Ammonia Absorption Band Research Articles

СРАВНИТЕЛЬНЫЙ АНАЛИЗ ПОВЕДЕНИЯ СЛАБЫХ ПОЛОС ПОГЛОЩЕНИЯ АММИАКА 552 И 645 НМ В СПЕКТРЕ ЮПИТЕРА

This article contains the first obtained results of measuring the latitudinal variations of the very weak ammonia (NH3) absorption band at 552 nm along the central Jupiter meridian based on spectral observations in 2019. This study is preliminary in nature as the first attempt to clarify the possibility and advisability of further detailed study of this weak ammonia band behavior on the Jupiter disk. We obtained the latitude variations of the equivalent widths and equivalent absorption paths for the 552 nm NH3 band and, for comparison, the same characteristics for the more intense, but also weak NH3 645 nm band. The absorption maximum in both bands is observed in the equatorial zone, where in the center of the disk their equivalent widths are: W (552 nm) = 2.58 ± 0.29 A and W (645 nm) = 6.77 ± 0.25 A. A feature that distinguishes the behavior of the 552 nm NH3 band from the 645 nm NH3 band is a steeper decrease in absorption towards high latitudes, as well as small differences in the positions of absorption extrema in latitudes. The effect of latitudinal differences in the position of the extrema of the methane absorption bands was discovered by us as early as 1999 observations and was later noted also in the ammonia absorption bands. These features reflect structural differences in the Jupiter cloud layer and its troposphere and can serve as one of the means for its (troposphere) optical sounding.

MOLECULAR ABSORPTION BANDS IN JUPITER TROPOSPHERE RESEARCH

Based on spectrophotometric observations of Jupiter, made since 2004, some features of the behaviorof the methane and ammonia absorption bands on the planet's disk, are discussed. In the meridional course of theobserved variations in the equivalent widths of both ammonia and methane in the 6000A - 8000A spectral range,there are both similar elements and differences. Variations in the equivalent widths of ammonia absorption bands at6450 A and 7870A show a decrease in absorption in the low and moderate latitudes of the northern hemisphere. Inthe 7870 A band, compared to the 6450 A band, the ammonia absorption depression in the region of the NorthernEquatorial Belt is narrower in latitudes and some more deeper. With similar latitudinal variations in methaneabsorption, systematic differences are observed in the position of extrema for different bands of ammonia andmethane. The importance of studying the behavior of absorption bands for optical probing of the troposphere ofJupiter is noted. The alternatives of the model of the troposphere structure and ammonia cloud on Jupiter arediscussed. The first model assumes the existence of a geometrically and optically thick ammonia cloud layer. In thislayer, the formation of the observed molecular absorption bands in the process of multiple scattering mainly occurs.An alternative model assumes the presence of a geometrically and optically thin layer of ammonia clouds. The bulkof the molecular absorption in this case is created in the troposphere between the cloud layers of ammonia andammonium hydrosulfide. The need for further research in this direction is noted. One of the important results, so farpreliminary, was the differences we found in the latitudinal position of the extremes of the intensities of themolecular absorption bands on Jupiter. This feature is most likely due to the difference in the conditions of formationof different absorption bands in the ammonia cloud layer and the underlying troposphere. The complexity andambiguity of the mechanism of formation of molecular absorption bands requires further consideration of bothvarious models of the structure of the Jovian atmosphere and further detailed spectral observations of Jupiter, withparticular emphasis on the study of weak and moderate absorption bands, which we would like to draw attention to inthis publication

Ammonia in Jupiter’s Atmosphere: Spatial and Temporal Variations of the NH3 Absorption Bands at 645 and 787 Nm

Based on the material of long-term spectrophotometric observations of Jupiter, we studied the weak absorption bands of ammonia at 645 and 878 nm, whose behavior had previously been little studied. A clearly expressed depression of ammonia absorption in the 787-nm band was found in the Northern Equatorial Belt (NEB) of Jupiter. In the Great Red Spot, this band also exhibits substantial weakening. The position of the depression in the NEB is similar to that of the enhanced brightness temperature detected in the observations of the millimeter-wave radio emission, which is considered to be a result of the reduced ammonia content in this belt. At the same time, the weakening of the 787-nm band in the Red Spot is most likely caused by the enhanced bulk density of clouds, which influences the formation of absorption bands in the multiple scattering by cloud particles. The brightness temperature in the Red Spot is relatively low, as seen from observations in the radio and thermal IR ranges. We studied the spatial and temporal variations of the 645- and 787-nm bands in five belts of Jupiter: the Equatorial Zone (EZ), both Equatorial Belts (SEB and NEB), and both Tropical Zones (STZ and NTZ). The observations covered the time interval from 2005 to 2015, i.e., almost a complete orbital period of Jupiter. These observations confirmed the systematic character of the depression of the 787-nm band in the NEB and the difference in the latitudinal variations of the 645- and 787-nm bands. The latter can be related to features of the vertical distribution of the cloud density, which has a different influence on bands of different intensity.

Analysis of gaseous ammonia (NH3) absorption in the visible spectrum of Jupiter

Observations of the visible/near-infrared reflectance spectrum of Jupiter have been made with the Very Large Telescope (VLT) Multi Unit Spectroscopic Explorer (MUSE) instrument in the spectral range 0.48–0.93 µm in support of the NASA/Juno mission. These spectra contain spectral signatures of gaseous ammonia (NH3), whose abundance above the cloud tops can be determined if we have reliable information on its absorption spectrum. While there are a number of sources of NH3 absorption data in this spectral range, they cover small sub-ranges, which do not necessarily overlap and have been determined from a variety of sources. There is thus considerable uncertainty regarding the consistency of these different sources when modelling the reflectance of the entire visible/near-IR range. In this paper we analyse the VLT/MUSE observations of Jupiter to determine which sources of ammonia absorption data are most reliable. We find that the band model coefficients of Bowles et al. (2008) provide, in general, the best combination of reliability and wavelength coverage over the MUSE range. These band data appear consistent with ExoMOL ammonia line data of Yurchenko et al. (2011), at wavelengths where they overlap, but these latter data do not cover the ammonia absorption bands at 0.79 and 0.765 µm, which are prominent in our MUSE observations. However, we find the band data of Bowles et al. (2008) are not reliable at wavelengths less than 0.758 µm. At shorter wavelengths we find the laboratory observations of Lutz and Owen (1980) provide a good indication of the position and shape of the ammonia absorptions near 0.552 µm and 0.648 µm, but their absorption strengths appear inconsistent with the band data of Bowles et al. (2008) at longer wavelengths. Finally, we find that the line data of the 0.648 µm absorption band of Giver et al. (1975) are not suitable for modelling these data as they account for only 17% of the band absorption and cannot be extended reliably to the cold temperatures and H2/He-broadening conditions found in Jupiter’s atmosphere. This work is of significance not only for solar system planetary physics, but also for future proposed observations of Jupiter-like planets orbiting other stars, such as with NASA’s planned Wide-Field Infrared Survey Telescope (WFIRST).

Giant many-body effects in liquid ammonia absorption spectrum.

In the present work, we accurately calculate the absorption spectrum of liquid ammonia up to 13 eV using many-body perturbation approach. The electronic bandgap of liquid NH3 is perfectly described as the combination of density functional theory, Coulomb-hole screened exchange, and G0W0 approximation to the electronic self-energy, yielding a direct gap (Γ → Γ) of 7.71 eV, fully consistent with the experimentally measured gap from photo-emission spectroscopy. With respect to the NH3 optical properties, the entire spectrum in particular the low lying first absorption band is extremely affected by electron-hole interactions, leading to a fundamental redistribution of spectral weights of the independent-particle spectrum. Three well separated but broad main peaks are identified at 7.0, 9.8, and 11.8 eV with steadily increasing intensities in excellent agreement with the experimental data. Furthermore, we observe a giant net blue-shift of the first absorption peak of about 1.4 eV from gaseous to liquid phase as the direct consequence of many-body effects, allowing the associated liquid ammonia absorption band exciton to delocalize and feel more effectively the repulsion effects imposed by the surrounding solvent shells. Further, the spectrum is insensitive to the coupling of resonant and anti-resonant contributions. Concerning electronic response structure of liquid NH3, it is most sensitive to excitations at energies lower than its electronic gap.

Implementing a method based on near infrared spectroscopy for the “in-situ” determination of ammonia/water composition in an absorber test bench

This study presents the first results of composition measurement in an ammonia/water mixture determined at real time using near-infrared (NIR) spectroscopy in an absorber test bench. Two cells were designed and constructed; a static cell for calibrating and a dynamic cell for measuring in the absorber test bench. The ammonia absorption band was not significantly affected by the solution flow rate and neither by small variations in the temperature and pressure. A linear regression model between the absorbance value at 1033 nm and the ammonia concentration was established. This model was used to estimate the concentration in an ammonia/water absorber test bench. The methodology was validated by density measurements of the samples. The value of the mean deviation was 0.0024.

Spatially resolved variations in the methane and ammonia absorption in the atmosphere of Jupiter

A brief review of recent studies of the variations in the methane and ammonia absorption band over the disc of Jupiter carried out at the Fesenkov Astrophysical Institute observatory, with a spectrometer equipped with a charge-coupled device camera is given.

Accurate jovian radio flux density measurements show ammonia to be subsaturated in the upper troposphere

The availability of new accurate radio flux densities of Jupiter in and around the λ ≃ 1.3 cm ammonia absorption band, one from ground-based radio data and five from the WMAP satellite, permits re-examination of the structure of the jovian upper troposphere. These flux densities, with accuracies of 1–3%, indicate that the jovian atmospheric ammonia is globally subsaturated within and above the ammonia cloud tops, 0.4 bar ⩽ P ⩽ 0.6 bar , and subsolar (by a factor of 2) below the cloud base, 0.6 bar ⩽ P ⩽ 2 bar .

CASSINI/VIMS-V at Jupiter: Radiometric calibration test and data results

During the Cassini–Huygens flyby of Jupiter in December 2000, VIMS-V acquired multispectral data cubes of Jupiter's atmosphere. The visual and infrared imaging spectrometer-visual channel (VIMS-V) is one of the principal contributions of Italian Space Agency (ASI) to the Cassini–Huygens mission to Saturn. VIMS-V is an imaging spectrometer operating in the wavelength range 300– 1050 nm , with a (nominal) spectral resolution of 7.3 nm , and a (nominal) spatial resolution of 500 μrad . VIMS-V is boresighted with the VIMS-IR channel operating in the wavelength range 0.8– 5.1 μm . During the early phases of the Cassini mission, the spacecraft encountered Venus (June 23, 1999), followed shortly thereafter by a flyby of the Earth. During the Earth flyby the Moon (August 17, 1999) was observed. Following the Earth–Moon flyby, the spacecraft encountered Jupiter (closest approach on December 31, 2000), and during the roughly 6 months prior to Jupiter closest approach a series of observations were made of most of the objects in the Jovian system. We have determined the instrumental transfer function of VIMS-V using the Moon and Venus day side data. This transfer function was then used to remove instrumental effects from the Jupiter data and to convert raw instrumental response numbers to spectral radiance from the target. It was thus possible to study the spectral variability of Jupiter's atmosphere across its disk using data from both the visual (V) and infrared (IR) channels of VIMS. In this paper we discuss the main results obtained by the V channel. We have analyzed the principal spectral features of Jupiter atmosphere, and in particular, the spatial variation of methane and ammonia absorption bands over the Jovian disk. Using the instrument's spatial mapping capabilities we have investigated the nature of the absorption band in the spectrum of Jupiter's atmosphere at 929 nm that is consistent with the presence of ammonia or water vapor. After comet Shoemaker–Levy 9 impacted Jupiter, water vapor was considered the most likely cause of the 929 nm absorption feature, but our data indicate that ammonia is the source of this band. Other analyses were performed using standard techniques such as forming band ratios and removal of the continuum. Our analyses confirm previous ground or satellite based observations. We were also able to verify the instrument radiometric calibration, using observations conducted during the close encounters with Venus and the Moon.

Comparative morphology of molecular absorption on the discs of Jupiter and Saturn

Results on spectral observations of Jupiter and Saturn in the range of the methane and ammonia absorption bands are briefly described. Some peculiarities in the latitudinal and longitudinal variations in those absorptions are noted and the latitudinal asymmetry of the absorption distribution on Saturn's disc clearly expressed. The latter is evidently connected with seasonal changes and differences in the insolation regime caused by the very large tilt of Saturn's rotation axis and its rings. The behaviour of the absorption bands may be used as an important characteristics of the atmospheric state and structure in the future planetary monitoring programme.

Interpretation of NIMS and SSI Images on the Jovian Cloud Structure

We present maps of jovian cloud properties derived from images taken simultaneously by the Galileo solid state imaging system (SSI) and the near-infrared mapping spectrometer (NIMS) at 26 visible and near-infrared wavelengths, ranging from 0.41 to 5.2μm. Three regions—the Great Red Spot (GRS), a 5-micron Hot Spot, and one of the White Ovals—were studied. We perform a principal component analysis (PCA) on the multispectral images. The principal components (PCs), also known as empirical orthogonal functions, depend only on wavelength. The first PC is that spectral function which, when multiplied by an optimally chosen number (amplitude factor) at each pixel location and subtracted from the spectrum there, minimizes the variance for the image as a whole. Succeeding PCs minimize the residual variance after the earlier PCs have been subtracted off. We find that the pixel-to-pixel variations at the different wavelengths are highly correlated, such that the first three PCs explain 91% of the variance in the spectra. Further, one can estimate the amplitudes of the first two PCs using only the four SSI wavelengths and still explain 62% of the variance of the entire spectrum. This can be an advantage when trying to classify features that are resolved in the SSI images but not in the NIMS images. The first PC in all three regions shows negative correlation between 5μm emission and reflected solar light in both atmospheric windows and the methane and ammonia absorption bands. Thus most of the bright, optically thick clouds blocking thermal emission are also extended vertically to the upper troposphere. The first PC at the GRS shows a negative correlation between the violet and all other bands except 5μm for which the correlation is positive. Thus in the GRS there is a red chromophore (absorbing in the violet, reflecting at longer wavelengths) which is associated with clouds that block 5-μm emission. There is no such correlation at the hot spot and white oval regions and therefore no chromophore associated with clouds. The second PC shows a positive correlation between the depth of the methane and ammonia absorption bands and brightness at other visible and near-IR wavelengths; there is also a negative correlation between these quantities and 5-μm emission. Thus some of the bright, optically thick clouds blocking thermal emission are deep and do not extend vertically to the upper troposphere. A color image composed using the first three PCs shows areas of unusual spectra, which appear in distinct colors. An example is the small convective stormlike cloud to the northwest of the GRS. This cloud is highly reflective at long wavelengths (4μm) and might indicate unusually large particles.

Line intensities in the 647.5-nm ammonia band at low temperatures determined by intracavity laser spectroscopy

Absolute intensities of three ro-vibrational lines in the 647.5-nm ammonia absorption band are investigated by intracavity laser spectroscopy as a function of temperature (ranging from 180 to 300 K). The absorption lines are at 645.584, 645.712, and 646.050 nm. Their intensities at 299 K are found to be 2.57, 4.01, and 1.94 cm−1atm−1km−1, respectively. A rotational analysis using the derived intensity data is also presented.

Line intensities in the 647.5-nm ammonia band at low temperatures determined by intracavity laser spectroscopy

Absolute intensities of three ro-vibrational lines in the 647.5-nm ammonia absorption band are investigated by intracavity laser spectroscopy as a function of temperature (ranging from 180 to 300 K). The absorption lines are at 645.584, 645.712, and 646.050 nm. Their intensities at 299 K are found to be 2.57, 4.01, and 1.94 cm −1atm −1km −1, respectively. A rotational analysis using the derived intensity data is also presented.

Saturn's upper troposphere 1986–1989

This work describes observations of Saturn's atmosphere in the visible and near-infrared (460–940 nm) including 4 hydrogen quadrupole lines, 17 methane absorption bands ranging over 3 orders of magnitude in absorption strength, an ammonia absorption band, and the absolute calibrated continuum spectrum. All observations have complete coverage of Saturn's disk, in latitude as well as in center-to-limb position. A new method describing center-to-limb information is presented. The accuracy of the data is comparable to or better than that of previous data. This data set gives a quite complete description of Saturn's atmosphere in the visible and near infrared at the spatial resolution of ground-based observations. While the main data were acquired in 1988, small changes between 1986 and 1989 were determined also. Weak absorption features of hydrogen, methane, and ammonia show a significant enhancement in the North Polar Region compared to the rest of the planet. An atmospheric model is given which fits all observations within estimated errors. It has clear gas at the top of the atmosphere, an extended haze layer, and a reflective cloud at the bottom. Pressure levels and the haze optical depth were determined as a function of latitude. The single-scattering albedo spectrum of the particles (most likely ammonia ice crystals) is also given for each latitude. The methane mixing ratio is (3.0 ± 0.6) × 10−3, the ammonia mixing ratio is (1.2 + 0.8/−0.6) × 10−3 below the ammonia condensation level. A cold temperature methane absorption spectrum is determined under the assumption that methane band strengths are temperature invariant. It indicates that the absorption coefficients in band centers are typically 20–30% stronger than at room temperature. This spectrum should be useful in the interpretation of methane observations of all the giant planets and Titan.

Spatially resolved spectroscopic observations of Jupiter during the 1987 opposition

Spectroscopic observations in the spectral region λλ 6000–6600 » of major belts and zones of Jupiter have been carried out at Calar Alto Observatory (Spain) during the 1987 opposition, in support to the International Jupiter Watch Program. More than a hundred long-slit, medium resolution spectra have been taken at the coude focus of the 1.52 m telescope. The longitudinal and latitudinal distribution of the equivalent widths of the methane and ammonia absorption bands at 6190 » and 6450 » have been investigated. Unlike the CH4-6190 » band, the longitudinal distribution in the NH3-6450 » band shows asymmetries in some of the Jovian regions analyzed, being the absorption greater at West than at East limbs. These results are interpreted in terms of inhomogeneous scattering models by the simultaneous variation of the NH3 cloud optical thickness and its altitude level. The north-south scans show little latitudinal dependence on absorption in both methane and ammonia bands, although some relationship could be established between the absorption and relative continuum reflectivity variation.

Investigations of the optical properties of Saturn's atmosphere carried out at the main astronomical observatory of the Ukrainian Academy of Sciences

The results of polarimetric, photometric, and spectroscopic investigations of Saturn are given. Polarimetric and photometric investigations show that in the long wavelength spectral range ( λ > 0.5 μm) spherical particles play the dominant role in the reflected radiation. But for λ < 0.5 μm, oriented nonspherical particles are revealed in the equatorial region within planetocentric latitudes ±30°. It is suggested that Saturn's rings may be the source of these particles. According to photometric measurements, the brightness coefficient depends on the phase angle and marked temporal variations of the brightness coefficient were absent during the period from 1977 to 1981. A number of weak methane absorption bands were measured for the first time, and the ammonia absorption band at λ = 0.648 μm is present in our low-resolution photoelectric spectrophotometry. The reduction of high-resolution spectrograms has confirmed the presence of 20 ammonia lines belonging to the band at 0.648 μm. The observations can be fit to spherical aerosols with 1.35 ≤ n r ≤ 1.42, r 0 = 1 μm, σ 2 = 0.12; or n r = 1.93, r 0 ≅ 0.4 μm, σ 2 = 0.10; volume concentration 20 cm −3; and ammonia abundance (relative to methane) ≅ 0.01; and the optical thickness of the overlying gas layer is τ s = 0.2 to 0.4 for λ = 0.316 μm.

Near infrared reflection spectra of ammonia frost: Interpretation of the upper clouds of Saturn

Models of the uppermost cloud layer on Saturn indicate that the cloud is composed of solid ammonia, with the cloud particles being initially formed at about 150 K, and subsequently transported upward by convection to colder regions of the atmosphere. For the purpose of comparison with Saturn's near-i.r. reflection spectrum, we have obtained laboratory reflection spectra of ammonia frost corresponding to a variety of deposition rates and deposition temperatures. We find that the spectral location of the strong 3 μm absorption band of solid ammonia is highly dependent upon the manner in which the frost layer is formed. For rapid deposition on a 150 K cryosurface, which closely simulates what we believe to be the cloud formation process on Saturn, the band is centered at 3.29 μm. This band location, together with the apparent absence of gaseous ammonia overlying the solid ammonia cloud, is consistent with Saturn's observed reflection spectrum in the vicinity of 3 μm. Comparison of Saturn's reflection spectrum with that of Jupiter, in the vicinity of 1.3 and 1.6 μm, also supports the hypothesis of a Saturnian ammonia ice cloud. Differences in the two observed reflection spectra are interpreted as due to gaseous ammonia absorption bands for Jupiter being replaced by solid ammonia absorption features, at displaced wavelengths, for Saturn. An important conclusion of this study is that, for the purposes of either cloud interpretation or cloud radiation modeling, the polycrystalline phase of solid ammonia, and in turn its spectrum, is highly dependent upon the manner in which the soled is formed.

The abundances of ammonia in the atmospheres of Jupiter, Saturn, and Titan

An investigation of low-resolution ratio spectra of Jupiter, Saturn, and Titan in the region 5400–6500 Å has permitted new evaluations of ammonia absorption bands. The distribution of ammonia over the disk of Jupiter is very inhomogeneous. The carbon-to-nitrogen ratio is distinctly different from the solar value, but this is probably a result of uneven mixing of methane and ammonia, as suggested previously by Kuiper, rather than a compositional anomaly. The abundance of ammonia on Saturn also shows spatial variations, but appears constant in time over a 3-yr period. Two weak, unidentified absorptions were discovered in the red region of Titan's spectrum, in the absence of any detectable ammonia. The new upper limit is ηN < 120 cm-am.

Observations of NH 3 in the atmosphere of Jupiter during 1973, 1974

The 6450 Å ammonia absorption band in the atmosphere of Jupiter was observed during the summers of 1973 and 1974. High-dispersion spectra of this band were obtained and analyzed on a line-by-line basis to derive ammonia abundances in the Jovian atmosphere. The abundances determined this way show strikingly large fluctuations.

The Temperature and Ammonia Profiles in the Jovian Atmospheres from Inversion of the Jovian Emission Spectrum

Inversion techniques developed for remote soundings of the Earth's atmosphere by meteorological satellites are applied to the few earth-based observations of the Jovian emission spectrum. With a minimum of assumptions, it is possible to obtain directly from the observed spectra the profiles of atmospheric temperature and ammonia abundance. The temperature profile is obtained from inversion of the radiances observed in the infrared bands of methane and hydrogen. With the use of the derived temperature profile, the ammonia abundance profile is obtained from radiances observed in the infrared and microwave absorption bands of ammonia. The temperature profile is characterized by a definite tropopause region with a temperature of about 11 v K and by a stratospheric region in which the temperature slowly increases with altitude. The derived ammonia profile indicates the presence of a saturated ammonia layer with a base temperature of 140 K. It is concluded that inversion of infrared and microwave emission spectra provides a powerful technique for sounding the Jovian atmosphere, and that more accurate measurements should be obtained from fly-bys and earth-based observations. Subject headings: abundances, planetary - atmospheres, planetary - Jupiter