46 research outputs found
Titan Atmospheric Chemistry Revealed by Low-temperature N2-CH4 Plasma Discharge Experiments
Chemistry in Titan's N2-CH4 atmosphere produces complex organic aerosols. The
chemical processes and the resulting organic compounds are still far from
understood, although extensive observations, laboratory, and theoretical
simulations have greatly improved physical and chemical constraints on Titan's
atmosphere. Here, we conduct a series of Titan atmosphere simulation
experiments with N2-CH4 gas mixtures and investigate the effect of initial CH4
ratio, pressure, and flow rate on the production rates and composition of the
gas and solid products at a Titan relevant temperature (100 K) for the first
time. We find that the production rate of the gas and solid products increases
with increasing CH4 ratio. The nitrogen-containing species have much higher
yield than hydrocarbons in the gas products, and the N-to-C ratio of the solid
products appears to be the highest compared to previous plasma simulations with
the same CH4 ratio. The greater degree of nitrogen incorporation in the low
temperature simulation experiments suggests temperature may play an important
role in nitrogen incorporation in Titan's cold atmosphere. We also find that H2
is the dominant gas product and serves as an indicator of the production rate
of new organic molecules in the experiment, and that CH2NH may greatly
contribute to the incorporation of both carbon and nitrogen into the solid
particles. The pressure and flow rate affect the amount of time of the gas
mixture exposed to the energy source and therefore impact the N2-CH4 chemistry
initiated by the plasma discharge, emphasizing the influence of the energy flux
in Titan atmospheric chemistry.Comment: Accepted in ACS Earth and Space Chemistry, 6 figure
Abundance Measurements of Titan's Stratospheric HCN, HCN, CH, and CHCN from ALMA Observations
Previous investigations have employed more than 100 close observations of
Titan by the Cassini orbiter to elucidate connections between the production
and distribution of Titan's vast, organic-rich chemical inventory and its
atmospheric dynamics. However, as Titan transitions into northern summer, the
lack of incoming data from the Cassini orbiter presents a potential barrier to
the continued study of seasonal changes in Titan's atmosphere. In our previous
work (Thelen et al., 2018), we demonstrated that the Atacama Large
Millimeter/submillimeter Array (ALMA) is well suited for measurements of
Titan's atmosphere in the stratosphere and lower mesosphere (~100-500 km)
through the use of spatially resolved (beam sizes <1'') flux calibration
observations of Titan. Here, we derive vertical abundance profiles of four of
Titan's trace atmospheric species from the same 3 independent spatial regions
across Titan's disk during the same epoch (2012 to 2015): HCN, HCN,
CH, and CHCN. We find that Titan's minor constituents exhibit large
latitudinal variations, with enhanced abundances at high latitudes compared to
equatorial measurements; this includes CHCN, which eluded previous
detection by Cassini in the stratosphere, and thus spatially resolved abundance
measurements were unattainable. Even over the short 3-year period, vertical
profiles and integrated emission maps of these molecules allow us to observe
temporal changes in Titan's atmospheric circulation during northern spring. Our
derived abundance profiles are comparable to contemporary measurements from
Cassini infrared observations, and we find additional evidence for subsidence
of enriched air onto Titan's south pole during this time period. Continued
observations of Titan with ALMA beyond the summer solstice will enable further
study of how Titan's atmospheric composition and dynamics respond to seasonal
changes.Comment: 15 pages, 16 figures, 2 tables. Accepted for publication in Icarus,
September 201
Ethyl cyanide on Titan: Spectroscopic detection and mapping using ALMA
We report the first spectroscopic detection of ethyl cyanide (CHCN)
in Titan's atmosphere, obtained using spectrally and spatially resolved
observations of multiple emission lines with the Atacama Large
Millimeter/submillimeter array (ALMA). The presence of CHCN in Titan's
ionosphere was previously inferred from Cassini ion mass spectrometry
measurements of CHCNH. Here we report the detection of 27
rotational lines from CHCN (in 19 separate emission features detected
at confidence), in the frequency range 222-241 GHz. Simultaneous
detections of multiple emission lines from HCN, CHCN and CHCCH were
also obtained. In contrast to HCN, CHCN and CHCCH, which peak in
Titan's northern (spring) hemisphere, the emission from CHCN is found
to be concentrated in the southern (autumn) hemisphere, suggesting a distinctly
different chemistry for this species, consistent with a relatively short
chemical lifetime for CHCN. Radiative transfer models show that most of
the CHCN is concentrated at altitudes 300-600 km, suggesting production
predominantly in the mesosphere and above. Vertical column densities are found
to be in the range (2-5) cm.Comment: Published in 2015, ApJL, 800, L1
Detection of Cyclopropenylidene on Titan with ALMA
We report the first detection on Titan of the small cyclic molecule cyclopropenylidene (c-C3H2) from high-sensitivity spectroscopic observations made with the Atacama Large Millimeter/submillimeter Array. Multiple lines of cyclopropenylidene were detected in two separate data sets: ~251 GHz in 2016 (Band 6) and ~352 GHz in 2017 (Band 7). Modeling of these emissions indicates abundances of 0.50 ± 0.14 ppb (2016) and 0.28 ± 0.08 (2017) for a 350 km step model, which may either signify a decrease in abundance, or a mean value of 0.33 ± 0.07 ppb. Inferred column abundances are (3–5) × 1012 cm−2 in 2016 and (1–2) × 1012 cm−2 in 2017, similar to photochemical model predictions. Previously the C3H ion has been measured in Titan's ionosphere by Cassini's Ion and Neutral Mass Spectrometer (INMS), but the neutral (unprotonated) species has not been detected until now, and aromatic versus aliphatic structure could not be determined by the INMS. Our work therefore represents the first unambiguous detection of cyclopropenylidene, the second known cyclic molecule in Titan's atmosphere along with benzene (C6H6) and the first time this molecule has been detected in a planetary atmosphere. We also searched for the N-heterocycle molecules pyridine and pyrimidine finding nondetections in both cases, and determining 2σ upper limits of 1.15 ppb (c-C5H5N) and 0.85 ppb (c-C4H4N2) for uniform abundances above 300 km. These new results on cyclic molecules provide fresh constraints on photochemical pathways in Titan's atmosphere, and will require new modeling and experimental work to fully understand the implications for complex molecule formation
ALMA measurements of the HNC and HC3N distributions in Titan's atmosphere
We present spectrally and spatially resolved maps of HNC and HC3N emission from Titan's atmosphere, obtained using the Atacama Large Millimeter/submillimeter Array on 2013 November 17. These maps show anisotropic spatial distributions for both molecules, with resolved emission peaks in Titan's northern and southern hemispheres. The HC3N maps indicate enhanced concentrations of this molecule over the poles, consistent with previous studies of Titan's photochemistry and atmospheric circulation. Differences between the spectrally integrated flux distributions of HNC and HC3N show that these species are not co-spatial. The observed spectral line shapes are consistent with HNC being concentrated predominantly in the mesosphere and above (at altitudes z ≳ 400 km), whereas HC3N is abundant at a broader range of altitudes (z ≈ 70-600 km). From spatial variations in the HC3N line profile, the locations of the HC3N emission peaks are shown to be variable as a function of altitude. The peaks in the integrated emission from HNC and the line core (upper atmosphere) component of HC3N (at z ≳ 300 km) are found to be asymmetric with respect to Titan's polar axis, indicating that the mesosphere may be more longitudinally variable than previously thought. The spatially integrated HNC and HC3N spectra are modeled using the NEMESIS planetary atmosphere code and the resulting best-fitting disk-averaged vertical mixing ratio profiles are found to be in reasonable agreement with previous measurements for these species. Vertical column densities of the best-fitting gradient models for HNC and HC3N are 1.9 × 1013 cm-2 and 2.3 × 1014 cm-2, respectively
Saturn's atmospheric response to the large influx of ring material inferred from Cassini INMS measurements
During the Grand Finale stage of the Cassini mission, organic-rich ring
material was discovered to be flowing into Saturn's equatorial upper atmosphere
at a surprisingly large rate. Through a series of photochemical models, we have
examined the consequences of this ring material on the chemistry of Saturn's
neutral and ionized atmosphere. We find that if a substantial fraction of this
material enters the atmosphere as vapor or becomes vaporized as the solid ring
particles ablate upon atmospheric entry, then the ring-derived vapor would
strongly affect the composition of Saturn's ionosphere and neutral
stratosphere. Our surveys of Cassini infrared and ultraviolet remote-sensing
data from the final few years of the mission, however, reveal none of these
predicted chemical consequences. We therefore conclude that either (1) the
inferred ring influx represents an anomalous, transient situation that was
triggered by some recent dynamical event in the ring system that occurred a few
months to a few tens of years before the 2017 end of the Cassini mission, or
(2) a large fraction of the incoming material must have been entering the
atmosphere as small dust particles less than ~100 nm in radius, rather than as
vapor or as large particles that are likely to ablate. Future observations or
upper limits for stratospheric neutral species such as HCN, HCN, and CO
at infrared wavelengths could shed light on the origin, timing, magnitude, and
nature of a possible vapor-rich ring-inflow event.Comment: accepted in Icaru
Spatial variations in Titan's atmospheric temperature:ALMA and <i>Cassini </i>comparisons from 2012 to 2015
Submillimeter emission lines of carbon monoxide (CO) in Titan's atmosphere provide excellent probes of atmospheric temperature due to the molecule's long chemical lifetime and stable, well constrained volume mixing ratio. Here we present the analysis of 4 datasets obtained with the Atacama Large Millimeter/Submillimeter Array (ALMA) in 2012, 2013, 2014, and 2015 that contain strong CO rotational transitions. Utilizing ALMA's high spatial resolution in the 2012, 2014, and 2015 observations, we extract spectra from 3 separate regions on Titan's disk using datasets with beam sizes ranging from 0.35 × 0.28'' to 0.39 × 0.34''. Temperature profiles retrieved by the NEMESIS radiative transfer code are compared to Cassini Composite Infrared Spectrometer (CIRS) and radio occultation science results from similar latitude regions. Disk-averaged temperature profiles stay relatively constant from year to year, while small seasonal variations in atmospheric temperature are present from 2012 to 2015 in the stratosphere and mesosphere (~100-500 km) of spatially resolved regions. We measure the stratopause (320 km) to increase in temperature by 5 K in northern latitudes from 2012 to 2015, while temperatures rise throughout the stratosphere at lower latitudes. We observe generally cooler temperatures in the lower stratosphere (~100 km) than those obtained through Cassini radio occultation measurements, with the notable exception of warming in the northern latitudes and the absence of previous instabilities; both of these results are indicators that Titan's lower atmosphere responds to seasonal effects, particularly at higher latitudes. While retrieved temperature profiles cover a range of latitudes in these observations, deviations from CIRS nadir maps and radio occultation measurements convolved with the ALMA beam-footprint are not found to be statistically significant, and discrepancies are often found to be less than 5 K throughout the atmosphere. ALMA's excellent sensitivity in the lower stratosphere (60-300 km) provides a highly complementary dataset to contemporary CIRS and radio science observations, including altitude regions where both of those measurement sets contain large uncertainties. The demonstrated utility of CO emission lines in the submillimeter as a tracer of Titan's atmospheric temperature lays the groundwork for future studies of other molecular species - particularly those that exhibit strong polar abundance enhancements or are pressure-broadened in the lower atmosphere, as temperature profiles are found to consistently vary with latitude in all three years by up to 15 K
Saturn’s atmospheric response to the large influx of ring material inferred from Cassini INMS measurements
During the Grand Finale stage of the Cassini mission, organic-rich ring material was discovered to be flowing into Saturn’s equatorial upper atmosphere at a surprisingly large rate. Through a series of photochemical models, we have examined the consequences of this ring material on the chemistry of Saturn’s neutral and ionized atmosphere. We find that if a substantial fraction of this material enters the atmosphere as vapor or becomes vaporized as the solid ring particles ablate upon atmospheric entry, then the ring-derived vapor would strongly affect the composition of Saturn’s ionosphere and neutral stratosphere. Our surveys of Cassini infrared and ultraviolet remote-sensing data from the final few years of the mission, however, reveal none of these predicted chemical consequences. We therefore conclude that either (1) the inferred ring influx represents an anomalous, transient situation that was triggered by some recent dynamical event in the ring system that occurred a few months to a few tens of years before the 2017 end of the Cassini mission, or (2) a large fraction of the incoming material must have been entering the atmosphere as small dust particles less than 100 nm in radius, rather than as vapor or as large particles that are likely to ablate. Future observations or upper limits for stratospheric neutral species such as HCN, HCN, and CO at infrared wavelengths could shed light on the origin, timing, magnitude, and nature of a possible vapor-rich ring-inflow event
In vivo biofunctional evaluation of hydrogels for disc regeneration
Purpose Regenerative strategies aim to restore the original
biofunctionality of the intervertebral disc. Different
biomaterials are available, which might support disc
regeneration. In the present study, the prospects of success
of two hydrogels functionalized with anti-angiogenic peptides
and seeded with bone marrow derived mononuclear
cells (BMC), respectively, were investigated in an ovine
nucleotomy model.
Methods In a one-step procedure iliac crest aspirates
were harvested and, subsequently, separated BMC were
seeded on hydrogels and implanted into the ovine disc. For
the cell-seeded approach a hyaluronic acid-based hydrogel
was used. The anti-angiogenic potential of newly developed
VEGF-blockers was investigated on ionically crosslinked
metacrylated gellan gum hydrogels. Untreated discs
served as nucleotomy controls. 24 adult merino sheep were
used. After 6 weeks histological, after 12 weeks histological
and biomechanical analyses were conducted.
Results Biomechanical tests revealed no differences
between any of the implanted and nucleotomized discs. All
implanted discs significantly degenerated compared to
intact discs. In contrast, there was no marked difference
between implanted and nucleotomized discs. In tendency,
albeit not significant, degeneration score and disc height
index deteriorated for all but not for the cell-seeded
hydrogels from 6 to 12 weeks. Cell-seeded hydrogels
slightly decelerated degeneration.
Conclusions None of the hydrogel configurations was
able to regenerate biofunctionality of the intervertebral
disc. This might presumably be caused by hydrogel
extrusion. Great importance should be given to the development
of annulus sealants, which effectively exploit the
potential of (cell-seeded) hydrogels for biological disc
regeneration and restoration of intervertebral disc
functioningThis work was supported by the EU-project Disc Regeneration (NMP3-LA-2008-213904). Technical assistance of Iris Baum and the whole animal surgery team of the Institute of Orthopaedic Research and Biomechanics, Ulm, are gratefully acknowledged. DDAHA hydrogels were kindly provided by Cristina Longinotti (DDAHA, Anika Therapeutics, Abano Therme, Italy)