共查询到20条相似文献，搜索用时 312 毫秒
1.
The response of the Earth's global mean vertical atmospheric temperature structure to large increases in the atmospheric CO2 concentration was examined using a 1-D radiative-convective atmospheric model. It was found that the greenhouse warming of the terrestrial surface can be strongly inhibited by the development of a more isothermal, moister and higher troposphere than at present. The saturation of the strong CO2 infrared bands for high CO2 concentrations further inhibits the greenhouse warming to such an extent that a runaway greenhouse fuelled only by a rise in the atmospheric CO2 is not possible. However, a continuously rising solar-constant does eventually lead to a runaway. 相似文献
2.
Jean Meeus 《Icarus》1975,26(2):257-267
Several statements, on which the recent book The Jupiter Effect is based, are invalid: The planets will not be “aligned” in 1982, and such an alignment has no effect on solar activity; planetary tides on the Sun are negligible; tides raised on the Sun by Venus, Earth, and Jupiter have a period of 4 mo, not 11 yr, and Wood's curve has no physical justification; there are not more sunspots visible at the eastern limb of the Sun than at the western one; the mean number of sunspots is the same at Venus' inferior and superior conjunctions; the influence of solar flares on sudden changes in the Earth's rotation and on earthquakes is not proved. This leads to the conclusion that there is no evidence for a correlation between planetary positions and earthquakes. The “Jupiter effect” does not exist. 相似文献
3.
Ved Mitra 《Planetary and Space Science》1974,22(4):559-568
A new theory of the superrotation of upper atmosphere is worked out on the basis of global deposition of meteoroids assuming that a certain constant influx of meteoroids is continually falling upon the Earth's atmosphere. On the average the meteoroids are shown to carry a greater amount of orbital angular momentum than that corresponding to the Earth's orbit about the Sun. It is argued that the excess of orbital angular momentum appears as extra spin angular momentum in the atmospheric layer in which the meteoroids are arrested and this is used to calculate the velocity difference which can be maintained across a certain layer of the atmosphere. It is found that a global deposition of 34 tons/day of meteoric material is required to account for the observed superrotation which agrees with the recent estimates on meteoric mass influx on the Earth. 相似文献
4.
The course of evolution of Earth's primitive reducing atmosphere is shown to possibly have been determined to a large extent by the effect of thunder shock waves, which is comparable to the effect of solar uv radiation. The major chemical reactions occurring during a thunderstorm in the troposphere were pyrolysis of hydrocarbons, their oxidation by water vapor and their reaction with molecular nitrogen. These reactions were studied by the single-pulse shock tube technique and their rates as well as their product distributions were determined.The greenhouse effect of water vapor and acetylene enhanced oxidation by water vapor and prevented the accumulation of large graphite and polymer deposits on the Earth's surface. This is in accordance with their absence on the contemporary Earth. Changes in the frequency and power of thunderstorms, within reasonable limits, affected the rate of evolution but caused only small changes in the concentration profiles of HCN and aldehydes, which are essential for further chemical evolution in the oceans. The surface temperature and relative hydrogen concentration are shown to be of prime importance in determining the course and outcome of atmospheric evolution. 相似文献
5.
James B. Pollack 《Icarus》1979,37(3):479-553
In this paper, we review the observational data on climatic change for the terrestrial planets, discuss the basic factors that influence climate, and examine the manner in which these factors may have been responsible for some of the known changes. Emphasis is placed on trying to understand the similarities and differences in both the basic factors and their climatic impacts on Venus, the Earth, and Mars. Climatic changes have occurred on the Earth over a broad spectrum of time scales that range from the elevated temperatures of Pre-Cambrian times (～109 years ago), through the alternating glacial and interglacial epochs of the last few million years, to the small but significant decadal and centurial variations of the recent past. Evidence for climatic change on Mars is given by certain channel features, which suggest an early to intermediate aged epoch of warmer and wetter climate, and by layered polar deposits, which imply more recent periodic climate variations. No evidence for climatic change on Venus exists as yet, but comparison of its present climate state with that of outer terrestrial planets offers important clues on some of the mechanisms affecting climate. The important determinants of climate for a terrestrial planet include the Sun's output, astronomical perturbations of its orbital and axial characteristics, the gaseous and particulate content of its atmosphere, its land surface, volatile reservoirs, and its interior. All these factors appear to have played major roles in causing climatic changes on the terrestrial planets. Despite a lower solar luminosity in the past, the Earth and Mars have had warmer periods in their early history. In both cases, a more reducing atmosphere may have been the responsible agent through an enhanced greenhouse effect. In this paper, we present detailed calculations of the effect of atmospheric pressure and composition on the temperature state of Mars. We find that the higher temperature period is easier to explain with a reducing atmosphere than with the current fully oxidizing one. Both the very high surface temperature and massive atmosphere of Venus may be the result of the solar flux being a factor of two higher at its orbit than at the Earth's orbit. This difference may have led to a runaway greenhouse effect on Venus, i.e., the emplacement of volatiles entirely in the atmosphere rather than mostly in surface reservoirs. But if Venus formed with relatively little or no water, it may have always had an oxidizing atmosphere. In this case, a lower solar luminosity would have led to a moderate surface temperature in Venus' early history. Quasi-periodic variations in orbital eccentricity and axial obliquity may have contributed to the alternation between Pleistocene glacial and interglacial periods in the case of the Earth and to the formation of the layered polar deposits in the case of Mars. In this paper, we postulate that two mechanisms, acting jointly, account for the creation of the laminated terrain of Mars: dust particles serve as nucleation centers for the condensation of water vapor and carbon dioxide. The combined dust-H2O-CO2 particle is much larger and so has a much higher terminal velocity than either a dust-H2O or a plain dust particle. As a result, dust and water ice are preferentially deposited in the polar regions. In addition, we postulate that the obliquity variations are key drivers of the periodic layering because of their impact on both atmospheric pressure and polar surface temperature, which, in turn, influence the amounts of dust and water ice in the atmosphere. But eccentricity and precessional changes probably also play important roles in creating the polar layers. The drifting of continents on the Earth has caused substantial climatic changes on individual continents and may have helped to set the stage for the Pleistocene ice ages through a positioning of the continents near the poles. While continental drift apparently has not occurred on Mars, tectonic distortions of its lithosphere may, in some circumstances, cause an alteration in the mean value of that planet's obliquity, which would significantly impact its climate. Atmospheric aerosols can influemce climate through their radiative effects. In the case of the Earth, volcanic aerosols appear to have contributed to past climatic changes, while consideration needs to be given to the future impact of man-generated aerosols. In the case of Mars, the atmospheric temperature structure and thereby atmospheric dynamics are greatly altered by suspended dust particles. The sulfuric acid clouds of Venus play a major role in its heat balance. Cometary impacts may have added substantial quantities of water vapor and sulfur gases to Venus' atmosphere and thus have indirectly affected its cloud properties. Calculations presented in this paper indicate substantial changes in surface temperature accompany these compositional changes. 相似文献
6.
Herbert Frey 《Icarus》1977,32(2):235-250
The Earth's original ocean basins are proposed to be mare-type basins produced 4 billion y.a. by the flux of asteroid-sized objects responsible for the lunar mare basins. Scaling upward from the observed number of lunar basins for the greater capture cross-section and impact velocity of the Earth indicates that at least 50% of an original global crust would have been converted to basin topography. These basins were flooded by basaltic liquids in times short compared to the isostatic adjustment time for the basin. The modern crustal dichotomy (60% oceanic, 40% continental crust) was established early in the history of the Earth, making possible the later onset of plate tectonic processes. These later processes have subsequently reworked, in several cycles, principally the oceanic parts of the Earth's crust, changing the configuration of the continents in the process. Ocean basins (and oceans themselves) may be rare occurences on planets in other star systems. 相似文献
7.
George Gatewood Lee Breakiron Ronald Goebel Steven Kipp Jane Russell John Stein 《Icarus》1980,41(2):205-231
Extensive testing suggests that astrometric techniques can be employed to detect and study virtually any planetary system that may exist within 40 light years (12.5 parsec) of the Sun. Following the conclusion of Paper I [G. Gatewood, Icarus27 (1976), 1–12], the astrometric group at the Allegheny Observatory began an intensive survey of 20 nearby stars to detect the nonlinear variations in their motion that planetary systems would induce. Several tests conducted to further our understanding of the limitations of this survey indicated that the photographic detector itself is responsible for the majority of the random error. A new photoelectric detector has been designed and a simplified prototype of it successfully tested. The new detector is expected to be able to utilize virtually all of the astrometric information transmitted through the Earth's atmosphere. This is sufficient to determine relative positions to within an accuracy of approximately 1 milliarcsec/hr. Such precisions exceed the design capabilities of the best existing astrometric telescopes, thus a feasibility study has been conducted for the design of an improved instrument. The study concludes that a new ground-based telescope and the new detector combined should be able to study stars as faint as the 17th magnitude with an annual accuracy of a few tenths of a milliarcsecond. However, to obtain the ultimate accuracy possible from current technology, we must place an astrometric system above the Earth's atmosphere. A space-borne instrument utilizing the new detector would in theory have sufficient accuracy to detect any Earth-like planet orbiting any of the several hundred stars nearest the Sun. 相似文献
8.
Climatic models are increasingly being used to answer “cosmic questions” such as the possibility of an ice-covered Earth or a runaway greenhouse effect, or to examine the coevolution of climate and life. Conclusions from these models on such issues, of course, rest on the physical parameterizations of the models. Some of the basic parameterizations are reexamined quantitatively, and it is concluded that presently believed uncertainties in these parameterizations lead to an order-of-magnitude uncertainty in estimates of the sensitivity of the present Earth's climate to external forcings (like a change in solar constant). However, seasonal simulations with present Earth models suggest that estimates of the overall sensitivity of the climate to external forcing may be narrowed (over decadal time scales) to, perhaps, a factor of 2. But the effects of glaciers, continental locations, and atmospheric composition, all of which can change on geological time scales, further enhance the uncertainties in long-term climatic sensitivity estimates from state-of-the-art models. But it is precisely these long-term estimates of climatic sensitivity which support quantitative conclusions on, for example, the possible existence of continuously habitable zones around main-sequence stars. We believe that those who draw cosmic conclusions from climatic models should at least attempt to bracket the final results by repeating their calculations over a plausible range of uncertainty in basic model parameterizations. 相似文献
9.
Using 1.55cm observations of the Earth made by the Electrically Scanned Microwave Radiometer experiment on Nimbus 5, the appearance of the Earth from Venus is simulated. A single antenna unable to resolve the Earth's disk would give a time averaged disk temperature of 183K. In one rotation, the disk temperature would vary from 194K to 172K. During the 1973 inferior conjunction, a radio telescope with 1 arcsec resolution would resolve most of the major surface features of the Earth. 相似文献
10.
Hydrodynamic and electrodynamic problems of solar wind interaction with the Earth's magnetosphere on the day-side are investigated.The initial fact, well established, is that the density of the magnetic field energy in the solar wind is rather small. Magnetic field intensity and orientation are shown to determine the character of the solar wind flow around the magnetosphere. For mean parameters of the wind, if the tangential component of the magnetic field is more or equal 5γ, the flow in the magneto-sheath will be laminar. For other cases the flow is of a turbulent type.For turbulent flow, typical plasma parameters are estimated: mean free path, internal scale of inhomogeneities and dissipated energy. The results obtained are compared with experimental data.For the case of laminar flow, special attention is paid to the situation when magnetic fields of the solar wind and Earth are antiparallel. It is suggested, on the basis of solid arguments, that the southward interplanetary field diffuses from the magnetosheath into the Earth's magnetosphere. These ideas are used for the estimation of the distance to the magnetopause subsolar point. A detailed comparison with results of observation is made. The coincidence is satisfactory. Theoretical investigation has been made to a great extent for thin magnetopause with thickness δ ～ RHe-gyroradius of an electron.It is shown that during magnetospheric substorms relaxation oscillations with the period τ = 100–300 sec must appear. A theorem is proved about the appearance of a westward electrical field during the substorm development, when the magnetosphere's day-side boundary moves Earthward and about the recovery phase, when the magnetopause motion is away from the Earth, when there is an eastward electrical field.In the Appendix, plasma wave exitation in the magnetopause is considered and conductivity magnitudes are calculated, including the reduction due to the scattering by plasma turbulence. 相似文献
11.
High angular-resolution measurements of directional fluxes of solar particles in space have been obtained with detectors aboard OGO-5 during the cosmic ray event of 18 November 1968. This is the only case on record for which sharply-defined directional observations of protons and electrons covering a wide rigidity range (0.3 MV to 1.5 GV) are available.The satellite experiment provided data for determining pitch-angle distributions with respect to the direction of the local interplanetary magnetic field lines during the lengthy highly anisotropic phase of the event. It was found that the unidirectional differential intensities j(θ) of 3- to 25-MeV protons varied in accordance with the relationship j(θ) = b0 + b1cosθ + b2cos2θ, where b0 and b1 ? 0, and b2, is positive, zero or negative. Soon after onset, 79–266-keV electrons arriving from the direction of the Sun displayed an anisotropic component with the intensity varying as cos θ. Later, a double-peaked distribution appeared at the lower energies, whereas the flux at the upper end of the range covered by the experiment became isotropic. These results have been interpreted in the light of the temporal flux profiles and the state of the interplanetary medium.The observation of the unusually large and long-lasting anisotropies lead to several conclusions including: (1) If injection of the solar particles was instantaneous, the diffusion coefficient was either constant or increasing with distance from the Sun. (2) If the solar source emitted particles over an extended period, and there is evidence to that effect, there was weak scattering in the region between the Sun and the Earth and a strong scattering region beyond the Earth's orbit. (3) Solar electrons were stored near the Sun. (4) The observed angular distribution of 200-MV protons in the magnetosheath was in good agreement with that deduced in an earlier analysis of polar orbiting satellite observations and trajectory calculations. 相似文献
12.
Existing instruments are unable to detect planets about stars other than the Sun but such detection would be important for the theory of origin of our solar system and in the search for extraterrestrial intelligence. Infrared offers an advantage of about 105 over visible light as regards the ratio of power received from star and planet. Infrared interferometry from Earth orbit would allow discrimination against the stellar infrared by the placement of an interference null on the star and a spinning infrared interferometer would modulate the planetary emission to permit extraction by synchronous detection from the background level. The limit to sensitivity will be set by thermal emission from the zodiacal light particles near the Earth's orbit unless the interferometer is launched out of the ecliptic or out to the orbit of Jupiter, in which case instrumental limitations will dominate. Technological developments in several fields will be required as also with astrometry, spectroscopic radial velocity measurement, and direct photography from orbit, three approaches with which infrared interferometry should be carefully compared. 相似文献
13.
The radiogenic and primordial noble gas content of the atmospheres of Venus, Earth, and Mars are compared with one another and with the noble gas content of other extraterrestial samples, especially meteorites. The fourfold depletion of 40Ar for Venus relative to the Earth is attributed to the outgassing rates and associated tectonics and volcanic styles for the two planets diverging significantly within the first billion or so years of their history, with the outgassing rate for Venus becoming much less than that for the Earth at subsequent times. This early divergence in the tectonic style of the two planets may be due to a corresponding early onset of the runaway greenhouse on Venus. The 16-fold depletion of 40Ar for Mars relative to the Earth may be due to a combination of a mild K depletion for Mars, a smaller fraction of its interior being outgassed, and to an early reduction in its outgassing rate. Venus has lost virtually all of its primordial He and some of its radiogenic He. The escape flux of He may have been quite substantial in Venus' early history, but much diminished at later times, with this time variation being perhaps strongly influenced by massive losses of H2 resulting from efficient H2O loss processes.Key trends in the primordial noble gas content of terrestial planetary atmospheres include (1) a several orders of magnitude decrease in 20Ne and 36Ar from Venus to Earth to Mars; (2) a nearly constant 20Ne/36Ar ratio which is comparable to that found in the more primitive carbonaceous chondrites and which is two orders of magnitude smaller than the solar ratio; (3) a sizable fractionation of Ar, Kr, and Xe from their solar ratios, although the degree of fractionation, especially for 36Ar/132Xe, seems to decrease systematically from carbonaceous chondrites to Mars to Earth to Venus; and (4) large differences in Ne and Xe isotopic ratios among Earth, meteorites, and the Sun. Explaining trends (2), (2) and (4), and (1) pose the biggest problems for the solar-wind implantation, primitive atmosphere, and late veneer hypotheses, respectively. It is suggested that the grain-accretion hypothesis can explain all four trends, although the assumptions needed to achieve this agreement are far from proven. In particular, trends (1), (2), (3), and (4) are attributed to large pressure but small temperature differences in various regions of the inner solar system at the times of noble gas incorporation by host phases; similar proportions of the host phases that incorporated most of the He and Ne on the one hand (X) and Ar, Kr, and Xe on the other hand (Q); a decrease in the degree of fractionation with increasing noble-gas partial pressure; and the presence of interstellar carriers containing isotopically anomalous noble gases.Our analysis also suggests that primordial noble gases were incorporated throughout the interior of the outer terrestial planets, i.e., homogeneous accretion is favored over inhomogeneous accretion. In accord with meteorite data, we propose that carbonaceous materials were key hosts for the primordial noble gases incorporated into planets and that they provided a major source of the planets' CO2 and N2. 相似文献
14.
The observed density of Venus is about 2% smaller than would be expected if Venus were a twin planet of the Earth, possessing an identical internal composition and structure. In principle, this could be explained by a process of physical segregation of metal particles from silicate particles in the solar nebula prior to accretion, so that Venus accreted from relatively metal-depleted material. However, this model encounters severe difficulties in explaining the nature of the physical segregation process and also the detailed chemical composition of the Earth's mantle. Two alternative hypotheses are examined, both of which attempt to explain the density difference in terms of chemical fractionation processes. Both of these hypotheses assume that the relative abundances of the major elements Fe, Si, Mg, Al, and Ca are similar in both planets. According to the first hypothesis, a larger proportion of the total iron in Venus is present as iron oxide in the mantle, so that the core-to-mantle ratio is smaller than in the Earth. This model implies that Venus is more oxidized than the Earth, with its lower intrinsic density (i.e., corrected to equivalent pressures and temperatures) due to the larger amount of oxygen present. The difference between oxidation states is attributed to differing degrees of accretional heating arising from the relatively smaller mass of Venus. On the other hand, the second hypothesis maintains that Venus is more reduced than the Earth, with its mantle essentially devoid of oxidized iron. The difference intrinsic densities is attributed to the Earth accreting at a lower temperature than Venus as a result of the Earth's greater distance from the center of the nebula. As a result, large amounts of sulfur accreted on the Earth but not on Venus. The sulfur, which entered the core, is believed to have increased the mean density of the Earth because of its relatively high atomic weight. The hypothesis also implies that most of the Earth's potassium, because of its chalcophile properties, entered the core.These hypotheses are evaluated in the light of existing data. The second hypothesis leads to an intrinsic density for Venus which is only 0.4% smaller than that of the Earth. This difference is much smaller than is believed to exist. A wide range of chemical evidence is found to be unfavorable to this second hypothesis, but to be consistent with the interpretation that Venus is more oxidized than the Earth, as required by the first hypothesis. 相似文献
15.
Marco Tantardini Elena Fantino Yuan Ren Pierpaolo Pergola Gerard Gómez Josep J. Masdemont 《Celestial Mechanics and Dynamical Astronomy》2010,108(3):215-232
Of the three collinear libration points of the Sun–Earth Circular Restricted Three-Body Problem (CR3BP), L3 is that located opposite to the Earth with respect to the Sun and approximately at the same heliocentric distance. Whereas
several space missions have been launched to the other two collinear equilibrium points, i.e., L1 and L2, taking advantage of their dynamical and geometrical characteristics, the region around L3 is so far unexploited. This is essentially due to the severe communication limitations caused by the distant and permanent
opposition to the Earth, and by the gravitational perturbations mainly induced by Jupiter and the close passages of Venus,
whose effects are more important than those due to the Earth. However, the adoption of a suitable periodic orbit around L3 to ensure the necessary communication links with the Earth, or the connection with one or more relay satellites located at
L4 or L5, and the simultaneous design of an appropriate station keeping-strategy, would make it possible to perform valuable fundamental
physics and astrophysics investigations from this location. Such an opportunity leads to the need of studying the ways to
transfer a spacecraft (s/c) from the Earth’s vicinity to L3. In this contribution, we investigate several trajectory design methods to accomplish such a transfer, i.e., various types
of two-burn impulsive trajectories in a Sun-s/c two-body model, a patched conics strategy exploiting the gravity assist of
the nearby planets, an approach based on traveling on invariant manifolds of periodic orbits in the Sun–Earth CR3BP, and finally
a low-thrust transfer. We examine advantages and drawbacks, and we estimate the propellant budget and time of flight requirements
of each. 相似文献
16.
Photochemical Chapman theory predicts that the square of peak electron density, Nm, in the dayside ionosphere of Mars is proportional to the cosine of solar zenith angle. We use Mars Global Surveyor Radio Science profiles of electron density to demonstrate that this relationship is generally satisfied and that positive or negative residuals between observed and predicted values of are caused by periods of relatively high or low solar flux, respectively.Understanding the response of the martian ionosphere to changes in solar flux requires simultaneous observations of the martian ionosphere and of solar flux at Mars, but solar flux measurements are only available at Earth. Since the Sun's output varies both in time and with solar latitude and longitude, solar flux at Mars is not simply related to solar flux at Earth by an inverse-square law. We hypothesize that, when corrected for differing distances from the Sun, solar fluxes at Mars and Earth are identical when shifted in time by the interval necessary for the Sun to rotate through the Earth–Sun–Mars angle.We perform four case studies that quantitatively compare time series of Nm at Mars to time series of solar flux at Earth and find that our hypothesis is satisfied in the three of them that used ionospheric data from the northern hemisphere. We define a solar flux proxy at Mars based upon the E10.7 proxy for solar flux at Earth and use our best case study to derive an equation that relates Nm to this proxy. We discuss how the ionosphere of Mars can be used to infer the presence of solar active regions not facing the Earth.Our fourth case study uses ionospheric observations from the southern hemisphere at latitudes where there are strong crustal magnetic anomalies. These profiles do not have Chapman-like shapes, unlike those of the other three case studies. We split this set of measurements into two subsets, corresponding to whether or not they were made at longitudes with strong crustal magnetic anomalies. Neither subset shows Nm responding to changes in solar flux in the manner that we observe in the three other case studies.We find many similarities in ionospheric responses to short-term and long-term changes in solar flux for Venus, Earth, and Mars. We consider the implications of our results for different parametric equations that have been published describing this response. 相似文献
17.
L. Trafton 《Icarus》1984,58(2):312-324
Triton's seasons differ materially from those of Pluto owing to four important differences in the governing physics: First, the obliquity of Triton is significantly less than Pluto's obliquity. Second, Triton's inclined orbit precesses rapidly about Neptune so that a complicated seasonal variation in the latitude of the Sun occurs for Triton. Third, Neptune's orbit is much more circular than Pluto's orbit so that the sunlight intercepted by Triton's disk does not vary seasonally. Finally, Triton's atmosphere cannot be saturated at the lower latitudes so that the mass of the atmosphere is controlled by the temperature of the high-latitude ices or liquids (polar caps), as for CO2 on Mars. The consequences of Triton's entire surface being covered with volatile substances have been examined. It is found that the circularity of Neptune's orbit then implies that Triton would have hardly any seasonal variation at all in surface temperature or atmospheric bulk, in spite of the complicated precessional effects of Triton's orbit. The only seasonal effect would be the migration of surface ices and liquids. This scenario is ruled out because it implies a column CH4 abundance much higher than that observed and because it quickly depletes the lower latitudes of volatiles. It is concluded that Triton's most volatile surface substances are probably relegated to latitudes higher than 35° and probably form polar caps. The temperature of the polar caps should be nearly equal, even during midwinter/midsummer when the insolation of the summer pole is greatest. If the summer pole completely sublimates during one of the “major” summers, Triton's atmosphere may begin to freeze out over the winter caps. It is therefore expected that Triton's atmosphere undergoes large and complex seasonal variations. Triton is currently approaching a “maximum southern summer”, and over the remainder of this century, a dramatic increase in CH4 abundance above the current upper limit of 1 m-Am may be witnessed. 相似文献
18.
Habitable zones about main sequence stars 总被引：1，自引：0，他引：1
Michael H. Hart 《Icarus》1979,37(1):351-357
Calculations show that a main sequence star which is less massive than the Sun has a continuously habitable zone about it which is not only closer in than the corresponding zone about the Sun, but is also relatively narrower. Let L(t) represent the luminosity after t billion years of a main sequence star of mass M, and let rinner and router represent the boundaries of the continuously habitable zone about such a star—that is, the zone in which an Earthlike planet will undergo neither a runaway greenhouse effect in the early stages of its history nor runaway glaciation after it develops an oxidizing atmosphere. Then our computer results indicate that is roughly proportional to . This ratio is smaller for stars less massive than the Sun (because they evolve more slowly), and the width of the continuously habitable zone about a main sequence star is therefore a strong function of the initial stellar mass. Our calculations show that rinner = router for (i.e., K1 stars), and it therefore appears that there is no continuously habitable zone about most K stars, nor any about M stars. 相似文献
19.
H.I. Schiff 《Planetary and Space Science》1981,29(9):935-950
An enhanced “greenhouse effect” due to an increase in atmospheric CO2 is expected to produce significant climatic changes. If the combustion of fossil fuels is the only anthropogenic source of atmospheric CO2, measurements show that 54% resides in the atmosphere. The largest reservoir for the remaining 46% is the oceans. Known oceanic processes can account for 35% and the major uncertainty appears to be the role played by the intermediate waters. If, however, deforestation is as large a source of additional atmospheric CO2 as some have suggested, carbon balance cannot be obtained with presently identified removal processes. Various computer models have been used to calculate the effects of increasing atmospheric CO2. These include energy balance, radiative-convective and general circulation models (GCM's). Many feedback mechanisms must be considered including water vapour, clouds, oceans and the cryosphere. Although representing a considerable advance over other models, GCM's still require many approximations, of which the treatment of oceans and clouds are the most questionable. These models predict, for the scenario of the doubling of atmospheric CO2, an increase in global surface temperature of about 3°C with larger increases, up to 10° at higher latitudes. Significant changes in evaporation and precipitation patterns are also indicated. 相似文献
20.
William K. Hartmann 《Icarus》1976,27(4):553-559
Significant fractions of each planet's late-accreted mass originated not at its own distance from the Sun, but from a neighboring planet's orbit, according to results that follow from calculations by Wetherill (1975). “Late-accreted” refers to a loosely defined period after planets acquired most of their present mass. In an idealized model, Mercury, Venus, Earth, and Mars received 47, 45, 37, and 52% of their late-accreted mass from planetesimals formed closer to other planets. Resulting compositional anomalies in outer parts of early planets could be significant; atmospheric tests of Lewis's predicted S deficiency on Venus may be inconclusive.The Moon's orbit around Earth puts it in a special category: sorting occurs between Moon-impacting and Earth-impacting material according to approach velocity. In the above model, the moon receives 60% of its late-accreted mass from planetesimals formed near Venus' orbit. Distant planetesimals could be perturbed into the Earth-Moon system and cause major changes in the Moon's composition with only minor effect on Earth. The entire lunar bulk composition anomaly could be explained by plausible reservoirs of distant low-density material. 相似文献