Pasiphae (moon)

Lore & Background

Pasiphae was discovered in 1908 by Philibert Jacques Melotte, first spotted on a plate taken at the Royal Greenwich Observatory on the night of 28 February 1908. Inspection of previous plates found it as far back as January 27. It received the provisional designation 1908 CJ, as it was not clear whether it was an asteroid or a moon of Jupiter; recognition of the latter case came by April 10. It was later named after the mythological Pasiphaë, wife of Minos and mother of the Minotaur from Greek legend. Pasiphae did not receive its present name until 1975; before then, it was simply known as Jupiter VIII, and it was sometimes called 'Poseidon' between 1955 and 1975.

Pasiphae orbits Jupiter on a high eccentricity (0.490) and high inclination (about 157°) on a retrograde orbit with an average distance of 24.1 million km, but it gets as far as 35.9 million km from Jupiter. The orbital elements are continuously changing due to solar and planetary perturbations. In December 2023 the moon reached 0.2494 AU (37.31 million km) from Jupiter, and in November 2024 approached within 0.0666 AU (9.96 million km) of Jupiter. It gives its name to the Pasiphae group, a group of retrograde moons of Jupiter with semi-major axes spread over 22–25 million km, inclinations between 141° and 158°, and higher eccentricities between 0.22 and 0.44. Pasiphae is also known to be in a secular resonance with Jupiter.

With a diameter estimated at 58 km (albedo 4.4%), Pasiphae is the largest retrograde and third largest irregular satellite after Himalia and Elara. Spectroscopical measurements in infrared indicate that Pasiphae is a spectrally featureless object, consistent with the suspected asteroidal origin. The satellite appears pale red (colour indices V=17.22 B-V=0.74, R-V=0.38) though it falls into the grey color-class of C-type asteroids. Pasiphae probably did not form near Jupiter but was captured by Jupiter later, and is believed to be a fragment from a captured asteroid along with other Pasiphae group satellites.

Reader's Guide

Pasiphae holds significance as the largest retrograde irregular satellite of Jupiter and the third largest irregular satellite overall, after Himalia and Elara. Its discovery in 1908 by Philibert Jacques Melotte at the Royal Greenwich Observatory marked an early step in identifying distant, irregular moons that do not follow the prograde, nearly circular orbits of the major Galilean moons. The initial uncertainty over whether it was an asteroid or a moon highlights the challenges of early 20th-century astronomy in distinguishing small solar system bodies. Pasiphae's high eccentricity (0.490) and high inclination (about 157°) on a retrograde orbit, with an average distance of 24.1 million km and a maximum of 35.9 million km, place it among the most dynamically extreme Jovian moons. Its orbital elements change continuously due to solar and planetary perturbations, and it is in a secular resonance with Jupiter. The moon lends its name to the Pasiphae group, a collection of retrograde moons with similar orbital parameters, suggesting a common origin. Spectroscopical measurements show it is spectrally featureless and pale red, consistent with a C-type asteroid classification, supporting the theory that Pasiphae and its group are fragments of a captured asteroid. Its legacy lies in informing models of irregular satellite capture and the evolution of the Jovian system.

Did You Know?

• It was provisionally designated 1908 CJ because it was unclear whether it was an asteroid or a moon of Jupiter.
• It is the largest retrograde irregular satellite of Jupiter, with a diameter estimated at 58 km.