排序方式: 共有16条查询结果,搜索用时 328 毫秒
11.
The composition of planetesimals depends upon the epoch and the location of their formation in the solar nebula. Meteorites
produced in the hot inner nebula contain refractory compounds. Volatiles were present in icy planetesimals and cometesimals
produced in the cold outer nebula. However, the mechanism responsible for their trapping is still controversial. We argue
for a general scenario valid in all regions of the turbulent nebula where water condensed as a crystalline ice (Hersant et al., 2004). Volatiles were trapped in the form of clathrate hydrates in the continuously cooling nebula. The epoch of clathration
of a given species depends upon the temperature and the pressure required for the stability of the clathrate hydrate. The
efficiency of the mechanism depends upon the local amount of ice available. This scenario is the only one so far which proposes
a quantitative interpretation of the non detection of N2 in several comets of the Oort cloud (Iro et al., 2003). It may explain the large variation of the CO abundance observed in comets and predicts an Ar/O ratio much less than
the upper limit of 0.1 times the solar ratio estimated on C/2001 A2 (Weaver et al., 2002). Under the assumption that the amount of water ice present at 5 AU was higher than the value corresponding to the
solar O/H ratio by a factor 2.2 at least, the clathration scenario reproduces the quasi uniform enrichment with respect to
solar of the Ar, Kr, Xe, C, N and S elements measured in Jupiter by the Galileo probe. The interpretation of the non-uniform
enrichment in C, N and S in Saturn requires that ice was less abundant at 10 AU than at 5 AU so that CO and N2 were not clathrated in the feeding zone of the planet while CH4, NH3 and H2S were. As a result, the 14N/15N ratio in Saturn should be intermediate between that in Jupiter and the terrestrial ratio.
Ar and Kr should be solar while Xe should be enriched by a factor 17. The enrichments in C, N and S in Uranus and Neptune
suggest that available ice was able to form clathrates of CH4, CO and the NH3 hydrate, but not the clathrate of N2. The enrichment of oxygen by a factor 440 in Neptune inferred by Lodders and Fegley (1994) from the detection of CO in the
troposphere of the planet is higher by at least a factor 2.5 than the lower limit of O/H required for the clathration of CO
and CH4 and for the hydration of NH3. If CO detected by Encrenaz et al. (2004) in Uranus originates from the interior of the planet, the O/H ratio in the envelope must be around of order of 260
times the solar ratio, then also consistent with the trapping of detected volatiles by clathration. It is predicted that Ar
and Kr are solar in the two planets while Xe would be enriched by a factor 30 to 70. Observational tests of the validity of
the clathration scenario are proposed. 相似文献
12.
13.
Prieto-Ballesteros O Fernández-Remolar DC Rodríguez-Manfredi JA Selsis F Manrubia SC 《Astrobiology》2006,6(4):651-667
Recent data from space missions reveal that there are ongoing climatic changes and erosive processes that continuously modify surface features of Mars. We have investigated the seasonal dynamics of a number of morphological features located at Inca City, a representative area at high southern latitude that has undergone seasonal processes. By integrating visual information from the Mars Orbiter Camera on board the Mars Global Surveyor and climatic cycles from a Mars' General Circulation Model, and considering the recently reported evidence for the presence of water-ice and aqueous precipitates on Mars, we propose that a number of the erosive features identified in Inca City, among them spiders, result from the seasonal melting of aqueous salty solutions. 相似文献
14.
S Franck W von Bloh C Bounama M Steffen D Schonberner H J Schellnhuber 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2001,28(4):695-700
We present a general modeling scheme for investigating the possibility of photosynthesis-based life on extrasolar planets. The scheme focuses on the identification of the habitable zone in main-sequence-star planetary systems with planets of Earth mass and size. Our definition of habitability is based on the long-term possibility of photosynthetic biomass production as a function of mean planetary surface temperature and atmospheric CO2-content. All the astrophysical, climatological, biogeochemical, and geodynamic key processes involved in the generation of photosynthesis-driven life conditions are taken into account. Implicitly, a co-genetic origin of the central star and the orbiting planet is assumed. The numerical solution of an advanced geodynamic model yields realistic look-up diagrams for determining the limits of photosynthesis in extrasolar planetary systems, assuming minimum CO2 levels set by the demand of C4 photosynthesis. 相似文献
15.
Khodachenko ML Ribas I Lammer H Griessmeier JM Leitner M Selsis F Eiroa C Hanslmeier A Biernat HK Farrugia CJ Rucker HO 《Astrobiology》2007,7(1):167-184
Low mass M- and K-type stars are much more numerous in the solar neighborhood than solar-like G-type stars. Therefore, some of them may appear as interesting candidates for the target star lists of terrestrial exoplanet (i.e., planets with mass, radius, and internal parameters identical to Earth) search programs like Darwin (ESA) or the Terrestrial Planet Finder Coronagraph/Inferometer (NASA). The higher level of stellar activity of low mass M stars, as compared to solar-like G stars, as well as the closer orbital distances of their habitable zones (HZs), means that terrestrial-type exoplanets within HZs of these stars are more influenced by stellar activity than one would expect for a planet in an HZ of a solar-like star. Here we examine the influences of stellar coronal mass ejection (CME) activity on planetary environments and the role CMEs may play in the definition of habitability criterion for the terrestrial type exoplanets near M stars. We pay attention to the fact that exoplanets within HZs that are in close proximity to low mass M stars may become tidally locked, which, in turn, can result in relatively weak intrinsic planetary magnetic moments. Taking into account existing observational data and models that involve the Sun and related hypothetical parameters of extrasolar CMEs (density, velocity, size, and occurrence rate), we show that Earth-like exoplanets within close-in HZs should experience a continuous CME exposure over long periods of time. This fact, together with small magnetic moments of tidally locked exoplanets, may result in little or no magnetospheric protection of planetary atmospheres from a dense flow of CME plasma. Magnetospheric standoff distances of weakly magnetized Earth-like exoplanets at orbital distances 相似文献
16.
How to characterize and implement point-to-point (i.e., unicast) routing algorithms is discussed. The characteristics of routing algorithms are first described as well as the specificities of routing in constellations of nongeostationary (NGO) satellites equipped with intersatellite links (ISLs). Then, some of the candidate algorithms displaying the characteristics identified before are simulated and evaluated for different constellations. From the simulation results, it appears that (1) from a routing perspective, constellations are not similar to terrestrial networks, and (2) the constellation geometry has a significant impact on the routing performance. 相似文献