Helene (moon)
Helene is a small icy moon of Saturn that shares the orbital region of Dione, travelling approximately 60° ahead of the larger moon. A body in this configuration is known as a Trojan moon. Helene has a volumetric mean diameter of about 36 km, roughly one thirty-first of Dione's diameter.[2][1]
Images obtained by Cassini revealed Helene's irregular shape and subdued topography. Loose material partly covers its craters, while its surface reflects substantially more light than Dione's.[2][6]
Size and physical characteristics
Helene has a volumetric mean radius of 18 km. Its volume equals that of a sphere 36 km across, although the moon itself has an irregular shape. Its mass is approximately 7.2 × 1015 kg, about one nineteenth of the mass of Pandora.[1]
The mean density is only 0.29 g/cm³, less than one third the density of solid water ice. If ice is its principal constituent, this value indicates a highly porous interior in which void space accounts for a substantial fraction of the total volume.[1][4]
A spherical estimate gives a surface gravity of approximately 0.0015 m/s², about 6,600 times weaker than Earth's. The escape velocity calculated from the mass and mean radius is approximately 7.3 m/s, or 26 km/h. Local gravity varies across the moon because of its irregular shape.[1]
Orbit and Trojan motion
Helene orbits Saturn at a mean distance of approximately 377,600 km from the planet's centre. One revolution takes 2.736916 Earth days, or about 2 days 17 hours 41 minutes. The orbit is slightly eccentric and inclined by approximately 0.2° to the local Laplace plane.[3]
The moon occupies the region around the L4 Lagrange point of the Saturn–Dione system. It has the same mean orbital period as Dione and librates around a position 60° ahead of it. This configuration is a 1:1 orbital resonance.[7]
Dione has a second Trojan moon, Polydeuces, which travels near the trailing L5 point. A similar arrangement accompanies Tethys: Telesto leads it and Calypso follows behind. These systems provide natural examples of small bodies sharing an orbit with a much more massive moon.[7]
Surface and topography
Large depressions and smooth regions are visible on Helene. Images obtained on 25 February 2006 showed a surface partly buried beneath its own impact debris. Telesto has a similarly subdued appearance.[2]
Impact structures occupy a substantial fraction of such a small body's surface. Loose deposits soften their outlines, so the visible terrain records both crater formation and the later redistribution of surface material.[2]
Reflectivity and dust covering
Helene and Calypso are substantially brighter than Dione and Tethys, the larger moons whose orbits they share. The contrast remains after accounting for the elongated shapes of the small moons, which affect the area illuminated during an observation.[6]
The ordinary flux of particles from Saturn's E ring is insufficient to explain this brightness. Helene and Calypso may receive additional dust, although its source remains uncertain. Possible explanations include distinctive dust-particle trajectories or a comparatively recent increase in the amount of material deposited on their surfaces.[6]
Discovery and name
Helene was discovered on 1 March 1980 through ground-based observations. Saturn's rings were then viewed almost edge-on from Earth, reducing their interference with searches for faint moons. NASA lists the French astronomers Pierre Laques and Jean Lecacheux as the discoverers.[4][5]
The moon initially received the designation S/1980 S6 and was also known as Dione B. Its official name refers to Helen of Troy in Greek mythology. Its numbered designation is Saturn XII.[4][5]
Cassini observations
On 3 March 2010, Cassini obtained images during its closest encounter with Helene. One frame, taken from a distance of approximately 1,900 km, shows the hemisphere facing away from Saturn.[8]
The spacecraft made its second-closest pass on 18 June 2011, approaching to within 6,968 km of the surface. The resulting sequence complemented the 2010 observations by revealing other parts of the small moon.[9] The image used in this article was obtained during that encounter; the published version was processed to reveal detail in shadowed regions.[10]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Planetary Satellite Physical Parameters // NASA JPL. Helene: GM = 0.00048 ± 0.00002 km³/s² from the SAT441 solution. Mass calculated using G = 6.67430 × 10−11 m³/(kg·s²).
- ↑ 2.0 2.1 2.2 2.3 2.4 Helene // NASA/JPL/Space Science Institute. 29 December 2006. PIA08335.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Planetary Satellite Mean Elements // NASA JPL. Helene: mean elements for 1 January 2000, 12:00 TDB.
- ↑ 4.0 4.1 4.2 4.3 Helene // NASA Science.
- ↑ 5.0 5.1 5.2 Saturn Moons // NASA Science. Table of moons and discovery circumstances.
- ↑ 6.0 6.1 6.2 Hedman M. M. et al. Photometric Analyses of Saturn’s Small Moons: Aegaeon, Methone, and Pallene Are Dark; Helene and Calypso Are Bright // The Astronomical Journal. 2020. Vol. 159. Article 129.
- ↑ 7.0 7.1 Robutel P., Rambaux N., Castillo-Rogez J. Influence of the Coorbital Resonance on the Rotation of the Trojan Satellites of Saturn // Celestial Mechanics and Dynamical Astronomy. 2012. Vol. 113. P. 1–22.
- ↑ Closest View of Helene // NASA/JPL/Space Science Institute. 11 June 2010. PIA12653.
- ↑ Cassini Captures Ice Queen Helene // NASA/JPL-Caltech/Space Science Institute. 20 June 2011.
- ↑ Leading Hemisphere of Helene — 20110618 // NASA/JPL/Space Science Institute; processed by WolfmanSF. Wikimedia Commons.
