Himalia
Himalia is a large irregular satellite of Jupiter orbiting approximately 11.4 million km from the centre of the planet. Its orbit is markedly eccentric and inclined to the ecliptic, and one revolution takes about 250 Earth days. Himalia gives its name to a group of smaller moons with similar orbits and surface properties.[3][5]
It is a dark, irregular body thought to be a remnant of a captured object. James Webb Space Telescope observations detected spectral signatures of hydrated minerals and ammonium-bearing material, linking Himalia with bodies whose rock was altered by water in the past.[5][2]
Size and physical characteristics
JPL lists a mean radius of 85 ± 10 km, corresponding to a diameter of approximately 170 km. Himalia is not spherical: Cassini images show an elongated outline, while its three-dimensional shape remains poorly constrained.[1][6]
The mass is estimated at approximately 2.3 × 1018 kg, with an uncertainty of about 38%. Its tabulated mean density is 0.88 ± 0.46 g/cm³. The large uncertainties in both mass and size prevent a reliable determination of the proportions of rock, ice and internal void space.[1]
A spherical estimate based on the central mass and radius values gives a surface gravity of approximately 0.021 m/s², about 470 times weaker than Earth's. The corresponding escape speed is about 60 m/s, or 215 km/h. These figures are likewise sensitive to the adopted mass and dimensions.[1]
Orbit
Himalia's orbital semi-major axis is approximately 11.439 million km, about six times greater than that of Callisto, the outermost Galilean moon. The orbital eccentricity is about 0.16 and the inclination to the ecliptic approximately 28.4°.[3]
These mean elements place its distance from Jupiter's centre between roughly 9.6 and 13.3 million km. Himalia moves in a prograde direction and completes one orbit in approximately 249.9 days.[3]
Its great distance, inclination and eccentricity distinguish Himalia from Jupiter's regular inner moons. The orbit is consistent with the capture of an object that once travelled independently around the Sun.[5]
Rotation
Himalia rotates far more rapidly than it orbits Jupiter. A combined analysis of two independent ground-based photometric data sets produced a synodic rotation period of 7.7819 ± 0.0005 hours, or about 7 hours 47 minutes. The brightness variation had an amplitude of 0.20 ± 0.01 magnitude.[4]
Published in 2012, this result replaced an earlier, incorrect estimate of about 9.5 hours. Himalia is not synchronously rotating, unlike many of the large moons that orbit close to their planets.[4]
Himalia group and origin
The Himalia group consists of prograde irregular moons with similar orbits. Its known members include Elara, Lysithea and Leda. Their orbital and surface similarities support a common origin in the breakup of a larger captured body.[5]
Himalia is the largest surviving member of the family. The group also contains much smaller bodies; two additional moons with orbital inclinations near 28° were announced in 2018.[7]
JWST spectra indicate that the parent body may have been compositionally heterogeneous and contained material resembling the ammonium-bearing asteroids represented by Ceres.[2]
Surface and spectral composition
Himalia reflects only a small fraction of incident sunlight. Its dark surface and spectral properties resemble those of carbonaceous asteroids. Existing images do not resolve individual craters or other detailed landforms.[5][6]
Himalia's spectrum, obtained with JWST's NIRSpec instrument, contains a complex absorption band between 2.7 and 3.6 μm and evidence of carbon dioxide bound within other materials.[2]
The broad band can be produced by hydrated silicates and ammonium compounds. Ammoniated phyllosilicates provide the best match, and similar features appear on Elara. They indicate that some of the moons' original material underwent alteration involving liquid water.[2]
Discovery and name
Charles Dillon Perrine discovered Himalia on 3 December 1904 in photographs taken with the 90-centimetre Crossley reflector at Lick Observatory on Mount Hamilton, California.[5]
The moon is also designated Jupiter VI. It is named after Himalia, a nymph of Rhodes and a lover of Zeus in Greek mythology.[5][8]
Cassini observations
On 19 December 2000, Cassini photographed Himalia during its Jupiter flyby on the way to Saturn. The spacecraft was approximately 4.4 million km from the moon, so only its general outline could be distinguished.[6]
The images resolved Himalia's disc and indicated an irregular shape. The illuminated body's vertical extent was estimated at approximately 160 km. Before this observation, Jupiter's outer moons had appeared as little more than point sources in spacecraft images.[6]
The image does not reveal detailed topography. Most current knowledge of Himalia's composition and rotation therefore comes from spectroscopy and variations in brightness rather than close surface imaging.[6][4][2]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Planetary Satellite Physical Parameters // NASA JPL. Himalia: GM = 0.15155 ± 0.05763 km³/s², JUP344 solution. Mass calculated with G = 6.67430 × 10−11 m³/(kg·s²).
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Sharkey B. N. L. et al. JWST Reveals Varied Origins Between Jupiter's Irregular Satellites // The Planetary Science Journal. 2025. DOI: 10.3847/PSJ/ae04dd.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 Planetary Satellite Mean Elements // NASA JPL. Himalia: mean elements for 1 January 2000, 12:00 TDB.
- ↑ 4.0 4.1 4.2 4.3 Pilcher F., Mottola S., Denk T. Photometric lightcurve and rotation period of Himalia (Jupiter VI) // Icarus. 2012. Vol. 219. P. 741–742. DOI: 10.1016/j.icarus.2012.03.021.
- ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 Himalia // NASA Science.
- ↑ 6.0 6.1 6.2 6.3 6.4 Himalia, a Small Moon of Jupiter // NASA/JPL/University of Arizona. 31 January 2001. PIA02881.
- ↑ Sheppard S. et al. New Jupiter Satellites and Moon-Moon Collisions // Research Notes of the AAS. 2018.
- ↑ All Moons of Jupiter // NASA Science. Table of moons and discovery circumstances.