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Global modelling of the early martian climate under a denser CO2 atmosphere: Water cycle and ice evolution

- Thu, 2012-12-13 08:24
January 2013
Publication year: 2013
Source:Icarus, Volume 222, Issue 1



We discuss 3D global simulations of the early martian climate that we have performed assuming a faint young Sun and denser CO2 atmosphere. We include a self-consistent representation of the water cycle, with atmosphere–surface interactions, atmospheric transport, and the radiative effects of CO2 and H2O gas and clouds taken into account. We find that for atmospheric pressures greater than a fraction of a bar, the adiabatic cooling effect causes temperatures in the southern highland valley network regions to fall significantly below the global average. Long-term climate evolution simulations indicate that in these circumstances, water ice is transported to the highlands from low-lying regions for a wide range of orbital obliquities, regardless of the extent of the Tharsis bulge. In addition, an extended water ice cap forms on the southern pole, approximately corresponding to the location of the Noachian/Hesperian era Dorsa Argentea Formation. Even for a multiple-bar CO2 atmosphere, conditions are too cold to allow long-term surface liquid water. Limited melting occurs on warm summer days in some locations, but only for surface albedo and thermal inertia conditions that may be unrealistic for water ice. Nonetheless, meteorite impacts and volcanism could potentially cause intense episodic melting under such conditions. Because ice migration to higher altitudes is a robust mechanism for recharging highland water sources after such events, we suggest that this globally sub-zero, ‘icy highlands’ scenario for the late Noachian climate may be sufficient to explain most of the fluvial geology without the need to invoke additional long-term warming mechanisms or an early warm, wet Mars.
Highlights ► Global 3D study of the early martian climate and water cycle. ► New general circulation model with accurate radiative transfer and dynamic clouds developed. ► Simulations show adiabatic effect at higher CO2 pressure causes ice to migrate to valley network regions. ► Seasonal melting insufficient to explain necessary erosion. ► Impacts, volcanism or basal melting may have caused episodic flooding events.



Planetary perturbations for Oort Cloud comets. I. Distributions and dynamics

- Thu, 2012-12-13 08:24
January 2013
Publication year: 2013
Source:Icarus, Volume 222, Issue 1



This paper is the first in a series, where we aim to model the injection of comets from the Oort Cloud so well that the shape of the energy distribution of long-period comets (i.e., the distribution of reciprocal semi-major axis) together with the observed rate of perihelion passages can be used to make serious inferences about the population size and energy distribution of the cloud. Here we explore the energy perturbations caused by the giant planets on long-period comets with perihelia inside or near the planetary system. We use a simplified dynamical model to integrate such perturbations for large samples of fictitious comets and analyse the statistics of the outcomes. After demonstrating the sensitivity of derived parameters to the sample size, when close encounters are involved, we derive a map of the RMS energy perturbation as a function of perihelion distance (q) and the cosine of the inclination (i), which compares well with the results of previous papers. We perform a critical analysis of the loss cone concept by deriving the “opacity” (chance of leaving the Oort spike by planetary perturbations per perihelion passage) as a function of q and cos i, concluding that the often made assumption of full opacity for q <15AU is seriously in error. While such a conclusion may also have been drawn from earlier studies, we provide the first full, quantitative picture. Moreover, we make a preliminary investigation of the long-term evolution of long-period comet orbits under the influence of planetary perturbations, neglecting the external effects of Galactic tides and stellar encounters. This allows us to make predictions about the production of decoupled objects like Halley-type comets and Centaurs from the injection of Oort Cloud comets, as well as of a related population of transneptunians deriving from the Oort Cloud with perihelia well detached from the planets.
Highlights ► An extensive study of planetary perturbations on Oort Cloud comets is performed. ► The probability for a comet to be ejected from the Oort spike is computed. ► This probability is not a step function from which the loss cone concept derives. ► The long term behaviour under planetary perturbations alone is also performed. ► Some inferences about the production of Halley type comets or Centaurs are made.



Constraint on the lunar core size from electromagnetic sounding based on magnetic field observations by an orbiting satellite

- Thu, 2012-12-13 08:24
January 2013
Publication year: 2013
Source:Icarus, Volume 222, Issue 1



We have attempted to constrain the lunar core size from electrical conductivity sounding by using magnetic field data from the Kaguya (KG) and Lunar Prospector (LP) satellites. As suggested by previous studies, the signature of induction in the core can sometimes be detected by the satellites as an internal dipole field when the Moon enters from the magnetosheath to the tail lobe of the Earth’s magnetosphere. Since the magnetic anomaly field is up to about 2nT at the orbital altitude (∼100km), we removed the anomaly field from the observed magnetic field on the basis of the magnetic anomaly analysis. A spherical harmonic analysis tuned for the satellite observations was applied for separation of the internal and external fields, and the internal to external dipole ratio was used to estimate the size of the core. We estimated the effect of lunar mantle induction due to the external field fluctuation in the tail lobe region for two typical conductivity models of the lunar interior. Simulation results show that mantle induction fields seriously disturb the core signal. However, data selection made with reference to statistical distribution of the dipole ratio is effective to minimize the influence of the mantle conductivity on the core size estimates. The internal to external dipole ratio obtained for the selected period of KG observation was 0.0023±0.0019. The mean value corresponded to a lunar core size of 290km, and the upper bound of the lunar core size was estimated to be about 400km with a 95% confidence limit. On the other hand, it was not possible to select a time interval that was suitable for successful application of the method adopted in the present work to the LP dataset. Larger external magnetic field variation during the period of LP observation caused this, although the stronger magnetic field at the LP observation period was expected to be suitable for the core size estimation. The larger external field variation during the period of LP observation was a consequence of moderately active solar conditions in 1998 relative to the deep solar minimum conditions in 2008.
Highlights ► We analyzed magnetic field data around the Moon observed by Kaguya and Lunar Prospector. ► The upper bound of a highly conducting lunar core radius is 400km. ► The induction signal by the lunar mantle is significant at 100-km altitude.



Asteroidal impacts and the origin of terrestrial and lunar volatiles

- Thu, 2012-12-13 08:24
January 2013
Publication year: 2013
Source:Icarus, Volume 222, Issue 1



Asteroids impacting the Earth partly volatilize, partly melt (O’Keefe, J.D., Ahrens, T.J. [1977]. Proc. Lunar Sci. Conf. 8, 3357–3374). While metal rapidly segregates out of the melt and sinks into the core, the vaporized material orbits the Earth and eventually rains back onto its surface. The content of the mantle in siderophile elements and their chondritic relative abundances hence is accounted for, not by the impactors themselves, as in the original late-veneer model (Chou, C.L. [1978]. Proc. Lunar Sci. Conf. 9, 219–230; Morgan, J.W. et al. [1981]. Tectonophysics 75, 47–67), but by the vapor resulting from impacts. The impactor’s non-siderophile volatiles, notably hydrogen, are added to the mantle and hydrosphere. The addition of late veneer may have lasted for 130Ma after isolation of the Solar System and probably longer, i.e., well beyond the giant lunar impact. Constraints from the stable isotopes of oxygen and other elements suggest that, contrary to evidence from highly siderophile elements, ∼4% of CI chondrites accreted to the Earth. The amount of water added in this way during the waning stages of accretion, and now dissolved in the deep mantle or used to oxidize Fe in the mantle and the core, may correspond to 10–25 times the mass of the present-day ocean. The Moon is at least 100 times more depleted than the Earth in volatile elements with the exception of some isolated domains, such as the mantle source of 74220 pyroclastic glasses, which appear to contain significantly higher concentrations of water and other volatiles.
Highlights ► Impacts on Earth lead to the volatilization and melting of the impactor. ► Impacts on the Moon lead to the fragmentation of the impactor. ► Volatiles in the Earth represent the accretion of ∼4% of CI-chondrites. ► Earth’s core formed 30Ma after SS, the late veneer arriving later than 130Ma. ► Up to 1% water has been added to the Earth, whereas the Moon is significantly drier.



Titan’s atmosphere and surface liquid: New calculation using Statistical Associating Fluid Theory

- Thu, 2012-12-13 08:24
January 2013
Publication year: 2013
Source:Icarus, Volume 222, Issue 1



The application of PC-SAFT equation of state (EOS) in analyzing the in situ measurement of atmospheric data by Huygens probe reveals new insights into Titan’s atmosphere and surface liquids. The EOS offers the most reliable and accurate calculations in fluid phase equilibria at the cryogenic conditions encountered in Titan and other extra-terrestrial bodies. This paper and a succeeding one pertaining to solid phases are foundational introductions to a new thermodynamics tool (new to planetary science) and will open the way for many diverse planetological applications, but here we limit applications to Titan. Titan’s lower stratosphere and lower troposphere are modeled as a well-mixed chemical solution with fixed overall composition of nine components. Using this model in the lower stratosphere, the dew point, below which condensation occurs, is calculated to be at an altitude of 65.3km (T =91.3K, P =0.031bar). The first drop of liquid at this point is almost pure propane, which would form a haze (not a dense cloud) due to the minor abundance of this species. Using this model in the lower troposphere, the atmospheric methane mole fractions measured by Huygens probe is well predicted up to an altitude of 29km, thus validates the model and the EOS. The surface liquid, which is assumed to be in thermodynamic equilibrium with the ground-level atmosphere, is dominated by C2H6, CH4, C3H8, and N2 with mole percents of 53%, 32%, 7%, and 7%, respectively, at a density of 614kg/m3 in the equator. The effects of the temperature on the surface liquid composition are also discussed. Despite the small surface temperature difference between equatorial and polar regions (3.7K), the composition of liquid in polar regions is very different from that in the equator: 68% CH4, 22% N2, and 8% C2H6 with the amount of liquid nine times larger than that in the equator at a 10%-smaller density of 551kg/m3. The system is accurately estimated using the binary of CH4 and N2 only at an altitude higher than 8km up to 29km. In this case the binary phase diagram can be applied to estimate that the methane-rich solid phase appears at an altitude of 21km and higher.
Highlights ► We apply Statistical Associating Fluid Theory for Titan’s fluid phase equilibria. ► Dew point of Titan’s atmosphere is estimated to be at an altitude of 65.3km. ► The model’s prediction matches the measured vertical profile of CH4 up to 29km. ► Equatorial surface liquids: 53% C2H6, 32% CH4, 7% C3H8, and 7% N2. ► Polar surface liquids: 8% C2H6, 68% CH4, and 22% N2.



Experimental studies of ice grain ejection by massive gas flow from ice and implications to Comets, Triton and Mars

- Thu, 2012-12-13 08:24
January 2013
Publication year: 2013
Source:Icarus, Volume 222, Issue 1



This is an experimental study of ice grain ejection when trapped gases are released from water ice. When ice is formed by adherence of water molecules at low temperatures, it forms an amorphous structure with many pores, where gas molecules can reside. When further ice layers are formed, the gases are trapped in the ice. Upon its warming-up, the ice structure changes, releasing fractions of the trapped gas. If they do not encounter obstacles, they are released quiescently by dynamic percolation. In a non-dense ice a huge flux of ice grains emanates from the ice, propelled by gas jets and covering its entire surface. When the overlying ice is denser, due to back-migration of water vapor during its sublimation, gas trying to escape from below cannot penetrate the dense ice and breaks it, producing non-circular craters and a chaotic terrain, as observed experimentally and in close encounters with Comets Wild 2, Tempel 1 and Hartley 2. These experimental findings explain several observations of Solar System bodies: ice grain ejection from Comets Temple 1 and Hartley 2. Also explained are the dark jets observed on Triton, where their ejection speed suggests a deep source. On Mars, dark streaks are observed in the southern pole in spring, most likely by plumes carrying dark dust, carried by winds and falling on the surface. As found by us experimentally, only frozen CO2 covered by water ice or mixed with it will work to form jets, whereas pure frozen CO2 will sublimate quiescently.
Highlights ► An experimental study of ice grain ejection when trapped gases are released from ice. ► The experimental findings explain ice grain ejection from comets. ► Also explained are the dark jets observed on Triton. ► The plumes observed in the martian pole in springtime require a mixture of CO2 and ice.



3D modelling of the early martian climate under a denser CO2 atmosphere: Temperatures and CO2 ice clouds

- Thu, 2012-12-13 08:24
January 2013
Publication year: 2013
Source:Icarus, Volume 222, Issue 1



On the basis of geological evidence, it is often stated that the early martian climate was warm enough for liquid water to flow on the surface thanks to the greenhouse effect of a thick atmosphere. We present 3D global climate simulations of the early martian climate performed assuming a faint young Sun and a CO2 atmosphere with surface pressure between 0.1 and 7bars. The model includes a detailed radiative transfer model using revised CO2 gas collision induced absorption properties, and a parameterisation of the CO2 ice cloud microphysical and radiative properties. A wide range of possible climates is explored using various values of obliquities, orbital parameters, cloud microphysic parameters, atmospheric dust loading, and surface properties. Unlike on present day Mars, for pressures higher than a fraction of a bar, surface temperatures vary with altitude because of the adiabatic cooling and warming of the atmosphere when it moves vertically. In most simulations, CO2 ice clouds cover a major part of the planet. Previous studies had suggested that they could have warmed the planet thanks to their scattering greenhouse effect. However, even assuming parameters that maximize this effect, it does not exceed +15K. Combined with the revised CO2 spectroscopy and the impact of surface CO2 ice on the planetary albedo, we find that a CO2 atmosphere could not have raised the annual mean temperature above 0°C anywhere on the planet. The collapse of the atmosphere into permanent CO2 ice caps is predicted for pressures higher than 3bar, or conversely at pressure lower than 1bar if the obliquity is low enough. Summertime diurnal mean surface temperatures above 0°C (a condition which could have allowed rivers and lakes to form) are predicted for obliquity larger than 40° at high latitudes but not in locations where most valley networks or layered sedimentary units are observed. In the absence of other warming mechanisms, our climate model results are thus consistent with a cold early Mars scenario in which nonclimatic mechanisms must occur to explain the evidence for liquid water. In a companion paper by Wordsworth et al. we simulate the hydrological cycle on such a planet and discuss how this could have happened in more detail.
Highlights ► We present 3D simulations of the possible early Mars climate. ► We assume a faint young Sun and a thick CO2 atmosphere with CO2 clouds. ► We explore various obliquities, orbits, cloud parameters and dust loading. ► The mean temperature cannot be raised above 0°C anywhere on the planet.



Retrieval of jovian cloud structure from the Cassini ISS limb-darkening data I. Continuum scattering phase functions for cloud and haze in the South Tropical Zone

- Thu, 2012-12-13 08:24
January 2013
Publication year: 2013
Source:Icarus, Volume 222, Issue 1



We have deduced the scattering properties of aerosols in the jovian upper troposphere by analyzing imaging data obtained at a wide variety of solar phase angles (4–140°) by the Cassini Imaging Science Subsystem (ISS) Narrow Angle Camera (NAC) during its flyby of Jupiter. The limb-darkening curves along the South Tropical Zone (STrZ) are extracted from CB2 images (effective wavelength: 750nm) to constrain the single scattering phase functions of aerosols. The best-fit Mie scattering phase function for cloud is obtained with the real part of the refractive index n r,cloud =1.85 and the effective radius r eff,cloud =0.3μm. The best-fit combination of n r,cloud and r eff,cloud would strongly suggest the idea that the abundant small particle population in the upper troposphere is not composed of pure NH3 ice. Although the optical properties of the stratospheric haze are not well constrained compared with those of cloud, the haze is found to be optically thin (<0.06) and to be strongly forward scattering (effective radius r eff,haze =0.5μm). We compare our results with the scattering phase function at red wavelength (640nm) for the STrZ derived by Tomasko et al. (Tomasko, M.G., West, R.A., Castillo, N.D. [1978]. Icarus 33, 558–592), which was deduced from analysis of the Pioneer 10 Imaging Photopolarimeter (IPP) data (12–150° for solar phase angles). Our new Mie scattering phase function can reproduce the Pioneer 10 observations well. In contrast, their scattering phase function described by the double Henyey–Greenstein function does not reproduce the Cassini ISS observations. This is attributed to the fact that their scattering phase function is underconstrained, primarily due to a considerable gap in observations for an intermediate solar phase angle range (34–109°). Our new Mie scattering phase function has advantages over that of Tomasko et al. (Tomasko, M.G., West, R.A., Castillo, N.D. [1978]. Icarus 33, 558–592). (1) Since the Cassini ISS data do not have a large gap in solar phase angle, the new Mie scattering phase function is better constrained. (2) The Mie scattering phase function can be applied easily to different wavelengths. With such characteristics, we now have a reliable baseline scattering phase function that can be used to interpret the ever-changing appearance of jovian clouds as changes of the vertical cloud structure and/or distribution of chromophores in the atmosphere.
Highlights ► Scattering properties of aerosols in Jupiter are retrieved from Cassini ISS data. ► The real refractive index of cloud (=1.85) is much higher than that for NH3 ice. ► Jovian upper troposphere is covered with a layer of small cloud particles (0.3μm). ► Stratospheric haze is optically thin and strongly forward scattering. ► Our Mie scattering phase function can reproduce the Pioneer 10 IPP observations.



Persistently illuminated regions at the lunar poles: Ideal sites for future exploration

- Thu, 2012-12-13 08:24
January 2013
Publication year: 2013
Source:Icarus, Volume 222, Issue 1



The Lunar Reconnaissance Orbiter Camera (LROC) provides multi-temporal and high resolution imaging of the north and south polar regions. These images delimit illuminated areas from those in shadow, and are used to analyze the illumination environment of the polar regions over the course of a lunar year. The Wide Angle Camera (WAC) provides repeat imaging of the north and south pole at a frequency of roughly 2h with a ground sampling distance of 100m. The LROC Narrow Angle Camera (NAC) acquires images with a ground sampling distance of 0.5–2.0m providing the means to construct high resolution maps that reveal illuminated terrain under varying lighting conditions. With the multi-temporal coverage provided by the WAC and the high resolution images from the NAC, the LROC dataset enables a more comprehensive analysis of the illumination conditions near the lunar poles than any previous image based dataset. Furthermore, these images are used to validate previously published numerical models that simulate polar illumination conditions. From our analysis of the LROC images, we identified localized regions where the lunar surface remains illuminated for nearly 94% of the year with the longest eclipsed period lasting only 43h. We also identified small illuminated peaks (tens of meters across) in areas previously modeled to be in shadow. Together, the WAC and NAC dataset provide direct, high resolution observations of the actual surface illumination environment of potential exploration sites near the lunar poles.
Highlights ► LROC has acquired over 24,000 WAC and 31,500 NAC images within 2° of the poles. ► 7818 WAC images were compiled into 100m/pixel illumination maps of each pole. ► We identify persistently illuminated regions that are ideal destinations for future missions. ► Outposts on the rim of Shackelton remain illuminated for 94% of a lunar year. ► The longest all three Shackleton outposts are eclipsed is 43h.



Mineralogy of Mare Serenitatis on the near side of the Moon based on Chandrayaan-1 Moon Mineralogy Mapper (M3) observations

- Thu, 2012-12-13 08:24
January 2013
Publication year: 2013
Source:Icarus, Volume 222, Issue 1



Spectral analysis of Mare Serenitatis has been carried out using Chandrayaan-1 Moon Mineralogy Mapper (M3) data in order to map the compositional diversity of the basaltic units that exist in the basin. Mare Serenitatis is characterized by multiple basaltic flows of different ages indicating a prolonged volcanism subsequent to the basin formation event. Reflectance spectra of fresh craters from the Mare Serenitatis have been analyzed to study the nature and location of the spectral absorption features around 1- and 2-μm respectively, arising due to the electronic charge transition of Fe2+ in the crystal lattice of pyroxenes and/or olivine. Chandrayaan-1 M3 data have been utilized to obtain an Integrated Band Depth (IBD) mosaic of the Serenitatis basin. Based on the spectral variations observed in the IBD mosaic, 13 spectral units have been mapped in the Mare Serenitatis. In the present study, we have also derived spectral band parameters, namely, band center, band strength, band area and band area ratio from the M3 data to study the mineralogical and compositional variations amongst the basaltic units of the studied basin. On the basis of spectral band parameter analysis, the pyroxene compositions of the basaltic units have been determined, which vary from low to intermediate end of the high-Ca pyroxene and probably represent a sub-calcic to calcic augite compositional range. Detailed spectral analyses reveal little variations in the mafic mineralogy of the mare basalts in terms of pyroxene chemistry. The uniformity in pyroxene composition across the basaltic units of Mare Serenitatis, therefore, suggest a probably stable basaltic source region, which might not have experienced large-scale fractionation during the prolonged volcanism that resulted in filling of the large Serenitatis basin.
Highlights ► IBD mosaic of Mare Serenitatis has been generated using Chandrayaan-1 M3 data. ► Thirteen spectral units have been mapped based on spectral variations in the IBD mosaic. ► Pyroxene chemistry has been obtained based on spectral band parameter analysis. ► Pyroxene chemistry varies from sub-calcic to calcic augite compositional range. ► No compositional trend has been observed in Mare Serenitatis.



Diffusion and thermal escape of H2 from Titan’s atmosphere: Monte Carlo simulations

- Thu, 2012-12-13 08:24
January 2013
Publication year: 2013
Source:Icarus, Volume 222, Issue 1



The Direct Simulation Monte Carlo (DSMC) technique is used here to describe the transition region in Titan’s atmosphere where the gas flow goes from being collisional to collisionless. We expand on our previous study (Tucker, O.J., Johnson, R.E. [2009]. Planet. Space Sci. 57, 1889–1894) by including H2 in addition to CH4 and N2. We again find that thermal escape of CH4 is Jeans-like, contrary to what has been suggested by some fluid/continuum models. However, we also show that the temperature of molecular hydrogen separates from the background gas well below the exobase, and its escape cools the background gas. This results in a non-isothermal CH4 density profile without requiring an upward CH4 flux and, therefore, fits using the diffusion equation can overestimate the escape flux. These simulations also reproduce the Cassini H2 density versus altitude data averaged over flybys for which Titan is orbiting in Saturn’s plasma sheet, but with a somewhat different escape rate than suggested by the diffusion equation. However, for flybys for which Titan is not in Saturn’s plasma sheet our simulations result in H2 densities that diffusively separate from the N2 densities at lower altitudes than typically indicated by the Cassini data. By tracking ballistic transport in the H2 corona we show that a global, as well as temporal description of the exobase region is required.
Highlights ► Direct Monte Carlo simulations (DSMCs) of thermal escape from Titan’s atmosphere. ► Our DSMC results are compared to diffusion models and Cassini data. ► Thermal escape of CH4 and H2 from Titan is similar to Jeans escape. ► Cooling by H2 escape will results in the CH4 density profile being non-barometric. ► A global and temporal description is required to model Titan’s upper atmosphere.



The origin of the non-mare mascon gravity anomalies in lunar basins

- Thu, 2012-12-13 08:24
January 2013
Publication year: 2013
Source:Icarus, Volume 222, Issue 1



Many lunar basins are characterized by prominent positive gravity anomalies over the basin interiors, referred to as mass concentrations or mascons. While a significant fraction of some near-side mascon anomalies can be explained as a result of the flexural support of the mare basalts within the basins, a number of basins, including Orientale, exhibit mascons in excess of those that can be plausibly ascribed to the mare. Some basins exhibit mascons but lack mare altogether. Lunar gravity and topography data are used to map the isostatic anomaly, or the height of the surface above or below its isostatic level. Orientale is representative of the majority of lunar basins, in which the super-isostatic basin center is surrounded by a sub-isostatic annulus of comparable magnitude but greater area. The basin structure as a whole is found to be strongly sub-isostatic. High-resolution crustal thickness models of Orientale confirm that it is surrounded by an annulus of thickened but sub-isostatic crust. It is proposed that the flexural uplift of the annulus causes the uplift and positive gravity anomalies within the basin center. Finite element models are used to examine the flexural uplift of the sub-isostatic annulus and the basin center for a range of lithosphere thicknesses both outside the basin and in the basin interior. The uplift of the basin center can exceed 2km, increasing the central gravity anomaly by ∼200mGal. This annular uplift explains a significant fraction of the Orientale mascon, and is likely a dominant cause of non-mare mascons globally.
Highlights ► A significant fraction of lunar mascons cannot be explained by mare loading. ► Mascon basins are surrounded by annuli of sub-isostatic thickened crust. ► Isostatic analysis, crustal thickness models, and flexural modeling are performed. ► Flexural uplift of the annulus results in uplift of the basin center of Orientale. ► Annular uplift can account for a significant fraction of the non-mare mascons.



Dications and thermal ions in planetary atmospheric escape

- Thu, 2012-12-13 08:24
January 2013
Publication year: 2013
Source:Icarus, Volume 222, Issue 1



In the recent years, the presence of dications in the atmospheres of Mars, Venus, Earth and Titan has been modeled and assessed. These studies also suggested that these ions could participate to the escape of the planetary atmospheres because a large fraction of them is unstable and highly energetic. When they dissociate, their internal energy is transformed into kinetic energy which may be larger than the escape energy. The goal of this study is to assess the impact of the doubly-charged ions in the escape of CO2-dominated planetary atmospheres and to compare it to the escape of thermal photo-ions. We solve a Boltzmann transport equation at daytime taking into account the dissociative states of CO 2 + + for a simplified single constituent atmosphere of a case-study planet. We compute the escape of fast ions using a Beer–Lambert approach. We study three test-cases. On a Mars-analog planet in today’s conditions, we retrieve the measured electron escape flux. When comparing the two mechanisms (i.e. excluding solar wind effects, sputtering, etc.), the escape due to the fast ions issuing from the dissociation of dications may account for up to 6% of the total and the escape of thermal ions for the remaining. We show that these two mechanisms cannot explain the escape of the atmosphere since the magnetic field vanished and even contribute only marginally to this loss. We show that with these two mechanisms, the atmosphere of a Mars analog planet would empty in another giga years and a half. At Venus orbit, the contribution of the dications in the escape rate is negligible. When simulating the hot Jupiter HD 209458 b, the two processes cannot explain the measured escape flux of C+. This study shows that the dications may constitute a source of the escape of planetary atmospheres which had not been taken into account until now. This source, although marginal, is not negligible. The influence of the photoionization is of course large, but cannot explain alone the loss of Mars’ atmosphere nor the atmospheric escape of HD 209458 b.
Highlights ► First study of the dissociation of doubly charged ions on Mars atmospheric escape. ► Computation of the exothermal ion escape at Mars, Venus, exoplanets. ► Forecasting of the future of Mars atmosphere.



The dual nature of the martian crust: Young lavas and old clastic materials

- Thu, 2012-12-13 08:24
January 2013
Publication year: 2013
Source:Icarus, Volume 222, Issue 1



Visible and thermal infrared spacecraft datasets are used to gain insight into the nature of the surface materials and upper martian crust, revealing a distinct transition in the physical properties of martian crustal materials that occurred during the Hesperian era. Contrary to a prevailing view of the martian crust as primarily composed of lava flows, we find that most older regions of Mars have morphological and thermophysical properties consistent with poorly consolidated fine-particulate materials that may have a volcaniclastic origin. By contrast, younger surfaces contain blocky materials and thermophysical properties consistent with effusive lava flows. Explosive volcanism is likely to have been dominant on early Mars and these findings have implications for the evolution of the volatile content of the crust and mantle and subsequent development of the surface morphology. This dual nature of the crust appears to be a defining characteristic of martian history.
Highlights ► Older martian regions are composed of poorly consolidated materials. ► A transition in the nature of crustal materials occurred during the Hesperian era. ► Explosive versus effusive volcanism may account for crustal differences. ► The nature of the crustal materials affects subsequent morphological development.



Lunar-forming impacts: High-resolution SPH and AMR-CTH simulations

- Thu, 2012-12-13 08:24
January 2013
Publication year: 2013
Source:Icarus, Volume 222, Issue 1



We present results of the highest-resolution simulations to date of potential Moon-forming impacts using a Lagrangian, particle-based method (smooth particle hydrodynamics, or SPH) and an Eulerian, grid-based method with adaptive mesh refinement (AMR-CTH). We consider a few candidate impacts advocated by recent works, directly comparing simulations performed at varying resolutions and with both numerical methods and their predictions for the properties of resulting protolunar disks. For a fixed set of impact conditions, simulations with either method and with different resolutions yield very similar results for the initial impact and the first few hours of the post-impact period. The subsequent disk properties in the ∼5–20h time period can vary substantially from case-to-case, depending on the orbits of and mutual interactions between large bound clumps of ejecta that often form after the initial impact. After such clumps have completed at least one orbit (which typically requires ∼25–50h), the predicted protolunar disk mass and its angular momentum converge to within about 10% for simulations of very similar impact conditions using different resolutions or methods. The disks produced by the CTH simulations are consistently about 10% less massive than those produced by SPH simulations, due presumably to inherent differences between the codes. The two methods predict broadly similar values for the fraction of the protolunar disk that originates from the target vs. the impactor, and for the initial disk radial surface density and temperature profiles.
Highlights ► We model lunar-forming impacts with high-resolution SPH and AMR-CTH simulations. ► We compare impacts simulated with two hydrodynamical methods and varied resolutions. ► We focus on the predicted properties of the protolunar disk. ► Resulting disk masses and angular momenta are similar to within ∼10%. ► The fraction of the disk originating from the target is also not strongly affected.



Massive identification of asteroids in three-body resonances

- Thu, 2012-12-13 08:24
January 2013
Publication year: 2013
Source:Icarus, Volume 222, Issue 1



An essential role in the asteroidal dynamics is played by the mean motion resonances. Two-body planet–asteroid resonances are widely known, due to the Kirkwood gaps. Besides, so-called three-body mean motion resonances exist, in which an asteroid and two planets participate. Identification of asteroids in three-body (namely, Jupiter–Saturn–asteroid) resonances was initially accomplished by Nesvorný and Morbidelli (Nesvorný D., Morbidelli, A. [1998]. Astron. J. 116, 3029–3037), who, by means of visual analysis of the time behaviour of resonant arguments, found 255 asteroids to reside in such resonances. We develop specialized algorithms and software for massive automatic identification of asteroids in the three-body, as well as two-body, resonances of arbitrary order, by means of automatic analysis of the time behaviour of resonant arguments. In the computation of orbits, all essential perturbations are taken into account. We integrate the asteroidal orbits on the time interval of 100,000yr and identify main-belt asteroids in the three-body Jupiter–Saturn–asteroid resonances up to the 6th order inclusive, and in the two-body Jupiter–asteroid resonances up to the 9th order inclusive, in the set of ∼250,000 objects from the “Asteroids – Dynamic Site” (AstDyS) database. The percentages of resonant objects, including extrapolations for higher-order resonances, are determined. In particular, the observed fraction of pure-resonant asteroids (those exhibiting resonant libration on the whole interval of integration) in the three-body resonances up to the 6th order inclusive is ≈0.9% of the whole set; and, using a higher-order extrapolation, the actual total fraction of pure-resonant asteroids in the three-body resonances of all orders is estimated as ≈1.1% of the whole set.
Highlights ► Algorithms and software for identification of resonant asteroids are developed. ► The set of about 250,000 asteroids from the AstDyS database is considered. ► Asteroids in three-body Jupiter–Saturn–asteroid resonances are identified. ► Objects in two-body resonances with Jupiter are also identified. ► The percentages of objects in pure and transient resonances are determined.



A visible and near-infrared photometric correction for Moon Mineralogy Mapper (M3)

- Thu, 2012-12-13 08:24
January 2013
Publication year: 2013
Source:Icarus, Volume 222, Issue 1



Observations of the Moon obtained by the Moon Mineralogy Mapper (M3) instrument were acquired at various local viewing geometries. To compensate for this, a visible near-infrared photometric correction for the M3 observations of the lunar surface has been derived. Images are corrected to the standard geometry of 30° phase angle with an incidence of 30° and an emission of 0°. The photometric correction is optimized for highland materials but is also a good approximation for mare deposits. The results are compared with ground-based observations of the lunar surface to validate the absolute reflectance of the M3 observations. This photometric model has been used to produce the v1.0 Level 2 delivery of the entire set of M3 data to the Planetary Data System (PDS). The photometric correction uses local topography, in this case derived from an early version of the Lunar Orbiter Laser Altimeter data, to more accurately determine viewing geometry. As desired, this photometric correction removes most of the topography of the M3 measurements. In this paper, two additional improvements of the photometric modeling are discussed: (1) an extrapolated phase function long ward of 2500nm to avoid possible misinterpretation of spectra in the wavelength region that includes possible OH/H2O absorptions and (2) an empirical correction to remove a residual cross-track gradient in the data that likely is an uncorrected instrumental effect. New files for these two effects have been delivered to PDS and can be applied to the M3 observations.
Highlights ► The photometric model of the Moon Mineralogy Mapper data is described in detail. ► The photometric model corrects the data over all the wavelengths range. ► Additional correction is provided for wavelengths after 2500nm. ► A cross-track correction is proposed to improve residuals in the images.



A new approach to determining asteroid masses from planetary range measurements

- Thu, 2012-12-13 08:24
January 2013
Publication year: 2013
Source:Icarus, Volume 222, Issue 1



We describe a new approach to estimate asteroid masses from planetary range measurements. The approach significantly simplifies the process of parameter estimation and allows an effective control of systematic errors introduced by the omission of asteroids from the dynamical model. All asteroid masses are adjusted individually thus avoiding the usual distinction between masses considered individually and masses based on densities within the C, S and M taxonomic classes. Regularization is achieved by accounting, on each mass, for a prior uncertainty determined from available estimations of asteroid diameters and densities. The new approach is used to fit the asteroid model of the JPL planetary ephemeris to Mars range data. The adjusted planetary solutions exhibit similar extrapolation capacity as previous releases of the JPL ephemeris. Up to 27 asteroid masses are determined to better than 35%. The masses agree well with estimates obtained independently by other authors. The determined masses are also robust with respect to cross-validation on a dataset with a shorter time-span and with respect to a different selection of asteroids in the model.
Highlights ► All asteroid masses are considered individually and adjusted using prior constraints. ► We avoid the usual distinction between individual masses and masses based on taxonomy. ► Simplified regularization. ► Quantitative estimation of systematic errors introduced by selection. ► 27 Asteroid masses determined to better than 35%.



Identification of C4H5, C4H4, C3H3 and CH3 radicals produced from the reaction of atomic carbon with propene: Implications for the atmospheres of Titan and giant planets and for the interstellar medium

- Thu, 2012-12-13 08:24
January 2013
Publication year: 2013
Source:Icarus, Volume 222, Issue 1



We observed the products C4H5, C4H4, C3H3 and CH3 of the C(3P)+C3H6 reaction using product time-of-flight spectroscopy and selective photoionization. The identified species arise from the product channels C4H5 +H, C4H4 +2H and C3H3 +CH3. Product isomers were identified via measurements of photoionization spectra and calculations of adiabatic ionization energy. Product C4H5 probably involves three isomers HCCCHCH3, H2CCCCH3 and H2CCCHCH2. In contrast, products C4H4 and C3H3 involve exclusively HCCCHCH2 and H2CCCH, respectively. Reaction mechanisms are unraveled with crossed-beam experiments and quantum-chemical calculations. The 3P carbon atom attacks the π orbital of propene (C3H6) to form a cyclic complex c-H2C(C)CHCH3 that rapidly opens the ring to form H2CCCHCH3 followed by decomposition to HCCCHCH3/H2CCCCH3/H2CCCHCH2 +H and H2CCCH+CH3; the corresponding branching ratios are 7:5:10:78 predicted with RRKM calculations at collision energy 4kcalmol−1. Nascent C4H5 with enough internal energy further decomposes to HCCCHCH2 +H. Ratios of products C4H5, C4H4 and C3H3 are experimentally evaluated to be 17:8:75. This work provides a comprehensive look at product channels of the title reaction and gives implications for the formation of hydrocarbons in extra-terrestrial environments such as Titan and carbon-rich interstellar media. We suggest that the title reaction, hitherto excluded in any chemical networks, needs to be taken into account at least in the atmosphere of Titan and carbon-rich molecular clouds where rapid neutral–neutral reactions are dominant and carbon atoms and propene are abundant.
Highlights ► We observed products C4H5, C4H4, C3H3 and CH3 in the reaction of C(3P)+C3H6. ► An ab initio potential-energy surface of the reaction C(3P)+C3H6 was established. ► Product isomers were identified via measurements of TOF/photoionization spectra. ► Products C4H4 and C3H3 involve exclusively HCCCHCH2 and H2CCCH, respectively. ► Branching ratios of channels C4H5 +H, C4H4 +2H and C3H3 +CH3 were determined.



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

- Thu, 2012-12-13 08:24
January 2013
Publication year: 2013
Source:Icarus, Volume 222, Issue 1



We report on the survivability in hypervelocity impacts of yeast in spore form, and as mature cultures, at impact velocities from 1 to 7.4kms−1, corresponding to an estimated peak shock pressure of ∼43GPa. 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 1kms−1, falling to ∼10−3% at 7.4kms−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 10GPa). 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.
Highlights ► Demonstration of the survival of yeast spores during hypervelocity impact. ► Demonstration of a new capability to fire frozen projectiles at hypervelocities. ► Literature review of the survival of microbes during hypervelocity impacts.



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