A possibly universal red chromophore for modeling color variations on Jupiter
Introduction
It has long been known that the condensable molecules near the visible cloud level in Jupiter’s atmosphere, including ammonia and ammonium hydrosulfide, are colorless at visible wavelengths, while Jupiter’s cloud features have an overall red coloration to varying degrees, as evident from the spectral samples shown in Fig. 1. Jupiter’s clouds presumably contain some unknown compound that absorbs blue light preferentially, with the Great Red Spot being a region of enhanced red coloration. A number of suggestions have been made over the years to explain the color of the GRS, including molecules involving nitrogen, sulfur, phosphorous, and various compounds generated by irradiation, and complex organics of unknown composition such as tholins, as summarized by West et al. (1986) and further reviewed by West et al. (2009). Recent arguments have been advanced for irradiated NH4SH by Loeffler et al. (2016). Until recently no accurate match to the GRS spectrum had ever been demonstrated. Judging on the basis of spectral fit quality, the most promising material suggested as the GRS coloring agent is a laboratory-generated chemical compound made from photodissociated ammonia (NH3) molecules reacting with acetylene (C2H2), described by Carlson et al. (2016). Baines et al. (2016) showed that the GRS spectrum measured by the visual channels of the Cassini VIMS instrument in 2000 could be accurately fit by a cloud model in which the chromophore appeared as a physically thin layer of small chromophore particles immediately above the main cloud layer of the GRS, which they referred to as the crème brûlée model because of the dessert’s analogous vertical structure. They also considered other models in which the chromophore appeared in a vertically detached stratospheric haze, which did not fit as well, or as a coating on the particles of the main cloud layer, for which the fit was significantly worse.
Here we use the same VIMS data set, but extend the analysis to other cloud features and consider more varied vertical structures, showing that models using the same chromophore can fit much of the color variation that is normally seen over Jupiter’s disk.
Section snippets
VIMS instrumental characteristics
The VIMS instrument (Brown et al., 2004) provides two overlapping spectral channels covering the ranges from 0.3 to 1.05 µm (VIS) and from 0.86 to 5 µm (IR) with an effective pixel size of 0.5 milliradians on a side and a near-IR spectral resolution of approximately 15 nm (sampled at intervals of approximately 16 nm). The IR channel uses a linear detector to record a spectrum for a single spatial pixel, so that an image must be acquired by scanning the FOV across the target. The visual
Atmospheric structure and composition
We used the tabulated results of Seiff et al. (1998) for Jupiter’s temperature structure down to the 22-bar level, and assumed a dry adiabatic extrapolation below 22 bars. We assumed an atmospheric composition of He/H2 = 0.157 ± 0.003 (von Zahn et al., 1998) and CH4/H2 = (Niemann et al., 1998), which are expressed as number density ratios. Because NH3 is a condensable gas and is vertically variable below the condensation level, as well as horizontally variable, we selected a
Chromophore optical properties
The imaginary index and scattering properties of the Carlson et al. (2016) chromophore are displayed in Fig. 6. This material is nearly an order of magnitude more absorbing than the tholin measured by Khare et al. (1993), also shown for comparison. The imaginary index of the new material also has a nearly constant logarithmic slope over the 0.4–0.6 µm wavelength range, which turns out to be a desirable feature in matching the visible spectra of Jovian clouds. Note the large change in slope of
Fit quality and parameter variations
Our initial fits to the low phase angle observations of the GRS showed that putting the chromophore in the stratospheric haze or at the top of the main cloud produce comparable and excellent fits. The model in which the chromophore appears as a coating on the main cloud particles provides an inferior fit. With the chromophore as a coating on all the particles in the main cloud, the UV–vis reflectivity gradient of the model can’t reach the observed gradient without making the longer wavelength
Results from fitting medium phase angle observations
As evident in Fig. 2, the medium phase angle observations from 31 December 2000 and 1 January 2001 offer the advantages of higher spatial resolution and two different viewing geometries that provide additional constraints on the vertical structure of clouds and hazes, as well as their scattering properties. By accounting for the zonal wind-induced drift of features during the 38.85 h between the two observations, we are able to extract spectral samples from the same atmospheric region, although
Speculation on physical mechanisms
Production of the Carlson et al. (2016) chromophore depends on UV flux, ammonia, and acetylene. Because ammonia falls off with altitude and UV flux increases with altitude it is expected that production of photolyzed ammonia would occur somewhat above the cloud tops and would be widespread over Jupiter. The availability of acetylene seems to be a controlling factor in the production rate and its flux is quite weak on average according to current photochemical models (Moses et al., 2010). Baines
Summary and conclusions
We used Jupiter’s 0.35–1.1 µm spectrum, as measured by the Cassini/VIMS instrument near the end of 2000, to constrain cloud structures for the GRS, the equatorial zone, and north and south equatorial belts. We used a simple model structure in which the main cloud was composed of conservative particles and covered by a thin layer of particles made of the chromophore produced by Carlson et al. (2016). Our main conclusions from this investigation are as follows.
- 1.
The substance described by Carlson
Acknowledgments
We thank Gordon Bjoraker and an anonymous reviewer for constructive reviews. LAS and PMF acknowledge support by NASA grant NNX14AH40G from NASA’s Planetary Atmospheres Program.
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2020, IcarusCitation Excerpt :One of the most important questions for this scenario is the production rate of acetylene for which thunderstorms were invoked as a possible explanation (Baines et al., 2019). Convincing as the case for the “crème brûlée” model mightbe, Braude et al. (2020) found it difficult to reproduce the observed limb-darkening of red features in the Jovian atmosphere with this model, as fits by Sromovsky et al. (2017) were only given at few locations on the disk. Braude et al. (2020) also argue that it is possible to find a universal chromophore, at least within observation and model uncertainties, but their results provide a steeper blue-absorption gradient than that using the Carlson et al. (2016) chromophore.
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2019, IcarusCitation Excerpt :In stating our model results below, the quoted 1-σ uncertainty for a model parameter is the change in that value produced by a unit change in χ2, allowing as well all of the other free parameters to be adjusted to minimize χ2 (cf. Press et al., 1992). We used our proven radiative transfer software incorporating the Levenberg–Marquardt parameter-fitting technique (Press et al., 1992) previously employed to analyze the vertically inhomogenous atmospheric structures of Jupiter and Saturn (e.g., Sromovsky and Fry, 2010a, b, Sromovsky et al., 2013, 2016, 2017). This software has the ability to rapidly and efficiently zero-in on the best-fitting parameters over the selected spectrum of 91 wavelengths.
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2018, IcarusCitation Excerpt :The fit to the HST data is comparable to our low dose data over this spectral range, and thus it would be interesting to know how the spectrum of the Carlson et al. (2016) photolyzed mixture changes below 400 nm, as the potential for a contributor other than one of the main cloud components is an interesting possibility. The need for more data points at not only the shorter wavelengths but also longer ones is evidenced by this group's most recent work (Sromovsky et al., 2017), where the Carlson et al. (2016) laboratory data (400 nm–740 nm) was linearly extrapolated so that it extended from 350 to 1060 nm. This data yielded adequate fits to multiple regions in Jupiter's clouds, yet it is important to note that the validity of such a linear extrapolation is unknown and can only be tested via laboratory measurements.