APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

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APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby APOD Robot » Mon Nov 28, 2011 5:06 am

Image A Landslide on Asteroid Vesta

Explanation: Asteroid Vesta is home to some of the most impressive cliffs in the Solar System. Pictured above near the image center is a very deep cliff running about 20 kilometers from top to bottom. The image was taken by the robotic Dawn spacecraft that began orbiting the 500-kilometer space rock earlier this year. The topography of the scarp and its surroundings indicates that huge landslides may have occurred down this slope. The scarp's origin remains unknown, but parts of the cliff face itself must be quite old as several craters have appeared in it since it was created. Dawn has now finished up its high altitude mapping survey and will spiral down to a lower altitude orbit to better explore the asteroid's gravitational field. During 2012, Dawn is scheduled to blast away from Vesta and begin a long journey to the only asteroid belt object known to be larger: Ceres.

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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby orin stepanek » Mon Nov 28, 2011 1:12 pm

Looks like we will get to see Pluto and Ceres both in 2015! 8-)
The cliff on Vesta looks awesome! With Vesta's week gravity; I would imagine watching a landslide there would be kind of slow motion though. :wink:
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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby TNT » Mon Nov 28, 2011 1:34 pm

There should be a before and after picture. It's hard to tell where the landslide occurred and what parts of the asteroid was affected.
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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby Chris Peterson » Mon Nov 28, 2011 1:55 pm

TNT wrote:There should be a before and after picture. It's hard to tell where the landslide occurred and what parts of the asteroid was affected.

That would be nice... assuming there had been a spacecraft in orbit around Vesta a few thousand, or million, or billion years ago, before the landslide happened!
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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby meeziedick » Mon Nov 28, 2011 5:18 pm

Why are all of the craters circular? Are there no craters caused by incoming objects at an angle?

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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby neufer » Mon Nov 28, 2011 6:57 pm

meeziedick wrote:
Why are all of the craters circular? Are there no craters caused by incoming objects at an angle?

    Craters should be thought of as explosions NOT as excavations.

    Such explosions are primarily circular even when the meteor
    (which is generally ~ 10 times smaller than the resulting crater)
    strikes with a glancing blow.
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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby Beetle » Mon Nov 28, 2011 6:59 pm

What is the surface gravity of Vesta? And what is the orbital period of Dawn?

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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby hexalm » Mon Nov 28, 2011 7:24 pm

orin stepanek wrote:Looks like we will get to see Pluto and Ceres both in 2015! 8-)
The cliff on Vesta looks awesome! With Vesta's week gravity; I would imagine watching a landslide there would be kind of slow motion though. :wink:

Only till it gets going! Gravity is about 45 times weaker on Vesta, but there's also no wind resistance.

Granted, it would take free falling for about a mile to reach 120 km/h, but I hear the cliffs on Vesta are very high!

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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby neufer » Mon Nov 28, 2011 7:30 pm

Beetle wrote:
What is the surface gravity of Vesta?

0.022 g: A 180 lb. man would weigh the equivalent of about 4 lb. on Vesta.

Escape velocity 0.35 km/s ~ 780 mph.

Beetle wrote:
And what is the orbital period of Dawn?

A month ago it was 12.3 hours in the high altitude mapping orbit (HAMO).

In less than a month, however, it will be ~ 4 hours in the low altitude mapping orbit (LAMO).

http://dawn.jpl.nasa.gov/mission/journal_10_31_11.asp wrote:
Dawn Journal: October 31, 2011

<<Dawn has completed another wonderfully successful phase of its exploration of Vesta, studying it in unprecedented detail during the past month. The high altitude mapping orbit (HAMO) includes the most intensive and thorough imaging of the entire year Dawn will reside at Vesta. Circling the colossus 680 kilometers (420 miles) beneath it in HAMO, the probe has spent most of its time over the illuminated side taking pictures and other scientific measurements and most of the time over the dark side beaming its findings back to eager Earthlings.

Dawn revolves in a polar orbit around Vesta, passing above the north pole, then traveling over the day side to the south pole, and then soaring north over the night side. Each circuit takes 12.3 hours. Meanwhile, Vesta completes a rotation on its axis every 5.3 hours. Mission planners choreographed this cosmic pas de deux by choosing the orbital parameters so that in 10 orbits, nearly every part of the lit surface would come within the camera’s field of view. (Because it is northern hemisphere winter on that world, a region around the north pole is hidden in the deep dark of night. Its appearance in Dawn’s pictures will have to wait for HAMO2.) A set of 10 orbits is known to Dawn team members (and now to you) as a mapping cycle.

Dawn's target after HAMO will be an altitude of around 180 kilometers (110 miles), closer to the surface beneath it than most satellites are that orbit Earth. It may take six to eight weeks to follow the winding path from HAMO to this low altitude mapping orbit (LAMO) under the delicate push of the ion thrust. While that may seem like a long time, a mission to Vesta that relied on conventional chemical propulsion would be quite unaffordable within NASA's Discovery Program, and a mission to both Vesta and Ceres would essentially be impossible. In a real sense then, the time to travel from one orbit to another is about as fast as possible given humankind's present selection of tools for probing the cosmos.

Dawn will spend about two months in LAMO, revolving around the rocky body once every four hours. The science camera will acquire many exciting pictures from this new vantage point, and while they certainly will afford a better view of some regions than could be obtained in HAMO, that is not the primary objective of LAMO. Images and observations with VIR will be a valuable bonus, but the primary science data will be in two other areas.>>
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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby hexalm » Mon Nov 28, 2011 7:41 pm

Beetle wrote:What is the surface gravity of Vesta? And what is the orbital period of Dawn?

To add some detail (since I already wrote this post!), I found a value of g on Vesta given as .22 m/s^2 (which is .022 of g on earth)
here: http://www-ssc.igpp.ucla.edu/dawn/pdf/Vesta_Current_Knowledge.pdf
calculated from
F=GMm/R^2 => g_vesta=GM/R^2

using these values:
M = 2.75x10^20 g - 2.99x10^20 kg (M as given by
R = 266 km (mean radius)
and then G = 6.67 * 10^(11) N (m/kg)^2

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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby JrzyGirl » Tue Nov 29, 2011 8:32 pm

It might not have been an "all-at-once" landslide. The rubble might also be a consequence of what is known in geologic circles as "downhill creep" - a form of 'mass wasting' charactererized by the downhill movement of soil and loose rock on a slope. The slope can be almost horizontal, but may also be quite steep (as in the photo). My driveway is doing this, and the surface looks quite a bit like Vesta's rubble pile.

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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby bwidner » Wed Nov 30, 2011 5:54 pm

This image, and all similar should include a size scale. If the cliff is 20 km high then maybe show an image of the Empire State building at the top or bottom.

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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby BMAONE23 » Wed Nov 30, 2011 6:14 pm

The empire state building at 381m tall would barely be noticeable in the image since the cliff is 20000m tall it is 52.6 times taller than the Empire State. Even the Tallest building, the Burj Khalifa in Dubai is 829m tall and still, at less than 1/20th the height, would appear as a blip in the image.

Considering that Olympus Mons is 22Km tall it is 10% taller than this cliff face

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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby neufer » Wed Nov 30, 2011 9:02 pm

BMAONE23 wrote:
The empire state building at 381m tall would barely be noticeable in the image since the cliff is 20000m tall it is 52.6 times taller than the Empire State. Even the Tallest building, the Burj Khalifa in Dubai is 829m tall and still, at less than 1/20th the height, would appear as a blip in the image.

Then maybe just show an image of the Empire State building at the top or bottom of a photo of JrzyGirl's driveway.
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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby JohnD » Wed Nov 30, 2011 10:21 pm

How steep is that 'cliff'?
What is its angle to a vertical, a line towards the centre of gravity of Vesta?

I ask, because it's diffciult to visualise that from the picture, AND there are rather small and faint craters in the cliff, almost dead centre to the pic, that show the same shadowing as craters in the scarp below and on the plateau above the cliff. So these craters must be in surfaces that are at a very similar angle to the Sun. In other words, the 'cliff' isn't that steep, it's more like this: http://www.photovanbeek.com/blog/?p=1849 No. 20 from the top - "about 2000m of smooth rock at an angle of about 45 degrees."

Or am I wrong?
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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby neufer » Wed Nov 30, 2011 10:58 pm

JohnD wrote:
How steep is that 'cliff'?
What is its angle to a vertical, a line towards the centre of gravity of Vesta?

I ask, because it's diffciult to visualise that from the picture, AND there are rather small and faint craters in the cliff, almost dead centre to the pic, that show the same shadowing as craters in the scarp below and on the plateau above the cliff. So these craters must be in surfaces that are at a very similar angle to the Sun. In other words, the 'cliff' isn't that steep, it's more like this: http://www.photovanbeek.com/blog/?p=1849 No. 20 from the top - "about 2000m of smooth rock at an angle of about 45 degrees."

For dry materials the critical angle of repose is seldom above 45 degrees.

http://en.wikipedia.org/wiki/Angle_of_repose wrote:
Image

<<The angle of repose or, more precisely, the critical angle of repose, of a granular material is the steepest angle of descent or dip of the slope relative to the horizontal plane when material on the slope face is on the verge of sliding. This angle is in the range 0°–90°. When bulk granular materials are poured onto a horizontal surface, a conical pile will form. The internal angle between the surface of the pile and the horizontal surface is known as the angle of repose and is related to the density, surface area and shapes of the particles, and the coefficient of friction of the material. Material with a low angle of repose forms flatter piles than material with a high angle of repose.>>

Here is a list of various materials and their angle of repose.
All measurements are approximated and not exact.

Code: Select all

Material (condition)    Angle of Repose (degrees)
-----------------------------------------------------------------------
Sand (wet)                     45°
Flour (wheat)                  45°
Bark (wood refuse)             45°
Chalk                          45°
Coconut (shredded)             45°
Coffee bean (fresh)         35–45°
Earth                       30–45°
Gravel (loose dry)          30–45°
Asphalt (crushed)           30–45°
Malt                        30–45°
Bran                        30–45°
Ashes                          40°
Granite                     35–40°
Clay (dry lump)             25–40°
Sand (dry)                     34°
Gravel (natural w/ sand)    25–30°
Sand (water filled)         15–30°
Clover seed                    28°
Wheat                          28°
Clay (wet excavated)           15°
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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby JohnD » Wed Nov 30, 2011 11:41 pm

If anything on Vesta, Art, is lying at an angle of repsoe, it's that slumped terrain below the 'cliff'.
Material that has slid, being stirred and mixed like falling sand, off the cliff.

And the Wiki article ignores 'g'. Under low gravity, you can expect a higher AoR. http://www.mendeley.com/research/determ ... nt-method/ So the slope of that slumped terrain will be closer to the slope of the 'cliff' than it would be on Earth.

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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby neufer » Thu Dec 01, 2011 12:51 am

JohnD wrote:
If anything on Vesta, Art, is lying at an angle of repose, it's that slumped terrain below the 'cliff'.
Material that has slid, being stirred and mixed like falling sand, off the cliff.

And the Wiki article ignores 'g'. Under low gravity, you can expect a higher AoR.
http://www.mendeley.com/research/determ ... nt-method/
So the slope of that slumped terrain will be closer to the slope of the 'cliff' than it would be on Earth.

. :?: .
http://www.mendeley.com/research/determ ... nt-method/ wrote:
The gravity effects on the angle of repose of the sand were negligible.

Which is compatible with the angle of repose being essentially the same for earth & gravel.
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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby JohnD » Thu Dec 01, 2011 9:15 am

Oooooops!
Note to self, read papers more slowly.

However, one paper does not a principle make.
de Vet et al, http://adsabs.harvard.edu/abs/2010AGUFM.P53A1510D found that, "static angle of repose increases about 5 deg with decreasing g (from 1 to 0.1 g)"

On the other hand, this theoretical study http://ascelibrary.org/aso/resource/1/j ... horized=no found no gravity effect in a 2D model.

But this student group found a definite effect - "On Earth, that dust settles at almost a 45-degree angle," Martin said. "In lunar gravity, we were watching the drum and just seeing the angle go up, and up, and up, until it was almost at a 90-degree angle."

A seems logical that AoR would be changed by gravity. The angle of repose reflects a balance between the shear strength of the material and gravity pulling it downwards. At some angle, the downslope vector of gravity becomes less than the force necessary to pull the particles past each other and slippage stops. As shear strength weakens, the AoR will become less and less - wet, sticky sand piles higher than dry sand - and conversely as gravity weakens, a higher pile should be possible.

After, gosh, whole minutes of scholarship, I wonder if electrostatic forces counteract the effects of low gravity in some experiments? This is referred to in one of the above papers.

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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby neufer » Thu Dec 01, 2011 2:43 pm

JohnD wrote:
de Vet et al, http://adsabs.harvard.edu/abs/2010AGUFM.P53A1510D found that,
"static angle of repose increases about 5 deg with decreasing g (from 1 to 0.1 g)"

Only for the finest glass beads in air :!:

http://adsabs.harvard.edu/abs/2010AGUFM.P53A1510D wrote:
<<Angular materials had time-averaged angles of about 40 deg and rounded materials about 25 deg for all g, except the finest glass beads in air, which is explained by static electricity. For all materials, the static angle of repose increases about 5 deg with decreasing g (from 1 to 0.1 g), whereas the dynamic angle decreases with about 10 deg. Consequently, the avalanche size increases with decreasing g.>>

JohnD wrote:
On the other hand, this theoretical study http://ascelibrary.org/aso/resource/1/j ... horized=no found no gravity effect in a 2D model.

But this student group found a definite effect - "On Earth, that dust settles at almost a 45-degree angle," Martin said. "In lunar gravity, we were watching the drum and just seeing the angle go up, and up, and up, until it was almost at a 90-degree angle."

I don't think that many 20 kilometer high cliffs are made solely of dust :!:

viewtopic.php?f=9&t=16775&p=104706#p104706
viewtopic.php?f=9&t=20810&p=130741#p130741
JohnD wrote:
A seems logical that AoR would be changed by gravity. The angle of repose reflects a balance between the shear strength of the material and gravity pulling it downwards. At some angle, the downslope vector of gravity becomes less than the force necessary to pull the particles past each other and slippage stops. As shear strength weakens, the AoR will become less and less - wet, sticky sand piles higher than dry sand - and conversely as gravity weakens, a higher pile should be possible.

If wet, sticky sand is observed anywhere in the solar system
we will immediately send a Viking there to invade!

Image

http://www.tulane.edu/~sanelson/geol111/masswasting.htm wrote:
Mass-Wasting
Prof. Stephen A. Nelson Tulane University




<<Mass-wasting is the down-slope movement of Regolith (loose uncemented mixture of soil and rock particles that covers the Earth's surface) by the force of gravity without the aid of a transporting medium such as water, ice, or wind. Still, as we shall see, water plays a key role.

In order for regolith to move in a mass wasting process it must be on a slope, since gravity will only cause motion if the material is on a slope. On a slope, the force of gravity can be resolved into two components: a component acting perpendicular to the slope, and a component acting tangential to the slope. The perpendicular component of gravity, gp, helps to hold the object in place on the slope. The tangential component of gravity, gt, causes a shear stress parallel to the slope and helps to move the object in the down-slope direction. On a steeper slope, the shear stress or tangential component of gravity, gt, increases, and the perpendicular component of gravity, gp, decreases. Another force resisting movement down the slope is grouped under the term shear strength and includes frictional resistance and cohesion among the particles that make up the object. Down-slope movement is favored by steeper slope angles (increasing the shear stress) and anything that reduces the shear strength (such as lowering the cohesion among the particles or lowering the frictional resistance.

Although water is not directly involved as the transporting medium in mass-wasting processes, it does play an important role. Think about building a sandcastle on the beach. If the sand is totally dry, it is impossible to build a pile of sand with a steep face like a castle wall. If the sand is somewhat wet, however, one can build a vertical wall. If the sand is too wet, then it flows like a fluid and cannot remain in position as a wall. Dry unconsolidated grains will form a pile with a slope angle determined by the angle of repose. The angle of repose is the steepest angle at which a pile of unconsolidated grains remains stable, and is controlled by the frictional contact between the grains. In general, for dry materials the angle of repose increases with increasing grain size, but usually lies between about 30 and 37º. Slightly wet unconsolidated materials exhibit a very high angle of repose because surface tension between the water and the grains tends to hold the grains in place. When the material becomes saturated with water, the angle of repose is reduced to very small values and the material tends to flow like a fluid. This is because the water gets between the grains and eliminates grain to grain frictional contact.>>
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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby BMAONE23 » Thu Dec 01, 2011 5:59 pm

Atmosphere or a lack thereof will also have an effect as Wind will also cause the particles to shift

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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby JohnD » Thu Dec 01, 2011 6:23 pm

This has been an interesting thread diversion!
But may we return to the point?
What is the slope of the 'cliff'?

No one has taken up my argument that craters on the plateau above, on the 'cliff 'and below it on the remains of the landslide, all show about the same degree of shadowing, as if they were at the same angle to the sun. The first and last would show the same shadow, if the plateau and the scree slope were flat, but a 'cliff' crater on a vertical wall would be different, if the sun is more or less overhead. If it were at sunrise or set, then the difference would be much less, but that isn't the case in the photo.

So I think that the 'cliff' face is far from vertical, and more like the picture I linked to, of a 2000m slope in the Himalayas. As it's a long way back now, here it is again:
http://www.photovanbeek.com/blog/?p=1849 The pic is No. 20 from the top - "about 2000m of smooth rock at an angle of about 45 degrees."
And so there would be no question of fallin down it. Rolling or sliding perhaps, but it might even be walkable!
John

PS Thnak you Art, for Prof. Nelson's explanation of the vectors on a sand particle, that I referred to but did not explain. I think that shows that in lower gravity, if other forces do not supervene, the Angle of Repose will be greater for a given substance. J.

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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby neufer » Thu Dec 01, 2011 6:33 pm

JohnD wrote:
This has been an interesting thread diversion!

    Diversion :!: :?:
This is as close as I get to staying on topic.
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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby Chris Peterson » Thu Dec 01, 2011 6:41 pm

JohnD wrote:What is the slope of the 'cliff'?

Anything you want it to be... it all depends on context.

If this APOD is showing a "cliff", it isn't a steep one. It's just a sloping surface, as is easily seen in the oblique view generated from the digital elevation model for Vesta.

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Re: APOD: A Landslide on Asteroid Vesta (2011 Nov 28)

Postby Chris Peterson » Thu Dec 01, 2011 10:35 pm

Very useful for getting a better sense of true form, here is a 3D video from JPL of several Vesta flybys.
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