true polar wander,” a theory that Europa’s outer ice shell is essentially free-floating and moves.
“True polar wander occurs if Europa’s icy shell is decoupled from its rocky interior, resulting in high stress levels on the shell, which lead to predictable fracture patterns,” said Candy Hansen, a Juno co-investigator who leads planning for JunoCam at the Planetary Science Institute in Tucson, Arizona. “This is the first time that these fracture patterns have been mapped in the southern hemisphere, suggesting that true polar wander’s effect on Europa’s surface geology is more extensive than previously identified.”
The high-resolution JunoCam imagery has also been used to reclassify a formerly prominent surface feature from the Europa map.
“Crater Gwern is no more,” said Hansen. “What was once thought to be a 13-mile-wide impact crater — one of Europa’s few documented impact craters — Gwern was revealed in JunoCam data to be a set of intersecting ridges that created an oval shadow.”
The Platypus
Although all five Europa images from Juno are high-resolution, the image from the spacecraft’s black-and-white SRU offers the most detail. Designed to detect dim stars for navigation purposes, the SRU is sensitive to low light. To avoid over-illumination in the image, the team used the camera to snap the nightside of Europa while it was lit only by sunlight scattered off Jupiter (a phenomenon called “Jupiter-shine”).
This innovative approach to imaging allowed complex surface features to stand out, revealing intricate networks of cross-cutting ridges and dark stains from potential plumes of water vapor. One intriguing feature, which covers an area 23 miles by 42 miles (37 kilometers by 67 kilometers), was nicknamed by the team “the Platypus” because of its shape.
Characterized by chaotic terrain with hummocks, prominent ridges, and dark reddish-brown material, the Platypus is the youngest feature in its neighborhood. Its northern “torso” and southern “bill” — connected by a fractured “neck” formation — interrupt the surrounding terrain with a lumpy matrix material containing numerous ice blocks that are 0.6 to 4.3 miles (1 to 7 kilometers) wide. Ridge formations collapse into the feature at the edges of the Platypus.
For the Juno team, these formations support the idea that Europa’s ice shell may give way in locations where pockets of briny water from the subsurface ocean are present beneath the surface.
About 31 miles (50 kilometers) north of the Platypus is a set of double ridges flanked by dark stains similar to features found elsewhere on Europa that scientists have hypothesized to be cryovolcanic plume deposits.
“These features hint at present-day surface activity and the presence of subsurface liquid water on Europa,” said Heidi Becker, lead co-investigator for the SRU at NASA’s Jet Propulsion Laboratory in Southern California, which also manages the mission. “The SRU’s image is a high-quality baseline for specific places NASA’s Europa Clipper mission and ESA’s (DOI: 10.3847/PSJ/ad24f4
Reference: “A Complex Region of Europa’s Surface With Hints of Recent Activity Revealed by Juno’s Stellar Reference Unit” by Heidi N. Becker, Jonathan I. Lunine, Paul M. Schenk, Meghan M. Florence, Martin J. Brennan, Candice J. Hansen, Yasmina M. Martos, Scott J. Bolton and James W. Alexander, 22 December 2023, Journal of Geophysical Research: Planets.
DOI: 10.1029/2023JE008105