A global-mean model of coupled neutral and ion chemistry on Titan has been developed. Unlike the previous coupled models, the model involves ambipolar diffusion and escape of ions, hydrodynamic ...escape of light species, and calculates the H
2 and CO densities near the surface that were assigned in some previous models. We tried to reduce the numbers of species and reactions in the model and remove all species and reactions that weakly affect the observed species. Hydrocarbon chemistry is extended to C
12H
10 for neutrals and C
10H
+
11 for ions but does not include PAHs. The model involves 415 reactions of 83 neutrals and 33 ions, effects of magnetospheric electrons, protons, and cosmic rays. UV absorption by Titan's haze was calculated using the Huygens observations and a code for the aggregate particles. Hydrocarbon, nitrile, and ion chemistries are strongly coupled on Titan, and attempt to calculate them separately (e.g., in models of ionospheric composition) may result in significant error. The model densities of various species are typically in good agreement with the observations except vertical profiles in the stratosphere that are steeper than the CIRS limb data. (A model with eddy diffusion that facilitates fitting to the CIRS limb data is considered as well.) The CO densities are supported by the O
+ flux from Saturn's magnetosphere. The ionosphere includes a peak at 80 km formed by the cosmic rays, steplike layers at 500–700 and 700–900 km and a peak at 1060 km (SZA = 60°). Nighttime densities of major ions agree with the INMS data. Ion chemistry dominates in the production of bicyclic aromatic hydrocarbons above 600 km. The model estimates of heavy positive and negative ions are in reasonable agreement with the Cassini results. The major haze production is in the reactions C
6H + C
4H
2, C
3N + C
4H
2, and condensation of hydrocarbons below 100 km. Overall, precipitation rate of the photochemical products is equal to 4–7 kg cm
−2 Byr
−1 (50–90 m Byr
−1 while the global-mean depth of the organic sediments is ∼3 m). Escape rates of methane and hydrogen are 2.9 and 1.4 kg cm
−2 Byr
−1, respectively. The model does not support the low C/N ratio observed by the Huygens ACP in Titan's haze.
A self-consistent chemical kinetic model of the Venus atmosphere at 0–47 km has been calculated for the first time. The model involves 82 reactions of 26 species. Chemical processes in the atmosphere ...below the clouds are initiated by photochemical products from the middle atmosphere (H
2SO
4, CO, S
x
), thermochemistry in the lowest 10 km, and photolysis of S
3. The sulfur bonds in OCS and S
x
are weaker than the bonds of other elements in the basic atmospheric species on Venus; therefore the chemistry is mostly sulfur-driven. Sulfur chemistry activates some H and Cl atoms and radicals, though their effect on the chemical composition is weak. The lack of kinetic data for many reactions presents a problem that has been solved using some similar reactions and thermodynamic calculations of inverse processes. Column rates of some reactions in the lower atmosphere exceed the highest rates in the middle atmosphere by two orders of magnitude. However, many reactions are balanced by the inverse processes, and their net rates are comparable to those in the middle atmosphere. The calculated profile of CO is in excellent agreement with the Pioneer Venus and Venera 12 gas chromatographic measurements and slightly above the values from the nightside spectroscopy at 2.3 μm. The OCS profile also agrees with the nightside spectroscopy which is the only source of data for this species. The abundance and vertical profile of gaseous H
2SO
4 are similar to those observed by the Mariner 10 and Magellan radio occultations and ground-based microwave telescopes. While the calculated mean S
3 abundance agrees with the Venera 11–14 observations, a steep decrease in S
3 from the surface to 20 km is not expected from the observations. The ClSO
2 and SO
2Cl
2 mixing ratios are ∼10
−11 in the lowest scale height. The existing concept of the atmospheric sulfur cycles is incompatible with the observations of the OCS profile. A scheme suggested in the current work involves the basic photochemical cycle, that transforms CO
2 and SO
2 into SO
3, CO, and S
x
, and a minor photochemical cycle which forms CO and S
x
from OCS. The net effect of thermochemistry in the lowest 10 km is formation of OCS from CO and S
x
. Chemistry at 30–40 km removes the downward flux of SO
3 and the upward flux of OCS and increases the downward fluxes of CO and S
x
. The geological cycle of sulfur remains unchanged.
The Atmospheric Chemistry Suite (ACS) package is an element of the Russian contribution to the ESA-Roscosmos ExoMars 2016 Trace Gas Orbiter (TGO) mission. ACS consists of three separate infrared ...spectrometers, sharing common mechanical, electrical, and thermal interfaces. This ensemble of spectrometers has been designed and developed in response to the Trace Gas Orbiter mission objectives that specifically address the requirement of high sensitivity instruments to enable the unambiguous detection of trace gases of potential geophysical or biological interest. For this reason, ACS embarks a set of instruments achieving simultaneously very high accuracy (ppt level), very high resolving power (>10,000) and large spectral coverage (0.7 to 17 μm—the visible to thermal infrared range). The near-infrared (NIR) channel is a versatile spectrometer covering the 0.7–1.6 μm spectral range with a resolving power of ∼20,000. NIR employs the combination of an echelle grating with an AOTF (Acousto-Optical Tunable Filter) as diffraction order selector. This channel will be mainly operated in solar occultation and nadir, and can also perform limb observations. The scientific goals of NIR are the measurements of water vapor, aerosols, and dayside or night side airglows. The mid-infrared (MIR) channel is a cross-dispersion echelle instrument dedicated to solar occultation measurements in the 2.2–4.4 μm range. MIR achieves a resolving power of >50,000. It has been designed to accomplish the most sensitive measurements ever of the trace gases present in the Martian atmosphere. The thermal-infrared channel (TIRVIM) is a 2-inch double pendulum Fourier-transform spectrometer encompassing the spectral range of 1.7–17 μm with apodized resolution varying from 0.2 to 1.3 cm
−1
. TIRVIM is primarily dedicated to profiling temperature from the surface up to ∼60 km and to monitor aerosol abundance in nadir. TIRVIM also has a limb and solar occultation capability. The technical concept of the instrument, its accommodation on the spacecraft, the optical designs as well as some of the calibrations, and the expected performances for its three channels are described.
Four hydrogen (H2) lines have been detected in a spectrum of Mars observed with the Far Ultraviolet Spectroscopic Explorer. Three of those lines are excited by the solar Lyman β photons. The line ...intensities correspond to a column H2abundance of$1.17 (\pm0.13) \times 10^{13}$per square centimeter above 140 kilometers on Mars. A photochemical model for the upper atmosphere that simulates the observed H2abundance results in an H2mixing ratio of 15 ± 5 parts per million in the lower atmosphere. The H2and HD mixing ratios agree with photochemical fractionation of D (deuterium) between H2Oand H2. Analysis of D fractionation among a few reservoirs of ice, water vapor, and molecular hydrogen on Mars implies that a global ocean more than 30 meters deep was lost since the end of hydrodynamic escape. Only 4% of the initially accreted water remained on the planet at the end of hydrodynamic escape, and initially Mars could have had even more water (as a proportion of mass) than Earth.
Self‐consistent models for 11 neutral and 18 ion species from 80 to 300 km on Mars have been developed by solving the continuity equations including ambipolar diffusion for ions. The models were ...calculated for the conditions of the HST, FUSE, and Mariner 6 and 7 observations of D, H2, and H, respectively, when the solar activity index was equal to 25, 61, and 88 at Mars orbit, respectively. Special care was taken to simulate the processes of H2 and HD dissociation in the reactions with CO2+, O+, CO+, N2+, N+, Ar+, and O(1D) and by photoelectrons. Thermal and nonthermal escape velocities were used as the upper boundary conditions for H2, H, HD, D, and He. The H2 and HD mixing ratios of 15 ppm and 11 ppb chosen to fit the FUSE and HST observations of H2 and D, respectively, result in (HD/H2)/(HDO/H2O) = 0.4. This value agrees with the depletion of D in H2 because of the smaller HDO photolysis cross section and the preferential condensation of HDO above the condensation level. Therefore the controversial problem of deuterium fractionation is solved throughout the atmosphere. The influx of cometary water was ≈0.5 m planetwide in the last 3.8 billion years. It cannot affect the estimates of more than 30 m of water lost by sputtering and nonthermal and thermal escape and more than 1.3 km of water lost in the reaction with iron with subsequent hydrodynamic escape of H2. The calculated ion density profiles at various solar activity and the column reaction rates provide complete quantitative information for behavior of each ion, its formation, and its loss. The HCO+ ion is abundant in Mars' ionosphere because it is a final product of many reactions of other ions with H2 and does not react with neutral species.
Using the Fourier Transform Spectrometer at the Canada–France–Hawaii Telescope, we observed a spectrum of Mars at the P-branch of the strongest CH
4 band at 3.3 μm with resolving power of 180,000 for ...the apodized spectrum. Summing up the spectral intervals at the expected positions of the 15 strongest Doppler-shifted martian lines, we detected the absorption by martian methane at a 3.7 sigma level which is exactly centered in the summed spectrum. The observed CH
4 mixing ratio is
10
±
3
ppb
. Total photochemical loss of CH
4 in the martian atmosphere is equal to
2.2
×
10
5
cm
−2
s
−1
, the CH
4 lifetime is 340 years and methane should be uniformly mixed in the atmosphere. Heterogeneous loss of atmospheric methane is probably negligible, while the sink of CH
4 during its diffusion through the regolith may be significant. There are no processes of CH
4 formation in the atmosphere, so the photochemical loss must therefore be balanced by abiogenic and biogenic sources. Outgassing from Mars is weak, the latest volcanism is at least 10 million years old, and thermal emission imaging from the Mars Odyssey orbiter does not reveal any hot spots on Mars. Hydrothermal systems can hardly be warmer than the room temperature at which production of methane is very low in terrestrial waters. Therefore a significant production of hydrothermal and magmatic methane is not very likely on Mars. The calculated average production of CH
4 by cometary impacts is 2% of the methane loss. Production of methane by meteorites and interplanetary dust does not exceed 4% of the methane loss. Methane cannot originate from an extinct biosphere, as in the case of “natural gas” on Earth, given the exceedingly low limits on organic matter set by the Viking landers and the dry recent history which has been extremely hostile to the macroscopic life needed to generate the gas. Therefore, methanogenesis by living subterranean organisms is a plausible explanation for this discovery. Our estimates of the biomass and its production using the measured CH
4 abundance show that the martian biota may be extremely scarce and Mars may be generally sterile except for some oases.
High-resolution spectra of Venus and Mars at the NO fundamental band at 5.3 μm with resolving power
ν
/
δ
ν
=
76
,
000
were acquired using the TEXES spectrograph at NASA IRTF on Mauna Kea, Hawaii. ...The observed spectrum of Venus covered three NO lines of the P-branch. One of the lines is strongly contaminated, and the other two lines reveal NO in the lower atmosphere at a detection level of 9 sigma. A simple photochemical model for NO and N at 50–112 km was coupled with a radiative transfer code to simulate the observed equivalent widths of the NO and some CO
2 lines. The derived NO mixing ratio is
5.5
±
1.5
ppb
below 60 km and its flux is
(
6
±
2
)
×
10
7
cm
−2
s
−1
. Predissociation of NO at the (0–0) 191 nm and (1–0) 183 nm bands of the
δ-system and the reaction with N are the only important loss processes for NO in the lower atmosphere of Venus. The photochemical impact of the measured NO abundance is significant and should be taken into account in photochemical modeling of the Venus atmosphere. Lightning is the only known source of NO in the lower atmosphere of Venus, and the detection of NO is a convincing and independent proof of lightning on Venus. The required flux of NO is corrected for the production of NO and N by the cosmic ray ionization and corresponds to the lightning energy deposition of
0.19
±
0.06
erg
cm
−2
s
−1
. For a flash energy on Venus similar to that on the Earth (∼
10
9
J
), the global flashing rate is ∼90 s
−1 and ∼6 km
−2 y
−1 which is in reasonable agreement with the existing optical observations. The observed spectrum of Mars covered three NO lines of the R-branch. Two of these lines are contaminated by CO
2 lines, and the line at 1900.076 cm
−1 is clean and shows some excess over the continuum. Some photochemical reactions may result in a significant excitation of NO (
v
=
1
) in the lowest 20 km on Mars. However, quenching of NO (
v
=
1
) by CO
2 is very effective below 40 km. Excitation of NO (
v
=
1
) in the collisions with atomic oxygen is weak because of the low temperature in the martian atmosphere, and we do not see any explanation of a possible emission of NO at 5.3 μm. Therefore the data are treated as the lack of absorption with a 2 sigma upper limit of 1.7 ppb to the NO abundance in the lower atmosphere of Mars. This limit is above the predictions of photochemical models by a factor of 3.
► New DCl and DF line lists. ► First ground-based observation of D/H in H2O above the Venus clouds. ► First observation of D/H in HF on Venus. ► Improved D/H in HCl on Venus. ► Observed D/H in water ...on Venus favors a constant D/H profile, in accord with theory.
Intensities of the spectral lines in the fundamental bands of D35Cl and DF were calculated using the semi-empirical dipole moment functions derived from the most accurate and precise measurements of intensities of the ro-vibrational lines of H35Cl and HF. Values obtained in this way for the deuterated species are superior to any available measured or calculated data to date.
Our study of the D/H ratios in H2O, HCl, and HF on Venus is based on spatially-resolved high-resolution spectroscopy using the CSHELL spectrograph at NASA IRTF. Search for DF on Venus using its R5 (1-0) line at 3024.054cm−1 results in a DF mixing ratio of 0.23±0.11ppb that corresponds to (D/H)HF=420±200 times that in the Standard Mean Ocean Water (SMOW). H2O abundances on Venus were retrieved using lines at 3022.366 and 3025.761cm−1 that were observed at an exceptionally low overhead telluric water abundance of 0.3pr.mm. The measured H2O mixing ratios at 74km vary insignificantly between 55°S and 55°N with a mean value of 3.2ppm. When compared with simultaneous observations of HDO near 2722cm−1, this results in (D/H)H2O=95±15 times SMOW. Reanalysis of the observation of the D35Cl R4 (1-0) line at 2141.540cm−1 (Krasnopolsky, V.A. 2012b. Icarus 219, 244–249) using the improved line strength and more thorough averaging of the spectra gives (D/H)HCl=190±50 times SMOW.
The similarity of the measured (D/H)H2O=95±15 at 74km with 120±40 observed by De Bergh et al. (De Bergh, C., Bezard, B., Owen, T., Crisp, D., Maillard, J.P., Lutz, B.L. 1991. Science 251, 547–549) below the clouds favors the constant (D/H)H2O from the surface to the mesosphere, in accord with the prediction by theory. D/H≈100 removes a difference of a factor of 2 between H2O abundances in the observations by Krasnopolsky (Krasnopolsky, V.A. 2010b. Icarus 209, 314–322) and the Venus Express nadir observations (Cottini, V., Ignatiev, N.I., Piccioni, G., Drossart, P., Grassi, D., Markiewicz, W.J. 2012. Icarus 217, 561–569). Equivalent widths of the HDO and H2O lines are similar in our observations; therefore some errors cancel out in their ratios. Photochemistry of HCl in the mesosphere tends to enrich D in HCl and deplete it in H2O. This may be an explanation of the twofold difference between the observed D/H in HCl and H2O.
An alternative explanation is based on (D/H)H2O≈200 observed in the mesosphere by Bjoraker et al. (Bjoraker, G.L., Larson, H.P., Mumma, M.J., Timmermann, R., Montani, J.L. 1992. Bull. Am. Astron. Soc. 24, 995) and Fedorova et al. (Fedorova, A. et al. J. Geophys. Res. 113, E00B22). This means an effective exchange of D between H2O and HCl and almost equal D/H in both species. However, this requires a twofold increase in D/H from the lower atmosphere to the mesosphere. This increase is not supported by theory; furthermore, condensation processes usually deplete D/H above the clouds.
Photochemistry of HF has not been studied; it proceeds mostly in the lower thermosphere, and D/H in HF may be very different from that in H2O. Overall, the observational data on D/H in all hydrogen-bearing species on Venus are helpful to solve the problem of deuterium fractionation on Venus.
Detection of Atomic Deuterium in the Upper Atmosphere of Mars Krasnopolsky, Vladimir A.; Mumma, Michael J.; Gladstone, G. Randall
Science (American Association for the Advancement of Science),
06/1998, Letnik:
280, Številka:
5369
Journal Article
Recenzirano
High-resolution spectroscopy of Mars' atmosphere with the Hubble Space Telescope revealed the deuterium Lyman α line at an intensity of 23 ± 6 rayleighs. This measured intensity corresponds to ...HD/H$_2$ = 1.5 ± 0.6 × 10$^{-4}$, which is smaller by a factor of 11 than HDO/H$_2$O. This indicates that fractionation of HD/H$_2$ relative to that of HDO/H$_2$O is not kinetically controlled by the rates of formation and destruction of H$_2$ and HD but is thermodynamically controlled by the isotope exchange HD + H$_2$O ↔ HDO + H$_2$. Molecular hydrogen is strongly depleted in deuterium relative to water on Mars because of the very long lifetime of H$_2$ (1200 years). The derived isotope fractionation corresponds to an estimate of a planetwide reservoir of water ice about 5 meters thick that is exchangeable with the atmosphere.
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