Daphnis (moon)

Daphnis
Image of Daphnis by the Cassini spacecraft on January 18, 2017
Provisional or systematic name	S / 2005 S 1
Central body	Saturn
Properties of the orbit
Major semi-axis	136,505.5 ± 0.1 km
Periapsis	136,501 km
Apoapsis	136,510 km
eccentricity	0.0000331 ± 0.0000062
Orbit inclination	0.0036 ± 0.0013 °
Orbital time	0.5940798 d
Mean orbital velocity	16.67 km / s
Physical Properties
Albedo	≈ 0.5
Apparent brightness	24.0 likes
Medium diameter	7.8 ± 1.6 (9 × 9 × 6) km
Dimensions	8.4 ± 1.2 x 10 13 kg
surface	≈ 200 km 2
Medium density	0.34 ± 0.21 g / cm 3
Sidereal rotation	0.5940798
Acceleration of gravity on the surface	≈ 0 m / s 2
Escape speed	≈ 0 m / s
Surface temperature	≈ −195 ° C (78) K
discovery
Explorer	Cassini Imaging Science Team
Date of discovery	May 6, 2005
Remarks	Shepherd Moon of the Keeler Gap

Daphnis (also Saturn XXXV ) is the third innermost and one of the smaller of the 82 known moons of the planet Saturn . It is the shepherd's moon of the Keeler Gap and the outer of two moons within the A-ring of Saturn's rings .

Discovery and naming

Daphnis was discovered on May 6, 2005 on images taken by the Cassini space probe by the Imaging Science Team of the Cassini mission. The images of the outer A-ring were taken on May 1, 2005 for 16 minutes in a time-lapse sequence with an exposure time of 0.18 seconds.

The existence of a moon within the Keeler Gap had been predicted before the recordings. Wave patterns generated by gravitational disturbances in the ring system at the outer edge of the gap indicated a possible moon at the position of Daphnis. The waves on the inner rim sped ahead of Daphnis' orbit while those on the outer rim lay behind him. The reason for this is the different relative speeds of the ring particles.

Daphnis was later found on 32 images of the F-ring from April 13, 2005, which were taken over 18 minutes. On two further high-resolution images (3.54 km / pixel ) from May 2, 2005, the diameter of the moon was finally resolved more closely.

The discovery was announced on May 6, 2005 by Carolyn Porco , Head of the Imaging Science Team for the Cassini Mission. The moon was initially given the provisional designation S / 2005 S 1 , according to the system of the International Astronomical Union .

On July 17, 2006, the moon was named after the shepherd and poet Daphnis from Greek mythology . Daphnis was the son of Hermes and a nymph, as well as a descendant of the titans , after whom the largest moons of Saturn are named. Daphnis went blind due to a breach of faith with a nymph. He wanted to defeat Eros by not giving in to his love for Xenea awakened by Aphrodite . Pan also fell in love with Daphnis and taught him to play the pan flute . Due to his blindness, he fell from a rock and was turned into a rock himself.

The name comes from the laurel grove in which Daphnis was born and abandoned by his mother. The moons Daphnis and Pan - the only other shepherd moons within Saturn's main rings - were both named after mythological figures associated with shepherds.

Track properties

Cassini image of Daphnis with shadows and wave patterns

Orbit

Daphnis orbits Saturn on a prograde , almost perfectly circular orbit at an average distance of around 136,505.5 km (approx. 2.265 Saturn radii) from its center, i.e. 73,237 km above its cloud ceiling. The orbital eccentricity is 0.0000331, the orbit is inclined 0.0036 ° to the equator of Saturn , so it lies almost exactly in the equatorial plane of the planet. Due to the small eccentricity, the distance to Saturn varies by only about 10 km, due to the low inclination in height by only about 17 km.

The orbit of the next inner moon Pan is on average 2,921 km from Daphnis' orbit, the distance of the orbit of the next outer moon Atlas is on average 1,165 km.

Daphnis orbits Saturn in 14 hours, 15 minutes and 28.5 seconds. This roughly corresponds to the orbital period of the Uranus moon Cupid . Daphnis needs about 27 minutes longer than its inner neighbor Pan to make one cycle.

The moon runs around the planet in the 42 km wide Keeler gap of the A-ring , named after James Edward Keeler , which is only about 270 km from the outer edge of the A-ring. It acts as a shepherd's moon and with its gravity causes the Keeler gap to remain largely free of ring particles. Scientists have not observed any orbital disruptions from moons moving further out.

The moon orbits Saturn within a critical distance, the so-called Roche limit , which would cause a larger moon in this area to burst. Daphnis is probably only saved from this fate because of its small size or a loose internal structure.

rotation

Daphnis is always facing Saturn with the same side, that is, he shows a synchronous rotation like the earth's moon . The rotation time is therefore the same as the cycle time , which is also 14 hours, 15 minutes and 28.5 seconds.

Physical Properties

Daphnis in the Keeler gap

size

Daphnis has a mean diameter of 7.8 km. In the images of the Saturn orbiter Cassini, Daphnis appears as an irregularly shaped, elongated object with dimensions of 9 × 9 × 6 km, with the longitudinal axis aligned with Saturn.

internal structure

At 0.34 g / cm³, the mean density of Daphnis is far lower than that of Earth and even considerably lower than the density of Saturn; it is so low that Daphnis would swim in water even better than Saturn. This indicates that the moon is predominantly composed of water ice .

The fact that Daphnis orbits Saturn within the Roche boundary indicates that it either has a very solid internal structure or that it belongs to the so-called rubble piles , which have cavities inside due to the comparatively weak gravity. Because of the extremely low mean density, the latter hypothesis is more likely.

surface

Daphnis has a high albedo of an estimated 0.5, which means that it has a bright surface that reflects 50% of the incident sunlight . The mean surface temperature of Daphnis is estimated to be approximately −195 ° C (78 K ).

exploration

Due to its small size, the apparent brightness of 24.0 ^m (which is 1: 2,000,000,000 that of the central planet) and the overexposure by Saturn due to its close proximity, Daphnis cannot be seen with earth-based telescopes.

So far, Daphnis has been visited by three space probes , namely the flyby probes Voyager 1 on November 12, 1980 and Voyager 2 on August 25, 1981 as well as the Saturn orbiter Cassini, which has orbited Saturn since July 2004. Daphnis has been targeted by Cassini several times, so its orbital parameters are now fairly well known. The closest flyby to Cassini occurred on July 5, 2010, when the probe passed the moon at a distance of 72,816 km.

Web links

Commons : Daphnis (moon)  - collection of images, videos and audio files

• IAUC 8524: S / 2005 S 1 May 6, 2005 (discovery; English)
• IAUC 8730: Saturn XXXV (Daphnis) = S / 2005 S 1 July 17, 2006 (name; English)

Individual evidence

1. ↑ Paul Rincon: Saturn overtakes Jupiter as planet with most moons. BBC , October 7, 2019, accessed March 20, 2020 . 

further inside	Saturn moons	further outside
Pan	Semi- major axis  (km) Daphnis 136.500	Atlas