Methone, Anthe and Pallene, the Alkyonides
Methone, Anthe and Pallene are three tiny moons of Saturn, each only a few kilometers across, that orbit between Mimas and Enceladus. Discovered by Cassini in 2004 and 2007, they are known collectively as the Alkyonides. Methone stands out for its smooth, egg-like shape with no visible craters, and all three moons are associated with faint arcs or rings of dust, held in place in two cases by resonances with Mimas.
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Discovery and naming
Methone and Pallene were found by the Cassini imaging team in images taken in 2004, shortly after the spacecraft entered orbit around Saturn, and announced in August 2004. Pallene turned out to have been captured unknowingly in a Voyager 2 image from 1981. Anthe was discovered in 2007 in Cassini images.
The International Astronomical Union named the three after the Alkyonides, the daughters of the giant Alkyoneus in Greek mythology, who, according to the legend, threw themselves into the sea after their father's death and were turned into kingfishers. The myth counts seven sisters; three of their names have so far been given to Saturn's moons, and the group name is used informally by researchers.
Methone, the egg-shaped moon
On 20 May 2012, Cassini flew within about 1,900 km of Methone, the closest it ever came to any of these moons. The images revealed something unexpected: a smooth, elongated body, about 3.9 × 2.6 × 2.4 km, with no craters visible at that resolution. Its shape resembles an egg, with one end slightly more pointed than the other.
The lack of craters and the regular shape have been interpreted in terms of very loose, fine material. One analysis, by Peter Thomas and colleagues in 2013, showed that Methone's shape is close to what would be expected for a fluid body in equilibrium with Saturn's tides and its own rotation. If so, its density would be very low, around 0.3 g/cm³, lower than that of any other known moon. The idea is that fine icy grains on the surface can migrate and settle, smoothing any craters, although the exact mechanism is still debated. The images also show subtle brightness variations, possibly related to how charged particles from Saturn's magnetosphere strike its surface.
Anthe and Pallene have not been seen at comparable resolution, so their shapes are not well known. Pallene appears somewhat elongated and is the largest of the three.
Ring arcs and a faint ring
Cassini found that each of these moons shares its orbit with dust. Methone and Anthe sit inside narrow arcs, portions of rings that extend only part of the way around the orbit, about 10 degrees or so for Methone's arc and somewhat more for Anthe's. Pallene, by contrast, is accompanied by a faint, complete ring, sometimes called a torus.
The dust probably comes from the moons themselves: micrometeorite impacts knock small particles off their surfaces, and the moons' gravity is too weak to recapture them. What keeps the material from spreading into a full ring around Methone and Anthe is Mimas. Methone is close to a 15:14 resonance with Mimas and Anthe close to an 11:10 resonance, which means that the dust and the moons are trapped in particular segments of the orbit, oscillating back and forth along it. Pallene is not in such a strong resonance with Mimas, so its dust is free to spread around the entire orbit.
These arcs are an example of how small moons, resonances and dust interact, a theme that also appears around Janus and Epimetheus, Atlas and the larger structures described on the ring names page.
The E ring environment
All three moons orbit inside the broad, tenuous E ring, which is fed by the plumes of Enceladus. E ring particles constantly strike their surfaces, which may contribute to their brightness and to the fine coating seen on Methone. The Alkyonides thus offer a glimpse of how small icy bodies are continuously reshaped by their environment.
FAQ
Why is Methone shaped like an egg?
Methone's smooth, egg-like shape may be the result of very fine, loose material that behaves almost like a fluid, settling into a shape set by Saturn's tides and the moon's rotation. If so, its density is very low, around 0.3 g/cm³.
How big are Methone, Anthe and Pallene?
They are tiny: Methone is about 3 km across, Anthe roughly 1 to 2 km, and Pallene about 5 km. All three are among the smallest named moons of Saturn.
Where are the Alkyonides located?
They orbit between Mimas and Enceladus, at distances of about 194,700 km (Methone), 197,700 km (Anthe) and 212,300 km (Pallene) from Saturn's center.
What are the ring arcs around Methone and Anthe?
They are narrow arcs of dust, probably knocked off the moons by impacts, that are confined to short segments of the moons' orbits by resonances with Mimas.
Sources
- NASA Science: Saturn's moons
- NASA: Cassini mission
- Hedman, M. M. et al., "Three tenuous rings/arcs for three tiny moons", Icarus 199 (2009)
- Thomas, P. C. et al., "The inner small satellites of Saturn: a variety of worlds", Icarus 226 (2013)