Kalyke

Lore & Background

Kalyke was discovered in 2000 by a team of astronomers from the University of Hawaiʻi led by Scott S. Sheppard, receiving the temporary designation S/2000 J 2. It was named in October 2002 after the Greek mythological figure Kalyke or Calyce. It orbits Jupiter at an average distance of 23,298,000 km in 726.70 days, with an inclination of 165° to the ecliptic, in a retrograde direction and with an eccentricity of 0.2140. Its orbital elements are continuously changing due to solar and planetary perturbations. It belongs to the Carme group, made up of tightly confined irregular retrograde moons orbiting Jupiter at a distance ranging between 22.7 and 23.5 million km, at an inclination of about 165°, and eccentricities between 0.24 and 0.28.

Kalyke appears red in the visible spectrum, similar to D-type asteroids. Its spectrum has been remarked to be like that of asteroid 5145 Pholus, another very reddish object. From infrared thermal measurements by the WISE spacecraft, Kalyke's albedo was measured at 2.9%, corresponding to a diameter of 6.9 kilometres, making it one of the least reflective bodies in the Solar System. A group of scientists observed Kalyke on 14 January 2002 with the Nordic Optical Telescope on the island of La Palma in the Canary Islands of Spain, obtaining colours of V−R=0.72, V−I=0.96. It was observed again on 13 March 2002 with the MMT Observatory on Mount Hopkins in Arizona, United States, with colours of B−V=0.94, V−R=0.70, V−I=0.88. From this they determined Kalyke was a very red object, consistent with the colours of a centaur or TNO (unlike other Carme group objects), but this classification was uncertain. The V−R value is indicative of a red object, while the V−I value is of a more moderate, light red and compatible with P and D-type asteroids. They suggested the moderate V−I value may not be real and an artifact of fringing of the CCDs used to take the images, and that more observations were needed to confirm Kalyke's red colour. However, a later paper disagreed and argued that the V−I value was real, because of its repeatability between observations. The spectral placement could indicate the V−I value was caused by an absorption feature of mafic silicate. If this absorption feature were to be found around the 1 μm mark in its spectrum, it would be just as likely for Kalyke to have originated in the main asteroid belt.

Kalyke probably did not form near Jupiter but was captured by Jupiter later. Like the other members of the Carme group, which have similar orbits, Kalyke is probably the remnant of a broken, captured heliocentric asteroid. The Carme group is thought to have been created when a D-type asteroid split into many fragments when it later suffered an impact. However, Kalyke is significantly redder in color than other moons of the Carme group. This suggests that it may be an interloper from the outer Solar System, as opposed to an origin with the Hilda group or Jupiter trojans that has been speculated for the rest of the Carme group. It was proposed that Kalyke was a remnant of the object that collided with the Carme group progenitor, as opposed to being a fragment of the progenitor itself. However, it would be unusual for such a remnant to settle into a post-collision orbit very similar to the progenitor it impacted.

Reader's Guide

Kalyke's significance lies in its unusual redness compared to other Carme group moons, which challenges the group's assumed common origin. While the Carme group is thought to have formed from a single D-type asteroid that broke apart after an impact, Kalyke's very red color—similar to centaurs or trans-Neptunian objects—suggests it may be an interloper from the outer Solar System. This has led to the proposal that Kalyke might be a remnant of the impacting object rather than a fragment of the progenitor, though such a scenario would be unusual because a remnant would not typically settle into an orbit so similar to the progenitor's. The debate over Kalyke's color measurements, with some scientists attributing its moderate V−I value to CCD fringing artifacts and others arguing it is real and possibly due to mafic silicate absorption, further complicates its classification. If the absorption feature is real and lies near 1 μm, Kalyke could have originated in the main asteroid belt. Its very low albedo of 2.9% makes it one of the least reflective bodies in the Solar System, and its diameter of 6.9 km was determined from WISE thermal measurements. Kalyke thus serves as a key object for understanding the diversity of captured irregular satellites and the complex collisional history of the Jupiter system.

Did You Know?

• Kalyke is the second-largest member of the Carme group.
• It was proposed that Kalyke may be a remnant of the object that collided with the Carme group progenitor, rather than a fragment of the progenitor itself.