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Swift
2014-Aug-14, 02:26 PM
Laboratory Equipment magazine (http://www.laboratoryequipment.com/news/2014/08/gravity-find-may-protect-earth-asteroids?et_cid=4099217&et_rid=54636800&type=headline)


Previous research has shown that asteroids are loose piles of rubble held together by gravity and friction. However, the UT team found that 1950 DA is spinning so quickly that it defies these forces. Ben Rozitis, a postdoctoral researcher, Eric MacLennan, a doctoral candidate and Joshua Emery, an assistant professor in the Department of Earth and Planetary Sciences, wanted to know what keeps the body from breaking apart.

Looking at thermal images and orbital drift to calculate thermal inertia and bulk density, the team detected the action of cohesive forces in an environment with little gravity.

“We found that 1950 DA is rotating faster than the breakup limit for its density,” said Rozitis. “So if just gravity were holding this rubble pile together, as is generally assumed, it would fly apart. Therefore, interparticle cohesive forces must be holding it together.”

NEOWatcher
2014-Aug-14, 02:39 PM
How do we know it's a loose pile of rubble?
Can it be rubble that somehow the surface fused with other rubble (like a layer of ice or something)?

I'm imagining a piece of rubble crashing int the other piece just enough for a slight surface melting to fuse them, and so on.

Or something like that bag of peas in the freezer that become hard as a rock that I have to slam against the counter to break them up. (just to have the remainder fuse again)

mkline55
2014-Aug-14, 02:46 PM
I was told many years ago that the asteroid belt was created by the collision/destruction of an entire planet or moon. I suspect that is not the only source of asteroids, though. I look at a chunk of granite and clearly see it is not held together by gravity alone. So not all asteroids are alike? Why is that surprising? If a few grains of sand, rocks, and whatever clump together because their orbits are closely matched, and gravity is sufficient, then they do. Others don't match orbits. Some smash into one another and create more smaller rocks. Some just bounce away. Some rocks are spinning rapidly. Some not so rapid. Some are big. Some are not. I guess I am just confused why this is news.

StupendousMan
2014-Aug-14, 04:03 PM
I was told many years ago that the asteroid belt was created by the collision/destruction of an entire planet or moon.

That origin story has not been part of the mainstream astronomical view for at least four decades.

mkline55
2014-Aug-14, 04:15 PM
That origin story has not been part of the mainstream astronomical view for at least four decades.

Well I never claimed to be young.

NEOWatcher
2014-Aug-14, 05:41 PM
I'm not sure what you are saying, nor do I know what was previously known.

But, the way I picture it is with this analogy:
A mop gets wet, you spin it to dry it out. There are some parts of that mop that stay together, while the water, dirt and maybe even some threads fly off.

I know a lot of that has to do with friction, surface tension, etc. But; I found an article that mentions this is about the Van Der Waal force. Since I am not too familiar with VDW, nor the math behind it, I really am not sure what's going on which is why I asked what I asked.

Swift
2014-Aug-14, 06:26 PM
I know a lot of that has to do with friction, surface tension, etc. But; I found an article that mentions this is about the Van Der Waal force. Since I am not too familiar with VDW, nor the math behind it, I really am not sure what's going on which is why I asked what I asked.
The wikipedia article about van der Waals forces (http://en.wikipedia.org/wiki/Van_der_Waals_force) is pretty good.

It is pretty important in molecular bonding and that is where I am personally most familiar with it. It is a group of weak intermolecular bonding mechanisms that arise from the electrostatic interactions between dipoles. It is much weaker than the attraction in ionic bonds (say between a positively charged sodium ion and a negatively charged chlorine ion). But when you make an ionic molecule like HCl, the hydrogen end will have a partial positive charge and the chlorine end will have a partial negative charge, forming a dipole. The positive end of one of these dipoles will have some attraction to the negative end of another molecule, and that is what gives you van der Waal attraction.

It can also take place in non-ionicly bonded systems, as long as there is any partial charge so as to form a dipole.

There are examples given in the article about where it becomes important in macroscopic systems. I didn't know it was being considered as a mechanism in these "rubble" asteroids, but thinking about it now, I'm not surprised.

As to the "how do they know these are piles of rubble", I should leave that answer to the astronomers here, but I thought one way was direct observation - that Japanese probe that visited an asteroid, for one.

NEOWatcher
2014-Aug-14, 06:39 PM
The wikipedia article about van der Waals forces (http://en.wikipedia.org/wiki/Van_der_Waals_force) is pretty good.
Yes; I tried reading that, and because I'm not all that versed on ionic, electrostatic, dipoles etc. I have a hard time with it.
I know about ionic bonds, but when it comes to other bonding, I start to get lost.
But; now that you gave me another description, it's a lot clearer. Thanks.

And; yes, given that, I also can't understand why this is new. Maybe it's the first example with the conditions that passed the threshold of gravity bonding where they were able to do the math.


As to the "how do they know these are piles of rubble", I should leave that answer to the astronomers here, but I thought one way was direct observation - that Japanese probe that visited an asteroid, for one.
We have only visited very few close up. I don't know if that's enough or understand why it's enough. So; I await.

transreality
2014-Aug-14, 11:09 PM
sounds like a job for Dark Matter

Hornblower
2014-Aug-15, 12:24 AM
sounds like a job for Dark Matter

We don't need dark matter to make the stuff of low-density asteroids sticky.

John Mendenhall
2014-Aug-15, 02:03 AM
Hm. Do you suppose that last year's comet came apart when the holding forces failled due to solar heating? Rotational forces would strew material widely and nothing would be detectable afterwards as observed.

Noclevername
2014-Aug-16, 11:50 AM
What about "vacuum welding"? Or maybe there are frozen volatiles inside the object cementing particles together with ice?

borman
2014-Aug-16, 03:26 PM
Here is a recent paper regarding the pebbles to planetismals problem:

Formation of pebble-pile planetesimals

http://arxiv.org/abs/1408.2535

To be considered is if the current too fast spinning pile may have been chipped off from part of a larger planetismal that offered stronger cohesive forces that formed the current fragment.

Squink
2014-Aug-18, 01:48 PM
This could help explain what appear to be dust plains in the apparently near vertical regions comet 67P/Churyumov–Gerasimenko. Something is keeping all the little stuff from pooling down near the neck.

Jerry
2014-Aug-20, 03:50 AM
The spinning does not tell us what binds 1950 DA together; it constrains the net force to be greater than gravity. Since Deep Impact had minimal effect upon surface features, this is also a minimal constraint. We don't know if these are 'rubble piles' as they appear to be, or a tightly welded nuclei with the appearance of a rubble pile on the surface.

Fun stuff.

Reality Check
2014-Aug-21, 04:09 AM
(29075) 1950 DA (http://en.wikipedia.org/wiki/(29075)_1950_DA) is a asteroid. Deep Impact impacted on a comet (Tempel 1). 67P/Churyumov–Gerasimenko is a comet.

ETA: As for why asteroids are likely to be rubble piles, it is because the observed density of asteroids is less than that of meteorites, some of which have origins in asteroids.

Jerry
2014-Aug-24, 06:10 PM
The mass is coming in roughly three-times higher than the flyby estimates of mass (another fly-by anomaly?). In any case, it looks to be slightly less than 1g/cc; so if there are minerals identified that dominate the surface that are much heavier than water-ice, this will imply a very high level of porousity.

Fascinating!

Jerry
2014-Aug-25, 02:47 PM
http://www.planetary.org/blogs/emily-lakdawalla/2014/08221148-philae-landing-site-selection.html




With a very crude estimate, by deomposing into two circular cylinders, and the august 21 NavCam I get a density of 311 kg/m³: http://i.imgur.com/EaooABk.jpg

The previous density estimate has been 102 kg/m³. The mass correction has been a factor of 10/3.14 = 3.18, leading to a density correction to 325 kg/m³, assuming an unchanged volume estimate. Hence a value between about 300 and 350 kg/m³ sounds plausible.

Ok, the prior estimates put the density between ~100 and 350 kg/m^3; so the current value is within the original margins gathered from initial fly-by data. It has got to be very difficult to hold this 'orbit'; the gravity of the comet more of a nuisance parameter than aid in maintaining what is more of an eccentric rendezvous than an orbit.

transreality
2014-Aug-25, 10:44 PM
They must be rubble piles since the particles are able to sort by size, so they must be mobile with respect to each other. This sorting is apparent in the fine dust plains with a few large boulders in a cluster. Also, this effect is eliminating the craters. If there is a frozen nucleus, then is there one in each of the lobes of the comet? If so, how did it seperate. The comet stretching into two lobes which will eventually themselves seperate is what is expected of a gravity bound clump of dust and rubble as it spins up.

Reality Check
2014-Aug-27, 02:57 AM
It has got to be very difficult to hold this 'orbit'; ....
At the moment Rosetta is not orbiting comet Churyumov-Gerasimenko. It is executing triangular "orbits" in front of the comet with the transfer to the actual orbit of the comet scheduled for 3 September.
What Rosetta does now (http://blogs.esa.int/rosetta/2014/08/06/what-rosetta-does-now/)

it is interesting that previous mass estimates, e.g. 67P/Churyumov-Gerasimenko mass determination based on a new method for modeling non-gravitiational forces and accelerations (http://www.lpi.usra.edu/meetings/acm2012/pdf/6282.pdf) at 3.14⨯1012±0.21⨯1012 kg, were about 3 times higher than what is estimated from the Rosetta trajectory. I wonder if the strange shape of the comet is the reason for the difference - the paper uses the standard of an triaxial ellipsoid.

Van Rijn
2014-Aug-27, 10:28 PM
Ah, yes, that makes sense. The gravity is low even at the surface, and even less important at distance. It does take a little getting used to though since it is "normal" for us flatlanders to think of worlds with significant mass.

Squink
2014-Aug-28, 08:25 PM
At the moment Rosetta is not orbiting comet Churyumov-Gerasimenko. It is executing triangular "orbits" in front of the comet with the transfer to the actual orbit of the comet scheduled for 3 September.Cutting kitties (https://www.youtube.com/watch?v=Og5VRqyM6o0).

Buttercup
2014-Aug-28, 09:02 PM
If it LOOKS solid, I don't know how it could be a "loose pile of rubble."

Then again, what do I know? {shrugs}

Noclevername
2014-Aug-28, 11:50 PM
If it LOOKS solid, I don't know how it could be a "loose pile of rubble."

Then again, what do I know? {shrugs}

What would you expect to look different?

Squink
2014-Aug-30, 11:18 PM
On the shapes and spins of "rubble pile" asteroids (http://www.asu.cas.cz/~asteroid/paris/material/binshape.pdf)
Something with two lobes with a massive overhang above the center of gravity does not look like a rubble pile.

Noclevername
2014-Aug-31, 02:23 AM
Don't forget the bodies are in near-zero gravity and almost no erosion. Things don't fall in on themselves, or sort into similar-sized particles, nearly as much as they would under the circumstances we're used to seeing. A wall of regolith looks like a solid wall.

Squink
2014-Aug-31, 02:14 PM
If that's regolith (http://www.esa.int/spaceinimages/Images/2014/08/Rosetta_s_comet_in_3D), it's cemented regolith, which I'd hesitate to call rubble. Center of gravity is near base of neck on larger lobe near bottom of pic.

Noclevername
2014-Aug-31, 02:55 PM
If that's regolith (http://www.esa.int/spaceinimages/Images/2014/08/Rosetta_s_comet_in_3D), it's cemented regolith, which I'd hesitate to call rubble. Center of gravity is near base of neck on larger lobe near bottom of pic.

Isn't that also near the center of spin, and thus nearly zero G?

Squink
2014-Aug-31, 04:30 PM
Spin's always about CG. Things away from it get pulled towards it, just like things get pulled towards to zero G spot at the center of the earth.

Reality Check
2014-Sep-01, 01:45 AM
On the shapes and spins of "rubble pile" asteroids (http://www.asu.cas.cz/~asteroid/paris/material/binshape.pdf)
Something with two lobes with a massive overhang above the center of gravity does not look like a rubble pile.
Firstly comet Churyumov-Gerasimenko (two lobes with a massive overhang) is a comet not an asteroid. So a citation to a "rubble pile" asteroids paper is not correct.
On the other hand, "the first unambiguous rubble pile to be photographed is 25143 Itokawa (http://en.wikipedia.org/wiki/Rubble_pile)," and 5143 Itokawa (http://en.wikipedia.org/wiki/25143_Itokawa) looks like a lumpy potato.

(29075) 1950 DA (http://en.wikipedia.org/wiki/(29075)_1950_DA) which is the subject of this thread though seems to be fairly spherical.

Squink
2014-Sep-01, 03:43 AM
Firstly comet Churyumov-Gerasimenko (two lobes with a massive overhang) is a comet not an asteroid. So a citation to a "rubble pile" asteroids paper is not correct. Would you claim rubble pile comets might have massive overhangs, just because the naming is different?
Gravity works on both comets and asteroids, as do van der Waals forces.
The comet can't be all rubble. It might be cemented regolith.

Of course, you give the thing another few hundred orbits around the sun, and it won't be called a comet anymore, it'll be an asteroid plain and simple. Then my citation will cause great problems for labeling it a "rubble pile" asteroid won't it?

Reality Check
2014-Sep-02, 12:29 AM
Would you claim rubble pile comets...

I will claim that "rubble pile comets" do not exist. The term "rubble pile" is applied to asteroids.
I will claim that the topic of this thread is the rapid rotation of an asteroid and the need for additional forces, e.g. van der Waals forces, to keep this asteroid intact.
I will claim that there is no regolith in comets because regolith is a layer of loose, heterogeneous material covering solid rock (http://en.wikipedia.org/wiki/Regolith). Comets do not have solid rock (average density of ~0.6 g/cc as compared to ~3.0 g/cc for asteroids).

I suspect that the maximum rotation speed for comets before they crack up could be smaller than asteroids from a personal expectation that balls of ice and dust are weaker than balls of rock.
We know that one comet has a "massive overhang": Churyumov-Gerasimenko (two lobes with a massive overhang).
In a few thousand years your citation may or may not be correct. Churyumov-Gerasimenko may stay intact, break up or impact a body. The point is that it is not relevant now because Churyumov-Gerasimenko is a comet now.

Jerry
2014-Sep-03, 11:09 PM
Our observational evidence regarding comets is very curious. The regoth, for lack of a better word, on Temple 1 was not water ice or low density organics; the off-gassing from other comets; and the stardust collections revealed asteroid-like surface compositions - "silicates, carbonates, smectite, metal sulfides (like fool's gold), amorphous carbon and polycyclic aromatic hydrocarbons". http://en.wikipedia.org/wiki/Tempel_1


No hydrous silicates or carbonate minerals were detected, which suggests a lack of aqueous processing of Wild 2 dust. Very few pure carbon (CHON) particles were found in the samples returned. A substantial amount of crystalline silicates such as olivine, anorthite and diopside were found,[12] materials only formed at high temperature. This is consistent with previous observations of crystalline silicates both in cometary tails and in circumstellar disks at large distances from the star http://en.wikipedia.org/wiki/81P/Wild

So the textbook predictions have taken a bad beating. On the other hand, the observed densities are completely inconsistent with the surface compositions; and they do out gas both water-ice and other volatiles, so the best answer for now is we don't have a good handle on comets - they merit further study.

Reality Check
2014-Sep-04, 01:50 AM
Our observational evidence regarding comets is very curious. ...
The complete Tempel 1 (http://en.wikipedia.org/wiki/Tempel_1) paragraph is:

The crater that formed was not visible to Deep Impact due to the cloud of dust raised by the impact, but was estimated to be between 100 and 250 meters in diameter[5] and 30 meters deep. The probe's spectrometer instrument detected dust particles finer than human hair, and discovered the presence of silicates, carbonates, smectite, metal sulfides (like fool's gold), amorphous carbon and polycyclic aromatic hydrocarbons. Water ice was detected in the ejecta.[6] The water ice came from 1 meter below the surface crust (the devolatized layer around the nucleus).[6]

You missed out the explanations for the observed crystalline silicates (http://en.wikipedia.org/wiki/81P/Wild#Exploration) (even before the results of Stardust):

Possible explanations for this high temperature material at large distances from Sun were summarised before the Stardust sample return mission by van Boekel et al.:[13]

Both in the Solar System and in circumstellar disks crystalline silicates are found at large distances from the star. The origin of these silicates is a matter of debate. Although in the hot inner-disk regions crystalline silicates can be produced by means of gas-phase condensation or thermal annealing, the typical grain temperatures in the outer-disk (2–20 au) regions are far below the glass temperature of silicates of approx 1,000 K. The crystals in these regions may have been transported outward through the disk or in an outward-flowing wind [14]. An alternative source of crystalline silicates in the outer disk regions is in situ annealing, for example by shocks or lightning. A third way to produce crystalline silicates is the collisional destruction of large parent bodies in which secondary processing has taken place. We can use the mineralogy of the dust to derive information about the nature of the primary and/or secondary processes the small-grain population has undergone.
The quotes do not mention any problem with "textbook predictions", Jerry.
I suspect that it is textbook predictions that
* surface compositions will not agree with the bulk properties of comet nuclei for the simple reason that the surface is where outgassing happens. Add in the "weathering" of the surface by the solar wind and even cosmic rays. Thus the surface will not have the same composition as the interior.
* comets out gas both water-ice and other volatiles since comets are predicted to be composed of water, CO2, methane, etc. (and dust!).
Comets do merit further study but we do have "a good handle" on them (for whatever you mean by "good" and "handle" :D).

Jerry
2014-Sep-05, 03:27 PM
* comets out gas both water-ice and other volatiles since comets are predicted to be composed of water, CO2, methane, etc. (and dust!).
Comets do merit further study but we do have "a good handle" on them (for whatever you mean by "good" and "handle" :D). It depends upon how old your textbook is.

In Carl Sagan's days, comets were thought to be 'dirty ice balls'; and yes, it is still thought that the observed dark surface layers (which are devoid of ice) are just that: Surface crap; but the contention that this is just a surface layer is not supported by limited hard data. When the Comet Wild 2 two was hammered by a 100kg copper hammer; the resulting crater had the predicted width (~100m) but not anywhere near the predicted depth. Since we did not get back to look at it for several years, it is possible the speculate that a deep crater was caused by the impact and later filled in; but frankly the net result was the same that we would see if we plowed a similar object into the moon - which is much, much denser than the measured density of comets.

A single data point does not a theory make, but this apparent lack of penetration by the Deep Impact probe should spawn other, more rigorous tests of the interior composition of comets. Rosetta won't be bashing anything into the surface (we hope); but it will be interesting to untangle the reasoning behind the varying predictions of the density of CG. Landing on this dusty duck (which has about the same net density of a rubber duck with 40% void interior volume) will be a challenge. What a cool mission!

Reality Check
2014-Sep-07, 11:45 PM
It depends upon how old your textbook is.

Not really: In Carl Sagan's days, comets were thought to be 'dirty ice balls' but even with a change to "icy dirt balls" the surface still gets "weathered". The change is not to "the surface is solid dust and so is dark", Jerry :D!

When "the Tempel 1 (http://en.wikipedia.org/wiki/Tempel_1) was hammered by a 100kg copper hammer" called Deep Impact (http://en.wikipedia.org/wiki/Deep_Impact_(spacecraft)), this did not change that evidence that surfaces are weathered. The density of the comet has little to do with surface weathering (a possible change in surface composition?). But consider that not all outgassing escapes a comet - some falls back to the surface. So we would expect the surface of a comet to be the nearer to composition of the jets.
As an aside: the Deep Impact (http://en.wikipedia.org/wiki/Deep_Impact_(spacecraft)) ejecta was 5 million kilograms of water and 10 to 25 million kilograms of dust but it sampled a bit more than the surface.

Jerry: Where is your citation for the predicted depth of Deep Impact (http://en.wikipedia.org/wiki/Deep_Impact_(spacecraft))?

Jerry
2014-Sep-08, 06:43 PM
Not really: In Carl Sagan's days, comets were thought to be 'dirty ice balls' but even with a change to "icy dirt balls" the surface still gets "weathered". The change is not to "the surface is solid dust and so is dark", Jerry :D!

When "the Tempel 1 (http://en.wikipedia.org/wiki/Tempel_1) was hammered by a 100kg copper hammer" called Deep Impact (http://en.wikipedia.org/wiki/Deep_Impact_(spacecraft)), this did not change that evidence that surfaces are weathered. The density of the comet has little to do with surface weathering (a possible change in surface composition?). But consider that not all outgassing escapes a comet - some falls back to the surface. So we would expect the surface of a comet to be the nearer to composition of the jets.
As an aside: the Deep Impact (http://en.wikipedia.org/wiki/Deep_Impact_(spacecraft)) ejecta was 5 million kilograms of water and 10 to 25 million kilograms of dust but it sampled a bit more than the surface.

Jerry: Where is your citation for the predicted depth of Deep Impact (http://en.wikipedia.org/wiki/Deep_Impact_(spacecraft))?

http://web.archive.org/web/20050712235105/http://www.nasa.gov/mission_pages/deepimpact/media/deepimpact-070805.html



"The major surprise was the opacity of the plume the impactor created and the light it gave off," said Deep Impact Principal Investigator Dr. Michael A'Hearn of the University of Maryland, College Park. "That suggests the dust excavated from the comet's surface was extremely fine, more like talcum powder than beach sand. And the surface is definitely not what most people think of when they think of comets -- an ice cube."... Scientists say the crater was at the large end of original expectations, which was from 50 to 250 meters (165 to 820 feet) wide.

http://www.nytimes.com/2005/09/07/science/07comet.html?ex=1183867200&en=0a4d2e3f70107d4f&ei=5070&_r=0


Dr. Lisse described Tempel 1 as an object the size of a city "that has the strength of lemon meringue."

It is still not clear how large a crater Deep Impact carved out, but Dr. A'Hearn said it was probably tens of yards deep and about 100 yards wide.


But they still don't know how big the crater is, so no one has won the pool at JPL. Is it as big as a house?

"I think it's bigger than that," Schultz said. ...
The science team has seen a dark feature where the crater would be, but they have not finessed the image enough to know if it's the crater or a shadow.

We now know it was a shadow, as the subsequent close-up images reveal a barely visible, very shallow crater with a typical nipple in the center - not what one would expect in a collision of a washing machine moving at 10,000km/s with a large lemon merigue pie.

http://www.newscientist.com/article/dn7961#.VA3h-fldU2E

"It's like a sponge, with a lot of cavities," agrees Horst Uwe Keller, an astronomer at the Max-Planck Institute for Solar System Research in Germany. He observed the event with Europe's Rosetta spacecraft and says the discovery confirms previous observations suggesting other comets are also porous. "When you touch it, it just crumbles under your hands."...The team estimates the impact blasted away a crater about 100 metres wide and up to 30 m deep...They found that the 72 trillion kilogram-nucleus was extremely porous, with as much as 80% of its volume taken up by empty space.

"That tells me there is no solid layer all the way down to the centre," says Mike A'Hearn, the mission's principal investigator at the University of Maryland in College Park, US

This was all stated before the revisit revealed the very shallow crater.

http://cen.acs.org/articles/84/i29/Comets-Chemical-Composition.html

Included are minerals such as magnesium-rich forsterite and iron-rich fayalite (both in the olivine family); ferrosilite, an iron-rich pyroxene; and nontronite, a smectite clay containing iron, aluminum, and sodium. The spectra also feature telltale signs of other minerals, amorphous carbon, water ice, sulfides, and polyaromatic hydrocarbons.

That 5 million kilograms of water, by the way is an upper limit, not a measured amount. Notice in the article they say the spectra also features "the telltale signs of ...amorphous carbon, water, sulfides and polyaromatic hydrocarbons" or in other words, hydrates, carbonates and sulfates. Very, very little water vapor was seen in the ejecta plume, but they calculated that up to 5 million kilograms of water could be hidden in the opaque dust raised during the impact.


http://arxiv.org/pdf/astro-ph/0604489v1.pdf


http://www.nasa.gov/mission_pages/deepimpact/media/deepimpact_water_ice.html#.VA3wKvldU2E

Reality Check
2014-Sep-11, 12:22 AM
http://web.archive.org/web/20050712235105/http://www.nasa.gov/mission_pages/deepimpact/media/deepimpact-070805.html

That is a media release, Jerry, but as you quote does mention that "Scientists say the crater was at the large end of original expectations, which was from 50 to 250 meters (165 to 820 feet) wide.".

Jerry: The Wikipedia article has news articles as sources unfortunately.
"A total of 5 million kilograms (11 million pounds) of water[38] and between 10 and 25 million kilograms (22 and 55 million pounds) of dust were lost from the impact.[36]"
[36] Deep Impact collision ejected the stuff of life (http://www.newscientist.com/article/dn7961#.VBDgu8IcRhE)
[38] Impactor ejects mighty water mass (http://news.bbc.co.uk/2/hi/science/nature/4871934.stm) (4 April 2006: "The Nasa projectile that slammed into Comet Tempel 1 last year kicked out at least 250,000 tonnes of water.")
This looks like the science: Swift ultraviolet photometry of the Deep Impact encounter with Comet 9P/Tempel 1 (http://www.sciencedirect.com/science/article/pii/S0019103506003411) (published March 2007)

...From our data we determine that a total of (1.4±0.2)×10^32 water molecules were ejected in the impact, ...

Radio observations of comet 9P/Tempel 1 with the Australia Telescope facilities during the Deep Impact encounter (http://arxiv.org/abs/astro-ph/0604489v1) (Apr 2006) is 3 days of ground-based observations to get an average OH production rate 2.8 x 10^{28} molecules/second.

But the point of my post was:
When the Tempel 1 comet was "hammered by a 100kg copper hammer" called Deep Impact, this did not change that evidence that surfaces are weathered. The density of the comet has little to do with surface weathering (a possible change in surface composition?). But consider that not all outgassing escapes a comet - some falls back to the surface. So we would expect the surface of a comet to be the nearer to composition of the jets.

Jerry
2014-Sep-11, 03:10 AM
That is a media release, Jerry, but as you quote does mention that "Scientists say the crater was at the large end of original expectations, which was from 50 to 250 meters (165 to 820 feet) wide.".

Jerry: The Wikipedia article has news articles as sources unfortunately.
"A total of 5 million kilograms (11 million pounds) of water[38] and between 10 and 25 million kilograms (22 and 55 million pounds) of dust were lost from the impact.[36]"
[36] Deep Impact collision ejected the stuff of life (http://www.newscientist.com/article/dn7961#.VBDgu8IcRhE)
[38] Impactor ejects mighty water mass (http://news.bbc.co.uk/2/hi/science/nature/4871934.stm) (4 April 2006: "The Nasa projectile that slammed into Comet Tempel 1 last year kicked out at least 250,000 tonnes of water.")
This looks like the science: Swift ultraviolet photometry of the Deep Impact encounter with Comet 9P/Tempel 1 (http://www.sciencedirect.com/science/article/pii/S0019103506003411) (published March 2007)


Radio observations of comet 9P/Tempel 1 with the Australia Telescope facilities during the Deep Impact encounter (http://arxiv.org/abs/astro-ph/0604489v1) (Apr 2006) is 3 days of ground-based observations to get an average OH production rate 2.8 x 10^{28} molecules/second. I'm still looking for sourches - but if my memory is correct (it may not be) yes, there was water vapor about the comet after the impact; but remember this thing was emitting vapor trails before the probe struck it, and there was no significant increase in vapor emitted before and after the impact.


But the point of my post was:
When the Tempel 1 comet was "hammered by a 100kg copper hammer" called Deep Impact, this did not change that evidence that surfaces are weathered. The density of the comet has little to do with surface weathering (a possible change in surface composition?). But consider that not all outgassing escapes a comet - some falls back to the surface. So we would expect the surface of a comet to be the nearer to composition of the jets. I quite agree; and this is why the water numbers are so vexing (and even more so with comet 67 CG; where we have not detected any surface ice). If the emitted particles showered back onto the comet, why is there so very little surface ice? (Less than 5% on Tempel 1, and they can't find any on 67 CG!) Seriously - how can a comet so clearly out-gas water, which we can clearly see, and we expect some to fall back on the nuclei of the comet and form either patches, crests - even a film of water-ice on the surface of this very cold body = Why can't we find any?

For what it is worth, I am really puzzled-baffled by these observations. The best I can come up with, is that the surface is charcoal bricket, and the water vapor that does not escape is adsorbed by active charcoal! This is consistent with the measured density, but provides absolutely no clue as to why such a charcoal brick would out-gas in the first place. Perhaps the varied surface morphology is the answer: Parts of the brick out-gas, and other parts - especially the surface - adsorb. Weird.

Reality Check
2014-Sep-11, 04:21 AM
I quite agree; and this is why the water numbers are so vexing

I am not sure why the water numbers would be "vexing", Jerry. They are what is detected from Deep Impact which of course is a sample from a crater extending from the surface to some tens of meters.
We have jets with some water of which a unknown, maybe tiny amount will return to the surface and
* Tempel 1 which was relatively active and had a small amount of surface ice.
* 67 CG with less outgassing and no evidence of surface ice.
The conclusion: without actual numbers we cannot say anything.

The puzzle about 67 CG and surface ice is that with a comet that distant from the Sun, we would expect surface ice from depositions during its time in the outer solar system:
Rosetta orbiter science data reveals surprising facts about comet 67P/Churyumov-Gerasimenko (http://www.techtimes.com/articles/15196/20140909/rosetta-orbiter-science-data-reveals-surprising-facts-about-comet-67p-churyumov-gerasimenko.htm)

Jerry
2014-Sep-12, 12:59 PM
I am not sure why the water numbers would be "vexing", Jerry. They are what is detected from Deep Impact which of course is a sample from a crater extending from the surface to some tens of meters.
We have jets with some water of which a unknown, maybe tiny amount will return to the surface and
* Tempel 1 which was relatively active and had a small amount of surface ice.
* 67 CG with less outgassing and no evidence of surface ice.
The conclusion: without actual numbers we cannot say anything.

The puzzle about 67 CG and surface ice is that with a comet that distant from the Sun, we would expect surface ice from depositions during its time in the outer solar system:
Rosetta orbiter science data reveals surprising facts about comet 67P/Churyumov-Gerasimenko (http://www.techtimes.com/articles/15196/20140909/rosetta-orbiter-science-data-reveals-surprising-facts-about-comet-67p-churyumov-gerasimenko.htm) You stated it better than I did, but that is one part of the equation. When we looked at the crater of deep impact, shouldn't we have expected water-ice to fall back to the surface along with the dust, and shouldn't outgassing from the interior in general deposit ice on the surface as long as the comet is far enough from the sun to to maintain subfreezing surface temperatures? We see water ice in the craters of Mercury, but not on either Temple 1 (at least very little) or 67P CG

I found the reference for the OH measurements on Temple 1:

http://arxiv.org/ftp/arxiv/papers/0904/0904.0592.pdf


Because the gas flowed outward at some finite speed (we assumed 0.55 km sec-1 but as we will show later in this paper, this is an upper limit to gas
velocity.), it did not reach the end of our 7 arcsec slit until after our fifth observation post impact. Thus, the ends of our slit during these first observations would still be an
accurate and concurrent measure of the ambient cometary spectrum. We therefore extracted the spectra from the ends of the slit, averaging over the spectra from both ends 3
of the slit from the first four spectra, and defined this spectrum to be the ambient cometary spectrum.

So they didn't actually see an increase in the amount of water in the spectrum until 4,000 seconds after the impact. (The fifth sampling period was at 4,000 seconds.) They then back-extrapolated to pencil in the volume of water that they think was released upon and during the first 4000 seconds after impact, but hidden by dust. There is nothing wrong with doing this, but the real error bars in such and extrapolation are very wide; and I think it indicates the comet is very thick skinned, with little water very near the surface.

Noclevername
2014-Sep-13, 01:42 AM
When we looked at the crater of deep impact, shouldn't we have expected water-ice to fall back to the surface along with the dust, and shouldn't outgassing from the interior in general deposit ice on the surface as long as the comet is far enough from the sun to to maintain subfreezing surface temperatures? We see water ice in the craters of Mercury, but not on either Temple 1 (at least very little) or 67P CG


Water ice sublimes. The icy craters on Mercury receive no solar exposure at all and keep a constant temp, the craters on tumbling comets are frequently exposed to varying temperatures and levels of sunlight. My opinion is that any ice deposits above the crust would not last long in a sunny vacuum. Outgassing may even push away ice particles that form.

The surface composition is also different. Having ice in a body, does not necessarily mean having ice on it.

Reality Check
2014-Sep-15, 12:25 AM
I found the reference for the OH measurements on Temple 1:

http://arxiv.org/ftp/arxiv/papers/0904/0904.0592.pdf.
The temporal changes in the emission spectrum of Comet 9P/Tempel 1 after Deep Impact (http://arxiv.org/abs/0904.0592) is about modeling the changing composition of the Deep Impact ejecta which gives clues as to the composition of the nucleus. Their model has large changes of parent gases with depth so the composition of surface outgassing is not a reliable indicator of internal composition. Section 5 ends with pervious attempts to look at the variation of composition looked at split comets and found little variation but this was s different situation (sudden impact versus equilibrium).