2022-12-15 テキサス大学オースチン校(UT Austin)
テキサス大学オースティン校の研究者は、彗星や小惑星が厚さ数十キロと推定される氷の殻に衝突した後の様子を観察するコンピュータ・モデルを開発した。このモデルによると、もし衝突が衛星の氷の殻の少なくとも半分まで到達することができれば、熱せられた融解水が残りの氷を突き破って沈み、生命に必要な化学物質である酸化剤が表面から海へと運ばれ、保護された海域で生命が維持される可能性があることが分かった。この研究は、『Geophysical Research Letters』に掲載された。
科学者たちはエウロパの表面に何十ものクレーターを発見しており、その多くは、凍った雪解け水とクレーターの下での衝突後の動きを示唆する、はっきりとした波紋のような外観をしている。
＜関連情報＞
- https://news.utexas.edu/2022/12/15/comet-impacts-could-bring-ingredients-for-life-to-europas-ocean/
- https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022GL100287
エウロパにおける衝撃で生成されたメルトチャンバーの沈降による地表-海洋間の交換現象 Surface-To-Ocean Exchange by the Sinking of Impact Generated Melt Chambers on Europa
Evan Carnahan, Steven D. Vance, Rónadh Cox, Marc A. Hesse
Geophysical Research Letters Published: 28 November 2022
DOI:https://doi.org/10.1029/2022GL100287
Abstract
Impacts into icy bodies often generate near-surface melt chambers and thermal perturbations that soften the ice. We explore the post-impact evolution of non-penetrating impacts into Europa’s ice shell. Simulations of viscous ice deformation show that dense impact melts founder before refreezing. If the transient cavity depth exceeds half the ice shell thickness, over 40% of the impact melt drains into the underlying ocean. Drainage of impact melts from the near-surface to the ocean occurs on timescales of 103–104 years. The drainage of melts to the ocean occurs for all plausible ice shell thicknesses and ice viscosities, suggesting that melt foundering is a natural consequence of impacts on icy worlds. Post-impact viscous deformation is an important process on icy worlds that affects cryovolcanism, likely modifies crater morphology, creates porous columns through the ice for surface-to-ocean exchange, and may supply the oxidants required for habitability to subsurface oceans.
Key Points
- Impacts into ice shells generate melt chambers that viciously deform and sink through the ice shell, likely modifying crater morphologies
- If transient cavity depth exceeds half the ice thickness, impact melts drain to the ocean and may deliver oxidants needed for habitability
- Draining impact melts form a stable porous channel that connects the ocean to near-surface and may allow for the escape of ocean fluids
Plain Language Summary
Europa is an ocean world with an outer ice shell in our Solar System. Outer ice shells on ocean worlds serve as both a barrier and facilitator for internal ocean habitability and the detectability of its potential inhabitants. A prominent feature we observe on the surface of ice shells is highly variable craters from impacts that indent but do not penetrate the ice shell. Here, we find that many of the impacts into Europa’s ice shell would have generated large near-surface melt chambers, which rapidly drain into the underlying ocean and form a continuous surface-to-ocean melt column. This melt drainage occurs with any entrained surface materials and provides a consistent means of exchange between the surface and ocean, which likely increases the habitability of internal oceans. Furthermore, this drainage displaces large volumes of water from below impact craters, on the order of 10’s of cubic kilometers. This large scale drainage may help to explain the wide range of collapsed, enigmatic crater morphologies observed on Europa, and other ocean worlds.
