APOD: Phobos: Doomed Moon of Mars (2015 Nov 22)

Comments and questions about the APOD on the main view screen.
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Chris Peterson
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Re: APOD: Phobos: Doomed Moon of Mars (2015 Nov 22)

Post by Chris Peterson » Mon Nov 23, 2015 7:53 pm

Cousin Ricky wrote:See my first response to you. Phobos' orbital speed could reverse the usual tidal effects, and so far, Chris has not corrected me.
Because I think you are correct.
Chris

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Re: APOD: Phobos: Doomed Moon of Mars (2015 Nov 22)

Post by Ron-Astro Pharmacist » Mon Nov 23, 2015 8:43 pm

http://www.popsci.com/falling-phobos-wi ... round-mars

Ooh - our descendants have something to look forward to. :ssmile:
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Re: APOD: Phobos: Doomed Moon of Mars (2015 Nov 22)

Post by ta152h0 » Mon Nov 23, 2015 8:47 pm

yeah, Eta Karinae, Betelgeuse, WR104 and Phobos
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Re: APOD: Phobos: Doomed Moon of Mars (2015 Nov 22)

Post by Czerno o » Mon Nov 23, 2015 11:32 pm

@Neufer, cousin Ricky & @others : Thank you !

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Berkeley: Mars to Lose Its Largest Moon, But Gain a Ring

Post by bystander » Tue Nov 24, 2015 5:58 pm

Mars to Lose Its Largest Moon, But Gain a Ring
University of California, Berkeley | 2015 Nov 24

The demise of Phobos and development of a Martian ring system - Benjamin A. Black & Tushar Mittal
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
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Re: APOD: Phobos: Doomed Moon of Mars (2015 Nov 22)

Post by Mentor » Wed Dec 02, 2015 4:42 pm

FLPhotoCatcher wrote:It seems clear to me that the grooves were created by a string of rocks crashing into Phobos. If so, I would guess there was an earlier moon that broke up into a ring that Phobos crashed through a few times.
I agree with you completely. I came to this same conclusion independently. Perhaps there were rings around Mars in the past, and these grooves resulted as the moon passed through those rings.

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Chris Peterson
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Re: APOD: Phobos: Doomed Moon of Mars (2015 Nov 22)

Post by Chris Peterson » Wed Dec 02, 2015 4:46 pm

Mentor wrote:
FLPhotoCatcher wrote:It seems clear to me that the grooves were created by a string of rocks crashing into Phobos. If so, I would guess there was an earlier moon that broke up into a ring that Phobos crashed through a few times.
I agree with you completely. I came to this same conclusion independently. Perhaps there were rings around Mars in the past, and these grooves resulted as the moon passed through those rings.
Passing through a ring system would result in craters, not grooves.
Chris

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Re: APOD: Phobos: Doomed Moon of Mars (2015 Nov 22)

Post by Mentor » Wed Dec 02, 2015 5:10 pm

Chris Peterson wrote:
Mentor wrote:
FLPhotoCatcher wrote:It seems clear to me that the grooves were created by a string of rocks crashing into Phobos. If so, I would guess there was an earlier moon that broke up into a ring that Phobos crashed through a few times.
I agree with you completely. I came to this same conclusion independently. Perhaps there were rings around Mars in the past, and these grooves resulted as the moon passed through those rings.
Passing through a ring system would result in craters, not grooves.
Some of the groove-like features on Phobos appear to be lines of craters. Perhaps the scale from groove-like to crater-like would depend on the granularity of impacting material.

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Re: APOD: Phobos: Doomed Moon of Mars (2015 Nov 22)

Post by geckzilla » Wed Dec 02, 2015 9:34 pm

Mentor wrote:
Chris Peterson wrote:
Mentor wrote: I agree with you completely. I came to this same conclusion independently. Perhaps there were rings around Mars in the past, and these grooves resulted as the moon passed through those rings.
Passing through a ring system would result in craters, not grooves.
Some of the groove-like features on Phobos appear to be lines of craters. Perhaps the scale from groove-like to crater-like would depend on the granularity of impacting material.
I find it far easier to accept the craters along the grooves as explained by Phobian regolith collapsing into voids opened up over long periods of stress than a simultaneous line of impacts. Think of it as an ongoing process. It would help explain why some of the craters appear younger and sharper along the grooves while others appear older and more irregular, blending almost smoothly into the grooves. Indeed, this makes much more sense than a single, simultaneous event.
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Re: APOD: Phobos: Doomed Moon of Mars (2015 Nov 22)

Post by neufer » Wed Dec 02, 2015 10:14 pm

geckzilla wrote:
I find it far easier to accept the craters along the grooves as explained by Phobian regolith collapsing into voids opened up over long periods of stress than a simultaneous line of impacts. Think of it as an ongoing process. It would help explain why some of the craters appear younger and sharper along the grooves while others appear older and more irregular, blending almost smoothly into the grooves. Indeed, this makes much more sense than a single, simultaneous event.
The only thing Phobos has to fear is Phobos itself.
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Re: APOD: Phobos: Doomed Moon of Mars (2015 Nov 22)

Post by suicidejunkie » Tue Dec 08, 2015 6:22 pm

Since there is some confusion on the first page about tidal forces dragging in vs pushing out:

You have a moon in orbit, and a planet below rotating.
In a lower orbit the moon will travel around the planet faster, and in a higher orbit the moon will travel around the planet slower.
In the middle, you have (geo)stationary orbits, where the moon goes around at the same rate the planet's surface does.

When the moon (such as luna) is in a high orbit and moving slower than the surface, the tides raised on the planet will be pushed slightly ahead of the moon by the planet's rotation. The mass being in front a bit will accelerate the moon. However, that means the moon will move to a slightly higher orbit, and take *longer* to go around than before, making things worse.

Conversely, in a low orbit (such as Phobos'), moving faster than the surface, the tides raised on the planet will lag behind the moon. The mass behind decelerates the moon, but that puts it into a lower orbit so it takes even less time to go around than before making things worse again. Not only that, but being closer increases the gravitational force and the height of the tides, for a vicious cycle spiraling down to its eventual doom.

TL;DR:
Geosynchronous orbit is the tipping point. Moons inside that orbit will spiral in, and moons outside that orbit will spiral out.
How to save (most of) Phobos:
1) Build a mass driver on the surface.
2) Fire rocks into space to boost its orbit to a synchronous one.
3) Watch out for Deimos and the mars probes; any traffic incidents will raise your insurance rates dramatically.