Daphnis (moon)
Daphnis is a small moon of Saturn orbiting inside the Keeler Gap near the outer edge of the planet's A ring. Its volumetric mean diameter is approximately 8 km. The moon's gravity produces waves along both edges of the gap, including structures that rise above and descend below the main plane of the rings.[2][4]
Detailed Cassini images reveal a smooth outer covering, several small craters and an equatorial ridge. Daphnis belongs to a group of small Saturnian moons whose appearance has been strongly altered by the accumulation of ring material.[4][3]
Size and physical characteristics
The shape model published in 2020 gives Daphnis a volumetric mean radius of 3.9 ± 0.5 km. The corresponding ellipsoid measures approximately 9.8 × 8.4 × 5.6 km. The moon is distinctly elongated and flattened: its longest dimension is almost 1.8 times its shortest.[2]
Its mass is estimated at (7.7 ± 1.5) × 1013 kg, while its mean density is approximately 0.27 ± 0.14 g/cm³. Both values remain uncertain. The low central density estimate is consistent with a porous interior containing a large volume of void space.[1][2]
Daphnis is substantially smaller than Pan and Atlas. Its proximity to the ring particles nevertheless allows this small body to produce structures that extend far beyond its own dimensions.[1][7]
Orbit and the Keeler Gap
Daphnis orbits at a mean distance of approximately 136,500 km from Saturn's centre. One revolution takes 0.594080 Earth days, or about 14 hours 15 minutes. Its orbit is nearly circular and lies close to the plane of the rings.[5]
The moon travels within the Keeler Gap, an opening approximately 42 km wide in the A ring. The gap lies closer to the ring's outer edge than the broader Encke Gap, which contains Pan.[8]
Daphnis gravitationally perturbs particles on both sides of its orbit and gives the gap its wavy edges. In spacecraft images, the moon appears as a small body surrounded by long bands of ring material.[7]
Waves along the ring edges
Particles at the inner edge of the gap orbit Saturn faster than Daphnis. The waves produced there therefore extend ahead of the moon in the direction of orbital motion. Particles at the outer edge move more slowly, and the corresponding waves trail behind it.[6]
The disturbances also have a vertical component: particles rise above the ring plane and descend below it. Near Saturn's 2009 equinox, when sunlight struck the rings at a very low angle, Cassini recorded shadows cast by these elevated structures.[6]
Images obtained on 30 April 2009 show Daphnis within the gap together with the bends and vertical undulations created by its gravity.[7]
Surface and accumulation of material
High-resolution images show a small equatorial ridge and a comparatively smooth outer layer. A second ridge runs approximately parallel to the main one north of the equator, while several craters remain visible across the surface.[4]
Small moons within and near Saturn's rings acquire dust and ice particles from their surroundings. This process has altered the outer layers of Pan and Daphnis particularly strongly.[3]
Loose material settles on a denser central body. The high porosity of the coating and preferential accumulation near the equator help explain the distinctive outlines of the small ring moons.[3]
The ridges of Pan, Atlas and Daphnis may all have grown through the accretion of ring particles. Their shapes depend on the distribution of material near the gap, the moon's orbit and changes in its orientation as particles are captured.[9]
Discovery and name
The Cassini imaging science team discovered Daphnis in images obtained on 1 May 2005. Waves along the edges of the Keeler Gap had already indicated that an unseen moon was orbiting within it.[6]
Its provisional designation was S/2005 S1, and its numbered designation is Saturn XXXV. The moon is named after Daphnis, a shepherd and musician in Greek mythology.[6][10]
Cassini close-up
On 16 January 2017, Cassini obtained a particularly detailed image of Daphnis from a distance of approximately 28,000 km.[4]
A narrow strand of ring material extends behind the moon in the image. Daphnis may have drawn a small clump from the edge of the ring before the material began to spread along its orbit.[4][11]
See also
References
- ↑ 1.0 1.1 1.2 Thomas P. C. Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission // Icarus. 2010. Vol. 208. P. 395–401. Table 3.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Thomas P. C., Helfenstein P. The small inner satellites of Saturn: Shapes, structures and some implications // Icarus. 2020. Vol. 344. Article 113355.
- ↑ 3.0 3.1 3.2 3.3 NASA’s Cassini Finds Saturn’s Rings Coat Tiny Moons // NASA/JPL-Caltech. 28 March 2019.
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 Daphnis Up Close // NASA/JPL-Caltech/Space Science Institute. 18 January 2017. PIA21056.
- ↑ 5.0 5.1 5.2 5.3 5.4 5.5 Planetary Satellite Mean Elements // NASA JPL. Daphnis: mean elements for 1 January 2000, 12:00 TDB.
- ↑ 6.0 6.1 6.2 6.3 6.4 6.5 Daphnis // NASA Science.
- ↑ 7.0 7.1 7.2 Gravity-Induced Undulations // NASA/JPL/Space Science Institute. 1 June 2009. PIA11504.
- ↑ Daphnis the Wavemaker // NASA Astronomy Picture of the Day. 21 January 2017.
- ↑ Quillen A. C., Zaidouni F., Nakajima M., Wright E. Accretion of Ornamental Equatorial Ridges on Pan, Atlas and Daphnis // Icarus. 2021. Vol. 357. Article 114260.
- ↑ Saturn Moons // NASA Science. Table of moons and discovery circumstances.
- ↑ PIA21056 — Daphnis Up Close, Enhanced Version // NASA/JPL-Caltech/Space Science Institute. Wikimedia Commons.
