APOD: Stickney Crater (2013 Jan 18)

[APOD Robot]

Stickney Crater

Explanation: Stickney Crater, the largest crater on the martian moon Phobos, is named for Chloe Angeline Stickney Hall, mathematician and wife of astronomer Asaph Hall. Asaph Hall discovered both the Red Planet's moons in 1877. Over 9 kilometers across, Stickney is nearly half the diameter of Phobos itself, so large that the impact that blasted out the crater likely came close to shattering the tiny moon. This stunning, enhanced-color image of Stickney and surroundings was recorded by the HiRISE camera onboard the Mars Reconnaissance Orbiter as it passed within some six thousand kilometers of Phobos in March of 2008. Even though the surface gravity of asteroid-like Phobos is less than 1/1000th Earth's gravity, streaks suggest loose material slid down inside the crater walls over time. Light bluish regions near the crater's rim could indicate a relatively freshly exposed surface. The origin of the curious grooves along the surface is mysterious but may be related to the crater-forming impact.

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[owlice]

I love HiRISE images. Enjoyed the link to Stickney information, too. It's not every day I read about someone whose sons were named Asaph, Samuel, Angelo, and Percival.

[Beyond]

Whats amazeing to me, is that there are many colored craters in that small area.Did the moon get hit by a rainbow once upon a time, and it got covered over until little craters revealed it

[revloren]

The eye is drawn to the smaller crater within Stickney, Limtoc crater. I'm not seeing any craters whithin craters here, so my guess is that Limtoc is quite young. Any estimates as to its age?

[saturno2]

I think at the impact, escaped water, that flowed over the edge and
cristallized ( blue part ) .

[Uwe]

First thought by looking at some areas was 'glass'.
Minerals molten in the blink of an eye at the impact and then set again only moments later.
Like the martian 'blueberries', in a way.

btw:
'Great picture and very fine article! I enjoy APOD every day.
(Although this one looks a bit familiar, doesn't it? )

[ErnieM]

Escape velocity is much less in the Martian moons so more debris must be blown into space from craters like this falling into Earth. I wonder how many "Martian rocks" are "Martian moon rocks" instead.

[rstevenson]

Whats amazeing to me, is that there are many colored craters in that small area.Did the moon get hit by a rainbow once upon a time, and it got covered over until little craters revealed it

and

I think at the impact, escaped water, that flowed over the edge and
cristallized ( blue part ) .

From the "stunning, enhanced-color image" link, I read that this image was made "by combining data from HiRISE's blue/green, red, and near-infrared channels. The color data accentuate details not apparent in the black and white images." This means that you can't necessarily assume, for example, that blue=ice. I'm not sure what the moon would look like in "natural" colour, but it would probably not be this colourful. Maybe more like this...

Rob

[neufer]

Stickney Crater

The origin of the curious grooves along the surface is mysterious but may be related to the crater-forming impact.

• Crater-forming impacts => Martian 'saturnian' rings => Phobos 'bathtub' rings

<<Faint dust rings produced by Phobos and Deimos have long been predicted but attempts to observe these rings have, to date, failed. 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. 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 Crater. 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.>>

[b][color=#0000FF]This backscattered electron image of one thin section of the Kaidun breccia meteorite reveals the complex mixture of millimeter-sized clasts of diverse compositions and textures.[/color][/b] - [url=http://www.psrd.hawaii.edu/Oct09/Kaidun_meteorite.html]Linda M. V. Martel[/url]

---

<<Kaidun is a meteorite that fell on 3 December 1980 on a Soviet military base near what is now Al-Khuraybah in Yemen. A fireball was observed travelling from the northwest to the southeast, and a single stone weighing about 2 kilograms was recovered from a small impact pit. It contains a uniquely wide variety of minerals, causing some confusion as to its origin. It is largely made up of carbonaceous chondrite material of type CR2, but it is known to contain fragments of other types, such as C1, CM1, and C3. Of the nearly 60 minerals found within the meteorite, several have not been found elsewhere in nature, such as florenskyite, which has the chemical formula: FeTiP.

In March 2004 it was suggested that the meteorite originated from the Martian moon Phobos. The reason Phobos has been suggested is the existence of two extremely rare alkaline-rich clasts visible in the meteorite, each of which entered the rock at different times. This suggests that the parent body would have been near a source of an alkaline-rich rock, which is in particular a product of deep differentiation. This points to Mars and one of its moons, and Phobos is more likely than Deimos because it is closer to Mars.

[Boomer12k]

Be great to stand there and look at Mars as you hurtle around it, what an awesome view that would be. And with a friend, even romantic.

APODs always spark my imagination....not always scientifically.

:---[===] *

[neufer]

Be great to stand there and look at Mars as you hurtle around it, what an awesome view that would be.
And with a friend, even romantic.

APODs always spark my imagination....not always scientifically.

The low equatorial orbit prevents you from observing the ice caps.

Better to sit at home (with a friend or night cap) and view Mars polar orbital satellite views:

....................................................
Mars Odyssey NASA Mars orbiter

Main website - THEMIS images - UMSF forum

Launched 7 Apr 2001
Entered orbit 24 Oct 2001
in extended mission
....................................................
Mars Express ESA Mars orbiter

Main website - science website - VMC image Flickr page - VMC blog - UMSF forum

Launched 2 Jun 2003
Entered orbit 25 Dec 2003
in extended mission
...................................................
Mars Reconnaissance Orbiter NASA Mars orbiter

Main website - HiRISE images - MARCI weather - CTX images via ASU global map website - UMSF forum

Launched 12 Aug 2005
Entered orbit 10 Mar 2006
in extended mission
....................................................
ISRO's Mars Orbiter Mission

[Bchristy]

Is it possible the grooves are the result of tidal forces pulling large boulders along in sort of a "surface orbit?" It seems like tidal forces along with the super low gravity coupled with the relatively small diameter of Phobos and the rotation of the little moon could cause a loose boulder to be dragged or rolled across the surface and create such a pattern. I have noticed patterns on Mercury and the asteroid Vesta that suggest heavy objects moving across their surface also.

[neufer]

Is it possible the grooves are the result of tidal forces pulling large boulders along in sort of a "surface orbit?" It seems like tidal forces along with the super low gravity coupled with the relatively small diameter of Phobos and the rotation of the little moon could cause a loose boulder to be dragged or rolled across the surface and create such a pattern. I have noticed patterns on Mercury and the asteroid Vesta that suggest heavy objects moving across their surface also.

What did you see on Mercury

[zbvhs]

Is it possible that the small crater (Limtoc?) is a subsidence crater? According to what I read, these asteroid-like bodies are not homogeneous like Earth or Mars but instead are rather loose agglomerations of smaller chunks with interior voids in their structure. Asteroids must reach a certain size before interior melting occurs to weld them into a more solid lump.

[JohnD]

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

[Wadsworth]

Death Star extraterrestrial base Phobos makes its way onto APOD yet again.

[Wadsworth]

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.

As you go inside a body, the change in gravity is relative to the change in density of the material. So your statement would be true if you said, "gravity gets less as you go inside a body of uniform density." Or, "gravity gets less as you go inside a body who's density decreases toward its core." However, if the core of an object is more dense than it's outskirts, gravity will increase as you move toward the center.
I'm not vindicating the subsidence theory, just talking about gravity.

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.

Why would they need so much energy? I don't see a problem with the sliding or rolling boulder theory. These lines are likely a combination of your two theories. Caused by some inpact event at a shallow relative angle forming 'crater chains'. The gravel string or loosend material would still need to slide, roll, or be pulled as well to form such long and well defined lines.

[Raven]

What if Phobos has a dense round core that's worked its way entirely loose from the thin shell? Should it be renamed "Tinklebell"?

Interesting that the "freshly uncovered" surface should be a silver-gray, unlike the Mars-reddish of the other impact areas. Almost as though the captured-asteroid itself were naturally silver-gray, while the smaller impacting material were of the same nature as the surface of Mars... and the large impact that caused Stickney Crater were something else entirely.

[neufer]

The surface gravity on Phobos is between 190 and 860 MICRO-g.

The escape velocity on Phobos is about 11m/sec, 25mph.

You could run into space!

Usain Bolt (at 10.44 m/sec) could, perhaps, run into space.

But it would be a real challenge getting any sort of footing with just 860 MICRO-g.

<<Usain Bolt (born 21 August 1986) is a Jamaican sprinter widely regarded as the fastest person ever. His 2009 record breaking margin for 100 m, from 9.69 seconds (his own previous world record) to 9.58 [= 10.44 m/s], is the highest since the start of fully automatic time measurements.>>

[Raven]

But it would be a real challenge getting any sort of footing with just 860 MICRO-g.

Magnetic shoe-soles and ferrous racetrack surface.

Second choice: velcro.

[neufer]

But it would be a real challenge getting any sort of footing with just 860 MICRO-g.

Magnetic shoe-soles and ferrous racetrack surface.

Second choice: velcro.

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

(A taut trampoline over Stickney Crater might work, however.)

[JohnD]

As you go inside a body, the change in gravity is relative to the change in density of the material. So your statement would be true if you said, "gravity gets less as you go inside a body of uniform density." Or, "gravity gets less as you go inside a body who's density decreases toward its core." However, if the core of an object is more dense than it's outskirts, gravity will increase as you move toward the center.
I'm not vindicating the subsidence theory, just talking about gravity.

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.

Why would they need so much energy? I don't see a problem with the sliding or rolling boulder theory. These lines are likely a combination of your two theories. Caused by some inpact event at a shallow relative angle forming 'crater chains'. The gravel string or loosend material would still need to slide, roll, or be pulled as well to form such long and well defined lines.

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

[Chris Peterson]

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.

As you move towards the crust, there is a slight increase in the acceleration of gravity, until you reach about 600 km. Then it decreases a bit for a while and reverses to a peak value of about 11 m/s² at a depth of 2800 km. Then it steadily decreases to zero at the center. The gross variation is completely the result of the differentiation of mantle and core.

EarthGravity.gif (11.52 KiB) Viewed 6016 times

[Raven]

If you think of an object falling through the atmosphere toward the surface of the Earth, much the same phenomenon is happening, at a different scale. The object accelerates toward the sold mass, also initially the mass of the atmosphere, but as it passes each layer/shell/height of atmosphere, the entire hollow sphere of that layer/shell/height and above of atmospheric mass is negated as attractive mass -- once you're inside a hollow sphere, it's zero-G to you; only the remainder "below" you, the sphere you're "outside", is still attractive mass. However, you're not losing much attractive mass, since atmosphere is so light compared to Earth's solid portion.

Likewise falling down the well: you accelerate as you pass the last layer of relatively light mantle before entering the denser core.

[Raven]

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.