The geology of Hotei Regio, Titan: Correlation of Cassini VIMS and RADAR

Joint Cassini VIMS and RADAR SAR data of ∼700-km-wide Hotei Regio reveal a rich collection of geological features that correlate between the two sets of images. The degree of correlation is greater than anywhere else seen on Titan. Central to Hotei Regio is a basin filled with cryovolcanic flows that are anomalously bright in VIMS data (in particular at 5 μm) and quite variable in roughness in SAR. The edges of the flows are dark in SAR data and appear to overrun a VIMS-bright substrate. SAR-stereo topography shows the flows to be viscous, 100-200 m thick. On its southern edge the basin is ringed by higher (∼1 km) mountainous terrain. The mountains show mixed texture in SAR data: some regions are extremely rough, exhibit low and spectrally neutral albedo in VIMS data and may be partly coated with darker hydrocarbons. Around the southern margin of Hotei Regio, the SAR image shows several large, dendritic, radar-bright channels that flow down from the mountainous terrain and terminate in dark blue patches, seen in VIMS images, whose infrared color is consistent with enrichment in water ice. The patches are in depressions that we interpret to be filled with fluvial deposits eroded and transported by liquid methane in the channels. In the VIMS images the dark blue patches are encased in a latticework of lighter bands that we suggest to demark a set of circumferential and radial fault systems bounding structural depressions. Conceivably the circular features are tectonic structures that are remnant from an ancient impact structure. We suggest that impact-generated structures may have simply served as zones of weakness; no direct causal connection, such as impact-induced volcanism, is implied. We also speculate that two large dark features lying on the northern margin of Hotei Regio could be calderas. In summary the preservation of such a broad suite of VIMS infrared color variations and the detailed correlation with features in the SAR image and SAR topography evidence a complex set of geological processes (pluvial, fluvial, tectonic, cryovolcanic, impact) that have likely remained active up to very recent geological time (<10⁴ year). That the cryovolcanic flows are excessively bright in the infrared, particularly at 5 μm, might signal ongoing geological activity. One study [Nelson, R.M., and 28 colleagues, 2009. Icarus 199, 429-441] reported significant 2-μm albedo changes in VIMS data for Hotei Arcus acquired between 2004 and 2006, that were interpreted as evidence for such activity. However in our review of that work, we do not agree that such evidence has yet been found.     

The time scales of magma mixing and mingling involving primitive melts and melt–mush interaction at mid-ocean ridges

We have studied the chemical zoning of plagioclase phenocrysts from the slow-spreading Mid-Atlantic Ridge and the intermediate-spreading rate Costa Rica Rift to obtain the time scales of magmatic processes beneath these ridges. The anorthite content, Mg, and Sr in plagioclase phenocrysts from the Mid-Atlantic Ridge can be interpreted as recording initial crystallisation from a primitive magma (~11 wt% MgO) in an open system. This was followed by crystal accumulation in a mush zone and later entrainment of crystals into the erupted magma. The initial magma crystallised plagioclase more anorthitic than those in equilibrium with any erupted basalt. Evidence that the crystals accumulated in a mush zone comes from both: (1) plagioclase rims that were in equilibrium with a Sr-poor melt requiring extreme differentiation; and (2) different crystals found in the same thin section having different histories. Diffusion modelling shows that crystal residence times in the mush were <140 years, whereas the interval between mush disaggregation and eruption was ≤1.5 years. Zoning of anorthite content and Mg in plagioclase phenocrysts from the Costa Rica Rift show that they partially or completely equilibrated with a MgO-rich melt (>11 wt%). Partial equilibration in some crystals can be modelled as starting <1 year prior to eruption but for others longer times are required for complete equilibration. This variety of times is most readily explained if the mixing occurred in a mush zone. None of the plagioclase phenocrysts from the Costa Rica Rift that we studied have Mg contents in equilibrium with their host basalt even at their rims, requiring mixing into a much more evolved magma within days of eruption. In combination these observations suggest that at both intermediate- and slow-spreading ridges: (1) the chemical environment to which crystals are exposed changes on annual to decadal time scales; (2) plagioclase crystals record the existence of melts unlike those erupted; and (3) disaggregation of crystal mush zones appears to precede eruption, providing an efficient mechanism by which evolved interstitial melt can be mixed into erupted basalts.     

A comprehensive numerical simulation of Io’s sublimation-driven atmosphere

Io’s sublimation-driven atmosphere is modeled using the direct simulation Monte Carlo (DSMC) method. These rarefied gas dynamics simulations improve upon earlier models by using a three-dimensional domain encompassing the entire planet computed in parallel. The effects of plasma heating, planetary rotation, inhomogeneous surface frost, molecular residence time of SO₂ on the exposed (non-volatile) rocky surface, and surface temperature distribution are investigated. Circumplanetary flow is predicted to develop from the warm dayside toward the cooler nightside. Io’s rotation leads to a highly asymmetric frost surface temperature distribution (due to the frost’s high thermal inertia) which results in circumplanetary flow that is not axi-symmetric about the subsolar point. The non-equilibrium thermal structure of the atmosphere, specifically vibrational and rotational temperatures, is also examined. Plasma heating is found to significantly inflate the atmosphere on both the dayside and nightside. The plasma energy flux causes high temperatures at high altitudes but plasma energy depletion through the dense gas column above the warmest frost permits gas temperatures cooler than the surface at low altitudes. A frost map (Douté, S., Schmitt, B., Lopes-Gautier, R., Carlson, R., Soderblom, L., Shirley, J., and the Galileo NIMS Team [2001]. Icarus 149, 107-132) is used to control the sublimated flux of SO₂ which can result in inhomogeneous column densities that vary by nearly a factor of four for the same surface temperature. A short residence time for SO₂ molecules on the “rock” component is found to smooth lateral atmospheric inhomogeneities caused by variations in the surface frost distribution, creating an atmosphere that looks nearly identical to one with uniform frost coverage. A longer residence time is found to agree better with mid-infrared observations (Spencer, J.R., Lellouch, E., Richter, M.J., López-Valverde, M.A., Jessup, K.L, Greathouse, T.K., Flaud, J. [2005]. Icarus 176, 283-304) and reproduce the observed anti-jovian/sub-jovian column density asymmetry. The computed peak dayside column density for Io assuming a surface frost temperature of 115 K agrees with those suggested by Lyman-α observations (Feaga, L.M., McGrath, M., Feldman, P.D. [2009]. Icarus 201, 570-584). On the other hand, the peak dayside column density at 120 K is a factor of five larger and is higher than the upper range of observations (Jessup, K.L., Spencer, J.R., Ballester, G.E., Howell, R.R., Roesler, F., Vigel, M., Yelle, R. [2004]. Icarus 169, 197-215; Spencer et al., 2005).     

The paleolimnology of a small waterbody in the Pocono Mountains of Pennsylvania,USA: reconstructing 19th–20th century specific conductivity trends in relation to changing land use

Remains of scaled chrysophytes, magnetic minerals and pollen were used to analyze the recent paleolimnological history of a small lake, Lake Waynewood, in the Pocono Mountains of Pennsylvania. Important shifts in all three variables were observed over the 133 years represented by the core. The most significant changes occurred near the turn of the century when the watershed was heavily logged. Before the logging event, species such asSynura sphagnicola, S. spinosa, Mallomonas galeiformis andM. duerrschmidtiae were co-dominant members of the flora. Subsequent to the deforestation of the watershed other taxa, includingM. crassisquama, M. caudata andS. petersenii, increased in relative importance. Concurrent with changes in the scaled chrysophytes was a six-fold increase in the concentrations of magnetic material, presumably the result of increased erosion caused by the logging. Changes in pollen grains also correlate well with the onset of the deforestation event. The scaled chrysophyte inferred specific conductivity of the lake has more than tripled, with the primary increase occurring concurrent with the commencement of logging and the increase in magnetic mineral material. The effects of other human-related disturbances are also discussed.     

Fluxes of metals to a manganese nodule radiochemical,chemical, structural,and mineralogical studies

Fluxes of metals to the top and bottom surfaces of a manganese nodule were determined by combining radiochemical (²³⁰Th,²³¹Pa,²³²Th,²³⁸U,²³⁴U) and detailed chemical data. The top of the nodule had been growing in its collected orientation at 4.7 mm Myr^?1 for at least 0.5 Myr and accreting Mn at 200 μg cm^?2 kyr^?1. The bottom of the nodule had been growing in its collected orientation at about 12 mm Myr^?1 for at least 0.3 Myr and accreting Mn at about 700 μg cm^?2 yr^?1. Although the top of the nodule was enriched in iron relative to the bottom, the nodule had been accreting Fe 50% faster on the bottom.²³²Th was also accumulating more rapidly in the bottom despite a 20-fold enrichment of²³⁰Th on the top.The distribution of alpha-emitting nuclides calculated from detailed radiochemical measurements matched closely the pattern revealed by 109-day exposures of alpha-sensitive film to the nodule. However, the shape and slope of the total alpha profile with depth into the nodule was affected strongly by²²⁶Ra and²²²Rn migrations making the alpha-track technique alone an inadequate method of measuring nodule growth rates.Diffusion of radium in the nodule may have been affected by diagenetic reactions which produce barite, phillipsite and todorokite within 1 mm of the nodule surface; however, our sampling interval was too broad to document the effect. We have not been able to resolve the importance of nodule diagenesis on the gross chemistry of the nodule.     

Post-depositional metal enrichment processes inside manganese nodules from the north equatorial Pacific

Interiors of manganese nodules from siliceous ooze beneath the Pacific equatorial high-productivity region, when examined by scanning electron microscopy (SEM) and electron microprobe, display post-depositional recrystallization textures and metalliferous oxide bands (diameter 1–10 μm, 30–40 wt.% Mn, 4–5% Ni, 3–4% Cu). SEM has revealed biogenic siliceous matter in all stages of degradation and dissolution within nodule interiors, creating cavities and voids. Often these miniature vugs contain authigenic phillipsite crystallites which are coated with delicate clusters of crystalline Mn-Fe oxides (todorokite) containing significant amounts of Ni and Cu. We postulate the following diagenetic processes and mechanism of uptake of transition metals inside manganese nodules: (1) palagonite + biogenic silica + pelagic clay → phillipsite + montmorillonite; (2) biogenic matter + amorphous FeOOH or δ-MnO₂ → Fe_aq²⁺ and/or Mn^IIMn^IV oxide (todorokite); (3) aerated seawater or δ-MnO₂ + Fe_aq²⁺ → FeOOH and/or todorokite (deposited on phillipsite); (4) (Ni^II and Cu^II) organic chelates (adsorbed on clays, etc.) + amorphous FeOOH or δ-MnO₂ → Ni-Cu-todorokite + phillipsite, etc.This mechanism explains the well-known positive Mn-Ni-Cu and negative Fe-Ni, Fe-Cu correlations in nodules. By analogy with terrestrial todorokites, which require about 8 wt.% Mn to be in the divalent state to stabilize the crystal structure, as much as 8 wt.% (Ni + Cu) could be accommodated in todorokite-bearing deep-sea manganese nodules. However, although such nodules beneficiate Ni and Cu with respect to marine sediments and seawater, they remain undersaturated in these divalent cations.     

Dynamic response of a staggered wall-beam structure

A method for the dynamic analysis of staggered wall-beam frames is developed using consistent mass terms which are derived and given in simple terms. The method uses effective stiffnesses for wall-beam elements developed in an earlier paper. Experiments using a nine storey 1 : 15 scale perspex model are described. The first three natural frequencies of the model were obtained using two methods: sinusoidal external excitation of the structure with the base fixed and white noise excitation employing a single degree-of-freedom shake table, in the latter method with and without the addition of mass throughout the model. Agreement between analytical predictions of the first three natural frequencies and mode shapes and experimentally determined values is considered satisfactory, particularly for the first two modes. The lumped mass assumption gave reasonable results for these two cases, whereas the consistent mass theory gave reasonable results for the first three natural frequencies.     

Résultats préliminaires sur la sédimentation pélagique de l'Atlantique tropical au Crétacé et au Tertiaire (plateau de Demerara,Leg ODP 207)

Five sites located on a bathymetric transect of the distal Demerara Rise were studied by ODP Leg 207. Albian sediments of essentially terrigenous nature (clay, siltstone, sandstone) are the oldest drilled stratigraphic levels and form apparently the top of the synrift sequence. They are overlain by Cenomanian to Santonian finely laminated black shales, rich in organic matter of marine origin, which accumulated on a thermally subsiding ramp. Early Campanian hiatuses are thought to be the result of final disjunction of Demerara Rise (South America) from Africa and the onset of deep water communication between the two Atlantic basins (south and central). The overlying Uppermost Cretaceous–Oligocene chalk includes rich and diversified calcareous plankton assemblages, as well as two radiolarian-rich intervals (Late Campanian and Middle Eocene). A complex erosional surface developed during the Late Oligocene–Early Miocene. Sedimentation was impeded since then on the intermediate and deep sites of Demerara Rise, possibly due to the action of deep submarine currents. To cite this article: T. Danelian et al., C. R. Geoscience 337 (2005).