Oral presentations and abstracts

Abstract

Observations from the Texas Echelon Cross Echelle Spectrograph (TEXES) on NASA’s IRTF and the VISIR instrument on the VLT are used to characterize the Saturn’s seasonal changes. Radiative transfer modelling (using NEMESIS [8]) provides the northern hemisphere temperature progression of the atmosphere over 10 years, both during and beyond the Cassini mission. Comparisons between imaging observations taken one Saturn year apart (1989-2018) show the extent of the interannual variability of Saturn’s northern hemisphere climate for the first time.

1. Introduction

With the culmination of Cassini's unprecedented 13-year exploration of the Saturn system in September 2017, and with no future missions currently scheduled to visit the ringed world, the requirement to build upon Cassini's discoveries now falls upon Earth-based observatories. Mid-infrared observations have been used to characterise features such as the extreme temperatures within an enormous storm system in 2011 [1,6], the cyclic variations in temperatures and winds associated with the 'Quasi-Periodic Oscillation' (QPO) in the equatorial stratosphere [2] and the onset of a seasonal warm polar vortex over the northern summer pole [3].

Saturn's axial tilt of 27º subjects its atmosphere to seasonal shifts in insolation [4], the effects of which are most significant at the gas giant's poles. The north pole emerged from northern spring equinox in 2009 (planetocentric solar longitude L_s=0º), and northern summer solstice in May 2017 (L_s=90º), providing Earth-based observers with their best visibility of the north polar region since 1987, with its warm central cyclone and long-lived hexagonal wave [5,6].

Studying these interconnected phenomena within Saturn's atmosphere (particularly those that evolve with time in a cyclic fashion) requires regular temporal sampling throughout Saturn's long 29.5-year orbit. We present here a showcase of research from the wealth of archived observations from both TEXES and VISIR obtained over the past decade.

1.1 Temperature progression

Methane (CH₄) is used to determine stratospheric temperatures due to its even distribution across the planet, as well as its well understood emissive behavior. Figure 1 shows the visible changes in the stratospheric and tropospheric conditions as seen by VISIR over 3 years; these images are representative of a range of filters between 7-20 µm, which can be stacked and inverted to derive the 3D temperature distribution in the upper troposphere and stratosphere. Using this technique, we probe changes in the atmospheric 3D temperature distribution across the planet disc in VISIR observations taken from April 2008 (L_s=343º) to July 2018 (L_s=102º); thereby discerning the spatial variability as well as temporal. VISIR observations concurrent with the Cassini/CIRS observations will be used to cross-check the time-series from Cassini, which can be extended beyond the end-of-mission with the newer VISIR observations. These profiles will provide a new measure of long-term temperature variability in the context of an established model.

1.2 Interannual Variability

Spectroscopic maps of the northern summer hemisphere from TEXES instrument on the IRTF collected in September 2018 have provided a unique opportunity, as they were acquired exactly one Saturn year apart from the 1989 observations of Gezari et al, (1989) [7], which were the first ever 2D images of Saturn in the mid-IR. Examining the differences in brightness temperatures and composition will indicate the extent of any interannual variation for Saturn’s northern hemisphere. This study will also provide unique insight into the timescale of the QPO which will be contrasted with a previously suggested biennial cycle [2]. The seasonal temperature progression measured in Section 1.1 also enables us to place this interannual variability in a wider context and provides further opportunity for insightful comparison with the comparatively shorter-term temperature variability.

Figure 1: VISIR observations from P95-102 sensing the troposphere (right) and stratosphere (left).  Polar warming is evident in the stratosphere; but is considerably smaller than that seen during southern summer and in the historical record of the 1980s.  The warm polar hexagon is seen at the north pole, the first such observation from the ground. Work to remove residual striping is ongoing and has been successfully applied to the 2017 images. 2015-16 images have been published by Fletcher et al., 2017 [2].

Acknowledgements

This research is funded by a European Research Council consolidated grant under the European Union’s Horizon 2020 research and innovation program, grant agreement 723890. We would like to thank co-author Mael Es-Sayeh for his significant contribution to this research.

References

[1] Fletcher et al., 2012, Icarus 221, p560-586

[2] Fletcher et al., 2017, Nature Astronomy, 1, p765-770

[3] Fletcher et al. 2015, Icarus. 251, 131-153

[4] Fletcher et al., 2015, https://arxiv.org/abs/1510.05690

[5] Fletcher et al., 2008, Science. 319, 79-81

[6] Fouchet et al., 2016, Icarus, 277, p196-214

[7] Gezari et al., 1989, Nature, 342, 777–780

[8] Irwin et al. 2008, JQSRT 109:1136-1150

[9] Orton et al., 2008, Nature 453, p198

How to cite: Blake, J., Fletcher, L., Antunano, A., Melin, H., Roman, M., Es Sayeh, M., Donelly, P., Rowe-Gurney, N., King, O., Greathouse, T., and Orton, G.: Saturn’s Seasonal Atmosphere: Cassini CIRS contrasts to VLT and IRTF observations, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-310, https://doi.org/10.5194/epsc2020-310, 2020.

To address questions about the driving mechanisms of Saturn's equatorial oscillation, our team at the Laboratoire de Météorologie Dynamique built the DYNAMICO-Saturn Global Climate Model to study tropospheric dynamics, tropospheric waves activity (Spiga et al. 2020) and equatorial stratospheric dynamics (Bardet et al. 2020) of Saturn. Previous studies (Guerlet et al. 2014, Spiga et al. 2020, Cabanes et al. 2020) have shown that our model produces consistent thermal structure and seasonal variability compared to Cassini CIRS measurements, mid-latitude eddy-driven tropospheric eastward and westward jets commensurate to those observed and following the zonostrophic regime, and planetary-scale waves such as Rossby-gravity (Yanai), Rossby and Kelvin waves in the tropical channel. Extending the model top toward the upper stratosphere allowed our model to produce an almost semi-annual equatorial oscillation with opposite eastward and westward phases. Associated temperature anomalies have a similar behavior than the Cassini/CIRS observations, but the amplitude of the temperature oscillation is twice smaller than the observed one. The absence of sub-grid-scale waves in the model produces an imbalance in eastward- and westward-wave forcing on the mean flow and could be an explanation to the irregularity in both the oscillating period and the downward rate propagation of the resolved Saturn equatorial oscillation.

To explore the impact of those small-scale waves on the spontaneous equatorial oscillation emerging in the DYNAMICO-Saturn GCM (Bardet et al. 2020), we add a sub-grid-scale non-orographic gravity waves drag parameterization in our model.
This parameterization is directly adapted from the stochastic terrestrial model of Lott et al. (2012). This formalism represents a broadband gravity wave spectrum, using the superposition of a large statistical set of monochromatic waves. As the time scale of the life cycles of gravity waves is much longer than the time step of our GCM, our parametrization can launch a few waves whose characteristics are randomly chosen at each time step. This stochastic gravity waves drag parameterization is applied in DYNAMICO-Saturn on all points of the horizontal grid.

A key parameter used in the non-orographic gravity waves drag parameterization is the maximum value of the Eliassen-Palm flux. The Eliassen Palm flux represents the momentum carried by waves that could be transferred to the mean flow. This value has never been measured in Saturn's atmosphere and it represents an important degree of freedom in the parameterization of gravity waves.

We performed several test simulations, lasting two Saturn years whose initial state is derived from Bardet et al (2020), with an horizontal resolution of 1/2° in longitude/latitude and a vertical resolution ranging between 3 bar to 1 μbar. For these test simulations, the maximum value of the Eliassen-Palm fulx is set to 10^-6, 10^-5, 10^-4 and 10^-3 kg m^-1 s^-2. 

Preliminary results show that the appropriate value of our main parameter is between 10^-5 and 10^-4 kg m^-1 s^-2. Eliassen-Palm flux value of 10^-3 kg m^-1 s^-2 demonstrates a too large impact: the equatorial oscillation is entirely vanished is this configuration. The simulation using the value of 10^-6 kg m^-1 s^-2 is equivalent to the control simulation without the gravity waves drag parameterization.  

The next step is to test other parameters, as phase velocity of the gravity waves, horizontal wavenumber, to understand how gravity waves impact the equatorial oscillation.

How to cite: Bardet, D., Spiga, A., Guerlet, S., Millour, E., and Lott, F.: Parameterizing gravity waves in the DYNAMICO-Saturn Global Climate Model to understand Saturn's equatorial oscillation, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-601, https://doi.org/10.5194/epsc2020-601, 2020.

The delivery of enriched icy grains has been proposed as a mechanism to explain the enrichment of Jupiter with noble gases [1]. The enrichment with noble gases imposes constraints on the formation temperature of these grains, with Ar in particular only adsorbing to amorphous ice below 30K [2]. While significant consideration has been given to the formation conditions of the ices, the release of species as the grain migrates inward toward the forming planets has been given less thought. The desorption of the noble gases Ar, Kr and Xe trapped in amorphous ice occurs largely below 80K [3], while Jupiter formed at a temperature of 130K. [4] The composition of the icy grains thus changes from formation to deposition. The accretion and release processes are visualised in Figure 1.

The accretion and subsequent desorption of noble gas species alongside water into an enriched icy grain has been simulated using a Monte Carlo model. Assuming a tetrahedral structure of amorphous water ice, particles are deposited onto a predefined grid at temperatures sufficiently low to retain Ar. The temperature is subsequently increased up to 150K, capturing the temperature range relevant for giant planet formation. Previously reported experimental measurements of evaporation rates are used to benchmark the model and constrain the diffusion and evaporation rates of each noble gas species. The accretion and heating phases are shown in Figure 2, alongside the relevant physical processes. The desorption of each species from the ice is tracked during heating, and used to compute the temperature-dependent enrichment profile of the grain.

The Ar/Xe ratio of an icy grain simulated to form at 20K and heated up to 150K is shown in Figure 3. The higher thermal velocity of Ar causes the grains to initially be excessively enriched with Ar relative to the other noble gases at deposition. The ratio of 1.6 upon delivery to Jupiter at 130K is in line with atmospheric probe measurements conducted by Galileo, shown as the shaded green area. For icy particles accreted from gas reservoirs with a limited water budget, a distinct dip in the Ar/Xe ratio is observed in the 30K-70K range in which the ice giants formed. In this temperature range, Ar readily desorbs from the ice while Xe is retained. The atmospheric signature of the amorphous ice delivery mechanism could thus differ for Neptune in particular, with a depletion of Ar relative to Jupiter a possibility. This feature can be taken into account during the interpretation of potential future composition measurements of ice giant atmospheres. In addition, the results suggest the delivery of 0.1-0.5m_⊕ of enriched ice is sufficient to provide the measured Jovian enrichment.

References:

[1] Tobias Owen, Paul Mahaffy, HB Niemann, Sushil Atreya, Thomas Donahue, Akiva Bar-Nun, and Imke de Pater. A low-temperature origin for the planetesimals that formed jupiter. Nature, 402(6759):269, 1999.

[2] A Bar-Nun, J Dror, E Kochavi, and D Laufer. Amorphous water ice and its ability to trap

gases. Physical Review B, 35(5):2427, 1987.

[3] R Scott Smith, R Alan May, and Bruce D Kay. Desorption kinetics of ar, kr, xe, n2, o2, co,

methane, ethane, and propane from graphene and amorphous solid water surfaces. The Journal

of Physical Chemistry B, 120(8):1979-1987, 2016.

[4] Takayuki Tanigawa and Hidekazu Tanaka. Final masses of giant planets. ii. jupiter formation

in a gas-depleted disk. The Astrophysical Journal, 823(1):48, 2016.

[5] Nikhil Monga and Steven Desch. External photoevaporation of the solar nebula: Jupiter's

noble gas enrichments. The Astrophysical Journal, 798(1):9, 2014.

How to cite: Dotinga, R., Cazaux, S., and Dulieu, F.: Atmospheric signatures of amorphous water ice delivery, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-1116, https://doi.org/10.5194/epsc2020-1116, 2020.

How to cite: Benmahi, B., Cavalié, T., Dobrijevic, M., Biver, N., Bermudez Diaz, K., Sandqvist, A., Lellouch, E., Moreno, R., Fouchet, T., Hue, V., Hartogh, P., Billebaud, F., Lecacheux, A., Hjalmarson, A., Frisk, U., and Olberg, M.: Monitoring of the temporal evolution of water vapor in the stratosphere of Jupiter with the Odin space telescope between 2002 and 2019, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-87, https://doi.org/10.5194/epsc2020-87, 2020.

Abstract

In the first 20 orbits of the Juno mission, over 150 waves and wave-like features have been detected by the JunoCam public-outreach camera. A wide variety of wave morphologies were detected over a wide latitude range, but the great majority were found near Jupiter’s equator.  By analogy with previous studies of waves in Jupiter’s atmosphere, most of the waves detected are likely to be inertia-gravity waves.

The Juno mission’s JunoCam instrument [1], has detected very small-scale waves. Our survey of JunoCam images revealed a surprising variety of features with wave-like morphologies.  They are presented in terms of differences in visual morphology, without implication that this differentiation arises from the associated responsible dynamics.

Long wave packets with short, dark wave fronts represent 79% of the types of waves in our inventory, especially in the Equatorial Zone (EZ) that were also detected in previous studies.  These include wave trains with orthogonal wave crests.  Even more commonly, we detected wave packets with tilted fronts that are not oriented orthogonally to the wave packet direction. Both the meridional extent and wavelength of these waves are much shorter than the Rossby deformation radius, so it is logical to assume that they are formed by and interact with small-scale turbulence, and thereby propagate the waves in all directions (Fig. 1). Sometimes the short wavefronts are aligned in curved wave packets, all associated with larger features, and located outside the EZ. 

Short wave packets with wide wave fronts were also detected.   In the Earth’s atmosphere, such waves are often associated with thunderstorms producing a brief impulse period with radiating waves.

Wave packets with bright features appear bright on a dark background rather than dark on a lighter background, like the waves described above.   Differences between darker and brighter wave crests could be the composition of the material affected.

Lee waves, stationary waves generated by the vertical deflection of winds over an obstacle, were also detected. Jupiter’s atmosphere no doubt possesses the dynamical equivalent of an obstacle (Fig. 2).

Waves associated with large vortices include compact cyclonic and anticyclonic features with extended radial wavefronts, resembling structures in terrestrial cyclonic hurricanes (Fig. 3). 

Long, parallel dark streaks are seen both with a non-uniform patterns and  in regularly spaced parallel bands.  Their orientation suggests that they are tracing out the direction of flow on streamlines.

Figure 4 plots the distribution of mean wavelengths for different types of waves and wave-like features as a function of latitude, co-plotted with mean zonal wind velocity.  The minimum distance between crests is 29.1 km. The variability of wavelengths within a single packet is typically no greater than 20-30%.  The equatorial waves with long packets and short crests in the EZ have wavelengths that are clustered between 30 km and 320 km, with most between 80 and 230 km in size.  No waves are found at latitudes associated with retrograde zonal flow unless associated with a larger atmospheric feature.

JunoCam, detected 157 waves or wave-like features in its first 20 perijove passes. Of these, 100 are waves with long, linear packets and short crests. Another 25were detected with short packets and long crests.  They are all likely to be truly propagating waves, which  form the vast majority of features detected, concentrated in a latitude range between 5ºS and 7ºN. Few of these appear to be associated with other features except for waves that appear to be oriented in lines of local flow. There were fewer waves in the EZ between the equator and 1ºN than there were immediately north and south of this band, which was different from the waves detected by Voyager imaging in 1979 that were more equally distributed.  Other waves outside the EZ are influenced by other features.  These include waves associated with an anticyclonic or cyclonic eddies, lee waves some 10 km above the surrounding cloud deck. Several features appeared within or emanating from vortices. Two sets of extremely long, curved features were detected near the edges of a southwestern extension of a region associated with high 5-µm radiances at the southern edge of the NEB.   No waves were detected south of 7ºS that were not associated with larger vortices, such as the GRS.  No waves or wave-like features were detected in regions of retrograde mean zonal flow that were not associated with larger features, similar to the waves detected by Voyager imaging.

Acknowledgements

The primary support for this research was provided by NASA, a portion of which was distributed to the Jet Propulsion Laboratory, California Institute of Technology. 

References

 [1] Hansen et al. Junocam: Juno’s outreach camera. Space Sci. Rev. 217, 475-506.  2017.

[2] Wong et al. High-resolution UV/optical/IR imaging of Jupiter in 2016–2019. Space Sci. Rev. 247, 58. 2020.