Starship Asterisk*

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.

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.

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.

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.

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.

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.

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

Nature Geoscience (online 23 Nov 2015) DOI: 10.1038/ngeo2583

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

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

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

yeah, Eta Karinae, Betelgeuse, WR104 and Phobos

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.

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.

by **neufer** » Mon Nov 23, 2015 7:00 pm

Czerno o wrote:
The cause is certainly NOT tidal forces,
since they tend to EXPAND orbits (seen as a secular increase of the mean semi-major axis) because of the orbital energy slowly but steadily lost forever due to friction inside the bodies (dissipated as heat).

If a moon's orbital revolution lags behind the planet's rotation then the planet will relinquish angular momentum to the moon such that the moon's orbit EXPANDs. This is the case with the Earth's Moon.

If a moon's orbital revolution exceeds the planet's rotation then the moon will relinquish angular momentum to the planet such that the moon's orbit SHRINKs. This is the case with Phobos.

by **ta152h0** » Mon Nov 23, 2015 6:29 pm

maybe the sun does significantly pull on it

by **Cousin Ricky** » Mon Nov 23, 2015 6:17 pm

Czerno o wrote:Thank you Ms. Ann, but this does not clear the mystery, what causes Phobos's orbit
to shrink, ...

Correct, it doesn't clear up the mystery.

Czerno o wrote:... if indeed it is shrinking ?

It is, by 2 meters per century. (Ann quoted Wikipedia. I'm citing Wikipedia's source.)

Czerno o wrote:The cause is certainly NOT tidal forces,
since they tend to EXPAND orbits (seen as a secular increase of the mean semi-major axis) because of the orbital energy slowly but steadily lost forever due to friction inside the bodies (dissipated as heat).

If it is true that Phobos is "falling" towards its companion, there has to be a celestial-mechanical explanation DIFFERENT than inelastic deformations due to the mutual tidal forces ! Could it be mutual N-body perturbations within the Mars-Phobos-Deimos system ?

See my first response to you. Phobos' orbital speed could reverse the usual tidal effects, and so far, Chris has not corrected me.

by **BMAONE23** » Mon Nov 23, 2015 6:16 pm

Czerno o wrote:
Ann wrote:
Czerno o wrote:« Gravitational tidal forces are dragging it down. »
That assertion is puzzling...
The mean distance between Mars and Phobos is 9377 kilometers, whereas the mean difference between the Earth and the Moon is 384,400 kilometers. That means that Phobos is ~40 times closer to Mars than the Moon is to the Earth. The proximity of Phobos to Mars causes a lot of tidal stress on this moon.

Also the Moon's orbit is growing:
(...)
By contrast, the orbit of Phobos is shrinking, increasing the tidal forces of Mars on Phobos:
Wikipedia wrote:
Phobos is drawing closer to Mars by 2 meters every one hundred years, and it is predicted that in 30 to 50 million years it will collide with the planet or break up into a planetary ring.
All these factors explain why Phobos may be about to break up, while our own Moon is not.
Thank you Ms. Ann, but this does not clear the mystery, what causes Phobos's orbit
to shrink, if indeed it is shrinking ? The cause is certainly NOT tidal forces,
since they tend to EXPAND orbits (seen as a secular increase of the mean semi-major axis) because of the orbital energy slowly but steadily lost forever due to friction inside the bodies (dissipated as heat).

If it is true that Phobos is "falling" towards its companion, there has to be a celestial-mechanical explanation DIFFERENT than inelastic deformations due to the mutual tidal forces ! Could it be mutual N-body perturbations within the Mars-Phobos-Deimos system ?

I would like to know if the long term orbital behaviour of that "dwarf planetary" system have been theorised/calculated satisfactorily by celestial mechanics experts and / or simulated (as in : computer integration; I will be much more confident in serious analytic derivation of long term orbital parameters à la Poincaré than modern numerical integrations, by the way),
and if so, whether observations are in accordance with calculated parameters

I always understood this as:
When the Tidal Force (Bulge) between the two bodies (Earth-Moon) leads the smaller body (Moon), it pulls the smaller body along giving energy to it. That extra energy causes the body to spiral outward.
When the Tidal Force between the two bodies trails the smaller body, it slowly takes energy from the smaller body causing it to spiral inward.
Where the Tidal Force is would depend upon the size/mass of the smaller body and the distance between them

by **Chris Peterson** » Mon Nov 23, 2015 2:35 pm

FLPhotoCatcher wrote:As for being tidal cracks, more possible in regard to the straightness. But it seems unlikely - especially since many of the lines cross each other without effecting one another (jogging).

Keep in mind that Phobos is not a solid, rigid structure. With a bulk density of only 1.8, and a probably material density of around 3, it is most likely a porous aggregate... what is commonly called a "rubble pile". How a thin crust on such a structure will respond to forces is not likely to be intuitive.

by **Czerno o** » Mon Nov 23, 2015 2:27 pm

Ann wrote:
Czerno o wrote:« Gravitational tidal forces are dragging it down. »
That assertion is puzzling...
The mean distance between Mars and Phobos is 9377 kilometers, whereas the mean difference between the Earth and the Moon is 384,400 kilometers. That means that Phobos is ~40 times closer to Mars than the Moon is to the Earth. The proximity of Phobos to Mars causes a lot of tidal stress on this moon.

Also the Moon's orbit is growing:
(...)
By contrast, the orbit of Phobos is shrinking, increasing the tidal forces of Mars on Phobos:
Wikipedia wrote:
Phobos is drawing closer to Mars by 2 meters every one hundred years, and it is predicted that in 30 to 50 million years it will collide with the planet or break up into a planetary ring.
All these factors explain why Phobos may be about to break up, while our own Moon is not.

Thank you Ms. Ann, but this does not clear the mystery, what causes Phobos's orbit
to shrink, if indeed it is shrinking ? The cause is certainly NOT tidal forces,
since they tend to EXPAND orbits (seen as a secular increase of the mean semi-major axis) because of the orbital energy slowly but steadily lost forever due to friction inside the bodies (dissipated as heat).

If it is true that Phobos is "falling" towards its companion, there has to be a celestial-mechanical explanation DIFFERENT than inelastic deformations due to the mutual tidal forces ! Could it be mutual N-body perturbations within the Mars-Phobos-Deimos system ?

I would like to know if the long term orbital behaviour of that "dwarf planetary" system have been theorised/calculated satisfactorily by celestial mechanics experts and / or simulated (as in : computer integration; I will be much more confident in serious analytic derivation of long term orbital parameters à la Poincaré than modern numerical integrations, by the way),
and if so, whether observations are in accordance with calculated parameters

by **Ann** » Mon Nov 23, 2015 4:27 am

As a Color Commentator, I have to ask this question: How did one patch of Phobos get to be so much brighter and than the rest of the moon and so non-red compared to the rest of it? This bright part of Phobos can in some ways be compared to Pluto's "heart" which is so much brighter than the rest of Pluto, but the "heart" - or at least one lobe of it, Sputnik Planum - is so different geologically than the rest of Pluto, too. It appears to be made up of large, rounded, crater-free segments of ice. But while the "heart", Tombaugh Regio, is a lot brighter than the rest of Pluto, it appears to be basically the same orange-ish hue.

The whitish part of Phobos also appears to be geologically different than the rest of Phobos. It is heavily cratered and very striated, and it looks "exposed", as if a layer of regolith had been removed from it. Much of the rest of Phobos appears softened, as if it was covered with sand.

Ann

by **FLPhotoCatcher** » Mon Nov 23, 2015 4:01 am

Boomer12k wrote:Ummmm.....me thinks a good way to understand the depressions on Phobos is to read up on Rilles of The Moon....which are mostly sunken Lava tubes or areas that collapsed. But that does not necessarily explain the "straightness" and "one direction-ness" of these grooves....however...there are grooves on the right side in the white area that are perpendicular, and contrary to the others. I suppose Impacts and Tidal Forces could "sink" these areas, also tubes can collapse from cooling and shrinking. Many of these grooves are very faint, and light on Phobos.

There is not enough mass, heat, or gravity in a small moon like Phobos to make lava tubes or volcanoes. And if there was, the lava tubes would not be so straight.

As for being tidal cracks, more possible in regard to the straightness. But it seems unlikely - especially since many of the lines cross each other without effecting one another (jogging).

I still think the moon must have orbited through a ring of debris that used to be there. It would explain the straightness of the lines, the lines of craters, the faint lines (smaller ring particles) and the way the lines cross each other.

But it would certainly be cool to observe Mars with Rings.

by **Boomer12k** » Sun Nov 22, 2015 11:49 pm

Ummmm.....me thinks a good way to understand the depressions on Phobos is to read up on Rilles of The Moon....which are mostly sunken Lava tubes or areas that collapsed. But that does not necessarily explain the "straightness" and "one direction-ness" of these grooves....however...there are grooves on the right side in the white area that are perpendicular, and contrary to the others. I suppose Impacts and Tidal Forces could "sink" these areas, also tubes can collapse from cooling and shrinking. Many of these grooves are very faint, and light on Phobos.

They seem to be a line of craters, some fainter ones just did not form craters, but just sunk...and remind ME, at least, of a Volcanic "Curtain of Fire" that leaves a rift or fault afterwards.
Some "appear" to be impacts and then filled in with debris.
On Phobos, this may not be volcanic fire, or lava...but outgasing like a Geyser...and then along the rilles as well...??? Out-gasing Pockets, so to speak.

Just reminds me of that.
Enceladus is another moon with Rilles, grooves, rifts, etc....lots of tidal forces, but its rilles meander all over the place.
Jupiter's Europa is another moon with strong tidal forces, and the grooves meander around and criss-cross, etc...and are also very long. But do not seem to have much if any cratering in or on them.

I don't think Tidal Forces alone explain Phobos Striations....Earth's water comes mostly from Asteroids, not comets. Phobos probably has water, it out-gases, and areas collapse. Some areas are more violent and cause some craters. But that does not necessarily explain the straightness of the grooves....hhmmm, but certainly neither do "Tidal Forces" as other moons exhibit "meandering" lines...just my opinion. My conclusion? Combination of things, and happenstance.

It would certainly be cool to observe Mars with a Ring.
:---[===] *

by **Boomer12k** » Sun Nov 22, 2015 11:32 pm

Jim Armstrong wrote:APOD's such as this always produce a wide and interesting range of comments.
I am surprised that no one has mentioned the brightly illuminated crater on the left,
It is probably sunshine, but one wonders if it is a self-lit area or perhaps the flash bulb on the MRO.

I think it is the Angle of the Crater...it is on a more sun facing hillside...so more exposed.

:---[===] *

by **Jim Armstrong** » Sun Nov 22, 2015 10:27 pm

APOD's such as this always produce a wide and interesting range of comments.
I am surprised that no one has mentioned the brightly illuminated crater on the left,
It is probably sunshine, but one wonders if it is a self-lit area or perhaps the flash bulb on the MRO.

by **Tszabeau** » Sun Nov 22, 2015 9:26 pm

Sink holes.

by **neufer** » Sun Nov 22, 2015 7:47 pm

geckzilla wrote:
I think that the appearance of the grooves being older than craters—craters being "on top" of the grooves—may an illusion. I so see some instances of grooves visibly crossing craters, but very faintly due to the lighting more heavily outlining the craters than the grooves. Stickney, for instance, has an entire rim covered in grooves. How could it be that Stickney is younger than those grooves? Why the grooves do not continue to the center of the crater, I do not know. It could have something to do with the way stress is applied to the moon.

Or it could be that the center of Stickney (and other groove-free Phobos craters) is filled with the "ashes" of a departed older brother of Phobos & Deimos whose ring has since been absorbed by Phobos.

by **George** » Sun Nov 22, 2015 7:44 pm

On another note: it seems that deimos is usually translated as dread or terror, NOT panic.
And if you look at the kitty, it doesn't seem panicky...

Starship Asterisk*

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

Post a reply

Expand view Topic review: APOD: Phobos: Doomed Moon of Mars (2015 Nov 22)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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