A bimodal distribution of haze in Pluto’s atmosphere

Abstract

Pluto, Titan, and Triton make up a unique class of solar system bodies, with icy surfaces and chemically reducing atmospheres rich in organic photochemistry and haze formation. Hazes play important roles in these atmospheres, with physical and chemical processes highly dependent on particle sizes, but the haze size distribution in reducing atmospheres is currently poorly understood. Here we report observational evidence that Pluto’s haze particles are bimodally distributed, which successfully reproduces the full phase scattering observations from New Horizons. Combined with previous simulations of Titan’s haze, this result suggests that haze particles in reducing atmospheres undergo rapid shape change near pressure levels ~0.5 Pa and favors a photochemical rather than a dynamical origin for the formation of Titan’s detached haze. It also demonstrates that both oxidizing and reducing atmospheres can produce multi-modal hazes, and encourages reanalysis of observations of hazes on Titan and Triton.

Impacts of Organic Ice Condensation on the Optical Properties of Haze on Pluto

“…Kutsop et al (2021) combined the UV extinction obtained by Alice and the scattering intensities at three wavelengths and eight phase angles obtained by MVIC and proposed that the haze size distribution follows either the bimodal or the powerlaw distribution. By testing a number of scenarios of homogeneous particles' size distributions of Pluto's haze, Fan et al (2022) confirmed that a bimodal distribution of "tholin"like particles, including a large population of smaller-sized spheres with a radius of ∼80 nm and a small population of larger-sized fractal aggregates with an effective radius of ∼1 μm, could result in scattering properties that agree with the observations obtained by the four New Horizons instruments.…”

“…As a benchmark, we adapt parameters of uncoated aggregates from Fan et al (2022), the best constraints on Pluto's haze available to date. As forward scattering is sensitive to the particle size parameter, we pick (for the initial guess) a set of parameters from Fan et al (2022) retrieved by only considering forward scattering; we then conduct further tests to evaluate the contribution of the ice coating. The parameters D f , R m , and N m , are assumed to be 1.9, 24 nm, and 1275, respectively.…”

“…To better constrain the morphology and component of haze particles with ice components, we consider observations from four instruments on board the New Horizons spacecraft (Fan et al 2022). A summary of the observations, including the observational wavelength, phase angle, and altitude range, is given in Table 2.…”

“…In 2015, the New Horizons spacecraft's flyby of Pluto measured its scattered radiation from the ultraviolet (UV) to the near-infrared (NIR) and confirmed the existence of haze in the atmosphere (Stern et al 2015;Gladstone et al 2016). Fundamental microphysical properties of these particles are then constrained using extensive information of extinction and scattering intensities from multiwavelength and full-phase observations obtained by four instruments on board the New Horizons spacecraft: the Multispectral Visible Imaging Camera (MVIC; Kutsop et al 2021;Fan et al 2022), the LOng-Range Reconnaissance Imager (LORRI; Cheng et al 2017), the Linear Etalon Imaging Spectral Array (LEISA; Grundy et al 2018), and the Alice UV Imaging Spectrograph (Young et al 2018).…”

“…However, considerable differences still exist in the backward scattering near the surface. Fan et al (2022) presented a sensitivity test by substituting the "tholin"-like component with organic ice and proposed that such replacement does not significantly influence their scattering properties. However, the aforementioned efforts either qualitatively discussed or oversimplified the influence of gas condensation on the haze scattering properties, leaving the characteristics of organic ice on Pluto's haze particles still an open question.…”