APOD: Phobos South Pole from Mars Express (2011 Jan 24)

[APOD Robot]

Phobos South Pole from Mars Express

Explanation: Where on this moon would you land? The moon pictured above is not Earth's moon but Phobos, the closest moon to the planet Mars. Phobos is so close to Mars that it is expected to break up and crash into the red planet within the next 100 million years. Earlier just this year, however, ESA's Mars Express mission took detailed images of the area surrounding Phobos' South Pole. Visible on the small moon's unusually dark surface are many circular craters, long chains of craters, and strange streaks. Large Stickney Crater, which looms on the far right, was also visible in the corresponding North Polar image taken last year. This and other similar images of Phobos are so detailed, resolving items even 10-meters across, that they are useful for examining proposed landing sites of the future Phobos-Grunt mission. The Russian Phobos-Grunt robotic spacecraft is scheduled to launch toward Phobos later this year and return surface samples in 2014.

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[bystander]

ESA: Mars Express close flybys of martian moon Phobos
http://asterisk.apod.com/vie ... 74#p141574

[agulesin]

When I see such photographs (thanks APOD team!), I two things come to mind:

1) How amazing it is that we have managed to send a craft such a distance and can receive photos from it... How ignorant we would be about the solar system if such things were not developed. A poke in the eye for those who shun NASA funding.
2) How beautiful our own moon is when we could have been orbited by something not at all photogenic like this!

[Boomer12k]

I say we start a "SAVE PHOBOS" club. We collect money starting now, then when we go there we strap on rockets to boost its speed! Over many times of doing that, (and this is why I suggest starting now, because it will cost a lot to do it by the time we get there), it will climb to a higher orbit and be saved!!!! If we wait until we terraform Mars with a thicker atmosphere it will come down all the sooner, (from friction), and ruin the terra-forming effort!!!! So, it needs to be done!!!!!

(Sponsored by the Mars and Phobos preservation society)

[workgazer]

Why no speed up the decent of Phobos , let it slam in to the surface, the resulting dust, possible melt, could kick start the terraforming effort?

[rstevenson]

Dust would more likely cool the planet. In fact, judging from the recorded results of "dusty" events here on Earth, like large volcanic eruptions, that's the only likely result.

Now if we were talking a few hundred large ice-ball comets, that might have a useful effect -- in the long run.

Rob

[zbvhs]

How are the poles of Phobos defined? Rotational axes perhaps? The body's long axis?

[hank jagt]

I propose that the long groves on the surface of Phobos where caused by other objects that where in close orbit and made slow contact, rolled and then their inertia carried them away. The alignment of Phobos's poles and its slow rotation bear this out. Throw a snow ball on a snowy roof and you will see a similarity to this.

[neufer]

I propose that the long groves on the surface of Phobos where caused by other objects that where in close orbit and made slow contact, rolled and then their inertia carried them away. The alignment of Phobos's poles and its slow rotation bear this out. Throw a snow ball on a snowy roof and you will see a similarity to this.

<<Stretch marks or striae (singular stria) are a form of scarring on the skin with an off-color hue. They are caused by tearing of the dermis, and over time can diminish but not disappear completely. Stretch marks are often the result of the rapid stretching of the skin associated with rapid growth (common in puberty) or weight gain (e.g. pregnancy, muscle building, or rapid gain of fat) or in some cases, severe pulling force on skin that overcomes the dermis' elasticity. Stretch marks can appear anywhere on the body, but are most likely to appear in places where larger amounts of fat are stored.>>

[JohnD]

Hank,
Please search this site for other Phobos threads.
The apparent grooves on Phobos are in fact crater chains, so close together that the indivdual craters are beyond discrimination. But look at many other 'grooves', you can see the craters and their nature is clear.

You recall the 'Chain of Pearls' that Comet Shoemaker-Levy turned into before impacting on Jupiter? A comet that was more fragile, or the 'orbiting sand shoal' that is the nature of many asteroids, would have broken up into many more pieces as they approached a much bigger body, and produced a crater chain like this on impact. There are many more examples of crater chains on other planetoids that we have imaged. For instance on:
Callisto: http://apod.nasa.gov/apod/ap950715.html
Mars itself: http://www.craterchains.com/ns/mars.html
The Moon: http://nssdc.gsfc.nasa.gov/imgcat/html/ ... _7485.html
and even Earth: http://www.boulder.swri.edu/~bottke/cra ... chain.html

To groove a planetoid would require the grooving body to be held down by gravity, while it trundled across the surface.
Sorry, but Phobos just doesn't have the mass for that. On its surface gravity is in micro-g!
860–190 µg, due to the irregular surface and masscons.

John

PS Ooops! I have inadvertantly pointed you to the website of "craterchains", who staunchly maintains that such markings are the result of interplanatary warfare between intelligent races that we know not. He has some good pics though! J.

[neufer]

<<[One] hypothesis is that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a large planetesimal. The high porosity of the interior of Phobos (based on the density of 1.88 g/cm³) is inconsistent with an asteroidal origin. Observations of Phobos in the thermal infrared suggest a composition containing mainly phyllosilicates, which are well known from the surface of Mars. The spectra are distinct from those of all classes of chondrite meteorites, again pointing away from an asteroidal origin. Both sets of findings support an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit, similar to the prevailing theory for the origin of Earth's moon.

Phobos is heavily cratered. The most prominent surface feature is Stickney crater, named after Asaph Hall's wife, Angeline Stickney Hall, Stickney being her maiden name. As with Mimas's crater Herschel, the impact that created Stickney must have nearly shattered Phobos. Many grooves and streaks also cover the oddly shaped surface. The grooves are typically less than 30 meters deep, 100 to 200 meters wide, and up to 20 kilometers in length, and were originally assumed to have been the result of the same impact that created Stickney. Analysis of results from the Mars Express spacecraft, however, revealed that the grooves are not in fact radial to Stickney, but are centered on the leading apex of Phobos in its orbit (which is not far from Stickney). Researchers suspect that they have been excavated by material ejected into space by impacts on the surface of Mars. The grooves thus formed as crater chains, and all of them fade away as the trailing apex of Phobos is approached. They have been grouped into 12 or more families of varying age, presumably representing at least 12 Martian impact events.>>

[ABlakey]

Interesting that the earlier APOD of 2010 March 17 estimated the break up and crash at 20 million years while this one estimates it at 100 millions years. I guess I won't wait for it after all.

[Sannox]

Just a thought, no idea if anyone else noticed, according to today's pic, 24th Jan 2011, Phobos will break up within 100 million years or so due to the moon spiraling closer to Mars and the gravity of Mars will break up Phobos, fine, I have no problem with that, I clicked on the link in the text to show Phobos from the North Pole aspect, only to find that Phobos will disintegrate within 20 million years and create a ring system that will then crash into the surface of Mars, hmmmmmmmm, guess it's like Betelgeuse will go supanova within the next 5 minutes or the next million years or so I know we can't guess very well over such lengths of time but some consistency please

[Guest]

Hank,
Please search this site for other Phobos threads.
The apparent grooves on Phobos are in fact crater chains, so close together that the indivdual craters are beyond discrimination. But look at many other 'grooves', you can see the craters and their nature is clear.

You recall the 'Chain of Pearls' that Comet Shoemaker-Levy turned into before impacting on Jupiter? A comet that was more fragile, or the 'orbiting sand shoal' that is the nature of many asteroids, would have broken up into many more pieces as they approached a much bigger body, and produced a crater chain like this on impact. There are many more examples of crater chains on other planetoids that we have imaged. For instance on:
Callisto: http://apod.nasa.gov/apod/ap950715.html
Mars itself: http://www.craterchains.com/ns/mars.html
The Moon: http://nssdc.gsfc.nasa.gov/imgcat/html/ ... _7485.html
and even Earth: http://www.boulder.swri.edu/~bottke/cra ... chain.html

To groove a planetoid would require the grooving body to be held down by gravity, while it trundled across the surface.
Sorry, but Phobos just doesn't have the mass for that. On its surface gravity is in micro-g!
860–190 µg, due to the irregular surface and masscons.

John

PS Ooops! I have inadvertantly pointed you to the website of "craterchains", who staunchly maintains that such markings are the result of interplanatary warfare between intelligent races that we know not. He has some good pics though! J.

John,

The 'Chain of Pearls' certainly makes sense for the pattern we see but to have dozens of these on a small body seems improbable. Also you would expect a 'Chain of Pearls' to be composed of a more verity in sizes leaving larger and smaller craters and have at least some larger gaps in the groves. Also with the 'Chain of Pearls' idea you would expect to see wider pattern rather than the very narrow groves we see. To create these groves would NOT require the grooving body to be held down by gravity. Its inertia alone as it contacted the leading edge of the planetoid that was slowly spinning would cause it to role against the spin direction slowing it down until it reached "top" of its climb at a higher orbit but at a slower speed. At this point the low gravity would allow it leave the surface of the planetoid. The energy needed to to make the groves in a very soft surface would all come from its loss of inertia. A close look at some of the groves will show evidence of a repetitive pattern that you would expect to see from a rolling object with an irregular shape. Some places even show were the object had almost left the surface during its role.

Hank

[neufer]

Just a thought, no idea if anyone else noticed, according to today's pic, 24th Jan 2011, Phobos will break up within 100 million years or so due to the moon spiraling closer to Mars and the gravity of Mars will break up Phobos, fine, I have no problem with that, I clicked on the link in the text to show Phobos from the North Pole aspect, only to find that Phobos will disintegrate within 20 million years and create a ring system that will then crash into the surface of Mars, hmmmmmmmm, guess it's like Betelgeuse will go supanova within the next 5 minutes or the next million years or so I know we can't guess very well over such lengths of time but some consistency please

Phobos currently orbits ~6,000 km above the surface and ~2,000 km above the Roche limit.

At the current rate of 20 metres per century decrease orbital radius it will take 10 million years to hit the Roche limit and some time shortly after that break up into a ring. More than likely, however, tidal forces will increase the current 20 metres per century decrease orbital radius and it will break up in less than 10 million years. (I suspect that the 100 million years figure was simply off by a factor of 10.)

<<Because Phobos's orbital period is shorter than a Martian day, tidal deceleration is decreasing its orbital radius at the rate of about 20 metres per century. In an estimated 11 million years it will either impact the surface of Mars or, more probably, break up into a planetary ring. Given Phobos' 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 2,000 kilometers to about 7,100 kilometers. Newer calculations suggests this will happen in just 7.6 million years from now. At this distance, Phobos will probably begin to break up and form a ring system which will continue to spiral slowly into Mars.>>

[starswarm magellan]

One problem with the "chain of pearls" theory. Now I can't get that ZZ Topp song "Pearl Necklace" out of my mind and I don't like the song. Thanks a lot guys.

[JohnD]

John,
The 'Chain of Pearls' certainly makes sense for the pattern we see but to have dozens of these on a small body seems improbable.

Less probable. Not impossible.

Also you would expect a 'Chain of Pearls' to be composed of a more verity in sizes leaving larger and smaller craters and have at least some larger gaps in the groves. Also with the 'Chain of Pearls' idea you would expect to see wider pattern rather than the very narrow groves we see.

Really. If the 'rubble pile' was a sand and gravel pile? What makes you expect the components of a impactor to be a certain size? The vast majority of meteors on earth are the size of sand grains.

To create these groves would NOT require the grooving body to be held down by gravity. Its inertia alone as it contacted the leading edge of the planetoid that was slowly spinning would cause it to role against the spin direction slowing it down until it reached "top" of its climb at a higher orbit but at a slower speed. At this point the low gravity would allow it leave the surface of the planetoid.

Do you not seen the illogic in your statement? First the body need not be held down by gravity, then the low gravity would allow it to leave. Can't have it both ways.

The energy needed to to make the groves in a very soft surface would all come from its loss of inertia. A close look at some of the groves will show evidence of a repetitive pattern that you would expect to see from a rolling object with an irregular shape.

Goody! Evidence! Please show.

Some places even show were the object had almost left the surface during its role.

Er, "roll"?


Hank,
I presented you with lots of evidence.
You answer with statement and assertion.
You have to do better to make your case.
You might start by investigating what evidence there is of glancing impacts.
Moon craters Messier and Messier A are well known, as glancing impact craters. No sign of the impactor 'rolling along until it is released by the Moon's gravity'. It caused two round craters, one, to be sure, a bit oval. Not a groove in site, not even in the rays from Messier. See: http://seds.org/messier/xtra/m-crater.html

John

[garry]

I think the lines of crater chains would be an interesting place to land a probe. Quite unusual. If they are caused by individual impacts, I mean, a bit far fetched? You would have to have a a group of objects, all the same size & velocity, in an exact straight line.And without any gravity effecting this close moving group. As it seems they almost overlap, this means that each individual crater was made within milliseconds of the other. We have an impact and the debris that is thrown up does not effect the second impact & so on? I do not think so. Each crater in the chain looks clean & defined, so what about all the debris that was ejected? If you tried to duplicate this with a rifle, it would not work. Try it!

[JohnD]

garry,
Follow up the clues above; the String of Pearls, crater chains, rubble pile.
Tidal forces in a strong gravity field will pull a object in a decaying orbit out into a series of small fragments. They impact in a straight line, all at the same velocity, though not necessarily of the same size.
Your example of a rifle bullet is not, I regret, relevant. It's too small, too strong and in too weakly contoured a gravity field. I mean - you wouldn't expect a bullet to break up in flight, would you? That's what an asteroid would do.

John

[neufer]

I propose that the long groves on the surface of Phobos where caused by other objects that where in close orbit and made slow contact, rolled and then their inertia carried them away. The alignment of Phobos's poles and its slow rotation bear this out. Throw a snow ball on a snowy roof and you will see a similarity to this.

They would certainly have to be made of snow (or dry ice) since absolutely nothing remains of them at the end of their trails.

<<Stretch marks or striae (singular stria) are a form of scarring on the skin with an off-color hue. They are caused by tearing of the dermis, and over time can diminish but not disappear completely. Stretch marks are often the result of the rapid stretching of the skin associated with rapid growth (common in puberty) or weight gain (e.g. pregnancy, muscle building, or rapid gain of fat) or in some cases, severe pulling force on skin that overcomes the dermis' elasticity. Stretch marks can appear anywhere on the body, but are most likely to appear in places where larger amounts of fat are stored.>>

Phobos & Deimos undergo significant seasonal cycles in their polar regions

Miranda Diapir Rash :?: (density 1.20 g/cm³)

---

Phobos Diapir Rash :?: (density 1.876 g/cm³)?

---

<<Miranda's surface may be mostly water ice, with the low density body also probably containing silicate rock and organic compounds in its interior. Miranda's surface has patchwork regions of broken terrain indicating intense geological activity in the moon's past, and is criss-crossed by huge canyons. Large 'racetrack'-like grooved structures, called coronae, may have formed via extensional processes at the tops of diapirs, or upwellings of warm ice. The ridges probably represent extensional tilt blocks. The canyons probably represent graben formed by extensional faulting. >>

<<A diapir (French, from Greek diapeirein, to pierce through) is a type of intrusion in which a more mobile and ductily-deformable material is forced into brittle overlying rocks. Depending on the tectonic environment, diapirs can range from idealized mushroom-shaped Rayleigh-Taylor instability-type structures in regions with low tectonic stress such as in the Gulf of Mexico to narrow dikes of material that move along tectonically-induced fractures in surrounding rock. The term was introduced by the Romanian geologist Ludovic Mrazek, who was the first to understand the principle of salt intrusion and plasticity. The term "diapir" may be applied to igneous structures, but it is more commonly applied to non-igneous, relatively cold materials, such as salt domes and mud diapirs.

In addition to Earth-based observations, diapirism is thought to occur on Jupiter's moon Europa, Saturn's moon Enceladus, and Uranus's moon Miranda.

Diapirs commonly intrude vertically upward along fractures or zones of structural weakness through denser overlying rocks because of density contrast between a less dense, lower rock mass and overlying denser rocks. The density contrast manifests as a force of buoyancy. The process is known as diapirism. The resulting structures are also referred to as piercement structures. In the process, segments of the existing strata can be disconnected and pushed upwards. While moving higher, they retain much of their original properties such as pressure, which can be significantly different from that of the shallower strata they get pushed into. Rock types such as evaporitic salt deposits, and gas charged muds are potential sources of diapirs.>>

[NoelC]

They look like cracks that opened up underneath an already powdery surface to me. Maybe it cracked a few times when something massive hit it hard.

Note the big crater near the top at about 11 o'clock. Sure looks like powder falling into the basin, no?

-Noel

[garry]

Thanks John.
But as the crater chains are parallel, this means that each individual segment, after it broke up would have to be exactly the same size to form a parallel, crater chain. What are the chances of that?

[BMAONE23]

I would say that the probability of similarly sized stones flowing over the surface after and in the same direction as a minor impact event are 1000 times more likely than
Mining Troughs
Extra Terrestrial Firing Practice (strafing runs with laser weapons)
Railways along the outside of a space station
and only slightly more plausible than parallel fracture lines with debris infall
or
sedimentary layering as in Phobos is debris from the Hellas Basin impact ejecta

[JohnD]

Thank you, BMAONE23, for that illuminating reply!
In the spectrum of possibility, rolling stones may be a bit off the median, while strafing runs are in the far, frozen north, in that there is little evidence for the first and none for the second. But rubble pile asteroids exist and we know that tidal forces will stretch them out into a string of, well, rubble rather than pearls. More than that, we've seen it happen, to the Shoemaker-Levy Comet. So I think that among causes of crater chains, that has to be the most probable one.

Garry,
What are the chances of the components of a 'string of pearls' being all the same size? Pretty good, I'd say, from the evidence of other crater chains. And if they are all small enough, with current imaging, the individual craters are below discrimination, so it looks more like a trench.
There are crater chains that look like pearl necklaces with the centre piece, the biggest crater in the middle. Beautifully graded!:
Callisto. Gomul Cantena . See: http://en.wikipedia.org/wiki/Gomul_Catena
Or Ganymede, Enki Catena. http://www.nature.com/news/specials/cos ... eshow.html (No.10)
Or Catena (crater chain) Davy on the Moon is a line of equal craters, with irregular bigger ones. http://commons.wikimedia.org/wiki/File:Davy_crater.gif

If you look at the latest Phobos pics, the 'grooves' can be interpreted in a similar way to the Davy chain. More or less all the same, with now and then a bigger or a smaller crater.

So there is a spectrum of size, which is a lot more likely than all being the same size!

John

[Hank]

John,
The following evidence you sent was not very useful.....

"These craters and the rays were probably created by a very narrow-angle grazing impact (1-5 degrees); the main impact projectile created Messier, another piece Messier A, and ejecta the rays. The scenario could be reasonably reproduced in laboratory simulations: Impact velocity has been estimated at 1.7 km/s. Previously, numerous speculations have been involved in attempt to explain the particularities of these two interesting craters; see e.g. Charles Wood 2002" The speed and angle of impact are incorrect to produce these groves.
The only evidence needed is to put this through a good simulator with the correct conditions!

You may remember I earlier said that the groves on Phobos may have been caused by a SLOW impact not 1.7 km/s.

My earlier statement that the " 'Chain of Pearls' certainly makes sense for the pattern we see but to have dozens of these on a small body seems improbable." received your assertion of "Less probable. Not impossible" is in direct opposition to the following information you sent to me as evidence!

"Integrating our results over the 3.8 Gyr that have passed since the Late Heavy Bombardment of the Moon, we find that the Moon has a 60% change of having a single crater chain on it, matching observations. Although this result is encouraging, especially given the uncertainties in many of our parameters, we caution that a datum of one is certainly within the realm of small number statistics. Our model also predicts that there is only a 6% chance that a single crater chain will be formed on the Earth over 3.8 Gyr, which is inconsistent with survey results that show one or two terrestrial crater chains younger than 360 Myr. Moreover, the probability of getting a single crater chain on Earth over the last 360 Myr is only 1 in 1000. Finally, to eliminate any unknown systematic effects in our parameter estimates, we have taken the ratio of the crater chain production frequency on the Earth and Moon. We find that, on average, the production rate of crater chains on the Moon should be 10 times higher than the rate on the Earth. Thus, it seems extremely unlikely that the terrestrial crater chains, if real, can be produced by the mechanism tested here."

As you may know the only real test that can be done is a good simulation of a SLOW impact of a solid body impact a low angle.
I'll wager a pint that you will be surprised.

Hank