Description: Publication date: 1 March 2014 Source: Icarus, Volume 231 Author(s): Leigh N. Fletcher , Imke de Pater , Glenn S. Orton , Heidi B. Hammel , Michael L. Sitko , Patrick G.J. Irwin Imaging and spectroscopy of Neptune’s thermal infrared emission from Keck/LWS (2003), Gemini-N/MICHELLE (2005); VLT/VISIR (2006) and Gemini-S/TReCS (2007) is used to assess seasonal changes in Neptune’s zonal mean temperatures between Voyager-2 observations (1989, heliocentric longitude L s = 236 ° ) and southern summer solstice (2005, L s = 270 ° ). Our aim was to analyse imaging and spectroscopy from multiple different sources using a single self-consistent radiative-transfer model to assess the magnitude of seasonal variability. Globally-averaged stratospheric temperatures measured from methane emission tend towards a quasi-isothermal structure (158–164 K) above the 0.1-mbar level, and are found to be consistent with spacecraft observations of AKARI. This remarkable consistency, despite very different observing conditions, suggests that stratospheric temporal variability, if present, is 〈±5 K at 1 mbar and 〈±3 K at 0.1 mbar during this solstice period. Conversely, ethane emission is highly variable, with abundance determinations varying by more than a factor of two (from 500 to 1200 ppb at 1 mbar). The retrieved C 2 H 6 abundances are extremely sensitive to the details of the T ( p ) derivation, although the underlying cause of the variable ethane emission remains unidentified. Stratospheric temperatures and ethane are found to be latitudinally uniform away from the south pole (assuming a latitudinally-uniform distribution of stratospheric methane), with no large seasonal hemispheric asymmetries evident at solstice. At low and mid-latitudes, comparisons of synthetic Voyager-era images with solstice-era observations suggest that tropospheric zonal temperatures are unchanged since the Voyager 2 encounter, with cool mid-latitudes and a warm equator and pole. A re-analysis of Voyager/IRIS 25–50 μm mapping of tropospheric temperatures and para-hydrogen disequilibrium (a tracer for vertical motions) suggests a symmetric meridional circulation with cold air rising at mid-latitudes (sub-equilibrium para-H 2 conditions) and warm air sinking at the equator and poles (super-equilibrium para-H 2 conditions). The most significant atmospheric changes have occurred at high southern latitudes, where zonal temperatures retrieved from 2003 images suggest a polar enhancement of 7–8 K above the tropopause, and an increase of 5–6 K throughout the 70–90°S region between 0.1 and 200 mbar. Such a large perturbation, if present in 1989, would have been detectable by Voyager/IRIS in a single scan despite its long-wavelength sensitivity, and we conclude that Neptune’s south polar cyclonic vortex increased in strength significantly from Voyager to solstice.

Description: Publication date: 1 March 2014 Source: Icarus, Volume 231 Author(s): Justin Filiberto Igneous diversity is common on terrestrial planets and has been experimentally investigated for the Earth and Mars, but only suggested for Venus. Since Venus and Earth are sister planets and have similar bulk chemistry, experiments on terrestrial basalts can place constraints on the formation of the Venera and Vega basalts. Furthermore, experimental results can suggest the types of magmas that should be present on Venus if processes of differentiation similar to those taking place within the Earth are occurring on Venus, as suggested by the Venera and Vega analyses. The interpretation of the Venera 13 analysis as an alkali basalt suggests deep partial melting of a carbonated source region; while the identification of Venera 14 and Vega 2 as tholeiites suggest relatively shallow melting of a hydrous lherzolitic or peridotite source region. The residual liquids produced by differentiation of a Venus tholeiite, based on experiments on analog compositions, range from rhyolites to phonolites, depending on pressure of crystallization and bulk water content. Results from these crystallization experiments on tholeiitic terrestrial compositions can constrain the type and quality of data needed from future missions to determine the petrologic history of surface igneous rocks.

Description: Publication date: 1 March 2014 Source: Icarus, Volume 231 Author(s): Sarah V. Badman , Caitriona M. Jackman , Jonathan D. Nichols , John T. Clarke , Jean-Claude Gérard We characterise the interaction between the solar wind and Saturn’s magnetosphere by evaluating the amount of ‘open’ magnetic flux connected to the solar wind. This is deduced from a large set of Hubble Space Telescope images of the ultraviolet aurora, using the poleward boundary of the main aurora as a proxy for the open-closed field line boundary in the ionosphere. The amount of open flux is found to be 10–50 GWb, with a mean of 35 GWb. The typical change in open flux between consecutive observations separated by 10–60 h is - 5 or +7 GWb. These changes are a result of imbalance between open flux creation at the dayside magnetopause and its closure in the magnetotail. The 5 GWb typical decrease in open flux is consistent with in situ measurements of the flux transported following a reconnection event. Estimates of average, net reconnection rates are found to be typically a few tens of kV, with some extreme examples of unbalanced magnetopause or tail reconnection occurring at ∼300 kV. The range of values determined suggest that Saturn’s magnetosphere does not generally achieve a steady state between flux opening at the magnetopause and flux closure in the magnetotail. The percentage of magnetic flux which is open in Saturn’s magnetosphere is similar to that measured at the Earth (2–11%), but the typical percentage that is closed between observations is significantly lower (13% compared to 40–70%). Therefore, open flux is usually closed in smaller (few GWb) events in Saturn’s magnetosphere. The exception to this behaviour is large, rapid flux closure events which are associated with solar wind compressions. While the rates of flux opening and closure should be equal over long timescales, they are evidently different on shorter (up to tens of hours) timescales. The relative independence of the magnetopause and tail reconnection rates can be attributed to the long loading timescales required to transport open field lines into the tail.

Description: Publication date: 1 March 2014 Source: Icarus, Volume 231 Author(s): M. Fouchard , H. Rickman , Ch. Froeschlé , G.B. Valsecchi We present Monte Carlo simulations of the dynamical history of the Oort cloud, where in addition to the main external perturbers (Galactic tides and stellar encounters) we include, as done in a companion paper (Fouchard, M., Rickman, H., Froeschlé, Ch., Valsecchi, G.B. [2013b]. Icarus, in press), the planetary perturbations experienced each time the comets penetrate to within 50 AU of the Sun. Each simulation involves an initial sample of four million comets and extends over a maximum of 5 Gyr. For better understanding of the outcomes, we supplement the full dynamical model by others, where one or more of the effects are left out. We concentrate on the production of observable comets, reaching for the first time a perihelion within 5 AU of the Sun. We distinguish between four categories, depending on whether the comet jumps across, or creeps through, the Jupiter–Saturn barrier (perihelion distances between 5 and 15 AU), and whether the orbit leading to the observable perihelion is preceded by a major planetary perturbation or not. For reasons explained in the paper, we call the strongly perturbed comets “Kaib–Quinn comets”. We thus derive a synthetic picture of the Oort spike, from which we draw two main conclusions regarding the full dynamical model. One is that 2/3 of the observable comets are injected with the aid of a planetary perturbation at the previous perihelion passage, and about half of the observable comets are of the Kaib–Quinn type. The other is that the creepers dominate over the jumpers. Due to this fact, the spike peaks at only 31 000 AU, and the majority of new comets have semi-major axes less than this value. The creepers show a clear preference for retrograde orbits as a consequence of the need to avoid untimely, planetary ejection before becoming observable. Thus, the new comets should have a 60/40 preference for retrograde against prograde orbits in apparent conflict with observations. However, both these and other results depend on our model assumptions regarding the initial structure of the Oort cloud, which is isotropic in shape and has a relatively steep energy distribution. We also find that they depend on the details of the past history of external perturbations including GMC encounters, and we provide special discussions of those issues.

Description: Publication date: 1 March 2014 Source: Icarus, Volume 231 Author(s): Paul O. Hayne , David A. Paige , Nicholas G. Heavens Wintertime observations of the martian polar regions by orbiting spacecraft have provided evidence for carbon dioxide clouds, which measurably alter the polar energy budget and the annual CO 2 cycle. However, it has remained unclear whether snowfall contributes a substantial quantity to the accumulating seasonal ice caps. We develop models to constrain precipitation rates based on observations of south polar CO 2 clouds by the Mars Climate Sounder (MCS), and show that snowfall contributes between 3% and 20% by mass to the seasonal deposits at latitudes 70–90°S. The lower bound on this estimate depends on a minimum effective cloud particle size of ∼50 μm, derived by comparing the short lifetimes (less than a few hours) of some clouds with calculated sedimentation velocities. Separate constraints from infrared spectra measured by MCS suggest CO 2 cloud particles in the size range 10–100 μm. Snow particles are not likely to re-sublime before reaching the surface, because the lower atmosphere in this region remains near saturation with respect to CO 2 . Based on cooling rate calculations, snowfall originating below 4 km altitude likely contributes a comparable or greater amount to the seasonal deposits than the rest of the atmosphere. Due to the positive feedback between cloud particle number density and radiative cooling, CO 2 snow clouds should propagate until they become limited by the availability of condensation nuclei or CO 2 gas. Over the south polar residual cap, where cloud activity is greatest, atmospheric radiative cooling rates are high enough to offset heat advected into the polar regions and maintain consistent snowfall. At latitudes of 60–80°S the lower atmosphere tends to be slightly sub-saturated and rapid cooling by mechanical lift driven by orography or convergent flow may be required to initiate a snowstorm, consistent with the more sporadic clouds observed by MCS in this region, and their correlation with topographic features. Snowfall and accumulation at the surface are found to be inevitable consequences of the polar energy budget, unless advection redistributes heat from lower latitudes in much greater quantities than expected.

Description: Publication date: 1 March 2014 Source: Icarus, Volume 231 Author(s): Peter Gao , Xi Zhang , David Crisp , Charles G. Bardeen , Yuk L. Yung Observations by the SPICAV/SOIR instruments aboard Venus Express have revealed that the upper haze (UH) of Venus, between 70 and 90 km, is variable on the order of days and that it is populated by two particle modes. We use a one-dimensional microphysics and vertical transport model based on the Community Aerosol and Radiation Model for Atmospheres to evaluate whether interaction of upwelled cloud particles and sulfuric acid particles nucleated in situ on meteoric dust are able to generate the two observed modes, and whether their observed variability are due in part to the action of vertical transient winds at the cloud tops. Nucleation of photochemically produced sulfuric acid onto polysulfur condensation nuclei generates mode 1 cloud droplets, which then diffuse upwards into the UH. Droplets generated in the UH from nucleation of sulfuric acid onto meteoric dust coagulate with the upwelled cloud particles and therefore cannot reproduce the observed bimodal size distribution. By comparison, the mass transport enabled by transient winds at the cloud tops, possibly caused by sustained subsolar cloud top convection, are able to generate a bimodal size distribution in a time scale consistent with Venus Express observations. Below the altitude where the cloud particles are generated, sedimentation and vigorous convection causes the formation of large mode 2 and mode 3 particles in the middle and lower clouds. Evaporation of the particles below the clouds causes a local sulfuric acid vapor maximum that results in upwelling of sulfuric acid back into the clouds. In the case where the polysulfur condensation nuclei are small and their production rate is high, coagulation of small droplets onto larger droplets in the middle cloud may set up an oscillation in the size modes of the particles such that precipitation of sulfuric acid “rain” may be possible immediately below the clouds once every few Earth months. Reduction of the polysulfur condensation nuclei production rate destroys this oscillation and reduces the mode 1 particle abundance in the middle cloud by two orders of magnitude. However, it better reproduces the sulfur-to-sulfuric-acid mass ratio in the cloud and haze droplets as constrained by fits to UV reflectivity data. In general we find satisfactory agreement between our nominal and transient wind results and observations from Pioneer Venus, Venus Express, and Magellan, though improvements could be made by incorporating sulfur microphysics.

Description: Publication date: 1 March 2014 Source: Icarus, Volume 231 Author(s): M. Fouchard , H. Rickman , Ch. Froeschlé , G.B. Valsecchi We present Monte Carlo simulations of the dynamical history of the Oort cloud, where in addition to the main external perturbers (Galactic tides and stellar encounters) we include, as done in a companion paper (Fouchard, M., Rickman, H., Froeschlé, Ch., Valsecchi, G.B. [2013b]. Icarus, in press), the planetary perturbations experienced each time the comets penetrate to within 50 AU of the Sun. Each simulation involves an initial sample of four million comets and extends over a maximum of 5 Gyr. For better understanding of the outcomes, we supplement the full dynamical model by others, where one or more of the effects are left out. In the companion paper we studied in detail how observable comets are injected from the Oort cloud, when account is taken of the planetary perturbations. In the present paper we concentrate on how the cloud may evolve in the long term and also on the production of decoupled comets, which evolve into semi-major axes less than 1000 AU. Concerning the long-term evolution, we find that the largest stellar perturbations that may statistically be expected during the age of the Solar System induce a large scale migration of comets within the cloud. Thus, comets leave the inner parts, but the losses from the outer parts are even larger, so at the end of our simulations the Oort cloud is more centrally condensed than at the beginning. The decoupled comets, which form a source of centaurs and Halley type comets (roughly in the proportions of 70% and 30%, respectively), are mainly produced by planetary perturbations, Jupiter and Saturn being the most efficient. This effect is dependent on synergies with the Galactic tide and stellar encounters, bringing the perihelia of Oort cloud comets into the planetary region. The star-planet synergy has a large contribution due to the strong encounters that produce major comet showers. However, outside these showers a large majority of decouplings may be attributed to the tide-planet synergy.

Description: Publication date: 1 March 2014 Source: Icarus, Volume 231 Author(s): Megan L. Smith , Mark W. Claire , David C. Catling , Kevin J. Zahnle In extremely arid regions on Earth, such as the Atacama Desert, nitrate, sulfate and perchlorate salts form in the atmosphere and accumulate on the surface from dry deposition according to diagnostic evidence in their oxygen isotopes. Salts of similar oxyanions should have formed in the atmosphere of Mars because of comparable photochemical reactions. We use a 1-D photochemical model to calculate the deposition rates of sulfate, nitrogen oxyanions, and perchlorate from Mars’ atmosphere, given a plausible range of volcanic fluxes of sulfur- and chlorine-containing gases in the past. To calculate integrated fluxes over time, we assume that throughout the last 3 byr (the Amazonian eon), the typical background atmosphere would have been similar to today’s cold and dry environment. If the soil has been mixed by impact perturbations to a characteristic depth of ∼2 m during this time, given a time-average volcanic flux 0.1% of the modern terrestrial volcanic flux, the model suggests that the soil would have accumulated 1.0–1.7 wt.% SO 4 2 - and 0.2–0.4 wt.% N in the form of pernitrate (peroxynitrate) or nitrate. The calculated sulfate concentration is consistent with in situ observations of soils from rovers and landers and orbital gamma ray spectroscopy. However, nitrates or pernitrates are yet to be detected. The modeled formation of perchlorate via purely gas-phase oxidation of volcanically-derived chlorine is insufficient by orders of magnitude to explain 0.4–0.6 wt.% ClO 4 - measured by NASA’s Phoenix Lander. The far smaller amount of ozone in the martian atmosphere compared to the terrestrial atmosphere and the colder, drier conditions are the cause of lower rates of gas phase oxidation of chlorine volatiles to perchloric acid. Our calculations imply that non-gas-phase processes not included in the photochemical model, such as heterogeneous reactions, are likely important for the formation of perchlorate and are yet to be identified.

Description: Publication date: 1 March 2014 Source: Icarus, Volume 231 Author(s): Yoshihiro Furukawa , Taro Samejima , Hiromoto Nakazawa , Takeshi Kakegawa Late heavy bombardment (LHB) of extraterrestrial objects supplied carbon with metals to the prebiotic Earth. The early oceans were the major target of these impacts, followed by interactions among the atmosphere, oceanic water, and meteorite constituent materials under high-temperature and high-pressure conditions. Post-impact reactions of these hypervelocity impacts have the potential to produce reduced volatiles and organic compounds, including amino acids. Therefore, understanding the reactions in post-impact plumes is of great importance for the investigation of prebiotic organic compounds. The composition of post-impact plumes has been investigated with thermochemical calculations. However, experimental evidence is still needed to understand the reactions in dynamic systems of post-impact plumes. The present study investigates the effects of reaction temperature and availability of water on products from iron, nickel, graphite, nitrogen, and water in a dynamic gas flow system to investigate reactions in post-impact plumes. Results of this study indicate the formation of CO, H 2 , NH 3 , and HCN by hypervelocity oceanic impacts of iron-rich extraterrestrial objects. The formation of methane was limited in the present experiments, suggesting that the quenching rate is an influential factor for methane formation in post-impact plumes. Availability of water vapor in the plume was also an influential factor for the formation of reduced volatiles that controlled the CO formation rate from graphite. These results provide experimental evidence for the formation of reduced volatiles in post-impact plumes, which influenced the formation of prebiotic organic compounds.

Description: Publication date: February 2014 Source: Icarus, Volume 229 Author(s): Suniti Karunatillake , Scott M. McLennan , Kenneth E. Herkenhoff , Jonathan M. Husch , Craig Hardgrove , J.R. Skok In a companion work, we bridge the gap between mature segmentation software used in terrestrial sedimentology and emergent planetary segmentation with an original algorithm optimized to segment whole images from the Microscopic Imager (MI) of the Mars Exploration Rovers (MER). In this work, we compare its semi-automated outcome with manual photoanalyses using unconsolidated sediment at Gusev and Meridiani Planum sites for geologic context. On average, our code and manual segmentation converge to within ∼10% in the number and total area of identified grains in a pseudo-random, single blind comparison of 50 samples. Unlike manual segmentation, it also locates finer grains in an image with internal consistency, enabling robust comparisons across geologic contexts. When implemented in Mathematica-8, the algorithm segments an entire MI image within minutes, surpassing the extent and speed possible with manual segmentation by about a factor of ten. These results indicate that our algorithm enables not only new sedimentological insight from the MER MI data, but also detailed sedimentology with the Mars Science Laboratory’s Mars Hand Lens Instrument.