Limits to Mercury's magnesium exosphere from MESSENGER second flyby observations

The discovery measurements of Mercury's exospheric magnesium, obtained by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) probe during its second Mercury flyby, are modeled to constrain the source and loss processes for this neutral species. Fits to a Chamberlain exosphere reveal that at least two source temperatures are required to reconcile the distribution of magnesium measured far from and near the planet: a hot ejection process at the equivalent temperature of several tens of thousands of degrees K, and a competing, cooler source at temperatures as low as 400 K. For the energetic component, our models indicate that the column abundance that can be attributed to sputtering under constant southward interplanetary magnetic field conditions is at least a factor of five less than the rate dictated by the measurements. Although highly uncertain, this result suggests that another energetic process, such as the rapid dissociation of exospheric MgO, may be the main source of the distant neutral component. If meteoroid and micrometeoroid impacts eject mainly molecules, the total amount of magnesium at altitudes exceeding ∼100 km is found to be consistent with predictions by impact vaporization models for molecule lifetimes of no more than two minutes. Though a sharp increase in emission observed near the dawn terminator region can be reproduced if a single meteoroid enhanced the impact vapor at equatorial dawn, it is much more likely that observations in this region, which probe heights increasingly near the surface, indicate a reservoir of volatile Mg being acted upon by lower-energy source processes.     

The subsidence history of Conoco well 15/30-1, central North Sea

The depth of deposition of the sediments from North Sea well Conoco 15/30-1 is determined using faunal and sedimentary evidence. The effects of sedimentary load, corrected for compaction and variations in water depth and sea level are removed, to obtain the subsidence of the basement when covered by water only. A crustal stretching and lithospheric thinning of 25–35% (β=1.35) can account for both the initial fault-controlled subsidence and the later uniform subsidence.     

Sodium D2 line profiles: clues to the temperature structure of Mercury’s exosphere

The velocity distribution of atoms in an exosphere can be used to constrain source and loss processes. The velocity distribution itself can be constrained by observing the line profile of an atmospheric emission line. The only atomic species known to exist in the Hermean exosphere that has a strong enough resonance transition in the visible spectrum that its line profile can be observed from the ground with existing instruments is sodium. Line profiles of the sodium D2 emission (5889 A) were obtained at two facilities. On 29 and 30 May 1997, we obtained line profiles with the 107 inch telescope at The McDonald Observatory at coude focus. On 6 January 1998, we obtained line profiles at the Anglo–Australian telescope. The line profiles show that the sodium in Mercury’s atmosphere is at least 700 K hotter than the surface temperature throughout the atmosphere. This implies that the interaction of the Na atoms with the surface is governed by energetic processes.     

Temporal variation of isotope and rare earth element abundances in volcanic rocks from Guam: implications for the evolution of the Mariana Arc

Volcanic rocks exposed on Guam were erupted during the Late Middle Eocene (Facpi Fm.), Late Eocene-Oligocene (Alutom Fm.) and Miocene (Umatac Fm.). Four magma series are recognized: the boninite series (44 m.y.b.p.), the tholeiite and calc-alkaline series, which were erupted along with boninite series lavas at 32–36 m.y.b.p. and high-K lavas of the Umatac Fm. (14 m.y.b.p.). Isotope and and rare earth element (REE) characteristics of the four magma series are distinct. Boninite series lavas have U-shaped REE patterns, relatively low ¹⁴³Nd/¹⁴⁴Nd (0.51294–0.51298), and high ²⁰⁶Pb/²⁰⁴Pb (19.0–19.2). Tholeiite series lavas are LREE (light REE) depleted, and have high ¹⁴³Nd/¹⁴⁴Nd (0.51304–0.51306) and low ²⁰⁶Pb/²⁰⁴Pb (18.4–18.5). Calc-alkaline series lavas have Sr, Nd and Pb isotope ratios similar to tholeiite series lavas, but flat to U-shaped REE patterns. Umatac Fm. lavas are strongly LREE-enriched, and have higher ⁸⁷Sr/ ⁸⁶Sr (0.70375–0.70380) and ²⁰⁷Pb/²⁰⁴Pb relative to ²⁰⁶Pb/ ²⁰⁴Pb than Facpi and Alutom Fm. lavas. Boninite and tholeiite series magmas, erupted in the position of the Palau-Kyushu Ridge, were probably derived from distinct mantle sources having OIB and N-MORB-like isotopic characteristics, together with fluids derived from subducted Pacific plate basalt. Calc-alkaline series lavas were most likely derived from the tholeiite series by extensive crystal fractionation, wallrock contamination and magma mixing. Lavas of the Umatac Fm., erupted in the position of the West Mariana Ridge, may include up to 2–3% subducted sediment, similar to some active Mariana arc lavas.     

Elasticity of superhydrous B

The adiabatic single-crystal elastic moduli of superhydrous B, Mg₁₀Si₃O₁₄(OH)₄, have been measured at ambient conditions using Brillouin spectroscopy. This material is the first hydrous phase found to be stable at the extreme conditions of 20 GPa and 1400 °C. The single-crystal moduli, in GPa, are: C ₁₁=280.0±1.5, C ₂₂=307.4±1.6, C ₃₃=293.4±1.4, C ₄₄=90.0±1.1, C ₅₅=99.2±0.8, C ₆₆=89.6±0.6, C ₁₂=66.1±2.2, C ₁₃=105.6±2.6, C ₂₃=81.8±2.6. With aggregate elastic properties of K _VRH =154.0±4.2 and μ _VRH =97.0±0.7 GPa, superhydrous B is approximately 16% suffer than forsterite and 20% softer than magnesium silicate spinel; it is also considerably more elastically isotropic than forsterite. The single-crystal moduli are compared to those of forsterite, magnesium silicate spinel and periclase, materials that are both structurally and compositionally similar to superhydrous B. The longitudinal moduli of superhydrous B and forsterite follow similar trends and appear to be dominated by the incompressibility and rotation of silicon tetrahedra. The shear and off-diagonal moduli more closely resemble those of periclase and spinel and may reflect the properties inherent to layers of magnesium octahedra.     

Seasonal and interannual evolution of the mixed layer in the Antarctic Zone south of Tasmania

Seasonal and interannual variations of the mixed layer properties in the Antarctic Zone (AZ) south of Tasmania are described using 7 WOCE/SR3 CTD sections and 8 years of summertime SURVOSTRAL XBT and thermosalinograph measurements between Tasmania and Antarctica. The AZ, which extends from the Polar Front (PF) to the Southern Antarctic Circumpolar Current Front (SACCF), is characterized by a 150 m deep layer of cold Winter Water (WW) overlayed in summer by warmer, fresher water mass known as Antarctic Surface Water (AASW). South of Tasmania, two branches of the PF divide the AZ into northern and southern zones with distinct water properties and variability. In the northern AZ (between the northern and southern branches of the PF), the mixed layer depth (MLD) is fairly constant in latitude, being 150 m deep in winter and around 40–60 m in summer. In the southern AZ, the winter MLD decreases from 150 m at the S-PF to 80 m at the SACCF and from 60 to 35 m in summer. Shallower mixed layers in the AZ-S are due to the decrease in the wind speed and stronger upwelling near the Antarctic Divergence. The WW MLD oscillates by ±15 m around its mean value and modest interannual changes are driven by winter wind stress anomalies.The mixed layer is on annual average 1.7 °C warmer, 0.06 fresher and 0.2 kg m⁻³ lighter in the northern AZ than in the southern AZ. The Levitus (1998) climatology is in agreement with the observed mean summer mixed layer temperature and salinity along the SURVOSTRAL line but underestimates the MLD by 10–20 m. The winter MLD in the climatology is also closed to that observed, but is 0.15 saltier than the observations along the AZ-N of the SR3 line. MLD, temperature and density show a strong seasonal cycle through the AZ while the mixed layer salinity is nearly constant throughout the year. During winter, the AZ MLD is associated with a halocline while during summer it coincides with a thermocline.Interannual variability of the AZ summer mixed layer is partly influenced by large scale processes such as the circumpolar wave which produces a warm anomaly during the summer 1996–1997, and partly by local mechanisms such as the retroflection of the S-PF which introduces cold water across the AZ-N.     

Effort changes around a marine reserve: The case of the California Rockfish Conservation Area

This article is an analysis of observed changes in the level of fishing effort and the spatial distribution of fishing effort following implementation of a marine reserve off the California coast. The Rockfish Conservation Area (RCA) closes the area between depth contour based boundaries to commercial trawl fishing. In this analysis, commercial fishing vessels are grouped based on their level of fishing effort within the closed area prior to the closure in order to compare changes in effort levels between groups. The results suggest that the RCA may have had a small effect on the level of fishing effort in the California groundfish trawl fishery. Spatial distribution of fishing effort before and after implementation of the marine reserve is also compared. Some concentration of effort occurred along parts of the closed area boundaries. This pattern suggests the “fishing the line” behavior noted in the marine reserve literature, but other possible explanations exist including the effects of coincident changes in other regulations and changing bioeconomic conditions.     

Exploring the Digital Divide in an Australian Regional City: a case study of Albury

The ‘digital divide’ is generally considered to be the gap between people who have access to information and communication technologies (ICT) and those who do not, and an issue of significant social justice. This paper presents findings from research that explored the digital divide within the regional city of Albury. The study focused on assessing whether there was a digital divide; identifying strategies to address any divide; and developing a methodology that could be used to explore the digital divide in other contexts. Data were gathered using semi-structured interviews, focus groups, and a telephone survey of city residents. Findings demonstrated that a digital divide existed in that there were differences in computer access in relation to income and different locations within the city, and for access to the Internet by age, education and income levels. Overall, the patterns indicated that those with lower education and income and the elderly reported lower levels of access to ICT. Drawing on the research findings we suggest some practical ways of addressing the digital divide that could be applied to other similar locations where a digital divide exists, such as providing technical support and training, improved access to and awareness of ICT services, and facilitating access to ICT services such as the provision of appropriate hardware and software for disadvantaged groups. Our view is that the mixed-method approach we employed provided helpful, reliable information at reasonable cost and could be considered by other researchers and local governments.     

Research Note: A unifying framework for the widely used stabilization of potential field inverse problems

We present a brief review of the widely used and well-known stabilizers in the inversion of potential field data. These include stabilizers that use L₂, L₁ and L₀ norms of the model parameters and the gradients of the model parameters. These stabilizers may all be realized in a common setting using two general forms with different weighting functions. Moreover, we show that this unifying framework encompasses the use of additional stabilizations which are not common for potential field inversion.