OPS4

The polar orbit of the Juno spacecraft provides an unprecedented view of Jupiter's atmosphere as it passes above the cloud tops every 53 days. The spectrum in the near infrared is dominated by reflected sunlight from aerosols (both condensate clouds and hazes) in the troposphere, as well as absorptions by the molecular species present. In addition, thermal emission longward of 4.5 µm provides access to the gaseous composition and aerosols below the top-most clouds.  Of particular importance in shaping the spectra are ammonia, phosphine and water, in addition to minor contributions from species such as arsine, germane and carbon monoxide. These regions also include emissions by ionospheric H₃⁺. Here, we produce meridionally averaged zonal profiles from the Juno-JIRAM observations obtained during PJ3, which provide almost complete latitude coverage. To analyse the observations, we use the radiative transfer and retrieval code NEMESIS (Irwin et al., 2008), which has been updated to cover this wavelength with the latest line-data from HITRAN. Our aim is to analyse both the reflected-sunlight region (2-4 µm) and the thermal emission region (4-5 µm) simultaneously for the first time, building on the work of Grassi et al. (2019) and Grassi et al. (2020).  We investigate the appropriate set of aerosol and haze layers, starting with NH4SH at 1.3 bars, NH3 and 0.7 bars and two grey hazes: one in the troposphere and one in the stratosphere.  The optical properties of these aerosols are tested to find the optimal cloud structure to reproduce the full JIRAM spectrum. From the retrievals of the zonally-averaged spectra we investigate whether spatial variations of tropospheric composition are truly required to fit the data, comparing gaseous contrasts to the expected circulation patterns associated with Jupiter’s belts and zones.

How to cite: Melin, H., Fletcher, L., Irwin, P., and Grassi, D.: Zonal Profiles of Jupiter's Tropospheric Abundances from Near-Infrared Juno JIRAM Spectroscopy, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-173, https://doi.org/10.5194/epsc2021-173, 2021.

Motivation

Recent close-up views of Jupiter’s polar regions by NASA’s Juno spacecraft are providing a wealth of information about the dynamics and composition of Jupiter’s poles. We present thermal imaging observations from a series of repeated observations over fifteen years.  These images complement the observations made by the Juno spacecraft by observing in a spectral region not covered by its instrumentation, providing a near-global context for the narrow fields-of-view of Juno’s Microwave Radiometer and JunoCam instruments and providing a long term baseline to study Jovian atmospheric evolution. In this work, we specifically focus on the distinct thermal signatures of Jupiter’s northern and southern polar regions in both the troposphere and stratosphere. Although Cassini CIRS maps of Jupiter in 2000 could see this transition to coll polar vortices (e.g. Simon-Miler et al. 2006, Fletcher et al. 2016), the spatial resolution of the 8-m facility provides access to higher latitudes.

Data

We present thermal imaging from the Subaru Telescope using the COoled Mid-Infrared Camera and Spectrometer (COMICS, Kataza et al., 2000), between 7.8 and 25 μm. These data map temperatures in the upper troposphere (100 – 500 mbar) and stratosphere (20 – 0.5 mbar), and they constrain the distribution of tropospheric gases and condensate aerosols. This data set covers 2005 to 2020, allowing for the investigation of long-term trends, as well as comparisons with Juno and Juno-supporting observations at other wavelengths from 2016 onward.   

Preliminary Results

Figure 1 illustrates the composite polar maps of brightness temperature for each hemisphere using images measured in January, 2017. The retrieved temperatures in each latitude circle were derived from these data were binned over  μ (= cosine of emission angle) and the centre-to-limb variation was inverted using the NEMESIS radiative transfer code (Irwin et al., 2008). The results demonstrated that tropospheric temperatures exhibit a steep decline of at least 2 K poleward of 60°N-63°N (planetocentric), which we believe marks the boundary of a tropospheric polar vortex. A similar drop in temperature takes place more gradually over 54°S-64°S. The precise latitude of the inferred vortex boundary varies slightly between observations in both hemispheres. No seasonal trends in these latitude variations were observed. Stratospheric polar temperatures are affected by auroral-related heating at 140° – 230°W (System III) at high-northern latitudes (>55°N) and 330° – 90°W at high-southern latitudes (<70°S).  Away from regions affected by auroral heating, the boundaries of the vortex in the stratosphere appear to be similar to those in the troposphere, as stated above.  However, unlike the uniformly colder temperatures of the polar vortices in the troposphere, temperatures in the polar vortices in the stratosphere appear be the same or even warmer than lower latitudes, with the south polar region appearing warmer and the north polar region cooler in 2016-2020.  To first order, the zonally variable latitude boundaries of the polar vortices appear to be coincident with the boundaries of distinct regions of overlying hazes known as the polar caps, which are detected in reflected sunlight.  This suggests the dynamical boundaries are controlled by a Rossby wave and important contributions by these aerosols to the radiative balance within these polar regions (see Zhang et al 2015, Guerlet et al. 2020).

Figure 1. Polar projection of brightness temperatures in Jupiter from observations made by the Subaru/ COMICS instrument in January of 2017.  The top panels for the north (left) and the south (right) display a composite brightness temperature map for observations made with a filter centered at 7.80 µm that is sensitive to stratospheric temperatures near 10 mbar of atmospheric pressure. The bottom panels for the north (left) and south (right) are made similar with a filter centered at 17.65 µm that is sensitive to temperatures near 190 mbar of atmospheric pressure.

The location of polar-vortex temperature change is indicated by the arrows.  The cooler temperatures associated with the north polar vortex at 190 mbar (upper left) is the most prominent. There is essentially no meridional stratospheric temperature change detectable at high-southern latitudes (lower right); the compact brightening close to 0° W longitude is the result of auroral-related heating of the stratosphere (see, for example, Sinclair et al., 2021, and references therein).

References

Fletcher, L. N. et al. 2016.  Icarus. 278, 128-161.

Guerlet, S., et al. 2020. Icarus, 351:113935. doi: https://doi.org/10.1016/j.icarus.2020.113935. 129

Irwin, P. G. J. et al. 2008. J. Quant. Spectr. Rad. Transfer 109, 1136-1150. 

Kataza, H. et al. 2000. Proc. SPIE 4008, 114-1152

Simon-Miller, et al. 2006. Icarus 180, 98-112.

Sinclair, J. A. et al. 2019. Nature Astronomy 3, 607-613.

Zhang, X., R. A. 2016. Nature Communications 6:10231. Doi 10.1038/ncomms10231.

How to cite: Orton, G., Sinclair, J., Burke, A., Fujiyoshi, T., Kasaba, Y., Momary, T., Chan, R., and Fletcher, L.: Jupiter’s  Polar Vortices in the Mid-Infrared as Observed by Subaru/COMICS Prior to and During the Juno Mission, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-59, https://doi.org/10.5194/epsc2021-59, 2021.

Abstract

The Saturn DYNAMICO Global Climate Model (GCM) is a high-resolution, multi-annual numerical simulation of Saturn’s atmospheric dynamics [1], combining a radiative-convective equilibrium model [2] and a hydrodynamical solver on an icosahedral grid. The model reproduces well the observed behaviour of jets and eddy-momentum transfer to the mean flow. Vortices arise naturally in the model over time but until now they have not been given direct consideration.

Analysis of large-scale vortices in the GCM is performed by investigating; (1) the long-term statistical distribution and organization of vortices and (2) the short-term evolution of individual discrete features. The metrics used in both studies provide direct comparison to observations, albeit with some methodological differences.

Long-term analysis

This study uses a vortex selection method similar to previous observational studies of Jupiter and Saturn [3, 4, 5, 6]. We seek to address whether the Saturn GCM replicates the hemispheric symmetries and asymmetries of vortices measured on Saturn by [4, 5, 6], as well as studying the formation conditions and long-term temporal evolution of vortex distributions.

Those studies collectively observe Saturn intermittently over roughly one half of a Saturnian year between northern winter and summer solstice. The Saturn GCM offers a much-expanded view with a timeseries spanning three full model years after spin-up (to be extended further by four years), with vortex measurements obtained at each seasonal peak.

The model offers consistent spatial resolution across each global map throughout the timeseries (½-degree latitude-longitude bins corresponding to ~10-500 km/pixel from pole-equator). It constrains well the size and location of vortices, the horizontal wind field components and the magnitude and sign of horizontal vorticity.

Figure 1: Overall distribution of high-latitude vortices at ~700 mbar level for entire timeseries. (Left) histogram of vortex count with mean atmospheric temperature in red. (Centre) instantaneous zonal wind speed at the vortex centre with the mean zonal wind profile. (Right) average vortex size (average of major and minor axes) and vorticity sign with Rossby deformation radius (L_D, orange) and the Rayleigh Kuo-criterion for barotropic instability (R-K, black). Mean profiles are averaged over the entire timeseries and shaded areas represent one standard deviation between all individual profiles.

Short-term analysis

This method is also applied to short term model outputs to investigate the appearance and disappearance, lifetime vs. size relationship, latitudinal and longitudinal drift and merging events. This analysis allows for the tracking of “ambiguous blobs” of vorticity with a cadence of 1 model day to study the thermal and dynamical conditions related to cyclogenesis and vortex evolution. This also provides insights into vortex behaviour and the proximity interactions with the local flow, jets, and other vortices.

Summary and Conclusions

The long-term analysis reveals that the Saturn GCM can largely replicate well the trends seen in previous observations. Some disagreement with the data can be explained methodologically. Otherwise, this presents avenues of future study like refinement of the method (employing a machine learning approach for vortex detection), inclusion of more detail in model (e.g., moist convection) and exploration of the primordial epoch of the model (for a deeper understanding of vortex behaviour in the absence of strong zonal circulations).

PTD and the France authors were supported by Agence Nationale de la Recherche (ANR) and the UK author acknowledges the Science and Technology Facilities Council (STFC).

References

[1] Spiga et al. (2020), Icarus, 335, http://dx.doi.org/10.1016/j.icarus.2019.07.011.

[2] Guerlet et al. (2014), Icarus, 238, https://doi.org/10.1016/j.icarus.2014.05.010.

[3] Li et al. (2004), Icarus, 172, https://doi.org/10.1016/j.icarus.2003.10.015.

[4] Vasavada et al. (2006), J. Geophys. Res. Planets, 111, https://doi.org/10.1029/2005JE002563.

[5] Trammell et al. (2014), Icarus, 242, https://doi.org/10.1016/j.icarus.2014.07.019.

[6] Trammell et al. (2016), J. Geophys. Res. Planets, 121, https://doi.org/10.1002/2016JE005122.

How to cite: Donnelly, P., Spiga, A., Guerlet, S., and James, M.: Statistical Analysis of Large-Scale Vortices in the Saturn DYNAMICO GCM, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-190, https://doi.org/10.5194/epsc2021-190, 2021.

Introduction

The polygons of cyclones at Jupiter’s poles – a pentagon at the south pole and an octagon at the north pole, each centred on another cyclone – were discovered at Juno’s first few perijoves by the JunoCam and JIRAM instruments [ref.1].  JunoCam images showed that the configurations remained stable for the next two years, up to perijove (PJ) 15 [ref.2].  Here we summarise their behaviour up to PJ33 in 2021 April; with the orbital period of 53 days, this covers 4½ years.

The data are polar projection maps produced from JunoCam images as described in [ref.2].

South polar cluster

The five southern circumpolar cyclones (CPCs) surround the central South Polar Cyclone (SPC).  However, the SPC is not at the pole, but displaced by 1°-3° latitude, always into the same quadrant of longitude; and there is a gap between two of the CPCs (CPC-1 & 2), in the same quadrant [ref.2].

This asymmetry of the pentagon has persisted until now (Figures 1&2).  At PJ23 a smaller, compact cyclone was present in the gap, suggesting that the pentagon might be turning into a hexagon, but at PJ24 it was replaced by a chaotic cyclonic region and the asymmetric pentagonal structure was then restored.

We proposed [ref.2] that the cluster is a ‘vortex crystal’ whose configuration could be explained geometrically, if we adopt two conjectures: that the cyclones have a maximum size that prevents them growing larger or merging, and that the central cyclone must be smaller than the mean size of the others (as observed).  In this case, tight packing of the cyclones  produces a pentagon with a gap; and to pack the cyclones as close as possible to the pole, the cluster must be displaced towards the side of the gap, with the largest cyclone on the opposite side, as observed. 

Consistent with this model, as the cluster wanders somewhat, the gap tends to be widest when the SPC is furthest from the pole (Figure 3).  However, when the SPC longitude is greatest, it lies alongside CPCs-2 & 3 rather than CPCs-1 & 2; and at these times a gap opens up between CPCs-2 & 3, supplementing or replacing the gap between CPCs-1 & 2, as the model would predict.  

We noted [ref.2] that the wandering of the SPC appeared to be cyclic.  The record over 4½ years confirms this, showing that it performs loops with a period of 11.5 (±1) months (Figure 4).  The loops are of varying sizes, but show a progressive drift in one direction.  The rate of this drift of cycles has been very uneven, but if it were interpreted as precession of the cycles around the pole, the rate would be ~8° (±8°) per year.

Independently of this cyclic motion, the whole pentagon shows a very slow rotation about the SPC, which is stable in the long term: we estimated +1.5°/PJ from PJ1 to PJ15 [ref.2].  Up to PJ32, the average rotation is  +1.21 (±0.07) °/PJ, i.e. 8.3 (±0.5) °/yr.