Venus-intercepting meteoroid streams     

Martin Beech 《Monthly notices of the Royal Astronomical Society》1998,294(2):259-264

This study is motivated by the possibility of determining the large-body meteoroid flux at the orbit of Venus. Towards this end, we attempt to estimate the times at which enhanced meteoric activity might be observed in the planet's atmosphere. While a number of meteoroid streams are identified as satisfying common Earth and Venus intercept conditions, it is not clear from the Earth-observed data if these streams contain large-body meteoroids. A subset of the Taurid Complex objects may produce fireball-rich meteor showers on Venus. A total of 11 short-period, periodic comets and 46 near-Earth asteroids approach the orbit of Venus to within 0.1 au, and these objects may have associated meteoroid streams. Comets 27P/Crommelin and 7P/Pons–Winnecke are identified as candidate parents to possible periodic meteor showers at the orbit of Venus.  相似文献   

THE IODINE FACTOR IN HEALTH AND EVOLUTION*     

JEROME E. DOBSON 《Geographical review》1998,88(1):3-28

ABSTRACT. Iodine is essential for modern humans and may have been essential for Neandertals as well. Today about 30 percent of the world's population is at risk of iodine deficiency disorders (idd ), 750 million people suffer from goiter, 43 million have idd -related brain damage and mental retardation, and 5.7 million are afflicted by cretinism, the most severe form of idd . Distinctive Neandertal skeletal traits are identical to those of modern humans who suffer from cretinism. Cro-Magnon Venus figurines also exhibit distinctive traits associated with cretinism among modern humans. This new evidence, coupled with recent mitochondrial dna findings, suggests that a single genetic alteration, which improved the ability of the thyroid gland to extract and utilize iodine, may account for differences between Neandertals and modern humans. Late Pleistocene human evolution, consequently, may be explained by several alternative interpretations involving iodine pathology and/or biological adaptation. Speciation may have resulted from the geographical isolation of inland populations.  相似文献   

金星非单调冷却热演化历史分析          下载免费PDF全文

张健  石耀霖 《地球物理学报》2007,50(1):146-152

金星表层年龄和构造活动特点表明其岩石层在最近的地质历史时期经历过广泛的更新.这种全球性的表层改造与其内部热演化历史进程密切相关.如果金星存在相变形成的上、下地幔,依据现今所了解的金星物理性质和参量化的热对流理论,并且考虑金星地幔相变边界层状态对对流的控制作用,我们计算了金星热演化历史.结果表明,金星的热演化历史是一种非单调的冷却过程,在这种非单调的热演化历史进程中,金星地幔会出现大体等周期的翻转.由于参数选取的不同,翻转时金星上、下地幔的温差随时间可能出现稳定变化、逐渐加强、逐渐减弱三种不同演化模式,目前尚不能确定实际金星热演化历史究竟是哪一种模式.金星地幔相变边界层的穿透对流可能是推动其表层岩石层全球性更新的关键,导致其表层火山活动和地表构造以大致500 Ma时间间隔更新和重造.  相似文献   

Modeling crater populations on Venus and Titan     

D.G. Korycansky  K.J. Zahnle 《Planetary and Space Science》2005,53(7):695-710

We describe a model for crater populations on planets and satellites with dense atmospheres, like those of Venus and Titan. The model takes into account ablation (or mass shedding), pancaking, and fragmentation. Fragmentation is assumed to occur due to the hydrodynamic instabilities promoted by the impactors’ deceleration in the atmosphere. Fragments that survive to hit the ground make craters or groups thereof. Crater sizes are estimated using standard laws in the gravity regime, modified to take into account impactor disruption. We use Monte Carlo methods to pick parameters from appropriate distributions of impactor mass, zenith angle, and velocity. Good fits to the Venus crater populations (including multiple crater fields) can be found with reasonable values of model parameters. An important aspect of the model is that it reproduces the dearth of small craters on Venus: this is due to a cutoff on crater formation we impose, when the expected crater would be smaller than the (dispersed) object that would make it. Hydrodynamic effects alone (ablation, pancaking, fragmentation) due to the passage of impactors through the atmosphere are insufficient to explain the lack of small craters. In our favored model, the observed number of craters (940) is produced by ∼5500 impactors with masses , yielding an age of (1-σ uncertainty) for the venusian surface. This figure does not take into account any uncertainties in crater scaling and impactor population characteristics, which probably increase the uncertainty to a factor of two in age.We apply the model with the same parameter values to Titan to predict crater populations under differing assumptions of impactor populations that reflect present conditions. We assume that the impactors (comets) are made of 50% porous ice. Predicted crater production rates are ≈190 craters . The smallest craters on Titan are predicted to be in diameter, and ≈5 crater fields are expected. If the impactors are composed of solid ice (density ), crater production rates increase by ≈70% and the smallest crater is predicted to be in diameter. We give cratering rates for denser comets and atmospheres 0.1 and 10 times as thick as Titan's current atmosphere. We also explicitly address leading-trailing hemisphere asymmetries that might be seen if Titan's rotation rate were strictly synchronous over astronomical timescales: if that is the case, the ratio of crater production on the leading hemisphere to that on the trailing hemisphere is ≈4:1.  相似文献   

Atmospheric impacts, fragmentation, and small craters on Venus     

D.G. Korycansky  Kevin J. Zahnle 《Icarus》2004,169(2):287-299

We use high-resolution three-dimensional numerical models of aerodynamically disrupted asteroids to predict the characteristic properties of small impact craters on Venus. We map the mass and kinetic energy of the impactor passing though a plane near the surface for each simulation, and find that the typical result is that mass and energy sort themselves into one to several strongly peaked regions, which we interpret as more-or-less discrete fragments. The fragments are sufficiently well separated as to imply the formation of irregular or multiple craters that are quite similar to those found on Venus. We estimate the diameters of the resulting craters using a scaling law derived from the experiments of Schultz and Gault (1985, J. Geophys. Res. 90 (B5), 3701-3732) of dispersed impactors into targets. We compare the spacings and sizes of our estimated craters with measured diameters tabulated in a Venus crater database (Herrick and Phillips, 1994a, Icarus 111, 387-416; Herrick et al., 1997, in: Venus II, Univ. of Arizona Press, Tucson, AZ, pp. 1015-1046; Herrick, 2003, http://www.lpi.usra.edu/research/vc/vchome.html) and find quite satisfactory agreement, despite the uncertainty in our crater diameter estimates. The comparison of the observed crater characteristics with the numerical results is an after-the-fact test of our model, namely the fluid-dynamical treatment of large impacts, which the model appears to pass successfully.  相似文献   

关于金星自转问题的商榷     

黄沅 《地球科学与环境学报》1997,(Z1)

本研究继哥白尼“日心说”理论，用一种新天体演化论揭示金星逆转是视运动，金星的真正运动是在内能释放、磁场减弱；体积增大、自转速率减慢；旋转轴趋于直立、公转轨道接近正圆；从自转天体向无自转天体（惰性天体）的演化过程。研究认为．天体是在物质聚集下形成，能量释放中发生演变（衰亡），所有的天体都将遵循这种规律。故本研究以水星向同步自转天体演化，月球恰是颗同步自转天体，金星向无自转天体演化为据，论证了金星自转是正转而不是逆转，同时通过模拟实验也证明这一结论的正确性，这都充分说明金星逆转违背了哥白尼体系（Ｃｏｐｅｒｎｉｃａｎｓｙｓｔｅｍ），而传统以地球为中心的认识应该废弃。  相似文献   

Tectonic and stratigraphic implications of the relative ages of venusian plains and wrinkle ridges     

George E. McGill 《Icarus》2004,172(2):603-612

A major ongoing controversy concerns the style of crustal evolution on Venus. At one extreme is a directional model that proposes a sequence of depositional and deformational events that occur at specific times in the evolution of the crust and that are global in extent. At the other extreme is a model that argues for different ages of these events in different places on the planet. A test of the directional model is here focused on whether wrinkle ridges formed at a single time in the recorded crustal history of Venus. Where sets of wrinkle ridges intersect it commonly is possible to determine that one set is older than the other. Also, the deformation responsible for wrinkle ridges is, in places, clearly progressive with respect to stratigraphic material units. These observations are not consistent with a specific single time for the formation of wrinkle ridges within the stratigraphic sequence. Within an area including about 1/3 of the surface of Venus 15% of craters that are younger than regional plains are older than wrinkle ridges, 85% are younger than wrinkle ridges. Taking 750 myr as a reasonable mean age for the regional plains, this implies that the mean age of wrinkle ridges is ∼110 myr younger than the mean age of plains. Solomon et al. (1999, Science 286, 87) propose that the emplacement of a large volume of plains lava would lead to a major atmospheric temperature increase. Their model predicts thermal stresses in the lithosphere that, at shallow depth, would reach peak compressive stresses in about 100 myr, a number very similar to the time lag between plains emplacement and wrinkle ridge formation indicated by the crater data. The thermal compressive stresses responsible for wrinkle ridges would be maintained at a level sufficient to deform basalt for at least 100 myr and possibly for as long as 350 myr. These time intervals are not really short compared to the mean age of the plains. Finally, because wrinkle ridges are demonstrably younger than the plains they deform, they cannot be related to the processes that formed the plains and thus should not be used to define a “plains with wrinkle ridges” unit.  相似文献   

Venus phase function and forward scattering from H₂SO₄     

Anthony Mallama  Dennis Wang 《Icarus》2006,182(1):10-22

Ground-based and spacecraft photometry covering phase angles from 2° to 179° has been acquired in wavelength bands from blue to near infrared. An unexpected brightness surge is seen in the B and V bands when the disk of Venus is less than 2% illuminated. This excess luminosity appears to be the result of forward scattering from droplets of H₂SO₄ (sulfuric acid) in the high atmosphere of Venus. The fully sunlit brightness of Venus, adjusted to a distance of one AU from the Sun and observer, was found to be V=−4.38, and the corresponding geometric albedo is 67%. The phase integral is 1.35 and the resulting spherical albedo is 90%. Comparison between our data and photometry obtained over the past 50 years indicates a bias in the older photoelectric results, however atmospheric abundance variations suggest that brightness changes may have occurred too.  相似文献   

Hydroxyl airglow on Venus in comparison with Earth     

A. Migliorini  G. Piccioni  P. Drossart 《Planetary and Space Science》2011,59(10):974-980

Hydroxyl nightglow is intensively studied in the Earth atmosphere, due to its coupling to the ozone cycle. Recently, it was detected for the first time also in the Venus atmosphere, thanks to the VIRTIS-Venus Express observations. The main Δν=1, 2 emissions in the infrared spectral range, centred, respectively, at 2.81 and 1.46 μm (which correspond to the (1-0) and (2-0) transitions, respectively), were observed in limb geometry (Piccioni et al., 2008) with a mean emission rate of 880±90 and 100±40 kR (1R=10⁶ photon cm⁻² s⁻¹ (4πster)⁻¹), respectively, integrated along the line of sight. In this investigation, the Bates-Nicolet chemical reaction is reported to be the most probable mechanism for OH production on Venus, as in the case of Earth, but HO₂ and O may still be not negligible as mechanism of production for OH, differently than Earth. The nightglow emission from OH provides a method to quantify O₃, HO₂, H and O, and to infer the mechanism of transport of the key species involved in the production. Very recently, an ozone layer was detected in the upper atmosphere of Venus by the SPICAV (Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus) instrument onboard Venus Express (Montmessin et al., 2009); this discovery enhances the importance of ozone to the OH production in the upper atmosphere of Venus through the Bates-Nicolet mechanism. On Venus, OH airglow is observed only in the night side and no evidence has been found whether a similar emission exists also in the day side. On Mars it is expected to exist both on the day and night sides of the planet, because of the presence of ozone, though OH airglow has not yet been detected.In this paper, we review and compare the OH nightglow on Venus and Earth. The case of Mars is also briefly discussed for the sake of completeness. Similarities from a chemical and a dynamical point of view are listed, though visible OH emissions on Earth and IR OH emissions on Venus are compared.  相似文献   

Tectonic evolution of Ovda Regio: An example of highly deformed continental crust on Venus?     

I. Romeo  R. Capote 《Planetary and Space Science》2011,59(13):1428-1445

A detailed structural analysis of several selected areas of Ovda Regio provides evidence of a complex tectonic evolution. We have reported thrusting in the marginal fold belts indicating together with the presence of short-wavelength folds a significant amount of shortening. Extensional tectonics postdate at least in some locations contraction, while the contrary was not observed. Both contraction and extension occur on a complex layered crust yielding contemporary structures of different wavelengths. The thrust and fold belts of the plateau margins are characterized by concentric contraction followed by concentric contraction with perpendicular extension and finally radial extension. Deformation in the thrust and fold belts of Ovda margins is gradually transmitted to the external plains. A complex tectonic history has been revealed in the internal area of Ovda, basically characterized by contraction in different directions generating basin and dome interference at different wavelengths. Small amounts of a non-coaxial component of deformation have been observed both in the margins and in the central area of the plateau. All the reported observations can be explained if Ovda Regio is a continent that survived a global subduction event.  相似文献