Charon's refractory factory

Abstract

We combine novel laboratory experiments and exospheric modeling to reveal that "dynamic" Ly-α photolysis of Plutonian methane generates a photolytic refractory distribution on Charon that increases with latitude, consistent with poleward darkening observed in the New Horizons images. The flux ratio of the condensing methane to the interplanetary medium Ly-α photons, φ, controls the distribution and composition of Charon's photoproducts. Mid-latitude regions are likely to host complex refractories emerging from low-φ photolysis, while high-φ photolysis at the polar zones primarily generate ethane. However, ethane being colorless does not contribute to the reddish polar hue. Solar wind radiolysis of Ly-α-cooked polar frost past spring sunrise may synthesize increasingly complex, redder refractories responsible for the unique albedo on this enigmatic moon.

Fig. 1.. Dynamic photolysis at Charon.

(A) Charon’s MVIC color image showing the distinct red polar cap region named Mordor Macula nearly circumscribed by the 75° latitude annulus in the sunlit northern hemisphere. The red color is attributed to tholin-like refractories resulting from the photolysis of Plutonian methane by the IPM Ly-α photons (3). Following arrival from Pluto, CH₄ molecules hop in random gravitationally bound trajectories (blue curves) across Charon’s surface until they are trapped in ultracold locations at polar latitudes in the winter hemisphere. CH₄ molecules are dissociated by the isotropic IPM Ly-α photons (orange arrows) as they accrete atop Charon’s cold polar surface, exemplified in the extruded zone. (B) Simplified illustration of the dynamic photolysis process that converts methane into refractories in the PAL during accretion at cold Charon polar locations, also reproduced in our laboratory experiments. The refractory fraction RF in films photolyzed during methane loading strongly depends on φ, the flux ratio of CH₄ to Ly-α photons. The photolyzed mixture (R + unprocessed CH₄) continues to accumulate on top of Charon’s subsurface until spring equinox when the winter pole emerges into sunlight. The surface temperature rises desorbing the unprocessed methane while leaving the less volatile refractories behind on the polar surface. QCM, quartz crystal microbalance.

Fig. 2.. Charon’s seasonally varying φ maps.

Modeled pole-centric maps of methane cold-trapping rate (A), and φ ratio, obtained by dividing the cold-trapping rate by the isotropic IPM Ly-α flux (B) over Charon’s winter hemisphere. The exosphere simulations track CH₄ molecules as they ballistically migrate to polar “cryotraps” following initial impact (9). Seasonal variations in methane escape from Pluto’s atmosphere are ignored in these simulations. Both CH₄ accretion rate and φ are time-averaged for several Pluto orbits at present-day precession and for an orbit with a ±90° offset in the solar longitude of perihelion relative to the present-day angle. Only the winter segment of the 248-year orbit where methane accretion rate values are positive is considered in the time average. Also shown are snapshot φ maps at autumn equinox (C) and winter solstice (D). The temperature in the polar zone >70° plummets quickly below 30 K as the hemisphere recedes into winter night to capture most of the methane desorbing from the opposite spring pole. This leads to rapid accretion of methane-rich polar cap, ~30 μm thick, within a year following autumn equinox. Methane from Pluto continues to accrete onto the cold pole forming a thinner (~0.3 μm) layer over thick equinotical frost over the 124-year winter (E). Very little methane is photolyzed during this high-φ accretion as indicated by the red dots in (E). More complex species result during low-φ photolysis indicated by multicolored dots present in the winter frost layer. Charon surface temperatures are modeled for thermal inertia of 10 J m⁻² K⁻¹ s^−1/2 and albedo and emissivity values of 0.3 and 0.9, respectively. The annuli (dotted circles) are latitudes in 15° increments, while dotted lines are longitudes in 45° increments referenced from the prime meridian (solid line).

Fig. 3.. Refractory fraction RF synthesized in dynamically photolyzed CH₄ films.

Mass loss during controlled thermal desorption of Ly-α processed methane films photolyzed at different φ values are shown in the top panel. The mass loss is normalized to total methane mass. The first decrease in film mass at ~40 K is due to desorption of the methane molecules unaffected by Ly-α photons. The second decrease at ~65 K is from C₂H₆ desorption. The mass fraction retained at 50 K (top, arrows) belongs to photolyzed refractories more complex than methane. We plot the refractory fraction against φ (black circles) in the bottom panel. Methane films photolyzed at higher φ contain fewer refractories. The solid black curve is a model fit (Eq. 2B) to the RF versus φ data, which gives the methane-to-refractory conversion cross section σ_{M → R}. The solid blue curve shows the dependence of the refractory production rate (RPR) on φ. RPR, which is RF × F_M, increases steadily with φ; the red dash indicates a maximum conversion rate.

Fig. 4.. Photoproduct distribution on Charon imprinted by IPM Lyman-α.

(A). Pole-centric view of the time- and precession-averaged refractory abundance (per orbit), which steadily increases with latitude to a polar maximum of ~80 ML. The spatial dependence of the refractory abundance is remarkably consistent with the latitudinal gradient of the albedo captured in the MVIC + LORRI composite map [from Schenk et al. (47)] shown in (B). The abundance, normalized to the polar maximum, is compared to 1 − R in (C). R is the reflectance ratio [Figure 2D of the study of Grundy et al. (3)] obtained by dividing latitudinally averaged brightness in the observed image by a modeled image assuming a uniform albedo.

Fig. 5.. Infrared spectra of dynamically processed films.

The top panel shows that a diverse, complex family of heavier hydrocarbons are synthesized in conditions of low φ. In contrast, high-φ photolysis result in formation of mainly ethane (bottom). These films were photolyzed at 10 K. Ethane therefore would be the dominant constituent of the polar refractories, while Charon’s lower latitudes would host increasingly complex refractories. Detailed assignments of absorption bands between 3 and 3.6 μm are given by Carrascosa et al. (18) and reference therein.