Engineering topics

Achterberg, R. K.

Publications and source records attributed to Achterberg, R. K..

At least 19 records

NASA NTRS2019

Titan's Neutral Atmosphere Seasonal Variations up to the End of the Cassini Mission

In this paper we report new results concerning the seasonal atmospheric evolution near Titan's poles and equator using the Cassini Composite Infrared Spectrometer (CIRS). In previous papers (Coustenis et al. 2016 and 2018), after the 2010 equinox, we have monitored Titan's stratosphere since the beginning of the mission and reported recently on the observed strong temperature decrease and compositional enhancement above Titan's southern polar latitudes since 2012 of several trace species such as complex hydrocarbons and nitriles, which were previously observed only at high northern latitudes. This effect followed the transition of Titan's seasons from northern winter in 2002 to northern summer in 2017, while at that latter time, the southern hemisphere was entering winter. From 2013 until 2016, the northern polar region has shown a temperature increase of 10 K, while the south has shown a more significant decrease (up to 25 K) in a similar period of time. The stratosphere over the south pole of Titan shows a change in shape with a steeper positive slope and an increase in temperature in 2017 with respect to 2014, by 10-50 K in the 0.5 mbar-0.05 mbar pressure range, the larger 2017 temperatures observed in the higher atmospheric levels around 0.05 mbar, while in average the 70Â°S values return closer to the warmer ones found in 2012. While the south polar region is continuously enhanced since 2012, the chemical content in the north is showing a clear depletion for all molecules only since 2015 (Coustenis et al., 2018). We present here a complementary analysis of nadir spectra acquired by Cassini/CIRS at high resolution during the last year of the Cassini mission in 2017 and describe the temperature and composition variations near Titan's poles and at the equator over almost two Titan seasons. The 2017 data we have acquired and analyzed here are important because they are the only nadir ones afforded since 2014 close to the south pole at high resolution. They show no significant changes in the north with respect to 2016, but a large temperature increase in the southern polar stratosphere and a change in the temperature profiles shape while the South is also finally showing a related significant decrease in most of the abundances.

Coustenis, A.↗

NASA NTRS2019

Titan Surface Temperatures During the Cassini Mission

By the close of the Cassini mission in 2017 the Composite Infrared Spectrometer had recorded surface brightnesstemperatures on Titan for 13 yr (almost half a Titan year). We mapped temperatures in latitude from pole to pole inseven time segments from northern mid-winter to northern summer solstice. At the beginning of the mission thewarmest temperatures were centered at 13 S where they peaked at 93.9 K. Temperatures fell off by about 4 Ktoward the north pole and 2 K toward the south pole. As the seasons progressed the warmest temperatures shiftednorthward, tracking the subsolar point, and at northern summer solstice were centered at 24 N. While moving norththe peak temperature decreased by about 1 K, reaching 92.8 K at solstice. At solstice the fall-off toward the northand south poles were 1 K and 3 K, respectively. Thus the temperature range was the same 2 K at the two poles. Ourobserved surface temperatures agree with recent general circulation model results that take account of methanehydrology and imply that hemispherical differences in Titan's topography may play a role in the north?southasymmetry on Titan.

Jennings, D. E.↗

NASA NTRS2018

Thermal Emission From Saturn's Polar Cyclones

We have used data from the Cassini Composite Infrared Spectrometer to map the temperatures in Saturn's polar cyclones at the highest spatial resolution obtained during the Cassini mission. We find temperature contrasts of 7 K in the upper troposphere within 1.4° of both poles, roughly 50 percent larger than earlier measurements at lower spatial resolution. The polar hot spots weaken with depth, disappearing near 500 mbar. In the stratosphere, the polar hot spot becomes broader, extending 4° from the poles, and weakens with altitude disappearing near 1 mbar. A thermal relaxation model shows that the tropospheric hot spot is consistent with adiabatic heating from subsidence with a vertical velocity of about −0.05 mm/s above 500 mbar. The observed temperature gradients imply that the winds in the polar cyclone decay with increasing altitude over roughly three pressure scale heights above the 200‐mbar level.

Polar↗

NASA NTRS2018

Titan's Cold Case Files - Outstanding Questions After Cassini-Huygens

The entry of the Cassini-Huygens spacecraft into orbit around Saturn in July 2004 marked the start of a golden era in the exploration of Titan, Saturn's giant moon. During the prime mission (2004-2008), ground-breaking discoveries were made by the Cassini orbiter including the equatorial dune fields (flyby T3, 2005), northern lakes and seas (T16, 2006), and the large positive and negative ions (T16 & T18, 2006), to name a few. In 2005 the Huygens probe descended through Titan's atmosphere, taking the first close-up pictures of the surface, including large networks of dendritic channels leading to a dried-up seabed, and also obtaining detailed profiles of temperature and gas composition during the atmospheric descent. The discoveries continued through the Equinox mission (2008-2010) and Solstice mission (2010-2017) totaling 127 targeted flybys of Titan in all. Now at the end of the mission, we are able to look back on the high-level scientific questions from the start of the mission, and assess the progress that has been made towards answering these. At the same time, new scientific questions regarding Titan have emerged from the new discoveries that have been made. In this paper we review a cross-section of important scientific questions that remain partially or completely unanswered, ranging from Titan's deep interior to the exosphere. Our intention is to help formulate the science goals for the next generation of planetary missions to Saturn and Titan, and to stimulate new experimental, observation and theoretical investigations in the interim, before such missions arrive again at Titan.

Nixon, C. A.↗

NASA NTRS2017

Mapping Vinyl Cyanide and Other Nitriles in Titan's Atmosphere Using ALMA

Vinyl cyanide (C2H3CN) is theorized to form in Titan's 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 C2H3CN on Titan by Palmer et al. with the detection of three new C2H3CN rotational emission lines at submillimeter frequencies. These new, high-resolution detections have allowed for the first spatial distribution mapping of C2H3CN on Titan. We present simultaneous observations of C2H5CN, HC3N, and CH3CN emission, and obtain the first (tentative) detection of C3H8 (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 HC3N and C2H5CN, which are theorized to be short-lived in Titan's atmosphere, C2H3CN is most abundant over the southern (winter) pole, whereas the longer-lived CH3CN 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 Titan's atmospheric circulation system following the recent transition from northern to southern winter. We confirm that C2H3CN and C2H5CN are most abundant at altitudes above 200 km. Using a 300 km step model, the average abundance of C2H3CN is found to be 3.03 +/- 0.29ppb, with a C2H5CN/C2H3CN abundance ratio of 2.43 +/- 0.26. Our HC3N and CH3CN spectra can be accurately modeled using abundance gradients above the tropopause, with fractional scale-heights of 2.05 +/- 0.16 and 1.63 +/- 0.02, respectively.

Lai, J. C.-Y.↗

NASA NTRS2017

Spatial Variations in Titan's Atmospheric Temperature: ALMA and Cassini 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 ( approx. 100-500 km) of spatially resolved regions. We measure the stratopause (320 km) to in- crease 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 ( approx. 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.

Thelen, Alexander E.↗

NASA NTRS2017

D/H Ratios on Saturn and Jupiter from Cassini CIRS

We present new measurements of the deuterium abundance on Jupiter and Saturn, showing evidence that Saturn's atmosphere contains less deuterium than Jupiter's. We analyzed far-infrared spectra from the Cassini Composite Infrared Spectrometer to measure the abundance of HD on both giant planets. Our estimate of the Jovian D/H = (2.95 +/- 0.55) x 10(exp -5) is in agreement with previous measurements by ISO/SWS: (2.25 +/- 0.35) x 10(exp -5), and the Galileo probe: (2.6 +/- 0.7) x 10(exp -5). In contrast, our estimate of the Saturn value of (2.10 +/- 0.13) x 10(exp -5) is somewhat lower than on Jupiter (by a factor of 0.71(sub -0.15, sup +0..22)), contrary to model predictions of a higher ratio: Saturn/ Jupiter = 1.05-1.20. The Saturn D/H value is consistent with estimates for hydrogen in the protosolar nebula (2.1 +/- 0.5) x 10(exp -5), but its apparent divergence from the Jovian value suggests that our understanding of planetary formation and evolution is incomplete, which is in agreement with previous work.

Pierel, J. D. R.↗

NASA NTRS2017

Composite Infrared Spectrometer (CIRS) on Cassini

The Cassini spacecraft orbiting Saturn carries the composite infrared spectrometer (CIRS) designed to study thermal emission from Saturn and its rings and moons. CIRS, a Fourier transform spectrometer, is an indispensable part of the payload providing unique measurements and important synergies with the other instruments. It takes full advantage of Cassini's 13-year-long mission and surpasses the capabilities of previous spectrometers on Voyager 1 and 2. The instrument, consisting of two interferometers sharing a telescope and a scan mechanism, covers over a factor of 100 in wavelength in the mid and far infrared. It is used to study temperature, composition, structure, and dynamics of the atmospheres of Jupiter, Saturn, and Titan, the rings of Saturn, and surfaces of the icy moons. CIRS has returned a large volume of scientific results, the culmination of over 30 years of instrument development, operation, data calibration, and analysis. As Cassini and CIRS reach the end of their mission in 2017, we expect that archived spectra will be used by scientists for many years to come.

Jennings, Donald E.↗

NASA NTRS2016

Surface Temperatures on Titan During Northern Winter and Spring

Meridional brightness temperatures were measured on the surface of Titan during the 2004-2014 portion of the Cassini mission by the Composite Infrared Spectrometer. Temperatures mapped from pole to pole during five two year periods show a marked seasonal dependence. The surface temperature near the south pole over this time decreased by 2 K from 91.7 plus or minus 0.3 to 89.7 plus or minus 0.5 K while at the north pole the temperature increased by 1 K from 90.7 plus or minus 0.5 to 91.5 plus or minus 0.2 K. The latitude of maximum temperature moved from 19 S to 16 N, tracking the subsolar latitude. As the latitude changed, the maximum temperature remained constant at 93.65 plus or minus 0.15 K. In 2010 our temperatures repeated the north-south symmetry seen by Voyager one Titan year earlier in 1980. Early in the mission, temperatures at all latitudes had agreed with GCM predictions, but by 2014 temperatures in the north were lower than modeled by 1 K. The temperature rise in the north may be delayed by cooling of sea surfaces and moist ground brought on by seasonal methane precipitation and evaporation.

Cassini↗

NASA NTRS2015

Seasonal Variability of Saturn's Tropospheric Temperatures, Winds and Para-H2 from Cassini Far-IR Spectroscopy

Far-IR 16-1000 micrometer spectra of Saturn's hydrogen-helium continuum measured by Cassini's Composite Infrared Spectrometer (CIRS) are inverted to construct a near-continuous record of upper tropospheric (70-700 mbar) temperatures and para-H2 fraction as a function of latitude, pressure and time for a third of a saturnian year (2004-2014, from northern winter to northern spring). The thermal field reveals evidence of reversing summertime asymmetries superimposed onto the belt/zone structure. The temperature structure is almost symmetric about the equator by 2014, with seasonal lag times that increase with depth and are qualitatively consistent with radiative climate models. Localised heating of the tropospheric hazes (100-250 mbar) create a distinct perturbation to the temperature profile that shifts in magnitude and location, declining in the autumn hemisphere and growing in the spring. Changes in the para-H2 (f(sub p)) distribution are subtle, with a 0.02-0.03 rise over the spring hemisphere (200-500 mbar) perturbed by (i) low-f(sub p) air advected by both the springtime storm of 2010 and equatorial upwelling; and (ii) subsidence of high-f(sub p) air at northern high latitudes, responsible for a developing north-south asymmetry in f(sub p). Conversely, the shifting asymmetry in the para-H2 disequilibrium primarily reflects the changing temperature structure (and hence the equilibrium distribution of f(sub p)), rather than actual changes in f(sub p) induced by chemical conversion or transport. CIRS results interpolated to the same point in the seasonal cycle as re-analysed Voyager-1 observations (early northern spring) show qualitative consistency from year to year (i.e., the same tropospheric asymmetries in temperature and f(sub p)), with the exception of the tropical tropopause near the equatorial zones and belts, where downward propagation of a cool temperature anomaly associated with Saturn's stratospheric oscillation could potentially perturb tropopause temperatures, para-H2 and winds. Quantitative differences between the Cassini and Voyager epochs suggest that the oscillation is not in phase with the seasonal cycle at these tropospheric depths (i.e., it should be described as quasi-periodic rather than 'semi annual'). Variability in the zonal wind field derived from latitudinal thermal gradients is small (less than 10 m/s per scale height near the tropopause) and mostly affects the broad retrograde jets, with the notable exception of large variability on the northern flank of the equatorial jet. The meridional potential vorticity (PV) gradient, and hence the 'staircase of PV' associated with spatial variations in the vigour of vertical mixing, has varied over the course of the mission but maintained its overall shape. PV gradients in latitude and altitude are used to estimate the atmospheric refractive index for the propagation of stationary planetary (Rossby) waves, predicting that such wave activity would be confined to regions of real refractivity (tropical regions plus bands at 35-45 in both hemispheres). The penetration depth of these regions into the upper troposphere is temporally variable (potentially associated with stratification changes), whereas the latitudinal structure is largely unchanged over time (associated with the zonal jet system).

Fletcher, Leigh N.↗

NASA NTRS2015

Evolution of the Far-Infrared Cloud at Titan's South Pole

A condensate cloud on Titan identified by its 220 cm−1 far-infrared signature continues to undergo seasonal changes at both the north and south poles. In the north, the cloud, which extends from 55 N to the pole, has been gradually decreasing in emission intensity since the beginning of the Cassini mission with a half-life of 3.8 years. The cloud in the south did not appear until 2012 but its intensity has increased rapidly, doubling every year. The shape of the cloud at the south pole is very different from that in the north. Mapping in 2013 December showed that the condensate emission was confined to a ring with a maximum at 80 S. The ring was centered 4deg from Titanʼs pole. The pattern of emission from stratospheric trace gases like nitriles and complex hydrocarbons (mapped in 2014 January) was also offset by 4deg, but had a central peak at the pole and a secondary maximum in a ring at about 70 S with a minimum at 80 S. The shape of the gas emission distribution can be explained by abundances that are high at the atmospheric pole and diminish toward the equator, combined with correspondingly increasing temperatures. We discuss possible causes for the condensate ring. The present rapid build up of the condensate cloud at the south pole is likely to transition to a gradual decline from 2015 to 2016. Key words: molecular processes - planets and satellites: atmospheres - planets and satellites: composition - planets and satellites: individual (Titan) - radiation mechanisms: thermal

Cassini↗

NASA NTRS2014

Seasonal Evolution of Saturn's Polar Temperatures and Composition

The seasonal evolution of Saturn's polar atmospheric temperatures and hydrocarbon composition is derived from a decade of Cassini Composite Infrared Spectrometer (CIRS) 7-16 micrometers thermal infrared spectroscopy. We construct a near-continuous record of atmospheric variability poleward of 60 deg from northern winter/southern summer (2004, Ls = 293 deg) through the equinox (2009, Ls= 0 deg) to northern spring/southern autumn (2014, Ls = 56 deg). The hot tropospheric polar cyclones that are entrained by pro-grade jets within 2-3 deg of each pole, and the hexagonal shape of the north polar belt, are both persistent features throughout the decade of observations. The hexagon vertices rotated westward by approx. equal to 30 deg longitude between March 2007 and April 2013, confirming that they are not stationary in the Voyager-defined System III longitude system as previously thought. Tropospheric temperature contrasts between the cool polar zones (near 80-85 deg) and warm polar belts (near 75-80 deg) have varied in both hemispheres, resulting in changes to the vertical wind shear on the zonal jets in the upper troposphere and lower stratosphere. The extended region of south polar stratospheric emission has cooled dramatically poleward of the sharp temperature gradient near 75 deg S (by approximately -5 K/yr), coinciding with a depletion in the abundances of acetylene (0030 +/- 0.005 ppm/yr) and ethane (0.35 +/- 0.1 ppm/yr), and suggestive of stratospheric upwelling with vertical wind speeds of w approx. equal to +0.1 mm/s. The upwelling appears most intense within 5 deg latitude of the south pole. This is mirrored by a general warming of the northern polar stratosphere (+5 K/yr) and an enhancement in acetylene (0.030 +/- 0.003 ppm/yr) and ethane (0.45 +/- 0.1 ppm/yr) abundances that appears to be most intense poleward of 75 deg N, suggesting subsidence at w approx. equal to -0.15 mm/ s. However, the sharp gradient in stratospheric emission expected to form near 75 deg N by northern summer solstice (2017, Ls = 90 deg) has not yet been observed, so we continue to await the development of a northern summer stratospheric vortex. The peak stratospheric warming in the north occurs at lower pressure levels (p less than 1 mbar) than the peak stratospheric cooling in the south (p greater than 1 mbar). Vertical motions are derived from both the temperature field (using the measured rates of temperature change and the deviations from the expectations of radiative equilibrium models) and hydrocarbon distributions (solving the continuity equation). Vertical velocities tend towards zero in the upper troposphere where seasonal temperature contrasts are smaller, except within the tropospheric polar cyclones where w approx. equal to +0.02 mm/s. North polar minima in tropospheric and stratospheric temperatures were detected in 2008-2010 (lagging one season, or 6-8 years, behind winter solstice); south polar maxima appear to have occurred before the start of the Cassini observations (1-2 years after summer solstice), consistent with the expectations of radiative climate models. The influence of dynamics implies that the coldest winter temperatures occur in the 75-80 deg region in the stratosphere, and in the cool polar zones in the troposphere, rather than at the poles themselves. In addition to vertical motions, we propose that the UV-absorbent polar stratospheric aerosols entrained within Saturn's vortices contribute significantly to the radiative budget at the poles, adding to the localized enhancement in the south polar cooling and north polar warming poleward of +/-75 deg.

polar temperatures↗

NASA NTRS2014

Changes to Saturn's Zonal-Mean Tropospheric Thermal Structure After the 2010-2011 Northern Hemisphere Storm

We use far-infrared (20-200 micrometers) data from the Composite Infrared Spectrometer on the Cassini spacecraft to determine the zonal-mean temperature and hydrogen para-fraction in Saturn's upper troposphere from observations taken before and after the large northern hemisphere storm in 2010-2011. During the storm, zonal mean temperatures in the latitude band between approximately 25 deg N and 45 deg N (planetographic latitude) increased by about 3 K, while the zonal mean hydrogen para-fraction decreased by about 0.04 over the same latitudes, at pressures greater than about 300 mbar. These changes occurred over the same latitude range as the disturbed cloud band seen in visible images. The observations are consistent with low para-fraction gas being brought up from the level of the water cloud by the strong convective plume associated with the storm, while being heated by condensation of water vapor, and then advected zonally by the winds near the plume tops in the upper troposphere.

Saturn↗

NASA NTRS2013

Evolution of the Stratospheric Temperature and Chemical Composition over One Titanian Year

Since the Voyager 1 (V1) flyby in 1980, Titans exploration from space and the ground has been ongoing for more than a full revolution of Saturn around the Sun (one Titan year or 29.5 Earth years was completed in May 2010). In this study we search for temporal variations affecting Titans atmospheric thermal and chemical structure within that year. We process Cassini CIRS data taken during the Titan flybys from 2006-2013 and compare them to the 1980 V1IRIS spectra (re-analyzed here). We also consider data from Earth-based and -orbiting observatories (such as from the ISO, re-visited). When we compare the CIRS 2010 and the IRIS data we find limited inter-annual variations, below the 25 or35 levels for the lower and middle, or the high latitudes, respectively. A return to the 1980 stratospheric temperatures and abundances is generally achieved from 50degN to 50degS, indicative of the solar radiation being the dominating energy source at 10 AU, as for the Earth, as predicted by GCM and photochemical models. However, some exceptions exist among the most complex hydrocarbons (C4H2 and C3H4), especially in the North. In the Southern latitudes, since 2012, we see a trend for an increase of several trace gases, possibly indicative of a seasonal atmospheric reversal. At the Northern latitudes we found enhanced abundances around the period of the northern spring equinox in mid-2009 (as in Bampasidis et al. 2012), which subsequently decreased (from 2010-2012) returning to values similar to those found in the V1 epoch a Titanian year before.

radiative transfer↗

NASA NTRS2013

Elusive Ethylene Detected in Saturns Northern Storm Region

The massive eruption at 40 deg. N (planetographic latitude) on Saturn in 2010 December has produced significant and lasting effects in the northern hemisphere on temperature and species abundances. The northern storm region was observed on many occasions in 2011 by Cassini's Composite Infrared Spectrometer (CIRS). In 2011 May, temperatures in the stratosphere greater than 200 K were derived from CIRS spectra in the regions referred to as "beacons" (warm regions in the stratosphere). Ethylene has been detected in the beacon region in Saturn's northern storm region using CIRS. Ground-based observations using the high-resolution spectrometer Celeste on the McMath-Pierce Telescope on 2011 May 15 were used to confirm the detection and improve the altitude resolution in the retrieved profile. The derived ethylene profile from the CIRS data gives a C2H4 mole fraction of 5.9 +/- 4.5 x 10(exp -7) at 0.5 mbar, and from Celeste data it gives 2.7 +/- 0.45 x 10(exp -6) at 0.1 mbar. This is two orders of magnitude higher than the amount measured in the ultraviolet at other latitudes prior to the storm. It is also much higher than predicted by photochemical models, indicating that perhaps another production mechanism is required or a loss mechanism is being inhibited.

Hesman, B. E.↗

NASA NTRS2012

The Evolution of Hydrocarbons in Saturn's Northern Storm Region

The massive storm at 40N on Saturn that began in December 2010 has produced significant and lasting effects in the northern hemisphere on temperature and species abundances (Fletcher et aL 2011). The northern storm region was observed on several occasions between March 2011 and April 2012 by Cassini's Composite Infrared Spectrometer (CIRS) at a spectral resolution (0.5/cm) which permits the study of trace species in Saturn's stratosphere. During this time period, stratospheric temperatures in regions referred to as "beacons" (warm regions at specific longitudes at the latitude of the storm) became significantly warmer than pre-storm values of 140K, peaking near 220K, and subsequently cooling. These warm temperatures led to greatly enhanced infrared emission due to C4H2, C3H4, C2H2, and C2H6 in the stratosphere as well as the first detection of C2H4 on Saturn in the thermal infrared (Hesman et al. 2012). Using CH4 as a thermometer of Saturn's stratosphere in the beacon regions, we can derive the mixing ratios of each of these molecules. The most common hydrocarbons (C2H2 and C2H6) serve as dynamical tracers on Saturn and their abundances may constrain vertical motion in the stratosphere. All of these hydrocarbons are products of methane photolysis. Since many of the photochemical reactions that produce heavier hydrocarbons such as C4H2 and C3H4 are temperature sensitive, the beacon region provides a natural laboratory for studying these reactions on Saturn. We will discuss the time evolution of the abundances of each of these hydrocarbons from their pre-storm values, through the period of maximum heating , and during the period of cooling that is taking place in Saturn's stratosphere.

Bjoraker, Gordon↗

NASA NTRS2012

Seasonal Changes in Titan's Southern Stratosphere

In August 2009 Titan passed through northern spring equinox, and the southern hemisphere passed into fall. Since then, the moon's atmosphere has been closely watched for evidence of the expected seasonal reversal of stratospheric circulation, with increased northern insolation leading to upwelling, and consequent downwelling at southern high latitudes. If the southern winter mirrors the northern winter, this circulation will be traced by increases in short-lived gas species advected downwards from the upper atmosphere to the stratosphere. The Cassini spacecraft in orbit around Saturn carries on board the Composite Infrared Spectrometer (CIRS), which has been actively monitoring the trace gas populations through measurement of the intensity of their infrared emission bands (7-1000 micron). In this presentation we will show fresh evidence from recent CIRS measurements in June 2012, that the shortest-lived and least abundant minor species (C3H4, C4H2, C6H6, HC3N) are indeed increasing dramatically southwards of 50S in the lower stratosphere. Intriguingly, the more stable gases (C2H2, HCN, CO2) have yet to show this trend, and continue to exhibit their 'summer' abundances, decreasing towards the south pole. Possible chemical and dynamical explanations of these results will be discussed , along with the potential of future CIRS measurements to monitor and elucidate these seasonal changes.

Nixon, C. A.↗

NASA NTRS2012

The Origin and Evolution of Saturn's 2011-2012 Stratospheric Vortex

The planet-encircling springtime storm in Saturn's troposphere (December 2010-July 2011, Fletcher, L.N. et al. [2011]. Science 332, 1413-1414; Sánchez-Lavega, A. et al. [2011]. Nature 475, 71-74; Fischer, G. et al. [2011]. Nature 475, 75-77) produced dramatic perturbations to stratospheric temperatures, winds and composition at mbar pressures that persisted long after the tropospheric disturbance had abated. Thermal infrared (IR) spectroscopy from the Cassini Composite Infrared Spectrometer (CIRS), supported by ground-based IR imaging from the VISIR instrument on the Very Large Telescope and the MIRSI instrument on NASA's IRTF, is used to track the evolution of a large, hot stratospheric anticyclone between January 2011 and March 2012. The evolutionary sequence can be divided into three phases: (I) the formation and intensification of two distinct warm airmasses near 0.5 mbar between 25 and 35degN (B1 and B2) between January-April 2011, moving westward with different zonal velocities, B1 residing directly above the convective tropospheric storm head; (II) the merging of the warm airmasses to form the large single 'stratospheric beacon' near 40degN (B0) between April and June 2011, disassociated from the storm head and at a higher pressure (2 mbar) than the original beacons, a downward shift of 1.4 scale heights (approximately 85 km) post-merger; and (III) the mature phase characterised by slow cooling (0.11 +/- 0.01 K/day) and longitudinal shrinkage of the anticyclone since July 2011. Peak temperatures of 221.6 +/- 1.4 K at 2 mbar were measured on May 5th 2011 immediately after the merger, some 80 K warmer than the quiescent surroundings. From July 2011 to the time of writing, B0 remained as a long-lived stable stratospheric phenomenon at 2 mbar, moving west with a near-constant velocity of 2.70 +/- 0.04 deg/day (24.5 +/- 0.4 m/s at 40degN relative to System III longitudes). No perturbations to visible clouds and hazes were detected during this period. With no direct tracers of motion in the stratosphere, we use thermal windshear calculations to estimate clockwise peripheral velocities of 200-400 m/s at 2 mbar around B0. The peripheral velocities of the two original airmasses were smaller (70-140 m/s). In August 2011, the size of the vortex as defined by the peripheral collar was 65deg longitude (50,000 km in diameter) and 25deg latitude. Stratospheric acetylene (C2H2) was uniformly enhanced by a factor of three within the vortex, whereas ethane (C2H6) remained unaffected. The passage of B0 generated a new band of warm stratospheric emission at 0.5 mbar at its northern edge, and there are hints of warm stratospheric structures associated with the beacons at higher altitudes (p < 0.1 mbar) than can be reliably observed by CIRS nadir spectroscopy. Analysis of the zonal windshear suggests that Rossby wave perturbations from the convective storm could have propagated vertically into the stratosphere at this point in Saturn's seasonal cycle, one possible source of energy for the formation of these stratospheric anticyclones.

evolution↗