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Are the Surface Textures of Pluto’s Wright Mons and its Surroundings Exogenic?

Presentation #105.05 in the session “Pluto System: Geology”.

Published onOct 26, 2020
Are the Surface Textures of Pluto’s Wright Mons and its Surroundings Exogenic?

The Wright Mons region of Pluto remains one of the most intriguing and enigmatic landscapes on the planet. Wright Mons is a large mountain ~150 km across, rises ~4 km above its surroundings, sports a large central depression, and was tentatively assigned a cryovolcanic origin immediately following the New Horizons encounter in 2015. But the absence of obvious lateral flow features and the lack of unambiguously diagnostic characteristics of calderas or vents within the central depression have challenged the development of specific formation hypotheses. The pervasive cover of pillow-like hummocks, typically 7-20 km across, on the flanks and surroundings of Wright Mons have naturally figured into various working explanations for the formation of this region. Individual hummocks themselves are covered by a km-scale blocky texture. The similarity in size and shape of the convex hummocks and the style of their organization, such as forming a vaguely concentric fabric near the summit, seem to lend themselves to an endogenic origin.

An alternative working hypothesis for the development of the hummocky terrain is that it represents accretion of volatiles from atmospheric precipitation. The closely packed dome-like structures form a botryoidal (or globular) texture that is similar to the convex outward crystalline deposits seen on cave walls. These form by uniform surface-normal accretion, whereby radial growth at the same rate around multiple nuclei causes individual formations to abut and merge with neighboring ones, creating convex outward bulges separated by narrow, deep seams. In this hypothesis, the formation process of the hummocks is comparable to that which has been proposed for the Bladed Terrain deposits in Pluto’s equatorial region.

The accretion hypothesis must explain the source of the volatiles, however, and why they deposited in the locations where they are observed. The methane ice of the bladed terrain deposits preferentially precipitated at high altitude (where nitrogen ice cannot precipitate), and is concentrated within the equatorial diurnal zone to the east of Sputnik Planitia. The hummocky terrain of Wright Mons and its vicinity, however, is much more localized relative to the bladed terrain deposits, being concentrated at the southern end of Sputnik Planitia and extending across an elevation range of 7 km. This distribution would imply that if the hummocky terrain does have an accretional origin, it likely does not stem from precipitation of volatiles as governed by global climate dynamics, but rather from precipitation of locally sourced volatiles akin to fumarolic activity.

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