Expert: James Gort - 2/12/2007

Question
Dear Prof. James Gort,
Thank you for the in depth answer. It helped to answer the question. However, I would still like to know more about how rocks can stick to each other from a molecular structural point of view.
The book you mentioned is a little pricey for me right now, but hopefully I could get it in the future. It looks like a great book.
Would you be able to direct me on where I might obtain this knowledge that I am seeking?
Again, thank you for your time.
Urania
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The text above is a follow-up to ...

-----Question-----
Dear James,
When a rocky planet is forming, how do the pieces of rock "stick" to each other? In the 70's I heard it called space welding, in which pieces of rock and metal will bump and stick to each other simply because they are not oxidized. Recently I was looking into it more to understand it better and I can't find any information on this. It seems that "space welding" refers to astronauts welding in vacuo.
So can two pieces of rock bump and stick, or is there more to this? Two pieces of metal would have an easier time sticking to each other, I imagine.
Thank you so much for your time.
-----Answer-----
Hello Urania,

The condensation process (in which planets were formed from the solar nebula) began to be studied seriously in the 1940's and 1950's, primarily by the Nobel laureate Harold C. Urey.  He showed that planets must have been formed at low temperatures (a few hundred degrees K) from solid grains, rather than from hot, gravitationally bound gases.

In the 1970's, researchers in Russia, Caltech and others have determined two mechanisms were mainly involved in having the small grains accrete into large bodies - collisional accretion and gravitational collapse.

In 1978, Hartmann tested collisions of grains in a cold vacuum.  At the "right" collisional speed - this depends on the material - the grains would stick together ("weld", if you will).  At lower collisional speeds, the bodies would bounce apart, but end up with lower rebound speeds than their initial speeds.  At higher collisional speeds, the bodies would break each other apart, but the resulting pieces would also have lower final speeds.  As the mass increased for some bodies (due to the welding effect), these larger bodies would grow rapidly due to collisions with smaller bodies, in which the rebounds would be less than the escape velocity.  Smaller bodies would not grow as fast, since they might be broken apart easier, and their escape velocities would be less.  Computer models indicate that accretion by collision alone can result in several kilometer-wide bodies.

The other technique is gravitational collapse.  Again in the 1970's, it was shown that this mechanism is not likely for a very diffuse gas and dust nebular cloud.  But several researchers showed that the dust (as opposed to the gas) would rapidly collect in a narrow band along the ecliptic plane (like Saturn's rings), and the density of dust would be much higher in the narrow band than in the rest of the cloud.  Inhomogeneities in the dense dust, then, would have enough gravity to slowly attract nearby dust particles, and this process would cause the planetoids to continually grow.

If you're really interested in this subject, I recommend the book "Moons & Planets" by William K. Hartmann, a leading researcher in this area.  The book is scientifically sound, but very readable.  It mentions the critical welding speeds for some materials, but not the exact mechanism (this requires a better understanding of the molecular structure of the materials).  

Hope that helps!

Prof. James Gort  

Answer
Hello Urania,

I think the best explanation may come from Wikipedia, under "cold welding".  That states:  "It is now known that the force of adhesion following first contact can be augmented by pressing the metals tightly together, increasing the duration of contact, raising the temperature of the workpieces, or any combination of the above. Research has shown that even for very smooth metals, only the high points of each surface, called asperities, touch the opposing piece. Perhaps as little as a few thousandths of a percent of the total surface is involved. However, these small areas of taction develop powerful molecular connections; electron microscope investigations of contact points reveal that an actual welding of the two surfaces takes place after which it is impossible to discern the former asperitic interface. If the original surfaces are sufficiently smooth the subatomic attractions between contact points eventually draw the two pieces completely together and eliminate even the macroscopic interface.

Exposure to oxygen or certain other reactive compounds produces surface layers which reduce or completely eliminate the cold welding effect. This is especially true if, say, a metal oxide has mechanical properties similar to those of the parent element (or softer), in which case surface deformations do not crack the oxide film."

As an example, a water droplet (liquid form) has a molecular cohesion at its surface ("surface tension").  If another droplet at very low speed collided with the first droplet, it would bounce off its surface.  At a little higher speed, it would break those molecular bonds and join with the first droplet to form a single larger droplet (this is called accretion).  At still higher collisional speeds, the droplet would break apart into much smaller droplets.

The same can be said of solid particles (e.g., ice) or even metal particles.  In a vacuum, it is easier to break bonds if no layer of air or oxide exists between the colliding particles.  Just as in conventional welding, any intervening material would contaminate the weld and make it weaker (or even prevent the weld from happening).  But collisional welding does not need a vacuum to occur.  If you fired a bullet into a thick sheet of steel, some bullet fragments would become welded into the steel sheet.  

So your guess that metals would be easier to weld in a vacuum is not necessarily true.  Liquids of all types (water, mercury, etc.) would be fairly easy to weld.  But of the solid metals, softer metals (such as sodium or caesium) would be easier to weld than harder metals such as iron, which has stronger molecular bonds.

As a final note, it's also possible for colliding materials to "stick together" in another way besides molecular fusion or welding and gravity.  Electrostatic attraction between slightly charged particles may also play a significant role.

The exact mechanism of planetary accretion is very much an area of current research, but I hope I've given you some background on the current thinking.

Prof. James Gort