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11.
An accurate assessment of the bulk chemical composition of Mars is fundamental to understanding planetary accretion, differentiation, mantle evolution, the nature of the igneous parent rocks that were altered to produce sediments on Mars, and the initial concentrations of volatiles such as H, Cl and S, important constituents of the Martian surface. This paper reviews the three main approaches that have been used to estimate the bulk chemical composition of Mars: geochemical/cosmochemical, isotopic, and geophysical. The standard model is one developed by Wänke and Dreibus in a series of papers, which is based on compositions of Martian meteorites. Since their groundbreaking work, substantial amounts of data have become available to allow a reassessment of the composition of Mars from elemental data, including tests of the basic assumptions in the geochemical models. The results adjust some of the concentrations in the Wänke–Dreibus model, but in general confirm its accuracy. Bulk silicate Mars has roughly uniform depletion of moderately volatile elements such as K (0.6 × CI), and strong depletion of highly volatile elements (e.g., Tl). The highly volatile elements are within uncertainties uniformly depleted at about 0.06 CI abundances. The highly volatile chalcophile elements are likewise roughly uniformly depleted, but with more scatter, with normalized abundances of 0.03 CI. Bulk planetary H2O is much higher than estimated previously: it appears to be slightly less than in Earth, but D/H is similar in Earth and Mars, indicating a common source of water-bearing material in the inner solar system. K/Th ranges from ∼3000 to ∼5000 among the terrestrial planets, a small range compared to CI chondrites (19,000). FeO varies throughout the inner solar system: ∼3 wt% in Mercury, 8 wt% in Earth and Venus, and 18 wt% in Mars. These differences can be produced by varying oxidation conditions, hence do not suggest the terrestrial planets were formed from fundamentally different materials. The broad chemical similarities among the terrestrial planets indicate substantial mixing throughout the inner solar system during planet formation, as suggested by dynamical models. 相似文献
12.
The giant planets of our solar system possess envelopes consisting mainly of hydrogen and helium but are also significantly enriched in heavier elements relatively to our Sun. In order to better constrain how these heavy elements have been delivered, we quantify the amount accreted during the so-called “late heavy bombardment”, at a time when planets were fully formed and planetesimals could not sink deep into the planets. On the basis of the “Nice model”, we obtain accreted masses (in terrestrial units) equal to for Jupiter, and for Saturn. For the two other giant planets, the results are found to depend mostly on whether they switched position during the instability phase. For Uranus, the accreted mass is with an inversion and without an inversion. Neptune accretes in models in which it is initially closer to the Sun than Uranus, and otherwise. With well-mixed envelopes, this corresponds to an increase in the enrichment over the solar value of 0.033±0.001 and 0.074±0.007 for Jupiter and Saturn, respectively. For the two other planets, we find the enrichments to be 2.1±1.4 (w/ inversion) or 1.2±0.7 (w/o inversion) for Uranus, and 2.0±1.2 (w/ inversion) or 2.7±1.6 (w/o inversion) for Neptune. This is clearly insufficient to explain the inferred enrichments of ∼4 for Jupiter, ∼7 for Saturn and ∼45 for Uranus and Neptune. 相似文献
13.
Claude Catala 《Experimental Astronomy》2009,23(1):329-356
The PLAnetary Transits and Oscillations of stars Mission (PLATO), presented to ESA in the framework of its “Cosmic Vision” programme, will detect and characterize exoplanets
by means of their transit signature in front of a very large sample of bright stars, and measure the seismic oscillations
of the parent stars orbited by these planets in order to understand the properties of the exoplanetary systems. PLATO is the
next-generation planet finder, building on the accomplishments of CoRoT and Kepler: i) it will observe significantly more stars, ii) its targets will be 2 to 3 magnitudes brighter (hence the precision of the measurements will be correspondingly greater
as will be those of post-detection investigations, e.g. spectroscopy, asteroseismology, and eventually imaging), iii) it will be capable of observing significantly smaller exoplanets. The space-based observations will be complemented by ground-
and space-based follow-up observations. These goals will be achieved by a long-term (4 years), high-precision, high-time-resolution,
high-duty-cycle monitoring in visible photometry of a sample of more than 100,000 relatively bright (m
V
≤ 12) stars and another 400,000 down to m
V
= 14. Two different mission concepts are proposed for PLATO: i) a “staring” concept with 100 small, very wide-field telescopes, assembled on a single platform and all looking at the same 26° diameter field,
and ii) a “spinning” concept with three moderate-size telescopes covering more than 1400 degree2.
See for The PLATO Consortium. 相似文献
14.
In the tandem planet formation regime,planets form at two distinct sites where solid particles are densely accumulated due to the on/off state of the magnetorotational instability(MRI).We found that tandem planet formation can reproduce the solid component distribution of the Solar System and tends to produce a smaller number of large planets through continuous pebble flow into the planet formation sites.In the present paper,we investigate the dependence of tandem planet formation on the vertical magnetic field of the protoplanetary disk.We calculated two cases of B_Z 3.4 × 10~(-3) G and B_Z = 3.4 × 10~(-5) G at 100 AU as well as the canonical case of B_Z = 3.4 × 10~(-4) G.We found that tandem planet formation holds up well in the case of the strong magnetic field(B_Z 3.4 × 10~(-3) G).On the other hand,in the case of a weak magnetic field(B_Z= 3.4 × 10~(-5) G) at 100 AU,a new regime of planetary growth is realized:the planets grow independently at different places in the dispersed area of the MRl-suppressed region of r-8-30 AU at a lower accretion rate of M 10~(-7.4)M_⊙yr~(-1).We call this the "dispersed planet formation" regime.This may lead to a system with a larger number of smaller planets that gain high eccentricity through mutual collisions. 相似文献
15.
Pastoralism is a major occupation in Africa’s Sahel region, which spans from Senegal in the west to Ethiopia in the east and has a semi-arid climate. Pastoralists from the region are usually nomadic, moving regularly in search of water and pasture, often for hundreds of kilometers. Traditionally, pastoralists have relied on historical knowledge and transhumance pathways to reach the water bodies (WB). Due to climatic conditions, a vast majority of the WBs in this region are small, sporadic, and ephemeral, therefore there is a need to map and frequently monitor water availability. Remote sensing observations can be used to complement current monitoring efforts. High-resolution near-daily imagery from the recently launched PlanetScope (PS) constellation of small satellites has the potential to complement the existing medium-resolution and infrequent but well established and extensively calibrated sensors like Landsat. The aim of this study is to investigate the performance and applicability of high-resolution PS data to detect and monitor WBs in the Ferlo region of West Africa, in comparison to Landsat-8 imagery. The results indicate that for cloud-free days, PS data was comparable (r >0.88; ubRMSE <0.01) to Landsat, but due to inadequate cloud masking, the results were poor for cloudy days (r < 0.49 and ubRMSE>0.058). The PS images were able to detect nearly 95% of the WBs, whereas Landsat was able to identify only 32% at the water fraction threshold >40%. Initial results from this study show that the high spatial and temporal resolution PS datasets show promise in monitoring WBs in arid regions of West Africa. 相似文献
16.
After five years of thorough analysis of data from the Huygens Probe that descended into Titan's atmosphere in January 2005, we report major findings inferred from measurements of low frequency waves and atmospheric conductivity. The data account for the observation of a Schumann-like resonance trapped within Titan's atmospheric cavity. On Earth, this phenomenon is triggered by lightning and was anticipated to be observed on Titan, as it provides a tool to reveal the presence of a ground conductive boundary to sustain the resonance of the cavity. The Huygens observations show that the major electric field component of the signal is horizontal, which is inconsistent with lightning sources. We interpret, however, the observed signal as a second spherical harmonic of Titan's cavity, triggered and sustained by strong electric currents induced in the ionosphere by Saturn's magnetospheric plasma flow. The present study describes the characteristics of such trapped modes that allow us to constrain the parameters of the cavity and to infer the presence of a conductive layer at 45 km (±15 km) below the surface. By comparison with the presence of subsurface conductive ocean in the Galilean icy satellites, we conclude that Titan should have pursued similar processes of internal dynamics. To date, this result represents the only evidence for a buried ocean in Titan. 相似文献
17.
D. Koch W. Borucki G. Basri T. Brown D. Caldwell J. Christensen-Dalsgaard W. Cochran E. Dunham T. N. Gautier J. Geary R. Gilliland J. Jenkins Y. Kondo D. Latham J. Lissauer D. Monet 《Astrophysics and Space Science》2006,304(1-4):391-395
The Kepler Mission is a photometric space mission that will continuously observe a single 100 square degree field of view (FOV) of the sky of more than 100,000 stars in the Cygnus-Lyra region for four or more years with a precision of 14 parts per million (ppm) for a 6.5 hour integration including shot noise for a twelfth magnitude star. The primary goal of the mission is to detect Earth-size planets in the habitable zone of solar-like stars. In the process, many eclipsing binaries (EB) will also be detected. Prior to launch, the stellar characteristics will have been determined for all the stars in the FOV with K<14.5. As part of the verification process, stars with transits (about 5%) will need to have follow-up radial velocity observations performed to determine the component masses and thereby separate grazing eclipses caused by stellar companions from transits caused by planets. The result will be a rich database on EBs. The community will have access to the archive for uses such as for EB modeling of the high-precision light curves. A guest observer program is also planned for objects not already on the target list. 相似文献
18.
李自新 《广东海洋大学学报》1994,(2)
传动比方案设计是行星变速箱ＣＡＤ技术中的关键问题之一。本文应用行星变速箱基本设计理论和计算机技术，建立了行星变速箱传动比方案ＣＡＤ软件，为有关科研部门及生产企业的产品开发提供了一种有效手段。 相似文献
19.
In the deep interior of the giant planets Jupiter and Saturn, ordinary hydrogen and helium are transformed into a conducting metallic liquid at extremely high pressure. It is likely that the giant planets' observed magnetic field is constantly generated in the metallic fluid core by magnetohydrodynamic processes, converting mechanic energy in the form of convection into magnetic energy. The maximum strength of their magnetic fields is likely to be limited by magnetic field instabilities which convert the magnetic energy back into convection. The parameter which governs the occurrence of magnetic instabilities is the Elsasser number, = B
2/2, where B is the field strength, is the electrical conductivity, is the rotation rate and is the density. Since magnetic instability will be very active when exceeds a critical value
c
10 (the precise value depending on the magnetic field distribution), this imposes an upper bound on the effective electrical conductivity of the metallic fluid which comprises the bulk of Jupiter's interior and much of Saturn's.Stability calculations including both toroidal (model) and poloidal (observed) components of the magnetic field in a rapidly rotating spherical shell, have been performed. The most stable configuration of the field is when the poloidal component of field is strong and the toroidal field is weak; in this case we obtain an upper bound for electrical conductivity of 3 × 106 S/m; while the most unstable configuration of the field is when the toroidal and poloidal fields are comparable, giving rise to
m
3 × 105 S/m. The implications of the results for general dynamo theory are also discussed. 相似文献
20.