Volatile Deposition in East Tombaugh Regio: What are the Origins of Pluto's Cold Heart ?

Introduction: East Tombaugh Regio is a bright highland region located east of Sputnik Planitia. Together, these regions form the iconic heart-shaped feature in Pluto’s equatorial zone (Fig. 1). East Tombaugh Regio is interpreted as a relatively high-elevation region with arcuate and bladed terrain deposits [1,2]. The region seems to be currently experiencing mantling by a continuous veneer of volatile ices, with nitrogen-rich ice accumulating in local depressions forming smooth plains and methane-rich deposits appearing on more elevated surfaces [1-5].  The region is geologically young (<300 Ma based on crater counts) and exhibits a remarkably high reflectivity, with a bolometric albedo exceeding 0.95, comparable to that of Sputnik Planitia [6,7]. While nitrogen ice is known to be stable within the deep depressions of this region [8], the origin and mechanism of the methane-rich mantling remain unclear. This is especially puzzling given that the adjacent methane-rich bladed terrains (at similar latitudes) are significantly darker (albedo ~0.5–0.6) and appear to be shaped primarily by sublimation processes [1,2,5].

The timing of bright deposits formation in East Tombaugh Regio also remains poorly constrained. A general brightening of both low and high areas of the surface is known to produce a strong albedo feedback effect [9], likely sufficient to induce yet further methane and nitrogen ice deposition in this region.  In this way, the bright deposits may have been established themselves as a permanent feature since soon after emplacement of the bladed terrain deposits [1,2]. 

 

Science question: Why is East Tombaugh Regio dominated by CH₄ condensation considering that the CH₄-rich bladed terrains seem to be dominated by sublimation and that Cthulhu is overall depleted in volatile ice ?

 

Method: We use a new version of the Pluto Planetary Climate Model to simulate Pluto as observed in 2015. The PCM takes into account the sublimation and condensation cycles of N₂, CH₄, and CO, their thermal and dynamical effects, cloud formation, vertical turbulent mixing, molecular thermal conduction, and a detailed surface thermal model with different thermal inertia for various timescales (diurnal, seasonal). The PCM includes the latest bolometric albedo map [7] and topography data [6]. Perennial CH₄ deposits were added on the sub-Charon side of Pluto (covered by low-resolution New Horizons imaging) wherever terrain with diagnostic characteristics of the bladed terrain was detected.

 

The initial state of the simulation was obtained for Earth year 1984 from a 30-million-year simulation performed with the Pluto volatile transport model, thus allowing a steady state for ice distribution, surface and soil temperatures to be reached. We used a high resolution of 2.5° in latitude and 3.75° in longitude (i.e. ~50 km).

Figure 1. Left: Part of the mosaic color map of Pluto. North is up; Pluto’s equator roughly bisects the band of dark red terrains running across the map. Sputnik Planitia glacier is at the center of this map (NASA/JHAPL/SwRI). Middle: Map of East Tombaugh Regio showing methane frost forming in the PCM in 1 Earth year (2015). Right: Map of the nighttime difference CH₄ mixing ratio (Q_CH4) minus the saturation mixing ratio (Q_sat), with near surface winds (at 5 m above local surface) superimposed.

 

Preliminary results: In 2015, the bladed terrain deposits were the main source for gaseous CH₄ in the equatorial regions, according to the model. Overall, sublimated CH₄ is transported westward by the winds (Fig. 2) but this transport is also strongly influenced by local topography in the first few hundred meters (particularly by katabatic winds, Fig. 1, right). The model predicts the formation of CH₄-rich frost in East Tombaugh Regio in 2015 (Fig. 1, left), driven by the following mechanisms :

 

1.     Large amounts of gaseous CH₄ are transported near the surface into East Tombaugh Regio, especially onto mountain tops. This is due to (1) the region’s proximity to and alignment with the bladed terrain deposits (a major CH₄ source) and (2) the general circulation carrying sublimated CH₄ westward and downward (Fig. 2).

• As a result, the CH₄ mixing ratio (Q_CH4) exceeds the saturation mixing ratio (Q_sat) during the night (Fig. 1, right), at latitudes where the bright deposits are observed.

 

• Consequently, nighttime condensation occurs in East Tombaugh Regio and in particular at higher elevations (where Q_CH4 is greater) and in areas with enhanced near-surface turbulence.

 

CH₄ condensation is also predicted to occur for a large part of the day between 30°S and 10°S. However, this is inconsistent with observations, which show dark, volatile-free terrains in these regions. This discrepancy may stem from an overestimation of gaseous CH₄ in the model, or from limitations in the representation of albedo feedback, surface properties, and near-surface wind dynamics.

Figure 2. Snapshot of CH₄ mixing ratio transported in the atmosphere in the equatorial regions, as predicted by the PCM in 2015.

Contrasts with the dark-volatile free Cthulhu: As highlighted by Fig. 2, CH₄ deposition is less likely in Cthulhu because upward transport of CH₄ west of Sputnik reduces the amount of near-surface gaseous CH₄ in Cthulhu. Detailed analysis of the dynamics behind this upward transport will be presented at the meeting.

 

Further work: We are currently working on high-resolution simulations of East Tombaugh Regio, extending over a full annual cycle. These simulations include localized N₂ ponding and a more realistic treatment of albedo feedback. In parallel, a more detailed analysis of the atmospheric dynamics is also underway.

 

Acknowledgments: This work was supported by funding from ANR "Programme de Recherche Collaborative" 2024-2028 (ANR-23-CE49-0006).

 

References:

[1] White, O. L., et al. (2021). The Pluto System After New Horizons, 55.

[2] Singer, K. N., et al. (2025). JGR: Planets, 130(1).

[3] Schmitt, B., et al. (2017). Icarus, 287, 229-260.

[4] Protopapa, S., et al. (2017). Icarus, 287, 218-228.

[5] Earle, A. M., et al. (2018). Icarus, 314.

[6] Schenk, P. M., et al. (2018). Icarus, 314, 400-433.

[7] Hofgartner, J. D., et al. (2023). PSJ, 4(7), 132.

[8] Bertrand, T., et al. (2020). Nature communications, 11(1), 5056.

[9] Earle, A. M., et al. (2018). Icarus, 303, 1-9.