The Strangest Moon In the Solar System

StartsWithABang writes Moons in our Solar System — at least the ones that formed along with the planets — all revolve counterclockwise around their planetary parents, with roughly uniform surfaces orbiting in the same plane as their other moons and rings. Yet one of Saturn's moon's, Iapetus, is unique, with a giant equatorial ridge, an orbital plane that doesn't line up, and one half that's five times brighter than the other. While the first two are still mysteries, the last one has finally been solved.

Comments

Medium.

[Anonymous Coward]

Linking to Medium is the new linking to LiveJournal.

Re:Medium.

[Anonymous Coward]

How appropriate that this is the Starts with a Bang blog: way too many sentences in the postings on this site end with a bang.

... one half of Iapetus is only about one-fifth as bright as the other!

But one of them has!

... density of about seven dust-sized grains per cubic kilometer!

... and finally know why it has a yin-yang coloration unlike anything else!

Zowie! Boom! Kablooie!

These articles read like a comic book. When I'm looking for scientific details, the style of these articles is really offputting.

The strangest moon in the solar system is ours.

[Eunuchswear]

It's HUGE.

Re:Counterclockwise?

[Keramos]

Viewed from which side? Counterclockwise does not apply here.

Viewed when looking down from the north pole. This is mentioned in TFA, per

Rather that [sic] (looking down from the north pole) orbiting counterclockwise around its parent planet, which all the other moons do, Phoebe revolves clockwise around Saturn.

To summarize.

[jellomizer]

I found this article to be rather long winded in order to create a story with suspense.
The moon has a side facing away from Saturn which is darker then the side facing saturn. It seems to be due to collecting dust from a larger ring that is on the border of its orbit.

Done, saved you a long and pointless naritive.

Re:To summarize.

[Keramos]

I found this article to be rather long winded in order to create a story with suspense. The moon has a side facing away from Saturn which is darker then the side facing saturn. It seems to be due to collecting dust from a larger ring that is on the border of its orbit.

Done, saved you a long and pointless naritive.

Actually, that's not quite correct. You've got two errors there, and missing the real mystery, although the article itself actually fails to explicitly specify what the solution is.

The darker side is actually the leading hemisphere, not the far or outer side (from Saturn). Dust doesn't onto the far side, the moon plows through it in places, getting dust on the leading side. No mystery here for quite a while though - telescopes have been able to make out "the dark patterns look a lot like dust" for quite a while. The Phoebe ring itself was only detected about 10 years ago, but it was expected that dust was coming from the outer moons for a while.

The thing is, if the only process happening was that dust was being swept up by Iapetus, then every time the dark side faced the Sun, the dark coating would heat up, cause the ice underneath it to sublime (think evaporate, if that doesn't mean anything - it's close enough) and freeze again over the dust, leaving behind a light surface again. But we see a dark surface. Why? Mystery!

The solution (which the article doesn't really explain fully) is that initially dust from the ring caused ice to turn to gas, leaving behind a dark residue that we now see (and the Cassini probe has been able to measure), but instead of just floating around above the (relatively) warm, dark surface until it faces away from the Sun and cools down, much of the vapour refreezes on the light side as it passes over it due to the lower temperature there.

The dark residue (not the original dust) now causes further heating each orbit, repeating the cycle. Over time, a large amount of ice from the leading side is being evaporated away, leaving that side to get darker and darker from the residue, with a certain amount of the ice migrating to the light side and refreezing (as light coloured ice) keeping it nice and bright.

TLDR: Mystery! Dust doesn't explain the dark leading side of Iapetus! Ice would cover it in a shiny coat each orbit. Planetary detectives trace the culprit to dark residues left behind as heated ice moves to a new neighbourhood on the cooler side of the moon. More dark areas means more solar heating, and more ice migrating away in a self-perpetuating cycle. Mystery solved! Good job, planetary scientists!

Re:Counterclockwise?

[Sique]

The only planet whose north pole is not on the same side of the Ecliptic as the Earth's north pole is Uranus. In Uranus' case, the north pole is nearly in the Ecliptic itself, tilted at 98. All the other planets have their rotation axis (axial tilt) either nearly vertical to their orbit plane (e.g. Mercury, Venus, Jupiter), or tilted at about 20-30 (Earth, Mars, Saturn, Neptune).

Re:Counterclockwise?

[tylersoze]

Or just use the right hand rule :) Where "north" is by definition the positive direction of the total angular momentum pseudo-vector of the solar system.

Re:Counterclockwise?

[rubycodez]

Towards Polaris, and in fact that gives the direction of "galactic north" too. Note due to precession of Earth's axis after 3000 AD Gamma Cephei will become the pole star, and Iota Cephei in 5200 AD. Thuban was the pole star in 3000 B.C. Polaris will again become the pole star about 27,800AD