Juno’s JunoCam Images of Europa

On 2022 September 29 the Juno spacecraft passed Europa at 355 km, the first close pass since the Galileo flyby in 2000. Juno’s visible-light imager, JunoCam, collected four images, enabling cartographic, topographic, and surface geology analysis. The topography along the terminator is consistent with previously reported features that may indicate true polar wander. A bright band was discovered, and indicates global symmetry in the stress field that forms bright bands on Europa. The named feature Gwern is shown not to be an impact crater. Surface change detection shows no changes in 22 yr, although this is a difficult task considering differences between the JunoCam and Galileo imagers and very different viewing geometries. No active eruptions were detected.


Introduction
The Juno mission to Jupiter launched in 2011 and entered orbit around Jupiter in 2016.Juno's polar elliptical orbit originally did not come close to any of the Galilean moons.As Juno's orbit evolved, the apojove of its 53 days orbit gradually moved south and the perijove (PJ) rotated north.As a result, the inbound portion of the trajectory ellipse started cutting across the orbits of the Galilean moons beginning in 2021 (Hansen et al. 2022).The project names each close flyby as PJnn, with the number (nn) increasing by one each perijove.In June of 2021 the spacecraft made a close pass by Ganymede a few hours before the Jupiter flyby on PJ34 (Hansen et al. 2022;Ravine et al. 2022).The orbit continued to evolve, bringing Juno relatively close to Europa on PJ37 and PJ40 at 81,752 km and 46,922 km altitude, respectively.
On 2022 September 29 the Juno spacecraft made its closest pass by Jupiter's moon Europa a few hours before PJ45.Juno approached Europa from the dark side, with closest approach at 355 km altitude.Four images were acquired from Juno's visible imager JunoCam, starting at 1515 km altitude on the illuminated side.
Mounted on the Juno spacecraft spinning at 2 rpm, JunoCam push-frame images are acquired as the spacecraft rotates and the field of view sweeps across the target (Hansen et al. 2017).The JunoCam field of view is 58°, with 1600 pixels across in the cross-rotation dimension.This wide field of view means that geometric parameters such as phase angle will change substantially from one side of the image to the other.JunoCam has four filters: three broadband red, green, blue (RGB) and one narrowband methane (methane was not used for the Europa flyby).An advantage with the push-frame imager is that all three colors are acquired simultaneously in one wide image.
Due to the very low altitude this pass was rapid, and just a small portion of Europa was imaged at low emission angles.JunoCam acquired four RGB images of Europa with coverage over an area spanning longitudes 15W-80E, centered just north of the equator, as shown in Figures 1 and 2. The first image was acquired near the sub-Jovian point at a phase angle of over 80°, excellent lighting for discerning topography along the terminator.One RGB image was acquired on one spacecraft rotation, extending ∼120°, sufficient to capture Europa across its edges visible to JunoCam.The spacecraft was so close to Europa that the polar areas were hidden by the limb of Europa (see Figure 2).
The images collected between PJ37 and PJ45 are the first well-resolved images of Europa since the New Horizons flyby of 2007, and the images collected just before PJ45 are the first high-resolution images resolving geologic features since the last Galileo images of Europa were acquired in 2000.Objectives for the Juno flyby included close examination of the surface and shallow interior using instruments not available on earlier spacecraft (e.g., Brown et al. 2023;Zhang et al. 2023) and a reexamination by JunoCam of a region not very well observed before (namely portions of the sub-Jovian hemisphere; see Figures 1-4), the subject of this report.Science objectives specifically for JunoCam included improvements in cartography, surface geology, surface change detection, and searching for evidence of active plumes.
The spatial resolution is comparable to or better than previous Galileo coverage, which had a resolution of 1-6 km.A mosaic of the four images, shown in Figure 1, includes the western extent of Annwn Regio (chaos terrain), and the large ringed feature Callanish.Rays from Pwyll are visible on the eastern limb in the fourth image.
Another projection of the images is shown in Figure 2(a).As each image gets larger (lower resolution), we see the added The start time of the first image was 2022-09-29T09:38:05.7, and each subsequent image is obtained 1 minute later than the previous one.The geometric properties of the images are listed in Table 1.
The viewing geometry of the flyby is shown in Figure 3.The close pass began on the nightside, and the closest approach took place at 09:36:28 on the dark side (left globe).The best image for JunoCam began ∼2 minutes later when the illuminated part of Europa came into view (right globe, not to scale).The spacecraft flyby velocity was 23.6 km s −1 .

Image Processing and Cartography
The four JunoCam image observations were acquired over the northern sub-Jovian hemisphere between ∼350°and ∼90°E longitudes.This region was partially covered by Galileo from E25 (ESGLOBAL01) and E4 (ESGLOMAP01) observations at ∼1000 and 1400 m pixel −1 resolutions, respectively, although it should be noted that the E25 observations were particularly affected by radiation noise during image readout.JunoCam was especially useful in bridging a significant gap in prior imaging coverage between ∼30°and 45°E longitudes that was observed by Galileo at only ∼7000 m ground pixel scale, imaged even more poorly by New Horizons and Voyager, thereby fulfilling a cartography objective and improving our understanding of feature locations (though the imaging gap at ∼260°E remains).
The nature of the JunoCam imaging system allowed us to use image-stacking techniques to extract full information content.In this context, "mosaic" refers to the collection of RGB framelets making up one image.Each of the four main mosaics was acquired in three colors.By map-projecting each color framelet into its own mosaic at resolutions 3× better than nominal pixel resolution and summing (or stacking) the red, green, and blue filter submosaics for each observation, we were able to substantially improve feature recognition, thus mitigating or aliasing effects that often otherwise make the nearly ubiquitous narrow lineaments (mostly ridges) across much of Europa's surface in these images appear jagged (Figure 4).These stacked versions of the four JunoCam observations are then recombined with the three-color mosaics to produce full-resolution color mosaics (Figure 5).Thus, image mosaic 1 (Table 1) was map-projected at 330 m pixel −1 (at the equator), although the inherent ground pixel scale was ∼1000 m.While this technique does not increase the inherent resolving power of the images to detect features, it does significantly improve the ability to recognize and characterize many features.
In order to utilize the super-resolution technique to maximum effect (as well as fulfill the search for surface change detection and other mapping objectives) it was necessary to accurately and precisely align the JunoCam framelets to each other and to the existing global control network for Europa using Galileo, Voyager, and New Horizons images (Schenk et al. 2011;Bland et al. 2021).Alignment of the JunoCam framelets was achieved using standard Integrated Software for Imagers and Spectrometers (ISIS) qnet and jigsaw tools, in which match points are identified linking surface features in the overlapping images and then adjusting the associated camera pointing kernels until the least-squares residuals are minimized.The four JunoCam and overlapping Galileo image sets were then map-projected and ratioed.Any misalignment of the overlapping mosaics revealed by the ratio products as severe contrast effects were then corrected by adding or editing match points in those areas.This process was repeated until no major misalignments could be identified.

Pits, Ridges, and Terrains along the Terminator
An important set of results comes from looking along the terminator zone between ∼345°and 360°E, shown in Figure 6(a).JunoCam imaging here is at much lower incidence angles than existing Voyager or Galileo (E25 and E4) observations, allowing easier identification of blocks, troughs, and depressions in this terminator region.The most prominent topographic features are a set of irregularly distributed, 20-50 km wide, ovoid, relatively steep-walled depressions from ∼37°N to 11°S.These are morphologically similar to the large pits observed elsewhere on Europa in association with true polar wander (TPW) described below (Schenk et al. 2009), but are the first to be mapped in this hemisphere.
The four JunoCam mosaics have a maximum parallax angle difference of ∼22°.This is sufficient to resolve vertical features on the Z-scale of ∼500 m.However, because most of the surface of Europa has relief of <500 m (P.Schenk & F. Nimmo 2023, in preparation), no geologic features are resolvable in the stereogrammetric digital elevation models processed to date.No large domical features such as the one mapped on Ganymede at the sub-Jovian point (Ravine et al. 2022;P. Schenk & F. Nimmo 2023, in preparation) have as yet been identified.The ovoid depressions are also not resolved in the JunoCam stereo, but shape from shading (and attendant shadow lengths) indicate depths for most of up to 500 m, consistent with prior depression measurements (Singer et al. 2021).The largest pit at 28°355°E has a depth of ∼900 m, similar to the depths of irregular steep-walled depressions associated with TPW (Schenk et al. 2009(Schenk et al. , 2020)).Even the super-resolution version of mosaic 1 is not sufficient to resolve whether structural deformation occurred within these depressions.Mapping of these features in the first image, which also includes a previously unrecognized arcuate trough, reveals that most occur along the extension of one of the outer rings of the concentric TPW pattern (i.e., the reorientation of a floating ice shell with respect to the "fixed" rotation axis; Schenk et al. 2020), suggesting that many depressions observed elsewhere on Europa may be part of this pattern (Figure 6(b)).The observed size range of these pits are all larger than those observed globally by Singer et al. (2021), which are in the <5-20 km range, but are in the range of those large pits observed in geometric association with TPW (Schenk et al. 2009(Schenk et al. , 2020)).This supports a TPW origin for the observed large pits, though we note that this is not a systematic survey of all pits in the mapping area, especially smaller pits, which were mapped at image pixel scales of ∼200-250 m by Singer et al. (2021) and are not reliably identified at JunoCam pixel scales.Full mapping of pits and other TPW features is limited by the fact that the terminator mapping, including Voyager, Galileo, JunoCam, covers only <25% of Europa's surface.Figure 5. Full-resolution RGB cylindrical map projection of Europa using the mosaic 1 imaging data from JunoCam.This projection was processed using the "superresolution" technique as described in the text.While the JunoCam colors are reasonable approximations to the human color range, each color band here has been given an optimum contrast stretch to maximize color and brightness contrast.

Filling in the Geologic Map
With the increase in spatial resolution between the Galileo images and the JunoCam images in the region imaged (Figure 7), we expect to see more detail on the surface of Europa.This enhanced level of detail allows for further identification of linear features and bright bands, and notable improvements in our identification of some chaos morphology.In addition to the resolution change, these new observations are The large difference in lighting conditions, particularly where the JunoCam image is near the terminator, causes the imaged terrain to appear significantly different.A high incidence angle highlights texture and topography over apparent albedo differences, whereas a low incidence angle highlights apparent albedo differences and causes topography and texture to appear less conspicuous.The global geologic map was created with the Galileo images where the incidence angle was low, and so the units were mapped primarily by apparent albedo (or relative brightness).Now, with the addition of the high-incidence-angle JunoCam images, we are able to see where the texture and topography line up with what was assumed in the lowerresolution and low-incidence-angle images.Because of the high incidence angle, the knobby texture of chaos terrain particularly jumps out (Figures 6(a) and 7).The southeastern portion of the previously identified chaos terrain is now uncertain, because this region does not contain the chaos-like texture, even though it has the chaos-like low relative brightness.Due to the high incidence angle of the JunoCam images, we would expect the texture of the chaos terrain to be apparent as in the example to the north, but this is not the case in the south.This terrain, previously identified as chaos based on its relative albedo, could still be chaos terrain where the texture is smoother or unresolved, but it could also be a case of "smooth plains"-a terrain type not previously identified at the global scale, but previously defined by regional mapping in other locations on Europa (e.g., Prockter & Schenk 2005).
JunoCam data improved the resolution of the image coverage from the E4 and E25 Galileo encounters (partially shown in Figure 7).Two arcuate troughs identified in the Galileo data set are shown to extend further into the JunoCam image.A third newly identified trough is visible near the terminator in the JunoCam image, aligned with the other two troughs (Figure 6(a)).Large lineaments can be seen reaching across the area, meeting on both sides.The interpretation of the linear features identified as undifferentiated lineae in the Galileo map has been updated to ridges in the JunoCam map.Smaller lineaments can also be observed.Some portions of the surface imaged by JunoCam still appear relatively featureless, but this is probably due to features that are still unresolved in the new image (Becker et al. 2023; the Juno Stellar Reference Unit image with a resolution of ∼300 m pixel −1 shows very fine linear structure that would look bland at JunoCam's resolution).

Tectonics of Bright Bands
Bright bands are some of the least common landforms on Europa.The common types of bands on Europa are generally lower albedo than the background plains, have symmetrical boundaries, and can be tectonically reconstructed as crustal spreading features.Bright bands are asymmetrical and do not appear to be formed by spreading (Schenk & McKinnon 1989;Prockter et al. 2002;Greenberg 2004;Kattenhorn 2004).The most well-studied example of a bright band is Agenor Linea, which trends east to west for approximately 1500 km in the southern anti-Jovian hemisphere (not imaged by JunoCam).Even though Agenor is referred to as "bright," it is only relatively brighter than the background ridged plains at moderate to high phase angles.Based on imaging obtained through Galileo orbit E6, Geissler et al. (1998) found that Agenor appeared darker than its surroundings at low phase angles, and then underwent a contrast reversal at about 40°p hase, appearing increasingly brighter than its surroundings at higher phase angles.Further imaging of Agenor in the Galileo mission bore out these conclusions; Agenor is seen as a relatively dark feature at 27°phase in imaging from orbit E10, neutral in tone in high-resolution imaging at 45°phase in some E17 images, and relatively bright in other images from orbits E12, E14, and E17 (100°, 78°, and 71°phase, respectively).This unusual photometric behavior led Geissler et al. (1998) to suggest that Agenor may be recently active.Agenor was then targeted for high-resolution imaging later in the Galileo mission, which showed that it is not the most recently active feature in its region (Phillips et al. 2000).Images acquired late in the Galileo mission during orbit E25 revealed another Agenor-like bright band, named Corick Linea, on the northern sub-Jovian hemisphere.Corick also trends mostly east-west, curving northeast along its 1400 km length, and curiously lies almost antipodal to Agenor (Greenberg 2004).
JunoCam imaging of Europa resolves the eastern end of Corick Linea, as well as the eastern end of a previously unrecognized bright band to the north of Corick, named Pasiphae Linea (Figure 8).Like Agenor, Pasiphae appears bright in JunoCam images at high phase angles (80.77°at the center of the first image, with higher values toward the terminator), but Pasiphae can be found as a prominent dark lineament at low phase angles in Galileo imaging of the sub-Jovian hemisphere in orbits G2 (2°phase), C9 (4°phase), and E10 (27°phase).The highest-resolution previous imaging of Pasiphae was obtained during Galileo orbit E25 between 33°a nd 41°phase angles, where it appears as a faint dark lineament or a neutral-toned band.Just like Agenor, Pasiphae appears to undergo a contrast reversal with the background plains near 40°p hase angle.Pasiphae can be traced for approximately 1500 km through Galileo and JunoCam imaging.
The eastern terminus of Agenor Linea was imaged at high resolution in the Galileo extended mission, and the data showed that the end of the bright band turned into a set of southwardcurving fractures and ridges.These features strongly resemble "tail cracks," which are found at the terminus of a strike-slip fault, where local extension caused by motion along the fault is accommodated by a set of fractures curving toward the side of the material that is under tension.Interpretation of the tail cracks at Agenor has led to the interpretation that the bright band slipped in a right-lateral direction (Prockter et al. 2000;Kattenhorn 2004;Hoyer et al. 2014).
At the eastern termini of Corick and Pasiphae Lineae, the JunoCam images show sets of ridges curving to the north, in a splay pattern much like the eastern terminus of Agenor.Unfortunately, the resolution of the new images is insufficient to demonstrate that these ridge splays are the same age as the bright bands, but we predict that future imaging of this area at higher resolution will show a relationship between terminus and tail cracks, much like that seen at high resolution at Agenor.If this is correct, the northward curve of the tail cracks indicates that Corick and Pasiphae both formed by leftlateral slip.
It is striking that Corick and Pasiphae show the opposite sense of shear from Agenor, and that the locations of their eastern termini-near (27°N, 2°E) and (39°N, 25°E), respectively-are very close to being antipodal to the eastern terminus of Agenor near (42°S, 182°E).Is this mirror-image symmetry merely a coincidence, or is there a global symmetry in the driving stresses forming bright bands on Europa?Models of diurnal tides (e.g., Rhoden et al. 2012) would predict that a fracture with the position and orientation of Agenor would slip in a right-lateral direction, and that fractures in the positions and orientations of Corick and Pasiphae would slip in a leftlateral direction.While this is a promising explanation, it does not explain why bright bands have only been observed in these two antipodal areas.Another globally symmetrical stress state could be induced by TPW, and the currently hypothesized TPW scenario for Europa would place all the known bright bands in zones of maximum tensile stress (Schenk et al. 2008).However, a single high-resolution Galileo image from orbit E25 which covers the middle of Pasiphae Linea shows that the fractures of Kermario Fossae, which are geometrically This cross-cutting relationship suggests that if both sets of features are related to TPW, then they formed sequentially at different stages of the event, with bright bands first followed by the fracturing.Alternatively, Pasiphae Linea is not geometrically related to the known TPW features.Global Europa mapping will be required to address the origins of bright bands with more confidence.

Preliminary Analysis of Color and Photometric Changes
One promising aspect of the new set of images is the potential to carry out regional color and stratigraphic analysis of lineae with images all acquired at the same time and lighting.The analysis of Galileo images on the anti-Jovian side of Europa done by Geissler et al. (1998) showed a likely evolution of lineae from cracks to ridges to triple bands to ancient bands.Very preliminary JunoCam analysis has shown fascinating detail: 1.A cycloidal band in the north has different-colored segments in image 1: blue-gray to the south, dark to the north.At lower phase angles in later images and as the sub-spacecraft point shifts, one of the cycloid segments changes from the dark to the blue-gray color.This observation may indicate a dependence of color and/or photometric behavior on viewing azimuth, perhaps due to aligned structures in the band (see Figure 9).2. Many of the old bands, primarily in the north and south and trending generally north-south, are dark in the higher-phase-angle image 1, and some of them fade to blend in with the neutral background color at lower phase angles.3. Other linear features (perhaps ridges, mostly east-west trending) are bright in image 1 at high phase, and either disappear or reverse contrast to look dark at lower phase angles.
This is a promising area for future investigation.

Europa's Craters
Europa's surface age has been estimated at 40-90 Myr (Bierhaus et al. 2009).A total of just 41 craters over 1 km diameter are named and listed in Doggett et al. (2009), and crater characteristics are summarized in Bierhaus et al. (2009) and Schenk & Turtle (2009).No previously unmapped or newly formed craters have been recognized in the new images.Two prominent craters mapped in the overlapping area between the Galileo and JunoCam image coverage are Gwern (previously reported diameter 21 km) and Midir (diameter 38 km).However, in the JunoCam images Gwern appears as a set of intersecting ridges rather than an impact crater (Figure 10).The new images thus show that Europa has "lost" one of its 41 named craters, Gwern, and it is now clear that the intersection of linear ridge features simply produced a quasi-circular pattern in previous images.
Midir was imaged by Galileo near the evening terminator and appears as concentric albedo rings with very little topographic shading (Figure 11).The outermost bright ring was interpreted to be the crater rim.Midir is also clearly visible in the JunoCam images but has an odd appearance.It was imaged by JunoCam near the dawn terminator, lit from the opposite direction of the Galileo imaging.Due to the apparent lack of topographic shading in the Galileo images, one would expect a similar appearance in the JunoCam images.Instead, the eastern edge of the structure (previously interpreted as the rim) appears bright, when standard crater morphology would lead one to expect that this should be an inward-facing slope which should be shaded dark by the lighting angle.The apparent structural rings appear bright and not as ridge-like, and the ejecta are not as evident either.One possible interpretation of the images is that Midir is instead a smaller impact structure atop a topographic pedestal, with the pedestal probably being an inner component of the continuous ejecta blanket (Schenk & Ridolfi 2002;Schenk & Turtle 2009).Other interpretations with various combinations of albedo and topographic patterns are also possible, highlighting the need for higher-resolution images at a variety of viewing geometries to confidently identify impact craters.

Surface Change Detection
Europa's young surface age (e.g., Bierhaus et al. 2009) makes it conceivable that we could detect surface changes in the new images.Surface changes are common on neighboring, active Io (Phillips et al. 2000;Geissler et al. 2004).Previous attempts to map surface changes on Europa (Phillips et al. 2000;Bramson et al. 2011;Phillips 2014;Schenk 2020) have been unsuccessful.Galileo mapping coverage was nearly global but at highly inconsistent pixel scales of ∼1-10 km (with very small areas at much higher resolution) while the New Horizons observations covered ∼70% of the surface at pixel scales of only ∼14-20 km.These provide a useful time base from which to confidently make comparisons, but only for  features larger than ∼25 km globally and perhaps down to ∼10 km in the JunoCam area.Furthermore, differences in imaging system filters, sensitivity, and other factors, as well as the dissimilar observation conditions for most observations, also conspire to make change detection a difficult task, as described by Phillips et al. (2000) and Schenk (2020).
Despite the challenges, attempts were made using the 2022 JunoCam observations to identify surface changes on Europa.We use the cartographically registered image products in map projection described above in Section 2, as any displacement of features between two of the images due to misregistration will produce artifacts that could be misinterpreted as changes.As previously described, iterative updates to the match points were made until all identifiable misalignments were removed.Difference and ratio images were then produced of the JunoCam and Galileo mosaics in which surface changes would appear as anomalous bright or dark features (Figures 12 and  13).The E25 and E4 Galileo mosaics that overlap with JunoCam included large terminator zones in which shading and shadows were abundant in those images but not JunoCam, introducing extensive artifacts that would make interpretation difficult.While produced, these difference images were essentially uninterpretable for change detection.
Difference imaging using the 1997 Galileo E2 and E10 7 km pixel scale global images as reference were used, instead, with the JunoCam images degraded in resolution by factors of ∼2 to provide a better match to the Galileo map products.These images provided a reasonably close match to the phase angle and incidence angle characteristics of the JunoCam 3 and 4 images (Table 1).Despite this, there were some mismatches in emission angles near the outer margins of the JunoCam mosaics due to the proximity to Europa and the much-widerangle optics of JunoCam, which resulted in some lineated terrains appearing darker in the difference product due to increased surface shading in the JunoCam perspective.These features are not candidates for change due to the fact that they are common in the outer high-emission zones of the differenceimage products.
Potential changes on Europa's surface due to tectonic or other surface activity were not detected, and this can be attributed to the limited resolution and differences in illumination and viewing conditions for the Galileo and JunoCam observations (or lack of any surface changes).The lack of mappable surface changes in the JunoCam imaging or in prior searches (Phillips et al. 2000;Schenk 2020;Phillips & Ireland 2023) may be due to several causes.The imaging library for Europa is severely limited, and this sharply reduces the number of imaging opportunities where the observation conditions are reasonably similar and a search can be optimally conducted.The differences between the Galileo and JunoCam observation in terms of illumination, wavelength, image noise, and other factors makes comparison of geologic features in the two image sets very difficult indeed.Filter color differences are important because of the terrain color differences on Europa, causing some terrain types to be darker or brighter in one of the imaging systems.Future missions may be able to image Europa's surface at wavelengths, locations, and viewing geometries that optimize for differences in tidal flexing, phase angle, and other factors that could increase our chances of detecting ongoing geologic activity, if acquired under similar conditions as the earlier observations.

Surface Changes Due to Eruption Deposition
Reports of putative eruptive plumes up to 100 or more km high at Europa (e.g., Roth et al. 2014;Sparks et al. 2016;Jia et al. 2018) suggest at least the possibility of surface changes due to activity between the JunoCam observations in 2022 and the New Horizons 2007 flyby and the Galileo mission observations (1996Galileo mission observations ( -2001)).No obvious plume-like deposits are apparent in the difference products (Figure 12), which we assume would take the form of circular or elongate diffuse patches or swaths from point or linear vent sources (irregular deposition patches could also result in the event of new chaos formation; Fagents et al. 2000).There are several 10-30 km scale irregular patches that appear brighter or darker in the difference products (arrow in Figure 12) but none can be directly attributed to surface changes as identical features are mappable in each image but merely change brightness.These can be attributable to differences in photometric properties with phase angle due to differences in roughness, particle sizes, or albedo.Because these features are not geologically resolved as any specific type of terrain or feature, interpretation is necessarily handicapped.
It is possible that any putative plumes may be much smaller in scale (e.g., Fagents et al. 2000;Quick & Hedman 2020;Schenk 2020) than the available imaging allows us to search, or the plumes may be operating continuously (over decadal timescale), resulting in no significant changes over the Voyager to Galileo to New Horizons to Juno 1979-2022 timescale.It is also possible that plume-related changes or deposits are not visible to the wavelengths used by those spacecraft and require other instruments-for example, thin plume deposits could be transparent at visible wavelengths, if they lack distinctive chromophores.Even thin deposits would be expected to have strongly different photometric characteristics than surfaces exposed to space for longer periods; however, no photometrically unusual plume-related deposits can be identified (Figure 12).
Another plume search approach is to look for unusual color signatures, similar to those observed in plume deposits on Io (McEwen et al. 1998) and Enceladus (Schenk et al. 2011, in Icarus Saturn color report).Here we take the JunoCam red and blue projected mosaics and ratio them to search for diffuse or other features with distinct visible color slopes (Figure 14).None were observed other than the well-known linear dark reddish lineaments associated with double ridges and dark reddish chaos, which show up as dark features in our ratio map.All such features in the ratio map are identifiable as known geologic features, and thus are not plume related (unless all double ridges were <10 km scale vent sources; Fagents et al. 2000;Schenk 2020).No large 100 km scale circular or linear diffuse color signatures are evident, even though the edge of the ratio mosaic is close to at least one such proposed plume source.Thus, while it seems evident that something unusual is occurring near Europa, possibly vapor eruptions (Roth et al. 2014;Sparks et al. 2016;Jia et al. 2018;Arnold et al. 2019; and see Section 3.8), the imaging results thus far provide no evidence as to its character.

No Active Plumes on Europa
Our surface-mapping searches focused on surface changes related to putative plume activity on Europa.There is also a great deal of interest in whether or not there are active eruptions  at Europa (Roth et al. 2014;Sparks et al. 2016;Jia et al. 2018;Arnold et al. 2019).A key test for such active venting is to search for bright plumes of material projecting into space, as seen at Io and Enceladus.All evidence to date implicating plumes is consistent with vapor eruptions; there is no data relevant to possible particulates.In spite of that, we looked carefully at the terminator and the limb for signs of an eruption, both of which are easier to process than comparison of mapped image products acquired under nonideal circumstances.
Figure 15 shows a highly stretched version of the first and highest-resolution JunoCam image.Jupiter shine illuminates the territory along the dark side of the terminator.This allows us to determine that bright spots along the terminator are not eruptions but rather are associated with high points of the terrain.No detached bright spots associated with sunlit tops of eruptive plumes can be identified in the anti-sunward direction away from the terminator in the JunoCam 1 or 2 images (the terminator was not observed in images 3 and 4).
The limb of Europa in the fourth image was at ∼90 E (Figure 16), within a few hundred kilometers of where plumes have been reported previously as listed in Table 2.However, an eruption would have to occur at just the right longitude (limb or terminator) and just the right time when Juno flew by, so the probability of a detection was never very high.Also, the phase angle was not ideal for detection of small particles, as optical detection of plumes against space was not possible at Enceladus except at phase angles >120°, and the highest-phase JunoCam observation was at ∼90°phase.Although not ideal, it may have been possible for JunoCam to observe a plume somewhere along the limb.Despite these challenges, processing of the limb region using low-pass smoothing and contouring to suppress low-signal noise in these dark regions (Figure 17) do not reveal recognizable local-scale deviations from the near-limb profile, indicating that, if present, plumes are not identifiable in the limb or terminator regions of these images.It is also possible that the optical density of particulates within the plume is too low for JunoCam to detect.With a very low signal-to-noise ratio, JunoCam has imaged Jupiter's main ring, which has an albedo of 0.015 and an optical depth of 5 × 10 −6 .9For comparison, Enceladus' jets are ∼0.0025(Ingersoll & Ewald 2017).

Other Data Sets: PJ37 and PJ40 Polar Coverage
On 2021 October 16, ∼8.5 hr before PJ37, JunoCam acquired 17 shots of Europa at an altitude of 81,752 km, primarily to test exposure times (image identifiers JNCE_2021289_37C_0001_V01 to JNCE_2021289_37C00017_V01).At a latitude/longitude of 51.3/228W and with a resolution of 55 km pixel −1 , the images show interesting detail of albedo units on the surface but did not yield any new scientific insights.On 2022 February 24, ∼8 hr before PJ40, Juno came closer to Europa, with an altitude of 46,911 km.The five JunoCam RGB images collected had a best resolution of 31.6 km pixel −1 (image identifiers JNCE_2022055_40C00001_V01 to JNCE_2022055_ 40C00005_V01).This flyby was quite interesting as the images were centered at 77.3/129.9W.The view of the north polar region fills in gaps in Galileo coverage, as shown in Figure 18.
The images covering the north pole from PJ40 do not show any remarkable changes from the surrounding terrain, with no significant color changes or visible major structural features (e.g., a prominent dark band).The apparent mottled color suggests that the terrain consists of a mixture of ridged plains and chaos.Additionally, one small area of higher-resolution imaging was acquired by the Galileo spacecraft near the north pole (25ESNPOLE01), which predominantly consists of chaos terrain (Collins & Nimmo 2009).Future imaging of the poles at higher resolution will further illuminate the terrain and any potential similarities or differences in morphology compared to the rest of the moon, which could grant insight into how terrain types vary with tidal heat or ice shell thickness.

Summary and Conclusions
These images of Europa from JunoCam are the first close-up images since Galileo (flyby E26, 2000 January).A number of new results are reported here.JunoCam sees a new arcuate trough and pits that add to the evidence for a TPW tectonic pattern.Cartography has been extended in a portion of the lower-resolution area in the Galileo map and more lineae connections can be traced in a new segment of the geological map.The tectonics of bright bands are being investigated and have promising implications for understanding Europa's global stress state.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.
Looking to the future, Europa Clipper will be the next NASA Flagship mission to Europa, arriving at Jupiter in 2030.The European Jupiter Icy Moons Explorer (JUICE) will arrive at Jupiter in 2031, and make multiple passes by Europa on its journey to Ganymede.The JunoCam data have reminded us of all we have to look forward to in the future, with observations of Europa from Europa Clipper and JUICE.
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.territory.Ejecta from the bright rayed crater Pwyll can be seen in the last image.Figure2(b) shows maps of phase angles, incidence angles, emission angles, and pixel scale for the four images.

Figure 1 .
Figure 1.The image on the left is a composite of the four images JunoCam received, from the perspective of the fourth image.On the right side the four individual images show the rapid change in altitude and coverage.The upper left on the right side is the first image, the upper right is the second image, the lower left is the third image, and the lower right is the fourth image.The diagram on the bottom right shows the small area covered by JunoCam images relative to the Galileo map.

Figure 2 .
Figure 2. (a) Map showing the four JunoCam color images coregistered and map-projected onto a monochrome Voyager-Galileo global mosaic in simple cylindrical projection (cropped to highlight the JunoCam coverage area).The view is centered on the sub-Jovian hemisphere with some additional terrains to the east.The bright rayed crater Pwyll is just beyond the eastern edge of the JunoCam coverage area at −30, 90E but its rays of ejecta are visible in the fourth image.The abundant dark lineaments are double ridges flanked by dark reddish deposits.(b) Cylindrical maps showing phase, incidence, and emission angles and pixel scales for the four JunoCam mosaics of Europa, with range of angles and pixel scales represented in the 0-256 brightness scale.Map coverage is 60S to 90N, −12E to 105E.

Figure 3 .
Figure 3. Juno viewing geometry at closest approach (left) and a few minutes later (right, not to scale) when JunoCam took the four images of the illuminated surface.The color bar shows the altitude of the spacecraft along the groundtrack, changing rapidly.Black dots along the groundtrack are spaced by 1 minute.The time in the center of the disk on the left globe is the time of closest approach to Io.

Figure 4 .
Figure 4. Portion of JunoCam Europa mosaic 1.(a) Green filter mosaic map-projected at native 1000 m pixel −1 resolution; (b) global integrated super-resolution mosaic map-projected at 330 m pixel −1 scales, illustrating the improvement in feature recognition using the technique described in the text.

Figure 6 .
Figure 6.(a) The high-incidence-angle perspective on images along the terminator shows new blocks, pits, and the arcuate arm of a new trough discovered along the terminator.The 900 m deep pit is ∼24 km across.The change in identification of chaos described in Section 3.2 is also annotated here.(b) This graphic of the features consistent with true polar wander, taken from Schenk et al. (2020), adds in the cluster of JunoCam pits and the new trough on the left side.The JunoCam features are marked with "JC."

Figure 7 .
Figure 7.The image on the left shows the Galileo coverage while the image on the right was taken by JunoCam.The Galileo image shows the band of much less differentiated terrain, filled in by the JunoCam image on the right.While some of the differences in the two geologic maps arise from an improved mapping scale, others are due to the improved resolution, less radiation noise, and different lighting conditions of the two basemaps.The 1:15M geologic map based on the Galileo data is shown on the bottom left (Leonard et al. 2018, 2024), with a more detailed version allowed by the JunoCam imagery (∼1:10M) shown on the bottom right.

Figure 8 .
Figure 8.The image on the left shows two northeast-trending bright bands emerging from the terminator of the color JunoCam image at 19°N and 39°N.The image on the right shows the locations of the bright bands Corick and Pasiphae in cyan.Interpreted tail-crack structures emanating from their eastern termini are mapped as dashed magenta lines.Simple cylindrical projection; background grayscale image is the global Voyager-Galileo image mosaic.The background mosaic is downloadable from the USGS online: https://astrogeology.usgs.gov/search/map/Ganymede/Voyager-Galileo/Ganymede_Voyager_GalileoSSI_global_mosaic_1km.

Figure 9 .
Figure 9.A subtle change in color of the prominent north-south trending curved cycloid in the center is seen between the first and third images, possibly due to phase function effects.The relatively bluish tint might be interpreted as an indication of relatively youthful age.

Figure 10 .
Figure 10.Gwern "crater" in the Galileo basemap (left) is shown to be a set of intersecting ridges in the JunoCam images (right).

Figure 11 .
Figure 11.Midir crater in the Galileo basemap (left) and the JunoCam image (right).Both images were obtained at high incidence angles, but with opposite lighting directions.The Galileo image is lit from the left and the JunoCam image is lit from the right.

Figure 12 .
Figure 12.Difference images between Galileo (1997) and JunoCam (2022) imaging of Europa.The map is a simple cylindrical map projection centered at 40°E longitude.Left: Galileo G2 observation to JunoCam observation 4; right: Galileo C10 observation to JunoCam 4. White arrows highlight linear ridges or belts with dark difference signatures due to increased emission angles in the JunoCam observation.Black arrow highlights unusual but unresolved features with brightness difference attributed to different photometric phase functions.

Figure 13 .
Figure13.A portion of JunoCam observation 2, green filter, as compared to Galileo clear image 6439 from orbit I25, has been coregistered and masked to include overlap areas only, and their ratio (right).The difference in terminator position between the two images (the terminator is just off to the right side of the Galileo image) reveals details of two large vertical troughs, and Midir crater is visible near the upper right.Differences between the two images can be attributed to differences in lighting and viewing geometry; no surface changes attributed to geologic activity are visible.

Figure 14 .
Figure 14.Red/blue color ratio map of Europa from JunoCam observation 4. While the known reddish lineaments and irregular-shaped chaos patches are apparent, no diffuse patterns are evident to suggest ongoing plume deposition.The circular feature on the lower left is Callanish at −16.7/334.5W,identified as a large ringed feature with a diameter of 107.0 km.

Figure 15 .
Figure 15.This highly stretched version of the first JunoCam image was processed by citizen scientist Brian Swift.We can see the portion of the planet illuminated by Jupiter shine adjacent to normally illuminated dayside topography.

Figure 16 .
Figure16.Location of four limb tracks of the JunoCam mosaics (1-4).The white and black dots across the map of Europa show the approximate locations of putative large plumes identified elsewhere and tabulated in Table2.White dots might have been visible to JunoCam while black dots were not.

Figure 17 .
Figure 17.These figures show RGB framelets 7 and 8 from the fourth JunoCam image, stretched to look for bright eruptions along the limb.None are apparent.Similar processing was carried on all framelets in all four images.

Figure 18 .
Figure 18.JunoCam had a north polar view of Europa on PJ40.The figure shows (left to right) the best PJ40 JunoCam image, a polar grid on the JunoCam image, and the current existing Galileo map with the same orientation.Some albedo units close to the pole can be filled in.

Table 1
JunoCam Europa Image Geometry at Center of Image Note.Ranges of values are from terminator to limb at the center row.

Table 2
References for Previous Prospective Plume Locations (Vapor Only)