Ammonia Cloud Research Articles

Vertical Structure of Jupiter's Atmosphere at the Galileo Probe Entry Latitude

The vertical structure of Jupiter's atmosphere is examined to better characterize the latitudinal region of the Galileo probe entry site: +5°–10° planetographic latitude. One-hundred Hubble Space Telescope images of Jupiter taken with the 893- and 953-nm filters on 13 and 17 February 1995, 5 October 1995, and 14 May 1996 are analyzed, and a two-cloud model of the jovian atmosphere (D. M. Kuehn and R. F. Beebe 1993,Icarus101,282–292) is used to interpret center-to-limb variations of bright plume heads and adjacent dark areas in the latitude region +2°–10°. For both the plume heads and dark areas, the top of the ammonia cloud deck reaches a pressure level of roughly 200 mbar, and the cloud has an optical depth of between 6 and 7. The optical depth of the stratospheric haze layer is 0.59 for the plumes and 0.34 for the dark regions, while the single scattering albedo of the ammonia cloud deck is 0.996 for the plumes and 0.950 for the dark regions. Sensitivity tests motivated by the preliminary results from the Galileo probe net flux radiometer and nephelometer experiments indicate that the ammonia cloud deck may have a patchy structure.

The depth of penetration by the fragments of Comet SL9 into Jupiter's atmosphere

Assuming the mass of the fragments to be 10 12, 10 13, 10 14 kg and the density to be 3.0, 1.0, 0.8, 0.5, 0.2 g cm −3, we calculate that the fragments can penetrate the ammonia cloud layer and explode at about the 1 bar level. This result agrees with the result by simulation of Ref. [1].

Solar and thermal radiation in Jupiter's atmosphere: initial results of the Galileo probe net flux radiometer.

The Galileo probe net flux radiometer measured radiation within Jupiter's atmosphere over the 125-kilometer altitude range between pressures of 0.44 bar and 14 bars. Evidence for the expected ammonia cloud was seen in solar and thermal channels down to 0.5 to 0.6 bar. Between 0.6 and 10 bars large thermal fluxes imply very low gaseous opacities and provide no evidence for a deep water cloud. Near 8 bars the water vapor abundance appears to be about 10 percent of what would be expected for a solar abundance of oxygen. Below 8 bars, measurements suggest an increasing water abundance with depth or a deep cloud layer. Ammonia appears to follow a significantly subsaturated profile above 3 bars. Unexpectedly high absorption of sunlight was found at wavelengths greater than 600 nanometers.

Galileo observations of the impacts

Galileo observations in the UV, visible, and infrared uniquely characterize the luminous phenomena associated primarily with the early stages of the impacts of SL9 fragments—the bolide and fireball phases—because of the spacecraft's direct view of the impact sites. The single luminous events, typically 1 min in duration at near-IR wavelengths, are interpreted as initial bolide flashes in the stratosphere followed immediately by development of a fireball above the ammonia clouds, which subsequently rises, expands, and cools from ∼ 8000 K to ∼ 1000 K over the first minute. The brightnesses of the bolide phases were remarkably similar for disparate events, including L and N, which were among the biggest and smallest of the impacts as classified by Earth-based phenomena. Subsequent fireball brightnesses differ much more, suggesting that the similar-sized fragments were near the threshold for creating fireballs and large dark features on Jupiter's face. Both bolides and fireballs were much dimmer than had been predicted before the impacts, implying that impactor masses were small (∼0.5km diameter). Galileo data clarify the physical interpretation of the “first precursor,” as observed from Earth: it probably represents a massive meteor storm accompanying the main fragment, peaking ∼10s before the fragment penetrates to the tropopause; hints of behind-the-limb luminous phenomena, recorded from Earth immediately following the peak of the first precursor, may be due to reflection of the late bolide/early fireball stages from comet debris very high in Jupiter's atmosphere.

The homogeneous equilibrium approximation in heavy gas dispersion models

The dispersion of heavier-than-air aerosol clouds is of great relevance to the analysis and assessment of hazards in the chemical and petrochemical industries. There are many simple mathematical models for heavy gas dispersion but, in most cases, two-phase dynamics are either ignored or treated in a very simple way, without any convincing scientific justification. In fact almost all of those models which do treat two-phase phenomena rely on the homogeneous equilibrium assumption, whether they be for jet dispersion, for gravity dominated dispersion, or for any other aspect of a two-phase release. Here the authors test the homogeneous equilibrium model against a more sophisticated aerosol approach, in the context of two-phase ammonia clouds released in dry and moist air. It is shown that the simpler model does indeed provide a good description for some envisaged release situations, and guidance is given on where the homogeneous equilibrium model is not likely to be adequate. The homogeneous equilibrium model seems to be adequate for evaluating the dispersion of heavier-than-air clouds containing droplets smaller than 100 micrometers. In this paper the dispersion model DRIFT is compared with the aerosol model AERCLOUD.

Galileo infrared observations of the Shoemaker‐Levy 9 G Impact Fireball: A Preliminary report

The Galileo spacecraft was fortuitously situated for a direct view of the impacts of the fragments of comet Shoemaker‐Levy 9 in Jupiter's atmosphere. The Galileo Near Infrared Mapping Spectrometer (NIMS) instrument observed several of the impact events in several discrete bands and with a temporal resolution of roughly five seconds. Data have been received for the G impact showing two phases of strong infrared emission. The first phase is approximately one minute in duration and corresponds to the initial fireball and early plume development. This is followed six minutes later by the onset of heating by plume ejecta falling back on the upper atmosphere. This report provides a preliminary description of the fireball phase. The first detection of the G fireball occurred at 07:33:37 UT on July 18, 1994, approximately five seconds after the initial signal recorded by the Galileo Photopolarimeter‐Radiometer (PPR) instrument. The preceding NIMS measurement, occurring approximately one second before the initial PPR signal, showed no evidence of fireball emission. The detected duration of the fireball at 4.38 µm was 70 seconds. Spectra in the first half of this period show blackbody‐like emission, with absorption features from overlying methane and molecular hydrogen. The strengths of these features place the fireball in the upper troposphere and lower stratosphere, above the ammonia cloud layer. The emitting surface rises and accelerates, achieving a velocity of 2–3 km/sec after 25 seconds, in qualitative agreement with that expected for an explosion in an inhomogeneous atmosphere.The Galileo spacecraft, en route to Jupiter, was in a position to obtain a direct view of the impacts of Comet Shoemaker‐Levy 9 fragments on the nightside of Jupiter, providing an opportunity to investigate the early temporal evolution of the impact events. It was predicted that the comet fragments would produce high temperature bolides as they entered the atmosphere and then explode, producing hot fireballs which would rise, expand, and cool (Sekanina, 1993; Zahnle and Mac Low, 1994; Chevalier and Sarazin, 1994; Ahrens et al., 1994; Boslough et al., 1994). Much of the predicted radiation occurs in the infrared region, and time‐resolved infrared spectral observations, obtained over a broad wavelength range, are ideal for studying these phenomena.

The Chemistry of Hydrocarbon Ions in the Jovian Ionosphere

We have modeled the chemistry of hydrocarbon ions in the jovian ionosphere. We find that a layer of hydrocarbon ions is formed in the altitude range 300-400 km above the ammonia cloud tops, due largely to direct ionization of hydrocarbons by photons in the wings of the H 2 absorption lines in the 912- to 1100-Å region that penetrate to below the methane homopause. We have explicitly included in the model 156 ion-neutral reactions involving hydrocarbon ions with up to two carbon atoms. Larger hydrocarbon ions are included as two pseudoions, C 3H + n and C 4H + n . The model shows that 15 reactions of H +, CH + 3, CH + 5, C 2H + 3, C 2H + 4, C 2H + 5, AND C 2H + 6 with hydrocarbon neutrals are the major processes that are responsible for the production and growth of C 1 -, C 2 - and C 3 - or C - 4ions in the hydrocarbon ion layer. The model also shows that ions initially produced in the hydrocarbon ion layer are converted into hydrocarbon ions with more than two carbon atoms with very little loss by recombination. It is likely that successive hydrocarbon ion-neutral reactions continue to produce even larger hydrocarbon ions, so the terminal ions probably have more than three or four carbon atoms. In the auroral regions, the chemistry of hydrocarbon ions may modify the densities of neutral hydrocarbons, especially C 2H 2 in the upper mesosphere, and may play a major role in the production of polar haze particles.

The homogeneous equilibrium approximation in models of aerosol cloud dispersion

The dispersion of heavier-than-air aerosol clouds is of great relevance ti the analysis and assessment of hazards in the chemical and petrochemical industries. There are many simple mathematical models for heavy gas dispersion but, in most cases, two-phase dynamics are either ignored or treated in a very simple way, without any convincing scientific justification. In fact almost all of those models which do treat two-phase phenomena rely on the “homogeneous equilibrium” assumption, whether they be for jet dispersion, for gravity dominated dispersion, or for any other aspect of a two-phase release. Here we test the homogeneous equilibrium model against a more sophisticated aerosol approach, in the context of two-phase ammonia clouds released in dry and moist air. It is shown that the simpler model does indeed provide a good description for some envisaged release situations, and guidance is given on where the homogeneous equilibrium model is not likely to be adequate. The homogeneous equilibrium model seems to be adequate for evaluating the dispersion of heavier-than-air clouds containing droplets smaller than 100 μm.

A model for mass and heat transfer in an aerosol cloud

In mathematical models for heavy gas clouds dispersing in the atmosphere, two-phase dynamics is usually modelled using assumptions of homogeneous equilibrium. This implies that the mixture of liquid droplets and gas is homogeneous and in thermodynamic equilibrium at all times. This paper discusses a more rigorous approach for modelling the thermodynamical aspects of heavy gas dispersion. A model is presented for the evaporation and condensational growth of a monodisperse binary droplet population, including also the influence of air entrainment. The model evaluates the thermodynamical evolution of a five-component aerosol mixture, consisting of two-component droplets and a three-component gas. The model has been applied for evaluating two-phase ammonia clouds released in both dry and moist air. The numerical results show the influence on contaminant concentration of the droplet size, the atmospheric moisture and the non-ideality of a liquid solution.

A model for mass and heat transfer in an aerosol cloud

In mathematical models for heavy gas clouds dispersing in the atmosphere, two-phase dynamics is usually modelled using assumptions of homogeneous equilibrium. This implies that the mixture of liquid droplets and gas is homogeneous and in thermodynamic equilibrium at all times. This paper discusses a more rigorous approach for modelling the thermodynamical aspects of heavy gas dispersion. A model is presented for the evaporation and condensational growth of a monodisperse binary droplet population, including also the influence of air entrainment. The model evaluates the thermodynamical evolution of a five-component aerosol mixture, consisting of two-component droplets and a three-component gas. The model has been applied for evaluating two-phase ammonia clouds released in both dry and moist air. The numerical results show the influence on contaminant concentration of the droplet size, the atmospheric moisture and the non-ideality of a liquid solution.

Photometry of Saturn's 1990 Equatorial Disturbance

Multicolor CCD images at selected wavelengths from 370 to 900 nm obtained at Pic-du-Midi Observatory during four different phases of the development of Saturn's equatorial disturbance are used to analyze its photometric properties. Three epochs and four morphologically different regions were studied: October 5, 1990, the source of the event, the Great White Spot properly; October 6 and 8, 1990, an undisturbed area in the same latitude far away from the GWS; November 17, 1990, the mature phase of the planetary disturbance; and July 7, 1991, the declined phase, but still active, disturbance. The absolute spectral reflectivity of the GWS single feature showed a marked increase at all wavelengths with respect to the undisturbed neighbourings. A simple interpretation of these data indicates that optically thick cloud tops in the GWS nucleus were located at average pressures ∼200 mbar, ∼two scale heights above the expected ammonia cloud deck at ∼1.8 bar. The particles forming the GWS were very bright in the red and less contaminated by the blue absorbing impurities. Limb to limb scans at latitudes 5°, 13°, and 20° N are used to characterize the temporal spectral dependence of the Minnaert's limb darkening coefficient K and the normalized albedo ( I/F) 0. Pronounced increases in K and ( I/F) 0 were noted at continuum wavelengths in November 1990, but only moderate changes were apparent in July 1991 (both with respect to the undisturbed area observed in early October 1990). The particles constituting the upper cloud of the disturbance during its mature phase in November 1990 showed changes in their vertical distribution, and had a higher single-scattering albedo at blue and yellow wavelengths, and were more forward scattering than those which normally form the equatorial haze and cloud of Saturn.

Detectability of water-ice clouds expected after the P/Shoemaker-Levy 9 impact with Jupiter at near-infrared molecular bands

The predicted crash of periodic comet Shoemaker-Levy 9 into Jupiter in mid-July 1994 is expected to release a considerable amount of cometary material, which is mainly composed of water, into Jupiter's atmosphere. This released water will form a water-ice cloud layer in the stratosphere. We estimated the reflected sunlight from the impact cloud at near-infrared wavelengths using a radiative transfer model. It was found that the cloud is likely to be observable when we choose some strong molecular absorption band such as methane for imaging observation, because we get higher contrast between the cometary ice cloud and background scattered light from the lower ammonia clouds at these wavelengths. This cometary cloud can be used as an atmospheric tracer of the upper troposphere and the stratosphere of Jupiter.

Clouds of ammonia ice: Laboratory measurements of the single-scattering properties

This work presents scattering measurements and photographs of ammonia ice crystals grown at temperatures from 130 to 180 K. The prime candidate for the material making up the visible clouds of Jupiter and Saturn is ammonia ice. Spacecraft observations of these planets have constrained the single-scattering properties of the cloud particles. To further investigate the nature of these particles, ammonia ice crystals were grown at temperatures occuring at the relevant levels of the atmospheres of Jupiter and Saturn. The experimental apparatus used to make these measurements has a glass-walled cylindrical chamber which permits measurement of the scattered light over a wide range of scattering angles and a temperature control system which uses a liquid nitrogen reservoir combined with heaters. The chamber is illuminated by a tungsten lamp through a rapidly spinning filter/polarizer wheel which yields measurements of intensity and linear polarization in each of three colors. A photographic record of the crystals is obtained with a microscope objective, and six linear array detectors measure the scattered light. Representative scattering measurements and photographs are presented, showing a variety of phase functions and crystal shapes. The data cannot be reproduced by theoretical calculations for ammonia clouds composed purely of cubic, tetrahedral, or octahedral crystals. The data appear similar to microwave analog measurements of the scattering by a mix of particles shapes and also by fluffy particles. The ammonia measurements fall into two groups: one has wavelength-dependent polarization and for size parameters up to about seven the scattering properties can be fit by Mie theory. The second group has wavelength-independent phase functions, implying size parameters of 10 to 50, and has a characteristic signature of polarization varying from - 10% to +10%. The data can be used to rule out some models for Jupiter's and Saturn's atmospheres and to guide future modeling efforts. For Jupiter, models with a cloud of ammonia crystals of size parameter equal to about 5 (in the red) are suggested. For Saturn, a model is suggested that has a thin layer of small ammonia crystals (in the Mie range) over a thicker ammonia cloud with the wavelength-independent polarization that is characteristic of larger crystals.

Water—cumulus in Jupiter's atmosphere: Numerical experiments with an axisymmetric cloud model

The formation and evolution of convective H 2O clouds in the deep Jovian troposphere are investigated using an axisymmetric model with parameterized microphysics to calculate the vertical and radial wind components and the liquid and ice contents in the clouds. A constant upwelling velocity from within the deep atmosphere was assumed, and the model was initiated with a slightly superadiabatic lapse rate below the lifting condensation level (LCL), which was located at 5160 mbar. The abundance of water in Jupiter's atmosphere was assumed to be solar, and several relative humidity profiles were evaluated. The initial temperature profile above the LCL was assumed to be dry-adiabatic. The role of condensate mass-loading in the dynamic evolution of the cloud was considered. Results show that for plausible values of superadiabatic deviations and upwelling velocities, the Jovian water clouds are 25 km deep and extend up to the 3-bar pressure level. These cumulus clouds contain 2.5–5 g/m 3 of liquid water and ice, with updrafts at the axis reaching 30 m/sec and a cloud top temperature of 235 K. Clouds could retain their vertical structure for long periods of time. According to the model the water clouds reach the ammonia condensation level only when the temperature and relative humidity profiles are coupled in such a way that instability (or at least neutrality) to moist convection is ensured over a large depth of the troposphere. Under intermediate conditions, at which the upper layers were assumed to be stable, clouds were 40–50 km deep with updrafts of 60–70 m/sec, but did not reach the ammonia cloud deck. Thus, they were probably not connected to the observed equatorial “plumes”.

Analysis of voyager 2 images of Jovian lightning

Voyager 2 images have been examined and found to contain bright spots due to lightning activity that is confined to two narrow latitude bands centered at 49° N and 13.5° N latitude and to a single region near 60° N latitude. No lightning activity is observed in the southern hemisphere even though a large area was observed with the same sensitivity as that used for northern hemisphere observations. The brightest band of lightning activity occurs at 49° N, but no unusual cloud features can be associated with this activity. This result is consistent with the depth of the lightning activity being 80 km below the ammonia clouds and thus not producing readily observed disturbances in the visible clouds. The secondary band of lightning activity that appears at 13.5° N is restricted in longitude to the “disturbed” zone. One of the two storms found in this tropical band is very close to the location of a bright cloud feature that extends some 3.5° in latitude. If it is assumed that the deflection of this feature is caused by the westerly flow of an air parcel over a hill in the potential temperature surface located at the 5- to 10-bar level, then the air parcel moving over the hill would be forced to rise sufficiently high that the free convection level would be reached. This situation could be the cause of the lightning activity in this region. The energy dissipation rate by lightning activity in the Voyager 2 images is 3200 W/km 2 when attenuation by clouds is considered. The energy dissipation rate is approximately 50% greater than that found in the Voyager 1 images. The ratio of the energy dissipated by Jovian lightning to the thermal flux available to drive convection motions is found to be three decades larger than the terrestrial ratio. This value is similar to the ratio for the two planets found by Ingersoll (1990) for the conversion of thermal energy flux to mechanical energy.

The Jovian ionospheric E region

We have constructed a model of the Jovian ionosphere that includes direct photoionization of hydrocarbon molecules. A high resolution solar spectrum was synthesized from Hinteregger's solar maximum spectrum (F79050N) and high resolution cross sections for photoabsorption by H2 bands in the range 842 to 1116 Å were constructed. Two strong solar lines and about 30% of the continuum flux between 912 and 1116 Å penetrate below the methane homopause despite strong absorption by CH4 and H2. We find that hydrocarbons (mainly C2H2) are ionized at a maximum rate of 55 cm−3 sec−1 at 320 km above the ammonia cloud tops. The hydrocarbon ions produced are quickly converted to more complex hydrocarbon ions through reactions with CH4, C2H2, C2H6, and C2C4. We find that a hydrocarbon ion layer is formed near 320 km that is about 50 km wide with a peak density in excess of 1 × 104 cm−3.

Role and Evolution of Ammonia Clouds in Bipolar Flow Sources

Many bipolar flow sources have been found around infrared sources with a wide luminosity range. The bipolar sources are thought to be a common stage of early stellar evolution. Recently, compact dense molecular clouds have been detected, just around the exciting infrared sources of several bipolar flow sources (e.g. Torrelles et al. 1983). An investigation of the nature of these surrounding dense molecular clouds should be important to study the acceleration and collimation mechanisms of the bipolar outflow.

A Highly Excited Ammonia Cloud at least 300 pc from the Galactic Centre

AbstractA cloud with a l.s.r. velocity of ∼ 160 km s-1 and a half-width of ∼ 25 km s-1 was mapped in the (1,1), (2,2) and (4,4) lines of para NH3 and the (3,3) and (6,6) lines of ortho NH3 using the 40″ arc beam of the 100-m Effelsberg telescope. The cloud was extended some 5′ in latitude and contained a central concentration ∼ 1′.5 in size at ℓ = 1°.59; b=0°.01. Line temperatures of the four higher transitions define a rotational temperature of ∼ 120 K, while the (1,1) and (2,2) alone yield the lower value of ∼ 40 K. The high temperatures, well above that of the dust, are too high to be caused by collisions with H2 molecules in the ground state, but might result from collisions with H2 in excited states. On a (ℓ, v) plot the cloud does not coincide with any generally accepted galactic centre feature but it is possibly located near the periphery of the ‘rotating nuclear disk’, where conditions would be favourable for the production of shocks, excited H2 and possibly high NH3 temperatures.

Mesoscale Waves as a Probe of Jupiter's Deep Atmosphere

Search of the Voyager images of Jupiter reveals a class of mesoscale waves occurring near the extrema of the zonal velocity profile between latitudes 30°S and 30°N. The average horizontal wavelength is 300 km, compared to an atmospheric scale height of 20 km; the standard deviation about the mean is only 20%. The alignment of most wave trains is nearly zonal, i.e., wave crests are north–south. A typical wave packet is 20 wave crests in length and 1300 km wide. Modeling shows that the waves propagate in a duct below the ammonia condensation level (the visible clouds). For selection of one dominant mode, the Richardson number in the duct must be of order unity; this is the first quantitative determination that has ever been made of the stability of Jupiter's atmosphere below its clouds. The duct is topped by a trapping layer, the ammonia clouds, where the Richardson number is less than 0.25. Meridional trapping of the wave packets is due to the shear in the zonal wind. It is speculated that the period of the waves is diurnal and that they do not propagate poleward of latitudes ±30° because the Coriolis frequency exceeds the duct Brunt frequency. The waves might be excited by diurnal variations of moist convective activity in the ammonia clouds.

Jupiter's zone-belt structure at radio wavelengths: II. Comparison of observations with model atmosphere calculations

Model atmosphere calculations are presented which simulate high-resolution maps of Jupiter's radio emission. They are compared with observations recently obtained at the Very Large Array at 1.3, 2.0, 6.1, and 20.5 cm with resolutions ranging from 0.075 to 0.218 Jovian radii ( I. de Pater and J. R. Dickel (1986). Jupiter's zone-belt structure at radio wavelengths. I. Observations. Astrophys. J., in press). The models indicate that ammonia gas is strongly depleted in the upper atmosphere with respect to the solar value both in zones and belts. At very high levels in the atmosphere ( P < 0.3−0.5 bar) the gas is undersaturated and distributed uniformly over the planet. In the cloud formation region (0.5 < P < 2 bar), the ammonia depletion is largest in the belts, where it extends down to depths corresponding to 1.8–2 bar. In the zones, the lower ammonia abundances are found down to pressures of 1 bar. Deeper into the Jovian atmosphere, at pressures ≥2.2 bar, the gas is overabundant relative to the solar value by nearly a factor of 2 in both zones and belts. The altitude distribution of the ammonia gas is explained in terms of chemistry, cloud physics, and atmospheric dynamics. The undersaturation at high levels in the atmosphere is attributed to photodissociation of ammonia gas under influence of solar UV photons, coupled with Jupiter's meteorology (up- and downward drafts in the atmosphere). The general depletion of this gas throughout Jupiter's upper atmosphere may be caused by trapping of the gas in a layer of NH 4SH particles, and/or in an aqueous ammonia cloud. The cloud deck responsible for trapping ammonia gas is thicker above zones than belts. If the observed depletion of ammonia gas is entirely due to trapping in an NH 4SH cloud, the difference in thickness of this cloud between zones and belts gives rise to a temperature difference of 3–4°K between the two regions. This temperature difference may trigger the zonal wind motions in Jupiter's atmosphere near the cloud tops.