Planetary Scientists Revealed Triton’s Haze Formation Mechanism (Planetary Science)


⦿ Ohno and colleagues presented the first microphysical model of haze formation on Triton.

⦿ Their results support the idea that Triton hazes are predominantly composed of hydrocarbon ices.

⦿ They simulated the formation of sphere and aggregate hazes with and without condensation of the C2H4 ice.

⦿ They found that haze particles can grow into fractal aggregates even in the Triton’s tenuous atmosphere.

⦿ UV occultation observations of Voyager 2 at short wavelength < 0.15 µm may slightly favor the icy aggregates on Triton.

⦿ They also answered why the haze formation on Triton is different from that of Pluto, even though they had similar atmosphere.

Organic aerosols (haze hereafter) produced via photochemistry of hydrocarbons are of great interest to study atmospheric properties and surface environment. The opacity of haze has crucial impacts on the radiative energy balance of atmospheres on Titan, Jupiter, and Pluto. It has been suggested that the haze veiled Archean Earth and played an important role to maintain warm climates. In the atmospheric chemistry context, hazes act as loss sites of gaseous species via condensation and heterogeneous reactions. Recent studies also have suggested that the presence of haze greatly impacts observations of exoplanetary atmospheres.

In the outer Solar System, observations of Titan, Pluto, and Triton provide important insights on organic haze formation in reduced (N2-CH4-CO) atmospheres. The presence of haze on Titan was discovered by ground-based observations and images from Voyager 1. Polarimetric and photometric observations of Titan’s haze particles by Pioneer 11 and Voyager 1 are consistent with fractal aggregates—nonspherical particles constituted by numerous spherical monomers. Cassini observations found that the Titan hazes extend from the ground to ionosphere above 1100 km.

Pluto’s hazes were discovered by the stellar occulations and recently have been investigated in detail by the New Horizons spacecraft. The UV extinction coefficients of hazes are nearly proportional to the atmospheric (N2) density from 26 to 100 km above the ground. The haze has a blue color that is consistent with Rayleigh scattering from particles with radii of ∼0.01 µm, whereas the strong forward scattering is consistent with particles with radii of ∼0.5 µm. This observational characteristic also indicates the aggregate nature of haze particles similar to the Titan haze.

Triton has also been found to possess near-surface haze layer in its thin atmosphere. The Voyager 2 imaging observations found that the optically thin hazes extend to an altitude of ∼30 km. Using the high phase angle images, Pollack et al. estimated the particle size of ∼0.1 µm, haze scattering optical depth of ∼0.003, and the particle production rate of ∼4.6 × 10¯15 g cm¯21. From disk-averaged photometry for a wavelength of λ = 0.414–0.561 µm, Hillier et al. reported that Triton hazes have single scattering albedo of nearly unity and cause strong forward scattering with an asymmetry factor of ∼0.6, although their results are highly influenced by discrete clouds near the ground. They also suggested that the scattering optical depth is nearly proportional to λ¯2. From the disk-resolved photometry at the similar wavelength range, Rages & Pollack later constrained the particle size of ∼0.17 µm and scattering optical depths of 0.001–0.01 that is higher at shorter wavelength. Solar occultation observations at UV wavelengths (λ = 0.14–0.165 µm) constrained the extinction optical depth to ∼0.024, significantly higher than the scattering optical depth at visible wavelength. In sum, hazes on Titan, Pluto, and Triton all exhibit the wavelength-dependent opacity and the strong forward scattering.

Microphysical models have been used to investigate the haze formation processes and constrain fundamental parameters, such as the haze production rate and charge to radius ratio. The models inferred the production rate of 0.5– 3 × 10¯14 g cm¯21 for Titan and 1.2 × 10¯14 g cm¯21 for Pluto, respectively. The microphysical models can also give insight on the degree of particle charge, which is associated with ionization processes in atmospheres. Previous studies suggested the charge-to-radius ratio of qe∼15 e µm¯1 for Titan and qe∼30 e µm¯1 for Pluto to explain the degree of forward scattering of haze particles.

In contrast to Titan and Pluto, Triton haze has not been thoroughly studied by a detailed microphysical model. Now, Ohno and colleagues presented the first microphysical model of haze formation on Triton.

Our model solves the evolution of both size and porosity distributions of haze particles in a self-consistent manner. We simulated the formation of sphere and aggregate hazes with and without condensation of the C2H4 ice.

— told Ohno, first author of the study

Figure 1. (Top) Vertical optical depth of the best models of ice balls (green line) and ice aggregates (black line). The gray, blue, and orange symbols denote the optical depth reported by Herbert & Sandel (1991), Krasnopolsky et al. (1992), and Rages & Pollack (1992), respectively. The extinction optical depth is identical to the scattering one since they have assumed the imaginary refractive index of zero for C2H4 ice. (Bottom) The visible I/F for all scattering angles in other best models. The gray shaded region denotes the I/F constrained in Rages & Pollack (1992). © Ohno et al.

Their model simulates the evolution of both size and porosity distributions of haze particles in a self-consistent manner. They have compared the model results with the observed UV extinction coefficient and visible scattered light intensity from Voyager 2 and showed that ice-free hazes, often assumed for Titan and Pluto hazes, cannot explain the Trion observations. Their results support the idea that Triton hazes are predominantly composed of hydrocarbon ices, which has been inferred from the Triton’s cold environment but not assessed in detail. They have proposed two possible models of haze formation with ice condensation, namely ice ball and ice aggregate scenarios, that can successfully explain the existing observations of Triton hazes. Their findings are summarized as follows.

⦿ Haze particles can grow into fractal aggregates even in the Triton’s tenuous atmosphere. The aggregates can grow into the mass-equivalent sphere radius of 0.2–1 µm, while the spheres can grow into only small sizes of 0.03–0.06 µm. Due to collisional growth, the fractal dimension of fractal aggregates is Df = 1.8–2.2, varying with the particle mass and altitude. The mass-dominating aggregates have the fractal dimension of Df ≈ 1.9. The obtained Df is in agreement with the outcome of cluster-cluster aggregation and similar to the fractal dimension of Titan hazes.

⦿ The ice-free hazes cannot simultaneously explain both UV and visible observations of Voyager 2, while including the condensation of C2H4 ices provides two better solutions. For ice aggregates, the required total haze mass flux is ∼2 × 10¯15 g cm¯21. For the icy sphere scenario, the column integrated C2H4 production rate is ∼8 × 10¯15 g cm¯21, and the ice-free mass flux of ∼6 × 10¯17 g cm¯21.

⦿ Future observations on the UV optical depth with greater wavelength coverage and scattering phase function with more phase angles would distinguish the ice balls and ice aggregates scenarios. The optical depth of ice aggregates increases with decreasing the wavelength at λ < 0.15 µm, while the optical depth of the ice balls is invariant at this wavelength range. The ice aggregates are slightly more consistent with the UV solar occultation observations of Voyager 2. The ice aggregates also cause forward scattering stronger than the ice balls do. These observational signatures would help to shed light on the nature of haze formation on Triton for future observations, such as NASA Ice Giants Mission and TRIDENT.

⦿ Lastly, although they did not focus on atmospheric thermal structure in this study, hazes may play an important role in controlling the temperature structure on Triton. In 2017, Zhang et al. suggested that radiative cooling by hazes is a key to explain the cold temperature on Pluto. It would be interesting to include the feedback of haze radiative effects on Triton’s temperature structure in future haze formation models. Since their results suggest that Triton hazes at the lower atmosphere are likely composed of hydrocarbon ices, the haze radiative feedback may have different effects as compared to that on Pluto, for which optical constants of Titan tholin is often assumed. On the other hand, since Triton hazes are likely ice-free in the hot upper atmosphere, they may act as coolants in the upper atmosphere, as suggested for Pluto.

Why are Triton and Pluto Hazes different?

Triton’s N2-CH4-CO atmosphere is similar to Pluto; nevertheless, Ohno and Colleagues results suggested that the properties of Triton hazes are quite different from those of Pluto hazes. According to their results, Triton hazes are predominantly composed of hydrocarbon ices, while Pluto hazes are suggested to be similar to Titan hazes, although there are no direct observational constraints on the Pluto haze compositions. They also note that a recent study of Lavvas et al. suggested that Pluto hazes may be composed of hydrocarbon ices, such as C4H2. Nonetheless, it would be important to understand the cause in terms of their composition and formation processes if Triton and Pluto hazes were indeed different.

One straightforward explanation of the difference is the temperature structure. Triton’s lower atmosphere is very cold (∼ 40–60 K), while the Pluto’s lower atmosphere is relatively hot (∼ 60–100 K). Thus, the ice condensation onto haze particles can be inhibited at Pluto’s lower hot atmosphere, explaining why Pluto hazes look different from Triton hazes. They note that the C2 hydrocarbons might condense or stick onto the haze particles in the cold upper atmosphere of Pluto.

CH4 abundance in the upper atmosphere might also cause the difference of Triton and Pluto haze properties. The Voyager 2 observation found that CH4 abundance steeply decreases with increasing altitudes on Triton, while the abundance rather increases with altitudes Pluto as revealed by the New Horizons spacecraft. The New Horizons observation also found that high order hydrocarbons, such as C2H2, C2H4, and C2H6, are abundantly present in the upper atmosphere of Pluto. Since photochemistry of hydrocarbons eventually yields photochemical haze as known from Titan, Pluto seems to be more favored to form Titan-like hazes than Triton does.

Water delivery by interplanetary dust particles (IDPs) might be an alternative factor that causes the different haze formation processes. In the outer solar system relevant to Triton and Pluto, IDPs are mostly coming from Edgeworth Kuiper belts and Oort Cloud comets. Based on dust dynamics simulations, Poppe & Horanyi suggested that meteoroidal water influx on Triton is two orders of magnitude higher than that on Pluto because of strong gravitational acceleration and focusing by Neptune’s gravity field. The estimated H2O mass flux is 1.8 × 10¯17 g cm¯21, which is about two orders of magnitude lower than the total haze mass flux suggested in this study. However, the deposited water can influence the photochemistry of hydrocarbons; for example, photolysis of H2O produces OH radicals that eventually form CO through reactions with carbon-based molecules. It would be interesting to study how the IDPs’ water delivery may affect hydrocarbon photochemistry and subsequent haze formation.

Reference: Kazumasa Ohno, Xi Zhang, Ryo Tazaki, Satoshi Okuzumi, “Haze Formation on Triton”, pp. 1-29, ArXiv, 2021.

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