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At least 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

A new spectrum of Triton near the time of the Voyager encounter

A 5200-10,000 A spectrum of Triton that was telescopically obtained during the summer of 1989, just before the Voyager II encounter with the Neptune system, exhibits a measurable 8900 A CH4 ice absorption band. A combination of these data with those of Voyager indicates that the absorption is caused solely by Triton surface CH4 ice. A Hapke-type model for the Triton spectrum (1) sets a 20-micron lower limit on the CH4 ice's mean grain size (although it is suspected that actual grain size is closer to 100 microns), and (2) indicates that CH4 ice is widely distributed on the southern-hemisphere surface of Triton.

Grundy, William M.↗
NASA NTRS1991

Non-LTE models of Titan's upper atmosphere

Models for the thermal structure of Titan's upper atmosphere, between 0.1 mbar and 0.01 nbar are presented. The calculations include non-LTE heating/cooling in the rotation-vibration bands of CH4, C2H2, and C2H6, absorption of solar IR radiation in the near-IR bands of CH4 and subsequent cascading to the nu-4 band of CH4, absorption of solar EUV and UV radiation, thermal conduction and cooling by HCN rotational lines. Unlike earlier models, the calculated exospheric temperature agrees well with observations, because of the importance of HCN cooling. The calculations predict a well-developed mesopause with a temperature of 135-140 K at an altitude of approximately 600 km and pressure of about 0.1 microbar. The mesopause is at a higher pressure than predicted by earlier calculations because non-LTE radiative transfer in the rotation-vibration bands of CH4, C2H2, and C2H6 is treated in an accurate manner. The accuracy of the LTE approximation for source functions and heating rates is discussed.

Yelle, Roger V.↗
NASA NTRS1992

Methane measurement by the Pioneer Venus large probe neutral mass spectrometer

The Pioneer Venus Large Probe Mass Spectrometer detected a large quantity of methane as it descended below 20 km in the atmosphere of Venus. Terrestrial methane and Xe-136, both originating in the same container and flowing through the same plumbing, were deliberately released inside the mass spectrometer for instrumental reasons. However, the Xe-136 did not exhibit behavior similar to methane during Venus entry, nor did CH4 in laboratory simulations. The CH4 was deuterium poor compared to Venus water and hydrogen. While the inlet to the mass spectrometer was clogged with sulfuric acid droplets, significant deuteration of CH4 and its H2 progeny was observed. Since the only source of deuterium identifiable was water from sulfuric acid, we have concluded that we should correct the HDO/H2O ratio in Venus water from 3.2 x 10(exp -2) to (5 plus or minus 0.7) x 10(exp -2). When the probe was in the lower atmosphere, transfer of deuterium from Venus HDO and HD to CH4 can account quantitatively for the deficiencies recorded in HDO and HD below 10 km, and consequently, the mysterious gradients in water vapor and hydrogen mixing ratios we have reported. The revision in the D/H ratio reduces the mixing ratio of water vapor (and H2) reported previously by a factor of 3.2/5. We are not yet able to say whether the methane detected was atmospheric or an instrumental artifact. If it was atmospheric, its release must have been episodic and highly localized. Otherwise, the large D/H ratio in Venus water and hydrogen could not be maintained.

Donahue, T. M.↗
NASA NTRS1992

Carbon monoxide and methane in the North American Arctic and Subarctic troposphere - July-August 1988

Enhanced concentrations of CH4 in the unpolluted atmospheric mixed layer over both Arctic and subarctic tundra landscapes are documented here using data from the NASA Arctic Boundary Layer Expedition (ABLE 3A). The CH4 concentration gradients were determined mainly by interactions of biogenic emission from wet tundra and turbulent mixing proceses. The gradient were most frequently associated with intrusion of upper tropospheric or stratospheric air into the midtroposphere, emissions from forest and tundra fires, and long-range transport of enhanced concentration of these gases from unidentified sources. Summertime haze layers exhibited midtropospheric enhancements of CH4 similar to those measured in winter Arctic events. The observations confirm the importance of Arctic and Subarctic wetland environments as a regional source of global atmospheric CH4.

Harriss, Robert C.↗
NASA NTRS1993

The distribution of hydrocarbons in Neptune's upper atmosphere

The distribution of hydrocarbons in the upper atmosphere of Neptune is determined on the basis of data from the Voyager UVS solar occultation experiment. Densities are inferred from the transmission properties of the atmosphere measured by the UVS. The CH4 mole fraction in the lower stratosphere is between 0.0006 and 0.005. It is inferred that CH4 in Neptune's lower stratosphere is oversaturated by at least a factor of 10, and possibly by as much as a factor of 100. The density of C2H6 reaches values of 3 x 10 exp 9/cu cm near 100 microbar; a C2H6 production efficiency of 35 +15/-5 percent is derived. The eddy diffusion coefficient is approximately constant at a value of 10,000 sq cm/s at altitudes below 300 km and increases at higher altitudes as the pressure to the 0.75 power. A mole fraction of 0.001 is found to be consistent with the CH4 vapor pressure at 3.4 mbar; consequently, CH4 ice particles must reach this level to produce the inferred oversaturation.

Yelle, Roger V.↗
NASA NTRS1992

Carbon cycling in high-latitude ecosystems

The carbon-rich soils and peatlands of high-latitude ecosystems could substantially influence atmospheric concentrations of CO2 and CH4 in a changing climate. Currently, cold, often waterlogged conditions retard decomposition, and release of carbon back to the atmosphere may be further slowed by physical protection of organic matter in permafrost. As a result, many northern ecosystems accumulate carbon over time (Billings et al., 1982; Poole and Miller, 1982), and although such rates of accumulation are low, thousands of years of development have left Arctic ecosystems with an extremely high soil carbon content; Schlesinger's (1984) average value of 20.4 kg C/m(sup 2) leads to a global estimate of 163 x 10(exp 15) g C. All GCM simulations of a doubled CO2 climate predict the greatest warming to occur in the polar regions (Dickinson, 1986; Mitchell, 1989). Given the extensive northern carbon pools and the strong sensitivity of decomposition processes to temperature, even a slight warming of the soil could dramatically alter the carbon balance of Arctic ecosystems. If warming accelerates rates of decomposition more than rates of primary production, a sizeable additional accumulation of CO2 in the atmosphere could occur. Furthermore, CH4 produced in anaerobic soils and peatlands of the Arctic already composes a good percentage of the global efflux (Cicerone and Oremlund, 1988); if northern soils become warmer and wetter as a whole, CH4 emissions could dramatically rise. A robust understanding of the primary controls of carbon fluxes in Arctic ecosystems is critical. As a framework for a systematic examination of these controls, we discussed a conceptual model of regional-scale Arctic carbon turnover, including CH4 production, and based upon the Century soil organic matter model.

Townsend, Alan↗
NASA NTRS1994

Lifetimes and eigenstates in atmospheric chemistry

The time scales and mode of the atmosphere's response to chemical perturbations are defined by the eigenvalues and eigenvectors of the system. The eigenstates of a simplified one-box CH4-CO-OH system are analyzed. The longest time constant (smallest eigenvalue) always exceeds the lifetime defined by the steady-state loss frequency for CH4, the longest lived gas. Thus, the extent of a CH4 perturbation -- the methane response time -- is always longer than predicted by the steady-state lifetime and is independent of size of the perturbation in the linear limit. This lengthening of the atmospheric recovery time can be diagnosed by how close we are to a chemically unstable troposphere, i.e., how much OH production exceeds that minimum needed to oxidize just the global emissions of CH4, CO, and other hydrocarbons and species.

Prather, Michael J.↗
NASA NTRS1994

An examination of the total hydrogen budget of the lower stratosphere

We analyze the hydrogen budget of the lower stratosphere using simultaneous in situ measurements of northern hemispheric water vapor (H2O) and methane (CH4) obtained during the spring Stratospheric Photochemistry, Aerosols, and Dynamics Expedition (SPADE), as well as previously published in situ H2 data. Based on this data, we conclude that approximately two H2O molecules are produced for each CH4 molecule destroyed. This implies that H2 production from CH4 oxidation is balanced by H2 oxidation. The uncertainty in this analysis is greatly reduced by the use of multiple data sets. Additionally, we infer that, on an annual and global average, H2O enters the stratosphere with a mixing ratio of 4.2 +/- 0.5 ppmv, and that the quasi-conserved quantity 2 x CH4 + H2O has a value of 7.6 +/- 0.6 ppmv in these northern hemishere air parcels (where xi denotes the mixing ratio of the constituent xi).

Dessler, A. E.↗
NASA NTRS1994

Infrared spectoscopy of Triton and Pluto ice analogs: The case for saturated hydrocarbons

The infrared transmission spectra and photochemical behavior of various organic compounds isolated in solid N2 ices, appropriate for applications to Triton ad Pluto, are presented. It is shown that excess absorption in the surface spectra of Triton and Pluto, i.e., absorption not explained by present models incorporating molecules already identified on these bodies (N2, CH4, CO, and CO2), that starts near 4450/cm (2.25 microns) and extends to lower frequencies, may be due to alkanes (C(n)H(2n+2)) and related molecules frozen in the nitrogen. Branched and linear alkanes may be responsible. Experiments in which the photochemstry of N2: CH4 and N2: CH4: CO ices was explored demonsrtrate that the surface ices of Triton and Pluto may contain a wide variety of additional species containing H, C, O, and N. Of these, the reactive molecule diazomethane, CH2N2, is particularly important since it may be largely responsible for the synthesis of larger alkanes from CH4 and other small alkanes. Diazomethane would also be expected to drive chemical reactions involving organics in the surface ices of Triton and Pluto toward saturation, i.e., to reduce multiple CC bonds. The positions and intrinsic strengths (A values) of many of the infrared absorption bands of N2 matrix-isolated molecules of relevance to Triton and Pluto have also been determined. These can be used to aid in their search and to place constraints on their abundances.

Bohn, Robert B.↗
NASA NTRS1994

The Spectrum of Titan near 3.4 Micron: Comparison of Spectral Features to Organic Molecules

In August 1993 we observed the spectrum of Titan from 2.9 to 4.1 micrometers at a resolving power of R=500-1000 (dw approx. 2-4 per cm). The spectrum shows, as expected, a strong absorption throughout this region from three strong bands Of CH4. However, comparison of the spectrum to simple radiative transfer models and to the spectrum of Jupiter reveal some interesting differences. A broad emission feature that we attribute to CH4 is centered at 3.3 micrometers. At 3.45 micrometers we see a distinct narrow absorption feature that is present in both the Jupiter and laboratory spectrum, but only when other features, absent in the Titan spectrum, are also present. In our attempt to understand the origin of this spectral feature we have compared the spectrum to CH4 and other organic molecules. Interesting similarities appear between the aliphatic hydrocarbons (CH2 and CH3 groups) seen in laboratory organics and the Titan spectrum, but only if we assume that the shorter wavelength feature (CH3) is masked by the broad CH4 emission. A second problem with the explanation is that like column abundance of organic haze (Toon et al. 1991) is insufficient to provide the required number of molecules to create a spectral feature with tau approx. 1. Laboratory organics produced through a variety of processes have been compared to the Titan spectrum and the results arc presented.

Pendleton, Yvonne↗
NASA NTRS2003

Using Neural Networks to Describe Tracer Correlations

Neural networks are ideally suited to describe the spatial and temporal dependence of tracer-tracer correlations. The neural network performs well even in regions where the correlations are less compact and normally a family of correlation curves would be required. For example, the CH4-N2O correlation can be well described using a neural network trained with the latitude, pressure, time of year, and CH4 volume mixing ratio (v.m.r.). In this study a neural network using Quickprop learning and one hidden layer with eight nodes was able to reproduce the CH4-N2O correlation with a correlation co- efficient of 0.9995. Such an accurate representation of tracer-tracer correlations allows more use to be made of long-term datasets to constrain chemical models. Such as the dataset from the Halogen Occultation Experiment (HALOE) which has continuously observed CH4, (but not N2O) from 1991 till the present. The neural network Fortran code used is available for download.

Lary, D. J.↗
NASA NTRS2004

Tropospheric Methane Over the Pacific Basin During TRACE-P

We received funding for the acquisition, statistical analysis, interpretation, and publication of atmospheric methane data collected during the Transport and Chemical Evolution over the Pacific (TRACE-P) mission, flown February through April of 2001. These high precision measurements were made aboard the NASA DC-8 and P-3B aircraft using a fast response, tunable diode laser system, the DACOM instrument. Our analysis focused on the vertical and geographical distribution of this key trace gas, particularly in the context of other data available from the region such as those from the Pacific NOAA CMDL surface sites, and earlier NASA aircraft missions (such as PEM-West A and B). The paper reporting these results, "Large-Scale Distribution of CH4 in the Western Pacific: Sources and Transport from the Asian Continent" by authors K.B. Bartlett, G.W. Sachse, T. Slate, C. Harward, and D.R. Blake, is attached. The dataset of roughly 13,800 CH4 measurements ranged between 1602 ppb in stratospherically impacted air and 2149 ppb in highly polluted air. Overall, we found that CH4 concentrations were highly correlated with trace gases characteristic of a mix of anthropogenic industrial and combustion sources. They were strikingly correlated with ethane (C2H6) in particular, whose major sources are natural gas leakage and biomass burning. Correlation between the two hydrocarbons indicated that these sources may dominate for CH4 as well at this time.

Bartlett, Karen B.↗
NASA NTRS1999

Methane concentration and isotopic composition measurements with a mid-infrared quantum-cascade laser

A quantum-cascade laser operating at a wavelength of 8.1 micrometers was used for high-sensitivity absorption spectroscopy of methane (CH4). The laser frequency was continuously scanned with current over more than 3 cm-1, and absorption spectra of the CH4 nu 4 P branch were recorded. The measured laser linewidth was 50 MHz. A CH4 concentration of 15.6 parts in 10(6) ( ppm) in 50 Torr of air was measured in a 43-cm path length with +/- 0.5-ppm accuracy when the signal was averaged over 400 scans. The minimum detectable absorption in such direct absorption measurements is estimated to be 1.1 x 10(-4). The content of 13CH4 and CH3D species in a CH4 sample was determined.

Non-NASA Center↗
NASA NTRS1996

Detection of abundant ethane and methane, along with carbon monoxide and water, in comet C/1996 B2 Hyakutake: evidence for interstellar origin

The saturated hydrocarbons ethane (C2H6) and methane (CH4) along with carbon monoxide (CO) and water (H2O) were detected in comet C/1996 B2 Hyakutake with the use of high-resolution infrared spectroscopy at the NASA Infrared Telescope Facility on Mauna Kea, Hawaii. The inferred production rates of molecular gases from the icy, cometary nucleus (in molecules per second) are 6.4 X 10(26) for C2H6, 1.2 X 10(27) for CH4, 9.8 X 10(27) for CO, and 1.7 X 10(29) for H2O. An abundance of C2H6 comparable to that of CH4 implies that ices in C/1996 B2 Hyakutake did not originate in a thermochemically equilibrated region of the solar nebula. The abundances are consistent with a kinetically controlled production process, but production of C2H6 by gas-phase ion molecule reactions in the natal cloud core is energetically forbidden. The high C2H6/CH4 ratio is consistent with production of C2H6 in icy grain mantles in the natal cloud, either by photolysis of CH4-rich ice or by hydrogen-addition reactions to acetylene condensed from the gas phase.

Non-programmatic↗
NASA NTRS2006

Sulfur during the Transition from Anoxic to Oxic Atmospheres

The invention of oxygenic photosynthesis was likely accompanied by the introduction of large amounts of O2 and complementary reduced gases (chiefly CH4) into the atmosphere. To first approximation the venting of O2 and CH4 are stochiometrically linked. We therefore present a suite of numerical photochemical models that address the anoxic-oxic transition in an atmosphere driven by large linked inputs of biogenic 02 and CH4. We find in general that, in steady state, there are two solutions, one oxic and the other anoxic. The anoxic solution appears to be linearly stable. If volcanic SO2 fluxes are large, S disproportionates into oxidized (H2S04) and reduced (S8) exit channels. As elemental sulfur is insoluble it provides a means of preserving photochemical mass-independent fractionation (MIF). On the other hand, if the source of volcanic SO2 is smaller than today, all S can leave the atmosphere as S8. Under these conditions there would be no MIF signal. The oxic solution appears to be linearly unstable. In the oxic solutions S is invariably oxidized to sulfate, and the MIF signal would be absent. The transitional atmosphere is relatively unstable and is also the most photochemically active. Consequently it is the transitional atmosphere, not the oxic or anoxic atmospheres, that has the lowest CH4 levels and weakest greenhouse warming. As a practical matter we expect the transitional atmospheres to vary strongly in response to diurnal and seasonal biological forcing.

Zahnle, Kevin↗
NASA NTRS2011

Methane and Environmental Change during the Paleocene-Eocene Thermal Maximum (PETM): Modeling the PETM Onset as a Two-stage Event

An atmospheric CH4 box model coupled to a global carbon cycle box model is used to constrain the carbon emission associated with the PETM and assess the role of CH4 during this event. A range of atmospheric and oceanic emission scenarios representing different amounts, rates, and isotopic signatures of emitted carbon are used to model the PETM onset. The first 3 kyr of the onset, a pre-isotope excursion stage, is simulated by the atmospheric release of 900 to 1100 Pg C CH4 with a delta C-13 of -22 to - 30 %. For a global average warming of 3 deg C, a release of CO2 to the ocean and CH4 to the atmosphere totalling 900 to 1400 Pg C, with a delta C-13 of -50 to -60%, simulates the subsequent 1 -kyr isotope excursion stage. To explain the observations, the carbon must have been released over at most 500 years. The first stage results cannot be associated with any known PETM hypothesis. However, the second stage results are consistent with a methane hydrate source. More than a single source of carbon is required to explain the PETM onset.

Carozza, David A.↗
NASA NTRS2014

Laboratory Investigations of the Complex Refractory Organic Material Produced from Irradiation of Pluto Ice Analogs

Much of Pluto's surface consists of N2 ice with smaller amounts of CH4 and CO ices. Despite the low temperature (approximately 45K), chemistry can be driven in the surface ices by radiation processing such as cosmic ray bombardment. When cosmic rays strike the surface, much of their energy is dispersed in the form of secondary electrons, which in turn drive much of the resulting chemical reactions. Laboratory experiments designed to simulate the conditions on these icy bodies may provide insight into this chemistry. Significant progress has been made in the laboratory toward understanding the smaller, simple compounds produced in the solid phase by radiation processing of (N2, CH4, CO) ices (Bohn et al. 1994; Moore & Hudson 2003; Hodyss et al. 2011; Kim and Kaiser 2012). Recently Materese et al. (2014) used a variety of techniques to better characterize the refractory materials produced from the UV photo-irradiation of N2:CH4:CO ices. However, because Pluto's atmosphere is optically thick to Lyman-alpha UV radiation it is important to re-examine the results using an alternate radiation source. Our latest work has consisted of the analysis of refractory materials produced from the electron bombardment of low temperature N2(‐), CH4(‐), and CO(‐)containing ices (100:1:1). The ice mixture was chosen to be analogous to the known surface ices on Pluto and the radiation source was chosen to mimic the secondary electrons produced by cosmic rays bombardment. The residues were studied using multiple chemical techniques including, infrared (IR) spectroscopy, X‐ray absorption near‐edge structure (XANES) spectroscopy, and gas chromatography coupled with mass spectrometry (GC‐MS). The organic residues produced in these experiments can be seen as an analog for the refractory component of the surface of Pluto, and are compared with the residues previously obtained from UV photo‐irradiation. UV and near‐ IR spectroscopy of the surfaces of Pluto and Charon during the encounter with NASA's New Horizons spacecraft in 2015, will give the first close‐up measurements of ices and their photoproducts. Laboratory measurements and experiments will provide a better context for the data returned by the spacecraft.

organics↗
NASA NTRS2014

Quaoar: New, Longitudinaly Resolved, Spectroscopic Characterization of Its Surface

(50000) Quaoar, one of the largest Trans-neptunian objects, is comparable in size to Pluto's moon Charon. However, while Charon's surface is rich almost exclusively in H2O ice, Quaoar's surface characterized by ices of CH4, N2, as well as C2H6, a product of irradiation of CH4 (Dalle Ore et al. 2009). Because of its distance from the Sun, Quaoar is expected to have preserved, to a degree, its original composition, however, its relatively small size did not make it a prime candidate for presence of volatile ices in the study by Schaller and Brown (2007). Furthermore, based on the Brown et al. (2011) study (Brown, Schaller, & Fraser, 2011. A Hypothesis for the Color Diversity of the Kuiper Belt. ApJL, 739, L60) its red coloration points to CH3OH as the ice which, when irradiated, might have produced the red material. We present new visible to near-infrared (0.3-2.48 micrometers) spectro-photometric data obtained with the XSHOOTER (Vernet et al. 2011, A&A, 536A, 105 ) instrument at the VLT-ESO facility at four different longitudes on the surface of Quaoar. The data are complemented by previously published photometric observations obtained in the near-infrared (3.6, 4.5 micrometers) with the Spitzer Space Telescope, which provide an extra set of constraints in the model calculation process in spite of the different observing times that preclude establishing the spatial consistency between the two sets. For each of the four spectra we perform spectral modeling of the entire wavelength range -from 0.3 to 4.5 micrometers- by means of a code based on the Shkuratov radiative transfer formulation of the slab model. We obtain spatially resolved compositional information for the surface of Quaoar supporting the presence of CH4 and C2H6, as previously reported, along with evidence for N2 and NH3OH. The albedo at the two Spitzer bands indicates the likely presence of CO and CO2. CH3OH, predicted on the basis of Quaoar's coloration (Brown et al. 2011), is not found at any of the four longitudes, implying that the presence of this ice is a sufficient, but not necessary condition for reddening of TNO surfaces. Other ices, in particular CH4 (Brunetto et al. 2006), have been shown to be plausible precursors for reddening of TNO surfaces.

Ices↗