Phobos: Irregular Shape? (APOD 3 Dec 2006)

[Al]

Thanks John. What you said makes sense.

Now, just suppose that Phobos ran into a bunch of pieces of something from the right as John suggests. One would think that pieces close enough to the moon to be attracted by gravity would 'roll' along the surface making tracks as we see them. I would expect the objects to roll up the hill to a crater and then take off because the hill would swing them upward. We see that on John's 4 o'clock trail where it shows to the right of the picture but not to the left of the crater. However, there is a track a short distance down the right crater wall that doesn't conform to this idea.

Also note that there is a track up near the top of the picture that doesn't conform to the above 'jumping' idea. The track can be seen across two craters--very evident in the first but not as clear in the 2nd. If there were pieces of material rolling across the surface of Phobos, would Phobos's gravity pull those pieces down to roll across the crater. I would think the gravity of a small moon is not that great.

Hope this creates some thoughts.

Al

[Andy Wade]

Could spectra be used to determine if, in fact, Mars' moons came about as a direct result of the Hellas Impact and are in fact part of the planet?

When I said I thought it looked like Phobos might be sedimentary in origin, I didn't actually mean I thought it was part of the Meridiani seabed!

[FieryIce]

You're forgetting a crucial fact about Phobos, it's density.
To quote from page four, Artificial Satellities of Mars, I. S. Shklovskiy, Riddle of the Martian Satellites, by I. S. Shklovskiy, FTD-TT-62-488/1, Wright-Patterson Air Force Base FOIA 88CG/SCCM

"Question: How do you explain these anomalies?
Answer: after analysis and rejection of all imaginable causes for the slowing down of Phobos I arrived at the following conclusion: most likely, it is the braking action of the upper, highly rarefied layers of the atmosphere that plays the determining part. But, in order to make this braking action so significant, and taking into account the extremely rarefied Martian atmosphere at this altitude, Phobos should have very small mass, i.e., very low average density, approximately one thousand times smaller than the density of water.
But can a continuous solid have such low density, probably smaller than that of air? Of course not! However, we can imagine that Phobos is not coherent but is rather a cloud of minute dust particles separated by appreciable distances from one another. But such a cloud, as calculations showed, would inevitably spread along its entire trajectory, changing into something resembling the famous ring of Saturn. There is only one way in which the requirements of coherence, constancy of shape of Phobos, and its extremely small average density can be reconciled. We must assume that Phobos is a hollow, empty body, resembling an empty tin can.
Well, can a natural celestial body be hollow? Never! Therefore, Phobos must have an artificial origin and be an artificial Martian satellite. The peculiar properties of Deimos, though less pronounced than those of Phobos, also point toward an artificial origin."

[iamlucky13]

Several sources I found list the density of Phobos to be roughly 2 g/cm^3, which is about twice the density of water. However, they note this is low for solid rock, indicating the moon is porous. One abstract mentioned using changes in the orbit of the Mars Express probe caused by the gravitational interaction with Phobos.

I did some more reading about the peculiar acceleration. Apparently Shklovskiy estimated the mass based purely on the assumption that the acceleration was due to atmospheric drag. It turns out that astronomers of his day calculated the acceleration wrong, and the actual magnitude is explained by tidal drag.

http://www.uapress.arizona.edu/onlinebk ... chap14.htm

[JohnD]

Al,
Rolling along the surface.
As it happens, all the craterchains I pointed out above are about 250 meters across. (I measure them on screen and compared with the scale in the picture)
A rock that rolled across the surface of Phobos to leave those marks would need to be at least that in diameter. If such a rock approximates to a sphere, its' volume would be about 8 million cubic meters. If it was the same density as Phobos (2 g/cm^3 - iamlucky13), then it would mass 16 million metric tonnes. (I think, check my arithmetic, please) That will not weigh very much on Phobos, but it would have to much more closely approximate a sphere than Phobos or any other non planetary object to roll far, let alone take off on an up-slope! friction would soon slow it down and irregularity deviate its' course - these are near straight.
And anyway, why would your object roll? What force pulls it sideways across the surface?

No, these are crater chains, the result of an object being torn into a string of smaller objects by the tidal force of an objst much larger than Phobos. The string then impacts on a body, could be the large body, could be another, just like a stick of bombs only more regular. If the objects are close enough together in the 'stick', the craters meld into each other to form a groove. The examples above all show close, but un-melded craters. Rather than suggest another cause, the fine grooves are due to the same effect, from smaller or older, melded crater chains.

Oh, and FieryIce. You do the maths. If Phobos is hollow, how thick must the walls be to survive the size of the old, major craters we see, without penetration? Or is the one at nine o'clock a Death Star weapon?

John

[FieryIce]

That’s interesting, the words acceleration and drag together in the same statement.

[JohnD]

What's wrong with that, FI?
Acceleration can be positive or negative. Drag is a force in opposition to motion, casuing negative acceleration.


Oh, and please answer my question about penetration of the hollow Phobos?

John

[Dr. Skeptic]

You're forgetting a crucial fact about Phobos, it's density.
To quote from page four, Artificial Satellities of Mars, I. S. Shklovskiy, Riddle of the Martian Satellites, by I. S. Shklovskiy, FTD-TT-62-488/1, Wright-Patterson Air Force Base FOIA 88CG/SCCM

"Question: How do you explain these anomalies?
Answer: after analysis and rejection of all imaginable causes for the slowing down of Phobos I arrived at the following conclusion: most likely, it is the braking action of the upper, highly rarefied layers of the atmosphere that plays the determining part. But, in order to make this braking action so significant, and taking into account the extremely rarefied Martian atmosphere at this altitude, Phobos should have very small mass, i.e., very low average density, approximately one thousand times smaller than the density of water.
But can a continuous solid have such low density, probably smaller than that of air? Of course not! However, we can imagine that Phobos is not coherent but is rather a cloud of minute dust particles separated by appreciable distances from one another. But such a cloud, as calculations showed, would inevitably spread along its entire trajectory, changing into something resembling the famous ring of Saturn. There is only one way in which the requirements of coherence, constancy of shape of Phobos, and its extremely small average density can be reconciled. We must assume that Phobos is a hollow, empty body, resembling an empty tin can.
Well, can a natural celestial body be hollow? Never! Therefore, Phobos must have an artificial origin and be an artificial Martian satellite. The peculiar properties of Deimos, though less pronounced than those of Phobos, also point toward an artificial origin."

I could create thousands of scenarios to explain the posed data anomalies and chances are they would all be at least partially wrong. Where is the hard core science to backup your speculative theory. It is quite a leap to go from a low density moon to "artificial origin" in one paragraph, I'll stick with icy- crystalline nugget center.

[FieryIce]

Doing the math for penetration is irrelevant; Stickney Crater isn’t a gaping hole. I have no interest in punching a hole in Phobos. Many paper houses are plastered with theories.
Acceleration in the negative is no longer acceleration.

[JohnD]

FI,
I'd feel proud if you weren't such an easy target.
But it wouldn't look good on a T-shirt:
"I got FI to bluster, rather than offer a reasoned argument for the theory of a hollow Phobos"

And in my physics, acceleration="rate of change of velocity". A rate of change can be positive or negative.
If you want to bluster again and say that negative acceleration is 'deceleration', okay, but that is just a constructed word "after 'acceleration'" as the OED says.

John

[Dr. Skeptic]

John you at absolutely correct, + or - acceleration is the standard term in a mathematical equations as m/s^2. there is no mathematical term exclusively for -acceleration.

[Al]

John, and all;

Yes, I see some of the 'lines' that look as if they are made up of small craters. But others don't seem to be.

Maybe I chose the wrong word--'rolling'. Drag may fit what I meant better. I pictured the moon running into a bunch of something, like from the right of the picture. The pieces of that something would leave marks on the moon as it went by. The force involved would simply be the force of the moon ant the individual items moving by each other--much like your car hitting a bunch of gravel thrown up from the roadway. The impact on your car would leave marks, some of which would be along the top and sides. And I bet the marks would tend to be parallel as in the Phobos picture.

Al

[JohnD]

Al,
Don't forget the resolution of this picture - about 7 meters (an astounding achievement!).
This may allow a chain of craters that are 250 meters wide to be seen as craters, but if they are c.25 meters, then the chain will appear as a groove.

I fear that even 'dragging' won't work! Gravel thrown against your car may leave a mark but will bounce off, even if it hits a glancing blow. The only example I can think of that equates to the car and gravel that could leave a definite line of marks would be a burst from an automatic weapon, that might leave a line of bullet holes, or marks if the surfaceis strong enough. So that takes us back to FI's interplanetary war idea, and to the hollow Phobos! A handful of gravel like a comet or, it is suspected, some asteroids would leave a random group, like that of single shots at a target.
But we have a perfectly reasonable mechanism for a 'line of bullets' that does not need to make assumptions about such science fiction - tidal disruption, what happened to the Shoemaker-Levy comet that impacted on Jupiter.

Something must hold the rock against the surface for a drag mark to show. Gravity would do, but then the friction of rock against surface would be immense, even between very low mass objects (in astronomical terms) like Phobos and a stray piece of asteroid. And what is to pull it along on the surface? That force too would need to be equally immense.

Imagine a whole bunch of 'gravel balls', orbiting together like the Trojan asteroids. For whatever reason, they get too close to a large object and are stretched into long lines of gravel by the tidal effect of that large object's gravity. Then the lines impact on Phobos, before their own gravity can pull them back into balls again.

I'm no expert - that story is speculation as much as FI's is. But mine has less assumptions than are required for interplanatery war, on hollow artificial satellites 22 kilometers across marked by strafing runs from unknown weapons.
John

[iamlucky13]

Orbital mechanics is a funny thing. As Phobos experiences drag, it will lose altitude, but actually gain velocity. It's overall energy (potential plus kinetic) decreases, but it's final velocity (kinetic energy) is higher. This is very counter-intuitive. Just remember, the velocity required to maintain an orbit is higher the smaller the radius, which is why the shuttle orbits at 17,500 mph, while a geosynchronous satellite orbits at 7,000 mph. I forgot about this when I asked why Phobos was ahead of position. If it helps to envision it as discrete rather than continuous process, you would see the satellite slow, then fall into a lower orbit, picking up speed by falling.

Also, a completely hollow Phobos could not explain the perturbations it causes to the orbit of the Mars Express probe. However, the calculated density of 2 g/cm^3 is definitely reasonable. The Tagish Lake meteorite, thought to be similar to Phobos, has a density of 1.7 g/cm^3.

[JohnD]

13,
Since you mention it, may I mention that it is the effect of which you write which causes tidal disruption?
That ball of gravel would orbit in a very large gravity field, like the Sun's, with the individual particles orbiting around their common centre of mass (CCoM), or stuck together loosely with ?ice. If the ball enters a smaller gravity field with a steeper gradient, like that of a planet, then objects on the far side of the CCoM from the planet are going too slow for their new orbit; they experience a force pulling them away from the planet into a higher slower moving orbit.
Gravel inside the CCoM have a similar force pulling them in towards the planet. Once this tidal force is greater than the force of gravity, or the ice, that holds the ball togther, then the ball will stretch like a putty. The process continues until the ball is stretched out in a line of gravel, each pursuing their own orbit around the planet. Of course that orbit may be hyperbolic, so that the line of gravel leaves the planet and moves on. If that is into a smooth gravity field, then the ball may reform, eventually as their mutual gravity pulls them togther again, but if it encounters another planet or satellite, then all or part of the line may impact, as a crater chain.

John

[KenGrok]

I, like AI, was struck by the layering in the APOD photo and thought "sediment!". Eager to find out more, I was eventually directed to this forum. It's nice to see there's some excitement about the matter.

I have followed JohnD's reasoning against AI's proposition, but I still think there's something to the idea. The parallelness of the lines is what gets me, especially where the same lines continue into and back out of the Stickney (though this is less apparent in the APOD photo).

In the APOD photo, in the northern half of the terrain I make out six grooves (showing up a bit darker) that are at roughly equal distances apart from each other. I grew up vacationing in the red sandstone of southern Utah and northern Arizona. I doubt that Phobos is a chunk of the Grand Canyon (grin). But I'm used to seeing millions of years of regular cycles and processes show up in the faces of the mountains I've walked up.

Previous research points to the relatively low density of this moon. I'm no expert, but let's say that rather than sediment turned to stone, Phobos is made up of, among other things, layers of frozen material laid down in cycles somewhere. Just because our Earth does not have any layers of sediment 20 km deep doesn't mean that something similar (i.e. from different materials) couldn't have formed elsewhere.

Regardless of its exact makeup, could it be erosion of softer, less hardy layers that has created the grooved appearance? The erosion could come from smaller particles of any origin. The effect would be most pronounced on the leading face of Phobos. This would not yet have taken place in the newer craters. In fact, whatever caused in-space erosion may no longer be occuring.

Another thought regarding the ideas of machine-gun hits from Mars ejecta or the one of rubbish rolling or gouging the grooves: if Phobos' axis has changed significantly over the eons, I would guess that we would see a set of parallel lines oriented in a different direction.

The idea of layers doesn't explain every feature on Phobos. For example, there are some parallel lines coming out of Stickney that head off in a different direction (they show up quite brighter in the APOD photo, and are more evident in a 1978 Viking I photo http://en.wikipedia.org/wiki/Image:Phobos-viking1.jpg). I’d say that multiple factors have shaped Phobos’ surface.

OK, my two cents. Anyone beside AI feel there's something to this idea?

An additional resource (not really supporting the layer idea):
http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2195.pdf (this one does talk about the lines on north half)

[JohnD]

Ken,
Thank you! What an interesting and informative paper!

It raises a Q for me - Assuming that grooves are just crater chains below the resolution of the photos, the authors suggest that ejecta from Mars impacts caused these markings.
Does (do?) 'ejecta' leave the impact site as a 'line of gravel', or is it spread out by tidal effect between the impact site and Phobos?

The former would remove the weakness in the ball to line by tides argument, in that you need a mighty gravity field to disrupt an asteroid. That argument might require encounters with Jupiter etc. AND then Mars/Phobos, an assumption not required for the ejecta theory.

JOhn

[philbur]

Hi,

I'm new here and have a question.

The reference photo shows that the moon has a large number of parallel striations that look like sedimentary deposits. There is no explanation in the text. Could anybody please explain to me what they are?

Thank You and Regards

Phil Burman
Stavanger, Norway.

[JohnD]

My apologies. I wrote above that:

If the ball enters a smaller gravity field with a steeper gradient, like that of a planet, then objects on the far side of the CCoM from the planet are going too slow for their new orbit; they experience a force pulling them away from the planet into a higher slower moving orbit.
Gravel inside the CCoM have a similar force pulling them in towards the planet. John

I should, of course, have said that "objects on the far side of the CCoM from the planet are going too FAST for their new orbit" and vice versa.

Orbital mechanics aren't easy -as Larry Niven put it in the "The Smoke Ring", ""East takes you out, out takes you west, west takes you in, and in takes you east."

John