Microphysical Model of Jupiter's Great Red Spot Upper Chromophore Haze

Abstract

The origin of the red colouration in Jupiter's Great Red Spot (GRS) is a long-standing question in planetary science. While several candidate chromophores have been proposed, no clear conclusions have been reached regarding its nature, evolution, or relationship to atmospheric dynamics. In this work, we perform microphysical simulations of the reddish haze over the GRS and quantify the production rates and timescales required to sustain it. Matching the previously reported chromophore column mass and effective radius in the GRS requires column-integrated injection fluxes in the range $1\times10^{-12}$-$7\times10^{-12}$ kg m$^{-2}$ s$^{-1}$, under low upwelling velocities in the upper troposphere ($v_{\mathrm{trop}}\lesssim 1.5\times10^{-4}$ m s$^{-1}$) and particle charges of at least 20 electrons per $μ$m. Such rates exceed the mass flux that standard photochemical models of Jupiter currently supply via NH$_3$-C$_2$H$_2$ photochemistry at 0.1-0.2 bar, the most popular chromophore pathway in recent literature. We find a lower limit of 7 years on the haze formation time. We also assess commonly used size and vertical distribution parameterisations for the chromophore haze, finding that eddy diffusion prevents the long-term confinement of a thin layer and that the extinction is dominated by particles that can be represented by a single log-normal size distribution.

Figures (16)

• Figure 1: Profiles of temperature (red) and air number density (blue) as a function of pressure and altitude in the GRS (solid lines). The red and blue dashed lines show NEB profiles from Gladstone1996, shown for comparison.
• Figure 2: Vertical profile of the eddy diffusion coefficient used for the GRS.
• Figure 3: Sedimentation velocities and timescales for particles with radii in the range 0.01--10 $\mu$m in the GRS according to the formulation of Kasten1968.
• Figure 4: Ratio between the coagulation kernel $K_{\text{coag}}(r_1, r_1)$ and the classical Smoluchowski constant $K_{\text{Smolu}}(r_1, r_1)$ as a function of the Knudsen number, computed under terrestrial conditions ($T = 298~\mathrm{K}$, $\eta = 1.85 \times 10^{-5}~\mathrm{Pa\cdot s}$, mean free path $\lambda_g = 6.86 \times 10^{-8}~\mathrm{m}$).The black line corresponds to the continuum regime with slip--flow corrections, the red line represents the Fuchs formulation, and the blue line indicates the free--molecule limit. Shaded regions highlight the continuum, transition, and free molecular regimes.
• Figure 5: Sticking efficiency $\alpha_s$ (the multiplicative factor applied to the coagulation kernel) at $T=150$ K for collisions between equal--sized particles ($r_1=r_2=r$) as a function of particle radius, shown for the range of $Q$ values explored in this work. Note that, although equal--size coagulation can be strongly inhibited as Q increases, growth can still occur via collisions with smaller particles for which $\alpha_s(r_1,r_2)$ is not strongly suppressed.