New Horizons Alice ultraviolet observations of a stellar occultation by Jupiter’s atmosphere

The Alice ultraviolet spectrograph onboard the New Horizons spacecraft observed two occultations of the bright star χ Ophiucus by Jupiter’s atmosphere on February 22 and 23, 2007 during the approach phase of the Jupiter flyby. The ingress occultation probed the atmosphere at 32°N latitude near the dawn terminator, while egress probed 18°N latitude near the dusk terminator. A detailed analysis of both the ingress and egress occultations, including the effects of molecular hydrogen, methane, acetylene, ethylene, and ethane absorptions in the far ultraviolet (FUV), constrains the eddy diffusion coefficient at the homopause level to be  cm² s⁻¹, consistent with Voyager measurements and other analyses (Festou, M.C., Atreya, S.K., Donahue, T.M., Sandel, B.R., Shemansky, D.E., Broadfoot, A.L. [1981]. J. Geophys. Res. 86, 5717-5725; Vervack Jr., R.J., Sandel, B.R., Gladstone, G.R., McConnell, J.C., Parkinson, C.D. [1995]. Icarus 114, 163-173; Yelle, R.V., Young, L.A., Vervack Jr., R.J., Young, R., Pfister, L., Sandel, B.R. [1996]. J. Geophys. Res. 101 (E1), 2149-2162). However, the actual derived pressure level of the methane homopause for both occultations differs from that derived by [Festou et al., 1981] and [Yelle et al., 1996] from the Voyager ultraviolet occultations, suggesting possible changes in the strength of atmospheric mixing with time. We find that at 32°N latitude, the methane concentration is  cm⁻³ at 70,397 km, the methane concentration is  cm⁻³ at 70,383 km, the acetylene concentration is  cm⁻³ at 70,364 km, and the ethane concentration is  cm⁻³ at 70,360 km. At 18°N latitude, the methane concentration is  cm⁻³ at 71,345 km, the methane concentration is  cm⁻³ at 71,332 km, the acetylene concentration is cm⁻³ at 71,318 km, and the ethane concentration is  cm⁻³ at 71,315 km. We also find that the H₂ occultation light curve is best reproduced if the atmosphere remains cold in the microbar region such that the base of the thermosphere is located at a lower pressure level than that determined by in situ instruments aboard the Galileo probe (Seiff, A., Kirk, D.B., Knight, T.C.D., Young, R.E., Mihalov, J.D., Young, L.A., Milos, F.S., Schubert, G., Blanchard, R.C., Atkinson, D. [1998]. J. Geophys. Res. 103 (E10), 22857-22889) - the Sieff et al. temperature profile leads to too much absorption from H₂ at high altitudes. However, this result is highly model dependent and non-unique. The observations and analysis help constrain photochemical models of Jupiter’s atmosphere.     

EUV spectroscopy of the Venus dayglow with UVIS on Cassini

We analyze EUV spatially-resolved dayglow spectra obtained at 0.37 nm resolution by the UVIS instrument during the Cassini flyby of Venus on 24 June 1999, a period of high solar activity level. Emissions from OI, OII, NI, CI and CII and CO have been identified and their disc average intensity has been determined. They are generally somewhat brighter than those determined from the observations made with the HUT spectrograph at a lower activity level, We present the brightness distribution along the foot track of the UVIS slit of the OII 83.4 nm, OI 98.9 nm, Lyman-ß + OI 102.5 nm and NI 120.0 nm multiplets, and the CO C-X and B-X Hopfield-Birge bands. We make a detailed comparison of the intensities of the 834 nm, 989 nm, 120.0 nm multiplets and CO B-X band measured along the slit foot track on the disc with those predicted by an airglow model previously used to analyze Venus and Mars ultraviolet spectra. This model includes the treatment of multiple scattering for the optically thick OI, OII and NI multiplets. It is found that the observed intensity of the OII emission at 83.4 nm is higher than predicted by the model. An increase of the O⁺ ion density relative to the densities usually measured by Pioneer Venus brings the observations and the modeled values into better agreement. The calculated intensity variation of the CO B-X emission along the track of the UVIS slit is in fair agreement with the observations. The intensity of the OI 98.9 nm emission is well predicted by the model if resonance scattering of solar radiation by O atoms is included as a source. The calculated brightness of the NI 120 nm multiplet is larger than observed by a factor of ∼2-3 if photons from all sources encounter multiple scattering. The discrepancy reduces to 30-80% if the photon electron impact and photodissociation of N₂ sources of N(⁴S) atoms are considered as optically thin. Overall, we find that the O, N₂ and CO densities from the empirical VTS3 model provide satisfactory agreement between the calculated and the observed EUV airglow emissions.     

Lunar atmospheric H2 detections by the LAMP UV spectrograph on the Lunar Reconnaissance Orbiter

We report on the detection of H₂ as seen in our analysis of twilight observations of the lunar atmosphere observed by the LAMP instrument aboard NASA’s Lunar Reconnaissance Orbiter. Using a large amount of data collected on the lunar atmosphere between September 2009 and March 2013, we have detected and identified, the presence of H₂ in the native lunar atmosphere, for the first time. We derive a surface density for H₂ of 1.2 ± 0.4 × 10³ cm⁻³ at 120 K. This is about 10 times smaller than originally predicted, and several times smaller than previous upper limits from the Apollo era data.     

The ecological and social basis for management of a Red Sea marine-protected area

The Farasan Islands in the southern Red Sea of Saudi Arabia have nationally and internationally significant conservation values, and are important for a range of marine-based resource uses. In preparation for the establishment of a marine protected area around the Farasan Islands and its management, surveys were undertaken to assess the state of the coastal and marine resources, and the issues associated with human activities. Stakeholders were interviewed about issues and their attitudes towards the proposed protected area, and constraints to planning and management were identified. Marine habitats included seagrass beds, mangroves, and extensive areas of fringing reef dominated by a diverse coral community or a mixture of coral and macroalgae. Although used for a diverse range of human activities (fishing, shipping, transport, military purposes, recreation, waste dumping, sand extraction) impacts were minimal and localized. The most immediate threat to the marine resources was over-exploitation by fisheries. The types of management activities appropriate to the MPA, and the scale of management, were constrained by a number of unique and important factors: declines in national financial support for conservation efforts, a lack of trained personnel, difficulties in attracting staff to this remote location, loss of community support, the absence of a tourist base from which economic instruments could be developed, and the lack of local non-governmental organizations. Management recommended for the Farasan Islands Marine-Protected Area included zoning, community participation in management, public awareness, and training as a first step, followed by site-specific management actions, research and monitoring, and infrastructure development.     

Hydrocarbon photochemistry in the upper atmosphere of Jupiter

The hydrocarbon photochemistry in the upper atmosphere of Jupiter is investigated using a one-dimensional, photochemical-diffusive, and diurnally averaged model. The important chemical cycles and pathways among the major species are outlined and a standard model for the North Equatorial Belt region is examined in detail. It is found that several traditionally dominant chemical pathways among the C and C2 species are replaced in importance by cycles involving C-C4 species. The pressure and altitude profiles of mixing ratios for several observable hydrocarbon species are compared with available ultraviolet- and infrared-derived abundances. The results of sensitivity studies on the standard model with respect to variations in eddy diffusion profile, solar flux, atomic hydrogen influx, latitude, temperature, and important chemical reaction rates are presented. Measured and calculated airglow emissions of He at 584 angstroms and H at 1216 angstroms are also used to provide some constraints on the range of model parameters. The relevance of the model results to the upcoming Galileo mission is briefly discussed. The model is subject to considerable improvement; there is a great need for laboratory measurements of basic reaction rates and photodissociation quantum yields, even for such simple species as methylacetylene and allene. Until such laboratory measurements exist there will be considerable uncertainty in the understanding of the C3 and higher hydrocarbons in the atmospheres of the jovian planets.     

The relative abundance of ethane to acetylene in the Jovian stratosphere

The observed ratio of C2H6 to C2H2 in the Jovian stratosphere increases from approximately 55 at 2 mbar to approximately 277 at 12 mbar. In current photochemical models this ratio typically increases between 2 and 12 mbar by a factor of < or = 3. Recent laboratory kinetics studies on the reaction between C2H3 and H2 to form C2H4 suggest an efficient chemical mechanism for hydrogenation of C2H2 to C2H6. Inclusion of this scheme as part of a comprehensive updated model for hydrocarbon photochemistry in the atmosphere of Jupiter provides an explanation of the altitude variation of the C2H6/C2H2 ratio. The sensitivity of these results to uncertainties in the key rate constants at low temperatures is illustrated, identifying needs for additional laboratory measurements. Since the key reaction rate constants decrease with decreasing temperature, the hydrogenation of C2H2 as proposed predicts a qualitatively decreasing trend in the C2H6/C2H2 value with decreasing distance from the Sun. The observed variation between Jupiter and Saturn is consistent with this prediction.     

Small ice cap instability in the presence of fluctuations

Phenomena associated with small ice cap instability (SICI) are investigated using a general circulation model (GCM: NCAR Community Climate Model version 0) and a noise-forced nonlinear energy balance model (EBM). Both make use of idealized boundary conditions consisting of an all-land planet without topography and mean annual insolation. Ice is prescribed to exist on surface areas for which the instantaneous temperature lies below freezing. The adjustable parameters of the EBM were chosen to match the GCM solutions. For the regions in parameter space where SICI might occur we do not find the corresponding icefree steady state solution with the GCM. Our simulations with the EBM show that SICI phenomena in the presence of fluctuations are strongly dependent on the amplitude of the noise forcing. When the strength of noise forcing is adjusted to match the fluctuations in the GCM, we do not find a SICI in the EBM. With weaker levels of forcing the SICI reappears. In all cases steady stable ice caps smaller than a critical size are not found to exist.     

An experimental investigation of density-stratified inertial gravity currents

Turbidity currents and pyroclastic density currents may originate as stratified flows or develop stratification during propagation. Analogue, density‐stratified laboratory currents are described, using layers of salt solutions with different concentrations and depths to create the initial vertical stratification. The evolving structure of the flow depends on the distribution of the driving buoyancy between the layers, B* (proportional to the layer volumes and densities), and their density ratio, ρ*. When the lower layer contains more salt than the upper layer, and so has a greater proportion of the driving buoyancy (B* < 0·5), this layer can run ahead leading to streamwise or longitudinal stratification (ρ*→0), or the layers can mix to produce a homogeneous current (ρ*→1). If the upper layer contains more salt and thus buoyancy (B* > 0·5), this layer travels to the nose of the current by mixing into the back of the head along the body/wake density interface to produce a homogeneous flow (ρ*→1) or overtaking, leading to streamwise stratification (ρ*→0). Timescales describing the mixing between the layers and the streamwise separation of the layers are used to understand these flow behaviours and are in accordance with the experimental observations. Distance–time measurements of the flow front show that strongly stratified flows initially travel faster than weakly stratified flows but, during their later stages, they travel more slowly. In natural flows that are stratified in concentration and grain size, internal features, such as stepwise grading, gradual upward fining and reverse grading, could be produced depending on B* and ρ*. Stratification may also be expected to affect interactions with topography and overall fan architecture.     

Integration of sedimentology and ground‐penetrating radar for high‐resolution imaging of a carbonate platform

Ground‐penetrating radar has not been applied widely to the recognition of ancient carbonate platform geometries. This article reports the results of an integrated study performed on an Upper Jurassic outcrop from the south‐east Paris basin, where coral bioherms laterally change into prograding depositional sequences. Ground‐penetrating radar profiles illustrate the different bedding planes and major erosional unconformities visible at outcrop. A ground‐penetrating radar profile conducted at the base of the cliff displays a palaeotopographic surface on which the outcropping bioherms settled. The excellent penetration depths of the ground‐penetrating radar (20 m with a monostatic 200 MHz antenna) images the carbonate platform geometries, ranging between outcrop workscale (a few metres) and seismic scale (several hundreds of metres). This study supports recent evidence of icehouse conditions and induced sea‐level fluctuations controlling the Upper Jurassic carbonate production.