Saturn’s Peculiar Flying Saucer Shaped Moons
Saturn has some pretty weird looking moons, something I
discovered a while back while reading a paper about crater counts and how these
related to the age of these moons (link below). Here are a few key paragraphs from the paper,
describing these unusual moons, and the processes that may have produced their
current appearance:
“Saturn’s small icy satellites
present a broad variety of shapes and have radii between 300 m and 135 km. They
have high porosities and very low densities, about half the density of water ice… Except for Hyperion, they are strongly
related to the ring system, being some of them embedded in them, others
producing a gap or just orbiting in the border of a ring. Others are related
even to the mid-sized satellites as trojans of Tethys or Dione.”
…
Observations for Pan and Atlas
show that these objects have a distinct equatorial ridge, which may be present
on Daphnis as well (Charnoz et al. 2007; Porco et al. 2007). These features can
be appreciated in the striking images obtained by the Cassini mission (Fig. 1). The origin of these formations is unclear. On one
hand, the rotational periods of these satellites are much too long
(approximately 14 hours) for centrifugal force to compensate the gravitational
force. On the other hand, tidal force generated by Saturn would deform bodies
in the radial direction generating an ellipsoidal object and not a "flying
saucer" object (Charnoz et al. 2007). According to Porco et al. (2007) one
possible explanation is that all three satellites likely grew from cores that
were one third to one half their present sizes by accumulation of A ring
material during an initial formation stage that took place inside a more
vertically extended ring. Once they had filled their Roche lobe, a secondary
stage of accretion formed their equatorial ridges in a disk that was already 20
meter thick (as the current rings), which would explain the accumulation of particles
along the equator. More recently, Leleu et al. (2018) found an alternative
explanation considering head-on merging collisions between similar-sized bodies
(also known as the pyramidal regime) as they migrated away from the rings.
Basically, the paper took advantage of Cassini’s high
resolution photographs of Saturn’s rings, to do crater counts and relate those
to theory regarding collision rates and erosion rates. The crater count evidence seems to indicate
that some of these moons (Pan, Daphnis, and Atlas among them) suffered from large
scale collisions within the past hundred million years or so. The cratering also indicates that the
surfaces undergo ongoing resurfacing processes (i.e. erosion, sculpting, matter
accumulation, etc.). Besides interacting
with the ring system, these moons probably were hit by objects originating in
the trans-Neptunian region (also known as Centaurs).
You can download the paper from the arXiv site for further
details. For my own part, I will focus
on these moons as interesting objects in their own right, as I like moons. And judging from blog counts, lots of other
people do, too.
Here are some pictures and descriptions of a few of these
moons, focussing on those known as “ring shepherds”. Most of these photos were taken by NASA’s Cassini
spacecraft, though some of the discoveries were made much earlier by the
Voyager spacecraft.
Saturn’s Moon Pan
This one has been variously described as a flying saucer, a
walnut, a ravioli or a poached egg. I
think it looks a bit like someone wearing a sunhat at a jaunty angle in this
photo – but lots of imagery could be applied to Pan. By the way, Pan was a Greek messenger god,
associated with shepherds. Pan, the
moon, is considered a “shepherd moon”, that herds particles in the ring system
(e.g. clears a gap).
Pan is the innermost
moon of Saturn that has been given a name.
It orbits at about 134,000 km, with a very small eccentricity (a very
circular orbit). It’s about 35 km across
and 23 km wide. So, obviously not big
enough to become spherical due to its own gravity.
It was predicted to exist from some disturbances noted in
the Encke Gap, which is an opens space in the ring system of about 325 km
wide. The predicted size and position of
Pan turned out to be very accurate.
The funny shape is due to Pan sweeping up material from the
ring system, in which it is embedded.
That material accreted preferentially in a plane, as a sort of ring
itself, around Pan. By now it is part of
Pan, but it is hard to say how solid and tightly bound it is to the surface.
Saturn’s Moon Atlas
Atlas is another small moon, at about 40 km by 35 km by 19
km. It is also somewhat poached egg
shaped, like Pan. At about 138,000 km,
it is somewhat farther out than Pan. The
orbit may be somewhat chaotic, though, as it is influenced by some of the other
inner moons. It is also considered to be a ring shepherd moon.
The moon was named Atlas, from Greek mythology, as it “held
up the rings, as the titan Atlas held up the Earth”. As the titan Atlas was supposed to be
standing on a giant Turtle, this moons somewhat turtle-like appearance may be
appropriate.
Atlas’s shape is due to picking up matter from the ring
system of Saturn, same as Pan. It has
been calculated that Atlas probably has picked up about all it can this way, as
it has about filled its Roche Lobe, meaning that any new matter would tend to
be flung off by the moon’s centrifugal force overcoming the rather small
gravity that it possesses.
Atlas’s “pancake” component, if we may call it that, appears
to be very smooth, but that may just be an artifact of the photography (i.e.
somewhat out of focus).
Saturn’s Moon Daphnis
Daphnis is a very small moon, only 8 km by 8 km by 6 km. It too resides in one of the gaps of the ring
system, at about 136,000 km. Like Pan,
it is a shepherd moon – in the photo above, you can see it peeking out, from
the gap that it produces in the ring system.
It’s name also relates to a shepherd in Greek mythology.
A closeup of Daphnis
reveals that it is not all that weirdly shaped, compared to Pan and Atlas,
though it does seem to have a bit of the flange like structure (equatorial ridge
seems to be the accepted term), similar to the other two moons, though much
reduced, probably because of its small gravitational field. It’s hard to say what it looks like – in my
mind its shape is a bit like a Star Trek shuttlecraft, or perhaps an elongated walnut.
Saturn’s Moon Prometheus
As the photos above show, the appearance of Prometheus
depends a lot on the perspective from which the picture was taken, as Prometheus
has an irregular shape of about 135 km by 80 km by 60 km. So, it can have a sort of flying saucer look,
as in the photo on the left, or an elongated potato shape, as on the right. It orbits at about 139,000 km, and is also a
ring shepherd.
It interacts
gravitationally with some other nearby moons and the ring system, so its orbit
seems to be a bit chaotic. Here is a
still photo from a time lapse video taken by Cassini, showing Prometheus going
through the F Ring, and dragging matter with it as it does so.
Prometheus is named after the Titan that stole fire from the
gods in Greek mythology. As a
punishment, he suffered from severe liver troubles.
Though it looks pretty elongated in the photo, it is
still nowhere nearly as stretched out as the interstellar body Oumuamua (shown below), which recently
visited the solar system. Oumuamua’s
extreme elongation is very unusual – there is speculation that it may have been
“sculpted” by collisions with interstellar dust during its long journey between
the stars, which is reminiscent of the way Saturn’s moons have had their shapes
altered by their long travel among the dust of Saturn’s ring system.
Sources:
1 - Cratering and age of Saturn’s small satellites. N.L. Rossignoli, R.
P. Di Sisto, M. Zanardi and A.
Dugaro1, arXiv:1904.13011
[astro-ph.EP]
2 Wiki, Saturn’s moons
3- Oumuamua’s Elongated Shape:
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Now that you have read some real science (astronomy and astrophysics), you should read some science fiction. Either of the Kati of Terra series or the Witch’s Stones series would be excellent choices. Alternatively, you could try the short story “The Magnetic Anomaly”, which has lots of physics, and plenty about magnetic fields, perhaps affecting the brain’s hippocampus. J
Now that you have read some real science (astronomy and astrophysics), you should read some science fiction. Either of the Kati of Terra series or the Witch’s Stones series would be excellent choices. Alternatively, you could try the short story “The Magnetic Anomaly”, which has lots of physics, and plenty about magnetic fields, perhaps affecting the brain’s hippocampus. J
Kati of Terra
How about trying Kati of Terra, the 3-novel story of a
feisty young Earth woman, making her way in that big, bad, beautiful universe
out there.
The Witches’ Stones
Or, you might prefer, the trilogy of the Witches’ Stones
(they’re psychic aliens, not actual witches), which follows the interactions of
a future Earth confederation, an opposing galactic power, and the Witches of
Kordea. It features Sarah Mackenzie,
another feisty young Earth woman (they’re the most interesting type – the novelist
who wrote the books is pretty feisty, too).
The Magnetic Anomaly: A Science Fiction Story
“A geophysical crew went into the Canadian north. There were
some regrettable accidents among a few ex-military who had become geophysical
contractors after their service in the forces. A young man and young woman went
temporarily mad from the stress of seeing that. They imagined things, terrible
things. But both are known to have vivid imaginations; we have childhood
records to verify that. It was all very sad. That’s the official story.”
