JANUS, whose full name Jovis, Amorum ac Natorum Undique Scrutator translates roughly as a scrutinizer of Jupiter, his loves and his children from all sides, is the workhorse imaging system on JUICE, the JUpiter ICy moons Explorer. The spacecraft launched in April 2023 on a mission that will ultimately deliver it to Jupiter in 2031, where it will spend years studying the gas giant and three of its large icy moons: Ganymede, Callisto and Europa. Before it can begin that science campaign, however, JUICE must execute a series of gravity assists, using the masses of Earth, the Moon and Venus to bend its trajectory and build up the enormous velocity needed to reach the outer solar system. The August 2024 Lunar–Earth flyby was the first of these crucial maneuvers, and mission planners recognized early on that it offered a rare opportunity to test the spacecraft’s instruments against a familiar target.

The choice of Earth as a calibration and validation target is more scientifically astute than it might first appear. Earth is, in effect, a rehearsal stage for planetary atmospheric imaging. Its cloud systems, storm dynamics, aerosol layers and day–night contrast phenomena operate under physical principles that also govern the atmospheres of the giant planets, albeit at different temperatures, compositions and rotation rates. By pointing JANUS at terrestrial weather systems, the science team could exercise the camera’s full observational chain, from pointing accuracy and exposure control to onboard data handling and image calibration, while simultaneously collecting genuine atmospheric data. The result is a two-for-one payoff: the instrument is validated for Jupiter, and researchers gain a benchmark dataset of Earth observations made with the very same hardware they will later use on the king of planets.

During the close portion of the flyby, JANUS acquired images of both the night side and the day side of Earth, a demanding test because the illumination conditions differ by many orders of magnitude. Day-side imaging exercises the camera’s ability to handle bright, sunlit scenes with fine spatial detail, revealing cloud structures, storm systems and surface features with the kind of contrast that will be essential for tracking Jovian weather. Night-side imaging, by contrast, pushes the sensitivity limits of the detector and tests how well the instrument can capture faint phenomena such as airglow, lightning-like flashes and the subtle glow of the upper atmosphere. Both regimes will matter enormously at Jupiter, where the planet’s rapid rotation, deep cloud decks and powerful auroral emissions create scenes of extreme dynamic range that few cameras in history have been asked to record.

The close flyby images allowed the team to characterize specific atmospheric systems in detail, and the analysis published in Annales Geophysicae focuses precisely on these features as relevant examples for future Jovian studies. Terrestrial cyclones, frontal systems and convective storm clusters observed by JANUS serve as controlled case studies: researchers know the ground truth of Earth’s weather from satellites, radar and surface observations, so they can assess how well JANUS resolves and measures such phenomena. When the same camera later images Jupiter’s anticyclones, its turbulent jet streams and its towering convective outbreaks, the team will be able to interpret the data with confidence grounded in this terrestrial validation. The methodology mirrors a long tradition in planetary science, in which flyby targets are used to cross-calibrate instruments against well-understood references before the main mission begins.

Beyond the close-ups, JUICE also captured a distant view of Earth that produced a striking low-resolution portrait of the entire planet. Such whole-disk images may seem less glamorous than detailed cloud mosaics, but they serve a distinct technical purpose. They test the camera’s performance at long range, verifying pointing, focus and radiometric calibration under conditions that resemble, in some respects, the approach phase at Jupiter and the observation of small moons from orbit. Whole-disk imaging also provides context for the higher-resolution work, and the Earth portrait stands as a poignant scientific artifact in its own right, a reminder of the pale blue dot that every interplanetary mission carries with it in memory as it ventures outward.

The validation exercise matters because JANUS faces one of the most challenging imaging assignments ever attempted at Jupiter. The planet’s atmosphere is a seething laboratory of fluid dynamics: alternating east–west jet streams reach hundreds of kilometers per hour, storms the size of Earth persist for decades or centuries, and lightning discharges far more energetic than any terrestrial flash illuminate the deep cloud layers. Understanding these phenomena requires high-resolution, time-resolved imaging that can track cloud motions and measure wind fields, and it demands a camera whose calibration is known with exquisite precision. Any systematic error in JANUS’s response, whether in geometric distortion, radiometric sensitivity or temporal stability, would propagate directly into errors in derived wind speeds and cloud heights. The Earth observations provide the empirical foundation for eliminating those errors before the spacecraft arrives at its destination.

There is also a deeper scientific continuity at play. Comparative planetology thrives on observing the same physical processes in different planetary settings, and Earth offers the most densely observed atmosphere in the solar system. Convective storms on Earth, driven by water condensation and latent heat release, have direct analogues in Jupiter’s water-cloud thunderstorms, which are thought to power the planet’s most vigorous eruptions. Cyclonic vortices on Earth can be compared with Jovian oval storms to test theories of vortex stability and decay. Even the night-side airglow observed by JANUS connects to upper-atmosphere physics that operates on Jupiter as well. By building a JANUS dataset of well-characterized terrestrial phenomena, the team has created a Rosetta stone that will help translate between what we know about Earth’s atmosphere and what we hope to learn about Jupiter’s.

The August 2024 flyby itself was a feat of celestial navigation. JUICE threaded a path around the Earth–Moon system, using the combined gravitational pull of both bodies to reshape its orbit, a maneuver type so delicate that only a handful of spacecraft have ever attempted it. Every instrument that observed during the encounter had to operate autonomously, with precise timing, because the geometry of a fast flyby leaves no room for second chances. That JANUS delivered calibrated, scientifically useful images of both the day and night sides of Earth on the first attempt speaks to the maturity of the instrument design and the thoroughness of the team led by Hueso, Antuñano, Lara and their international collaborators spanning Spain, Germany, Italy, France, Israel, Japan, the United Kingdom and the European Space Agency itself.

As JUICE continues its interplanetary cruise, with further gravity assists still ahead before its 2031 arrival at Jupiter, the JANUS Earth observations will remain a touchstone for the mission’s imaging science. They confirm that the camera is ready to tackle the gas giant’s atmosphere, and they have already begun producing science, offering fresh perspectives on our own planet from a vantage point few cameras have shared. When JANUS finally turns its gaze from Earth to Jupiter, every cloud deck it resolves and every storm it tracks will rest on the foundation laid during those August days in 2024, when a Jupiter camera came home briefly, looked at its makers’ world, and proved itself worthy of the giant that awaits.

Subject of Research: JANUS camera observations of Earth during the JUICE mission's 2024 Lunar–Earth gravity assist as validation for imaging Jupiter's atmosphere

Article Title: JUICE-JANUS observations of Earth in preparation for the JANUS investigation of Jupiter's atmosphere

Article References: Hueso, R., Antuñano, A., Lara, L. M., Stephan, K., Zinzi, A., Coustenis, A., Yair, Y., Sato, M., Haruyama, J., Simon, A., Tubiana, C., Penasa, L., Agostini, L., Lucchetti, A., Aboudan, A., Aye, M., Kersten, E., Matz, K.-D., Politi, R., … Palumbo, P. (2026). JUICE-JANUS observations of Earth in preparation for the JANUS investigation of Jupiter's atmosphere. Annales Geophysicae, 44(2), 1043-1073. https://doi.org/10.5194/angeo-44-1043-2026

Image Credits: AI Generated

DOI: 10.5194/angeo-44-1043-2026

Keywords: JUICE, JANUS, ESA, Jupiter, Earth flyby, gravity assist, planetary science, atmospheric science, space instrumentation, camera calibration, comparative planetology, Annales Geophysicae

The exoplanet census has grown remarkably since the first potentially rocky world, the roughly six-Earth-mass CoRoT-7 b, was confirmed in 2009. Today, planets smaller than Neptune appear to be roughly ten times more common than giant planets, and recent reviews suggest such worlds orbit about a third of all Sun-like stars. Yet characterizing them remains extraordinarily difficult. For most exoplanets, astronomers measure only orbital period and radius; only with both mass and radius can bulk density be calculated, and even then inferring whether a planet is genuinely dominantly rock and metal is far from conclusive. The field relies on a rule-of-thumb radius threshold of about 1.6 times Earth’s radius, below which planets are typically classified as rocky, but the authors caution that such “terrestrial” worlds may hold substantial fractions of volatiles relative to Earth, making them terra incognitae to a Solar System geoscientist. Two so-called Earth twins, Kepler-62 f and the questioned Kepler-452 b, have been proposed, with estimated occurrence rates of such temperate twins falling between 0.37 and 0.88 per Sun-like star, though these figures carry considerable extrapolation uncertainty.

The heart of the review is a tour of the Solar System’s rocky bodies and the lessons each offers. Earth, the best-understood planet, is presented not as a static blue marble but as a world whose character has shifted dramatically through time. Its interior—consisting of a solid inner core exceeding 6000 K and at least 330 GPa in pressure, a molten outer core that powers the geodynamo, an ultramafic mantle comprising about 67 percent of the planet’s mass, and a chemically differentiated crust—cooled by some 200 to 300 K since the Hadean. Plate tectonics, the authors emphasize, is likely not the planet’s original operating mode. Instead, Earth probably passed through an early “squishy-lid” regime characterized by shallow intrusive bodies and a warm, deformable lithosphere, before rigid mobile-lid tectonics emerged with the stabilization of Archean cratons between roughly 3.8 and 3.2 billion years ago. Continental crust, with its felsic, granitic composition, appears to require liquid water or some similar agent to lower rock melting temperatures, and today covers only about 40 percent of the surface while constituting nearly 70 percent of total crustal volume.

Earth’s atmosphere and climate have been equally mutable. The Faint Young Sun Paradox arises because the Sun’s luminosity has risen nearly 30 percent since it entered the main sequence, yet geological proxies indicate Earth remained temperate from early in its history, likely sustained by elevated carbon dioxide partial pressures. Reconstructions from fossil raindrop imprints, gas bubbles in ancient basalts, and nitrogen and argon isotopes trapped in 3-to-3.5-billion-year-old quartz suggest late-Archean surface pressures of roughly 0.5 bar or less. The rise of atmospheric oxygen around 2.4 billion years ago, driven by cyanobacterial photosynthesis and organic carbon burial, coincided with the first documented snowball Earth, possibly triggered by the collapse of methane greenhouse warming. Later, large igneous province eruptions such as the Siberian Traps injected 1 to 10 teratonnes of carbon dioxide into the atmosphere, causing the Permian-Triassic mass extinction that eliminated roughly 90 percent of land species. Even Earth’s color may have changed, perhaps from an early orange haze to its familiar blue, a reminder that a living world need not always look the way ours does today.

Venus receives particular scrutiny because, to first order, very little distinguishes it from Earth: a mass of 0.82 Earth masses, a radius of 0.95 Earth radii, and presumably similar bulk composition, yet surface conditions of an astonishing 750 K under 92 atmospheres of pressure, in air that is 93.5 percent carbon dioxide beneath global sulphuric acid clouds. A deuterium-to-hydrogen ratio roughly 100 times Earth’s indicates substantial water loss at some point in the planet’s past. The review highlights two competing scenarios: either Venus never cooled enough to condense its primordial steam atmosphere, or it once possessed genuine oceans before a runaway greenhouse desiccated the world. Which is correct has profound implications for interpreting Earth-size exoplanets, since the former would suggest that worlds close to their stars are reliably post-runaway greenhouses, while the latter would imply stellar distance matters less than assumed and that climate catastrophe can strike even a clement world. Notably, the authors warn that as an ocean evaporates into a warming atmosphere, water reaching stratospheric altitudes can be photodissociated, allowing hydrogen to escape and atmospheric oxygen to rise—potentially fooling remote observers into inferring a habitable, even inhabited, world when the opposite is true.

Mars, at 0.53 Earth radii and 0.11 Earth masses, illustrates the fate of smaller worlds. Its bimodal hypsometry, expressed as the hemispheric crustal dichotomy, may reflect degree-one mantle convection, crustal growth feedback, or a gigantic ancient impact rather than the buoyant felsic rocks that produce Earth’s two-tier topography. The planet’s geology is written largely in its past: the Noachian eon saw the construction of the Tharsis and Elysium volcanic rises, the Valles Marineris canyon system, and extensive fluvial activity, but with no evidence of subduction or transform faults, Mars has operated under a stagnant-lid regime for essentially its entire history. Its small size gave it a high surface-to-volume ratio, rapid interior cooling, and a thick lithosphere; critically, its mantle and crust, only 15 percent the size of Earth’s combined silicate reservoirs, exhausted their degassable volatile budget long ago, leaving the atmosphere unable to be replenished after solar wind stripping. Whether early Mars sustained long-term temperate conditions or was mostly an ice-house world with punctuated warm intervals remains actively debated, with three-dimensional climate models supporting scenarios from persistent lakes and even a northern ocean to a cold, icy highlands state.

Mercury and the Moon, the smallest rocky bodies of the inner Solar System, share histories of early volcanism followed by global contraction. Mercury’s crust was built by voluminous effusive eruptions that largely ceased around 3.5 billion years ago as secular cooling put the lithosphere into compression, producing a worldwide network of thrust faults and several kilometres of radial contraction that continues today. Its outsize core, roughly 0.8 of its body radius, may result from formation in a highly reduced inner disk or from mantle-stripping impacts; intriguingly, elevated potassium-thorium and potassium-uranium ratios suggest the innermost planet is nonetheless somewhat volatile-rich. The Moon, coalesced from debris of the proto-Earth–Theia collision, crystallized an anorthositic flotation crust from its magma ocean around 4.35 billion years ago, with later basaltic mare volcanism concentrated between 3.8 and 3.2 billion years ago and persisting to at least 2 billion years in the radiogenic Procellarum KREEP Terrain, as confirmed by Chang’e-5 and Chang’e-6 sample returns. Io, the Solar System’s most volcanically active body, demonstrates a different heat source entirely: tidal dissipation driven by its eccentric orbit within the Laplace resonance with Europa and Ganymede sustains surface heat flux 15 to 40 times Earth’s, with eruption temperatures up to 1600 K and a resurfacing rate of about 1.5 centimetres per year.

The review’s final section confronts the exoplanet classes with no Solar System analogues. Super-Earths, planets of roughly 1 to 1.6 Earth radii below the empirically observed “radius valley,” may be bare rock worlds like LHS 3844 b, or remnant cores of formerly puffier planets that lost primordial hydrogen envelopes—a scenario under which some rocky planets effectively form billions of years after their systems establish, thermally blanketed for eons by their departed atmospheres. Tidally locked planets, especially common around M-dwarf stars, could host hemisphere-scale differences in volatile deposition and volcanic activity. Lava worlds, whose daysides reach rock-melting or even rock-evaporating temperatures, sustain long-lived magma oceans heated more strongly from above than within, offering potential insight into bulk rock compositions through thermal emission spectroscopy. Other exotic outcomes include long-lived internally heated planets driven by tidal dissipation, radiogenic abundance, or magnetic induction heating; super-Mercuries with outsize iron cores; low-density “super-Ganymedes” cloaked in thick water or ice layers; and worlds whose bulk compositions diverge from chondritic norms due to protoplanetary disk processes.

The authors distill these comparisons into rules of thumb tempered by caution. Relatively large, ancient planets are more likely to retain moderate atmospheres, ongoing volcanism, and perhaps magnetic fields and liquid-water conditions, while smaller worlds cool, contract, and lose geological activity faster—yet tidal and induction heating complicate even this elementary guidance, as does the stochastic influence of giant impacts, orbital arrangement, and formation location. The starkest lesson remains the Earth–Venus contrast: two worlds within a single mass class with wildly divergent histories, raising the uncomfortable possibility that Venus’s fate could one day befall Earth, either through catastrophic volcanic outgassing or under a steadily brightening Sun. The team closes with a charge to the field: planetary evolution is not a predictable linear path but a wending, stochastic trail, and anyone interpreting a rocky exoplanet from a handful of bulk measurements should be circumspect and prepared to be surprised. That, they argue, is the core lesson the Solar System offers the search for other worlds.