Planetary atmosphere observation analysis

Abstract:

We present a reanalysis (using the Minnaert limb-darkening approximation) of visible/near-infrared (0.3–2.5 μm) observations of Uranus and Neptune made by several instruments. We find a common model of the vertical aerosol distribution i.e., consistent with the observed reflectivity spectra of both planets, consisting of: (a) a deep aerosol layer with a base pressure >5–7 bar, assumed to be composed of a mixture of H2S ice and photochemical haze; (b) a layer of photochemical haze/ice, coincident with a layer of high static stability at the methane condensation level at 1–2 bar; and (c) an extended layer of photochemical haze, likely mostly of the same composition as the 1–2-bar layer, extending from this level up through to the stratosphere, where the photochemical haze particles are thought to be produced. For Neptune, we find that we also need to add a thin layer of micron-sized methane ice particles at ∼0.2 bar to explain the enhanced reflection at longer methane-absorbing wavelengths. We suggest that methane condensing onto the haze particles at the base of the 1–2-bar aerosol layer forms ice/haze particles that grow very quickly to large size and immediately “snow out” (as predicted by Carlson et al. (1988), https://doi.org/10.1175/1520-0469(1988)045<2066:CMOTGP>2.0.CO;2), re-evaporating at deeper levels to release their core haze particles to act as condensation nuclei for H2S ice formation. In addition, we find that the spectral characteristics of “dark spots”, such as the Voyager-2/ISS Great Dark Spot and the HST/WFC3 NDS-2018, are well modelled by a darkening or possibly clearing of the deep aerosol layer only.

Abstract:

Here we present new constraints on Uranus’s and Neptune’s externally sourced stratospheric water abundance using disk-averaged observations of the 557 GHz emission line from Herschel’s Heterodyne Instrument for the Far-Infrared. Derived stratospheric column water abundances are × 1014 cm−2 for Uranus and ×1014 cm−2 for Neptune, consistent with previous determinations using ISO-SWS and Herschel-PACS. For Uranus, excellent observational fits are obtained by scaling photochemical model profiles or with step-type profiles with water vapor limited to ≤0.6 mbar. However, Uranus’s cold stratospheric temperatures imply a ∼0.03 mbar condensation level, which further limits water vapor to pressures ≤0.03 mbar. Neptune’s warmer stratosphere has a deeper ∼1 mbar condensation level, so emission-line pressure broadening can be used to further constrain the water profile. For Neptune, excellent fits are obtained using step-type profiles with cutoffs of ∼0.3–0.6 mbar or by scaling a photochemical model profile. Step-type profiles with cutoffs ≥1.0 mbar or ≤0.1 mbar can be rejected with 4σ significance. Rescaling photochemical model profiles from Moses & Poppe to match our observed column abundances implies similar external water fluxes for both planets: × 104 cm−2 s−1 for Uranus and ×104 cm−2 s−1 for Neptune. This suggests that Neptune’s ∼4 times greater observed water column abundance is primarily caused by its warmer stratosphere preventing loss by condensation, rather than by a significantly more intense external source. To reconcile these water fluxes with other stratospheric oxygen species (CO and CO2) requires either a significant CO component in interplanetary dust particles (Uranus) or contributions from cometary impacts (Uranus, Neptune)

Abstract:

<jats:title>Abstract</jats:title> <jats:p>We present an analysis of all currently available ground-based imaging of Neptune in the mid-infrared. Dating between 2003 and 2020, the images reveal changes in Neptune’s mid-infrared (∼8–25 <jats:italic>μ</jats:italic>m) emission over time in the years surrounding Neptune’s 2005 southern summer solstice. Images sensitive to stratospheric ethane (∼12 <jats:italic>μ</jats:italic>m), methane (∼8 <jats:italic>μ</jats:italic>m), and CH<jats:sub>3</jats:sub>D (∼9 <jats:italic>μ</jats:italic>m) display significant subseasonal temporal variation on regional and global scales. Comparison with H<jats:sub>2</jats:sub> S(1) hydrogen quadrupole (∼17.035 <jats:italic>μ</jats:italic>m) spectra suggests that these changes are primarily related to stratospheric temperature changes. The stratosphere appears to have cooled between 2003 and 2009 across multiple filtered wavelengths, followed by a dramatic warming of the south pole between 2018 and 2020. Conversely, upper-tropospheric temperatures—inferred from ∼17 to 25 <jats:italic>μ</jats:italic>m imaging—appear invariant during this period, except for the south pole, which appeared warmest between 2003 and 2006. We discuss the observed variability in the context of seasonal forcing, tropospheric meteorology, and the solar cycle. Collectively, these data provide the strongest evidence to date that processes produce subseasonal variation on both global and regional scales in Neptune’s stratosphere.</jats:p>

Abstract:

&lt;p&gt;We present the primary results from our recent analyses of mid-infrared observations of Neptune and Uranus from ground-based telescopes, including VLT-VISIR, Subaru-COMICS, and Gemini-TEXES.&amp;#160; We discuss our recent discovery that Neptune&amp;#8217;s stratospheric temperatures appear to be changing dramatically in just the past few years, following decades of cooling. &amp;#160;In contrast, we show that no evidence yet exists of long-term thermal changes in Uranus&amp;#8217; stratosphere, but mid-IR observations of Uranus are still extremely limited. We share new observations from VLT-VISIR, express the need for continued ground-based imaging, and discuss how the James Webb Space Telescope MIRI observations will help greatly advance our understanding of the Ice Giants in the years ahead. &amp;#160;&lt;/p&gt;

Abstract:

&lt;p&gt;Jupiter's Great Red Spot (GRS) is a remarkable phenomenon among solar system atmospheres. In addition to its unique dynamical properties, the vertical structure of its clouds and hazes is a relevant subject of study, being of particular interest the unknown chromophore species responsible for the GRS characteristic reddish color. In a recently published paper (Anguiano-Arteaga et al., 2021) we showed the existence of a stratospheric haze (P &lt; 100 mbar) that seemed to be compatible with the chromophore-candidate proposed by Carlson et al. (2016), although a second coloring agent located in the upper tropospheric levels (P &lt; 500 mbar) was also suggested.&lt;/p&gt;&lt;p&gt;In this study, we have analyzed high-resolution images obtained with the Hubble Space Telescope&amp;#8217;s Wide Field Camera 3 between 2015 and 2021, with a spectral coverage from the UV to the near IR, including two methane absorption bands. Following the same procedure as in our previous paper, we have obtained the spectral reflectivity of the GRS and a few dynamically interesting regions in the surrounding area under different viewing geometries.&lt;/p&gt;&lt;p&gt;From the measured spectra, and following the scheme proposed by Anguiano-Arteaga et al. (2021), we retrieved several key atmospheric parameters (optical depths, particle vertical and size distributions and refractive indices) for each of the regions using the NEMESIS radiative transfer suite (Irwin et al., 2008). We show the spatial and temporal variations on these parameters, including the evolution of the properties of the chromophore species.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;- Anguiano-Arteaga, A., P&amp;#233;rez-Hoyos, S., S&amp;#225;nchez-Lavega, A., Sanz-Requena, J. F., &amp; Irwin, P. G. J. (2021). Vertical distribution of aerosols and hazes over Jupiter's Great Red Spot and its surroundings in 2016 from HST/WFC3 imaging. &lt;em&gt;J. Geophys. Res. Planets., &lt;/em&gt;126&lt;em&gt;,&lt;/em&gt;&lt;em&gt; &lt;/em&gt;e2021JE006996&lt;em&gt; &lt;/em&gt;https://doi.org/10.1029/2021JE006996&lt;/p&gt;&lt;p&gt;- Carlson, R.W., Baines, K.H., Anderson, M.S., Filacchione, G., &amp; Simon, A.A. (2016). Chromophores from photolyzed ammonia reacting with acetylene: Application to Jupiter&amp;#8217;s Great Red Spot. &lt;em&gt;Icarus, 274&lt;/em&gt;, 106-115. https://doi.org/10.1016/j.icarus.2016.03.008&lt;/p&gt;&lt;p&gt;- 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., &amp; Parrish, P. D. (2008). The NEMESIS planetary atmosphere radiative transfer and retrieval tool. &lt;em&gt;J. of Quant. Spec. and Radiative Transfer&lt;/em&gt;, &lt;em&gt;109&lt;/em&gt; , 1136-1150. https://doi.org/10.1016/j.jqsrt.2007.11.006&lt;/p&gt;