On the icy Enceladus, cracks led to the boiling of the internal ocean

(ORDO NEWS) — In 2006, the Cassini spacecraft recorded curtains of geysers erupting from cracks near the south pole of Saturn’s moon Enceladus sometimes up to 200 kg of water per second.

A new study shows how expanding ice during millennia-long cooling cycles can crack open the moon’s icy shell and release its inner ocean, providing a possible explanation for the geysers.

The diameter of Enceladus is about 504 kilometers. The moon is covered in ice 20-30 kilometers thick and the surface temperature is about -201 Celsius (-330 Fahrenheit), but decades of data from NASA’s Cassini-Huygens mission have provided evidence of a deep ocean inside the ice shell escaping into space through continuous “cryovolcanism”. “. How such a small, cold world can support such violent geological activity remains a mystery to scientists.

“This has captured the attention of both scientists and the general public,” said Max Rudolf, assistant professor of geophysics at the University of California, Davis and lead author of the new study.

The scientists used a physics-based model to map conditions that could allow cracks from the surface to reach the ocean and trigger eruptions. The model takes into account cycles of warming and cooling, which last on a scale of hundreds of millions of years and are associated with changes in the orbit of Enceladus around Saturn.

During each cycle, the ice shell undergoes periods of thinning and thickening. According to Rudolf, the thickening occurs due to the freezing of the base of the ice shell, which grows downwards, like ice on a lake.

The pressure exerted by downward expanding ice on the ocean below is one of the possible mechanisms researchers have proposed to explain the geysers of Enceladus.

As the outer ice shell cools and thickens, the pressure on the ocean underneath increases because the ice has more volume than water. The rising pressure also creates stresses in the ice, which can become pathways for fluid to reach the surface at a distance of 20 to 30 kilometers.

A new study has found that ocean pressure would be enough to cause tiger-stripe cracks to form on the surface of Enceladus. But the pressure will never be large enough to push water to the surface when both ocean pressure and thermal contraction are taken into account, they found, excluding this explanation of geysers.

“The proposed model could explain the formation of an initial fissure that could lead to the formation of multiple fissures (tiger stripes) at the south pole of Enceladus,” said Miki Nakajima, assistant professor of astronomy at the University of Rochester.

According to Rudolf, the mechanism first proposed by Nakajima and Andrew Ingersoll in a 2016 study could explain the eruptions. These researchers suggested that water entering the cracks is exposed to space (Enceladus has no atmosphere) and spontaneously boils when it enters a vacuum.

This is consistent with the appearance of the surface of Enceladus, which shows no evidence of cryo lava flows seeping from cracks in the surface.

Meanwhile, some evidence suggests that Jupiter’s moon Europa, another icy world the size of Earth’s moon, could also be experiencing similar eruptions, although less is known about the activity taking place there.

This mechanism of ocean pressure and spontaneous eruption cannot explain the cryovolcanism that may occur on Europa.

Further research and observations of this moon are needed to determine the potential causes of eruptions. Rudolf is looking forward to the Europa Clipper mission, for which NASA is assembling a spacecraft to learn more about the geological processes on Europa.

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