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Register a new userALMA measurements of the HNC and HC$_3$N distributions in Titans atmosphere
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Martin Cordiner PhD
Publication date
2014
fields
Physics
and research's language is
English
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We present spectrally and spatially-resolved maps of HNC and HC$_3$N emission from Titans atmosphere, obtained using the Atacama Large Millimeter/submillimeter Array (ALMA) on 2013 November 17. These maps show anisotropic spatial distributions for both molecules, with resolved emission peaks in Titans northern and southern hemispheres. The HC$_3$N maps indicate enhanced concentrations of this molecule over the poles, consistent with previous studies of Titans photochemistry and atmospheric circulation. Differences between the spectrally-integrated flux distributions of HNC and HC$_3$N 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 $zgtrsim 400$ km), whereas HC$_3$N is abundant at a broader range of altitudes ($zapprox70$-600 km). From spatial variations in the HC$_3$N line profile, the locations of the HC$_3$N 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 HC$_3$N (at $zgtrsim300$ km) are found to be asymmetric with respect to Titans polar axis, indicating that the mesosphere may be more longitudinally-variable than previously thought. The spatially-integrated HNC and HC$_3$N spectra are modeled using the NEMESIS planetary atmosphere code and the resulting best-fitting disk-averaged vertical mixing ratio (VMR) 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 HC$_3$N are $1.9times10^{13}$ cm$^{-2}$ and $2.3times10^{14}$ cm$^{-2}$, respectively.
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Previous investigations have employed more than 100 close observations of Titan by the Cassini orbiter to elucidate connections between the production and distribution of Titans 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 Titans 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 Titans 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 Titans trace atmospheric species from the same 3 independent spatial regions across Titans disk during the same epoch (2012 to 2015): HCN, HC$_3$N, C$_3$H$_4$, and CH$_3$CN. We find that Titans minor constituents exhibit large latitudinal variations, with enhanced abundances at high latitudes compared to equatorial measurements; this includes CH$_3$CN, 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 Titans 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 Titans south pole during this time period. Continued observations of Titan with ALMA beyond the summer solstice will enable further study of how Titans atmospheric composition and dynamics respond to seasonal changes.
Titan harbors a dense, organic-rich atmosphere primarily composed of N$_2$ and CH$_4$, with lesser amounts of hydrocarbons and nitrogen-bearing species. As a result of high sensitivity observations by the Atacama Large Millimeter/submillimeter Array (ALMA) in Band 6 ($sim$230-272 GHz), we obtained the first spectroscopic detection of CH$_3$C$_3$N (methylcyanoacetylene or cyanopropyne) in Titans atmosphere through the observation of seven transitions in the $J = 64rightarrow63$ and $J = 62rightarrow61$ rotational bands. The presence of CH$_3$C$_3$N on Titan was suggested by the Cassini Ion and Neutral Mass Spectrometer detection of its protonated form: C$_4$H$_3$NH$^+$, but the atmospheric abundance of the associated (deprotonated) neutral product is not well constrained due to the lack of appropriate laboratory reaction data. Here, we derive the column density of CH$_3$C$_3$N to be (3.8-5.7)$times10^{12}$ cm$^{-2}$ based on radiative transfer models sensitive to altitudes above 400 km Titans middle atmosphere. When compared with laboratory and photochemical model results, the detection of methylcyanoacetylene provides important constraints for the determination of the associated production pathways (such as those involving CN, CCN, and hydrocarbons), and reaction rate coefficients. These results also further demonstrate the importance of ALMA and (sub)millimeter spectroscopy for future investigations of Titans organic inventory and atmospheric chemistry, as CH$_3$C$_3$N marks the heaviest polar molecule detected spectroscopically in Titans atmosphere to date.
We present the first maps of cyanoacetylene isotopologues in Titans atmosphere, including H$^{13}$CCCN and HCCC$^{15}$N, detected in the 0.9 mm band using the Atacama Large Millimeter/submillimeter array (ALMA) around the time of Titans (southern winter) solstice in May 2017. The first high-resolution map of HC$_3$N in its $v_7=1$ vibrationally excited state is also presented, revealing a unique snapshot of the global HC$_3$N distribution, free from the strong optical depth effects that adversely impact the ground-state ($v=0$) map. The HC$_3$N emission is found to be strongly enhanced over Titans south pole (by a factor of 5.7 compared to the north pole), consistent with rapid photochemical loss of HC$_3$N from the summer hemisphere combined with production and transport to the winter pole since the April 2015 ALMA observations. The H$^{13}$CCCN/HCCC$^{15}$N flux ratio is derived at the southern HC$_3$N peak, and implies an HC$_3$N/HCCC$^{15}$N ratio of $67pm14$. This represents a significant enrichment in $^{15}$N compared with Titans main molecular nitrogen reservoir, which has a $^{14}$N/$^{15}$N ratio of 167, and confirms the importance of photochemistry in determining the nitrogen isotopic ratio in Titans organic inventory.
Vinyl cyanide (C$_2$H$_3$CN) is theorized to form in Titans atmosphere via high-altitude photochemistry and is of interest regarding the astrobiology of cold planetary surfaces due to its predicted ability to form cell membrane-like structures (azotosomes) in liquid methane. In this work, we follow up on the initial spectroscopic detection of C$_2$H$_3$CN on Titan by Palmer et al. (2017) with the detection of three new C$_2$H$_3$CN rotational emission lines at submillimeter frequencies. These new, high-resolution detections have allowed for the first spatial distribution mapping of C$_2$H$_3$CN on Titan. We present simultaneous observations of C$_2$H$_5$CN, HC$_3$N, and CH$_3$CN emission, and obtain the first (tentative) detection of C$_3$H$_8$ (propane) at radio wavelengths. We present disk-averaged vertical abundance profiles, two-dimensional spatial maps, and latitudinal flux profiles for the observed nitriles. Similarly to HC$_3$N and C$_2$H$_5$CN, which are theorized to be short-lived in Titans atmosphere, C$_2$H$_3$CN is most abundant over the southern (winter) pole, whereas the longer-lived CH$_3$CN is more concentrated in the north. This abundance pattern is consistent with the combined effects of high-altitude photochemical production, poleward advection, and the subsequent reversal of Titans atmospheric circulation system following the recent transition from northern to southern winter. We confirm that C$_2$H$_3$CN and C$_2$H$_5$CN are most abundant at altitudes above 200 km. Using a 300 km step model, the average abundance of C$_2$H$_3$CN is found to be $3.03pm0.29$ ppb, with a C$_2$H$_5$CN/C$_2$H$_3$CN abundance ratio of $2.43pm0.26$. Our HC$_3$N and CH$_3$CN spectra can be accurately modeled using abundance gradients above the tropopause, with fractional scale-heights of $2.05pm0.16$ and $1.63pm0.02$, respectively.
We apply a 1D upper atmosphere model to study thermal escape of nitrogen over Titans history. Significant thermal escape should have occurred very early for solar EUV fluxes 100 to 400 times higher than today with escape rates as high as $approx 1.5times 10^{28}$ s$^{-1}$ and $approx 4.5times 10^{29}$ s$^{-1}$, respectively, while today it is $approx 7.5times 10^{17}$ s$^{-1}$. Depending on whether the Sun originated as a slow, moderate or fast rotator, thermal escape was the dominant escape process for the first 100 to 1000 Myr after the formation of the solar system. If Titans atmosphere originated that early, it could have lost between $approx 0.5 - 16$ times its present atmospheric mass depending on the Suns rotational evolution. We also investigated the mass-balance parameter space for an outgassing of Titans nitrogen through decomposition of NH$_3$-ices in its deep interior. Our study indicates that, if Titans atmosphere originated at the beginning, it could have only survived until today if the Sun was a slow rotator. In other cases, the escape would have been too strong for the degassed nitrogen to survive until present-day, implying later outgassing or an additional nitrogen source. An endogenic origin of Titans nitrogen partially through NH$_3$-ices is consistent with its initial fractionation of $^{14}$N/$^{15}$N $approx$ 166 - 172, or lower if photochemical removal was relevant for longer than the last $approx$ 1,000 Myr. Since this ratio is slightly above the ratio of cometary ammonia, some of Titans nitrogen might have originated from refractory organics.
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