surface voltage. These results are verified with full numerical simulations for wide ranges of dust properties. We find that supernova dust grains traverse the inner solar system nearly undeflected, if the incoming grain velocity – which we take to be that of the incident supernova remnant – is comparable to the solar wind speeds and much larger than the escape velocity at 1 AU. Consequently, the dust penetration to 1 AU has essentially 100% transmission probability and the dust capture onto the earth should have a geometric cross section. Our results cast in a new light the terrestrial deposition of radioisotopes from nearby supernovae in the geological past. For explosions beyond ～10 pc from earth, dust grains can still deliver supernova ejecta to earth, and thus the amount of supernova material deposited is set by the efficiency of dust condensation and survival in supernovae. Turning the problem around, we use observations of live ⁶⁰Fe in both deep-ocean and lunar samples to infer a conservative lower bound iron condensation efficiency of M_dust,Fe/M_tot,Fe ? 4  × 10^?4 for the supernova which apparently produced these species 2–3 Myr ago.     

Restitution coefficients and sticking velocities of a chondrule analogue colliding on a porous silica layer at impact velocities between 0.1 and 80 m s−1

To study the accretional growth of rimmed chondrules and their agglomerates in the solar nebula, we measured the restitution coefficients, ε, and the sticking velocities to a porous silica layer, v_c, by impacting the silica layer with a glass ball at velocities from 0.1 to 80 m s^?1. We used a porous silica layer covering a basalt block with thicknesses ranging from 1/5 of the glass ball radius to equal to the glass ball radius as a rimmed chondrule analogue, and the porosity of the silica layer was set to be 70%, 80%, 85%, and 90%. Collisional experiments were conducted by means of the free fall method or by the use of a spring gun or a gas gun, allowing us to vary the impact velocity. We used a laser displacement meter to estimate the impact and rebound velocities as well as the acceleration during the collision at impact velocities below 1 m s^?1. As a result, the sticking velocity, v_c, of 90%- and 85%- porosity layers with a thickness equal to 1/2 of the glass ball diameter was 0.44 and 2.4 m s^?1, respectively. On the other hand, we found a distinct barrier to sticking for smaller-porosity layers: the silicate layer with a porosity smaller than 80% never exhibited sticking at any impact velocity below 1 m s^?1. Instead, we observed a rebound effect with restitution coefficients larger than 0.2. In the case of a silica layer with a porosity smaller than 80%, we observed the sub-sticking condition defined by ε < 0.1 at velocities extending from 5 m s^?1 to 70 m s^?1.     

Constraints on Saturn’s tropospheric general circulation from Cassini ISS images

An automated cloud tracking algorithm is applied to Cassini Imaging Science Subsystem high-resolution apoapsis images of Saturn from 2005 and 2007 and moderate resolution images from 2011 and 2012 to define the near-global distribution of zonal winds and eddy momentum fluxes at the middle troposphere cloud level and in the upper troposphere haze. Improvements in the tracking algorithm combined with the greater feature contrast in the northern hemisphere during the approach to spring equinox allow for better rejection of erroneous wind vectors, a more objective assessment at any latitude of the quality of the mean zonal wind, and a population of winds comparable in size to that available for the much higher contrast atmosphere of Jupiter. Zonal winds at cloud level changed little between 2005 and 2007 at all latitudes sampled. Upper troposphere zonal winds derived from methane band images are ～10 m s^?1 weaker than cloud level winds in the cores of eastward jets and ～5 m s^?1 stronger on either side of the jet core, i.e., eastward jets appear to broaden with increasing altitude. In westward jet regions winds are approximately the same at both altitudes. Lateral eddy momentum fluxes are directed into eastward jet cores, including the strong equatorial jet, and away from westward jet cores and weaken with increasing altitude on the flanks of the eastward jets, consistent with the upward broadening of these jets. The conversion rate of eddy to mean zonal kinetic energy at the visible cloud level is larger in eastward jet regions (5.2 × 10^?5 m² s^?3) and smaller in westward jet regions (1.6 × 10^?5 m² s^?3) than the global mean value (4.1 × 10^?5 m² s^?3). Overall the results are consistent with theories that suggest that the jets and the overturning meridional circulation at cloud level on Saturn are maintained at least in part by eddies due to instabilities of the large-scale flow near and/or below the cloud level.     

Regional differences in gully occurrence on Mars: A comparison between the Hale and Bond craters

The observation of gullies on Mars raised questions about the presence of liquid water in the recent past. In some regions like Hale and Bond crater, gullies occur in one crater (Hale) but do not in another crater nearby (Bond). These regional differences have been interpreted as an argument for a formation of the gullies related to groundwater. The formation of gullies on Earth depends on rainfall and/or melting of snow as well as on several parameters such as the presence of steep slopes and sufficient amounts of fines and debris. We investigated the Hale/Bond region for differences in crater wall morphology and texture, slopes, and thermal properties to determine whether the gully formation is dependent on factors such as steep slope angles and availability of fine-grained material. Morphologically there exist two kinds of gullies in the Hale crater: Gullies on the south- and east-facing crater slopes have a pristine appearance with deep channels eroded into the talus material and well-preserved aprons. Gully-like features on the north- and west-facing slopes are degraded and superposed by craters, indicating that they are old in comparison to the pristine ones. However, their formation process is unclear and might be due to debris flows, surface runoff or dry mass wasting processes or a combination of these processes. The crater walls of Bond do not show gullies. Their morphology is most likely consistent with a degraded mantle deposit. Slope measurements reveal that the gullies in Hale crater occur on slopes between ～20° and ～30° in contrast to the slopes without gullies in Bond that are between ～10° and ～20° steep. Mean thermal inertia values on slopes with younger gullies are ～175 J m^?2 K^?1 s^?1/2 corresponding to higher amounts of fine-grained material. At slopes with older gully-like features mean thermal inertia values are ～315 J m^?2 K^?1 s^?1/2 corresponding to higher amounts of bedrock or possibly indurated grain sizes. Mean thermal inertia values of the Bond crater walls are ～230 J m^?2 K^?1 s^?1/2 indicating more consolidated terrain possibly due to the cementation of the dissected mantle material. From our investigation we conclude that the occurrence of gullies in the Hale/Bond region most likely depends on the distribution of unconsolidated material and steep slopes. The regional and local gully distribution on Mars likely varies due to differences in topography and surface material properties. Their proposed clustered distribution on Mars is not an argument for a groundwater formation mechanism of the gullies.     

Star forming activities in W75N and DR21 – Are the two regions in collision?

We have observed the massive star formation region W75N in ¹²CO J = 3 ? 2 with KOSMA. The profile of ¹²CO J = 3 ? 2 indicated that besides the 9 km s^?1 component, there is another component of ?3 km s^?1, which is associated with another star formation region, DR21N, located to the north of DR21. We derived the physical and dynamical parameters of the core and high velocity gas associated with the two components separately. Star forming activities were investigated, including outflows and infall analysis. The two regions overlap in space and are not connected in velocity. We found that the cloud–cloud collision scenario may not apply for the DR21/W75N case.     

Chandra X-ray observations of two unusual BAL quasars

We report sensitive Chandra X-ray non-detections of two unusual, luminous Iron Low-Ionization Broad Absorption Line Quasars (FeLoBALs). The observations do detect a non-BAL, wide-binary companion quasar to one of the FeLoBAL quasars. We combine X-ray-derived column density lower limits (assuming solar metallicity) with column densities measured from ultraviolet spectra and CLOUDY photoionization simulations to explore whether constant-density slabs at broad-line region densities can match the physical parameters of these two BAL outflows, and find that they cannot. In the “overlapping-trough” object SDSS J0300+0048, we measure the column density of the X-ray absorbing gas to be N_H ? 1.8 × 10²⁴ cm^?2. From the presence of Fe ii UV78 absorption but lack of Fe ii UV195/UV196 absorption, we infer the density in that part of the absorbing region to be n_e ? 10⁶ cm^?3. We do find that a slab of gas at that density might be able to explain this object’s absorption. In the Fe iii-dominant object SDSS J2215–0045, the X-ray absorbing column density of N_H ? 3.4 × 10²⁴ cm^?2 is consistent with the Fe iii-derived N_H ? 2 × 10²² cm^?2 provided the ionization parameter is log U > 1.0 for both the n_e = 10¹¹ cm^?3 and n_e = 10¹² cm^?3 scenarios considered (such densities are required to produce Fe iii absorption without Fe iiabsorption). However, the velocity width of the absorption rules out its being concentrated in a single slab at these densities. Instead, this object’s spectrum can be explained by a low density, high ionization and high temperature disk wind that encounters and ablates higher density, lower ionization Fe iii-emitting clumps.     

Kinetics of methane clathrate formation and dissociation under Mars relevant conditions

Spectral observations have detected methane within the martian atmosphere (Formisano, V., Atreya, S., Encrenaz, T., Ignatiev, N., Giuranna, M. [2004]. Science 306, 1758–1761; Mumma, M.J. et al. [2009]. Science 323, 1041–1045), however, the origin of the methane has not been determined. Methane clathrate (also referred to as methane hydrate) has been suggested as a potential subsurface reservoir, storing and releasing biologic and/or abiogenic methane. In this study, rates of methane hydrate formation and dissociation were measured experimentally at 234–264 K and 1.4–4.7 MPa to test the clathrate reservoir hypothesis. Initial formation rates range from 4.3 × 10^?6 to 8.1 × 10^?5 mol m^?2 s^?1. Results show decreasing rates of formation over time in individual experiments, indicating initial rapid clathration, followed by diffusion-limited transport of methane into the ice through the previously formed hydrate. These experiments indicate increased pressure results in increased formation rates, likely the result of higher concentration gradients, enhancing the methane diffusion flux into the solid phase. Experiments conducted at elevated temperatures produced faster initial rates of formation, resulting from increased kinetic energy of methane molecules and/or thickening of the Quasi-Liquid Layer. Based on this temperature dependence, the activation energy for methane hydrate formation from ice was determined to be 35.9 kJ/mol. Hydrate dissociation experiments initiated by depressurization or warming at conditions between 222 K and 265 K and 0.1–2.0 MPa were conducted following each formation experiment, yielding methane hydrate dissociation rates from 3.01 × 10^?6 to 9.92 × 10^?5 mol m^?2 s^?1. While both hydrate dissociation and formation showed decreasing instantaneous rates over the course of each experiment, the transition between the initial rate of dissociation and the interpreted diffusion-limited period of continued dissociation was more abrupt than that observed in formation experiments, supporting an ice shielding effect. The initial concentration of methane in the solid phase had a significant effect on hydrate dissociation rates. Higher methane concentrations in the solid phase produce faster initial rates, likely due to increased concentration gradients, thus increasing the diffusion component of dissociation. Increased temperatures also produced faster dissociation rates, yielding an activation energy for dissociation of 32.7 kJ/mol. The rates determined within this study suggest that small near-surface methane hydrate reservoirs are a feasible source for recent methane plumes detected on Mars. Rates of methane release from gas hydrates also indicate that gas hydrate dissociation may have played a role in forming ancient chaos terrain and associated outflow channels.     

High resolution investigation of the 7 μm region of the ethane spectrum

Building upon previous studies, we re-investigated the ethane spectrum between 1330 and 1610 cm^?1 by combining unapodized spectra obtained at room temperature with a Bruker Fourier transform spectrometer (FTS) in Brussels and at 131 K with a Bruker FTS in Pasadena. The maximum optical path differences (MOPD) of the two datasets were 450 and 323.7 cm, corresponding to spectral resolutions of 0.0020 and 0.0028 cm^?1, respectively. Of the 15,000 lines observed, over 4592 transitions were assigned to the ν₆ (at 1379 cm^?1), ν₈ (at 1472 cm^?1), ν₄+ν₁₂ (at 1481 cm^?1) and 2ν₄+ν₉ (at 1388 cm^?1) bands, and another 1044 transitions were located for the ν₄+ν₈?ν₄ hot band (at 1472 cm^?1). Our new analysis included an improved implementation of the Hamiltonian calculation needed to interpret the complex spectral structures caused by numerous interactions affecting these four modes of vibration. From these results, we created the first line-by-line database containing the molecular parameters for over 20,000 ¹²C₂H₆ transitions at 7 μm.     

Survival of yeast spores in hypervelocity impact events up to velocities of 7.4 km s−1

We report on the survivability in hypervelocity impacts of yeast in spore form, and as mature cultures, at impact velocities from 1 to 7.4 km s^?1, corresponding to an estimated peak shock pressure of ～43 GPa. Spores from a yeast strain (BY4743), deficient in an enzyme required for uracil production, were fired into water (to simulate oceanic impact from space) using a light gas gun. The water was then retrieved and filtered and the resulting retentate and filtrate cultured to determine viability and survival rates of remnant spores. Yeast growth (confirmed as coming from the original sample as it had the same enzyme deficiency) was found in recovered samples at all impact speeds, albeit in smaller quantities at the higher speeds. The survival probabilities were measured as ～50% at 1 km s^?1, falling to ～10^?3% at 7.4 km s^?1. This follows the pattern observed in previous work on survival of microbial life and spores exposed to extreme shock loading, where there is reasonable survival at low peak shock pressures with more severe lethality above a critical shock pressure at the GPa scale (here between 2 and 10 GPa). These results are explained in the context of a general model for survival against extreme shock and are relevant to the hypotheses of panspermia and litho-panspermia, showing that extreme shocks during transfer across space are not necessarily sterilising.     

Main Belt Comet P/2008 R1 Garradd: Duration of activity

In this work we investigated changes of the water emission from a model comet of the size and orbital elements of Comet P/2008 R1 (Garradd). We performed simulations for model cometary nuclei of different compositions and two different orientations in space. Our simulations indicate, that the emission of water decreases from one orbital period to another one, but in some cases slowly. When the rotation axis of the nucleus lies in the orbital plane the seasonal maximum of water production during the first two orbital periods can be as high as about 10²⁶ mol s^?1, but decreases by two orders of magnitude during only 50 orbital periods. The highest rate of water production after many orbital periods is expected when the rotation axis is perpendicular to the orbital plane – the seasonal maximum of water production can be about 5 × 10²⁵ mol s^?1 during the first two orbital periods after activation of the comet and no more than 0.8 × 10²⁵ mol s^?1 500 orbital periods later. The upper estimate for the production of water derived from observations of P/2008 R1 (Garradd) by Jewitt et al. (Jewitt, D., Yang, B., Haghighipour, N. [2009]. Astron. J. 137, 4313–4321) is 5 × 10²⁵.     

Observation of DCl and upper limit to NH3 on Venus

To search for DCl in the Venus atmosphere, a spectrum near the D³⁵Cl (1–0) R4 line at 2141.54 cm^?1 was observed using the CSHELL spectrograph at NASA IRTF. Least square fitting to the spectrum by a synthetic spectrum results in a DCl mixing ratio of 17.8 ± 6.8 ppb. Comparing to the HCl abundance of 400 ± 30 ppb (Krasnopolsky [2010a] Icarus, 208, 314–322), the DCl/HCl ratio is equal to 280 ± 110 times the terrestrial D/H = 1.56 × 10^?4. This ratio is similar to that of HDO/H₂O = 240 ± 25 times the terrestrial HDO/H₂O from the VEX/SOIR occultations at 70–110 km. Photochemistry in the Venus mesosphere converts H from HCl to that in H₂O with a rate of 1.9 × 10⁹ cm^?2 s^?1 (Krasnopolsky [2012] Icarus, 218, 230–246). The conversion involves photolysis of HCl; therefore, the photochemistry tends to enrich D/H in HCl and deplete in H₂O. Formation of the sulfuric acid clouds may affect HDO/H₂O as well. The enriched HCl moves down by mixing to the lower atmosphere where thermodynamic equilibriums for H₂ and HCl near the surface correspond to D/H = 0.71 and 0.74 times that in H₂O, respectively. Time to establish these equilibriums is estimated at ～3 years and comparable to the mixing time in the lower atmosphere. Therefore, the enriched HCl from the mesosphere gives D back to H₂O near the surface. Comparison of chemical and mixing times favors a constant HDO/H₂O up to ～100 km and DCl/HCl equal to D/H in H₂O times 0.74.Ammonia is an abundant form of nitrogen in the reducing environments. Thermodynamic equilibriums with N₂ and NO near the surface of Venus give its mixing ratio of 10^?14 and 6 × 10^?7, respectively. A spectrum of Venus near the NH₃ line at 4481.11 cm^?1 was observed at NASA IRTF and resulted in a two-sigma upper limit of 6 ppb for NH₃ above the Venus clouds. This is an improvement of the previous upper limit by a factor of 5. If ammonia exists at the ppb level or less in the lower atmosphere, it quickly dissociates in the mesosphere and weakly affects its photochemistry.     

Neutral sodium atoms release from the surfaces of the Moon and Mercury induced by meteoroid impacts

In this work, we calculate the neutral Na production rates on the Moon and Mercury, as due to the impacts of meteoroids having an impact probability on the surface that can influence the daily observations of the exosphere: the meteoroids radius range considered for the Moon and Mercury are 10⁻⁸–0.15 and 10⁻⁸–0.10 m, respectively. We also estimate the mass of meteoroids that has impacted the surfaces of the Moon and Mercury in the last 3.8 Gy (after the end of the Late Heavy Bombardment).The results of our model are that (i) the Na production rates are ∼(3–4.9)×10⁴ and ∼(1.8–2.3)×10⁶ atoms cm⁻² s⁻¹, for Moon and Mercury, respectively, and (ii) in the last 3.8 Gy, the mass of meteoroids that has impacted the whole surface of the Moon and Mercury has been 8.86×10¹⁸ and 2.66×10¹⁹ g, respectively.