APOD: An Unusually Smooth Surface on... (2010 Feb 17)

[APOD Robot]

An Unusually Smooth Surface on Saturns Calypso

Explanation: Why is this moon of Saturn so smooth? This past weekend, humanity's Saturn-orbiting Cassini spacecraft passed as close to Saturn's small moon Calypso as it ever has, and imaged the small moon in unprecedented detail. Pictured above is an early return, raw, unprocessed image of the 20-km long irregularly shaped moon. Like its sister moon Telesto and the shepherd moon Pandora, Calypso has shown itself to be usually smooth, much smoother than most of Saturn's larger moons. A leading hypothesis for Calypso's smoothness is that much of the moon's surface is actually a relatively loose jumble of rubble -- making Calypso a rubble-pile moon. The loose nature of the small ice pieces allows them to fill in many small craters and other surface features. Calypso orbits Saturn always behind Saturn's much larger moon Tethys, whereas Telesto's orbit always precedes Tethys. Calypso's extremely white surface -- not unlike fresh snow -- may result from the continuous accumulation of fresh ice particles falling in from Saturn's E ring.

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[neufer]

An Unusually Smooth Surface on Saturn's Calypso

<<Ogygia (Greek: Ὠγυγίη or Ὠγυγία), is an island mentioned in Homer's Odyssey, Book V, as the home of the nymph Calypso, the daughter of the Titan Atlas and Tethys. In Homer's Odyssey Calypso detained Odysseus on Ogygia for seven years and kept him from returning to his home of Ithaca. According to Hesiod, Calypso bore Odysseus two children, Nausithous and Nausinous. Athena complained about Calypso's actions to Zeus, who sent the messenger Hermes to Ogygia to order Calypso to release Odysseus. Calypso finally allowed Odysseus to build a small raft and depart the island.

The Odyssey describes Ogygia as follows:

• ...and he (Hermes) found her within. A great fire was burning in the hearth, and from afar over the isle there was a fragrance of cleft cedar and juniper as they burned. But she within was singing with a sweet voice as she went to and fro before the loom, weaving with a golden shuttle. Round about the cave grew a luxuriant wood, alder and poplar and sweet-smelling cypress, wherein birds long of wing were wont to nest, owls and falcons and sea-crows with chattering tongues, who ply their business on the sea. And right there about the hollow cave ran trailing a garden vine, in pride of its prime, richly laden with clusters. And fountains four in a row were flowing with bright water hard by one another, turned one this way, one that. And round about soft meadows of violets and parsley were blooming...

Plutarch also gives an account of the location of Ogygia:

• First I will tell you the author of the piece, if there is no objection, who begins after Homer’s fashion with, an isle Ogygian lies far out at sea, distant five days’ sail from Britain, going westwards, and three others equally distant from it, and from each other, are more opposite to the summer visits of the sun. The great continent by which the great sea is surrounded on all sides, they say, lies less distant from the others, but about five thousand stadia from Ogygia, for one sailing in a rowing-galley; for the sea is difficult of passage and muddy through the great number of currents, and these currents issue out of the great land, and shoals are formed by them, and the sea becomes clogged and full of earth, by which it has the appearance of being solid.

Johannes Kepler in his Kepleri Astronomi Opera Omnia estimated that “the great continent” was America and attempted to locate Ogygia and the surrounding islands.>>

[BMAONE23]

Cassini has again paid for itself. What a great new image of this moon. I even see 12 stars in the background.

[jacklap]

"A leading hypothesis for Calypso's smoothness is that much of the moon's surface is actually a relatively loose jumble of rubble -- making Calypso a rubble-pile moon."

May I suggest......"making Calypso an astro-riprap moon." In the interest, of course, of not disparging the fine Rubble family name in the least.

[biddie67]

I wonder if the little moon's odd shape causes it to tumble or spin in its orbit .....

[Chris Peterson]

I wonder if the little moon's odd shape causes it to tumble or spin in its orbit .....

No, due to tidal locking it rotates synchronously with its orbit.

[forkboy1965]

I had originally sent an e-mail regarding the APOD submission of Calypso (17 Feb. 2010) to Dr. Nemiroff who suggested I post my "..interesting question." here on The Asterisk. So here goes...

While neither agreeing nor disagreeing with the demotion of Pluto from the status of planet, I have often wondered why it is the powers that be in astronomy haven't yet created a concensus for what is a "moon".

It seems, in my opinion, that any old hunk of rock or rubble is called a moon as long as it orbits a planet (which leaves the status of the "moon" Charon in question seeing how Pluto is no longer a full-on planet), but this seems like a very lazy definition, especially in light of the work done to create a more concise definition of what is a planet.

Does anyone think there should be an endeavor to better define a "moon" as opposed to any ol hunk of rock?

Thanks,
Forkboy

[emc]

Hi Forkboy, Makes sense to me… isn’t a big part of Science to define things for us and refine definitions as evidence dictates. But I may be a bit over the top with that comment since we have known that there are a lot of tiny moons for quite a while now. I like your question. I think it follows suite with the planet reclassification.

[bystander]

Natural Satellite

A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify non-artificial satellites of planets, dwarf planets, and minor planets.

As of July 2009, 336 bodies are formally classified as moons. They include 168 orbiting six of the eight planets, 6 orbiting three of the five dwarf planets, 104 asteroid moons, and 58 satellites of Trans-Neptunian objects, some of which will likely turn out to be dwarf planets. Some 150 additional small bodies were observed within Saturn's ring system, but they were not tracked long enough to establish orbits. Planets around other stars are likely to have natural satellites as well, although none have been observed.

The Definition of a Moon

There is not an established lower limit on what is considered a moon. Every body with an identified orbit, some as small as a kilometer across, has been identified as a moon, though objects a tenth that size within Saturn's rings, which have not been directly observed, have been called moonlets. Small asteroid moons, such as Dactyl, have also been called moonlets.

The upper limit is also vague. When the masses of two orbiting bodies are similar enough that one cannot be said to orbit the other, they are described as a double body rather than primary and satellite. Asteroids such as 90 Antiope are considered double asteroids, but they have not forced a clear definition of what constitutes a moon. Some authors consider the Pluto-Charon system to be a double (dwarf) planet. The most common dividing line on what is considered a moon rests upon whether the barycentre is below the surface of the larger body, though this is somewhat arbitrary, as it relies on distance as well as relative mass.

[BMAONE23]

Thoughts on Moon type categories
"Moon" (major moon) Ours for one Titan sized through Tethys sized
"Minor Moon" Mimas, Enceladus sized
"Moonlet" Hyperion, Phoebe, Janus sized
"Moontesimal" Pandora, Epimetheus, Atlas sized
"Moosteroid or Asteroon" Telesto, Calypso, Helene sized
"Dusteroids" Saturnian Rings

[neufer]

Thoughts on Moon type categories
"Moon" (major moon) Ours for one Titan sized through Tethys sized
"Minor Moon" Mimas, Enceladus sized
"Moonlet" Hyperion, Phoebe, Janus sized
"Moontesimal" Pandora, Epimetheus, Atlas sized
"Moosteroid or Asteroon" Telesto, Calypso, Helene sized
"Dusteroids" Saturnian Rings

Please send your irate emails about
the demotion of your favorite moon to:
"BMAONE23" , California, U.S.A.

[Chris Peterson]

While neither agreeing nor disagreeing with the demotion of Pluto from the status of planet, I have often wondered why it is the powers that be in astronomy haven't yet created a concensus for what is a "moon".

There is no consensus for the definition of a planet. All the IAU did in creating a formal definition was to create dissent (and not a small amount of ridicule).

I think any attempt to define "moon" would lead to similar issues.

We don't require a formal definition of "moon" any more than we require a formal definition of "planet". Defining either in a rigorous way does not serve to advance science in the slightest, nor to make communication between scientists more clear. There are words best left loosely defined.

IMO, "moon" is a perfectly reasonable term to use for any old hunk of rock in orbit around any other old hunk of rock, so long as the barycenter of the pair is inside the more massive object.

[BMAONE23]

Realistically,
Moon would only refer to a "Naturally occuring object orbiting a Parent Body" and should include the smaller of a Binary pair.
This would exclude all Man Made Satellites and Include Charon

[bystander]

I think a binary pair should be considered just that, a binary pair. Pluto and Charon are a binary dwarf planet, with moons that orbit the pair, Nix and Hydra.

[Chris Peterson]

Realistically,
Moon would only refer to a "Naturally occuring object orbiting a Parent Body" and should include the smaller of a Binary pair.
This would exclude all Man Made Satellites and Include Charon

I would exclude Charon because the barycenter of the Pluto-Charon pair lies outside either body. I would call Pluto-Charon a binary planet, which does, in fact, have two moons, Nix and Hydra, orbiting around the pair's barycenter.

[neufer]

IMO, "moon" is a perfectly reasonable term to use for any old hunk of rock in orbit around any other old hunk of rock, so long as the barycenter of the pair is inside the more massive object.

1) The average centre-to-centre distance from the Earth to the Moon is 384,403 kilometres.

2) The common centre of mass of the system (the barycentre) is located at about a quarter the Earth's radius beneath the surface of the Earth.

3) As a result of the conservation of angular momentum, [tidal drag on the] the Earth's rotation is accompanied by an increase of the mean Earth-Moon distance of about 3.8 m per century [or ~10% per billion years].>>

• Ergo: in 4 or 5 billion years the barycenter may well be located above
  the surface of the Earth and The Moon will cease to be "a moon."

--------------------------------------------------------
Please send your irate emails about
the upcoming demotion of our own Moon to:
Chris Peterson , Cloudbait Observatory, Colorado

[Chris Peterson]

Ergo: in 4 or 5 billion years the barycenter may well be located above
the surface of the Earth and The Moon will cease to be a moon

I can live with that. But who knows... in 5 billion years my thoughts on the matter might change.

[Chris Peterson]

Please send your irate emails about the upcoming demotion of our own Moon...

I'd call a conversion from "moon" to "planet" a promotion, not a demotion!

[GaryR]

• Ergo: in 4 or 5 billion years the barycenter may well be located above
  the surface of the Earth and The Moon will cease to be "a moon."

Well, in 4 or 5 billion years, the Earth and the Moon will be either vaporized or burned to crisp by the Sun in its red giant stage. So the matter will be moot.

Gary

[BMAONE23]

In 5 billion years we could be crispie crittered so instead of
Moot it could be Soot