New evidence suggests Europa’s icy shell floats on the moon’s subsurface ocean

New evidence suggests Europa's icy shell floats on the moon's subsurface ocean
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During NASA’s Juno spacecraft’s recent flyby of Jupiter’s moon Europa in September 2022, it discovered intriguing findings. Apart from detecting saltwater pockets linked to the deep subsurface ocean, Juno also captured potential signatures of tall plumes of water vapor on camera.

The Juno mission successfully captured a series of impressive images using the JunoCam instrument. These images provide scientists with valuable insights into Europa’s surface, as they were taken from a close distance of only 355 kilometers above the icy moon. The spacecraft also utilized its Stellar Reference Unit (SRU), typically employed for imaging dim stars, to assist Juno in navigation. This time, the SRU’s low-light capabilities have been optimized to capture a stunning image of the night side of Europe. This side is expertly illuminated solely by the light bouncing off of Jupiter’s cloud tops.

“Platypus” in the lower right corner

The SRU stumbled upon a peculiar structure that they affectionately dubbed “Platypus” because of its unique shape. This is what experts would call a chaotic landscape – a combination of ice blocks, ridges, humps, and brownish-red patches. Europa’s surface has been showcasing chaotic landscapes ever since the Voyager missions. Planetary scientists believe that these regions might be where salty liquid emerges, causing the icy crust to partially melt.

These features indicate modern surface activity and the presence of subsurface liquid water on Europa.

— Heidi Becker, SRU Lead Co-Investigator at NASA’s Jet Propulsion Laboratory

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Becker also mentioned that Platypus will be a significant focus for NASA’s Europa Clipper mission, set to launch this year, as well as Europe’s JUICE mission, which is already en route to Jupiter.

Located 50 kilometers north of Platypus, there are even more breathtaking features to behold. These include a remarkable pair of ridges, which are surrounded by intriguing dark patches on the surface. Similar features have been observed on Europa, suggesting they may serve as the origins of water vapor plumes that shoot into space, reaching heights of 200 kilometers.

There has been some controversy surrounding these plumes ever since Hubble first observed them in 2012. However, in contrast to Saturn’s moon Enceladus, where the plumes follow a predictable and regular pattern, the plumes on Europa were found to be less consistent. This inconsistency has led some researchers to question the existence of plumes on Europa. Discovering trenches that resemble the “tiger stripes” on Enceladus, which are the starting points of plumes on that moon, will give the Europa Clipper and JUICE missions specific areas to investigate for plumes.

However, Juno also discovered evidence of the movement of these features and the surface as a whole across Juno’s ocean. Scientists refer to this phenomenon as “true polar wander,” where the poles shift their geographic location as the icy crust floats on a subsurface global ocean.

True polar wander occurs when Europa’s icy shell is separated from its rocky interior, resulting in high levels of stress in the shell, causing predictable cracking patterns.

—Candy Hansen, Juno co-investigator at the Arizona Planetary Science Institute

Juno captured these fracture patterns as steep, irregular depressions ranging in size from 20 to 50 kilometers.

This is the first time these fracture patterns have been mapped in Europe’s southern hemisphere, indicating that the impact of true polar wander on Europa’s surface geology is more extensive than previously thought.

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