Sinope (moon)
Outermost retrograde Jovian moon, possibly independently captured.
Sinope (Jupiter IX) is a moon of Jupiter and one of its outermost irregular satellites, meaning it follows a distant, highly tilted, and elongated path around the planet. Discovered by Seth Barnes Nicholson at Mount Wilson Observatory in 1914, Sinope moves in a retrograde direction—opposite to Jupiter’s rotation—at an average distance of about 23.6 million kilometers (14.7 million miles).
The moon is a member of the Pasiphae group, a collection of irregular Jovian moons thought to be fragments of an asteroid or trans-Neptunian object that Jupiter captured and shattered billions of years ago. However, Sinope’s average orbital inclination and its different color suggest it might have been captured on its own, unrelated to that collision. Most of its physical traits are unknown, but it is estimated to be about 35 kilometers in diameter with a dark, red surface.
Sinope was originally designated Jupiter IX and was sometimes called "Hades" between 1955 and 1975, before receiving its current name from Greek mythology in 1975. It orbits at a mean distance of 23,679,300 kilometers, completing one revolution in 741.8 days. Its orbit has an inclination of 159.3° relative to the ecliptic and an eccentricity of 0.269, and it is constantly altered by gravitational pulls from the Sun and other planets. Sinope is in a secular resonance with Jupiter, similar to Pasiphae, but can drop out of this resonance, alternating between resonant and non-resonant behavior over timescales of 10 million years.
As part of the Pasiphae group, Sinope shares orbital traits with other members—semimajor axes between 22 and 25 million km, inclinations from 141° to 158°, and eccentricities of 0.22 to 0.44. Yet its larger size and distinct inclination sometimes set it apart, and its spectral features differ from Pasiphae’s, supporting a separate origin. The small irregular moon Aoede has a similar orbit and may be a remnant of Sinope.
Thermal measurements give Sinope a diameter of about 35 km and an albedo of 4.2%, making it one of the larger irregular Jovian moons. Its shape is unknown, but given its small size, it is likely irregular. In visible light, Sinope appears red (color indices B−V=0.84, R−V=0.46), and its infrared spectrum resembles D-type asteroids, unlike Pasiphae’s. Thermal modeling by WISE yielded a beaming parameter of 0.82 ± 0.02. Its rotation period is roughly 13 hours and 9.
Quick Facts
- Pronounced
- s · ə · ˈ · n · oʊ · p · iː
- Adjective
- Sinopean s · aɪ · n · ə · ˈ · p · iː · ə · n
- Named After
- Σινώπη Sinōpē
- Mpc Name
- Jupiter IX
- Discoverer
- Seth B. Nicholson
- Discovery Site
- Lick Observatory
- Discovered
- 21 July 1914
- Satellite Of
- Jupiter
- Group
- Pasiphae group
- Epoch
- 2026-01-01
- Observation Arc
- 103.87 yr (37,938 days)
- Periapsis
- 17.3 million km
Facts from the source article.
Lore & Background
Sinope was discovered by Seth Barnes Nicholson at Mount Wilson Observatory in 1914. It was originally known simply as Jupiter IX until 1975, when it received its present name from Greek mythology; between 1955 and 1975 it was sometimes called 'Hades'. The moon orbits Jupiter in the retrograde direction at an average distance of 23.6 million km, completing one orbit in 741.8 days. Its orbit has an inclination of 159.3° and an eccentricity of 0.269, and is subject to continuous gravitational perturbations from the Sun and other planets. Sinope is known to be in a secular resonance with Jupiter, similar to Pasiphae, but can drop out of this resonance, exhibiting periods of both resonant and non-resonant behavior on time scales of 10^7 years.
Sinope belongs to the Pasiphae group, a collection of irregular retrograde Jovian moons believed to be fragments of an asteroid or trans-Neptunian object captured by Jupiter and destroyed by a collision billions of years ago. However, due to its large size, relatively distinct inclination, and dissimilar spectral features from Pasiphae, Sinope is sometimes considered an independent object captured separately. The small irregular moon Aoede has a similar orbit and could be a remnant of Sinope. Most of Sinope's physical properties are unknown; it is estimated to be about 35 km in diameter with a dark and red surface. Its visible spectrum is red, and its infrared spectrum resembles D-type asteroids, differing from that of Pasiphae. The rotation period is approximately 13 hours and 9.6 minutes, though later papers regard this as uncertain due to short observation periods.
Reader's Guide
Sinope's significance lies in its role as one of the outermost irregular satellites of Jupiter, providing insights into the capture and fragmentation history of the Jovian system. Its membership in the Pasiphae group suggests a common origin as a captured and shattered asteroid or trans-Neptunian object, yet its distinct inclination and red color raise the possibility that Sinope was captured independently, unrelated to the collision that formed the group. This ambiguity makes Sinope a key object for studying the diversity of irregular moon origins. The moon has not been imaged up close by any space probe, but it is planned as a distant observation target for the European Space Agency's Jupiter Icy Moons Explorer (Juice) mission, which will measure its rotation period and shape by monitoring brightness changes over time. Sinope's secular resonance with Jupiter, which it intermittently enters and leaves, adds a dynamic complexity that helps scientists understand orbital evolution under gravitational perturbations. Its estimated diameter of 35 km and low albedo of 4.2% place it among the larger irregular Jovian moons, yet its shape remains unknown, likely irregular due to its small size.
Did You Know?
- Sinope's orbit is in a secular resonance with Jupiter, but it can drop out of this resonance on time scales of 10^7 years.
- The small moon Aoede has a similar orbit to Sinope and could be a remnant of the latter.
More in Moons of Jupiter 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced