What a pleasure to see new images from JunoCam, the visible-light camera aboard the Juno spacecraft that has now imaged in its peregrinations around Jupiter the surface of its most interesting moon. Our probing of Europa’s secrets has depended heavily upon the imagery returned by the Galileo spacecraft. That mission made its last flyby in 2000, and we have another wait while ESA’s Juice mission and Europa Clipper make the journey, the former enroute, the latter scheduled for an October launch.

Juno’s 2022 flyby thus gave us a helpful visual update, one that is complemented by an informative snapshot taken by the spacecraft’s Stellar Reference Unit (SRU) star camera. While we have five high resolution images to work with, the Stellar Reference Unit’s black-and-white image has produced the most detail. The image is intriguing because of its method, for bear in mind that the SRU is designed to track stars for navigation purposes. That makes it a dim light instrument, one that must be handled carefully to avoid washing out the image. The Juno team used it on Europa’s nightside, where the ambient light was sunlight reflected off Jupiter itself and the Sun was safely hidden.

Image: From Juno’s SRU, this image shows the location of a double ridge running east-west (blue box) with possible plume stains and the chaos feature the team calls ‘the Platypus” (orange box). These features hint at current surface activity and the presence of subsurface liquid water on the icy Jovian moon. Credit: NASA/JPL-Caltech/SwRI.

What emerges is a jumble of chaotic terrain cut by ridges and laden with a reddish-brown material familiar from Galileo imagery of the moon. These dark stains have been hypothesized to be the deposits of cryovolcanic plumes. Amidst this terrain, a new feature emerges that interrupts different forms of terrain. The Juno team has christened it the Platypus. Here the ridge topography breaks down as it encounters what is clearly younger material laden with ice blocks, a disrupted area that is some 37 kilometers by 67 kilometers in size. A double ridge line north of the Platypus is also apparent, the complex terrain suggesting the kind of surface change that researchers believe may allow ocean water to come close to the surface in isolated pockets.

The mention of plumes is intriguing because of the possibility of one day collecting samples from a spacecraft during a flyby, although no plumes are evident in the Juno imagery. Both the Platypus and the double ridges suggest recent activity. On the possibility of plumes, the SRU paper notes:

Diffuse discontinuous low-albedo deposits flank double ridges ∼50 km north of the “Platypus” chaos margin, extending radially outward from the lineaments. The morphology of these deposits is similar to features observed elsewhere on Europa that have been associated with hypothesized plume activity, the discontinuous low-albedo spots flanking Rhadamanthys Linea being a prominent example (Quick & Hedman, 2020). Quick and Hedman (2020) surmise that 1–10 m thick deposits can be emplaced by sufficiently compact plumes and detected by high-resolution visible wavelength cameras. The radii of the deposits observed by the SRU are ∼2–5 km, which Quick & Hedman’s models associate with <10 km high plumes.

We can also compare the Juno imagery with that of Galileo, as the JunoCam paper does:

The number of documented craters larger than 1 km on Europa has gone from 41 to 40 craters. Careful comparisons of the JunoCam images with overlapping images from Galileo show no surface changes due to plume deposits or ongoing geologic activity over time intervals of 23–26 yr, though admittedly the images are not well matched in resolution, viewing geometry, and wavelength. No active eruptions were detected. Finally, from the Europa data set taken on 2022 February 24, we can say that the north polar cap of Europa at this image scale looks similar to lower latitudes.

It’s worth adding here that a recent search using the Atacama Large Millimeter/submillimeter Array (ALMA) collected data over four days to examine the moon’s entire surface, coming up with no evidence of plume activity. We’re clearly not dealing with a geyser phenomenon anywhere as active as what we find at Enceladus, and thus far evidence from the Hubble instrument has been the most compelling, but even the data from its 2013 observations remain at the edge of detection. Clearly the search for active plumes will continue given their exciting implications.

Meanwhile, evidence for surface activity of other kinds on Europa continues to emerge, presenting new targets for Europa Clipper as well as Juice. Juice (Jupiter Icy Moons Explorer) launched on April 14, 2023 and will arrive in July of 2031, while Europa Clipper is scheduled to reach the giant planet in April of 2030. The new imagery suggests that Europa’s outer ice shell moves freely over the ocean (“true polar wander”), capturing steep depressions up to 50 kilometers wide near the equator. These ovoid features are similar to those found in other parts of Europa. Candy Hansen, who leads JunoCam planning at the Planetary Science Institute in Tucson, AZ, notes their relevance:

“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. 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 landscape of ice blocks and troughs near Europa’s equator broken by depressions tells a tale that must be interpreted in terms of light and shadow. The feature called Gwern, for example, an apparent impact crater found in Galileo imagery, turns out under different lighting to be nothing more than an oval shadow caused by the intersection of prominent ridges. Cross-cut ridges and the dark stains that may mark the residue from ancient (or recent) plumes offer a compelling landscape. New features like the Platypus will get a particularly hard look from our incoming spacecraft.

Image: Jupiter’s moon Europa was captured by the JunoCam instrument aboard NASA’s Juno spacecraft during the mission’s close flyby on Sept. 29, 2022. The images show the fractures, ridges, and bands that crisscross the moon’s surface. Credit: Image data: NASA/JPL-Caltech/SwRI/MSSS. Image processing: Björn Jónsson (CC BY 3.0).

The SRU paper is Becker et al., “A Complex Region of Europa’s Surface With Hints of Recent Activity Revealed by Juno’s Stellar Reference Unit,” JGR Planets 22 December 2023 (full text). The paper on the JunoCam imagery is Hansen, “Juno’s JunoCam Images of Europa,” Planetary Science Journal Vol. 5, No. 3 (21 March 2024), 76. Full text. The paper on the ALMA observations is Cordiner et al., “ALMA Spectroscopy of Europa: A Search for Active Plumes,” submitted to IAU Symposium 383 conference proceedings (preprint).