24,182 research outputs found
Mars: Seasonally variable radar reflectivity
The 1971/1973 Mars data set acquired by the Goldstone Solar System Radar was analyzed. It was established that the seasonal variations in radar reflectivity thought to occur in only one locality on the planet (the Solis Lacus radar anomaly) occur, in fact, over the entire subequatorial belt observed by the Goldstone radar. Since liquid water appears to be the most likely cause of the reflectivity excursions, a permanent, year-round presence of subsurface water (frozen or thawed) in the Martian tropics can be inferred
Mars: Seasonally variable radar reflectivity
Since reflectivity is a quantity characteristic of a given target at a particular geometry, the same (temporally unchanging) target examined by radar on different occasions should have the same reflectivity. Zisk and Mouginis-Mark noted that the average reflectivities in the Goldstone Mars data increased as the planet's S hemisphere passed from the late spring into early summer. The same data set was re-examined and the presence of the phenomenon of the apparent seasonal variability of radar reflectivity was confirmed. Two objections to these findings are addressed: (1) reflectivity variations may be present in the Goldstone Mars data as a result of an instrument/calibration error; and (2) the variations were introduced into the analysis through comparing reflectivities from two incompatible subsets of the data
Response of Bose gases in time-dependent optical superlattices
The dynamic response of ultracold Bose gases in one-dimensional optical
lattices and superlattices is investigated based on exact numerical time
evolutions in the framework of the Bose-Hubbard model. The system is excited by
a temporal amplitude modulation of the lattice potential, as it was done in
recent experiments. For regular lattice potentials, the dynamic signatures of
the superfluid to Mott-insulator transition are studied and the position and
the fine-structure of the resonances is explained by a linear response
analysis. Using direct simulations and the perturbative analysis it is shown
that in the presence of a two-colour superlattice the excitation spectrum
changes significantly when going from the homogeneous Mott-insulator the quasi
Bose-glass phase. A characteristic and experimentally accessible signature for
the quasi Bose-glass is the appearance of low-lying resonances and a
suppression of the dominant resonance of the Mott-insulator phase.Comment: 20 pages, 9 figures; added references and corrected typo
Repulsive Casimir Pistons
Casimir pistons are models in which finite Casimir forces can be calculated
without any suspect renormalizations. It has been suggested that such forces
are always attractive. We present three scenarios in which that is not true.
Two of these depend on mixing two types of boundary conditions. The other,
however, is a simple type of quantum graph in which the sign of the force
depends upon the number of edges.Comment: 4 pages, 2 figures; RevTeX. Minor additions and correction
Integral field spectroscopy of QSO host galaxies
We describe a project to study the state of the ISM in ~20 low redshift
(z<0.3) QSO host galaxies observed with the PMAS integral field spectrograph.
We describe method developement to access the stellar and gas component of the
spectrum without the strong nuclear emission to access the host galaxy
properties also in the central region. It shows that integral field
spectroscopy promises to be very efficient to study the gas distribution and
its velocity field, and also spatially resolved stellar population in the host
galaxies also of luminous AGN.Comment: 4 pages, 6 figures, Euro3D Science Workshop, Cambridge, May 2003, AN,
accepte
Alkali oxide-tantalum, niobium and antimony oxide ionic conductors
The phase equilibrium relations of four systems were investigated in detail. These consisted of sodium and potassium antimonates with antimony oxide and tantalum and niobium oxide with rubidium oxide as far as the ratio 4Rb2O:llB2O5 (B=Nb, Ta). The ternary system NaSbO3-Sb2O4-NaF was investigated extensively to determine the actual composition of the body centered cubic sodium antimonate. Various other binary and ternary oxide systems involving alkali oxides were examined in lesser detail. The phases synthesized were screened by ion exchange methods to determine mobility of the mobility of the alkali ion within the niobium, tantalum or antimony oxide (fluoride) structural framework. Five structure types warranted further investigation; these structure types are (1) hexagonal tungsten bronze (HTB), (2) pyrochlore, (3) the hybrid HTB-pyrochlore hexagonal ordered phases, (4) body centered cubic antimonates and (5) 2K2O:3Nb2O5. Although all of these phases exhibit good ion exchange properties only the pyrochlore was prepared with Na(+) ions as an equilibrium phase and as a low porosity ceramic. Sb(+3) in the channel interferes with ionic conductivity in this case, although relatively good ionic conductivity was found for the metastable Na(+) ion exchanged analogs of RbTa2O5F and KTaWO6 pyrochlore phases
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