Telling time on Iapetus and other general questions about Iapetus

[DiamondTiara]

TL;DR

I have some things I am curious about when it comes to Iapetus - and how clocks would work on Iapetus.

I have some general questions about Iapetus here that I came up with. I have been very interested in Iapetus, the third largest moon of Saturn lately.

1 - How would clocks work on Iapetus? It orbits Saturn once every 79.3 days and is tidally locked to the planet. What would a time keeping system, if not a calendar be like for a hypothetical time keeping system?

2 - How large does Saturn appear in the sky when seen from Iapetus?

3 - How visible would the other moons be from Iapetus and how bright would they be?

4 - Iapetus's gravity is very low (0.0228 G) - would it be possible to walk, or would it be more akin to floating, like on the ISS, for movement?

 

[Drakkith]

1. You could still have a 'year', the time in which it takes Iapetus to orbit the Sun, it would just be much longer than ours. Divisions into orbits around Saturn would probably be important, but in reality you could have whatever calendar you like.

2. On average, 1.866 degrees, about 3.75x as large as the full Moon appears to us here on Earth.

3. Almost certainly visible to the naked eye. Exactly how bright, I can't say.

4. You would not be able to walk. Anything but the smallest upward push would send you floating off the surface for extended periods of time.

 

[DiamondTiara]

@Drakkith

Thank you for responding to my questions. I have another - would it be probable that Iapetus has some sort of internal ocean?

I imagine that there could be a Saturnian year that consists of Saturn's orbital period, and then each orbit of Iapetus could be considered "Months", with there being a third set between "Months" and Years, being like some sort of arbitrary number of "Months" added up to each other. Something like that. What do you think?

 

[Drakkith]

Thank you for responding to my questions. I have another - would it be probable that Iapetus has some sort of internal ocean?

No idea.

I imagine that there could be a Saturnian year that consists of Saturn's orbital period, and then each orbit of Iapetus could be considered "Months", with there being a third set between "Months" and Years, being like some sort of arbitrary number of "Months" added up to each other. Something like that. What do you think?

The biggest problem that I see is that Iapetus' orbital period doesn't divide into Saturn's orbital period very well. There are about 135.596 'months' per year in this calendar. So you'd need some way to sync your months up with your year to have a whole number per year. Here on Earth we sync our days to our years by having a leap day every 4 years. Something similar could be done on a larger scale, or some other method could be used.

 

[DiamondTiara]

@Drakkith

That makes sense, and that is largely what I thought would be the case. I imagine there would be something similar to leap years, on a grander scale.

So we have the Saturnian Year, and the Orbital period of Iapetus as a "Month", though it may be more accurate to refer to it as a "Day" because it is tidally locked, and the Month could be the arbitrary dividing point between a certain amount of "Days" per year.

Do you have any suggestions for what that would be?

 

[Drakkith]

Nope. I can't think of any convenient measures to use other than orbital periods and rotations.

 

[DiamondTiara]

Nope. I can't think of any convenient measures to use other than orbital periods and rotations.

Then let's use those, and perhaps there could be a somewhat clean use of them? Though, there would definitely be "leap months" involved, I would bet.

 

[Drakkith]

We already used them. Saturn's orbit, Iapetus' orbit, and Iapetus' rotation. The latter two are equal since Iapetus is tidally locked, so we really only have two. Now, we could get creative and do something with the periods of Saturn's other moons perhaps, but that is far more complicated and would take some non-trivial work to determine how to use them.

 

[Orodruin]

Nope. I can't think of any convenient measures to use other than orbital periods and rotations.

You could probably use the orbits of any of the other moons to get convenient time scales of the order of one Earth day.

 

[Baluncore]

I have another - would it be probable that Iapetus has some sort of internal ocean?

Compare the escape velocity, with the thermal velocity of molecules in the ocean.
With such low gravity, I would expect a liquid to evaporate into space.
What liquid do you expect between 90 K and 130 K ?
What is the source of internal heat?

 

[Vanadium 50]

Why wouldn't visitors to Iapetus just use GMT? It's not like they need to know when to plant their crops.

 

[berkeman]

I have been very interested in Iapetus lately, the third largest moon of Saturn lately.

Fixed that for you...

 

[DiamondTiara]

We already used them. Saturn's orbit, Iapetus' orbit, and Iapetus' rotation. The latter two are equal since Iapetus is tidally locked, so we really only have two. Now, we could get creative and do something with the periods of Saturn's other moons perhaps, but that is far more complicated and would take some non-trivial work to determine how to use them.

Yes, that's an entirely valid assessment.

You could probably use the orbits of any of the other moons to get convenient time scales of the order of one Earth day.

I had the idea of using the orbital period of Mimas (which is rather close to one day) for that, yes.

 

[snorkack]

1. You could still have a 'year', the time in which it takes Iapetus to orbit the Sun, it would just be much longer than ours. Divisions into orbits around Saturn would probably be important, but in reality you could have whatever calendar you like.

2. On average, 1.866 degrees, about 3.75x as large as the full Moon appears to us here on Earth.

3. Almost certainly visible to the naked eye. Exactly how bright, I can't say.

Have a look at https://en.wikipedia.org/wiki/Moons_of_Saturn - the big table about "Confirmed" at the bottom. It has a column for "Abs. magn"

For solar system objects the absolute magnitude is the magnitude the body would have at 1 AU from Sun and observer, and at opposition. Which only Earth and Moon are, for an observer on the Sun.

Saturn is almost 10 AU from Sun. That is one component. Iapetus is less than 1/40 AU from Saturn. Another component. Between these two, an inner satellite should be about 3 magnitudes brighter than absolute. Outer satellites would vary more.

That provided full phase. We on Earth only see Saturn and satellites at full. Iapetus would see all phases, which would be dimmer. How detailed phase curves do the probes have for Saturn, rings and satellites?

So basically anything with quoted magnitude below +9 would be naked eye bodies (at opposition - and all inner satellites would have close to the same phase). At 3 million km, one arc minute would be around 1000 km - so Rhea would be barely an extended body when closer, Titan consistently, the others would be wandering stars.