Long-term atmospheric variability on Uranus and Neptune

Hammel, Heidi B.; Lockwood, G. W.

doi:10.1016/j.icarus.2006.08.027

Cited by 41 publications

(34 citation statements)

References 54 publications

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“…The Voyager Ultraviolet Spectrometer (UVS) observations (e.g., Broadfoot et al, 1986;Herbert et al, 1987;Yelle et al, 1987Yelle et al, , 1989Bishop et al, 1990) first made it clear that K zz was comparatively small on Uranus, at least at summer southern solstice at the time of the Voyager encounters. Our Spitzer observations confirm that stratospheric K zz values are still low on Uranus in 2007, despite several other observations that suggest that stronger tropospheric convective activity or other dynamical changes on Uranus may have occurred from the 1986 Voyager encounter to at least the 2007 equinox over a wide range of latitudes (e.g., Karkoschka, 2001;Rages et al, 2004;Klein and Hofstadter 2006;Hammel and Lockwood 2007;Norwood and Chanover 2009;Irwin et al, 2008Irwin et al, , 2011Irwin et al, , 2012Sromovsky et al, 2009;Sromovsky et al, 2012a;Sromovsky et al, 2012b).…”

Section: Implications For Trace-constituent Chemistry and Mixingsupporting

confidence: 69%

Mid-infrared spectroscopy of Uranus from the Spitzer infrared spectrometer: 2. Determination of the mean composition of the upper troposphere and stratosphere

Orton

Moses

Fletcher

et al. 2014

Icarus

106

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a b s t r a c tMid-infrared spectral observations Uranus acquired with the Infrared Spectrometer (IRS) on the Spitzer Space Telescope are used to determine the abundances of C 2 H 2 , C 2 H 6 , CH 3 C 2 H, C 4 H 2 , CO 2 , and tentatively CH 3 on Uranus at the time of the 2007 equinox. For vertically uniform eddy diffusion coefficients in the range 2200-2600 cm 2 s À1 , photochemical models that reproduce the observed methane emission also predict C 2 H 6 profiles that compare well with emission in the 11.6-12.5 lm wavelength region, where the t 9 band of C 2 H 6 is prominent. Our nominal model with a uniform eddy diffusion coefficient K zz = 2430 cm 2 s À1 and a CH 4 tropopause mole fraction of 1.6 Â 10 À5 provides a good fit to other hydrocarbon emission features, such as those of C 2 H 2 and C 4 H 2 , but the model profile for CH 3 C 2 H must be scaled by a factor of 0.43, suggesting that improvements are needed in the chemical reaction mechanism for C 3 H x species. The nominal model is consistent with a CH 3 D/CH 4 ratio of 3.0 ± 0.2 Â 10 À4 . From the best-fit scaling of these photochemical-model profiles, we derive column abundances above the 10-mbar level of 4.5 + 01.1/À0.8 Â 10 19 molecule-cm À2 for CH 4 , 6.2 ± 1.0 Â 10 16 molecule-cm À2 for C 2 H 2 (with a value 24% higher from a different longitudinal sampling), 3.1 ± 0.3 Â 10 16 molecule-cm À2 for C 2 H 6 , 8.6 ± 2.6 Â 10 13 molecule-cm À2 for CH 3 C 2 H, 1.8 ± 0.3 Â 10 13 molecule-cm À2 for C 4 H 2 , and 1.7 ± 0.4 Â 10 13 molecule-cm À2 for CO 2 on Uranus. A model with K zz increasing with altitude fits the observed spectrum and requires CH 4 and C 2 H 6 column abundances that are 54% and 45% higher than their respective values in the nominal model, but the other hydrocarbons and CO 2 are within 14% of their values in the nominal model. Systematic uncertainties arising from errors in the temperature profile are estimated very conservatively by assuming an unrealistic ''alternative'' temperature profile that is nonetheless consistent with the observations; for this profile the column abundance of CH 4 is over four times higher than in the nominal model, but the column abundances of the hydrocarbons and CO 2 differ from their value in the nominal model by less than 22%. The CH 3 D/CH 4 ratio is the same in both the nominal model with its uniform K zz as in the vertically variable K zz model, and it is 10% lower with the ''alternative'' temperature profile than the nominal model. There is no compelling evidence for temporal variations in global-average hydrocarbon abundances over the decade between Infrared Space Observatory and Spitzer observations, but we cannot preclude a possible large increase in the C 2 H 2 abundance since the Voyager era. Our results have implications with respect to the influx rate of exogenic oxygen species and the production rate of stratospheric hazes on Uranus, as well as the C 4 H 2 vapor pressure over C 4 H 2 ice at low temperatures.

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Section: Implications For Trace-constituent Chemistry and Mixingsupporting

confidence: 69%

Mid-infrared spectroscopy of Uranus from the Spitzer infrared spectrometer: 2. Determination of the mean composition of the upper troposphere and stratosphere

Orton

Moses

Fletcher

et al. 2014

Icarus

106

View full text Add to dashboard Cite

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“…Voyager observations of Uranus in 1986 recorded for the first time the presence of a bright south polar cap extending southward from about 45°S. Although observations made between 1994 and 2003 showed temporal variations in brightness in the south polar region; they did not record signs of a northern polar cap with comparable brightness to that observed in the south pole (Hammel & Lockwood, ; Rages et al, ). Near equinox in 2007, however, Uranus' atmosphere underwent a number of seasonal changes that led to a reversal in the polar brightness: a south polar collar at 45°S diminishing in brightness relative to midlatitudes and a north polar collar at 45°N becoming steadily brighter with time (Irwin, Teanby, et al, ).…”

Section: Introductionmentioning

confidence: 98%

“…In August 2014, a large bright polar‐cap‐like feature was identified in Uranus's northern pole from Keck telescope images (de Pater et al, ), whose progressive formation over 2013 was observed by amateur telescopes (see PVOL database, Hueso et al, ). Based on long‐term records of Uranus brightness variations before the 2007 equinox (Hammel & Lockwood, ; Lockwood & Jerzykiewicz, ), it was thought that the polar cap could be a seasonal formation or redistribution of aerosols (de Pater et al, ; Sromovsky et al, ).…”

Section: Introductionmentioning

confidence: 99%

Uranus's Northern Polar Cap in 2014

Toledo

Irwin

Teanby

et al. 2018

Geophysical Research Letters

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In October and November 2014, spectra covering the 1.436 to 1.863‐μm wavelength range from the SINFONI Integral Field Unit Spectrometer on the Very Large Telescope showed the presence of a vast bright north polar cap on Uranus, extending northward from about 40°N and at all longitudes observed. The feature, first detected in August 2014 from Keck telescope images, has a morphology very similar to the southern polar cap that was seen to fade before the 2007 equinox. At strong methane‐absorbing wavelengths (for which only the high troposphere or stratosphere is sampled) the feature is not visible, indicating that it is not a stratospheric phenomenon. We show that the observed northern bright polar cap results mainly from a decrease in the tropospheric methane mixing ratio, rather than from a possible latitudinal variation of the optical properties or abundance of aerosol, implying an increase in polar downwelling near the tropopause level.

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“…Unlike Uranus, with its unusual inclination and negligible internal heat source (e.g., Pearl and Conrath, 1991), Neptune's weather layer exhibits rapidly varying cloud activity, zonal banding, dark ovals and sporadic orographic clouds. Furthermore, the extent of this meteorological activity and the planet's global visible albedo appear to vary with time (Lockwood and Jerzykiewicz, 2006;Hammel and Lockwood, 2007). In this study we attempt to connect this cloud-level activity to changes in Neptune's thermal structure and atmospheric chemistry between Voyager observations (1989) and ground-based observations close to Neptune's 2005 summer solstice.…”

Section: Introductionmentioning

confidence: 99%

Neptune at summer solstice: Zonal mean temperatures from ground-based observations, 2003–2007

Fletcher

Pater²,

Orton

et al. 2014

Icarus

108

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• ). Our aim was to analyse imaging and spectroscopy from multiple different sources using a single self-consistent radiative-transfer model to assess the magnitude of seasonal variability. Globally-averaged stratospheric temperatures measured from methane emission tend towards a quasi-isothermal structure (158-164 K) above the 0.1-mbar level, and are found to be consistent with spacecraft observations of AKARI. This remarkable consistency, despite very different observing conditions, suggests that stratospheric temporal variability, if present, is < ±5 K at 1 mbar and < ±3 K at 0.1 mbar during this solstice period. Conversely, ethane emission is highly variable, with abundance determinations varying by more than a factor of two (from 500 to 1200 ppb at 1 mbar). The retrieved C 2 H 6 abundances are extremely sensitive to the details of the T (p) derivation, although the underlying cause of the variable ethane emission remains unidentified. Stratospheric temperatures and ethane are found to be latitudinally uniform away from the south pole (assuming a latitudinally-uniform distribution of stratospheric methane), with no large seasonal hemispheric asymmetries evident at solstice. At low and mid-latitudes, comparisons of synthetic Voyager-era images with solstice-era observations suggest that tropospheric zonal temperatures are unchanged since the Voyager 2 encounter, with cool mid-latitudes and a warm equator and pole. A re-analysis of Voyager/IRIS 25-50 µm mapping of tropospheric temperatures and para-hydrogen disequilibrium (a tracer for vertical motions) suggests a symmetric meridional circulation with cold air rising at mid-latitudes (sub-equilibrium para-H 2 conditions) and warm air sinking at the equator and poles (super-equilibrium para-H 2 conditions). The most significant atmospheric changes have occurred at high southern latitudes, where zonal temperatures retrieved from 2003 images suggest a polar enhancement of 7-8 K above the tropopause, and an increase of 5-6 K throughout the 70 − 90• S region between 0.1 and 200 mbar. Such a large perturbation, if present in 1989, would have been detectable by Voyager/IRIS in a single scan despite its long-wavelength sensitivity, and we conclude that Neptune's south polar cyclonic vortex increased in strength significantly from Voyager to solstice.

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Long-term atmospheric variability on Uranus and Neptune

Cited by 41 publications

References 54 publications

Mid-infrared spectroscopy of Uranus from the Spitzer infrared spectrometer: 2. Determination of the mean composition of the upper troposphere and stratosphere

Mid-infrared spectroscopy of Uranus from the Spitzer infrared spectrometer: 2. Determination of the mean composition of the upper troposphere and stratosphere

Uranus's Northern Polar Cap in 2014

Neptune at summer solstice: Zonal mean temperatures from ground-based observations, 2003–2007

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