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Aerosols in the atmospheres of the Giant Planets

Copernicus Publications (2022)

Authors:

Patrick Irwin, Nicholas Teanby, Leigh Fletcher, Daniel Toledo, Glenn Orton, Michael Wong, Michael Roman, Santiago Pérez-Hoyos, Jose Franciso Sanz Raquena, Arjuna James, Charlotte Alexander, Jack Dobinson
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Comparing atmospheric models of Jupiter, can we reduce the degeneracy of this problem? 

(2022)

Authors:

Charlotte Alexander, Patrick Irwin

Abstract:

<p>The grand appearance of Jupiter’s banded atmosphere, coloured with many shades from white to red, whose cloud structure currently remains elusive with no clear single cloud model responsible for this varied appearance. With a general pattern of alternating bright cloudy zones and darker belts, as well as unique regions such as the Great Red Spot, finding a way to model all of these differing appearances with a single model has proven difficult. Jupiter’s atmosphere provides continual challenges when attempting to characterise its cloud structure due to its frequently varying appearance. This leads to differences between every observation meaning that models are constantly having to adapt to be explain these changes. Such changes can be seen in Figure 1, both of these spectra have been extracted for the same region of the Equatorial Zone (EZ) but are not identical due to the changes in appearance over the 3 years.<span class="Apple-converted-space"> </span></p> <p>Recent works [1,2,3,4] have all attempted to model Jupiter’s atmosphere using a universal chromophore (cloud colouring compound), combined with a deeper conservatively-scattering cloud and also a stratospheric haze layer. These works have all been able to model the atmosphere successfully but it has currently not been possible to determine between these differing solutions to find the most likely representation of the atmosphere. As all the different sets ups have several ways to vary cloud structure and chromophore properties among other parameters, they have been able to fit the changes in the observations. Therefore keeping each set up viable even as the atmosphere changes. The current inability to conclude on a favoured cloud structure highlights the highly degenerate nature of this problem.</p> <p>Utilising new observations and techniques to analyse the data highlights the delicacy of these results as the previous set ups have to be altered in order to produce the desired fit to the new observations once the input have varied slightly. An unchanged fit is shown in Figure 1, where the ideal fit of the EZ in 2018 has been used for 2021 data and is unable to model the spectra as well as for the spectra which it was derived from. Furthermore introduction of a limb viewing technique, as used in [2], has been done for this data. Here we attempt to fit multiple viewing angles simultaneously, which also begins to question the robustness of these results, due to an inability of nadir derived set ups to reproduce the multiple observations as successfully.<span class="Apple-converted-space"> </span></p> <p>Therefore in this work we have begun to attempt to reduce the degeneracy of the problem before utilising the Non-linear optimal Estimator for Multi-variatE spectral analySIS (NEMESIS) radiative-transfer retrieval algorithm [5], to fit to our observations. From this we want to find a vertical cloud structure which is more reproducible using different observations and techniques. It is hoped that reducing one of the degenerate parameters before fitting will allow us to constrain the atmospheric structure more decisively. Furthermore combining this with the limb darkening technique will hopefully allow us to rule out some of the solutions to this highly degenerate problem to find more confidence in the proposed models.<span class="Apple-converted-space"> </span></p> <p>In this work we will present the preliminary results taken from the application of these methods to observations to derive a new atmospheric model which can be compared with past work. Additionally we will present the use of new techniques to determine the ability of previous models to adapt to new observations to see if they are still viable.<span class="Apple-converted-space"> </span></p> <p><span class="Apple-converted-space"><img src="" alt="" width="640" height="464" /></span></p> <p>Figure 1: Spectra of the Equatorial Zone in both 2018 and 2021 and the spectral fit using the model from [1] derived for the 2018 data.<span class="Apple-converted-space"> </span></p> <p>[1] Braude, A. S., Irwin, P. G., Orton, G. S., and Fletcher, L. N. (2020). Colour and tropospheric cloud structure of jupiter from muse/vlt: Retrieving a universal chromophore. Icarus, 338:113589.<span class="Apple-converted-space"> </span></p> <p>[2] Pérez-Hoyos, S., Sánchez-Lavega, A., Sanz-Requena, J., Barrado-Izagirre, N., Carrión-González, O., Anguiano-Arteaga, A., Irwin, P., and Braude, A. (2020). Color and aerosol changes in jupiter after a north temperate belt disturbance. Icarus, 352:114031.</p> <p>[3] Dahl, E. K., Chanover, N. J., Orton, G. S., Baines, K. H., Sinclair, J. A., Voelz, D. G., Wijerathna, E. A., Strycker, P. D., and Irwin, P. G. J. (2021). Vertical structure and color of jovian latitudinal cloud bands during the juno era. The Planetary Science Journal, 2(1):16.<span class="Apple-converted-space"> </span></p> <p>[4] Baines, K., Sromovsky, L., Carlson, R., Momary, T., and Fry, P. (2019). The visual spectrum of jupiter’s great red spot accurately modeled with aerosols produced by photolyzed ammonia reacting with acetylene. Icarus, 330:217–229.</p> <p>[5] Irwin, P. G. J., Teanby, N. A., de Kok, R., Fletcher, L. N., Howett, C. J. A., Tsang, C. C. C., Wilson, C. F., Calcutt, S. B., Nixon, C. A., and Parrish, P. D. (2008). The NEMESIS planetary atmosphere radiative transfer and retrieval tool. , 109:1136–1150<span class="Apple-converted-space"> </span></p>
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Investigating the properties of a near-surface cloud layer from Venera 13 and 14 descent probe data

(2022)

Authors:

Shubham Kulkarni, Colin Wilson, Patrick Irwin

Abstract:

<div> <p><span data-contrast="auto">The scanning spectrophotometers (IOAV-2) onboard Venera 13 and 14 probes recorded the internal radiation field from an altitude of 62 km down to the surface, covering a wavelength range of 0.48 to 1.14 μm. The radiation was recorded from six directions with a field of view of 20°. The original data from the magnetic tapes were lost. However, a secondary dataset was created using the graphic material published earlier that contains the radiation only from two directions (one close to the zenith and one close to the nadir). While analysing the secondary dataset, [1] reported a rapid change in radiances indicative of a near-surface cloud layer. The presence of such a cloud layer could be indicative of aeolian or condensing species; furthermore, it would affect the viability of surface imaging from a balloon and other missions. Motivated by upcoming Venus missions, we re-analyze this dataset to learn more about a possible near-surface cloud layer.</span><span data-ccp-props="{"134233117":true,"134233118":true,"201341983":0,"335551550":6,"335551620":6,"335559740":240}"> </span></p> </div> <div> <p><span data-contrast="auto">The secondary dataset is available in two formats: (a) High spectral resolution with low vertical resolution (e.g. 68 wavelengths x 11 altitudes for Venera 13) and (b) Low spectral resolution with high vertical resolution (e.g. 28 wavelengths x 52 altitudes for Venera 13). In this work, we use the second format to capture more information about the vertical structure of the atmosphere. We modify the NEMESIS radiative transfer and retrieval tool [2] for simultaneous fitting of upward and downward internal radiation at all altitudes. For example, the downward and upward spectra captured by Venera 13 [3] at twelve predefined altitudes, are shown in Figure 1 and Figure 2 respectively.</span><span data-ccp-props="{"201341983":0,"335551550":6,"335551620":6,"335559739":200,"335559740":276}"> </span></p> </div> <div> <p><span data-contrast="auto">Before assessing the presence and properties of a near-surface cloud layer, it is necessary to first match the downwelling radiance from the main cloud deck in the atmosphere of Venus. Hence, NEMESIS is first used to retrieve particle properties in the main cloud deck. Using retrieved cloud abundances we set up the model atmosphere which is used to retrieve the properties of a near-surface cloud layer. Subsequently, the simulations are run for different particle sizes, abundances, and compositions to find the best match with the measured spectra. The results are used to comment on the existence of the near-surface cloud layer and on the properties of its constituent particles. Uncertainties associated with the particularities of the Venera probe measurements and their effects on assessing the presence of near-surface cloud layer and subsequent retrievals are briefly discussed in the end.</span><span data-ccp-props="{"201341983":0,"335551550":6,"335551620":6,"335559739":200,"335559740":276}"> </span></p> <p><span data-ccp-props="{"201341983":0,"335551550":6,"335551620":6,"335559739":200,"335559740":276}"><img src="" alt="" width="825" height="619" /></span></p> <p><span data-ccp-props="{"201341983":0,"335551550":6,"335551620":6,"335559739":200,"335559740":276}"><img src="" alt="" width="825" height="619" /></span></p> <p> </p> <div> <p><strong><span data-contrast="auto">References:</span></strong><span data-ccp-props="{"201341983":0,"335551550":6,"335551620":6,"335559739":200,"335559740":276}"> </span></p> </div> <div> <p><span data-contrast="auto">[1] Grieger, B., Ignatiev, N. I., Hoekzema, N. M., and Keller, H. U., </span><em><span data-contrast="auto">in European Space Agency, (Special Publication) ESA SP</span></em><span data-contrast="auto">, number 544, 63–70 (ESA Publications Division, Noordwijk, Netherlands, 2004).</span><span data-ccp-props="{"201341983":0,"335551550":6,"335551620":6,"335559739":200,"335559740":276}"> </span></p> </div> <div> <p><span data-contrast="auto">[2] Irwin, P. G., Teanby, N. A., de Kok, R., Fletcher, L. N., Howett, C. J., Tsang, C. C., Wilson, C. F., Calcutt, S. B., Nixon, C. A., and Parrish, P. D., </span><em><span data-contrast="auto">Journal of Quantitative Spectroscopy and Radiative Transfer 109(6),</span></em><span data-contrast="auto"> 1136–1150 (2008). </span><span data-ccp-props="{"201341983":0,"335551550":6,"335551620":6,"335559739":200,"335559740":276}"> </span></p> </div> <div> <p><span data-contrast="auto">[3] Moroz, V. I., Moshkin, B. E., Ekonomov, A. P., Golovin, Y. M., Gnedykh, V. I., and Grigorev, A. V., </span><em><span data-contrast="auto">Soviet Astronomy Letters</span></em><span data-contrast="auto"> (1982).</span><span data-ccp-props="{"201341983":0,"335551550":6,"335551620":6,"335559739":200,"335559740":276}"> </span></p> </div> </div>
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Monsters of rock: are Uranus and Neptune rock giants?

(2022)

Authors:

Nicholas Teanby, Patrick Irwin, Lucy Wright, Robert Myhill

Abstract:

<p><strong>Introduction</strong></p> <p>To understand solar system formation it is critical to know how Uranus and Neptune formed. This requires knowledge of internal composition. Uranus and Neptune are generally referred to as ”ice-giants” in recent literature, as it has been inferred that their interiors are ice-dominated. Physical measurements from the Voyager 2 flybys include mass, radius, oblateness, low order gravity coefficients, moments of inertia, and magnetic field snapshots. One fundamental issue is that high temperature and pressure ice mixtures have similar densities to silicates mixed with hydrogen and helium. Therefore, existing physical constraints cannot by themselves distinguish between ice or rock-dominated interiors and almost any interior model fits the observations [5,10]. Measurements of atmospheric composition and temperature provide a possible complementary window into these planets’ interiors and may provide a way to break the degeneracy [12]. Here we consider the case for ice and rock-dominated interiors and attempt to propose a consistent explanation.</p> <p><strong>The case for Ice Giants</strong></p> <p>Internally-generated magnetic fields are observed at Uranus and Neptune [11]. Fields are highly non-dipolar, suggesting a shallow origin. Magnetic field generation requires conducting fluid and the conventional explanation is super-ionic water at high temperature and pressure, implying ice-dominated interiors.</p> <p>Spectroscopic atmospheric CO observations provide further evidence favouring the ice giant model. CO has higher abundance in Uranus’ and Neptune’s stratospheres than in their tropospheres, indicating an external source [7]. Uranus has ~8 ppb stratospheric CO and <2 ppb tropospheric CO, which can mostly be accounted for with background interplanetary dust particle flux. Conversely, Neptune’s stratospheric CO abundance is the largest of any giant planet at ~1000 ppb. The only way to feasibly explain this is with a kilometre-scale ancient comet impact and shock chemistry, where cometary water reacts with methane in Neptune’s atmosphere to form CO [7,9].</p> <p>More relevant to the interior is that Neptune appears to have ~100 ppb tropospheric CO. Conventionally, this is explained by quenching CO dredged up from the deep interior by Neptune’s vigorous tropospheric mixing. Thermochemical models predict ~400 x O/H enrichment over solar abundance is required to reproduce this CO amount [2,8]. This extreme enrichment requires ~90% water ice in Neptune’s interior, again implying an ice-dominated interior. It is usually extrapolated that Uranus is also an ice giant with a similarly extreme oxygen and ice abundance, where the lack of CO in Uranus’ troposphere is conveniently explained by more sluggish tropospheric mixing.</p> <p><strong>Issues with the Ice Giant model</strong></p> <p>Although ice-dominated interiors can explain many observational aspects of Uranus and Neptune, there are also some worrying discrepancies.</p> <p>1) Most icy bodies in the outer solar system have rock fractions of ~70%. If Uranus and Neptune formed from similar objects, then we require some explanation of where the missing rock fraction has gone or why the planetesimals that formed Uranus and Neptune are different to anything we observe today.</p> <p>2) Measurements of atmospheric methane on Uranus and Neptune suggest deep abundances of a few percent [6]. This implies a C/H enrichment of ~50–100 x solar [1], which is much lower than that inferred for O/H from tropospheric CO.</p> <p>3) D/H is ~4x10<sup>-5</sup> on both Uranus and Neptune [3]. This is much lower than D/H observed in modern solar system icy objects such as comets, which typically have D/H ∼15–60x10<sup>-5</sup>. If interiors of Uranus and Neptune are well mixed and equilibrated, this implies only ~15% of the interiors can be ice, suggesting ~50–100 x solar enrichment [12]. Again, much lower than inferred from CO. A way around this is for interiors to only be partially mixed and equilibrated, with more D hiding in the unobservable deep atmosphere. Alternatively, some form of extinct exotic ices with lower D/H could be the source material.</p> <p>In summary, exotic ices, incomplete interior mixing, and unusually ice-rich planetesimals have all been invoked to make atmospheric observation consistent with the ice giant model. Not impossible, but also not entirely convincing as an explanation.</p> <p><strong>Rock Giant interiors as a potential solution</strong></p> <p>The alternative is that Uranus and Neptune’s interiors are rock-dominated. In this case we need to explain magnetic field generation and Neptune’s tropospheric CO.</p> <p>Recent work shows mixtures of silicates, hydrogen, and helium may be conductive at relevant pressures and temperatures, so super-ionic water is not necessarily required to generate magnetic fields [4]. Alternatively, there is no-doubt some ice in Uranus and Neptune’s interiors, which may form thin shell dynamos and explain non-dipolar field structures.</p> <p>Recent work also shows tropospheric CO may not actually be present throughout the troposphere and may be limited to the upper troposphere [12,13]. In this case, CO could be entirely sourced externally from comets.</p> <p>Profiles with CO limited to pressures <1 bar can fit spectroscopic observations very well, but require reduced upper troposphere eddy mixing to allow CO to survive long enough post-comet-impact to still be observable today. This seems plausible, as inspection of the Voyager 2 temperature profile and lapse rate suggest the upper troposphere is relatively stable [12]. Furthermore, Far-IR brightness temperatures suggest the boundary between radiative and convective zones may be ~1 bar.</p> <p><strong>Conclusion</strong></p> <p>Recent advances in our understanding of CO profiles on Neptune and high-pressure conductivity of silicate/hydrogen/helium mixtures suggests that rock-dominated interiors for Uranus and Neptune are becoming more plausible than conventional ice giant scenarios. Such a rock giant could be formed from planetesimals with similar rock:ice ratios and D/H ratios to modern-day outer solar system comets, Kuiper belt objects, and icy moons. Interiors could also be well mixed and equilibrated. This opens the possibility of simpler formation mechanisms for Uranus and Neptune, with both planets forming in similar ways, and avoiding any requirements for dubious ice compositions.</p> <p><strong>References</strong></p> <p>[1] Atreya+ 2020. https://ui.adsabs.harvard.edu/abs/2020SSRv..216...18A/abstract</p> <p>[2] Cavalié+ 2017. https://ui.adsabs.harvard.edu/abs/2017Icar..291....1C/abstract</p> <p>[3] Feuchtgruber+ 2013. https://ui.adsabs.harvard.edu/abs/2013A%26A...551A.126F/abstract</p> <p>[4] Gao+ 2022. https://ui.adsabs.harvard.edu/abs/2022PhRvL.128c5702G/abstract</p> <p>[5] Helled+ 2020. https://ui.adsabs.harvard.edu/abs/2020RSPTA.37890474H/abstract</p> <p>[6] Irwin+ 2019. https://ui.adsabs.harvard.edu/abs/2019Icar..331...69I/abstract</p> <p>[7] Lellouch+ 2005. https://ui.adsabs.harvard.edu/abs/2005A%26A...430L..37L/abstract</p> <p>[8] Luszcz-Cook+de Pater 2013. https://ui.adsabs.harvard.edu/abs/2013Icar..222..379L/abstract</p> <p>[9] Moreno+ 2017. https://ui.adsabs.harvard.edu/abs/2017A%26A...608L...5M/abstract</p> <p>[10] Neuenschwander+Helled 2022. https://ui.adsabs.harvard.edu/abs/2022MNRAS.512.3124N/abstract</p> <p>[11] Soderlund+Stanley 2020. https://ui.adsabs.harvard.edu/abs/2020RSPTA.37890479S/abstract</p> <p>[12] Teanby+ 2020. https://ui.adsabs.harvard.edu/abs/2020RSPTA.37890489T/abstract</p> <p>[13] Teanby+ 2019. https://ui.adsabs.harvard.edu/abs/2019Icar..319...86T/abstract</p> <p> </p>
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Preliminary atmospheric study of Jupiter using ISO/SWS data

(2022)

Authors:

José Ribeiro, Pedro Machado, Santiago Pérez-Hoyos, João Dias, Patrick Irwin

Abstract:

<p>The study of the thermal spectrum of Jupiter gives us the possibility to study the elements that constitute the Jovian atmosphere, allowing us to infer the formation history and conditions of the giant planet (Taylor et al., 2004). Determining the abundance of chemical species and isotopic ratios is fundamental in this regard. For this, we reanalyse 1997 Jupiter data obtained by the ESA mission Infrared Space Observatory (ISO) (Kessler et al., 1996) in the 793.65-3125 cm-1 (3.2-12.6 µm) region using the Short-Wave Spectrometer (SWS) (de Graauw et al., 1996).  Despite the age of this data, we argue that it warrants a revisit and reanalysis since it was an important step in the study of Jupiter’s atmosphere and there have since been advancements in atmospheric models and line data.</p><p>In this work we used the NEMESIS radiative transfer suite (Irwin et al. 2008) to reproduce the observations from Encrenaz et al. (1999), which will also work as a validation of our method. Using the Cassini/CIRS model as a starting point, we adapted the template for the ISO/SWS data. We compiled correlated k-tables from the spectral line database from Fletcher et al. (2018) for a NH<sub>3</sub>, PH<sub>3</sub>, <sup>12</sup>CH<sub>3</sub>D, <sup>12</sup>CH<sub>4</sub>, <sup>13</sup>CH<sub>4</sub>, C<sub>2</sub>H<sub>2</sub>, C<sub>2</sub>H<sub>6</sub>, C<sub>2</sub>H<sub>4</sub>, C<sub>4</sub>H<sub>2</sub>, He and H<sub>2 </sub>atmosphere.</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.ece30d39628269873172561/sdaolpUECMynit/2202CSPE&app=m&a=0&c=967973decff35ea7a1891e34420767cf&ct=x&pn=gnp.elif&d=1" alt="" width="792" height="462"></p><p><em>Figure 1: Plot of ISO/SWS and CIRS observations showing the discrepancy between both (no offset applied)</em></p><p>We first compare the spectrum obtained by ISO/SWS with the a priori model in order to find discrepancies between them as well as how each molecule individually impacts the forward model (Figure 1).</p><p>Our current work is focused on the 793.65-1500 cm<sup>-1</sup> (6.7-12.6 µm) region of the spectrum, for comparison reasons between the CIRS and ISO-SWS data, with the 793.65-1200 cm<sup>-1</sup> (8.3-12.6 µm) region showing the best fit.</p><p>We present here our preliminary results of the study of abundances of <sup>12</sup>CH<sub>3</sub>D, <sup>12</sup>CH<sub>4</sub>, <sup>13</sup>CH<sub>4</sub>, C<sub>2</sub>H<sub>2</sub> and C<sub>2</sub>H<sub>6</sub> of Jupiter’s atmosphere as well as our study of the pressure-temperature profile of Jupiter obtained using NEMESIS retrievals. We also compare our results with the profiles and abundances from Neimann et al. (1998) and Fletcher et al. (2016) with the aim of constraining the number of possible best fit profiles.</p><p>As consequence of the former study, we also present our initial study of the H/D and <sup>12</sup>C/<sup>13</sup>C isotopic ratio of the Jovian atmosphere from the abundances of <sup>12</sup>CH<sub>3</sub>D, <sup>13</sup>CH<sub>4</sub> and <sup>12</sup>CH<sub>4</sub> following the methodology from Fouchet et al. (2000).</p><p>We hope with this work to advance the understanding of the atmosphere of Jupiter and the physical and chemical processes that occur, as well as better determining its vertical distribution of chemical species and thermal profile. As future work, we expect to extend our frequency domain to the full range of ISO/SWS observations, study the <sup>15</sup>N/<sup>14</sup>N ratio and compare our finding with other relevant results.</p><p> </p><p> </p><p><strong>References:</strong></p><ul><li>de Graauw et al., Observing with the ISO short-wavelength spectrometer, A&A 315, L49-L54, 1996</li> <li>Encrenaz et al., The atmospheric composition and structure of Jupiter and Saturn form ISO observations: a preliminary review, Planetary and Space Science 47, 1225-1242, 1999</li> <li>Fletcher et al., Mid-infrared mapping of Jupiter’s temperatures, aerosol opacity and chemical distributions with IRTF/TEXES, Icarus 278, 128–161, 2016</li> <li>Fletcher et al., A hexagon in Saturn's northern stratosphere surrounding the emerging summertime polar vortex, Nature Communications, Volume 9, 2018.</li> <li>Fouchet et al., ISO-SWS Observations of Jupiter: Measurement of the Ammonia Tropospheric Profile and of the 15N/14N Isotopic Ratio, Icarus 143, 223–243, 2000</li> <li>Irwin et al., The NEMESIS planetary atmosphere radiative transfer and retrieval tool, Journal of Quantitative Spectroscopy & Radiative Transfer 109, 1136–1150, 2008</li> <li>Kessler et al., The Infrared Space Observatory (ISO) mission, A&A 315, L27, 1996</li> <li>Neimann et al., The composition of the Jovian atmosphere as determined by the Galileo probe mass spectrometer, Journal of Geophysical Research Atmospheres 103(E10):22831-45, 1998</li> <li>Taylor et al., Jupiter, The Planet, Satellites and Magnetosphere, Ch.4, Cambridge Planetary Science, Eds. Bagenal, Dowling, McKinnon, 2004</li> </ul><p> </p><p><strong>Acknowledgements:</strong></p><p>We thank Thérèse Encrenaz, from LESIA, Observatoire de Paris, for providing the data for this work, Patrick Irwin, from the University of Oxford (UK), for the help with the NEMESIS radiative transfer suite and Maarten Roos-Serote for guidance and help in analysing the data and retrieval results.</p><p>We acknowledge support from the Portuguese Fundação Para a Ciência e a Tecnologia (ref. PTDC/FIS-AST/29942/2017) through national funds and by FEDER through COMPETE 2020 (ref. POCI-01-0145 FEDER-007672) and through a grant of reference 2021.04584.BD. </p>
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