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1.
Strong evidence that Io's sodium emission is due to resonant scattering is given by our observations which show a monotonic increase of emission intensity with residual solar intensity. In addition we detected no emission during three eclipse observations of Io. We propose a resonant scattering model with two spacial components comprising an optically thick atmosphere extending 103 km above Io's surface surrounded by an optically thin cloud which forms a partial torus around Jupiter. In this model a flux of 107 cm?2 sec?1 sodium atoms are sputtered from Io's surface by heavy energetic ions which are accelerated in a plasma sheath around Io. The atoms sputtered from the surface collide with atoms in Io's atmosphere so the equipartition of kinetic energy is established. The total sodium abundance is about 3 × 1013 cm?2. During Io's day, sodium and other atmospheric constituents are ionized, giving rise to the ionosphere observed by Pioneer 10. Atoms escape by means of Jeans escape from the critical level, which is at the top of the atmosphere and the base of the cloud. We have observed sodium emission 6arcsec (6 Io diameters) above and below Io's orbital plane and 23arcsec toward Jupiter in Io's orbital plane. No emission was detected at maximum elongation 180° from Io. We interpret these results to mean that atoms escaping from Io form a partial torus whose thickness is about 12 arcsec and whose length is at least one-fifth of Io's orbital circumference. 相似文献
2.
The Io plasma torus, composed of mostly heavy ions of oxygen and sulfur, is sustained by an Iogenic mass loading rate of ∼1030 amu s−1 = 1.6 × 1028 SO2 s−1 or approximately 103 kg s−1(A.L. Broadfoot et al., 1979, Science 204, 979-982). We argue on the basis of available power sources, reanalysis of F. Bagenal (1997, Geophys. Res. Lett. 24, 2111-2114), HST UV remote sensing, and detailed model calculations that at most 20% of this mass leaves Io in the form of ions, i.e., ≤3 × 1027 × (ne,0/3600 cm−3) ions s−1, where ne,0 is the average torus electron density. For the Galileo spacecraft Io pass in December 1995, the ion mass loading rate was ≤3 × 1027 ions s−1, whereas for the Voyager epoch with lower ne,0 (=2000 cm−3), this rate would be ≤1.7 × 1027 ions s−1, consistent with the D.E. Shemansky (1980, Astrophys. J. 242, 1266-1277) mass loading limit of ≤1 × 1027 ions s−1. We investigate the processes that control Io’s large scale electrodynamic interaction and find that the elastic collision rate exceeds the ionization/pickup rate by at least a factor of 5 for all atmospheric column densities considered (1016-1021 m−2) and by a factor of ∼100 for the most realistic column density. Consequently, elastic collisions are mostly responsible for Io’s high conductances and thus generate Io’s large scale electrodynamic interaction such as the generation of Io’s electric current system and the slowing of the plasma flow. The electrodynamic part of Io’s interaction is thus best described as an ionosphere-like interaction rather than a comet-like interaction. An analytic expression for total electron impact rates is derived for Io’s atmosphere, which is independent of any particular model for the 3D interaction of torus electrons with its atmosphere. 相似文献
3.
Douglas B. Nash 《Icarus》1983,54(3):511-523
The role of adsorbed SO2 on Io's surface particles in producing the observed spectral absorption band near 4 μm in Io's reflectance spectrum is explored. Calculations show that a modest 50% monolayer coating of adsorbed SO2 molecules on submicron grains of sulfur of alkali sulfide, assumed to make up Io's uppermost optical surface (“radialith”), will result in a ν1 + ν3 absorption band near 4 μm with depth ～30% below the adjacent continuum, consistent with the observed strength of the Io band. The precise wavelength position of the ν1 + ν3 band of SO2 in different phase states such as frost, ice, adsorbate, and gas are summarized from the experimental literature and compared with the available telescopic measurements of the Io band position. The results suggest that the 4-μm band in Io's full disk spectrum can best be explained by the presence on Io's surface of widespread SO2 in the form of adsorbate rather than ice or frost. 相似文献
4.
Models for the distribution of sodium in Io's vicinity and in a disk in Io's orbital plane, compared with observational data, support arguments (1) that Io is the source of the sodium, (2) that sodium is ejected from the inside hemisphere and most of the high velocity sodium which is observed is ejected from the leading inside quadrant, (3) that most of the sodium leads Io in Io's vicinity but follows Io at distances of more than 7Rj from Jupiter, (4) that a significant fraction of the sodium flux is ejected at large angles with respect to Io's orbital plane, (5) that the source velocity distribution has a pronounced high-velocity tail, and (6) that impact ionization by electrons is significant at large distances from Io. 相似文献
5.
Thomas R. McDonough 《Icarus》1975,24(4):400-406
The Jovian hydrogen torus associated with Io, that was observed by Judge and Carlson, has been found by them to be a third of a torus rather than a complete torus. It is shown that the energetic particles observed by Pioneer 10 do not ionize atomic hydrogen sufficiently fast to erode the torus as observed. It is proposed that the reason an incomplete torus exists is the presence of a corotating cold magnetospheric plasma. If this explanation is correct, the angular extent of the fractional torus is a measure of the density of the magnetospheric plasma near Io's orbit, which is found to be ～102cm?3. It is shown that such a plasma may provide an adequate input to Io, where it can recombine and escape, to form enough hydrogen atoms to explain the number of observed torus atoms. Thus the magnetospheric plasma may serve as both the source and the sink of the torus. However, while it is not difficult to make the plasma be the sink of the toroidal hydrogen, it is difficult (although perhaps possible) to self-consistently make it the source. It may be necessary to invoke some other mechanism to generate the hydrogen. 相似文献
6.
The unexpectedly large scale height of Io's ionosphere (Kliore, A., et al., 1975, Icarus24, 407–410) together with the relatively large molecular weight of the likely principal constituent, SO2 (Pearl, J., et al., 1979, Nature280, 755–758), suggest a high ionospheric temperature. Electrical induction in Io's ionosphere due to the corotating plasma bound to the Jovian magnetosphere is one possible source for attainment of such high temperatures. Accordingly, unipolar induction models were constructed to calculate ionospheric joule heating numerically. Heating rates produced by highly simplified models lie in the range 10?9 to 10?8 W/m3. These heating rates are lower than those determined from uv photodissociative heating models (Kumar, S., 1980, Geophys. Res. Lett.7, 9–12) at low levels in the ionosphere but are comparable in the upper ionosphere. The low electrical heating rate throughout most of the ionosphere is due to the power limitation imposed by the Alfvén wings which complete the electrical circuit (Neubauer, F.M., 1980, J. Geophys. Res.85, 1171–1178). Contrary to the pre-Voyager calculations of Cloutier, P. A., et al. (1978, Astrophys. Space Sci.55, 93–112), our numerical results show that the J × B force density due to unipolar induction currents in the ionosphere is much less than the gravitational force density when the combined mass of the neutral species is included. The binding and coupling of the ionosphere is principally due to the relatively dense (possibly localized) neutral SO2 atmosphere. In regions where the ions and neutrals are collisionally coupled the ionosphere will not be stripped off by the J × B forces. However at a level above that (to which the ions move by diffusion only) the charged species would be removed. Thus there appears to be no need to postulate the existence of an intrinsic Ionian magnetic field as suggested by Kivelson, M. G., et al. (79, Science 205, 491–493) and Southwood, S. J., et al. (1980, J. Geophys. Res., in press) in order to retain the observed ionosphere. 相似文献
7.
The available full-disk reflectance spectra of Io in the range 0.3 to 2.5 μm have been interpreted by comparison with new laboratory spectra of a wide variety of natural and synthetic mineral phases in order to determine a surface compositional model for Io that is consistent with Io's other known chemical and physical properties. Our results indicate that the dominant mineral phases are sulfates and free sulfur derived from them, which points toward a low temperature and initially water-rich surface assemblage. Our current preferred mineral phase mixture that best matches the Io data and is simultaneously most consistent with other constraints, consists of a fine-grained particulate mixture of free sulfur (55 vol%), dehydrated bloedite [Na2Mg(SO4)2·xH2O] (30 vol%) ferric sulfate [Fe2(SO4)3·xH2O] (15 vol%), and trace amounts of hematite [Fe2O3]. Other salts may be present, such as halite and sodium nitrate, as well as clay minerals. Such a model is consistent with a probable pre- and post-accretion thermal history of Io-forming material and Io's observed Na emission and other properties. These results further support the evaporite surface hypothesis of Fanale et al'; while not precluding the presence of certain silicate phases such as montmorillonite.The average surface of Io's leading hemisphere appears to contain less free sulfur and more salts and to be finer grained than that of the trailing hemisphere. Since Io is immersed in Jupiter's magnetosphere, irradiation damage effects from low-energy proton bombardment were studied. Irradiation damage of lattices is estimated to be a relatively minor but operative process on the surface of Io; irradiation darkening by sulfate reduction to free sulfur and by F-center production in salts may be partly responsible for the differences in albedo of leading and trailing hemispheres and equatorial and polar regions of Io, but slight regional differences in relative intrinsic phase concentration on the surface may likewise account for these global variations in albedo.Possible unusual surface properties predicted by this model include: posteclipse darkening in certain wavelenghts, limb brightening in certain wavelengths, and unusual surface electrical properties. Further refinement of Io's surface composition model and better understanding of surface irradiation effects will be possible when observational data in the range 0.20 to 0.30 μm are obtained and when improved spectra in the range 0.30 to 5.0 μm are obtained having increased spectral, spatial, and temporal resolution. 相似文献
8.
A.J. Dessler 《Icarus》1980,44(2):291-295
Theoretical arguments have been presented to the effect that both plasma and energy are supplied to the Jovian magnetosphere primarily from internal sources. If we assume that Io is the source of plasma for the Jovian magnetosphere and that outward flow of plasma from the torus is the means of drawing from the kinetic energy of rotation of Jupiter to drive magnetospheric phenomena, we can obtain a new, independent estimate of the rate of mass injection from Io into the Io plasma torus. We explicitly assume the solar wind supplies neither plasma nor energy to the Jovian magnetosphere in significant amounts. The power expended by the Jovian magnetosphere is supplied by torus plasma falling outward through the corotational-centrifugal-potential field. A lower limit to the rate of mass injection into the torus, which on the average must equal the rate of mass loss from the torus, is therefore derivable if we adopt a value for the power expended to drive the various magnetospheric phenomena. This method yields an injection rate of at least 103 kg/sec, a value in agreement with the results obtained by two other independent methods of estimating mass injection rate. If this injection rate from Io and extraction of energy from Jupiter's kinetic energy of rotation has been maintained over geologic time, then approximately 0.1% of Io's mass (principally in the form of sulfur and oxygen) has been lost to the Jovian magnetosphere, and Jupiter's spin rate has been reduced by less than 0.1%. 相似文献
9.
D.F. Strobel 《Planetary and Space Science》1982,30(8):839-848
The chemistry and evolution of Titan's atmosphere is reviewed in the light of the scientific findings from the Voyager mission. It is argued that the present N2 atmosphere may be Titan's initial atmosphere rather than photochemically derived from an original NH3 atmosphere. The escape rate of hydrogen from Titan is controlled by photochemical production from hydrocarbons. CH4 is irreversibly converted to less hydrogen rich hydrocarbons, which over geologic time accumulate on the surface to a layer thickness of ～0.5 km. Magnetospheric electrons interacting with Titan's exosphere may dissociate enough N2 into hot, escaping N atoms to remove ～0.2 of Titan's present atmosphere over geologic time. The energy dissipation of magnetospheric electrons exceeds solar e.u.v. energy deposition in Titan's atmosphere by an order of magnitude and is the principal driver of nitrogen photochemistry. The environmental conditions in Titan's upper atmosphere are favorable to building up complex molecules, particularly in the north polar cap region. 相似文献
10.
An understanding of the rates of frost grain growth is essential to the goal of relating spectral data on surface mineralogy to the physical history of a planetary surface. Models of grain growth kinetics have been constructed for various frosts based on their individual thermodynamic properties and on the difference in binding energy between molecules on plane vs curved faces. A steady state situation can occur on planetary surfaces in which thermal elimination of small grains competes with their creation, usually by meteorite impact. We utilize predicted grain growth rates to explain telescopic spectral data on condensate surfaces throughout the solar system. On Pluto, predicted CH4 ice grain growth rates are very high despite the low temperature, resulting in a multicentimeter optical path. This explains the strong CH4 absorption band depths, which otherwise would require large amounts of CH4 gas. On the Uranian and Saturnian satellites, extremely slow grain growth rates are predicted because of the low vapor pressure of H2O at the existing average surface temperatures. This may explain evidence for fine grain size and peculiar microstructure. On Io, ordinary thermal exchange is more effective than sputtering in promoting grain growth because of the properties of SO2. Over much of Io's disk, submicron size grains of SO2 could plausibly reconfigure into a surface glaze on a timescale comparable to the resurfacing rate. This may explain the relatively strong SO2 signature in Io's infrared absorption spectrum as opposed to its weaker manifestation in the visible spectrum. In spite of lower sputtering fluxes, sputtering plays a more important role in grain growth for Europa, Ganymede, and Callisto than on Io. This is a result of high rates of thermally activated grain growth and resurfacing on Io. The sequence of H2O-ice absorption band depths (related to the mean grain size) is J2(T) ～ J3(T) > J2(L) > J3(L) ～ J4(T) ～ J4(L), where L = leading and T = trailing. This is to be expected if sputtering were dominant. The calculations show, however, that neither thermalized exchange fluxes nor sputtering exchange fluxes can produce the implied grain growth or the ordering by ice absorption band depths of the six satellite hemispheres. Only sputtering control by simple ejection of H2O from the satellites, as the dominant cause of shorter mean lifetimes for smaller exposed grains, can satisfactorily explain the data. Some observations, which suggest that there are vertical grain size gradients, may result from a steady state balance between intense near surface production of fine frost by comminution, coupled with ongoing ubiquitous grain growth in the vertical column. In certain cases, e.g., Europa and Enceladus, the possibility exists that endogenic activity as well as comminution could affect grain size—at least locally. It is concluded that not only ice identification and mapping, but ice grain size mapping is an important experiment to be conducted on future missions. 相似文献
11.
Shailendra Kumar 《Icarus》1985,61(1):101-123
Models of Io's ionosphere at the time of the Pioneer 10 encounter are constructed in the presence of an SO2Na atmosphere on Io. The formation of the observed ionosphere on the downstream side requires precipitation of electrons; solar EUV alone is inadequate. Electron impact in the range 500–800 eV on an SO2 atmosphere with a surface density of 14 × 1010 cm?3 provides the best fit to the Pioneer 10 radio occultation entry data. The SO2+, the major ion produced, is converted rapidly to SO+ and in turn to S+ by reactions with the dissociation products of SO2. Ion chemistry leads to the formation of S+ as the dominant ion at and above the ionospheric peak. Na+ would dominate the ion composition near the surface, and it provides important constraints on the amount of Na allowed in the atmosphere. The relatively narrow energy range and flux required for incident electrons suggests that a fraction of torus plasma is accelerated in the wake region and penetrates deep into the atmosphere. On the upstream side the torus plasma compresses the ionosphere. These characteristics support the possible presence of a weak magnetic field associated with Io. S+ ions would escape from Io in the wake region at a rate of up to 1026 sec?1. 相似文献
12.
Io's neutral sodium emission cloud was monitored during the period of Voyager 1 encounter from two independent ground-based sites. Observations from Table Mountain Observatory verified the continued existence of the “near-Io cloud” (d < 1.5 × 105 km, for 4πI > 1 kR; R denotes Rayleigh) while those from Wise Observatory showed a deficiency in the weaker emission at greater distances from Io. The sodium cloud has been monitored from both observatories for several years. These and other observations demonstrate that the behavior of the cloud is complex since it undergoes a variety of changes, both systematic and secular, which can have both time and spatial dependencies. The cloud also displays some characteristics of stability. Table Mountain images and high-dispersion spectra (resolution ) indicate that the basic shape and intensity of the “near cloud” have remained relatively constant at least since imaging observations began in 1976. Wise Observatory low-dispersion spectra (resolution ) which have been obtained since 1974 demonstrate substantial variability of the size and intensity of the “far cloud” (d ? 1.5 × 105 km) on a time scale of months or less. Corresponding changes in the state of the plasma associated with the Io torus are suggested, with the period of Voyager 1 encounter represented as a time of unusually high plasma temperature and/or density. Dynamic models of the sodium cloud employing Voyager 1 plasma data provide a reasonable fit to the Table Mountain encounter images. The modeling assumptions of anisotropic ejection of neutral sodium atoms from the leading, inner hemisphere of Io with a velocity distribution characteristic of sputtering adequately explain the overall intensity distribution of the “near cloud”. During the Voyager 1 encounter period there appeared a region of enhanced intensity projecting outward from Io's orbit and inclined to the orbital plane. This region is clearly distinguished from the sodium emission normally aligned with the plane of Io's orbit. The process responsible for this phenomenon is not yet understood. Similar but less pronounced features are also present in several Table Mountain images obtained over the past few years. 相似文献
13.
The thermal response of the Earth's ionospheric plasma is calculated for various suddenly applied electron and ion heat sources. The time-dependent coupled electron and ion energy equations are solved by a semi-automatic computational scheme that employs Newton's method for coupled vector systems of non-linear parabolic (second order) partial differential equations in one spatial dimension. First, the electron and composite ion energy equations along a geomagnetic field line are solved with respect to a variety of ionospheric heat sources that include: thermal conduction in the daytime ionosphere; heating by electric fields acting perpendicular to the geomagnetic field line; and heating within a stable auroral red are (SAR-arc). The energy equations are then extended to resolve differential temperature profiles, first for two separate ion species (H+, O+) and then for four separate ion species (H+, He+, N+, O+) in addition to the electron temperature. The electron and individual ion temperatures are calculated for conditions within a night-time SAR-arc excited by heat flowing from the magnetosphere into the ionosphere, and also for typical midlatitude daytime ionospheric conditions. It is shown that in the lower ionosphere all ion species have the same temperature; however, in the topside ionosphere above about 400 km, ion species can display differential temperatures depending upon the balance between thermal conduction, heating by collision with electrons, cooling by collisions with the neutrals, and energy transfer by inter-ion collisions. Both the time evolution and steady-state distribution of such ion temperature differentials are discussed.The results show that below 300km both the electrons and ions respond rapidly (<30s) to variations in direct thermal forcing. Above 600 km the electrons and ions display quite different times to reach steady state, depending on the electron density: when the electron density is low the electrons reach steady state temperatures in 30 s, but typically require 700 s when the density is high; the ions, on the other hand, reach steady state in 700 s when the density is high, and 1500–2500 s when the density is low. Between 300 and 600 km, a variety of thermal structures can exist, depending upon the electron density and the type of thermal forcing; however steady state is generally reached in 200–1000 s. 相似文献
14.
J.S. Morgan 《Icarus》1985,62(3):389-414
Spectrographic data on the Io torus from 15 nights of observations spread over a 4-month period in 1981 are presented here. The [SII] λλ6716, 6731; [SII] λλ4069, 4076; [OII] λλ3726, 3729; and [SIII] λ3722 lines were simultaneously measured on each spectrogram. An east-west asymmetry was observed in the optical emissions, showing larger western intensities and a more diffuse and radially extensive nebula to the east. Two configurations of [SII] longitudinal asymmetry that were stable over at least 4 days were observed. The magnetic longitudes of ～ 180 and 300° are shown to be of particular interest. Longitudinal structure was not detected in either the [OII] intensity or the plasma density as measured by the [OII] and [SII] doublet ratios. Errors in the line ratios could mask density variations as large as a factor of ～ 1.5. A radial variation in the ratio of OII/SII was observed, with the ratio being largest near Io's orbit. Monthly variabilitywas detected in both the intensity and density of the tours. The [SII] line ratios indicated an increase in ne over the 4-month period that was accompanied by increased intensities. For single measurements, no correlation between the [SII] intensities and the [SII] λ6716/λ6731 line ratio was detected, but this could be a result of errors in the line ratio determinations. Extremely low values of this same ratio were measured. These appear to indicate errors in the presently accepted [SII] transition probabilities. These [SII] line ratios indicate that very-high-density regions are present in the torus, and it is shown how these regions could have significantly influenced these measurements. 相似文献
15.
Dark paterae on the jovian satellite Io are evidence of recent volcanic activity. Some paterae appear to be entirely filled with dark volcanic material, while others have only partially darkened floors. Dark paterae have area and heat flow longitudinal distributions that are bimodal as well as anti-correlated with the longitudinal distribution of mountains on Io at a global scale. As part of our study of Io’s total heat flow, we have examined the darkest paterae and quantified their thermal emission in order to assess their contribution. This is the first time that the areas of the dark material in these paterae have been measured with such precision and correlated with their thermal emission. Dark paterae yield a significantly larger contribution to Io’s heat flow than dark volcanic fields. Dark paterae (including Loki Patera) yield at least ∼4 × 1013 W or ∼40% of Io’s total heat flow. In comparison, dark flow fields yield ∼1013 W or ∼10% of Io’s total heat flow. Of the total heat loss from dark paterae, Loki Patera alone yields ∼1013 W or ∼10% of Io’s total thermal emission. 相似文献
16.
We investigate the contribution of scattering in the telescope to our measurements of the size of Io's sodium cloud and to the distribution of emission intensity in the cloud. The brightest regions, within 30″ of Io near opposition and along the equatorial plane, are relatively undistorted but regions further than 45″ away and not close to the equatorial plane are very likely to consist of mainly scattered light. Portions of the cloud in the vicinity of the magnetic equator are also mostly scattered light when Io is near extreme magnetic latitude. The equatorial torus, however, extends up to 20 arcmin from Jupiter. The large size of the cloud is thus confirmed. High-resolution line profile shapes indicate that sodium streams from Io preferentially in the forward direction with velocities distributed up to 18 km sec?1. The observed wavelength shifts of the peak intensities from Io's rest frame are compatible with a cloud streaming through a bound atmospheric component but they could also be caused by a velocity distribution peaked at very low velocities. 相似文献
17.
The baroclinic stability of Jupiter's zonal flow is investigated using a model consisting of two continuously stratified fluid layers. The upper layer, containing a zonal shear flow and representing the Jovian cloudy regions above p ～ 5 bars, is the same as Eady's (1949) model for the Earth. The lower layer has a relatively large but finite depth with a quiescent basic state, representing the deep Jovian fluid bulk below p ～ 5 bars. Due to the presence of the lower layer, the linearized non-dimensional growth rates are drastically reduced from the O(1) growth rates of the original Early model. Only very long wavelengths relative to the upper fluid's radius of deformation L1 are unstable. Eddy transports of heat are also reduced relative to estimates based on scaling arguments alone. Since the hydrostatic approximation for the lower-layer perturbation breaks down at great depths, a second model is presented in which energy propagates downward in an infinitely deep lower fluid obeying the full linearized fluid equations. In this model, the growth rates are again very small, but now all wavelengths are unstable with maximum growth rates occurring for wavelengths O(1) relative to L1. These results illustrate the importance for the upper-layer meteorology of the interface boundary condition with the lower fluid, which is radically different from the rigid lower boundary of the Earth's troposphere. 相似文献
18.
Ground-based optical observations of D1 and D2 line emissions from Jupiter’s sodium nebula, which extend over several hundreds of jovian radii, were carried out at Mt. Haleakala, Maui, Hawaii using a wide field filter imager from May 19 to June 21, 2007. During this observation, the east-west asymmetry of the nebula with respect to the Io’s orbital motion was clearly identified. Particularly, the D1+D2 brightness on the western side of Jupiter is strongly controlled by the Io phase angle. The following scenario was developed to explain this phenomenon as follows: First, more ionospheric ions like NaX+, which are thought to produce fast neutral sodium atoms due to a dissociative recombination process, are expected to exist in Io’s dayside hemisphere rather than in the nightside one. Second, it is expected that more NaX+ ionospheric ions are picked up by the jovian co-rotating magnetic field when Io’s leading hemisphere is illuminated by the Sun. Third, the sodium atom ejection rate varies with respect to Io’s orbital position as a result of the first two points. Model simulations were performed using this scenario. The model results were consistent with the observation results, suggesting that Io’s ionosphere is expected to be controlled by solar radiation just like Earth. 相似文献
19.
《Icarus》1987,70(1):78-98
The discovery of large volcanic eruptions on Io suggests that Io is one of the most geologically active planetary bodies. The energy source of this geologic activity is believed to be tidal heating induced by Jupiter. A number of thermal history calculations were done to investigate the effect of tidal heating on the thermal history of Io taking into account solid state convection and advective heat transfer. These simulations show that the total tidal heating energy in Io is almost equal to the advectively transferred heat, indicating that the observed heat flow from Io is nearly equal to the total tidal heating energy. Since total tidal heating energy is dependent on the radius of the liquid mantle and the internal dissipation factor (Q), the radius of the liquid mantle can be estimated for a given value of Q. Some reasonable thermal history models of Io were obtained using a model with Q ≈ 25–50 in which the magma source of Ionian volcanism is at a depth of 100–300 km. The models satisfy the heat flow data and the existence of a thick lithosphere. Using a model with Q = 25 and L = 300 km (thickness of the advective region) as the standard model (model II), we then studied the effect of convective heat transfer and the initial temperature distribution on the Ionian thermal history. In these calculations, the other parameters are the same as in the standard model (model II). These calculations show that although the temperature distribution in the central region reflects the difference in the efficiency of convective heat transfer and initial temperature distribution, the temperature distribution in the outer region does not changes appreciably. 相似文献
20.
Radiation damage and luminescence, caused by magnetospheric charged particles, have been suggested by several authors as mechanisms for explaining some of the peculiar spectral/albedo features of Io. We have pursued this possibility by measuring the uv-visual spectral reflectance and luminescent efficiency of several proposed Io surface constituents during 2 to 10-keV proton irradiation at room temperature and at low temperature (120 < T < 140°K). The spectral reflectance of NaCl and KCl during proton irradiation exhibits the well-known F-center absorption bands at 4580 and 5560 Å. Na2SO4 shows a generalized darkening which increases toward longer wavelengths. NaNO3 shows a spectral reflectance change indicative of the partial alteration of NaNo3 to NaNo2. NaNO2 shows no change. The luminescent efficiencies of NaCl and KCl are ～10?4 at 300°K and increase by one-half order of magnitude at ～130°K. The efficiencies of K2CO3, Na2CO3, Na2SO4, and NaNO3 are 10?4, 10?4, 10?5 and 10?6, respectively, at 300°K and they all decrease by one-half order of magnitude at ～130°K. These results indicate that magnetospheric proton irradiation of Io could cause spectral features in its observed ultraviolet and visible reflection spectrum if salts such as those studied here are present on its surface. However, because the magnitude of these spectral effects is dependent on competing factors such as surface temperature, incident particle energy flux, solar bleaching effects, and trace element abundance, we are unable at this time to make a quantitative estimate of the strength of these spectral effects on Io. The luminescent efficiencies of pure samples that we have studied in the laboratory suggest that charged-particle induced luminescence from Io's surface might be observable by a spacecraft such as Voyager when viewing Io's dark side. 相似文献