APOD: Phobos: Doomed Moon of Mars (2022 Jul 03)

Comments and questions about the APOD on the main view screen.
User avatar
johnnydeep
Captain
Posts: 1412
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Phobos: Doomed Moon of Mars (2022 Jul 03)

Post by johnnydeep » Tue Jul 05, 2022 6:05 pm

Chris Peterson wrote: Tue Jul 05, 2022 3:08 pm
johnnydeep wrote: Tue Jul 05, 2022 3:00 pm
Chris Peterson wrote: Tue Jul 05, 2022 2:17 pm
The Roche limit only applies to objects that are held together by their own gravity. It isn't some clearly defined distance from the parent. It's related to the structure of the satellite, to its shape, to its material strength, to its rotation. The nature of the tidal forces on an object don't change with distance.
So I guess there's no "tendency" for the lower orbit parts of bodies - whatever their structure - to "want" go faster than the higher orbit parts? (The rationale being that bodies in lower orbit travel faster.)
There is. But all that does is slightly alter the fixed orientation of a tidally locked body very slightly. It can't create any spin (except in a limited sense before the body becomes tidally locked).
Ok, got it - thanks for being patient. :ssmile:
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

User avatar
MarkBour
Subtle Signal
Posts: 1284
Joined: Mon Aug 26, 2013 2:44 pm
Location: Illinois, USA

Re: APOD: Phobos: Doomed Moon of Mars (2022 Jul 03)

Post by MarkBour » Wed Jul 13, 2022 6:08 pm

VictorBorun wrote: Tue Jul 05, 2022 5:06 am
MarkBour wrote: Mon Jul 04, 2022 7:58 pm
Capture1.png
I'm just looking at this image from the Wikipedia "Roche Limit" page and thinking about it intuitively (https://en.wikipedia.org/wiki/Roche_limit)
As the object approaches the Roche limit, its inner chunks are wanting to orbit angularly more rapidly than the rest, and its outer chunks are wanting to orbit angularly more slowly than the rest. If there is a period of time in which this is appreciable, but during which the body can still "hold it together", then I think it would induce a rotational motion. You might even have some chunks breaking loose, but then rolling along the surface in that direction as well.
Well, if there is a ring, with some differential rotation, and a large globe orbiting along the median circle of the ring, the globe does get some spinning up, though far from infinitely large. The friction will work just until the equatorial velocity of the globe catches up with the headwind/tailwind of the matter of the ring at orbits higher/lower by the radius of the globe.
Very interesting. Has this been observed, say for any of the moonlets of Saturn? I spent a little time on Wikipedia and Google, but did not uncover any such observations. I found some papers discussing simulations with lots of mathematical work by: Heikki Salo, Keiji Ohtsuki, and Ryuji Morishima. It would be a huge effort to get full copies of these and try to understand them. The most basic conclusion, though, is that under some circumstances, the moonlets would spin a bit, just as you said.

This is different than what I was thinking about, but it suggests that for Phobos, it could happen in stages, then. It could reach its Roche limit and begin to break apart, but it might have a good solid core that holds together while the rest of the body turns into a ring. At that point, you'd have a moonlet within a ring and it might get spin-up from collisions with the other material, over time. But it would seem that collisions with particles that broke free in this way might be pretty rare, so I would not expect much spin up, if all of the particles came from the same source, as would be the case when I'm thinking about Phobos.
Image

Around Saturn, looking at moonlets like
Pan, Daphnis, Methone, Anthe, and Pallene,
I don't find any mention of observed rotations,
although a couple of them have those very
curious equatorial ridges.
Mark Goldfain

User avatar
VictorBorun
Commander
Posts: 571
Joined: Fri Oct 16, 2020 10:25 pm

Re: APOD: Phobos: Doomed Moon of Mars (2022 Jul 03)

Post by VictorBorun » Thu Jul 14, 2022 3:46 am

Saturn's moonlets in the rings are small in radius even doubled with their ridges so their spinning up by the rings' differential rotation would be tiny.
They should migrate to Saturn faster than the rings' particles, too.
Not sure they live long enough to spin up before they crash into Saturn.

Saturn itself is spinning fast. Was Saturn spinned up by its rings' and satellites' shower?

User avatar
MarkBour
Subtle Signal
Posts: 1284
Joined: Mon Aug 26, 2013 2:44 pm
Location: Illinois, USA

Re: APOD: Phobos: Doomed Moon of Mars (2022 Jul 03)

Post by MarkBour » Thu Jul 14, 2022 5:36 pm

VictorBorun wrote: Thu Jul 14, 2022 3:46 am Saturn's moonlets in the rings are small in radius even doubled with their ridges so their spinning up by the rings' differential rotation would be tiny.
They should migrate to Saturn faster than the rings' particles, too.
Not sure they live long enough to spin up before they crash into Saturn.

Saturn itself is spinning fast. Was Saturn spinned up by its rings' and satellites' shower?
Interesting. Why will they migrate in faster? Are they likely to dissolve in the process?

As to Saturn spinning fast, I know that it and Jupiter are similar, with Jupiter spinning even faster. A basic idea would be that they got it from their accretion and formation process. That's consistent with your suggestion, that angular momentum comes from what has fallen inward.

This one effect I've been thinking about in this thread about Phobos, would seem to induce a retrograde spin in the satellite. However, almost all of the planets in our solar system have a prograde spin in their orbits around the sun. I don't know how to make any sense of that fact.
Mark Goldfain

User avatar
VictorBorun
Commander
Posts: 571
Joined: Fri Oct 16, 2020 10:25 pm

Re: APOD: Phobos: Doomed Moon of Mars (2022 Jul 03)

Post by VictorBorun » Tue Jul 19, 2022 6:48 pm

MarkBour wrote: Thu Jul 14, 2022 5:36 pm As to Saturn spinning fast, I know that it and Jupiter are similar, with Jupiter spinning even faster. A basic idea would be that they got it from their accretion and formation process. That's consistent with your suggestion, that angular momentum comes from what has fallen inward.

This one effect I've been thinking about in this thread about Phobos, would seem to induce a retrograde spin in the satellite. However, almost all of the planets in our solar system have a prograde spin in their orbits around the sun. I don't know how to make any sense of that fact.
On one hand, Jupiter is spinning faster
Equatorial rotation velocity = 12.6 km/s = Escape velocity 59.5 km/s * 0.212
than Saturn
Equatorial rotation velocity = 9.87 km/s = Escape velocity 35.5 km/s * 0.278

On the other hand a more massive giant like Jupiter would have taken longer to slow down from initial spin when Equatorial rotation velocity was Escape velocity.

On the other hand still Jupiter is more slowed by Sun's tides.

I am lost here. Is Saturn spun up by the ring shower? Was Saturn even more spun up at the event that created the ring?

User avatar
MarkBour
Subtle Signal
Posts: 1284
Joined: Mon Aug 26, 2013 2:44 pm
Location: Illinois, USA

Re: APOD: Phobos: Doomed Moon of Mars (2022 Jul 03)

Post by MarkBour » Thu Jul 21, 2022 5:59 pm

VictorBorun wrote: Tue Jul 19, 2022 6:48 pm
MarkBour wrote: Thu Jul 14, 2022 5:36 pm As to Saturn spinning fast, I know that it and Jupiter are similar, with Jupiter spinning even faster. A basic idea would be that they got it from their accretion and formation process. That's consistent with your suggestion, that angular momentum comes from what has fallen inward.

This one effect I've been thinking about in this thread about Phobos, would seem to induce a retrograde spin in the satellite. However, almost all of the planets in our solar system have a prograde spin in their orbits around the sun. I don't know how to make any sense of that fact.
On one hand, Jupiter is spinning faster
Equatorial rotation velocity = 12.6 km/s = Escape velocity 59.5 km/s * 0.212
than Saturn
Equatorial rotation velocity = 9.87 km/s = Escape velocity 35.5 km/s * 0.278

On the other hand a more massive giant like Jupiter would have taken longer to slow down from initial spin when Equatorial rotation velocity was Escape velocity.

On the other hand still Jupiter is more slowed by Sun's tides.

I am lost here. Is Saturn spun up by the ring shower? Was Saturn even more spun up at the event that created the ring?
Search me! I looked around on the Internet again for a while about planetary rotation.

The rationale for planets to generally rotate in the prograde direction (for their orbit) is not a topic I can find much discussed. Perhaps nobody knows why that would be the most likely scenario -- or why it might not be that likely in general, yet would have occurred in our solar system. Of course there is a tendency toward tidal locking, which would be slowly prograde, but for our system of planets, almost all of them are not at all near tidal locking.

I don't know if this pattern (spins are prograde) holds for most of the asteroids for which we can detect spin thus far. There is a reference to a study of this (ref in https://en.wikipedia.org/wiki/Retrograd ... ade_motion -- to Poznan Observatory), but it was hard to get to the data or any conclusions from this study.

I did find one general theory put forth by an Ian Miller at:
https://www.researchgate.net/post/Plane ... exceptions.
I would have to buy his eBook to go further into it. He summarizes a theory that as a planet forms, it accumulates mass from the surrounding gas (of the circumstellar disk) in a way that it picks up most of the mass on its leading side. And then he further theorizes that this will, for some reason, impart a prograde spin on the growing planet. I have no idea why, or whether or not other people are finding this theory reasonable.

So, in summary -- I have not found much in the way of explanation.
Mark Goldfain

User avatar
VictorBorun
Commander
Posts: 571
Joined: Fri Oct 16, 2020 10:25 pm

Re: APOD: Phobos: Doomed Moon of Mars (2022 Jul 03)

Post by VictorBorun » Thu Jul 21, 2022 7:30 pm

MarkBour wrote: Thu Jul 21, 2022 5:59 pm Search me! I looked around on the Internet again for a while about planetary rotation.

The rationale for planets to generally rotate in the prograde direction (for their orbit) is not a topic I can find much discussed. Perhaps nobody knows why that would be the most likely scenario -- or why it might not be that likely in general, yet would have occurred in our solar system. Of course there is a tendency toward tidal locking, which would be slowly prograde, but for our system of planets, almost all of them are not at all near tidal locking.

I don't know if this pattern (spins are prograde) holds for most of the asteroids for which we can detect spin thus far. There is a reference to a study of this (ref in https://en.wikipedia.org/wiki/Retrograd ... ade_motion -- to Poznan Observatory), but it was hard to get to the data or any conclusions from this study.

I did find one general theory put forth by an Ian Miller at:
https://www.researchgate.net/post/Plane ... exceptions.
I would have to buy his eBook to go further into it. He summarizes a theory that as a planet forms, it accumulates mass from the surrounding gas (of the circumstellar disk) in a way that it picks up most of the mass on its leading side. And then he further theorizes that this will, for some reason, impart a prograde spin on the growing planet. I have no idea why, or whether or not other people are finding this theory reasonable.

So, in summary -- I have not found much in the way of explanation.
But is it not true that the spin of a proto-disk is blocking gravitational collapses, and after some spin is shed to evaporating (i.e. partially to kicked-out clumps in the plane of the disk and partially to a pair of conical jets from every new star or a large planet), what finally forms is spinning at maximum, all in the same direction?

User avatar
MarkBour
Subtle Signal
Posts: 1284
Joined: Mon Aug 26, 2013 2:44 pm
Location: Illinois, USA

Re: APOD: Phobos: Doomed Moon of Mars (2022 Jul 03)

Post by MarkBour » Thu Jul 21, 2022 9:19 pm

VictorBorun wrote: Thu Jul 21, 2022 7:30 pm But is it not true that the spin of a proto-disk is blocking gravitational collapses, and after some spin is shed to evaporating (i.e. partially to kicked-out clumps in the plane of the disk and partially to a pair of conical jets from every new star or a large planet), what finally forms is spinning at maximum, all in the same direction?
Sorry to be obstinate, Victor, but I just don't get it.
"spinning at maximum, all in the same direction"
does make sense to me in terms of the orbits. But I can't see why it would induce forward spins.

I wonder if the JWST can look at this: CAHA J23056+6016
https://www.circumstellardisks.org/show.php?id=20
it might be an exciting target.

And maybe watching it for a million years would help me better understand solar system planetary formation.
Mark Goldfain

User avatar
VictorBorun
Commander
Posts: 571
Joined: Fri Oct 16, 2020 10:25 pm

Re: APOD: Phobos: Doomed Moon of Mars (2022 Jul 03)

Post by VictorBorun » Thu Jul 28, 2022 6:32 pm

the spin conservation law works like this.

When a bunch of massive things spin and travel without any external interaction, the sum of every spin along an axis is constant however you choose an axis to calculate the spins.
If a thing is travelling along a line, it has an orbital spin of mvR, where m is the mass, v is the velocity component perpendicular to your chosen axis and R is the distance between the line and your axis.
If a thing is spinning and the spin is parallel to your chosen axis, the spin along your chosen axis is mvr, where m is the mass, v is an effective linear velocity or just the linear velocity of the spinning in case the thing is a thin ring, and r is an effective radius or just the radius in case the thing is a thin ring. If the spin is at an angle with your chosen axis, only its component along your axis is counted as an investment in the total spin along your axis.

Both kinds of spins invest in the total spin of the bunch.

Example: Moon+Earth bunch is internally tidally exchanging spins, Earth slowing down to smaller mvr and Moon retreating to higher orbits with greater mvR.

A protoplanetary disk collapses to star (or several stars), planets, satellites and secondary satellites. All the collapsing progress only as much as the protoplanetary disk sheds its total spin