743 research outputs found
Tectonic setting of Martian volcanoes and deep-seated intrusives
More than 50 volcanoes have been mapped on Mars, and recent geologic studies indicate structural evidence of deep seated intrusive bodies. Most volcanoes in the Tharsis region are volcanotectonic features; they have been associated with large scale tectonic and volcanic processes. They occur along complex systems of faults and grabens having a dominant northwest to southwest trend closely coincident with a great circle, which extends along 90 deg of arc from Tempe Patera to probable volcanic mountains near lat. 40 deg S, long. 150 deg. Deep seated intrusive bodies are also concentrated in the Tharsis region and are recognized mostly where faults have been deflected around their cores. The Elysium Mons-Amphitrites Patera volcanic alignment is subparallel to that of Tharsis but is longer, extending through about 120 deg of arc; it transects the dichotomy boundary and is radial to the Hellas basin. Volcanoes in the Tharsis region have the widest age range of all volcanoes on Mars, as determined by the size-frequency distribution of their craters having diameters of 2, 5, and 16 km
Paleolakes and lacustrine basins on Mars
The problems of how warm and wet Mars once was and when climate transitions may have occurred are not well understood. Mars may have had an early environment similar to Earth's that was conducive to the emergence of life. In addition, increasing geologic evidence indicates that water, upon which terrestrial life depends, has been present on Mars throughout its history. This evidence does not detract from the possibility that life may have originated on early Mars, but rather suggests that life could have developed over longer periods of time in longer lasting, more clement local environments than previously envisioned. It is suggested herein that such environments may have been provided by paleolakes, located mostly in the northern lowlands and probably ice covered. Such lakes probably would have had diverse origins. Glacial lakes may have occupied ice eroded hollows or formed in valleys obstructed by moraines or ice barriers. Unlike Earth, the Martian record of the origin and evolution of possible life may have not been erased by extensive deformation of the surface. Thus the basins that may have contained the paleolakes are potential sites for future biological, geological, and climatological study
Critical free energy and Casimir forces in rectangular geometries
We study the critical behavior of the free energy and the thermodynamic
Casimir force in a block geometry in
dimensions with aspect ratio above, at, and below on
the basis of the O symmetric lattice model with periodic boundary
conditions (b.c.). We consider a simple-cubic lattice with isotropic
short-range interactions. Exact results are derived in the large - limit
describing the geometric crossover from film () over cubic to
cylindrical () geometries. For , three perturbation
approaches are presented that cover both the central finite-size regime near
for and the region outside the central
finite-size regime well above and below for arbitrary . At bulk
of isotropic systems with periodic b.c., we predict the critical Casimir
force in the vertical direction to be negative (attractive) for a slab
(), and zero for a cube
. We also present extrapolations to the cylinder limit
() and to the film limit () for and . Our
analytic results for finite-size scaling functions in the minimal
renormalization scheme at fixed dimension agree well with Monte Carlo
data for the three-dimensional Ising model by Hasenbusch for and by
Vasilyev et al. for above, at, and below .Comment: 23 pages, 14 figure
Five-loop additive renormalization in the phi^4 theory and amplitude functions of the minimally renormalized specific heat in three dimensions
We present an analytic five-loop calculation for the additive renormalization
constant A(u,epsilon) and the associated renormalization-group function B(u) of
the specific heat of the O(n) symmetric phi^4 theory within the minimal
subtraction scheme. We show that this calculation does not require new
five-loop integrations but can be performed on the basis of the previous
five-loop calculation of the four-point vertex function combined with an
appropriate identification of symmetry factors of vacuum diagrams. We also
determine the amplitude functions of the specific heat in three dimensions for
n=1,2,3 above T_c and for n=1 below T_c up to five-loop order. Accurate results
are obtained from Borel resummations of B(u) for n=1,2,3 and of the amplitude
functions for n=1. Previous conjectures regarding the smallness of the resummed
higher-order contributions are confirmed. Borel resummed universal amplitude
ratios A^+/A^- and a_c^+/a_c^- are calculated for n=1.Comment: 30 pages REVTeX, 3 PostScript figures, submitted to Phys. Rev.
Geologic controls of erosion and sedimentation on Mars
Because Mars has had a history of diverse erosional and depositional styles, a variety of erosional landforms and sedimentary deposits can be seen on Viking orbiter images. Here we review how geologic processes involving rock, water, and structure have controlled erosion and sedimentation on Mars. Additionally, we review how further studies will help refine our understanding of these processes
Singularity in the boundary resistance between superfluid He and a solid surface
We report new measurements in four cells of the thermal boundary resistance
between copper and He below but near the superfluid-transition
temperature . For fits of to the data yielded ,
whereas a fit to theoretical values based on the renormalization-group theory
yielded . Alternatively, a good fit of the theory to the data could
be obtained if the {\it amplitude} of the prediction was reduced by a factor
close to two. The results raise the question whether the boundary conditions
used in the theory should be modified.Comment: 4 pages, 4 figures, revte
Dwarf galaxies beyond our doorstep: the Centaurus A group
The study of dwarf galaxies in groups is a powerful tool for investigating
galaxy evolution, chemical enrichment and environmental effects on these
objects. Here we present results obtained for dwarf galaxies in the Centaurus A
complex, a dense nearby (~4 Mpc) group that contains two giant galaxies and
about 30 dwarf companions of different morphologies and stellar contents. We
use archival optical (HST/ACS) and near-infrared (VLT/ISAAC) data to derive
physical properties and evolutionary histories from the resolved stellar
populations of these dwarf galaxies. In particular, for early-type dwarfs we
are able to construct metallicity distribution functions, find population
gradients and quantify the intermediate-age star formation episodes. For
late-type dwarfs, we compute recent (~1 Gyr) star formation histories and study
their stellar distribution. We then compare these results with properties of
the dwarfs in our Milky Way and in other groups. Our work will ultimately lead
to a better understanding of the evolution of dwarf galaxies.Comment: 6 pages, 5 figures; to appear in the proceedings of the conference "A
Universe of dwarf galaxies" (Lyon, June 14-18, 2010
Maturation of mammalian H/ACA box snoRNAs: PAPD5-dependent adenylation and PARN-dependent trimming
Small nucleolar and small Cajal body RNAs (snoRNAs and scaRNAs) of the H/ACA box and C/D box type are generated by exonucleolytic shortening of longer precursors. Removal of the last few nucleotides at the 3' end is known to be a distinct step. We report that, in human cells, knock-down of the poly(A) specific ribonuclease (PARN), previously implicated only in mRNA metabolism, causes the accumulation of oligoadenylated processing intermediates of H/ACA box but not C/D box RNAs. In agreement with a role of PARN in snoRNA and scaRNA processing, the enzyme is concentrated in nucleoli and Cajal bodies. Oligo(A) tails are attached to a short stub of intron sequence remaining beyond the mature 3' end of the snoRNAs. The noncanonical poly(A) polymerase PAPD5 is responsible for addition of the oligo(A) tails. We suggest that deadenylation is coupled to clean 3' end trimming, which might serve to enhance snoRNA stability
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