Seasonal variations of temperature, acetylene and ethane in Saturn’s atmosphere from 2005 to 2010, as observed by Cassini-CIRS

doi:10.1016/j.icarus.2013.03.011

Icarus

Volume 225, Issue 1, July 2013, Pages 257-271

https://doi.org/10.1016/j.icarus.2013.03.011 Get rights and content

Highlights

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Strong cooling at high-southern latitudes as the region falls into autumnal darkness.
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The magnitude of this cooling is matched by a radiative climate model.
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Changes in hydrocarbon concentrations imply downwelling at 25°N and upwelling at 15°S.
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These vertical motions are consistent with a general circulation model.
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A general enrichment of ethane in the northern hemisphere from 2005 to 2009.

Abstract

Acetylene (C₂H₂) and ethane (C₂H₆) are by-products of complex photochemistry in the stratosphere of Saturn. Both hydrocarbons are important to the thermal balance of Saturn’s stratosphere and serve as tracers of vertical motion in the lower stratosphere. Earlier studies of Saturn’s hydrocarbons using Cassini-CIRS observations have provided only a snapshot of their behaviour. Following the vernal equinox in August 2009, Saturn’s northern and southern hemispheres have entered spring and autumn, respectively, however the response of Saturn’s hydrocarbons to this seasonal shift remains to be determined. In this paper, we investigate how the thermal structure and concentrations of acetylene and ethane have evolved with the changing season on Saturn. We retrieve the vertical temperature profiles and acetylene and ethane volume mixing ratios from $Δ \tilde{ν} = 15.5 {cm}^{- 1}$ Cassini-CIRS observations. In comparing 2005 (solar longitude, L_s ∼ 308°), 2009 (L_s ∼ 3°) and 2010 (L_s ∼ 15°) results, we observe the disappearance of Saturn’s warm southern polar hood with cooling of up to 17.1 K ± 0.8 K at 1.1 mbar at high-southern latitudes. Comparison of the derived temperature trend in this region with a radiative climate model (Section 4 of Fletcher et al., 2010 and Greathouse et al. (2013, in preparation)) indicates that this cooling is radiative although dynamical changes in this region cannot be ruled out. We observe a 21 ± 12% enrichment of acetylene and a 29 ± 11% enrichment of ethane at 25°N from 2005 to 2009, suggesting downwelling at this latitude. At 15°S, both acetylene and ethane exhibit a decrease in concentration of 6 ± 11% and 17 ± 9% from 2005 to 2010, respectively, which suggests upwelling at this latitude (though a statistically significant change is only exhibited by ethane). These implied vertical motions at 15°S and 25°N are consistent with a recently-developed global circulation model of Saturn’s tropopause and stratosphere(Friedson and Moses, 2012), which predicts this pattern of upwelling and downwelling as a result of a seasonally-reversing Hadley circulation. Ethane exhibits a general enrichment at mid-northern latitudes from 2005 to 2009. As the northern hemisphere approaches summer solstice in 2017, this feature might indicate an onset of a meridional enrichment of ethane, as has been observed in the southern hemisphere during/after southern summer solstice.

Introduction

Saturn’s stratosphere is host to complex photochemistry. Photolysis of methane (CH₄) by solar ultraviolet light initiates a chain of photochemical reactions that produce a variety of larger hydrocarbons. The hydrocarbon photochemistry is triggered by high-altitude photolysis of methane by Lyman-Alpha photons at pressures less than 1 μbar, and the large net production rates of hydrocarbons at these altitudes can lead to strong vertical gradients in concentration (Moses et al., 2000). Of the various products of the photochemistry, acetylene and ethane are the most stable with predicted net photochemical lifetimes on the order of ∼100 and ∼700 years, respectively, in the lower stratosphere (Table 1). Their relatively long lifetimes and strong vertical gradients in concentration allow for their use as tracers of vertical motion in Saturn’s stratosphere.

Acetylene and ethane have been extensively studied on Saturn from a variety of ground-based and space-borne observations since their initial detection (Gillett and Forrest, 1974, Moos and Clarke, 1979). Subsequent observations showed that there was a poleward increase in ethane’s infrared emission in the southern hemisphere, but it remained a challenge to determine whether this was a thermal effect in the stratosphere or if there was in fact a higher concentration of stratospheric ethane towards the south pole (Tokunaga et al., 1978, Tokunaga et al., 1979, Sinton et al., 1980, Ollivier et al., 2000). Greathouse et al. (2005) first disentangled these two effects and determined that there was both a thermal enhancement and an enrichment of ethane in the lower stratosphere towards the south (summer) pole. This result was later confirmed by observations from the CIRS (Composite Infrared Spectrometer) instrument aboard the Cassini spacecraft (Flasar et al., 2004), which arrived at Saturn in 2004. Howett et al. (2007) determined the meridional variations of acetylene and ethane from Cassini-CIRS observations taken in late 2004 and also found an enrichment of ethane in the lower stratosphere (at ∼2 mbar) towards the south pole while acetylene’s meridional profile gradually decreased polewards. Hesman et al. (2009) found a similar result by analysing both Cassini-CIRS observations and IRTF-Celeste (a cryogenic grating spectrometer, Moran et al. (2007)) observations. Guerlet et al. (2009) analysed Saturn’s hydrocarbons from Cassini-CIRS limb observations. Although they did not find a southern increase in ethane in the lower stratosphere significant with respect to their uncertainty, they did show that its concentration is approximately uniform with latitude in the southern hemisphere, in contrast to prediction. Moses and Greathouse (2005) show the photochemical lifetimes of both hydrocarbons to exceed the length of a Saturn year (29.5 years) in the lower stratosphere. Their meridional profiles are therefore expected to be maximum at the equator – where the most sunlight is received annually – and to decrease to higher latitudes. While this is generally observed to be true in acetylene, ethane’s deviation from this predicted trend indicates a further mechanism at work: a meridional wind system has been proposed in previous studies.

Saturn has an obliquity of 26.7° and therefore experiences seasons like Earth. Following the vernal equinox in August 2009, Saturn’s northern hemisphere has shifted from winter to spring, and the southern hemisphere from summer to autumn and Cassini-CIRS observations, from Cassini’s arrival at Saturn in 2004 to present day, provide coverage of this seasonal shift. Fletcher et al. (2010) determined seasonal changes in Saturn’s thermal structure from 2004 to early 2009, using Cassini-CIRS observations. Observations of the northern hemisphere in 2011/2012 have recently been used to study perturbations to Saturn’s thermal structure and composition in the intense northern thunderstorm which erupted in late 2010 (Fletcher et al., 2012, Fletcher et al., 2011, Hesman et al., 2012). However, the evolution of Saturn’s thermal structure after the vernal equinox, outside the storm region, has yet to be determined. Previous studies of Saturn’s hydrocarbons have only provided a snapshot of their behaviour, with their temporal variations remaining to be determined. Cassini-CIRS observations of acetylene and ethane’s emission features sound the lower stratosphere (∼2 mbar) of Saturn. Acetylene’s photochemical production and loss timescales at such altitudes are comparable with the length of a saturnian season (Table 1) and so its evolution may be the result of both dynamical and chemical processes. Ethane’s production and loss timescales at this altitude are, however, comparably much longer at 700 years and 3000 years, respectively (Table 1). Temporal changes in ethane’s concentration are therefore a result of dynamical perturbations alone. The behaviour of acetylene and ethane from 2005 to 2010 will provide insight into the changing dynamics and photochemistry of the stratosphere as a result of Saturn’s seasonal change.

In this study, we determine the temporal variations of temperature and the concentrations of acetylene and ethane in Saturn’s atmosphere from 2005 to 2010. In Section 2, we detail the observations used and summarises the retrieval process used to determine the vertical temperature profile and hydrocarbon concentrations from them in Section 3. Section 4 summarises our results and we provide a discussion in Section 5.

Section snippets

Observations

The CIRS instrument aboard Cassini provides observations of the saturnian system in the thermal infrared (10 cm⁻¹ to 1500 cm⁻¹). The instrument features two interferometers: one for the far-infrared (‘FP1’, 10–600 cm⁻¹) and the second, comprising two arrays measuring in the mid-infrared (‘FP3’, 600–1100 cm⁻¹ and ‘FP4’, 1100–1500 cm⁻¹). Observations are available at different spectral resolutions. We chose to use ‘FIRMAP’ $(Δ \tilde{ν} = 15.5 {cm}^{- 1})$ near-nadir observations where the spacecraft is positioned in

Analysis

Atmospheric profiles were retrieved using the NEMESIS radiative transfer inverse retrieval tool (Irwin et al., 2008). NEMESIS is now able to model planetary spectra using either the line-by-line method or the correlated-k method (Lacis and Oinas, 1991). The latter involves grouping absorption coefficients by strength within a wavenumber interval for calculation of spectral absorption. The correlated-k version of NEMESIS was used given its faster computation time and its suitability for the

Temperature

Fig. 2 shows the retrieved temperature structures in 2005, 2009 and 2010 and their differences. We observe:

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A cooling of up to 17.1 ± 0.8 K at 1.1 mbar at high-southern latitudes, in comparing 2005 and 2010 results.
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A maximum in the equatorial temperature profile of 147.2 ± 0.6 K at 1.1 mbar in 2005, indicative of Saturn’s semi-annual oscillation (SSAO). From 2005 to 2010, we observe equatorial cooling of 5.8 ± 0.9 K at this altitude.
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Vertically-localised cooling of ∼2 ± 0.6 K at ∼200 mbar, extending south of

The atmosphere in 2005 (L_s ∼ 308°)

Our retrieved temperatures and concentrations of acetylene and ethane in 2005 are generally consistent with previous studies which analysed observations at a similar time. Saturn exhibits a thermal hemispheric asymmetry in 2005, with warmer temperatures in the southern hemisphere, as expected following the aftermath of southern summer solstice in 2002. There is a pole-to-pole temperature contrast of approximately 40 K at 2.1 mbar (Fig. 3), which is consistent with Fletcher et al. (2010) who also

Summary

The thermal structure and concentrations of acetylene and ethane have been retrieved from Cassini-CIRS observations from 2005 to 2010. In general, observed temperature trends are comparable with those in previous studies (Fletcher et al., 2010).

At the equator, whilst we also observe a cooling indicative of a downward propagation of Saturn’s semi-annual oscillation (SSAO) as also concluded in Guerlet et al., 2011, Schinder et al., 2011, the magnitude of cooling observed in this study (∼5 K at 1

Acknowledgments

UK authors acknowledge the STFC (Science and Technology Facilities Council) who have made this study possible. Fletcher was supported by a Royal Society research fellowship. We thank the Cassini-CIRS team for their planning and calibration of this data. Many thanks also to Sandrine Guerlet for providing results from limb observations in 2005/2006 and for also discussing unpublished results from limb observations in 2010. We also thank both anonymous reviewers for their helpful and constructive

References (39)

L.N. Fletcher et al.
Characterising Saturn’s vertical temperature structure from Cassini/CIRS
Icarus
(2007)
L.N. Fletcher et al.
Seasonal change on saturn from Cassini/CIRS observations, 2004–2009
Icarus
(2010)
L.N. Fletcher et al.
The origin and evolution of Saturn’s 2011–2012 stratospheric vortex
Icarus
(2012)
A.J. Friedson et al.
General circulation and transport in Saturn’s upper troposphere and stratosphere
Icarus
(2012)
A.J. Friedson et al.
A global climate model of Titan’s atmosphere and surface
Planet. Space Sci.
(2009)
T.K. Greathouse et al.
Meridional variations of temperature, C₂H₂ and C₂H₆ abundances in Saturn’s stratosphere at southern summer solstice
Icarus
(2005)
S. Guerlet et al.
Vertical and meridional distribution of ethane, acetylene and propane in Saturn’s stratosphere from CIRS/Cassini limb observations
Icarus
(2009)
S. Guerlet et al.
Meridional distribution of CH₃C₂H and C₄H₂ in Saturn’s stratosphere from CIRS/Cassini limb and nadir observations
Icarus
(2010)
B.E. Hesman et al.
Saturn’s latitudinal C₂H₂ and C₂H₆ abundance profiles from Cassini/CIRS and ground-based observations
Icarus
(2009)
C.J.A. Howett et al.
Meridional variations in stratospheric acetylene and ethane in the southern hemisphere of the saturnian atmosphere as determined from Cassini/CIRS measurements
Icarus
(2007)

J. Hurley

Observations of upper tropospheric acetylene on Saturn: No apparent correlation with 2000 km-sized thunderstorms

Planet. Space Sci.

(2012)

P.G.J. Irwin

The NEMESIS planetary atmosphere radiative transfer and retrieval tool

J. Quant. Spectrosc. Rad. Trans.

(2008)

E. Karkoschka et al.

Saturn’s vertical and latitudinal cloud structure 1991–2004 from HST imaging in 30 filters

Icarus

(2005)

J.I. Moses et al.

Photochemistry of Saturn’s atmosphere. I. Hydrocarbon chemistry and comparisons with ISO observations

Icarus

(2000)

W.M. Sinton et al.

Infrared scans of Saturn

Icarus

(1980)

A.T. Tokunaga et al.

Spatially resolved infrared observations of Saturn. II – The temperature enhancement at the south pole of Saturn

Icarus

(1978)

A.T. Tokunaga et al.

Spatially resolved infrared observations of Saturn. iii – 10- and 20-micron disk scans at B′ = −11.8°

Icarus

(1979)

R.V. Yelle et al.

Structure of the jovian stratosphere at the Galileo probe entry site

Icarus

(2001)

M.P. Baldwin

The quasi-biennial oscillation

Rev. Geophys.

(2001)

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Seasonal variations of temperature, acetylene and ethane in Saturn’s atmosphere from 2005 to 2010, as observed by Cassini-CIRS

Highlights

Abstract

Introduction

Section snippets

Observations

Analysis

Temperature

The atmosphere in 2005 (Ls ∼ 308°)

Summary

Acknowledgments

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Planet. Space Sci.

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Planet. Space Sci.

J. Quant. Spectrosc. Rad. Trans.

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The quasi-biennial oscillation

Rev. Geophys.

The atmosphere in 2005 (L_s ∼ 308°)