APOD: Phobos from Mars Express (2010 Mar 17)

[Amir]

wow, data are inconsistent with each other!
this

North pole declination: 67.16°

obviously points northward.
and then we have this:

... North pole of Venus is pointed 'downward' (our southward)...

why is that?

[wonderboy]

Could the marks on this small moon not be caused by the tidal forces being exerted upon it? If the moon is going to be "pulled apart" then these cracks would surely be best explained as stress marks or cracks on the surface of the moon?

[JohnD]

wonderboy,
If Phobos is as cracked up as that (all that it is cracked up to be!), what is holding it together?
The tidal forces would still be acting on it, so why hasn't it spread out into a 'string of pearls"?
OK, maybe it made a close approach to the (Chandresekar-delete) ROCHE limit at one time, and nearly broke up, but got away to a less stressful orbit where its teeny, tiny gravity can hold it together, but that would need some special pleading to explain the orbitals.
AND
Did you look at the new, hires pics from Express?
Magnified, lines that look like grooves can be seen to be crater chains.

JOhn

[wonderboy]

Just checked them out, I was wondering what hires was? I was thinking it was some moon I hadn't even heard of until I realised you meant Hi-Res haha. I'm a fool. Anyway, I really don't think that they are sedimentary markers. if the gravitational forces being exerted are only strong enough to sloooooooooooooooow the moon down over a long period of time then surely whats holding the moon together is its own weak gravity. All I'm asking is, if the forces involved are so weak with the force from mars being slightly stronger, then surely lines like this would appear due to the stress being exerted on the moon itself. I think it makes sense.

Then again, if it is sedimentary lets explore it and see what we find.

[GaryR]

wonderboy,
If Phobos is as cracked up as that (all that it is cracked up to be!), what is holding it together?
The tidal forces would still be acting on it, so why hasn't it spread out into a 'string of pearls"?
OK, maybe it made a close approach to the Chandresekar limit at one time, and nearly broke up, but got away to a less stressful orbit where its teeny, tiny gravity can hold it together, but that would need some special pleading to explain the orbitals.
AND
Did you look at the new, hires pics from Express?
Magnified, lines that look like grooves can be seen to be crater chains.

JOhn

Chandresekar limit? You mean Roche limit?

Gary

[JohnD]

I do!
Thank you, Gary. I have edited the above post in a obvious way.

John

[Case]

I have edited the above post in a obvious way.

Alternatively you might use the strikethrough strike tag for such...

[JohnD]

Delighted. Show mewhere to find it. Shown!
JOhn

[owlice]

JOhn, it's the "s" button on the menu you see when you click Post Reply:

Picture 2.jpg (9.01 KiB) Viewed 13887 times

Or use BBCode:

Code: Select all

[s]this will appear in strikeout once a Quick Reply is submitted[/s] and this will not

As shown here: this will appear in strikeout once a Quick Reply is submitted and this will not

[JohnD]

Thnak you!
John

[wonderboy]

I don't see what the big fuss is here. Everyone agrees that there are tidal forces slowing phobos and pulling it apart. When you pull something apart signs of such damage appear on the surface momentarily before it snaps completely (based on the fact that a human pulling something apart would be a short sharp process). Now, if you spread this "pulling apart" over many MANY years on a moon like phobos, then you will see signs of the stress arising. Some of the groves will have been created through rock and lunar dust falling into cracks and fissures, others will be created by meteorites bouncing along the surface of the moon. I believe we're all right, its just a case of determining what are cracks and what are meteorite trails. Its not really science, its just astronomical brute force.

[JohnD]

" Everyone agrees " do they?
But what do they agree? That Phobos is influenced by Mars tides, and that it is close to the Roche limit. But how close?
About
See: http://www.absoluteastronomy.com/topics/Phobos_(moon)
"Future destruction
Because Phobos's orbital period is shorter than a Martian day, tidal locking is decreasing its orbital radius at the rate of about 20 meters per century. In 11 million years it will either impact the surface of Mars or more likely break up into a planetary ring. Given Phobos's irregular shape and assuming that it is a pile of rubble (specifically a Mohr-Coulomb body) it has been calculated that Phobos is currently stable with respect to tidal forces. But it is estimated that Phobos will pass the Roche Limit for a rubble pile when its orbital radius drops by a little over km to about km. At this distance Phobos will likely begin to break up and form a short lived ring system around Mars. The rings themselves will then continue to spiral slowly into Mars."

And from: http://burro.astr.cwru.edu/stu/advanced/mars_moons.html
"For Mars-Phobos, the Roche Limit is estimated as 10,800 km. However, Phobos is about 1400 km inside of this. This is because the Roche Limit does not take into account the strength of the material of the object. With this in mind, the best estimate is that the Roche Limit is about 5000 km. Once Phobos comes within this, the moon will be destroyed by tidal forces."

The above give different rates for Phobos' orbital decay, but agree on the idea that Phobos is a long, long way, and a long, long time from being pulled apart by Mars tides.

John

[Pretzelogic]

It seems like everyone's forgotten the question originally posed in this entry:

Why is this small object orbiting Mars?

The similarity to C-type asteroids was pointed out, along with the difficulty that would be inherent in gravitationally capturing such asteroids, circularizing their orbits, etc.

Forgive me if this suggestion is naive; I have no more than an interested layman's degree of expertise in astronomy - but, is it possible that Mars' two moons were formed out of Mars itself? Perhaps thrown into orbit by an enormous impact (maybe the one that led to the formation of Hellas Planitia)? Over time, other debris thrown into orbit by that same impact, which might have formed a ring system for a time, could either have fallen back to the planetary surface already (or impacted with Deimos & Phobos, causing many of the craters now seen)...

Those with more expertise than I have (i.e., probably almost everyone else "here") may now begin explaining why my idea couldn't possibly be correct...

Thanks

[jacklap]

Check the March 25 ESA Phobos Flyby link at http://webservices.esa.int/blog/post/7/1085. At 30% +/- 5% porosity, this moon is a chunk of swiss cheese, figuratively speaking.

[Pretzelogic]

So, from that degree of porosity, and the statement (at the link jacklap cited) that "these results [new values for gravitation and density] are inconsistent with the proposition that Phobos is a captured asteroid", may I take it that experts (at least, at the ESA) are also leaning toward the notion that Phobos (maybe Deimos, too?) accumulated in Mars orbit from debris blasted out in an impact on the planet? My apologies if it begins to seem like my "density" may exceed that of Phobos...

[wonderboy]

What about Olympus Mons, is it possible that there was such a massive eruption from this huge volcano that chunks from the martian surface ended up in orbit around the planet? This is a very VERY distant possibility but it should be noted that volcano's on our planet can blast stuff pretty high into the atmosphere, why couldn't a volcano the size of Olympus Mons blast something that high from the interior of the planet. It could explain the idea of sedimentary deposits on the moon. I still stand by my original idea though .

[BMAONE23]

Olympus Mons is also almost directly opposite the Hellas Impact site see the Global Relief Map

[wonderboy]

Olympus Mons is also almost directly opposite the Hellas Impact site see the Global Relief Map

So are you saying it may be a possibility? I dunno if it would be possible, but a 15 mile high volcanoe volcano has a chance of doing something like that doesn't it?

[Chris Peterson]

So are you saying it may be a possibility? I dunno if it would be possible, but a 15 mile high volcanoe has a chance of doing something like that doesn't it?

If it were a stratovolcano, I'd think it a lot more likely. But Olympus Mons is a shield volcano, like those found in Hawaii. Such volcanoes are associated with slow, gentle flows of very liquid lava, not with explosive eruptions. Also, it is hard to understand how two bodies ejected from a latitude of 18° would end up in circular, low inclination orbits. Finally, you'd expect the moons to be basaltic in composition, which they don't appear to be.

I think some sort of capture hypothesis makes better sense.

[neufer]

Olympus Mons is also almost directly opposite the Hellas Impact site see the Global Relief Map

So are you saying it may be a possibility? I dunno if it would be possible, but a 15 mile high volcanoe has a chance of doing something like that doesn't it?

If it were a stratovolcano, I'd think it a lot more likely. But Olympus Mons is a shield volcano, like those found in Hawaii. Such volcanoes are associated with slow, gentle flows of very liquid lava, not with explosive eruptions. Also, it is hard to understand how two bodies ejected from a latitude of 18° would end up in circular, low inclination orbits. Finally, you'd expect the moons to be basaltic in composition, which they don't appear to be.

I think some sort of capture hypothesis makes better sense.

1) If Olympus Mons was first formed by the concentrated stress from Hellas Impact on the opposite side then certainly at that time it could have spewed out non basaltic matter (as might the splash back at Hellas itself).

2) Any capture model would still have to explain the circular, low inclination orbits.

Certainly the circular, low inclination orbits are due, in part, to the same tidal forces which are draining energy & angular momentum from the moons today.. since, if for no other reason, both the 124,000-year obliquity cycle and the 171,000-year precession are constantly changing where the Martian equator is situated:

<<Mars' "obliquity" -- the tilt of its spin axis -- is known to slowly increase and decrease between about 15 degrees and 35 degrees over a 124,000-year cycle (unlike Earth's tilt, which slowly rocks through a range of only 4 degrees thanks to the stabilizing tuggings of our large Moon). Indeed, recent studies suggest that occasionally -- at intervals of a few tens of millions of years -- Mars' obliquity may swing from 0 degrees all the way up to 60 degrees. At present -- by sheer chance -- Mars is about halfway through one of its obliquity cycles, at a tilt of about 25 degrees.>>

<<As on Earth, the effect of precession causes the north and south celestial poles to move in a very large circle, but on Mars the cycle is 171,000 Earth years rather than 26,000 years as on Earth.>>

[Chris Peterson]

1) If Olympus Mons was first formed by the concentrated stress from Hellas Impact on the opposite side then certainly at that time it could have spewed out non basaltic matter (as might the splash back at Hellas itself).

I don't see how. The material produced by a volcano, regardless of the nature of the volcano, will be basaltic. If Olympus Mons was created by an impact on the opposite side, ejecta from the collision seems far more likely than any volcanic material as a source for the moons.

2) Any capture model would still have to explain the circular, low inclination orbits.

That's true, but you can construct plausible scenarios for that. The low inclination comes fairly naturally; the circularization can be explained by tidal effects- assuming there has been sufficient time.

For me, the main argument for capture is that the composition appears consistent with asteroidal material and not with planetary debris.

[neufer]

For me, the main argument for capture is that the composition appears consistent with asteroidal material and not with planetary debris.

It has a superficial asteroidal regolith but inside it appears to be mostly ice.

<<Spectroscopically [Phobos] appears to be similar to the D-type asteroids, and is apparently of composition similar to carbonaceous chondrite material. Phobos' density is too low to be solid rock, however, and it is known to have significant porosity. These results led to the suggestion that Phobos might contain a substantial reservoir of ice. Recent images from Mars Global Surveyor indicate that Phobos is covered with a layer of fine-grained regolith at least 100 meters thick; it is hypothesized to have been created by impacts from other bodies, but it is not known how the material stuck to an object with almost no gravity. Spectral observations indicate that the surface regolith layer lacks hydration, but ice below the regolith is not ruled out.>>

Did the water/ice originally come from Mars? A comet?

[Chris Peterson]

It has a superficial asteroidal regolith but inside it appears to be mostly ice.

It doesn't appear that way to me. There is simple speculation about this, with no real evidence. I think a loose pile of rubble makes more sense- the same structure many asteroids appear to have.

[Seljuk]

I have a question and theory regarding the grooves (streaks) along Phobos. My question is why are they lateral in only two directions? It is as if they were combed along the surface And my humble view of all this is that they may have been formed similar to stretch marks (Striae) on our skin, such as those we see in abdominal enlargement of pregnancy ("can appear when there is rapid stretching of the skin. Stretch marks appear as parallel streaks...." http://www.nlm.nih.gov/medlineplus/ency ... s/8949.htm). The truth maybe anyones guess...

I want to include a few links to images that may prove this point: http://www.virtualmedicalcentre.com/dis ... sp?did=868

It is evident from the lack of grooves in the craters that these were formed early on.

[BMAONE23]

This image demonstrates that the "Grooves" don't always travel in the same linear direction. Look at the Orbit 748 image (upper right) And This surface map also indicates cross banding in the left hand region. And Here is a nice red/blue 3D image showing Cross Banding