The calcium exosphere of Mercury

A tenuous calcium atmosphere at Mercury, principally seen in the polar regions, was first observed in July, 1998, using the High Resolution Echelle Spectrograph (HIRES) at the W.M. Keck I telescope (Bida et al., Nature 404, 159, 2000). We report four years of observations of the calcium exosphere of Mercury, confirming the initial findings of a very tenuous atmosphere. These observations show a persistent but spatially variable blue shift, indicating an excess velocity toward the observer of up to 3 km s⁻¹, with an average excess velocity of 2.2 km s⁻¹ above the south pole. In addition, the line profiles reveal a hot corona at the equivalent of 12,000-20,000 K in a thermalized atmosphere, indicating a large range of motion with respect to the observer. The calcium is not confined to the polar-regions: rare and low Ca abundance is seen in the equatorial regions. Strong emission was seen anti-sunward on 3 May 2002. Apparent weak emission on the sunward hemisphere may be due to scattered light from the surface, or may indicate a high latitude source. We show that the likely source of the calcium is either impact vaporization in the form of CaO and clusters, which are subsequently photo-dissociated, or ion-sputtering of atoms, molecules and ions. The column abundance is somewhat, but not strongly, correlated with solar activity. We predict a very hot (probably escaping) oxygen component to the hermean exosphere.     

A search program for objects in the vicinity of the Earth

Using a Super-Schmidt camera installed at the Cesco Observatory we are developing a program conducted to detect asteroids (especially members of the Aten Group) and comets approaching the Earth. The search strategy is based on simulations of the population of objects in this kind of orbits.     

Basalt and tonalite from the Amami Plateau, northern West Philippine Basin: New Early Cretaceous ages and geochemical results, and their petrologic and tectonic implications

Abstract Basalts and tonalites dredged from the Amami Plateau in the northern West Philippine Basin have the geochemical characteristics of intraoceanic island arc rocks: low ⁸⁷Sr/⁸⁶Sr (0.70297–0.70310), intermediate ¹⁴³Nd/¹⁴⁴Nd (0.51288–0.51292), moderate light rare earth element (LREE) enrichment (La/Yb = 4.1–6.6) and high La/Nb (1.4–4.3). The incremental heating of hornblende from tonalites yielded well‐defined plateaus and ⁴⁰Ar/³⁹Ar isochron ages of 115.8 ± 0.5 Ma and 117.0 ± 1.1 Ma, while plagioclase yielded disturbed Ar release patterns, with ages ranging from 70 to 112 Ma. Taken together, these results show that the Amami Plateau was formed by subduction‐related magmatism in the Early Cretaceous period, earlier than indicated by prior K/Ar results. The results support tectonic models in which the West Philippine Basin was opened within a complex of Jurassic–Paleocene island arc terranes, which are now scattered in the northern West Philippine Basin, the Philippine Islands and Halmahera. The Amami Plateau tonalites and basalts have higher Sr/Y and lower Y and ⁸⁷Sr/⁸⁶Sr compared with younger tonalitic rocks from the northern Kyushu–Palau Ridge and the Tanzawa complex, which were formed by the subduction of the Pacific Plate beneath the Philippine Sea Plate. Based on the geochemical characteristics of the basalts, the Early Cretaceous subduction zone that formed the Amami Plateau may have been the site of slab melting, which suggests that a younger and hotter plate was being subducted at that time. However, the Amami tonalites were probably formed from basaltic magma by fractional crystallization or by partial melting of basaltic arc crust, rather than by melting of the subducted slab.