Mars Methane Sources in Northwestern Gale Crater Inferred From Back Trajectory Modeling

During its first seven years of operation, the Sample Analysis at Mars Tunable Laser Spectrometer (TLS) on board the Curiosity rover has detected seven methane spikes above a low background abundance in Gale crater. The methane spikes are likely sourced by surface emission within or around Gale crater. Here, we use inverse Lagrangian modeling techniques to identify upstream emission regions on the Martian surface for these methane spikes at an unprecedented spatial resolution. Inside Gale crater, the northwestern crater floor casts the strongest influence on the detections. Outside Gale crater, the upstream regions common to all the methane spikes extend toward the north. The contrasting results from two consecutive TLS methane measurements performed on the same sol point to an active emission site to the west or the southwest of the Curiosity rover on the northwestern crater floor. The observed spike magnitude and frequency also favor emission sites on the northwestern crater floor, unless there are fast methane removal mechanisms at work, or either the methane spikes of TLS or the non-detections of ExoMars Trace Gas Orbiter cannot be trusted.

Long-term drying of Mars by sequestration of ocean-scale volumes of water in the crust

Geological evidence shows that ancient Mars had large volumes of liquid water. Models of past hydrogen escape to space, calibrated with observations of the current escape rate, cannot explain the present-day deuterium-to-hydrogen isotope ratio (D/H). We simulated volcanic degassing, atmospheric escape, and crustal hydration on Mars, incorporating observational constraints from spacecraft, rovers, and meteorites. We found that ancient water volumes equivalent to a 100 to 1500 meter global layer are simultaneously compatible with the geological evidence, loss rate estimates, and D/H measurements. In our model, the volume of water participating in the hydrological cycle decreased by 40 to 95% over the Noachian period (~3.7 billion to 4.1 billion years ago), reaching present-day values by ~3.0 billion years ago. Between 30 and 99% of martian water was sequestered through crustal hydration, demonstrating that irreversible chemical weathering can increase the aridity of terrestrial planets.

Boolean logic by convective obstacle flows

We present a potential new mode of natural computing in which simple, heat-driven fluid flows perform Boolean logic operations. The system comprises a two-dimensional single-phase fluid that is heated from below and cooled from above, with two obstacles placed on the horizontal mid-plane. The obstacles remove all vertical momentum that flows into them. The horizontal momentum extraction of the obstacles is controlled in a binary fashion, and constitutes the 2-bit input. The output of the system is a thresholded measure of the energy extracted by the obstacles. Due to the existence of multiple attractors in the phase space of this system, the input–output relationships are equivalent to those of the OR, XOR or NAND gates, depending on the threshold and obstacle separation. The ability to reproduce these logical operations suggests that convective flows might have the potential to perform more general computations, despite the fact that they do not involve electronics, chemistry or multiple fluid phases.

Convective flow in the presence of a small obstacle: Symmetry breaking, attractors, hysteresis, and information

This work explores the stability and hysteresis effects that occur when a small sink of momentum is introduced into a heat-driven, two-dimensional convective flow. As per standard fluid mechanical intuition, the system minimizes work generation and dissipation when one component of momentum is extracted. However, when the sink absorbs all incoming momentum, the system configures itself such that one of the convection plumes aligns directly with the sink. This state is the most hydrodynamically stable, but it maximizes, rather than minimizes extracted mechanical work. Furthermore, in the case of only vertical momentum extraction, there are two attractors, with different stabilities. Numerical experiments involving slow variations of the horizontal momentum extraction show a clear history dependence. This hysteresis preserves information about the system's past states, and hence represents a primitive memory. The momentum sink can also be used to manipulate the horizontal position of the flow field, with potential applications in microfluidics and laminar convection systems. This simple system exhibits the phenomena of autocatalysis (during the initial growth of the convection plumes), negative feedback (the attractors are either fully or quasistable), memory, and elementary computation.

Fixation of nitrogen in the prebiotic atmosphere

Reactions between nitrogen and water in the air surrounding lightning discharges can provide an important source of nitric oxide even under conditions where oxygen is a minor atmospheric constituent. Estimates are given for the associated source of soluble nitrite and nitrate. It is shown that lightning and subsequent atmospheric chemistry can provide a source of nitrate for the primitive ocean as large as 106 tons of nitrogen per year, sufficient to fill the ocean to its present level of nitrate in less than 10(6) years.

Ethane ocean on titan

It is proposed that Saturn's satellite Titan is covered by an ocean one to several kilometers deep consisting mainly of ethane. If the ocean is in thermodynamic equilibrium with an atmosphere of 3 percent (mole fraction) methane, then its composition is roughly 70 percent ethane, 25 percent methane, and 5 percent nitrogen. Photochemical models predict that ethane is the dominant end product of methane photolysis so that the evolving ocean is both the source and sink for continuing photolysis. The coexisting atmosphere is compatible with Voyager data.

Potential environmental impact of a hydrogen economy on the stratosphere

The widespread use of hydrogen fuel cells could have hitherto unknown environmental impacts due to unintended emissions of molecular hydrogen, including an increase in the abundance of water vapor in the stratosphere (plausibly by as much as approximately 1 part per million by volume). This would cause stratospheric cooling, enhancement of the heterogeneous chemistry that destroys ozone, an increase in noctilucent clouds, and changes in tropospheric chemistry and atmosphere-biosphere interactions.

Keeping Mars warm with new super greenhouse gases

Our selection of new super greenhouse gases to fill a putative "window" in a future Martian atmosphere relies on quantum-mechanical calculations. Our study indicates that if Mars could somehow acquire an Earth-like atmospheric composition and surface pressure, then an Earth-like temperature could be sustained by a mixture of five to seven fluorine compounds. Martian mining requirements for replenishing the fluorine could be comparable to current terrestrial extraction.

Jupiter: Aerosol Chemistry in the Polar Atmosphere

Aromatic compounds have been considered a likely candidate for enhanced aerosol formation in the polar region of Jupiter. We develop a new chemical model for aromatic compounds in the Jovian auroral thermosphere/ionosphere. The model is based on a previous model for hydrocarbon chemistry in the Jovian atmosphere and is constrained by observations from Voyager, Galileo, and the Infrared Space Observatory. Precipitation of energetic electrons provides the major energy source for the production of benzene and other heavier aromatic hydrocarbons. The maximum mixing ratio of benzene in the polar model is 2x10-9, a value that can be compared with the observed value of 2+2-1x10-9 in the north polar auroral region. Sufficient quantities of the higher ring species are produced so that their saturated vapor pressures are exceeded. Condensation of these molecules is expected to lead to aerosol formation.

Atmospheric energy for subsurface life on Mars?

The location and density of biologically useful energy sources on Mars will limit the biomass, spatial distribution, and organism size of any biota. Subsurface Martian organisms could be supplied with a large energy flux from the oxidation of photochemically produced atmospheric H(2) and CO diffusing into the regolith. However, surface abundance measurements of these gases demonstrate that no more than a few percent of this available flux is actually being consumed, suggesting that biological activity driven by atmospheric H(2) and CO is limited in the top few hundred meters of the subsurface. This is significant because the available but unused energy is extremely large: for organisms at 30-m depth, it is 2,000 times previous estimates of hydrothermal and chemical weathering energy and far exceeds the energy derivable from other atmospheric gases. This also implies that the apparent scarcity of life on Mars is not attributable to lack of energy. Instead, the availability of liquid water may be a more important factor limiting biological activity because the photochemical energy flux can only penetrate to 100- to 1,000-m depth, where most H(2)O is probably frozen. Because both atmospheric and Viking lander soil data provide little evidence for biological activity, the detection of short-lived trace gases will probably be a better indicator of any extant Martian life.

Water on Mars: isotopic constraints on exchange between the atmosphere and surface

Using a new measurement of the D/H fractionation efficiency and new estimates of the water loss, we calculate that Mars has the equivalent of a approximately 9 m global water layer in a reservoir that exchanges with the atmosphere. The measured D/H enrichment is about 5 times the terrestrial value, but without exchange, the atmosphere converges on an enrichment of 50 in about 0.5 Ma. Due to the large buffering reservoir and the rapid loss rate (10(-3) pr-micrometers yr-1), the small atmospheric reservoir, averaging 10 pr-micrometers, is unlikely to be in continuous isotopic equilibrium with the full 9 m exchangeable reservoir. Instead, it presumably equilibrates during periods of high obliquity; the atmospheric D/H ratio is expected to be enriched in between such periods. If isotopic exchange with a small (4 mm global layer) reservoir occurs under current conditions, it possible for the atmospheric D/H ratio to be within 10% of its long term equilibrium.

Photo-induced fractionation of water isotopomers in the Martian atmosphere

The history and size of the water reservoirs on early Mars can be constrained using isotopic ratios of deuterium to hydrogen. We present new laboratory measurements of the ultraviolet cross-sections of H2O and its isotopomers, and modeling calculations in support of a photo-induced fractionation effect (PHIFE), that reconciles a discrepancy between past theoretical modeling and recent observations. This supports the hypothesis that Mars had an early warm atmosphere and has lost at least a 50-m global layer of water. Likely applications of PHIFE to other planetary atmospheres are sketched.

On the volatile inventory of Titan from isotopic abundances in nitrogen and methane

We analyze recently published nitrogen and hydrogen isotopic data to constrain the initial volatile abundances on Saturn's giant moon Titan. The nitrogen data are interpreted in terms of a model of non-thermal escape processes that lead to enhancement in the heavier isotope. We show that these data do not, in fact, strongly constrain the abundance of nitrogen present in Titan's early atmosphere, and that a wide range of initial atmospheric masses (all larger than the present value) can yield the measured enhancement. The enrichment in deuterated methane is now much better determined than it was when Pinto et al. (1986. Nature 319, 388-390) first proposed a photochemical mechanism to preferentially retain the deuterium. We develop a simple linear theory to provide a more reliable estimate of the relative dissociation rates of normal and deuterated methane. We utilize the improved data and models to compute initial methane reservoirs consistent with the observed enhancement. The result of this analysis agrees with an independent estimate for the initial methane abundance based solely on the present-day rate of photolysis and an assumption of steady state. This consistency in reservoir size is necessary but not sufficient to infer that methane photolysis has proceeded steadily over the age of the solar system to produce large quantities of less volatile organics. Our analysis indicates an epoch of early atmospheric escape of nitrogen, followed by a later addition of methane by outgassing from the interior. The results also suggest that Titan's volatile inventory came in part or largely from a circum-Saturnian disk of material more reducing than the surrounding solar nebula. Many of the ambiguities inherent in the present analysis can be resolved through Cassini-Huygens data and a program of laboratory studies on isotopic and molecular exchange processes. The value of, and interest in, the Cassini-Huygens data can be greatly enhanced if such a program were undertaken prior to the prime phase of the mission.

Isotopic fractionation of stratospheric nitrous oxide

We propose an isotopic fractionation mechanism, based on photolytic destruction, to explain the 15N/14N and 18O/16O fractionation of stratospheric nitrous oxide (N2O) and reconcile laboratory experiments with atmospheric observations. The theory predicts that (i) the isotopomers 15N14N16O and 14N15N16O have very different isotopic fractionations in the stratosphere, and (ii) laboratory photolysis experiments conducted at 205 nanometers should better simulate the observed isotopic fractionation of stratospheric N2O. Modeling results indicate that there is no compelling reason to invoke a significant chemical source of N2O in the middle atmosphere and that individual N2O isotopomers might be useful tracers of stratospheric air parcel motion.

Carbon dioxide in the atmosphere: isotopic exchange with ozone and its use as a tracer in the middle atmosphere

Atmospheric heavy ozone is enriched in the isotopes 18O and 17O. The magnitude of this enhancement, of the order of 100%, is very large compared with that commonly known in atmospheric chemistry and geochemistry. The heavy oxygen atom in heavy ozone is therefore useful as a tracer of chemical species and pathways that involve ozone or its derived products. As a test of the isotopic exchange reactions, we successfully carry out a series of numerical experiments to simulate the results of the laboratory experiments performed by Wen and Thiemens [1993] on ozone and CO2. A small discrepancy between the experimental and the model values for 17O exchange is also revealed. The results are used to compute the magnitude of isotopic exchange between ozone and carbon dioxide via the excited atom O(1D) in the middle atmosphere. The model for 18O is in good agreement with the observed values.

Calculated hydroxyl A2 sigma --> X2 pi (0, 0) band emission rate factors applicable to atmospheric spectroscopy

A calculation of the A2 sigma --> X2 pi (0, 0) band emission rate factors and line center absorption cross sections of OH applicable to its measurement using solar resonant fluorescence in the terrestrial atmosphere is presented in this paper. The most accurate available line parameters have been used. Special consideration has been given to the solar input flux because of its highly structured Fraunhofer spectrum. The calculation for the OH atmospheric emission rate factor in the solar resonant fluorescent case is described in detail with examples and intermediate results. Results of this calculation of OH emission rate factors for individual rotational lines are on average 30% lower than the values obtained in an earlier work.

CO2 greenhouse in the early martian atmosphere: SO2 inhibits condensation

Many investigators of the early martian climate have suggested that a dense carbon dioxide atmosphere was present and warmed the surface above the melting point of water (J.B. Pollack, J.F. Kasting, S.M. Richardson, and K. Poliakoff 1987. Icarus 71, 203-224). However, J.F. Kasting (1991. Icarus 94, 1-13) pointed out that previous thermal models of the primitive martian atmosphere had not considered the condensation of CO2. When this effect was incorporated, Kasting found that CO2 by itself is inadequate to warm the surface. SO2 absorbs strongly in the near UV region of the solar spectrum. While a small amount of SO2 may have a negligible effect by itself on the surface temperature, it may have significantly warmed the middle atmosphere of early Mars, much as ozone warms the terrestrial stratosphere today. If this region is kept warm enough to inhibit the condensation of CO2, then CO2 remains a viable greenhouse gas. Our preliminary radiative modeling shows that the addition of 0.1 ppmv of SO2 in a 2 bar CO2 atmosphere raises the temperature of the middle atmosphere by approximately 10 degrees, so that the upper atmosphere in a 1 D model remains above the condensation temperature of CO2. In addition, this amount of SO2 in the atmosphere provides an effective UV shield for a hypothetical biosphere on the martian surface.

Balloon observations of organic and inorganic chlorine in the stratosphere: the role of HClO4 production on sulfate aerosols

Simultaneous observations of stratospheric organic and inorganic chlorine were made in September 1993 out of Fort Sumner, New Mexico, using JPL balloon-borne MkIV interferometer. Between 15 and 20 km, a significant fraction (20-60%) of the inorganic chlorine could not be accounted for by the sum of measured HCl, ClONO2, and HOCl. Laboratory measurements of the reaction of ClO radicals on sulfuric acid solutions have indicated that, along with HCl, small amounts of perchloric acid, HClO4, were formed. Very little is known about the fate of HClO4 in the stratosphere and we use a photochemical box model to determine the impact of this new species on the partitioning of inorganic chlorine in the stratosphere. Assuming that HClO4 is photochemically stable, it is shown that in the enhanced aerosol loading conditions resulting from Mt. Pinatubo's eruption, HClO4 could represent a significant reservoir of chlorine in the lower stratosphere, sequestering up to 0.2 ppbv (or 50%) of the total inorganic chlorine at 16 km. The occurrence of this new species could bring to closure the inorganic chlorine budget deficiency made apparent by recent ER-2 aircraft in situ measurements of HCl.

Concentrations of Tropospheric Ozone from 1979 to 1992 over Tropical Pacific South America from TOMS Data

An estimate of tropospheric ozone concentrations was obtained from the difference in the Total Ozone Mapping Spectrometer (TOMS) data between the high Andes and the Pacific Ocean. From 1979 to 1992 the tropospheric ozone concentration apparently increased by 1.48 ± 0.40 percent per year or 0.21 ± 0.06 Dobson unit per year over South America and the surrounding oceans. An increase in biomass burning in the Southern Hemisphere can account for this trend in tropospheric ozone concentrations.

Decadal evolution of the Antarctic ozone hole

Ozone column amounts obtained by the total ozone mapping spectrometer (TOMS) in the southern polar region are analyzed during late austral winter and spring (days 240-300) for 1980-1991 using area-mapping techniques and area-weighted vortex averages. The vortex here is defined using the -50 PVU (1 PVU = 1.0 x 10(-6) K kg-1 m2 s-1) contour on the 500 K isentropic surface. The principal results are: (1) there is a distinct change after 1985 in the vortex-averaged column ozone depletion rate during September and October, the period of maximum ozone loss, and (2) the vortex-averaged column ozone in late August (day 240) has dropped by 70 Dobson units (DU) in a decade due to the loss in the dark and the dilution effect. The mean ozone depletion rate in the vortex between day 240 and the day of minimum vortex-averaged ozone is about 1 DU d-1 at the beginning of the decade, increasing to about 1.8 DU d-1 by 1985, and then apparently saturating thereafter. The vortex-average column ozone during September and October has declined at the rate of 11.3 DU yr-1 (3.8%) from 1980 to 1987 (90 DU over 8 years) and at a smaller rate of 2 DU yr-1 (0.9%) from 1987 to 1991 (10 DU over 5 years, excluding the anomalous year 1988). We interpret the year-to-year trend in the ozone depletion rate during the earlier part of the decade as due to the rise of anthropogenic chlorine in the atmosphere. The slower trend at the end of the decade indicates saturation of ozone depletion in the vortex interior, in that chlorine amounts in the mid-1980s were already sufficiently high to deplete most of the ozone in air within the isolated regions of the lower-stratospheric polar vortex. In subsequent years, increases in stratospheric chlorine may have enhanced wintertime chemical loss of ozone in the south polar vortex even before major losses during the Antarctic spring.

Methyl bromide: ocean sources, ocean sinks, and climate sensitivity

The oceans play an important role in the geochemical cycle of methyl bromide (CH3Br), the major carrier of O3-destroying bromine to the stratosphere. The quantity of CH3Br produced annually in seawater is comparable to the amount entering the atmosphere each year from natural and anthropogenic sources. The production mechanism is unknown but may be biological. Most of this CH3Br is consumed in situ by hydrolysis or reaction with chloride. The size of the fraction which escapes to the atmosphere is poorly constrained; measurements in seawater and the atmosphere have been used to justify both a large oceanic CH3Br flux to the atmosphere and a small net ocean sink. Since the consumption reactions are extremely temperature-sensitive, small temperature variations have large effects on the CH3Br concentration in seawater, and therefore on the exchange between the atmosphere and the ocean. The net CH3Br flux is also sensitive to variations in the rate of CH3Br production. We have quantified these effects using a simple steady state mass balance model. When CH3Br production rates are linearly scaled with seawater chlorophyll content, this model reproduces the latitudinal variations in marine CH3Br concentrations observed in the east Pacific Ocean by Singh et al. [1983] and by Lobert et al. [1995]. The apparent correlation of CH3Br production with primary production explains the discrepancies between the two observational studies, strengthening recent suggestions that the open ocean is a small net sink for atmospheric CH3Br, rather than a large net source. The Southern Ocean is implicated as a possible large net source of CH3Br to the atmosphere. Since our model indicates that both the direction and magnitude of CH3Br exchange between the atmosphere and ocean are extremely sensitive to temperature and marine productivity, and since the rate of CH3Br production in the oceans is comparable to the rate at which this compound is introduced to the atmosphere, even small perturbations to temperature or productivity can modify atmospheric CH3Br. Therefore atmospheric CH3Br should be sensitive to climate conditions. Our modeling indicates that climate-induced CH3Br variations can be larger than those resulting from small (+/- 25%) changes in the anthropogenic source, assuming that this source comprises less than half of all inputs. Future measurements of marine CH3Br, temperature, and primary production should be combined with such models to determine the relationship between marine biological activity and CH3Br production. Better understanding of the biological term is especially important to assess the importance of non-anthropogenic sources to stratospheric ozone loss and the sensitivity of these sources to global climate change.

Dust: a diagnostic of the hydrologic cycle during the last glacial maximum

Dust concentrations in ice of the last glacial maximum (LGM) are high in ice cores from Greenland and Antarctica. The magnitude of the enhancements can be explained if the strength of the hydrologic cycle during the LGM was about half of that at present. This notion is consistent with a large decrease (5 degrees Celsius) in ocean temperature during the LGM, as recently deduced from measurements of strontium and calcium in corals.

Hydrocarbon photochemistry in the upper atmosphere of Jupiter

The hydrocarbon photochemistry in the upper atmosphere of Jupiter is investigated using a one-dimensional, photochemical-diffusive, and diurnally averaged model. The important chemical cycles and pathways among the major species are outlined and a standard model for the North Equatorial Belt region is examined in detail. It is found that several traditionally dominant chemical pathways among the C and C2 species are replaced in importance by cycles involving C-C4 species. The pressure and altitude profiles of mixing ratios for several observable hydrocarbon species are compared with available ultraviolet- and infrared-derived abundances. The results of sensitivity studies on the standard model with respect to variations in eddy diffusion profile, solar flux, atomic hydrogen influx, latitude, temperature, and important chemical reaction rates are presented. Measured and calculated airglow emissions of He at 584 angstroms and H at 1216 angstroms are also used to provide some constraints on the range of model parameters. The relevance of the model results to the upcoming Galileo mission is briefly discussed. The model is subject to considerable improvement; there is a great need for laboratory measurements of basic reaction rates and photodissociation quantum yields, even for such simple species as methylacetylene and allene. Until such laboratory measurements exist there will be considerable uncertainty in the understanding of the C3 and higher hydrocarbons in the atmospheres of the jovian planets.

Loss of atmosphere from Mars due to solar wind-induced sputtering

Because Mars does not have a strong intrinsic magnetic field, the atmosphere is eroded by interactions with the solar wind. Early solar-system conditions enhanced the sputtering loss. It is calculated that approximately 3 bars of carbon dioxide (CO2) have been sputtered over the last 3.5 billion years. This significant increase over the previous estimate by Luhmann et al. of approximately 0.14 bar of CO2 is the result of the development of a more complete model. The model also predicts slightly greater loss of water--approximately 80 meters instead of the approximately 50 meters predicted by Luhmann et al. Because estimates of CO2 on early Mars range from 0.5 to 5 bars, the 0.14-bar estimate is insignificant but the approximately 3-bar estimate will have a large effect on our understanding of the planet's evolution.

A photochemical model of the martian atmosphere

The factors governing the amounts of CO, O2, and O3 in the martian atmosphere are investigated using a minimally constrained, one-dimensional photochemical model. We find that the incorporation of temperature-dependent CO2 absorption cross sections leads to an enhancement in the water photolysis rate, increasing the abundance of OH radicals to the point where the model CO abundance is smaller than observed. Good agreement between models and observations of CO, O2, O3, and the escape flux of atomic hydrogen can be achieved, using only gas-phase chemistry, by varying the recommended rate constants for the reactions CO + OH and OH + HO2 within their specified uncertainties. Similar revisions have been suggested to resolve discrepancies between models and observations of the terrestrial mesosphere. The oxygen escape flux plays a key role in the oxygen budget on Mars; as inferred from the observed atomic hydrogen escape, it is much larger than recent calculations of the exospheric escape rate for oxygen. Weathering of the surface may account for the imbalance. Quantification of the escape rates of oxygen and hydrogen from Mars is a worthwhile objective for an upcoming martian upper atmospheric mission. We also consider the possibility that HOx radicals may be catalytically destroyed on dust grains suspended in the atmosphere. Good agreement with the observed CO mixing ratio can be achieved via this mechanism, but the resulting ozone column is much higher than the observed quantity. We feel that there is no need at this time to invoke heterogeneous processes to reconcile models and observations.

Fractionation of hydrogen and deuterium on Venus due to collisional ejection

The fractionation factor f is important for interpreting the current escape fluxes of H and D on Venus and how the D/H ratio has evolved. The escape flux is currently governed by the two processes of charge exchange and collisional ejection by fast oxygen atoms. Using a best-fit parameterized equation for the O-H scattering angle phase function, more accurate branching ratios for the oxygen ion dissociation and including the effects of the initial energy and momentum of the ions and electrons, as well as for the hydrogen and deuterium gas, we have reanalyzed the collisional ejection process. Our analysis produces improved values for the efficiency of H and D escape as a function of the ionospheric temperature. From our results we propose the reduction of the hydrogen flux for collisional ejection from 8 to 3.5 x 10(6) cm-2 s-1. Assuming that collisions leading to escape occur mostly in the region between 200 and 400 km, the revised D/H fractionation factor due to collisional ejection is 0.47, where previously the process had been considered completely discriminating against deuterium escape (or f approximately 0.) The resulting deuterium flux is 3.1 x 10(4) cm-2 s-1, roughly 6 times the flux due to charge exchange, making collisional ejection the dominant escape mechanism for deuterium on Venus.

The relative abundance of ethane to acetylene in the Jovian stratosphere

The observed ratio of C2H6 to C2H2 in the Jovian stratosphere increases from approximately 55 at 2 mbar to approximately 277 at 12 mbar. In current photochemical models this ratio typically increases between 2 and 12 mbar by a factor of < or = 3. Recent laboratory kinetics studies on the reaction between C2H3 and H2 to form C2H4 suggest an efficient chemical mechanism for hydrogenation of C2H2 to C2H6. Inclusion of this scheme as part of a comprehensive updated model for hydrocarbon photochemistry in the atmosphere of Jupiter provides an explanation of the altitude variation of the C2H6/C2H2 ratio. The sensitivity of these results to uncertainties in the key rate constants at low temperatures is illustrated, identifying needs for additional laboratory measurements. Since the key reaction rate constants decrease with decreasing temperature, the hydrogenation of C2H2 as proposed predicts a qualitatively decreasing trend in the C2H6/C2H2 value with decreasing distance from the Sun. The observed variation between Jupiter and Saturn is consistent with this prediction.

Hydrocarbon nucleation and aerosol formation in Neptune's atmosphere

Photodissociation of methane at high altitude levels in Neptune's atmosphere leads to the production of complex hydrocarbon species such as acetylene (C2H2), ethane (C2H6), methylacetylene (CH3C2H), propane (C3H8), diacetylene (C4H2), and butane (C4H8). These gases diffuse to the lower stratosphere where temperatures are low enough to initiate condensation. Particle formation may not occur readily, however, as the vapor species become supersaturated. We present a theoretical analysis of particle formation mechanisms at conditions relevant to Neptune's troposphere and stratosphere and show that hydrocarbon nucleation is very inefficient under Neptunian conditions: saturation ratios much greater than unity are required for aerosol formation by either homogeneous, heterogeneous, or ion-induced nucleation. Homogeneous nucleation will not be important for any of the hydrocarbon species considered; however, both heterogeneous and ion-induced nucleation should be possible on Neptune for most of the above species. The relative effectiveness of heterogeneous and ion-induced nucleation depends on the physical and thermodynamic properties of the particular species, the abundance of the condensable species, the temperature at which the vapor becomes supersaturated, and the number and type of condensation nuclei or ions available. Typical saturation ratios required for observable particle formation rates on Neptune range from approximately 3 for heterogeneous nucleation of methane in the upper troposphere to greater than 1000 for heterogeneous nucleation of methylacetylene, diacetylene, and butane in the lower stratosphere. Thus, methane clouds may form slightly above, and stratospheric hazes far below, their saturation levels. When used in conjunction with the results of detailed models of atmospheric photochemistry, our nucleation models place realistic constraints on the altitude levels at which we expect hydrocarbon hazes or clouds to form on Neptune.

Chloryl nitrate: a novel product of the OClO + NO3 + M recombination

The products of the reaction of OClO with NO3 were investigated between 220 and 298 K using a flow reactor and infrared, visible, and ultraviolet analysis. At temperatures below 250 K new infrared and ultraviolet absorption features were observed and assigned to the novel compound chloryl nitrate (O2ClONO2). Additionally, ClO and NO2 were observed as reaction products, indicating the existence of a second reaction channel. O2ClONO2 formation predominates at temperatures below 230 K. The reaction rate constant at 220 K is estimated to be on the order of 10(-14) cm3 molecule-1 s-1 in 1-5 Torr of helium. These observations suggest that O2ClONO2 may exist in the terrestrial stratosphere.

Solar control of the upper atmosphere of Triton

If the upper atmosphere and ionosphere of Triton are controlled by precipitation of electrons from Neptune's magnetosphere as previously proposed, Triton could have the only ionosphere in the solar system not controlled by solar radiation. However, a new model of Triton's atmosphere, in which only solar radiation is present, predicts a large column of carbon atoms. With an assumed, but reasonable, rate of charge transfer between N2(+) and C, a peak C+ abundance results that is close to the peak electron densities measured by Voyager in Triton's ionosphere. These results suggest that Triton's upper atmospheric chemistry may thus be solar-controlled. Measurement of key reaction rate constants, currently unknown or highly uncertain at Triton's low temperatures, would help to clarify the chemical and physical processes occurring in Triton's atmosphere.

Enhancement of atmospheric radiation by an aerosol layer

The presence of a stratospheric haze layer may produce increases in both the actinic flux and the irradiance below this layer. Such haze layers result from the injection of aerosol-forming material into the stratosphere by volcanic eruptions. Simple heuristic arguments show that the increase in flux below the haze layer, relative to a clear sky case, is a consequence of "photon trapping." We explore the magnitude of these flux perturbations, as a function of aerosol properties and illumination conditions, with a new radiative transfer model that can accurately compute fluxes in an inhomogenous atmosphere with nonconservative scatterers having arbitrary phase function. One calculated consequence of the El Chichon volcanic eruption is an increase in the midday surface actinic flux at 20 degrees N latitude, summer, by as much as 45% at 2900 angstroms. This increase in flux in the UV-B wavelength range was caused entirely by aerosol scattering, without any reduction in the overhead ozone column.

Isotopic exchange between carbon dioxide and ozone via O(1D) in the stratosphere

We propose a novel mechanism for isotopic exchange between CO2 and O3 via O(1D) + CO2 --> CO3* followed by CO3* --> CO2 + O(3P). A one-dimensional model calculation shows that this mechanism can account for the enrichment in 18O in the stratospheric CO2 observed by Gamo et al. [1989], using the heavy O3 profile observed by Mauersberger [1981]. The implications of this mechanism for other stratospheric species and as a source of isotopically heavy CO2 in the troposphere are briefly discussed.

Troposphere-stratosphere interactions in a one-dimensional model of Jovian photochemistry

A simple one-dimensional model of the Jovian atmosphere including the coupling between a rapidly mixed troposphere and a stagnant stratosphere is presented. We treat analytically the case of a chemically unreactive species flowing downward through the stratosphere and troposphere with a constant flux. The calculated concentration profile has a maximum value approximately one atmospheric scale height above the tropopause. The corresponding mixing ratio rapidly decreases in the lower stratosphere. The contrast between the peak stratosphere and tropopause concentrations reflects the variation between the lower stratosphere and upper troposphere eddy diffusion coefficients. Numerical simulations of unreactive CO and C2H6, considering high-altitude photochemical sources (yielding downward fluxes to the troposphere) and deep troposphere thermochemical sources, demonstrate that upper troposphere abundances may have a large photochemical contribution. Recent observations of CO near 5 bar can be reproduced by a model using any one of three different tropospheric eddy diffusion profiles: a constant value of approximately 10(8) cm2 sec-1, a constant value of < or approximately 10(4) cm2 sec-1, or a two-layer model with a rapidly mixed (10(8) cm2 sec-1) layer below 20 bar and a slower mixing layer (10(4) cm2 sec-1) between 100 mbar and 20 bar. In the latter two scenarios, the photochemical source is an important and/or dominant source of upper tropospheric CO. However, the upper tropospheric C2H6 abundances are distinctly different among the three cases. These calculations suggest objectives for higher spectral resolution observations, critical vertical scales for planning experiments on future missions to Jupiter, and predictions that can be tested with the Galileo probe mass spectrometer.

Triton: topside ionosphere and nitrogen escape

The principal ion in the ionosphere of Triton is N+. Energetic electrons of magnetospheric origin are the primary source of ionization, with a smaller contribution due to photoionization. To explain the topside plasma scale height, we postulate that N+ ions escape from Triton. The loss rate is 3.4 x 10(7) cm-2 s-1 or 7.9 x 10(24) ions s-1. Dissociative recombination of N2+ produces neutral exothermic fragments that can escape from Triton. The rate is estimated to be 8.6 x 10(6) N cm-2 s-1 or 2.0 x 10(24) atoms s-1. Implications for the magnetosphere of Neptune and Triton's evolution are discussed.

Two-dimensional atmospheric transport and chemistry model: numerical experiments with a new advection algorithm

Extensive testing of the advective scheme, proposed by Prather (1986), has been carried out in support of the California Institute of Technology-Jet Propulsion Laboratory two-dimensional model of the middle atmosphere. We generalize the original scheme to include higher-order moments. In addition, we show how well the scheme works in the presence of chemistry as well as eddy diffusion. Six types of numerical experiments including simple clock motion and pure advection in two dimensions have been investigated in detail. By comparison with analytic solutions it is shown that the new algorithm can faithfully preserve concentration profiles, has essentially no numerical diffusion, and is superior to a typical fourth-order finite difference scheme.

Spatial variation of ozone depletion rates in the springtime Antarctic polar vortex

An area-mapping technique, designed to filter out synoptic perturbations of the Antarctic polar vortex such as distortion or displacement away from the pole, was applied to the Nimbus-7 TOMS (Total Ozone Mapping Spectrometer) data. This procedure reveals the detailed morphology of the temporal evolution of column O3. The results for the austral spring of 1987 suggest the existence of a relatively stable collar region enclosing an interior that is undergoing large variations. There is tentative evidence for quasi-periodic (15 to 20 days) O3 fluctuations in the collar and for upwelling of tropospheric air in late spring. A simplified photochemical model of O3 loss and the temporal evolution of the area-mapped polar O3 are used to constrain the chlorine monoxide (ClO) concentrations in the springtime Antarctic vortex. The concentrations required to account for the observed loss of O3 are higher than those previously reported by Anderson et al. but are comparable to their recently revised values. However, the O3 loss rates could be larger than deduced here because of underestimates of total O3 by TOMS near the terminator. This uncertainty, together with the uncertainties associated with measurements acquired during the Airborne Antarctic Ozone Experiment, suggests that in early spring, closer to the vortex center, there may be even larger ClO concentrations than have yet been detected.

SME observations of O2 (1 delta g) nightglow: an assessment of the chemical production mechanisms

Solar Mesosphere Explorer (SME) observations of the 3 a.m. 1.27 micrometers nightglow at 45 N latitude, averaged over the period 10-31 July 1984, are reported. From the deduced volume emission rates, we derive the O2(a1 delta g) night-time production rates for the 80-100 km altitude range. Utilizing the mean SME-acquired 3 p.m. ozone profile for the same latitude and time period and an updated photochemical model, we determine night-time O, O3, H, OH, HO2, and H2O2 profiles. These are used in calculating the rates of reactions which are sufficiently exothermic to produce O2(1 delta) or excited states of OH or HO2, which could transfer their energy to O2 to form O2(1 delta). Of these reactions, most have rates that are quite small compared with the observed night-time O2(1 delta) production rate. For several others, laboratory experiments have found O2(1 delta) yields which are insufficient for simulating the observed O2(1 delta). Using yields of O2(1 delta) based on published laboratory and observational studies, we find that the sum of two reaction sequences can approximate the SME measurements: (1) O+O+M and (2) H+O3 followed by OH*+O2.

El Chichon volcanic aerosols: impact of radiative, thermal, and chemical perturbations

We examine the consequences of the eruption of the El Chichon volcano on the Earth's stratospheric chemistry. Formed after the eruption, the volcanic aerosol cloud, with a peak particle density at 27 km, was very efficient at altering the radiation field. The results of a one-dimensional radiative transfer model show that the total radiation increased by 8% within the aerosol layer longward of 3000 angstroms. At certain altitudes and wavelengths below 3000 angstroms, the total radiation decreased by 15%. Consequently, there are changes in the photolysis rates obtained with a one-dimensional photochemical model: for example, O2 photodissociation rate constants decrease by 10%, while O3 photodissociation rate constants increase by a comparable amount. A combination of this radiation change and the effect of a temperature variation of a few degrees causes the abundance of O3 to decrease by 7% at 24 km, in good agreement with the Solar Backscattered Ultraviolet experiment (SBUV) measurements of a 5-10% decrease. The combined radiative and thermal perturbations on the concentrations of O, O(1 D), OH, HO2, H2O2, NO, NO2, NO3, N2O5, HNO3, HO2NO2, Cl, ClO, ClO2, HOCl, ClNO3, and HCl are computed and presented in detail. However, these changes as calculated are insufficient to explain the observations of significant decreases in NO and NO2 and increases in HCl. A heterogeneous reaction catalyzed by aerosol surfaces which transforms ClNO3 into HCl provides a pathway for sequestering NOx, and at the same time reduces ClNO3 in favor of HCL. The inclusion of this reaction in the model leads to a satisfactory single-step explanation of the otherwise puzzling observations of NO, NO2, and HCl. The observed lack of change in HNO3 cannot be explained by this hypothesis. The effects of a number of heterogeneous reactions, some believed to be important for the Antarctic stratosphere, have been assessed with our model. We also examine the hypothesis of direct injection of gases from the volcano into the stratosphere. Only an unrealistically large injection (60% column increase above 12 km) results in an HCl increase in agreement with observations. An equally large water injection decreases HCl, and decreases the NO and NO2 by as much as 20%, but still does not simulate the observed NOx decrease.