The existence of methane in Titan's atmosphere (~6% level at the surface) presents a unique enigma, as photochemical models predict that the current inventory will be entirely depleted by ...photochemistry in a timescale of ~20 Myr. In this paper, we examine the clues available from isotopic ratios ( super(12)C/ super(13)C and D/H) in Titan's methane as to the past atmosphere history of this species. We first analyze recent infrared spectra of CH sub(4) collected by the Cassini Composite Infrared Spectrometer, measuring simultaneously for the first time the abundances of all three detected minor isotopologues: super(13)CH sub(4), super(12)CH sub(3)D, and super(13)CH sub(3)D. From these we compute estimates of super(12)C/ super(13)C = 86.5 + or - 8.2 and D/H = (1.59 + or - 0.33) x 10 super(-4), in agreement with recent results from the Huygens GCMS and Cassini INMS instruments. We also use the transition state theory to estimate the fractionation that occurs in carbon and hydrogen during a critical reaction that plays a key role in the chemical depletion of Titan's methane: CH sub(4) + C sub(2)H arrow right CH sub(3) + C sub(2)H sub(2). Using these new measurements and predictions we proceed to model the time evolution of super(12)C/ super(13)C and D/H in Titan's methane under several prototypical replenishment scenarios. In our Model 1 (no resupply of CH sub(4)), we find that the present-day super(12)C/ super(13)C implies that the CH sub(4) entered the atmosphere 60-1600 Myr ago if methane is depleted by chemistry and photolysis alone, but much more recently-most likely less than 10 Myr ago-if hydrodynamic escape is also occurring. On the other hand, if methane has been continuously supplied at the replenishment rate then the isotopic ratios provide no constraints, and likewise for the case where atmospheric methane is increasing. We conclude by discussing how these findings may be combined with other evidence to constrain the overall history of the atmospheric methane.
This paper reports on the results from an extensive study of all nadir-looking spectra acquired by Cassini/CIRS during the 44 flybys performed in the course of the nominal mission (2004–2008). With ...respect to the previous study (Coustenis, A., and 24 colleagues 2007. Icarus 189, 35–62, on flybys TB–T10) we present here a significantly richer dataset with, in particular, more data at high northern and southern latitudes so that the abundances inferred here at these regions are more reliable. Our enhanced high-resolution dataset allows us to infer more precisely the chemical composition of Titan all over the disk. We also include improved spectroscopic data for some molecules and updated temperature profiles. The latitudinal distributions of all of the gaseous species are inferred. We furthermore test vertical distributions essentially for acetylene (C
2H
2) from CIRS limb-inferred data and from current General Circulation Models for Titan and compare our results on all the gaseous abundances with predictions from 1-D photochemical-radiative models to check the reliability of the chemical reactions and pathways.
Since the Voyager 1 (V1) flyby in 1980, Titan's exploration from space and the ground has been ongoing for more than a full revolution of Saturn around the Sun (one Titanian year or 29.5 Earth years ...had elapsed in 2010 May). In this study, we search for temporal variations affecting Titan's atmospheric thermal and chemical structure within that year.We process Cassini/CIRS data taken during the Titan flybys from 2006-2013 and find a rather uneventful equatorial evolution. Conversely, at northern latitudes, we found enhanced abundances around the period of the northern spring equinox in mid-2009, which subsequently decreased (from 2010 to 2012), returning to values similar to those found in the V1 epoch, one Titanian year before. In the southern latitudes, since 2012, we see a trend for an increase of several trace gases (C sub(4)H sub(2), C sub(3)H sub(4), and HCN), indicative of a seasonal atmospheric reversal setting in. When we compare the CIRS 2010 and the 1980 V1/IRIS spectra (reanalyzed here), we find limited inter-annual variations. A return to the 1980 stratospheric temperatures and abundances is generally achieved from 50degreesN to 50degreesS, indicative of the solar radiation being the dominating energy source at 10 AU, as for the Earth, as predicted by general circulation and photochemical models. Exceptions concern the most complex hydrocarbons (C sub(4)H sub(2) and C sub(3)H sub(4)). We also consider data from ground-based and Earth-orbiting observatories (such as from the Infrared Space Observatory, revisited here) and discuss possible atmospheric composition trends during a Titanian year.
By the close of the Cassini mission in 2017 the Composite Infrared Spectrometer had recorded surface brightness temperatures on Titan for 13 yr (almost half a Titan year). We mapped temperatures in ...latitude from pole to pole in seven time segments from northern mid-winter to northern summer solstice. At the beginning of the mission the warmest temperatures were centered at 13 S where they peaked at 93.9 K. Temperatures fell off by about 4 K toward the north pole and 2 K toward the south pole. As the seasons progressed the warmest temperatures shifted northward, tracking the subsolar point, and at northern summer solstice were centered at 24 N. While moving north the peak temperature decreased by about 1 K, reaching 92.8 K at solstice. At solstice the fall-off toward the north and south poles were 1 K and 3 K, respectively. Thus the temperature range was the same 2 K at the two poles. Our observed surface temperatures agree with recent general circulation model results that take account of methane hydrology and imply that hemispherical differences in Titan's topography may play a role in the north-south asymmetry on Titan.
Titan's Surface Brightness Temperatures Jennings, D. E; Flasar, F. M; Kunde, V. G ...
The Astrophysical journal,
02/2009, Letnik:
691, Številka:
2
Journal Article
We have developed a line-by-line Atmospheric Radiative Transfer for Titan code that includes the most recent laboratory spectroscopic data and haze descriptions relative to Titan's stratosphere. We ...use this code to model Cassini Composite Infrared Spectrometer data taken during the numerous Titan flybys from 2006 to 2012 at surface-intercepting geometry in the 600-1500 cm super(-1) range for latitudes from 50degreesS to 50degreesN. We report variations in temperature and chemical composition in the stratosphere during the Cassini mission, before and after the Northern Spring Equinox (NSE). We find indication for a weakening of the temperature gradient with warming of the stratosphere and cooling of the lower mesosphere. In addition, we infer precise concentrations for the trace gases and their main isotopologues and find that the chemical composition in Titan's stratosphere varies significantly with latitude during the 6 years investigated here, with increased mixing ratios toward the northern latitudes. In particular, we monitor and quantify the amplitude of a maximum enhancement of several gases observed at northern latitudes up to 50degreesN around mid-2009, at the time of the NSE. We find that this rise is followed by a rapid decrease in chemical inventory in 2010 probably due to a weakening north polar vortex with reduced lateral mixing across the vortex boundary.
A portable monoenergetic 24 keV neutron source based on the 124Sb-9Be photoneutron reaction and an iron filter has been constructed and characterized. The coincidence of the neutron energy from SbBe ...and the low interaction cross-section with iron (mean free path up to 29 cm) makes pure iron specially suited to shield against gamma rays from 124Sb decays while letting through the neutrons. To increase the 124Sb activity and thus the neutron flux, a >1 GBq 124Sb source was produced by irradiating a natural Sb metal pellet with a high flux of thermal neutrons in a nuclear reactor. The design of the source shielding structure makes for easy transportation and deployment. A hydrogen gas proportional counter is used to characterize the neutrons emitted by the source and a NaI detector is used for gamma background characterization. At the exit opening of the neutron beam, the characterization determined the neutron flux in the energy range 20–25 keV to be 6.00±0.30 neutrons per cm2 per second and the total gamma flux to be 245±8 gammas per cm2 per second (numbers scaled to 1 GBq activity of the 124Sb source). A liquid scintillator detector is demonstrated to be sensitive to neutrons with incident kinetic energies from 8 to 17 keV, so it can be paired with the source as a backing detector for neutron scattering calibration experiments. This photoneutron source provides a good tool for in-situ low energy nuclear recoil calibration for dark matter experiments and coherent elastic neutrino-nucleus scattering experiments.
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