Rosaly M. C. Lopes

One of the Jupiter Icy Moons Explorer (JUICE) mission goals of the European Space Agency, launched in 2023 April, is the detailed characterization of Ganymede, Jupiter’s largest moon and a confirmed ocean world. Among its science objectives, JUICE will investigate Ganymede’s past or possibly ongoing cryovolcanic and tectonic activity and assess material exchange between the surface, subsurface, and internal ocean. The JUICE Science Working Team has identified potential cryovolcanic regions as targets of high scientific interest, very relevant for astrobiological investigations. Using Voyager and Galileo images, G. C. Collins et al. identified 20 paterae and other candidate cryovolcanic regions, yet their origin and composition remain poorly constrained due to the limited spatial and spectral resolution of existing datasets. Here, we present a comprehensive reassessment of these 20 regions to support JUICE mission preparation. We integrate detailed analysis with a systematic spectral investigation using reprocessed Galileo Near-Infrared Mapping Spectrometer data and a consistent linear spectral unmixing approach which enables comparative assessment of compositional trends and spectral variability. Our results reveal distinct spectral groups among the candidate paterae, ranging from ice-dominated terrains to salt-enriched assemblages, showing systematic differences across model runs that use temperature-dependent laboratory end-members. Several regions display coherent morphological characteristics and compositional signatures consistent with cryovolcanic resurfacing or brine-related processes. These regions emerge as the most compelling targets for high-resolution imaging and spectroscopic observations by JUICE instruments, including with the camera JANUS and with MAJIS. This integrated analysis refines the prioritization of candidate cryovolcanic regions and provides a framework for interpreting future JUICE observations of Ganymede.

Recent observations by the JIRAM instrument on board NASA’s Juno spacecraft have confirmed that many of Io’s volcanic hotspots are active lava lakes, characterized by a colder central crust surrounded by a hotter peripheral ring. In this study, we investigate the thermal properties of 30 such lava lakes, providing new constraints on their structure and energy budget. We find that most of the total power from Io’s lava lakes comes from their low-temperature crusts rather than the hotter peripheral rings, suggesting that previous estimates underestimated lava-lake power by up to a factor of 10. Io’s paterae undergo stochastic resurfacing on timescales of roughly a decade, with each lake possibly following its own characteristic cycle. We also explore the relationship between the average temperature of the crust and the evolutionary state of each lake, offering insights into the frequency of resurfacing processes. Finally, we propose an improved assessment of Io’s global thermal output, emphasizing that only full-surface observation of Io with sufficient spatial and spectral resolution can yield realistic values for the moon’s volcanic total heat flux.

Saturn's moon Titan exhibits remarkable parallels to the Earth in many geophysical and geological processes not found elsewhere in the solar system at the present day. These include a nitrogen atmosphere with a condensible gas—methane—replacing the Earth's water, leading to an active meteorology with rainfall and surface manifestations including rivers, lakes and seas, and the dissolution of karstic terrain. Other phenomena such as craters, dunes, and tectonic features are found elsewhere—for example on Mars and Venus—but their continuing alteration by pluvial, fluvial and lacustrine processes can be studied only on Earth and Titan. Meanwhile Titan also hosts an interior liquid water ocean with similarities to the Earth as well as to ocean worlds such as Europa and Enceladus. Our focus in this review paper is twofold: to describe the geophysical and geological parallels between Earth and Titan, and to evaluate the yet‐underexploited possibilities for field analog research to gain new knowledge about these processes. To date, Titan's much colder temperature and different atmospheric and crustal materials have led to a skepticism that useful analogs can be found on Earth. Our conclusion, however, is that a much larger range of useful analog field work is possible and this work will substantially enhance our knowledge of both worlds. Such investigation will supplement the existing sparse data for Titan returned by space missions, will greatly enhance our understanding of such data sets, and will help to provide science impetus and goals for future missions.

The Jupiter and Icy Moons Explorer (JUICE) mission of the European Space Agency (ESA) will investigate the Jovian system with multiple instruments over several years, beginning in early 2031. This paper describes the historical context and state of knowledge, as well as JUICE’s scientific goals and measurement techniques of the satellites that will not be encountered in close flybys. These include the large volcanically active moon Io, the four small inner moons Metis, Adrastea, Amalthea, and Thebe, and the numerous small Irregular (outer) moons. JUICE will provide multiple opportunities to observe Io from relatively remote distances of hundreds of thousands of kilometers. These observations will enable monitoring of Io’s surface for changes, and for the study of its neutral clouds and plasma torus. Io observations will be performed with the four optical remote sensing instruments and with the Particle Environment Package. For the small inner moons it is planned to obtain complete geographic longitude (scales up to 8 km/px), solar-phase and multi-color coverage, oblique polar views, and UV to near-IR spectra. Astrometric measurements will also be performed. The Irregular moons will mostly appear unresolved to the JUICE instruments. Nonetheless, long-duration disk-integrated lightcurves will be acquired to derive rotation periods, object dimensions, pole-axis orientations, and colors for most objects for the first time. From these data, convex-shape models will be generated and phase curves determined. Furthermore, the precision of the orbital elements will be improved via accurate astrometry. UV and near-IR measurements will be attempted for the largest of these objects.

Surface observations of Saturn's moon Titan revealed features characterized as dissected, elevated plateaus with high valley density known as labyrinth terrains. Of this terrain class, a subtype referred to as radial labyrinth is described as dome‐shaped uplifts with radial channel patterns. Uplift of these radial labyrinths has previously been explained as cryomagmatic intrusions at the brittle–ductile transition zone. Here we propose an alternative hypothesis that crustal heterogeneities in Titan's upper clathrate crust introduce density differentials due to ethane‐methane substitution, as ethane‐rich liquids percolate into methane clathrate, inducing solid state flow and generating domal topography. This mechanism is analogous to salt tectonics on Earth and has similarly been evoked for dome formation on the dwarf planet Ceres. We show that the elevation and width of the observed radial labyrinths are consistent with domal uplift driven by a hydraulic head within the uppermost portion of Titan's crust, given a plausible set of elastic parameters for clathrate hydrates. Additionally, the insulating effect of clathrate, combined with partial mixing with water‐ice, allows for sufficiently low viscosity for geologic flow on a relevant timescale: uplift of the domes could have occurred within the last billion years.

On 27 December 2024, Juno's JIRAM (Jovian InfraRed Auroral Mapper) instrument observed an unprecedented volcanic event in Io's southern hemisphere, covering a vast region of ∼65,000 km ² , near 73°S, 140°E. Within the imaged region, only one hot spot was previously known (Pfd454). This feature was earlier estimated to cover an area of 300 km ² with a total power output of 34 GW. JIRAM results show that the region produces a power output of 140–260 TW, over 1,000 times higher than earlier estimates and likely exceeding the brightest eruption ever recorded on Io, that of Surt in 2001 (∼80 TW). Three adjacent hot spots also exhibited dramatic power increases: P139, PV18, and an unnamed feature south of the main one that surged to ∼1 TW, placing all of them among the top 10 most powerful hot spots observed on Io. A temperature analysis of the features supports the simultaneous onset of these brightenings and suggests a single eruptive event propagating beneath the surface across hundreds of kilometers; this is the first time this has been observed on Io. This in turn would imply a connection among the hotspots' magma reservoirs, while other nearby hotspots that have been known to be active in the recent past, such as Kurdalagon Patera, appear unaffected. The simultaneity supports models of massive, interconnected magma reservoirs. The topology of these regional magma systems may resemble that of a large‐scale sponge, in which the massive reservoirs are the pores, interconnected through a largely solid outer shell.

We investigated the surface geophysical properties of Titan through a multi-angular backscattering analysis of Cassini Ku-band radar data acquired in altimetry, SAR, scatterometry and radiometry modes. We test quasi-specular scattering models in combination with a volume scattering formulation to model the backscatter coefficient as a function of incidence angle over specific areas identified from topographic and radar imaging data. From data inversion we found that the exponential model provides the best fit to the observed data. We present our results in terms of effective permittivity, surface roughness and volume scattering albedo across different backscattering uniform regions at Titan’s northern hemisphere, where both radar-bright and -dark terrains have been observed from radar images acquired during the mission. Our results indicate that these bright regions belong to a large low-lying plain made of material with a relatively high dielectric constant, high-volume scattering albedo, and moderate wavelength-scale roughness inferred from the model inversion. In contrast, dark regions are consistent with solid hydrocarbons accumulations, reaching thicknesses of up to hundreds of meters above this plain, and they exhibit a lower dielectric constant, lower volume albedo, and slightly higher wavelength-scale surface roughness together with additional higher meter- to tens-of-meter-scale roughness inferred from altimetry. Finally, we propose some of the most probable scenarios based on a list of candidate materials that may be present at the surface of Titan. Although our analysis focuses on a specific, albeit extensive, region near the North Pole, our findings may offer broader insights into the global distribution of complex organics on Titan surface, which are likely superimposed on top of a mixture of ice and organic material.

We use data from the Jovian InfraRed Auroral Mapper (JIRAM) instrument on the Juno spacecraft, acquired during close fly‐bys of Jupiter's moon Io during 2022–2024, to identify new characteristics of active lava flows and changes since observations from previous missions (Voyager, Galileo, and New Horizons). At least two new vents as well as a new flow field are identified along with some significant changes seen at other sites. Our new JIRAM observations of Culann, Tvashtar, Daedalus, Kanehekili, Masubi, Amirani, Prometheus, and Girru show that volcanism continues at these sites since previous missions, in some cases suggesting long duration (>45 years) eruptions. A large (∼130 × 40 km) active flow field was detected to the NE of Seth Patera as well as a probable new ∼120 km long active flow at a previously unrecognized site designated “Unknown 13.” Flows were also detected by JIRAM at Quzah, Xihe, and a large, recently emplaced flow was observed in the south polar region at Illyrikon Regio. Comparison with locations of plumes detected by Juno's camera shows that plumes coincide with distal margins of flow fields where brightness temperatures are high, indicating Promethean‐type flow and plume activity. The spatial distribution of hotspots on most of the observed flows is consistent with earlier suggestions that the flows are inflated pāhoehoe‐type flow fields, although activity at Kanehekili and Quzah Flucti may well display characteristics of higher‐discharge rate, ‘a‘ā, eruptions.

During multiple Juno encounters with Io in 2023 and 2024, the spacecraft's imaging system, JunoCam, acquired approximately one hundred images of Io. These images have a range of scales down to 1 km per pixel, and covered more than half of Io's surface, including the previously poorly resolved northern and southern polar regions. Some of the highest resolution images were acquired of Io's nightside, illuminated by Jupiter‐shine. Here we provide an overview of the JunoCam Io data set and discuss the peculiarities of JunoCam's “pushframe” imaging. Using JunoCam's high resolution coverage of the northern polar region, we have developed an improved geologic map of this previously poorly resolved area, and estimated the heights of 16 previously unidentified or poorly imaged mountains. In the higher resolution images, 14 new or modified plume deposits were identified, as were at least 20 probable new lava flows. Eight “Prometheus‐type” volcanic plumes were identified, five of which were associated with areas showing flow‐like surface changes.

We use data from the Jovian InfraRed Auroral Mapper (JIRAM) instrument on the Juno spacecraft, acquired during close fly-bys of Jupiter's moon Io during 2022-2025 to identify new characteristics of active lava flows. This time period includes observations of an unprecedented large outburst in the south polar region on December 27, 2024 (Juno's orbit 68) and March 19, 2025 (orbit 70). During the previous close flybys in 2022-2024 (orbits from 51 to 62), at least two new vents as well as a new flow field were identified along with some significant changes seen at other sites. The lavas from Amirani, Prometheus, and Culann Paterae continue to be active suggesting decades-long eruptions. The activity at Amirani has changed since the Galileo mission’s observations (1996-2001); at a spatial resolution of 1-10 km/pixel the northern part of the flow is no longer active. The JIRAM images clearly show that the flow now displays multiple points of activity in the southern region and within the dark patera and fracture south of the flow. The activity at Prometheus did not show major changes comparable to those observed between the Voyager (1979) and Galileo (1996-2001) missions. However, Juno data show that the hottest area has now shifted to the region to the south of the source patera at the eastern end of the 100 km flow field, in the location of the main flow region observed during Voyager. At Culann, the changes since Galileo were not as significant, with the strongest thermal features detected by JIRAM associated with the darkest flow areas seen by Galileo. A new, large (~130 x 40 km) active flow field was detected to the NE of Seth Patera. This area had been detected as active from Galileo data, but at the time it was unclear, due to low spatial resolution, if the activity was from the patera or from flows emanating from it. JIRAM data show that the patera, presumably the vent, has elevated thermal emission, while the distal portion of the flow seen to the NW is also hot. Most interestingly, Seth has a new flow near the source as observed in the Galileo Era but to the ENE of the volcano. JIRAM data also show an active area, probably a new ~120 km long active flow at a previously unrecognized site designated "Unknown 13". This feature is ~100 km long and is associated with no recognizable landforms in the Voyager or Galileo optical images map and no hot spot had previously been detected in this area before Juno. The spatial distribution of hotspots on each of the observed flows is consistent with the earlier suggestions that the activity produced inflated pāhoehoe-type flow fields. A new, large flow was seen in JunoCam images in December 2024, when JIRAM detected a major outburst in the south polar region over a vast region centered at 73°S, 140°E, covering an area of ~65,000 km². Comparing JIRAM images taken during orbit 68 with those from orbit 70, reveals that this major eruption produced a lava flow that traveled 50 to 100 km eastward over the intervening two months (at an average rate of ~800 – 1,600 m/day), gradually cooling along its path. Such flow velocities are more consistent with terrestrial a’a-type (rather than pāhoehoe-type) flows. Comparative measurements from JIRAM’s two filters clearly show the hottest edge of the flow front extending eastward. Further observations by both JIRAM and JunoCam may be able to reveal the full extent of the flow.