The rotational temperature and column density of H3 in Uranus

H₃⁺ emission from Uranus has been observed repeatedly for over a decade. However, the details of the emission mechanisms are still poorly understood. In this paper, we discuss our findings from the observations we made in September 2000 and September 2001. The spectrum of Uranus was recorded at the NASA Infrared Telescope Facility using the SpeX instrument between 3 and 5 μm, with a resolving power of 1000. The 3.4–4.1 μm range permits a determination of both the H₃⁺ column density and its rotational temperature. The H₃⁺ emission, measured at 3.986 μm in the 0.8×3.7 arcsec aperture, was 0.031 Jy in September 2000 and 0.053 Jy in September 2001. The rotational temperature was found to be 560±40 K and 640±40 K in 2000 and 2001 respectively, with corresponding column densities of 5.1 (+3.2,−1.4) 10¹¹ and 4.0 (+1.8,−1.0) 10¹¹ cm⁻². These results extend the baseline for the variability study of the H₃⁺ emission (Astrophys. J. 524 (1999) 1059). Previous observations between 1992 and 1998 seemed to indicate a correlation between the H₃⁺ intensity and the solar cycle. The current data for 2000 and 2001 appear to be consistent with this general tendency.     

Measurements of the H2 4-0 quadrupole bands of Uranus and Neptune

We find that the equivalent widths of the lines of the 4-0 H₂ quadrupole band on Uranus and Neptune are substantially smaller than the values found by some previous observers. An analysis of our results based on a range of atmospheric models yields H₂ abundances of 240 ± 60 km-amagats for Uranus and ?200 km amagats for Neptune.     

Pressure-induced absorption by H2 in the atmosphere of Jupiter and Saturn

The S(1) line of the pressure-induced fundamental band of H₂ was identified and measured in the spectra of Saturn and Jupiter. This broad line at 4750 cm^?1 lies in a region free from telluric and planetary absorptions. It is about 99% absorbing in the core; the high-frequency wing extends to at least 5100 cm^?1. We compare the obseved line shape to the predictions of both a reflecting-layer model (RLM) and a homogeneous scattering model (HSM). The RLM provides a good fit to the Saturn line profile for temperatures near 150K; the derived base-level density is 0.52 (+0.26, ?0.17) amagat and the H₂ abundance is 25 (+10, ?9) km-amagat, assuming a scale height of 48 km. The Jupiter line profile is fit by both the RLM and HSM, but for widely differing temperatures, neither of which seems probable. The precise fitting of the observed S(1) line profile to computed models depends critically on the determination of the true continuum level; difficulties encountered in finding the continuum, especially for Jupiter, are discussed. Derived RLM densities and abundances for both planets are substantially lower than those derived from RLM analyses of the H₂ quadrupole lines, the 3ν₃ band of CH₄, and from other sources.     

The significance of pressure shifts for the interpretation of H2 quadrupole lines in planetary spectra

The effects of pressure shifts on the formation of H₂ quadrupole absorption lines in the atmospheres of the major planets have not previously been considered. It is shown that, although pressure shifts have not been measured for the 3-0 and 4-0 H₂ bands, they can be estimated from existing experimental and theoretical knowledge. Using these estimates, it is shown that the effect of pressure shifts is negligible for Jupiter and small for Saturn, but quite large for Uranus. Consideration of the shifts reduces H₂ abundances determined for Uranus by from 25% to 50% as compared to calculations in which the shifts are ignored. The effect may be even larger for Neptune.     

On the detectability of H2S in Jupiter

The influence of hydrogen sulfide, a still-undetected key molecule for the Jovian atmospheric chemistry in the infrared spectrum, was investigated. Synthetic spectra including various vertical distribution profiles of H₂S have been computed and compared with observational data for Jupiter in the 2- to 15-cm^?1 and 1160- to 1200-cm^?1 spectral ranges. No firm conclusion about the presence of H₂S can be drawn from the latter spectral region because of large uncertainties in gaseous opacities. In the microwave range, H₂S is found to be a possible candidate to explain the measurements. Constraints to its vertical distribution which would imply a significant supersaturation in the troposphere are derived. Physical and chemical processes involving H₂S in the atmosphere are discussed in the light of this hypothesis.     

Jupiter: An inhomogeneous atmospheric model analysis of spatial variations of the H2 4-0 S(1) line

An analysis of the structure of the Jovian atmosphere, primarily based on center-to-limb variations (CTLV) of the equivalent width of the hydrogen quadrupole 4-0 S(1) line, is presented. These data require that the atmosphere have regions of both long- and short- scattering mean free paths. Two alternative cloud structures which fit the data are developed. The first is a two-cloud model (TCM) consisting of a thin upper cloud and a lower semi-infinite cloud, with absorbing gas between the clouds and above the upper cloud. The second model is a reflecting-scattering model (RSM), in which a gas layer lies above a haze consisting of scattering particles and absorbing gas. The cloud-scattering phase function in both models must have a strong forward peak. The CTLV data require, however, the presence of a backscattering lobe on the phase function, with the backscattering intensity about 4% of the forward scattering. The decrease in reflectivity of all regions from the visible to the ultraviolet is explained by the presence of dust particles mixed with the gas. Most of the ultraviolet absorption in the atmosphere must occur above the upper cloud layer. Particles with a uniform distribution of radii from 0.0 to 0.1 μm with a complex index of refraction varying as λ^?2.5 are used. The contrast in reflectivity between belts and zones may be explained by the larger concentration of dust in the belts than in the zones. Spatially resolved ultraviolet limb-darkening curves will help to determine the dust distribution of the Jovian atmosphere. The visible methane bands at λλ 6190, 5430, and 4860 Å are analyzed in terms of these models. We derive a methane-to-hydrogen mixing ratio of 2.8 × 10^?3, which is about 4.5 times the value for solar composition.     

A measurement of the 362 GHz absorption line of Mars atmospheric H2O2

The 362.156 GHz absorption spectrum of H₂O₂ in the Mars atmosphere was observed on September 4 of 2003, employing the James Clerk Maxwell Telescope (JCMT) sub-millimeter facility on Mauna Kea, Hawaii. Radiative transfer analysis of this line absorption yields an average volume mixing ratio of 18±0.4 ppbv within the lower (0-30 km) Mars atmosphere, in general accordance with standard photochemical models (e.g., Nair et al., 1994, Icarus 111, 124-150). Our derived H₂O₂ abundance is roughly three times greater than the upper limit retrieved by Encrenaz et al. (2002, Astron. Astrophys. 396, 1037-1044) from infrared spectroscopy, although part of this discrepancy may result from the different solar longitudes (L_s) of observation. Aphelion-to-perihelion thermal forcing of the global Mars hygropause generates substantial (>200%) increases in HO_x abundances above ∼10 km altitudes between the L_s=112° period of the Encrenaz et al. upper limit measurement and the current L_s=250° period of detection (Clancy and Nair, 1996, J. Geophys. Res. 101, 12785-12590). The observed H₂O₂ line absorption weakens arguments for non-standard homogeneous (Encrenaz et al., 2002, Astron. Astrophys. 396, 1037-1044) or heterogeneous (Krasnopolsky, 2003a, J. Geophys. Res. 108; 2003b, Icarus 165, 315-325) chemistry, which have been advocated partly on the basis of infrared (8 μm) non-detections for Mars H₂O₂. Observation of Mars H₂O₂ also represents the first measurement of a key catalytic specie in a planetary atmosphere other than our own.     

Saturn: Long-term variation of H2 and CH4 absorptions

We present equivalent widths of various H₂ quadrupole lines in Saturn's spectrum monitored from the 1969 apparition to the present. Since the 1971 apparition, the S₃(0), S₃(1), Q₃(1), S₄(0), and S₄(1) lines have all been monitored with the slit set along the central meridian and excluding the rings. Comparison with published values obtained in 1967 suggests a seasonal variation correlated with the shading of Saturn by the rings and with the distance of Saturn from the Sun. Also, observations of CH₄ were collected over a long time interval. In spite of their inhomogeneity, they show an increase in absorption over the past 3 yr which is greater than any diurnal or short-term variation present. At the same time, the H₂ absorptions were relatively constant. This behavior might be explained by the descent of a high-altitude haze later during this period.     

The rotational excitation of H2 by H2

We present rate coefficients for rotational transitions induced in collisions between H₂ molecules. Rotational levels J  ≤ 8 and kinetic temperatures T  ≤ 1000 K are considered. The interaction potential computed by Schwenke has been used, together with the quantal coupled channels method of calculating the cross-sections. Comparison is made with the more recent of previous results.     

Simultaneous mapping of H2O and H2O2 on Mars from infrared high-resolution imaging spectroscopy

New maps of martian water vapor and hydrogen peroxide have been obtained in November-December 2005, using the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infra Red Telescope facility (IRTF) at Mauna Kea Observatory. The solar longitude Ls was 332° (end of southern summer). Data have been obtained at 1235-1243 cm⁻¹, with a spectral resolution of 0.016 cm⁻¹ (R=8×¹⁰⁴). The mean water vapor mixing ratio in the region [0°-55° S; 345°-45° W], at the evening limb, is 150±50 ppm (corresponding to a column density of 8.3±2.8 pr-μm). The mean water vapor abundance derived from our measurements is in global overall agreement with the TES and Mars Express results, as well as the GCM models, however its spatial distribution looks different from the GCM predictions, with evidence for an enhancement at low latitudes toward the evening side. The inferred mean H₂O₂ abundance is 15±10 ppb, which is significantly lower than the June 2003 result [Encrenaz, T., Bézard, B., Greathouse, T.K., Richter, M.J., Lacy, J.H., Atreya, S.K., Wong, A.S., Lebonnois, S., Lefèvre, F., Forget, F., 2004. Icarus 170, 424-429] and lower than expected from the photochemical models, taking in account the change in season. Its spatial distribution shows some similarities with the map predicted by the GCM but the discrepancy in the H₂O₂ abundance remains to be understood and modeled.     

Radiation chemistry of H2O + O2 ices

The chemistry and spectroscopy of proton-irradiated H₂O + O₂ ices have been investigated in relation to the production of oxidants in icy satellite surfaces. Hydrogen peroxide (H₂O₂), ozone (O₃), and the hydroperoxy (HO₂) and hydrogen trioxide (HO₃) radicals have all been observed, and their temperature and dose dependent production trends have been measured. We find that O₂ aggregates form during the growth of H₂O + O₂ ice films, and the presence of these aggregates greatly affects the HO₂ and H₂O₂ yields. In addition, we have found that the position of the spectral maximum of the ν₃ vibration of O₃ shifts with ice composition, giving an indication of the degree of dispersion of O₃ molecules within the ice. We discuss the relevance of these measurements to icy satellite surfaces.     

Release of N2, CH4, CO2, and H2O from surface ices on Enceladus

We vapor deposit at 20 K a mixture of gases with the specific Enceladus plume composition measured in situ by the Cassini INMS [Waite, J.H., Combi, M.R., Ip, W.H., Cravens, T.E., McNutt, R.L., Kasprzak, W., Yelle, R., Luhmann, J., Niemann, H., Gell, D., Magee, B., Fletcher, G., Lunine, J., Tseng, W.L., 2006. Science 311, 1419-1422] to form a mixed molecular ice. As the sample is slowly warmed, we monitor the escaping gas quantity and composition with a mass spectrometer. Pioneering studies [Schmitt, B., Klinger, J., 1987. Different trapping mechanisms of gases by water ice and their relevance for comet nuclei. In: Rolfe, E.J., Battrick, B. (Eds.), Diversity and Similarity of Comets. SP-278. ESA, Noordwijk, The Netherlands, pp. 613-619; Bar-Nun, A., Kleinfeld, I., Kochavi, E., 1988. Phys. Rev. B 38, 7749-7754; Bar-Nun, A., Kleinfeld, I., 1989. Icarus 80, 243-253] have shown that significant quantities of volatile gases can be trapped in a water ice matrix well above the temperature at which the pure volatile ice would sublime. For our Enceladus ice mixture, a composition of escaping gases similar to that detected by Cassini in the Enceladus plume can be generated by the sublimation of the H₂O:CO₂:CH₄:N₂ mixture at temperatures between 135 and 155 K, comparable to the high temperatures inferred from the CIRS measurements [Spencer, J.R., Pearl, J.C., Segura, M., Flasar, F.M., Mamoutkine, A., Romani, P., Buratti, B.J., Hendrix, A.R., Spilker, L.J., Lopes, R.M.C., 2006. Science 311, 1401-1405] of the Enceladus “tiger stripes.” This suggests that the gas escape phenomena that we measure in our experiments are an important process contributing to the gases emitted from Enceladus. A similar experiment for ice deposited at 70 K shows that both the processes of volatile trapping and release are temperature dependent over the temperature range relevant to Enceladus.     

Lunar atmospheric H2 detections by the LAMP UV spectrograph on the Lunar Reconnaissance Orbiter

We report on the detection of H₂ as seen in our analysis of twilight observations of the lunar atmosphere observed by the LAMP instrument aboard NASA’s Lunar Reconnaissance Orbiter. Using a large amount of data collected on the lunar atmosphere between September 2009 and March 2013, we have detected and identified, the presence of H₂ in the native lunar atmosphere, for the first time. We derive a surface density for H₂ of 1.2 ± 0.4 × 10³ cm⁻³ at 120 K. This is about 10 times smaller than originally predicted, and several times smaller than previous upper limits from the Apollo era data.     

Near-infrared laboratory spectra of solid H2O/CO2 and CH3OH/CO2 ice mixtures

We present near-IR spectra of solid CO₂ in H₂O and CH₃OH, and find they are significantly different from that of pure solid CO₂. Peaks not present in either pure H₂O or pure CO₂ spectra become evident when the two are mixed. First, the putative theoretically forbidden CO₂ (2ν₃) overtone near 2.134 μm (4685 cm⁻¹), that is absent from our spectrum of pure solid CO₂, is prominent in the spectra of H₂O/CO₂=5 and 25 mixtures. Second, a 2.74-μm (3650 cm⁻¹) dangling OH feature of H₂O (and a potentially related peak at 1.89 μm) appear in the spectra of CO₂-H₂O ice mixtures, but are probably not diagnostic of the presence of CO₂. Other CO₂ peaks display shifts in position and increased width because of intermolecular interactions with H₂O. Warming causes some peak positions and profiles in the spectrum of a H₂O/CO₂=5 mixture to take on the appearance of pure CO₂. Absolute strengths for absorptions of CO₂ in solid H₂O are estimated. Similar results are observed for CO₂ in solid CH₃OH. Since the CO₂ (2ν₃) overtone near 2.134 μm (4685 cm⁻¹) is not present in pure CO₂ but prominent in mixtures, it may be a good observational (spectral) indicator of whether solid CO₂ is a pure material or intimately mixed with other molecules. These observations may be applicable to Mars polar caps as well as outer Solar System bodies.     

On the chemistry of H2O H2 and meteoritic ions in the mesosphere and lower thermosphere

In the Earth's lower thermosphere and mesosphere, water vapor is photodissociated by absorption of Lyman alpha radiation. The hydrogen containing free radicals produced by this process lead to the formation of molecular hydrogen. Therefore, very small water vapor mixing ratios are expected at high altitudes, particularly in summer, when photolysis is especially rapid. We present one and two-dimensional model calculations regarding the distribution of H₂O and H₂ in the upper atmosphere.The ion chemistry of meteor ions in the lower thermosphere is also examined and it is shown that silicon ion densities can be used to infer water vapor concentrations near 100 km. The water vapor mixing ratios obtained are generally well below one part per million and are in good agreement with the model calculations.     

Sources and sinks for atmospheric H2: A current analysis with projections for the influence of anthropogenic activity

Oxidation of CH₄ provides the major source for atmospheric H₂ which is removed mainly by reaction with OH. Biological activity at the Earth's surface appears to represent at most a minor sink for H₂. Anthropogenic activity is a significant source for both H₂ and CO in the present atmosphere and may be expected to exert a growing influence in the future. Models are presented which suggest a rise in the mixing ratio of H₂ from its present value of 5.6 × 10^?7 to about 1.8 × 10^?6 by the year 2100. The mixing ratio of CO should grow from 9.7 × 10^?8 to 2.3 × 10^?7 over the same time period and there should be a rise in CH₄ by about a factor of 1.5 associated with anthropogenically induced reductions in tropospheric OH.     

Stratospheric trace gas analysis from ions: H2O and HNO3

A new method for stratospheric trace gas analysis based upon in situ ion composition measurements is presented. The strength of the method lies in its extremely high sensitivity which at present allows for detection limits of the order of 100–1000 molecules cm^?3 corresponding to volume mixing ratios of the order of 10^?15–10^?14 around 35 km altitude. The typical uncertainty of derived trace gas abundances is plus or minus a factor of about two. A disadvantage lies in the selectivity of the method which is restricted to trace gases having either large proton affinities, dipole moments or gas phase acidities. Analyses of water vapor and nitric acid vapor in the upper stratosphere are presented.     

Near-infrared spectra of laboratory H2O-CH4 ice mixtures

We present 1.25-19 μm infrared spectra of pure solid CH₄ and H₂O/CH₄=87, 20, and 3 solid mixtures at temperatures from 15 to 150 K. We compare and contrast the absorptions of CH₄ in solid H₂O with those of pure CH₄. Changes in selected peak positions, profiles, and relative strength with temperature are presented, and absolute strengths for absorptions of CH₄ in solid H₂O are estimated. Using the two largest (ν₃+ν₄) and (ν₁+ν₄) near-IR absorptions of CH₄ at 2.324 and 2.377 μm (4303 and 4207 cm⁻¹), respectively, as examples, we show that peaks of CH₄ in solid H₂O are at slightly shorter wavelength (higher frequency) and broader than those of pure solid CH₄. With increasing temperature, these peaks shift to higher frequency and become increasingly broad, but this trend is reversible on re-cooling, even though the phase transitions of H₂O are irreversible. It is to be hoped that these observations of changes in the positions, profiles, and relative intensities of CH₄ absorptions with concentration and temperature will be of use in understanding spectra of icy outer Solar System bodies.     

Role of H2O and CO2 Ices in Martian Climate Changes

In order to study the stability of martian climate, we constructed a two-dimensional (horizontal-vertical) energy balance model. The long-term CO₂ mass exchange process between the atmosphere and CO₂ ice caps is investigated with particular attention to the effect of planetary ice distribution on the climate stability. Our model calculation suggests that high atmospheric pressure presumed for past Mars would be unstabilized if H₂O ice widely prevailed. As a result, a cold climate state might have been achieved by the condensation of atmospheric CO₂ onto ice caps. On the other hand, the low atmospheric pressure, which is buffered by the CO₂ ice cap and likely close to the present pressure, would be unstabilized if the CO₂ ice albedo decreased. This may have led the climate into a warm state with high atmospheric pressure owing to complete evaporation of CO₂ ice cap. Through the albedo feedback mechanisms of H₂O and CO₂ ices in the atmosphere-ice cap system, Mars may have experienced warm and cold climates episodically in its history.     

H2O2 production by high-energy electrons on icy satellites as a function of surface temperature and electron flux

Chemistry on the icy surface of Europa is heavily influenced by the incident energetic particle flux from the jovian magnetosphere. The majority (>75%) of this energy is in the form of high energy electrons (extending to >10 MeV). We have simulated the electron irradiation environment of Europa with a vacuum system containing a high-energy electron gun for irradiation of ice samples formed on a gold mirror cooled with a cryostat. Pure water films of ∼2.6 μm thickness were grown at 100 K and then either cooled (to 80 K), warmed (to 120 K) or left at 100 K and subsequently irradiated with 10 keV electrons. The production of hydrogen peroxide (H₂O₂) was monitored by observation of the 2850 cm⁻¹ (3.5 μm) band. Equilibrium concentrations of H₂O₂, in units of percent by number H₂O₂ relative to water, were found to be 0.043% (80 K), 0.029% (100 K), and 0.0063% (120 K). These values are 33%, 22%, and 5%, respectively, that of the reported surface concentration on the leading hemisphere of Europa (Carlson, R.W., Anderson, M.S., Johnson, R.E., Smythe, W.D., Hendrix, A.R., Barth, C.A., et al. [1999]. Science 283(5410), 2062-2064) and less than the equilibrium concentrations formed by ion irradiation. In addition to the ice film temperature, the current of electrons was varied between different experiments to determine the production and destruction of H₂O₂ as a function of both electron flux and ice temperature. Variation in current was found to have little effect on the results other than accelerating arrival at radiolytic equilibrium.