APOD: Stickney Crater (2013 Jan 18)

[Bchristy]

Subsidence? Powered by what? The surface gravity on Phobos is between 190 and 860 MICRO-g, according to the Wiki, varying because Phobos has too little gravity to pull itself into a sphere and the distance of the suface from the middle varies. Its density is so low that a Russian once suggested it was hollow, but gravity gets less as you go inside a body, so even if there was an empty space inside it, there would so little gravitational pull that an internal collapse is unlikely.
And rocks rolling about the surface? The escape velocity on Phobos is about 11m/sec, 25mph. You could run into space! So a boulder big enough to groove the surface would need so much energy to do so that it would fly off, rather than roll or gouge.
These 'grooves are crater chains, the result of bodies being broken up by tidal forces into a line of small rocks, like the Shoemaker-Levy comet that broke into a 'string of pearls' and impacted on Jupiter. A typical 'gravel-pile' asteroid would turn into a string of gravel, and the line of impacts would meld into a groove when it hit Phobos.
The apparent curvature of the grooves as they move across the lip of Stickney is because they came from different directions. Those grooves that are aligned with the camera can be seen to be straight.
John

I would not expect each of the many crater chains to form such a uniform lines. That does sound like a neat and convenient explanation. However rather than at speeds of 25mph, perhaps the grooves were made over periods of many years.

[zbvhs]

Allow me one more shot at the subsidence idea. The sides of the crater are steep consistent with terrestrial landslides or slumps. The crater looks like material has slid downhill into something. Impacts spray material outward. This crater appears to have collapsed in on itself. The surface material is probably exceedingly fine, so not much force would be needed to move it downslope. The movement wouldn't be very fast so Limtoc might have taken millions of years to form. The dynamics of landslides on Phobos would be the same as here on Earth - just much slower.

[neufer]

I'm having a mental block trying to envision Usain Bolt
hitting 11 m/s wearing either magnetic or velcro shoe-soles.

A slight refinement: electromagnetic soles, with a future high-power battery-pack at the waist, and sensors connected to automatic toggles to turn off each shoe's toe and heel magnets at the appropriate moments for lifting them when running -- just to reduce magnetic "drag".

Or, without any magnetic or velcro shoe-gear at all, Usain could wear a "weight" harness that connects by flexible rod to rollers on inverted rails to his right and left, rollers he can release when he reaches escape velocity.

A slight refinement: a somewhat over-sized (20m in diameter?) trampoline connected to a restraining bungee cord pack around one's waist. There would be sensors connected to automatic toggles in order to drop off the restraining bungee cord pack at the appropriate moment of maximum (3g ?) trampoline acceleration. The bungee cord pack keeps one from prematurely going kilometers into the air in the preliminary bounces but then missing the trampoline on the way down.

[Beyond]

Sheeeeeeeeeeesh

[Wadsworth]

Thank you for that correction, Wadsworth! But worthwile thinking about the Earth. I am informed - correct this too, please - that an observer falling down an enormous well would feel a reducing gravitational acceleration as they approached the Center of the Earth, and none at all at the centre. Yet the core of the Earth is about four times denser than the crust.

I believe this was answered with the Earth gravity vs radius chart, but just in case (thanks for the chart by the way Chris, that explains it well): Gravitational acceleration definitely would feel reduced once you get through any less dense material into the relatively uniform density of our core. And then you would wizz right past the center of the core until the negative acceleration caught up with you and pulled you back down. It would be a fun ride, if we could survive the molten metals in there.

The energy is needed to keep the rolling, sliding rock in contact with the surface. With such low G, an impacting object will blast off again if it survives contact. Granted, that gravity is so low that the impacting object will be baarely accelerated as it approaches, so a softer landing is possible, compared with an object appraoching a planet. But if it strikes glancing blow, it will not come back again, any more than two billard balls strike each other repeatedly, unless they bounce off the side of the table, of course.

But they aren't 'grooves'. They are confluent lines of impacts, crater chains. Look at the Wiki entry for Phobos. That includes some very good Viking pictures of the moon, and one from the Reconnaisance Orbiter, all happenign to show 'grooves' in glancing light.
See: http://en.wikipedia.org/wiki/File:Phobo ... r_2008.jpg
and
http://en.wikipedia.org/wiki/File:Phobos-viking1.jpg
where this nature of the grooves is immediately obvious.

John

I definitely see your argument, and the other photos show some nicely defined creator chain impacts on Phobos. But that said, there sure are a lot of smooth lines on the surface. I wonder if NASA would post their billion year timelaps video showing the formations?..

[JohnD]

I don't want to flog a dead horse, or to try to build up a well established case, but I had wondered if the Comet Shoemaker break-up was an unique case, or that it required the enormous gravity field of a Jupiter to happen, so that the case had a fatal flaw. Then, through another APoD, http://apod.nasa.gov/apod/ap060513.html I came across Comet Schwassmann-Wachmann in 2010. Allegedly, warming of its icy components by the Sun caused it to break up into more than 60 parts, so there must have been many, many more smaller bits, all in a line due to tidal forces. If those parts were stony, as is conjectured about many asteroidal or trans-Uranian objects, this is a crater chain looking for an impact.

John

[neufer]

I had wondered if the Comet Shoemaker break-up was an unique case, or that it required the enormous gravity field of a Jupiter to happen, so that the case had a fatal flaw. Then, through another APoD, http://apod.nasa.gov/apod/ap060513.html I came across Comet Schwassmann-Wachmann in 2010. Allegedly, warming of its icy components by the Sun caused it to break up into more than 60 parts, so there must have been many, many more smaller bits, all in a line due to tidal forces. If those parts were stony, as is conjectured about many asteroidal or trans-Uranian objects, this is a crater chain looking for an impact.

Schwassmann-Wachmann will probably return to impact the object that initially broke it apart (i.e., the Sun).

A cometary nucleus will break apart if its orbit falls within the Roche limit of the Sun, Jupiter or any other planet.

The Roche limit "d" is generally a couple of Sun/planet radii distance from the center of the Sun/planet:



where is the radius of the Sun/planet,

is the density of the Sun/planet, and

(~ 500 kg/m³) is the density of the cometary nucleus.

<<Comet Shoemaker–Levy 9 (formally designated D/1993 F2) was a comet that broke apart and collided with Jupiter in July 1994, providing the first direct observation of an extraterrestrial collision of Solar System objects. Calculations showed that its unusual fragmented form was due to a previous closer approach to Jupiter in July 1992. At that time, the orbit of Shoemaker–Levy 9 passed within Jupiter's Roche limit, and Jupiter's tidal forces had acted to pull the comet apart. The comet was later observed as a series of fragments ranging up to 2 km in diameter. These fragments collided with Jupiter's southern hemisphere between July 16 and July 22, 1994, at a speed of approximately 60 km/s. The prominent scars from the impacts were more easily visible than the Great Red Spot and persisted for many months.>>

[Chris Peterson]

I don't want to flog a dead horse, or to try to build up a well established case, but I had wondered if the Comet Shoemaker break-up was an unique case, or that it required the enormous gravity field of a Jupiter to happen, so that the case had a fatal flaw. Then, through another APoD, http://apod.nasa.gov/apod/ap060513.html I came across Comet Schwassmann-Wachmann in 2010. Allegedly, warming of its icy components by the Sun caused it to break up into more than 60 parts, so there must have been many, many more smaller bits, all in a line due to tidal forces. If those parts were stony, as is conjectured about many asteroidal or trans-Uranian objects, this is a crater chain looking for an impact.

Comets (and many asteroids) are fragile bodies which are easily broken up by tidal forces (which is probably the dominant cause of Schwassmann-Wachmann's disruption, not solar heating). Events like these are common. However, the disrupted bodies do not stay close together in the same orbit for very long- not more than a few years to a few centuries, so the probability of a collision with another body while they are still close enough to create a crater chain is extremely small.

I'm not saying it doesn't happen, but it must be incredibly rare. It's more likely that crater chains occur when the colliding body is broken up by tidal interaction with the body it hits, and only shortly before the collision.

[JohnD]

Thnak you,Chris, and neufer.
The gravity field of Phobos is very small indeed, so an unlikely mechanism. Would that of Mars be sufficient, do you think?
John

[Chris Peterson]

Thnak you,Chris, and neufer.
The gravity field of Phobos is very small indeed, so an unlikely mechanism. Would that of Mars be sufficient, do you think?

Certainly, Mars has a strong enough gravitational field to tidally disrupt a comet or asteroid passing very nearby. Potentially, even Phobos does, given that some asteroids are nothing more than loose clumps of rock, and very small stresses can disrupt them. But disruption by Mars is much more likely.