冥王星の海を覗く(Peering into Pluto’s ocean)

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2024-05-21 ワシントン大学セントルイス校

Pluto
In a paper published in the journal Icarus, WashU graduate student Alex Nguyen used mathematical models and images from the New Horizons spacecraft to take a closer look at the ocean that likely covers Pluto beneath a thick shell of nitrogen, methane and water ice. (Image: NASA)

ワシントン大学の大学院生アレックス・グエンは、冥王星の氷の下にある液体の海について新たな計算を行いました。2015年に冥王星を通過したニュー・ホライズンズ探査機の画像と数学モデルを用い、グエンと共著者のパトリック・マクガヴァンは、冥王星の表面の割れ目や隆起を説明するモデルを作成しました。彼らの計算によると、冥王星の海は40~80kmの氷の層の下にあり、氷が内部の海を凍結から守っています。この海の密度は地球の海水の最大8%増しで、ユタ州のグレートソルト湖に似ています。この密度が冥王星の表面の割れ目の多さを説明します。

<関連情報>

冥王星の海がスプートニク平原流域の窒素氷を支える塩分濃度の役割 The role of Pluto’s ocean’s salinity in supporting nitrogen ice loads within the Sputnik Planitia basin

P.J. McGovern, A.L. Nguyen
Icarus  Available online 28 January 2024
DOI:https://doi.org/10.1016/j.icarus.2024.115968

Highlights

•The density of Pluto’s interior ocean affects the stress state in the icy shell above.

•The Sputnik Planitia impact basin is filled with nitrogen ice, stressing the shell.

•Ocean density (salinity) affects the response of the shell to loads.

•Models with high density predict stress states inconsistent with observed faulting.

•The density of Pluto’s ocean is likely <1100 kg/m3.

Abstract

The existence of a subsurface water ocean on Pluto has been inferred on numerous grounds. The characteristics of such an ocean are vital inputs for various types of models that are carried out to illuminate the interior structure and evolution of Pluto, especially in light of the complementary relationship between the dimensions of the ocean and an outer water ice shell. In particular, models of loading of the Sputnik Planitia (SP) impact basin by nitrogen ice infill require knowledge of ocean properties such as density, and previous studies did not consider the potential densification of Pluto’s ocean from salinity. Here we carry out analytic and Finite Element Method (FEM) modeling of loading deformations and stresses in an ice shell lithosphere, to determine the conditions, including ocean density, that best account for the observed distributions of tectonic features around the SP basin. Our results, while generally consistent with those of previous studies, provide more complete constraints on the initial basin depth (no deeper than 6 km, with most likely values in the 3–4 km range). Further, we find that increasing density (salinity) generally increases disagreement between model tectonic predictions and observations, and therefore we infer modest salinity-based densification of Pluto’s ocean (ρocean < 1100 kg/m3) relative to values seen or inferred on other worlds.

1701物理及び化学
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