Aegaeon (moon)
Aegaeon is a small moon of Saturn that travels within the bright arc of the tenuous G ring. Its volume equals that of a sphere approximately 660 metres across, although the body itself is strongly elongated. Aegaeon completes one orbit in about 19.4 hours; both its motion and the distribution of nearby ring particles are controlled by an orbital resonance with Mimas.[2][4][8]
Aegaeon is among the smallest individually observed moons of Saturn. It provides a natural example of how a sub-kilometre body interacts with surrounding dust and participates in the dynamics of a much larger ring structure.[3][8]
Size and physical characteristics
Aegaeon has a volumetric mean radius of 0.33 ± 0.06 km. Its equivalent diameter is approximately 660 metres, or about 0.7 km. This value describes the moon's volume rather than its greatest dimension.[2]
Close Cassini observations revealed a strongly elongated shape. A preliminary analysis of the 2011 images estimated a length of 1.2–1.6 km and a width of 0.3–0.6 km. The long axis pointed towards Saturn during the observation, consistent with synchronous rotation.[3]
Dynamical models use a density of approximately 0.54 g/cm³, estimated from the body's shape. With a mean radius of 330 m, this gives a mass of roughly 8 × 1010 kg. Both values depend on the structural model; Aegaeon's mass has not been measured with the precision available for larger moons.[1]
Orbit and resonance with Mimas
Aegaeon's semi-major axis is approximately 167,500 km from Saturn's centre. One orbit takes 0.808115 Earth days, or about 19 hours 24 minutes. The orbit is nearly circular and lies close to the plane of the rings.[4]
The moon is trapped in the 7:6 corotation eccentricity resonance with Mimas. During an interval close to six orbits of Mimas, Aegaeon completes approximately seven revolutions around Saturn. The resonance confines its motion relative to a particular region of the ring and keeps a concentration of particles nearby, forming the arc.[8]
The influence of Mimas links Aegaeon with two other small moons, Anthe and Methone, which are also associated with corotation resonances and dusty arcs. The precise orbital configuration and the ratio between the moon's mass and the surrounding material differ in each system.[8]
G-ring arc
The G ring lies outside Saturn's bright main rings. A denser arc occupies part of its inner region, and Aegaeon travels within it. The arc is approximately 250 km wide in the radial direction and about 150,000 km long along the orbit, roughly one-sixth of the ring's circumference.[5]
Dust scatters sunlight efficiently, so the extended arc is visible in images even though Aegaeon itself is extremely small. In early long-exposure frames, the moon appears as a short bright streak moving with the ring.[6]
Interplanetary particles striking Aegaeon can eject material from its surface, replenishing the arc before the dust spreads through the rest of the G ring. Estimates of the amount of dust and its loss rate, however, leave room for additional sources.[6][9]
Surface and reflectivity
Aegaeon has a dark surface. Close-image analysis found that it reflects substantially less light than many other icy moons inside Titan's orbit. Observations through different filters also indicate a reddish spectral slope.[3]
Aegaeon, Methone and Pallene are all darker than expected in comparison with neighbouring larger moons. This difference remains after their elongated shapes, which affect apparent brightness, are taken into account.[10]
All three moons are darkest where the flux of high-energy protons is greatest. Irradiation probably alters and darkens their surface material, although existing images do not reveal the chemical composition of the dark component.[10]
Dust supply and loss
Particles in the arc are affected not only by the gravity of Saturn and its moons but also by solar radiation pressure and interactions with plasma. These processes alter the paths of small dust grains and shorten the time they remain within the arc.[9]
A 2020 model compared particle losses with the amount of material produced by impacts on the small moons. Under the adopted conditions, Aegaeon alone could not replenish its arc over long periods. The result indicates that such structures may be temporary or require additional sources of material.[9]
Discovery and exploration
Aegaeon was found in Cassini images obtained on 15 August 2008 and then identified in earlier frames. The discovery was announced in March 2009. Its provisional designation was S/2008 S1, and it is also known as Saturn LIII.[5][6][7]
The first orbital solution used 21 images taken between June 2007 and February 2009. The moon was unresolved in these frames, so its size was initially estimated from its brightness.[8]
On 27 January 2011, Cassini passed approximately 15,000 km from Aegaeon. The resulting images revealed its elongated shape.[3][11]
The moon is named after Aegaeon, a hundred-handed giant of Greek mythology who took part in the struggle against the Titans.[5]
See also
References
- ↑ 1.0 1.1 1.2 Long-Term Evolution and Stability of Saturnian Small Satellites: Aegaeon, Methone, Anthe and Pallene // Monthly Notices of the Royal Astronomical Society. 2017. Vol. 470. P. 3750–3764. Table 1 gives the adopted mean radius and model density of Aegaeon. The mass is calculated as 4πρR³/3 and is not an independent gravitational measurement.
- ↑ 2.0 2.1 2.2 2.3 Thomas P. C. et al. The Inner Small Satellites of Saturn: A Variety of Worlds // Icarus. 2013. Vol. 226. P. 999–1019.
- ↑ 3.0 3.1 3.2 3.3 3.4 Hedman M. M. et al. Physical Properties of the Small Moon Aegaeon (Saturn LIII) // European Planetary Science Congress. 2011.
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 Planetary Satellite Mean Elements // NASA JPL. Aegaeon: mean elements for 1 January 2000, 12:00 TDB.
- ↑ 5.0 5.1 5.2 5.3 Aegaeon // NASA Science.
- ↑ 6.0 6.1 6.2 6.3 Tiny Moonlet Within G Ring Arc // NASA/JPL/Space Science Institute. 3 March 2009.
- ↑ 7.0 7.1 Saturn Moons // NASA Science. Table of moons and discovery circumstances.
- ↑ 8.0 8.1 8.2 8.3 8.4 Hedman M. M. et al. Aegaeon (Saturn LIII), a G-Ring Object // Icarus. 2010. Vol. 207. P. 433–447.
- ↑ 9.0 9.1 9.2 Madeira G., Giuliatti Winter S. M. Effects of Immersed Moonlets in the Ring Arc Particles of Saturn // The European Physical Journal Special Topics. 2020. Vol. 229. P. 1527–1543.
- ↑ 10.0 10.1 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.
- ↑ Aegaeon N1643264379 // NASA/JPL. Image description on Wikimedia Commons.
