A compound Ca‐, Al‐rich inclusion from CV3 chondrite Northwest Africa 3118: Implications for understanding processes during CAI formation

A calcium‐aluminum‐rich inclusion 3N from the Northwest Africa (NWA) 3118 CV3 carbonaceous chondrite is a unique cm‐sized compound object, primarily a forsterite‐bearing type B (FoB) CAI, that encloses at least 26 smaller CAIs of different types, including compact type A (CTA), B, C, and an ultra‐refractory inclusion. Relative to typical type A and B CAIs found elsewhere, the bulk compositions of the types A and B CAIs within 3N more closely match the bulk compositions predicted by equilibrium condensation of a gas of solar composition. Being trapped within the FoB melt may have protected them from melt evaporation that affected most “stand‐alone” CAIs. 3N originated either as an aggregate of many smaller (mostly types A, B, C) CAIs plus accreted Fo‐bearing material (like an amoeboid olivine aggregate) which experienced partial melting of the whole, or else as a FoB melt droplet that collided with and trapped many smaller solid CAIs. In the former case, 3N recorded the earliest accretion of pebble‐sized bodies known. In the latter case, the presence of a large number of individual refractory inclusions within 3N suggests a very high local density of refractory solids in the immediate region of the host CAI during the brief time while it was melted. Collisions would have occurred on time scales of hours at most, assuming a melt solidification interval for the host CAI of 300–400 °C (maximum) and a cooling rate of ~10 °C/h.     

Cassini RADAR observations of Enceladus, Tethys, Dione, Rhea, Iapetus, Hyperion, and Phoebe

Cassini 2.2-cm radar and radiometric observations of seven of Saturn's icy satellites yield properties that apparently are dominated by subsurface volume scattering and are similar to those of the icy Galilean satellites. Average radar albedos decrease in the order Enceladus/Tethys, Hyperion, Rhea, Dione, Iapetus, and Phoebe. This sequence most likely corresponds to increasing contamination of near-surface water ice, which is intrinsically very transparent at radio wavelengths. Plausible candidates for contaminants include ammonia, silicates, metallic oxides, and polar organics (ranging from nitriles like HCN to complex tholins). There is correlation of our targets' radar and optical albedos, probably due to variations in the concentration of optically dark contaminants in near-surface water ice and the resulting variable attenuation of the high-order multiple scattering responsible for high radar albedos. Our highest radar albedos, for Enceladus and Tethys, probably require that at least the uppermost one to several decimeters of the surface be extremely clean water ice regolith that is structurally complex (i.e., mature) enough for there to be high-order multiple scattering within it. At the other extreme, Phoebe has an asteroidal radar reflectivity that may be due to a combination of single and volume scattering. Iapetus' 2.2-cm radar albedo is dramatically higher on the optically bright trailing side than the optically dark leading side, whereas 13-cm results reported by Black et al. [Black, G.J., Campbell, D.B., Carter, L.M., Ostro, S.J., 2004. Science 304, 553] show hardly any hemispheric asymmetry and give a mean radar reflectivity several times lower than the reflectivity measured at 2.2 cm. These Iapetus results are understandable if ammonia is much less abundant on both sides within the upper one to several decimeters than at greater depths, and if the leading side's optically dark contaminant is present to depths of at least one to several decimeters. As argued by Lanzerotti et al. [Lanzerotti, L.J., Brown, W.L., Marcantonio, K.J., Johnson, R.E., 1984. Nature 312, 139-140], a combination of ion erosion and micrometeoroid gardening may have depleted ammonia from the surfaces of Saturn's icy satellites. Given the hypersensitivity of water ice's absorption length to ammonia concentration, an increase in ammonia with depth could allow efficient 2.2-cm scattering from within the top one to several decimeters while attenuating 13-cm echoes, which would require a six-fold thicker scattering layer. If so, we would expect each of the icy satellites' average radar albedos to be higher at 2.2 cm than at 13 cm, as is the case so far with Rhea [Black, G., Campbell, D., 2004. Bull. Am. Astron. Soc. 36, 1123] as well as Iapetus.     

Simulation of Io’s auroral emission: Constraints on the atmosphere in eclipse

We study the morphology of Io’s aurora by comparing simulation results of a three-dimensional (3D) two-fluid plasma model to observations by the high-resolution Long-Range Reconnaissance Imager (LORRI) on-board the New Horizons spacecraft and by the Hubble Space Telescope Advanced Camera for Surveys (HST/ACS). In 2007, Io’s auroral emission in eclipse has been observed simultaneously by LORRI and ACS and the observations revealed detailed features of the aurora, such as a huge glowing plume at the Tvashtar paterae close to the North pole. The auroral radiation is generated in Io’s atmosphere by collisions between impinging magnetospheric electrons and various neutral gas components. We calculate the interaction of the magnetospheric plasma with Io’s atmosphere-ionosphere and simulate the auroral emission. Our aurora model takes into account not only the direct influence of the atmospheric distribution on the morphology and intensity of the emission, but also the indirect influence of the atmosphere on the plasma environment and thus on the exciting electrons. We find that the observed morphology in eclipse can be explained by a smooth (non-patchy) equatorial atmosphere with a vertical column density that corresponds to ∼10% of the column density of the sunlit atmosphere. The atmosphere is asymmetric with two times higher density and extension on the downstream hemisphere. The auroral emission from the Tvashtar volcano enables us to constrain the plume gas content for the first time. According to our model, the observed intensity of the Tvashtar plume implies a mean column density of ∼5 × 10¹⁵ cm⁻² for the plume region.     

A Barometric Survey of Dust-Devil Vortices on a Desert Playa

Dust devils, and other columnar vortices, are associated with local surface pressure drops that can be observed in time-series data on both Earth and Mars. High cadence measurements are needed to resolve these small structures, and we report a month-long survey (June/July 2012) on a Nevada desert playa using microbarographs sampled multiple times per second. Candidate dust-devil signatures are classified, with detections being robust at about one per day for pressure drops exceeding 0.3 hPa (roughly a 5:1 signal-to-noise threshold, where the observed noise level corresponds reasonably well with the dynamic pressure associated with the estimate convective velocity scale). The vortex population is evaluated and compared with those observed on Mars: a broken power law or a more convex distribution describes the terrestrial data. A single station observes about three events per week (for normalized pressure drops of 0.06 %), about three times fewer than Mars observations for the same normalized drop. We find evidence for clustering of vortex events in a pseudo-periodic manner with a 20-min period, consistent with the size of boundary-layer convection cells.     

A comparison of the BALTIMOS coupled climate model with atmospheric and sea surface parameters derived from AMSR-E

The BALTEX Integrated Model System (BALTIMOS) coupled atmosphere ocean model was compared to passive microwave observations of the Advanced Microwave Scanning Radiometer (AMSR-E). Emphasis was put on quantifying the uncertainties associated with the different variables based on data screening both in the model and observations. Monthly means of three atmospheric parameters, as well as sea surface temperature, were compared for a period of 1 year. Sea ice extent was also derived from AMSR-E and compared to the model data on a daily basis. It is shown that the accuracy of the comparisons on a monthly mean basis is limited by precipitation screening. Out of the three atmospheric parameters, surface wind speed and water vapor column amount agree with the model data to within the accuracy of the comparison. The vertically integrated cloud liquid water content diagnosed from BALTIMOS is systematically higher than the liquid water content derived from satellite, even if potential systematic errors are accounted for. In terms of coupling, the two most relevant variables discussed are sea surface temperature and sea ice extent. The temporal extent of sea ice in the investigation area is well represented, as are the periods of the main growing and decay periods. The total sea ice cover appears to be underestimated by BALTIMOS, especially in the peak season between January and the beginning of March. The amplitude of the annual cycle of sea surface temperature in BALTIMOS appears to be too weak compared to the observations, leading to too cold sea surface temperatures in summer and too warm sea surface temperatures in winter. This might also partially explain the underestimation of sea ice cover by BALTIMOS.     

A comparison of solar radiative flux above clouds from MODIS with BALTIMOS regional climate model simulations

A comparison study for the solar radiative flux above clouds is presented between the regional climate model system BALTEX integrated model system (BALTIMOS) and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite observations. For MODIS, an algorithm has been developed to retrieve reflected shortwave fluxes over clouds. The study area is the Baltic Sea catchment area during an 11-month period from February to December 2002. The intercomparison focuses on the variations of the daily and seasonal cycle and the spatial distributions. We found good agreement between the observed and the simulated data with a bias of the temporal mean of 13.6 W/m² and a bias of the spatial mean of 35.5 W/m². For summer months, BALTIMOS overestimates the solar flux with up to 90 W/m² (20%). This might be explained by the insufficient representation of cirrus clouds in the regional climate model.     

Recently induced anoxia leading to the preservation of seasonal laminae in two NE-German lakes

The recent sediments of two lakes in the NE German lowland became seasonally laminated at different times. Anoxic bottom conditions resulted from a surplus of organic matter (OM), in the early stage indicated by irregularly laminated sediments comprising abundant iron-sulfide framboids. Their diagenetic formation predates the preservation of biochemical calcite varves. In the larger, deeper Lake Tiefer See near Klocksin, anoxia developed stepwise. A first anoxic pulse was contemporary with inflow narrowing by railway-dam construction and accumulation of OM. It was favored by a decrease of the intensity of lake circulation (turnover). Nutrients introduced from artificial fertilizer then increased the primary production (diatoms) to the point of OM surplus and seasonal laminae formation started 40 years later in 1924. In the smaller, shallower Lake Tiefer See in the Uckermark, a massive pulse of iron sulfide was centered around 1960, seven years after installation of piped field drainage into the lake. Anoxia developed rapidly with the nutrients drained from a fertilized groundwater catchment that is 10 times larger than the surface catchment, while surface erosion was reduced. Reducing bottom conditions became regular and the seasonal lamination was preserved after 1967. Morphological criteria to screen lakes for varved sediments should include reductions of natural lake inflow and catchment increase, such as by inflow of field drainage. Similar developments of increased nutrient input or intensity decrease of lake circulation may result from historical human activities but also from natural processes.     

The growth of wind-waves in Titan’s hydrocarbon seas

Ocean wave growth on Titan is considered. The classic Sverdrup–Munk theory for terrestrial wave growth is applied to Titan, and is compared with a simple energy balance model that exposes the effect of Titan’s environmental parameters (air density, gravity, and fluid density). These approaches are compared with the only previously-published (semi-empirical) model (Ghafoor, N.A.-L., Zarnecki, J.C., Challenor, P., Srokosz, M.A. [2000] J. Geophys. Res. 105, 12,077–12,091, hereafter G2k), and allow the impact of various parameters such as atmospheric density to be transparently explored.Our model, like G2k, suggests fully-developed significant wave heights on Titan H_s = 0.2 U², where U is the windspeed (SI units): in dimensionless terms this is rather close to H_s = 0.2 U²/g, a rule of thumb previously noted for terrestrial waves (we find various datasets where the prefactor varies by ～2). It is noted that liquid and air densities affect the growth rate of waves, but not their fully-developed height: for 1 m/s winds wave amplitude reaches 0.15 m (75% of fully-developed) with a fetch of only 1 km, rather faster than predicted by G2k. Liquid viscosity has no major effect on gravity wave growth, but does influence the threshold windspeed at which gravity–capillary waves form in the first place.The model is used to develop predicted ranges for wave height to guide the design of the Titan Mare Explorer (TiME), a proposed Discovery-class mission to float a capsule on Ligeia Mare in 2023. For the expected maximum 1 m/s winds, a significant wave height of 0.2 m and wavelength of ～4 m can be expected. Assuming that wave heights follow Rayleigh statistics as they do on Earth, then given the wave period of ～4 s, individual waves of ～0.6 m might be encountered over a 3 month period.For predicted Titan winds at Kraken Mare, significant wave heights may reach ～0.6 m in the peak of summer but do not exceed the tidal amplitude at its northern end, consistent with the area around Mayda Insula being a tidal flat, while elsewhere on Kraken and Ligeia and at Ontario Lacus, shorelines may be wave- or tidally-dominated, depending on the specific location.