Carme (moon)

Lore & Background

Carme was discovered by Seth Barnes Nicholson at Mount Wilson Observatory in California in July 1938. It is named after the mythological Carme, mother by Zeus of Britomartis, a Cretan goddess. The moon did not receive its present name until 1975; before then, it was simply known as Jupiter XI. It was sometimes called 'Pan' between 1955 and 1975 (Pan is now the name of a satellite of Saturn).

Carme orbits Jupiter at an average distance of 22,579,859 km in 693.17 days, at an inclination of 165° to the ecliptic, in a retrograde direction and with an eccentricity of 0.23. These orbital parameters are continuously changing due to solar and planetary perturbations. It gives its name to the Carme group, made up of retrograde irregular moons orbiting Jupiter at distances ranging between 22.7–23.5 million km, at an inclination of about 165°, and eccentricities between 0.24 and 0.28.

With a diameter of 46.7±0.9 km (albedo 3.5%), it is the largest member of the Carme group and the fourth-largest irregular moon of Jupiter. Like the other members of the Carme group (except for Kalyke) it is light red in color (B−V=0.76, V−R=0.47), similar to D-type asteroids. Its spectral shape and absorption features are special and make it unique compared to other satellites, with a strong upturn in the ultraviolet range and a CN emission at 0.388 μm, possibly indicative of low-level cometary activity. One study found that modelling its surface composition as 4% low-Fe serpentine and 96% kerogen provided a good match; another suggested it was composed mainly of amorphous carbon, minnesotaite, and ilmenite. The rotation period was found to be approximately 10 hours and 24 min, regarded by a later paper as uncertain due to short observation periods; another group found a different possible rotation period of 6.48 hours.

Reader's Guide

Carme is notable as the largest member of the Carme group and the fourth-largest irregular moon of Jupiter. Its discovery in July 1938 by Seth Barnes Nicholson at Mount Wilson Observatory added to the known irregular satellite population. The moon's naming history—first known as Jupiter XI, then sometimes called 'Pan' between 1955 and 1975, and finally named Carme in 1975—reflects evolving conventions in satellite nomenclature. Its orbit, at an average distance of 22,579,859 km with a retrograde inclination of 165°, places it among the distant irregular moons believed to be captured heliocentric asteroids. The Carme group, which shares similar orbital parameters, likely originated from a single captured asteroid that later broke apart. Carme's unique spectral features, including a CN emission possibly indicating cometary activity, and its surface composition models (kerogen and serpentine, or amorphous carbon with minnesotaite and ilmenite) distinguish it from other Jovian moons. The uncertain rotation period (approximately 10 hours 24 min or possibly 6.48 hours) highlights ongoing observational challenges. The moon's significance extends to potential future exploration, as the small Carme group moon Kallichore might be visited by the European Space Agency's Jupiter Icy Moons Explorer (Juice) mission in 2031.

Did You Know?

• It was known as Jupiter XI until 1975, and was sometimes called 'Pan' between 1955 and 1975.
• Its spectral shape shows a CN emission at 0.388 μm, possibly indicating low-level cometary activity.

Place in the Jovian Satellite System

Carme occupies a modest but fascinating niche within Jupiter's sprawling family of natural satellites. As of April 2026, astronomers have catalogued 115 known moons orbiting the gas giant, and Carme belongs to the outer irregular cohort rather than the celebrated Galilean quartet. These irregular satellites trace their lineage to asteroids that Jupiter snared from their original solar orbits, and they orbit at far greater distances, with highly inclined and eccentric paths, many moving in retrograde fashion opposite to the planet's spin. In terms of sheer size, Carme and its irregular kin are dwarfed by the Galilean moons; while Ganymede stretches beyond 3,100 kilometres in diameter, nearly every other Jovian satellite measures under 250 kilometres, and most barely clear the 5-kilometre mark. The naming tradition for these smaller bodies draws from Roman and Greek mythology, assigning them the names of lovers or daughters of Jupiter or Zeus. Notably, 58 of the 107 known irregular moons still lack official designations, a reminder that the outer reaches of the Jovian system remain only partially charted.

The Carme Group and Its Collisional Birth

The Carme group stands as a collisional family, a cluster of fragments sharing similar orbital parameters that likely originated when a single parent body was struck by a passing planetesimal. Numerical modeling suggests the impactor measured roughly 0.59 kilometres in diameter, a size that was extraordinarily common during the early assembly of the Solar System. Crucially, the blow was powerful enough to carve large craters into the progenitor and shatter it into multiple kilometer-sized pieces, yet not so devastating that the largest surviving fragment lost more than a sliver of the original mass. In fact, the biggest piece of the Carme group retains approximately 99 percent of the parent body's mass, a remarkably high fraction. This sets Carme apart from the Ananke group, where the largest fragment holds about 96 percent, and even more so from the Himalia group, whose largest piece accounts for only 78 percent of its parent. The latter's more energetic disruption would have required an impactor around 13 kilometres across, a size too rare among early planetesimals to be a likely explanation. While a moon-to-moon collision remains a theoretical possibility if Jupiter once hosted far more retrograde satellites, the passing-planetesimal scenario fits the Carme group's gentle fragmentation pattern most naturally.

Orbital Environment and the Rarity of Collisions

Carme and its fellow retrograde irregular satellites inhabit a region of Jupiter's gravitational domain that is vast, sparsely populated, and dynamically quiet. Their orbits sit much farther from the planet than the regular inner moons, tracing elongated, tilted paths that often run counter to Jupiter's rotation. Because of these long orbital periods and the great distances involved, encounters between retrograde satellites are exceedingly rare. Numerical integrations spanning the full 4.5-billion-year history of the system estimate that the expected number of collisions among all retrograde moons combined is only about one. This scarcity of impacts creates an interesting tension with the very existence of collisional families like the Carme group: if moons barely ever meet, how did they shatter into families? One resolution is that the parent bodies were struck not by sibling moons but by unrelated planetesimals still drifting through the outer Solar System. Another possibility, though less favored for Carme specifically, is that Jupiter's retrograde population was once far more numerous, raising the odds of moon-to-moon encounters before most of those bodies were lost or dispersed over billions of years.

Capture, the Proto-Disk, and a Place Among the Irregulars

The story of how Carme came to orbit Jupiter is inseparable from the planet's early accretion history. The outer irregular satellites are believed to have been asteroids on independent solar orbits that Jupiter's gravity gradually captured. At the time, Jupiter was still surrounded by a massive circumplanetary disk of gas and solid debris, and this disk played a critical role: it absorbed much of the incoming asteroid's orbital momentum, allowing the body to be trapped into a bound orbit around the planet. Once captured, many of these bodies experienced mechanical stresses that fractured them, and subsequent collisions with other small bodies further broke them into the kilometer-sized fragments we observe today. The regular satellites, by contrast, formed from the disk itself, and simulations indicate that only about 2 percent of the proto-disk's total mass was needed to account for the satellites we see now. This implies that several earlier generations of Galilean-mass moons may have formed, spiraled inward, and been lost before the current set settled into its stable configuration. Carme, as a captured and later shattered asteroid, represents a fundamentally different origin story from those inner worlds.