Monitoring the spin up in RX J0806+15     

Pasi Hakala  Gavin Ramsay  Kristiina Byckling 《Monthly notices of the Royal Astronomical Society》2004,353(2):453-456

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Radiative heat conduction and the magnetorotational instability     

Rafael A. Araya-Góchez  Ethan T. Vishniac 《Monthly notices of the Royal Astronomical Society》2004,355(2):345-351

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Resurfacing history of Europa from pole-to-pole geological mapping     

Patricio H. Figueredo  Ronald Greeley 《Icarus》2004,167(2):287-312

We produced geologic maps from two regional mosaics of Galileo images across the leading and trailing hemispheres of Europa in order to investigate the temporal distribution of units in the visible geologic record. Five principal terrain types were identified (plains, bands, ridges, chaos, and crater materials), which are interpreted to result from (1) tectonic fracturing and lineament building, (2) cryovolcanic reworking of surface units, with possible emplacement of sub-surface materials, and (3) impact cratering. The geologic histories of both mapped areas are essentially similar and reflect some common trends: Tectonic resurfacing dominates the early geologic record with the formation of background plains by intricate superposition of lineaments, the opening of wide bands with infilling of inter-plate gaps, and the buildup of ridges and ridge complexes along prominent fractures in the ice. It also appears that lineaments are narrower and more widely spaced with time. The lack of impact craters overprinted by lineaments indicate that the degree of tectonic resurfacing decreased rapidly after ridged plains formation. In contrast, the degree of cryovolcanic resurfacing appears to increase with time, as chaos formation dominates the later parts of the geologic record. These trends, and the transition from tectonic- to cryovolcanic-dominated resurfacing could be attributed to the gradual thickening of Europa's cryosphere during the visible geologic history, that comprises the last 2% or 30-80 Myr of Europa's history: An originally thin, brittle ice shell could be pervasively fractured or melted through by tidal and endogenic processes; the degree of fracturing and plate displacements decreased with time in a thickening shell, and lineaments became narrower and more widely spaced; formation of chaos regions could have occurred where the thickness threshold for solid-state convection was exceeded, and can be aided by preferential tidal heating of more ductile ice. In a long-term context it is not clear at this point whether this inferred thickening trend would reflect a drastic change in the thermal evolution of the satellite, or cyclic or irregular episodes of tectonic and cryovolcanic activity.  相似文献   

The energy balance of polars revisited     

Gavin Ramsay  Mark Cropper 《Monthly notices of the Royal Astronomical Society》2004,347(2):497-507

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Long term evolution and chaotic diffusion of the insolation quantities of Mars   总被引：1，自引：0，他引：1  

J. Laskar  A.C.M. Correia  M. Gastineau  B. Levrard 《Icarus》2004,170(2):343-364

As the obliquity of Mars is strongly chaotic, it is not possible to give a solution for its evolution over more than a few million years. Using the most recent data for the rotational state of Mars, and a new numerical integration of the Solar System, we provide here a precise solution for the evolution of Mars' spin over 10 to 20 Myr. Over 250 Myr, we present a statistical study of its possible evolution, when considering the uncertainties in the present rotational state. Over much longer time span, reaching 5 Gyr, chaotic diffusion prevails, and we have performed an extensive statistical analysis of the orbital and rotational evolution of Mars, relying on Laskar's secular solution of the Solar System, based on more than 600 orbital and 200,000 obliquity solutions over 5 Gyr. The density functions of the eccentricity and obliquity are specified with simple analytical formulas. We found an averaged eccentricity of Mars over 5 Gyr of 0.0690 with standard deviation 0.0299, while the averaged value of the obliquity is 37.62° with a standard deviation of 13.82°, and a maximal value of 82.035°. We find that the probability for Mars' obliquity to have reached more than 60° in the past 1 Gyr is 63.0%, and 89.3% in 3 Gyr. Over 4 Gyr, the position of Mars' axis is given by a uniform distribution on a spherical cap limited by the obliquity 58.62°, with the addition of a random noise allowing a slow diffusion beyond this limit. We can also define a standard model of Mars' insolation parameters over 4 Gyr with the most probable values 0.068 for the eccentricity and 41.80° for the obliquity.  相似文献   

The non-radially pulsating primary of the cataclysmic variable GW Librae     

L. van Zyl  B. Warner  D. O'Donoghue  C. Hellier  P. Woudt  D. Sullivan  J. Pritchard  J. Kemp  J. Patterson  W. Welsh  J. Casares  T. Shahbaz  F. van der Hooft  S. Vennes 《Monthly notices of the Royal Astronomical Society》2004,350(1):307-316

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Equilibria of a self-gravitating, rotating disc around a magnetized compact object     

J. Ghanbari  S. Abbassi 《Monthly notices of the Royal Astronomical Society》2004,350(4):1437-1444

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Extinction due to amorphous carbon grains in red quasars from the Sloan Digital Sky Survey     

B. Czerny  J. Li  Z. Loska  R. Szczerba 《Monthly notices of the Royal Astronomical Society》2004,348(3):L54-L57

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Hydrogen peroxide on Mars: evidence for spatial and seasonal variations     

Th Encrenaz  B Bézard  M.J Richter  S.K Atreya  S Lebonnois  F Forget 《Icarus》2004,170(2):424-429

Hydrogen peroxide (H₂O₂) has been suggested as a possible oxidizer of the martian surface. Photochemical models predict a mean column density in the range of 10¹⁵-10¹⁶ cm⁻². However, a stringent upper limit of the H₂O₂ abundance on Mars (9×10¹⁴ cm⁻²) was derived in February 2001 from ground-based infrared spectroscopy, at a time corresponding to a maximum water vapor abundance in the northern summer (30 pr. μm, Ls=112°). Here we report the detection of H₂O₂ on Mars in June 2003, and its mapping over the martian disk using the same technique, during the southern spring (Ls=206°) when the global water vapor abundance was ∼10 pr. μm. The spatial distribution of H₂O₂ shows a maximum in the morning around the sub-solar latitude. The mean H₂O₂ column density (6×10¹⁵ cm⁻²) is significantly greater than our previous upper limit, pointing to seasonal variations. Our new result is globally consistent with the predictions of photochemical models, and also with submillimeter ground-based measurements obtained in September 2003 (Ls=254°), averaged over the martian disk (Clancy et al., 2004, Icarus 168, 116-121).  相似文献   

A 2dF survey of the Small Magellanic Cloud     

Christopher J. Evans  Ian D. Howarth  Michael J. Irwin  Adam W. Burnley  Timothy J. Harries 《Monthly notices of the Royal Astronomical Society》2004,353(2):601-623

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