Starship Asterisk*

Phobos: Doomed Moon of Mars

Explanation: This moon is doomed. Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These martian moons may well be captured asteroids originating in the main asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of the Solar System. The larger moon, Phobos, is indeed seen to be a cratered, asteroid-like object in this stunning color image from the robotic Mars Reconnaissance Orbiter, recorded at a resolution of about seven meters per pixel. But Phobos orbits so close to Mars - about 5,800 kilometers above the surface compared to 400,000 kilometers for our Moon - that gravitational tidal forces are dragging it down. A recent analysis of the long grooves indicates that they may result from global stretching caused by tides -- the differing force of Mars' gravity on different sides of Phobos. These grooves may then be an early phase in the disintegration of Phobos into a ring of debris around Mars.

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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 find the explanation accompanying this APOD a bit problematic.

If the origin of the long grooves is the result of tidal stretching then these should be the youngest surface features on Phobos.

The cratering pattern of the surface suggests otherwise:

The groves are overprinted by craters.

The cratering of grooved and non-grooved surfaces is comparable.

The linear groves do not disrupt, or transect crater walls, indicating that the craters post-date the grooves.

Phobos:

I think that the moon has had a huge hit on the right side and the debris has rolled around the moon causing the trails and craters.

« Gravitational tidal forces are dragging it down. »

That assertion is puzzling, at best : isnt'it well known
from the observation and analysis of our own Earth-Moon binary system
that the effect of tidal forces is to cause mechanical stress and deformation
in the bodies which disispate energy in turn leading to
secularly increasing - not decreasing - mutual distances ?

Please someone explain how similar causes yield opposite effects ?

Nigma, I suppose that the grooves happened before Phobos became tidally locked presenting one face to Mars. Considering Phobos' small size, that happened early on. When Phobos tumbled the groves opened showing its direction of spin. Once it became tidally locked the gravitational tide froze into the rock and became unchanged. Tidal forces can't work if they never change. I suppose they imply that as Phobos sinks down the tidal force increases, but the face lock is still in affect so the only pull is towards Mars. I suppose Phobos is being stretched as a droplet of water is being stretched from the tap down into the sink.
I suppose it would be interesting to know which part of the picture is facing Mars. Obviously Mars can't be seen in the picture, so this camera must be somewhat between Phobos and Mars. But is it high noon?

Czerno o wrote:« Gravitational tidal forces are dragging it down. »

That assertion is puzzling, at best : isnt'it well known
from the observation and analysis of our own Earth-Moon binary system
that the effect of tidal forces is to cause mechanical stress and deformation
in the bodies which disispate energy in turn leading to
secularly increasing - not decreasing - mutual distances ?

Please someone explain how similar causes yield opposite effects ?

I was wondering the same thing myself. ISTR reading, decades ago, that Phobos is in a retrograde orbit, which would cause a decreasing orbital distance. But what I read is wrong, or at least a misunderstanding.

The misunderstanding may have come from the speed of Phobos' revolution. Because Phobos orbits faster than Mars rotates, it appears to orbit in the opposite direction than most Moons, from the perspective of a point on Mar's surface. Because the Martian rotation lags behind Phobos' revolution, the tidal interaction slows down the moon, causing its orbit to shrink. That's my understanding, anyway.

However, Phobos is so light that its tidal effect on Mars should be minuscule. With the Earth-Moon system, the tidal boost to the Moon's orbit is induced by the Moon's deformation of the Earth as the Earth rotates ahead of the Moon's orbit. Is Phobos' effect on Mars enough to decrease its orbit by 2 meters per century?

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.

Furthermore, the appearance that some grooves are formed by chains of craters—that is a line of objects striking the surface—may also be false. Another way a row craters could form along the grooves is material collapsing into subsurface voids. Stress might build up along the grooves and then the Phobian regolith collapses into these cavities. It may having nothing to do with an impacting body. Expanding upon this hypothesis, one might also conclude that these cavities are somehow disturbed by real impact events and perhaps that would make it less likely for a groove to run continuously through a crater.

Ah Domingo, futebol e Phobos, bacana

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...

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.

Sink holes.

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.

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.

:---[===] *

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.
:---[===] *

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.

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

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.

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

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.

maybe the sun does significantly pull on it

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.