Astronomy Picture of the Day: Oceans inside icy moons managed to "survive" giant collisions!

 

#NASA, #Space, #astronomy, #διάστημα

Black Hole

Subsurface oceans on icy worlds have proven more resilient to cosmic catastrophes than one might think. Computer simulations show that they can persist even after a devastating collision—though certainly not on every moon. ⠀ Many moons of the Solar System's giant planets have undergone major collisions. Scientists hypothesize, for instance, that Uranus's moon system may have been drastically altered by the giant impact that tilted the planet itself. Saturn's moons, meanwhile, may have been destroyed and subsequently reassembled from the resulting debris. Yet, some of these icy bodies—including Mimas, Enceladus, Dione, Miranda, and Ariel—may harbor subsurface oceans of liquid water. But what exactly happens to the ocean if its icy moon survives a catastrophic collision? Theoretically, the heat generated by the impact could melt the ice. However, such a catastrophe simultaneously alters the moon's internal structure, allowing it to lose heat more rapidly. To answer this question, Marc Neveu of NASA's Goddard Space Flight Center and his colleagues combined collision models with calculations regarding the long-term thermal evolution of moons. To do this, the researchers examined icy moons of two different sizes. In the first scenario, a moon with a radius of 500 kilometers collided with a body half its size. In the second, a larger moon with a radius of 1,000 kilometers was struck by a body with a radius of 500 kilometers. The scientists then tracked how the moon—reassembled from debris—evolved over billions of years following the impact and compared it to a similar moon that had avoided such a catastrophe. ⠀ It turned out that size was the decisive factor: within a large moon, the ocean survived the collision. Moreover, the heat generated by the impact melted a significant portion of the icy shell, thereby increasing the volume of liquid water. Such a vast ocean persisted for approximately two billion years. The situation is different for smaller moons. A collision altered their internal structure: rocky material sank into the interior, while water was pushed closer to the cold surface, where it froze more rapidly. Consequently, an ocean that might have persisted for about a billion years in the absence of a collision failed to form at all following the impact. Scientists also investigated whether a collision alone could transform a completely frozen satellite into an ocean world. This did not occur in any of the models. Even a slight orbital shift following the impact did not help; the associated tidal heating proved insufficient to melt the ice. Thus, collisions alone could not create an ocean where none had existed before. Their consequences depended on the moon's size: for large satellites, the additional heat helped an existing ocean persist longer, whereas for small moons, the restructuring of the interior prevented an ocean from forming. The study's findings have been published in the journal *Nature Astronomy*. Researchers proposed testing this scenario using specific satellites of gas giants. One candidate is Rhea, a moon of Saturn with a radius of approximately 760 kilometers. According to one theory, this celestial body may have formed from debris following a massive collision. Modeling such an impact will reveal whether it could have heated the moon's interior sufficiently to trigger the formation of a subsurface ocean.