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1.
Shock wave and thermodynamic data for rock-forming and volatile-bearing minerals are used to determine minimum impact velocities (vcr) and minimum impact pressures (pcr) required to form a primary H2O atmosphere during planetary accretion from chondritelike planetesimals. The escape of initially released water from an accreting planet is controlled by the dehydration efficiency. Since different planetary surface porosities will result from formation of a regolith, vcr and pcr can vary from 1.5 to 5.8 km/sec and from 90 to 600 kbar, respectively, for target porosities between 0 and ～45%. On the basis of experimental data, hydration rates for forsterite and enstatite are derived. For a global regolith layer on the Earth's surface, the maximum hydration rate equals 6 × 1010 g H2O sec?1 during accretion of the Earth. Attenuation of impact-induced shock pressure is modeled to the extent that the amount of released water as a function of projectile radius, impact velocity, weight fraction of water in the target, target porosity, and dehydration efficiency can be estimated. The two primary processes considered are the impact release of water bound in hydrous minerals (e.g., serpentine) and the subsequent reincorporation of free water by hydration of forsterite and enstatite. These processes are described in terms of model calculations for the accretion of the Earth. Parameters which lead to a primary atmosphere/hydrosphere are: an accretion time of ? 1.6 × 108years, the use of an accretion model defined by Weidenschilling (1974, 1976), a mean planetesimal radius of 0.5 km, a hydration rate of 6 × 1010 g H2O sec?1 inferred from a mean porosity of ～ 10% for the upper 1 km of the accreting Earth, and values for the dehydration efficiency, DE, of 0.55 and 0.07 for the maximum and minimum pressure decay model, respectively. Conditions which prohibit the formation of a primary atmosphere include an accretion time much longer than 1.6 × 108 years, a hydration rate for forsterite and enstatite well in excess of 6 × 1010 g H2O sec?1, and a dehydration efficiency DE < 0.07. We conclude that the concept of dehydration efficiency is of dominant importance in determining the degree to which an accreting planet acquires an atmosphere during its formation. 相似文献
2.
F.P. Fanale 《Icarus》1976,28(2):179-202
Observations of Mars and cosmochemical considerations imply that the total inventory of degassed volatiles on Mars is 102 to 103 times that present in Mars' atmosphere and polar caps. The degassed volatiles have been physically and chemically incorporated into a layer of unconsolidated surface rubble (a “megaregolith”) up to 2km thick. Tentative lines of evidence suggest a high concentration (～5g/cm2) of 40 Ar in the atmosphere of Mars. If correct, this would be consistent with a degassing model for Mars in which the Martian “surface” volatile inventory is presumed identical to that of Earth but scaled to Mars' smaller mass and surface area. The implied inventory would be: (40Ar) = 4g/cm2, (H2O) = 1 × 105g/cm2, (CO2) = 7 × 103g/cm2, (N2) = 3 × 102g/cm2, (Cl) = 2 × 103g/cm2, and (S) = 2 × 102g/cm2. Such a model is useful for testing, but differences in composition and planetary energy history may be anticipated between Mars and Earth on theoretical grounds. Also, the model demands huge regolith sinks for the volatiles listed.If the regolith were in physical equilibrium with the atmosphere, as much as 2 × 104g/cm2 of H2O could be stored in it as hard-frozen permafrost, or 5 × 104g/cm2 if equilibrium with the atmosphere were inhibited. Spectral measurements of Martian regolith material and laboratory measurement of weathering kinetics on simulated regolith material suggest large amounts of hydrated iron oxides and clay minerals exist in the regolith; the amount of chemically bound H2O could be from 1 × 104 to 4 × 104g/cm2. In an Earth-analogous model, a 2 km mixed regolith must contain the following concentrations of other volatile-containing compounds by weight: carbonates = 1.5%, nitrates = 0·3%, chlorides = 0.6%, and sulfates = 0.1%. Such concentrations would be undetectable by current Earth-based spectral reflectance measurements, and (except the nitrates) formation of the “required” amounts of these compounds could result from interaction of adsorbed H2O and ice with primary silicates expected on Mars. Most of the CO2 could be physically adsorbed on the regolith.Thus, maximum amounts of H2O and other volatiles which could be stored in the Mars regolith are marginally compatible with those required by an Earth-analogous model, although a lower atmospheric 40Ar concentration and regolith volatile inventory would be easier to reconcile with observational constraints. Differences in the ratios of H2O and other volatiles to 40Ar between surface volatiles on the real Mars and on an Earth-analogous Mars could result from and reflect differences in bulk composition and time history of degassing between Mars and Earth. Models relating Viking-observable parameters, e.g., (40Ar) and (36Ar), to the time history and overall intensity of Mars degassing are given. 相似文献
3.
Models are developed for the photochemistry of a CO2H2ON2 atmosphere on Mars and estimates are given for the concentrations of N, NO, NO2, NO3, N2O5, HNO2, HNO3, and N2O as a function of altitude. Nitric oxide is the most abundant form of odd nitrogen, present with a mixing ratio relative to CO2 of order 10?8. Deposition rates for nitrite and nitrate minerals could be as large as 3× 105 N equivalent atoms cm?2 sec?1 under present conditions and may have been higher in the past. 相似文献
4.
We used thermochemical equilibrium calculations to predict stabilities of pure rock-forming hydrous silicates on Venus' surface as a function of elevation, atmospheric H2O and SO2concentrations, and oxygen fugacity (fO2). About 50 different hydrous silicates were included in our calculations. We find that many of these are unstable on Venus's surface because of the low atmospheric H2O content of 30–45 parts per million by volume (ppmv) and the high surface temperatures (660 K on Maxwell Montes to 740 K in the plains). Hydrous Fe2+-bearing silicates are unstable due to oxidation to magnetite and/or hematite at the fO2of the near-surface atmosphere. Ca-bearing hydrous silicates are unstable because of sulfatization to anhydrite. Some Fe-free micas (e.g., eastonite, eastonite–phlogopite micas), and some alkali amphiboles might be stable on Venus' surface, especially in the lower temperature highlands. We discuss hydrous mineral formation in the interior and on the surface of Venus. We review the literature on mica and amphibole thermal decomposition and find that dehydration of phlogopitic micas and fibrous amphiboles produces (metastable) dehydroxylated anhydrides that decompose to more stable minerals at temperatures hundreds of degrees higher than the onset of dehydroxylation. These observations raise the possibility that anhydrides formed from hydrous silicates, which may have been present during a wetter period in Venus' history, may persist somewhere on Venus' present surface. We discuss experiments that could be used on future spacecraft missions to detect hydroxyl in rocks and hydrous silicates on Venus. Finally, we review estimates of the amount of water and OH (hydroxyl) in the Earth's mantle. Based on this review, we suggest that even if no hydrous silicates are stable on Venus, significant amounts of water are plausibly present in surface rocks as OH in nominally anhydrous minerals. 相似文献
5.
In situ measurements of stratospheric H2SO4 and HSO3vapors using passive chemical ionization mass spectrometry were made in October 1982 after the eruption of volcano El Chichon. Data were obtained between about 20 and 41 km showing [H2SO4 + HSO3] sum concentrations between about 104 and 2 × 105 cm?3 below 29 km and a steep rise above this altitude. Maximum [H2SO4 + HSO3] values of about 3 × 106 cm?3 are reached above 35 km.Partial [HSO3] concentrations increase above 34 km reaching about around 40 km. From the measurements it is concluded that H2SO4 and probably HSO3photolysis have an important influence above 34 km leading to the observed increase of [HSO3] and a depletion of H2SO4vapor.It also seems that the data support the view of heterogeneous HSO3 removal. If correct, this would imply that stratospheric aerosols are formed primarily from HSO3 rather than H2SO4vapor. 相似文献
6.
The photochemistry of the stratosphere of Venus was modeled using an updated and expanded chemical scheme, combined with the results of recent observations and laboratory studies. We examined three models, with H2 mixing ratio equal to 2 × 10?5, 5 × 10?7, and 1 × 10?13, respectively. All models satisfactorily account for the observations of CO, O2, O2(1Δ), and SO2 in the stratosphere, but only the last one may be able to account for the diurnal behavior of mesospheric CO and the uv albedo. Oxygen, derived from CO2 photolysis, is primarily consumed by CO2 recombination and oxidation of SO2 to H2SO4. Photolysis of HCl in the upper stratosphere provides a major source of odd hydrogen and free chlorine radicals, essential for the catalytic oxidation of CO. Oxidation of SO2 by O occurs in the lower stratosphere. In the high-H2 model (model A) the OO bond is broken mainly by S + O2 and SO + HO2. In the low-H2 models additional reactions for breaking the OO bond must be invoked: NO + HO2 in model B and ClCO + O2 in model C. It is shown that lightning in the lower atmosphere could provide as much as 30 ppb of NOx in the stratosphere. Our modeling reveals a number of intriguing similarities, previously unsuspected, between the chemistry of the stratosphere of Venus and that of the Earth. Photochemistry may have played a major role in the evolution of the atmosphere. The current atmosphere, as described by our preferred model, is characterized by an extreme deficiency of hydrogen species, having probably lost the equivalent of 102–103 times the present hydrogen content. 相似文献
7.
H. Levy 《Planetary and Space Science》1973,21(4):575-591
Altitude profiles for the number densities of NO, NO2, NO3, N2O5, HNO2, CH3O, CH3O2, H2CO, OH, and HO2 are calculated as a function of time of day with a steady-state photochemical model in which the altitude profiles for the number densities of H2O, CH4, H2, CO, O3, and the sum of NO and NO2 are fixed at values appropriate to a summer latitude of 34°. Average daily profiles are calculated for the long-lived species, HNO3, H2O2, and CH3O2H.The major nitrogen compound HNO3 may have a number density approaching 5 × 1011 molecules cm?3 at the surface, although an effective loss path due to collisions with particulates could greatly reduce this value.The number density of OH remains relatively unchanged in the first 6 km and reaches 1 × 107 molecules cm?3 at noon, while the number density of HO2 decreases throughout the lower troposphere from its noontime value of 8 × 108 molecules cm?3 at the surface.H2O2 and H2CO both have number densities in the ppb range in the lower troposphere.Owing to decreasing temperature and water concentration, the production of radicals and their steady-state number densities decrease with altitude, reaching a noontime minimum of 1 × 108 molecules cm?3 for OH and 3 × 107 molecules cm?3 for HO2 at the tropopause. The related minor species show even sharper decreases with increasing altitude.The primary path for interconverting OH and HO2 serves as the major sink for CO and leads to a tropospheric lifetime for CO of ~0.1 yr.Another reaction cycle, the oxidation of CH4, is quite important in the lower troposphere and leads to the production of H2CO along with the destruction of CH4 for which a tropospheric lifetime of ~2 yr is estimated.The destruction of H2CO that was produced in the CH4 oxidation cycle provides the major source of CO and H2 in the atmosphere. 相似文献
8.
Current evidence indicates that the Martian surface is abundant with water presently in the form of ice, while the atmosphere was at one time more massive with a past surface pressure of as much as 1 atm of CO2. In an attempt to understand the Martian paleoclimate, we have modeled a past CO2H2O greenhouse and find global temperatures which are consistent with an earlier presence of liquid surface water, a finding which agrees with the extensive evidence for past fluvial erosion. An important aspect of the CO2H2O greenhouse model is the detailed inclusion of CO2 hot bands. For a surface pressure of 1 atm of CO2, the present greenhouse model predicts a global mean surface temperature of 294°K, but if the hot bands are excluded, a surface temperature of only 250°K is achieved. 相似文献
9.
Nonthermal escape processes responsible for the escape of hydrogen and deuterium from Venus are examined for present and past atmospheres. Three mechanisms are important for the escape of hydrogen from the present atmosphere: (a) charge exchange of plasmaspheric H+ with exospheric H, (b) impact of exospheric hot O atoms on H, and (c) ion molecule reactions involving O+ and H2. However, in the past when the H abundance was higher, the charge-exchange mechanism would be the strongest. The H escape flux increases rapidly with increasing hydrogen abundance in the upper atmosphere and saturates at a value of 1 × 1010 cm?2 sec?1 emerging primarily from the day side when the H mixing ratio at the homopause is 2 × 10?3. This corresponds to an H2O mixing ratio of 1 × 10?3 at the cold trap and ～15% at the surface. Deuterium would also escape by the charge-exchange mechanism and a D/H enrichment by a factor of ～1000 over the nonthermal escape regime is expected, which could have lasted over the last 3 billion years. Coincidentally, the onset of hydrodynamic flow leading to efficient H escape occurs just at the H2O mixing ratio at which the charge-exchange escape flux saturates. Thus it is possible that Venus has lost an Earth-equivalent ocean of water over geologic time. If so, either the D/H enrichment has been kept low by modest outgassing of juvenile water or Venus started out with a D/H ratio of ～4.0 × 10?6. 相似文献
10.
For purified binary gas mixtures like NH3H2O or HClH2O, partial pressures appreciably greater than the two saturation partial pressures are needed to condense the gas mixture into small solution droplets (“homogeneous hetero-molecular nucleation”). Thus without foreign nuclei, clouds are not as easily formed as in the theories of Lewis; the latter should be valid only if large condensation nuclei are available. We calculate here from classical homogeneous heteromolecular nucleation theory the threshold partial pressures necessary to achieve droplet nucleation for the gas mixtures NH3H2O (Jupiter,…), HClH2O (Venus), H2SO4H2O (Venus), and C2H5OHH2O (laboratory). In the last case, theory and experiment agree satisfactorily. If no “dust” particles are available as condensation nuclei, then we expect in Jupiter's atmosphere the cloud base level to be around 40 km above the 400K level instead of 10–25 km in Lewis' models (1969) (similar upward shifts for the outer Jovian planets). For Venus, our corrections make the formation of HClH2O clouds less probable for the 60-km layer at 0°C. If H2SO4 is formed by (photo-)chemical oxidation of SO2 and if clouds are formed at that level where the H2SO4 production is largest, then the cloud base levels for H2SO4H2O mixture clouds will not be shifted by our nucleation effects. For more reliable predictions, one needs more accurate data on the water vapor content of the planetary atmospheres and laboratory experiments testing the theoretically predicted nucleation behavior of these gaseous mixtures. 相似文献
11.
William C. Maguire 《Icarus》1977,32(1):85-97
The Mariner 9 infrared spectrometer obtained data over a large part of Mars for almost a year beginning late in 1971. Mars' infrared emission spectrum was measured from 200 to 2000 cm?1 with an apodized resolution of 2.4 cm?1. No significant deviation from terrestrial ratios of carbon (12C/13C) or oxygen (16O/18O; 16O/17O) isotopes was observed on Mars. The 12C/13C isotopic ratio was found to be terrestrial with an uncertainty of 15%. Upper limits have been calculated for several minor constituents. With an effective noise equivalent radiance of 1.2 × 10?9 W cm?2 sr?1/cm?1, new upper limits in centimeter-atmospheres of 2 × 10?5 for C2H2, 4 × 10?3 for C2H4, 3 × 10?3 for C2H6, 2 × 10?4 for CH4, 1 × 10?3 for N2O, 1 × 10?4 for NO2, 4 × 10?5 for NH3, 1 × 10?3 for PH3, 7 × 10?4 for SO2, and 1 × 10?4 for OCS have been derived. 相似文献
12.
A spectroscopic search for H2O and CH4 in Comet Kohoutek (1973f) was made using a Pepsios interferometer. No evidence was found for either molecule, allowing us to set an upper limit on their production rates (on about 21 January 1974) of Q(H2O) < 6.2 × 1028 sec?1 and Q(CH4) < 2.0 × 1030 sec?1. If the cometary surface is water-ice, this production rate leads to a product (1 ? A)·(πR02) < 2.2 km2, where A is the Bond albedo, R0 is the nuclear radius, and we assume that all the absorbed solar energy is used to evaporate H2O. 相似文献
13.
The exosphere of an atmosphereless icy moon is the result of different surface release processes and subsequent modification of the released particles. At Europa icy moon, water molecules are directly released, but photolysis and radiolysis due to solar UV and Jupiter’s magnetospheric plasma, respectively, can result in OH, H, O and (possibly) H2 production. These molecules can recombine to reform water and/or new chemical species. As a consequence, Europa’s neutral environment becomes a mixture of different molecules, among which, H2O dominates in the highest altitudes and O2, formed mainly by radiolysis of ice and subsequent release of the produced molecules, prevails at lower altitudes. In this work, starting from a previously developed Monte Carlo model for the generation of Europa’s exosphere, where the only considered species was water, we make a first attempt to simulate also the H2 and O2 components of the neutral environment around Europa, already observed by the Hubble Space Telescope and the Ultraviolet Imaging Spectrograph on board Cassini, during its flyby of Jupiter. Considering a specific configuration where the leading hemisphere coincides with the sunlit hemisphere, we estimate along the Europa–Sun line an O2 column density of about 1.5 × 1019 m?2 at the dayside and 3 × 1018 m?2 at the nightside. In this work we also improve our previous estimation of the sputtered H2O exosphere of this moon, taking into consideration the trailing–leading asymmetry in the magnetospheric ion bombardment and the energy and temperature dependences of the process yields. We find that a density of 1.5 × 1012 H2O/m3 is expected at altitudes ～0.1RE above the surface of the trailing hemisphere. Additionally, we calculate the escape of H2O, O2 and H2. The total number of neutral atoms in Europa’s neutral torus, is estimated to be in the range 7.8 × 1032–3.3 × 1033. 相似文献
14.
Interference filter photometry was taken of Comet Encke on June 14, 1974 (1.07 AU heliocentric distance, postperihelion) at the CTIO (Cerro Tololo Interamerican Observatory) 150-cm reflector. Production rates were calculated of 4.1 × 1023 mol sec?1 of CN, 5.3 × 1023 mol sec?1 of C3, and 4.3 × 1024 mol sec?1 of C2. These are about three times smaller than at comparable heliocentric distance preperihelion, assuming a value of 100 for the ratio H2O/ (C2 + C3 + CN). An upper limit was placed on the production of nonvolatiles at about one-third that of volatiles in mass by assuming a bulk density of 1 g cm?3, a particle geometric albedo of 0.1, and a phase function of 0.2. 相似文献
15.
James L. Gooding 《Icarus》1978,33(3):483-513
Chemical weathering on Mars is examined theoretically from the standpoint of heterogeneous equilibrium between solid mineral phases and gaseous O2, H2O, and CO2 in the Martian atmosphere. Thermochemical calculations are performed in order to identify important gas-solid decomposition reactions involving the major mineral constituents of mafic igneous rocks. Where unavailable in the thermochemical literature, Gibbs free energy and enthalpy of formation are estimated for certain minerals and details of these estimation procedures are given. Partial pressure stability diagrams are presented to show pertinent mineral reaction boundaries at 298 and at 240°K. In the present Martian environment, the thermodynamically stable products of gas-solid weathering of individual minerals at 240°K should be Fe2O3, as hematite or maghemite (from fayalite, magnetite, and Fe-bearing pyroxenes), quartz (from all silicates), calcite (from Ca-bearing pyroxenes and plagioclase), magnesite (from forsterite and Mg-bearing pyroxenes), corundum (from all Al-bearing silicates), Ca-beidellite (from anorthite), and szomolnokite, FeSO4 or FeSO4·H2O (from iron sulfides). Albite, microcline, and apatite should be stable with respect to gas-solid decomposition, suggesting that gas-solid weathering products on Mars may be depleted in Na, K, and P (and, possibly, Cl and F). Certain montmorillonite-type clay minerals are thermodynamically favorable intermediate gas-solid decomposition products of Al-bearing pyroxenes and may be metastable intermediate products of special mineral surface reaction mechanisms. However, the predicted high thermodynamic susceptibility of these clay minerals to subsequent gas-solid decomposition implies that they should ultimately decompose in the present Martian surface environment. Kaolinite is apparently the only clay mineral which should be thermodynamically stable over all ranges of temperature and water vapor abundance in the present environment at the Martian surface. Considering thermodynamic criteria, including possible gas-solid decomposition reactions, it is doubtful that significant amounts of goethite and clay minerals can be currently forming on Mars by mechanisms known to operate to Earth. If major amounts of goethite and clay minerals occur on Mars, they probably owe their existence to formation in an environment characterized by the presence of liquid water or by mechanism possibly unique to Mars. In any case, any goethite or montmorillonite-type clay mineral on Mars must ultimately decompose. 相似文献
16.
Robert L. Huguenin 《Icarus》1976,28(2):203-212
Photostimulated oxidation weathering irreversibly removes both oxygen and hydrogen from the atmosphere at a rate of 108 to 1011 cm?2sec?1. This corresponds to a net loss of 1025 to 1028 molecules cm?2 (102 to 105 g cm?2 of H2O, assuming a uniform rate over geologic time. Additional H2O is removed through hydration of Fe2O3 and clay minerals, but the loss is reversible and the extent of regolith storage is uncertain. CO2 is irreversibly removed from the atmosphere through the formation of CaCO3 at a rate of 107?1010cm?2sec?1. Over geologic time this corresponds to a net loss of 1024?1027 molecules cm?2 (101?104g cm?2) of CO2. Previously, it was proposed that exospheric escape was the principal irreversible volatile sink, amounting to only 102g cm?2 of H2O and 100g cm?2 of CO2 over geologic time. A recent tentative identification of abundant argon on Mars suggests that the planet may have degassed up to 105g cm?2 of H2O and 104g cm?2 of CO2. If the amounts of H2O and CO2 removed by photostimulated oxidation are close to the upper limits proposed here, it is possible that chemical weathering may have had a major effect on limiting the supply of H2O and CO2 trapped in the regolith and polar caps. 相似文献
17.
The abundance of H2O is derived from the 1900- to 2100-cm?1 region of the Voyager 1 IRIS spectra. Scale variations of about a factor of 2 are seen in the water abundance between the North and South Equatorial Belts. Averaged over the full disk, the mixing ratio is , if H2O is uniformly mixed in the atmospheric region having temperatures of 230 to 270°K; this result implies a solar depletion by a factor of 100 in this region. In the belts, the best agreement is obtained for a H2O/H2 mixing ratio of 4.0 × 10?6 in the NEB and 7.2 × 10?6 in the SEB, assuming a constant mixing ratio. 相似文献
18.
Edward A. Cloutis Frank C. Hawthorne Katherine Krenn Dionne Marcino Johnathon Strong Diana L. Blaney Faith Vilas 《Icarus》2006,184(1):121-157
A suite of sulfate minerals were characterized spectrally, compositionally, and structurally in order to develop spectral reflectance-compositional-structural relations for this group of minerals. Sulfates exhibit diverse spectral properties, and absorption-band assignments have been developed for the 0.3-26 μm range. Sulfate absorption features can be related to the presence of transition elements, OH, H2O, and SO4 groups. The number, wavelength position, and intensity of these bands are a function of both composition and structure. Cation substitutions can affect the wavelength positions of all major absorption bands. Hydroxo-bridged Fe3+ results in absorption bands in the 0.43, 0.5, and 0.9 μm regions, while the presence of Fe2+ results in absorption features in the 0.9-1.2 μm interval. Fundamental SO bending and stretching vibration absorption bands occur in the 8-10, 13-18, and 19-24 μm regions (1000-1250, 550-770, and 420-530 cm−1). The most intense combinations and overtones of these fundamentals are found in the 4-5 μm (2000-2500 cm−1) region. Absorption features seen in the 1.7-1.85 μm interval are attributable to HOH/OH bending and translation/rotation combinations, while bands in the 2.1-2.7 μm regions can be attributed to H2O- and OH-combinations as well as overtones of SO bending fundamentals. OH- and H2O-bearing sulfate spectra are fundamentally different from each other at wavelengths below ∼6 μm. Changes in H2O/OH content can shift SO band positions due to change in bond lengths and structural rearrangement. Differences in absorption band wavelength positions enable discrimination of all the sulfate minerals used in this study in a number of wavelength intervals. Of the major absorption band regions, the 4-5 μm region seems best for identifying and discriminating sulfates in the presence of other major rock-forming minerals. 相似文献
19.
William D. Smythe 《Icarus》1975,24(4):421-427
Reflectance spectra from 1 to 6 microns were taken of CH4 and CO2 gas hydrates and were found to be very similar to H2O frost spectra over the entire wavelength region. H2O clathrates have a gas to H2O ratio of about , hence a surface may contain 17% (by number) gas and appear spectroscopically similar to an H2O frost covered surface. This is important in the pressure-temperature regime of the outer solar system where hydrates, which often have vapor pressures 10?5 (or less) that of the pure gas component, are marginally stable as solids (e.g., the vapor pressure in Torr at 60 K for CH4·6H2O = 10?8 while for CH4 = 10?1). We may conclude that reflectance spectroscopy (especially Earth-based) is useful for positive identification of some components of the surface, but does not set stringent limits for spectroscopically active hydrate forming substances in the presence of water frost. 相似文献
20.
Storage of hydrogen atoms in or on a planetary surface can take place via several different mechanisms. If the hydrogen atom reacts to form a hydroxyl (OH) group or water molecule, an absorption band near 3 μm will be present. Many possible mechanisms for sequestering atomic hydrogen are discussed: internal hydrogen in the form of non-structural OH and H2O in nominally-anhydrous minerals, structural OH in minerals, structural H2O in minerals, H2O in fluid inclusions, and OH and H2O in glasses; bulk H2O as either liquid water or ice; and surficial hydrogen that is either physisorbed as H2O, chemisorbed as an H2O surface complex, or chemically-bound as an OH group on surface terminal sites and grain boundary regions. Understanding the spectroscopic distinctions among these various phenomena is of critical importance in constraining both the evolution of planetary interiors and the cycling of water on planetary surfaces. Proper interpretation of 3-μm bands in reflectance spectra is shown to depend upon the relative contributions from surficial vs. interior hydrogen, which vary with effective surface area (i.e., the grain size and surface roughness) and the volume sampled by the spectrometer. 相似文献