Observations of metallic species in Mercury’s exosphere

From observations of the metallic species sodium (Na), potassium (K), and magnesium (Mg) in Mercury’s exosphere, we derive implications for source and loss processes. All metallic species observed exhibit a distribution and/or line width characteristic of high to extreme temperature - tens of thousands of degrees K. The temperatures of refractory species, including magnesium and calcium, indicate that the source process for the atoms observed in the tail and near-planet exosphere are consistent with ion sputtering and/or impact vaporization of a molecule with subsequent dissociation into the atomic form. The extended Mg tail is consistent with a surface abundance of 5-8% Mg by number, if 30% of impact-vaporized Mg remains as MgO and half of the impact vapor condenses. Globally, ion sputtering is not a major source of Mg, but locally the sputtered source can be larger than the impact vapor source. We conclude that the Na and K in Mercury’s exosphere can be derived from a regolith composition similar to that of Luna 16 soil (or Apollo 17 orange glass), in which the abundance by number is 0.0027 (0.0028) for Na and 0.0006 (0.0045) for K.     

Monte Carlo modeling of sodium in Mercury’s exosphere during the first two MESSENGER flybys

We present a Monte Carlo model of the distribution of neutral sodium in Mercury’s exosphere and tail using data from the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft during the first two flybys of the planet in January and September 2008. We show that the dominant source mechanism for ejecting sodium from the surface is photon-stimulated desorption (PSD) and that the desorption rate is limited by the diffusion rate of sodium from the interior of grains in the regolith to the topmost few monolayers where PSD is effective. In the absence of ion precipitation, we find that the sodium source rate is limited to ∼10⁶-10⁷ cm⁻² s⁻¹, depending on the sticking efficiency of exospheric sodium that returns to the surface. The diffusion rate must be at least a factor of 5 higher in regions of ion precipitation to explain the MASCS observations during the second MESSENGER flyby. We estimate that impact vaporization of micrometeoroids may provide up to 15% of the total sodium source rate in the regions observed. Although sputtering by precipitating ions was found not to be a significant source of sodium during the MESSENGER flybys, ion precipitation is responsible for increasing the source rate at high latitudes through ion-enhanced diffusion.     

The tectonic generation of the Limpopo Mobile Belt,and a definition of its western extremity

A new mechanism is suggested for the generation of the interference fold pattern which characterizes the Limpopo Mobile Belt. The mechanism is directly related to shear movement along the Tuli-Sabi Shear Zone, renamed the Tuli-Sabi Straightening Zone. The mobile belt is regarded as a taphrogenic lineament (McConnell, 1974) and its generation is compatible with the tectonic environment active in Proterozoic times according to Sutton and Watson (1974). Field evidence shows that the Tuli-Sabi Straightening Zone dies out in Botswana at Moshakabela, and it is reasoned that the mobile belt as a whole also disappears in this vicinity. It does not extend into central and western Botswana beneath the Karroo and Kalahari cover. Detailed examination of ERTS-1 imagery of northeastern Botswana strengthens these deductions. The Tuli-Sabi Straightening Zone and the characteristic fold patterns of the mobile belt can be seen quite clearly on the satellite imagery. Furthermore, the Tuli-Sabi Straightening Zone appears to be displaced southwards at the international boundary between Botswana and Rhodesia. The existence of a fold belt trending about N150° superimposed on the Limpopo Mobile Belt in the west of the area is postulated which is not the Shashe Mobile Belt (Crockett, 1967).     

Sodium exchange in a NaCl waste disposal case (Lobatse,Botswana): Implications to mineral water studies

In the reconstruction of the chemical evolution of mineral waters, sodium exchange for calcium and magnesium, upon contact with soil and rocks, is often suggested. Such a process has been observed to take place on a large (and rapid) scale in NaCl waste disposal: that infiltrated to the local groundwater at Lobatse, Botswana. The process was repeated in laboratory experiments. Visiting Scientist at the Nuclear Physics Research Unit, University of the Witwatersrand, Johannesburg, South Africa, during 1972.     

A scaling law for the occurrence of aftershocks in southern California

A scaling law for the occurrence of aftershocks in southern California is proposed which suggests that the number of aftershocks is independent of the magnitude of the mainshock if aftershocks are counted in the magnitude interval from (Mm ? Δ) to Mm.     

NMR and EPR studies of the chemically intermediate plagioclase feldspars

The NMR spectra of ²⁷Al and ²³Na and the EPR spectra of Fe³⁺ and Mn²⁺ have been studied in a range of plagioclase feldspars, of intermediate composition. Many of these crystals have previously been investigated by TEM methods (McLaren and Marshall, 1974). The broad, central ²⁷Al signals observed in labradorites have been interpreted in terms of an overall order/disorder of the Al/Si distribution. Spin Hamiltonian parameters have been determined for the Fe³⁺ spectra observed in labradorites and bytownites; these spectra were similar and yielded evidence of disordering of the Al/Si distribution. The severe limitations imposed by the variety of sites, the complex microstructures and the disordering, limit the structural information available from the resonance methods.     

Dounreay oil spill: Major implications of a minor incident

Few reports of oil spills on an exposed rocky shore have taken into account the background of natural events and the period over which the possible consequences of pollution and its treatment may come to light. This small incident has served as a cautionary tale, illustrating the pitfalls of instant assessment, and the type of baseline experience required for biological interpretation. It has also underlined the continuing inadequacy of the ecological advice available when treatment decisions have to be made.