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Pretty good science has nailed down the fact that a significant fraction of asteroids have macro-porosity of 10% or more. It seems this was more or less established as of a 2003 paper, and here are some updated graphs in a 2012 paper.

My understanding is that they establish the material density (including microporosity) from material characterization (including meteorites), and then get the mass of the body from other factors (like gravitational influence). Since the average density (or "bulk density") is found to be much lower, large empty spaces are predicted. Words that have been used (in 2003 at least) to describe categories of low and high macroporosity are:

  • fractured
  • rubble piles

But what does that mean? 253 Mathilde must be the poster child for a rubble pile, with a well-established macro-porosity of 50%, and it looks nothing like what I would colloquially call a rubble pile.

253 Mathilde

Half of this is empty space? How??

Is it filled with caves? Are they connected? Swiss cheese? If we travel to such an asteroid, what kind of structures would we expect to encounter?

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This is an answer from email communication with the author of the 2012 paper.

The way we understand catastrophic collisions (= events that disrupt entirely the target body), the larger blocks will re-accrete first. They are the most massive, and will therefore have a) a lower ejection speed, b) have more gravity to be pulled together.

So after the impact, you start first by gluing together the large blocks, with possible large voids between them. The smaller rock and dust fall back on the already rubble-pile structure, possibly generating smaller voids.

Now, we could imagine that the small rocks and dust would fill in the voids... It's not obvious at all: 1) the deeper they enter into the structure the less gravity pull would they feel toward the center; 2) whatever happens, they need to be pulled stronger than the friction stops them from percolating inside.

The (scarce) theoretical works on that point predict an internal structure where most of the void are deep inside, while the shell around is pretty-much packed. Unfortunately, we have absolutely NO measurements of mass distribution inside these bodies (e.g., Mathilde). It is definitively something we'd like to do on a future space mission to an asteroids (= orbit a high macroporosity asteroids and map its gravitational field as a proxy of 3D mass distribution inside).

Benoit Carry

http://benoit.carry.free.fr/

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Correct me if I'm wrong, but I believe asteroid macroporosity is meant to represent the amount (usually a percentage of its total volume) of void space between separated or semi-attached pieces of asteroid's body, what we consider a single mass body but might in fact be a combination of several larger chunks of materials held together by their microgravity and low radial velocity (axial rotation), or finer grained materials, be it on its body or formed between the cracks at the time of the impact (not necessarily a spectacular one, mind you) of one chunk of the asteroid into another.

That's why the word rubble is used so frequently in such documents. If these individual chunks would be somehow embedded into one another, I guess the word conglomerate would be more frequently used, but in geology, conglomerates are not necessarily highly porous, or not at all.

But to rather leave this description to astrogeologists that study the geology of celestial bodies, here's an excerpt from the document on Asteroid rubble piles: How big are the pieces? (PDF) prepared by the Department of Geological Sciences, University of Tennessee, and Vatican Observatory, of the Vatican City State for their 62nd Annual Meteoritical Society Meeting:

Introduction: Many lines of evidence lead to the conclusion that asteroids are strengthless bodies, piles of rubble held together only by their own self gravity. Among the indications leading to this conclusion are the observations that all asteroids spin slowly enough to be held together by their own self gravity; that near-Earth asteroids have odd shapes matching those predicted for strengthless bodies undergoing tidal distortion; and that the densities of asteroids are significantly lower than corresponding meteorite bulk densities.

This latter work allows one to put numerical estimates on the amount of macroporosity in asteroids. If S-type asteroids like Ida and Eros with a density on the order of 2.6 g/cm3 are made up of material similar to that in ordinary chondrites, then they must be at least 30% empty space. Given the 10% porosity typical of ordinary chondrites, these asteroids must have void spaces on a scale comparable to the meteorites representing another 20% of the volume of the asteroid. For C type objects like Mathilde, Phobos, and Deimos, the observed low density (1.5 g/cm3) implies a macroporosity of at least 20% if they are made of rocky material similar to low density hydrated carbonaceous chondrites. However, these dark asteroids are observed to be anhydrous; to make them out of water-free dark meteoritic material would require that they were 50% or more void space

I believe this Google-fu find describes well enough what macroporosity in asteroids is. So, sadly no Swiss cheese, or even grains of instant coffee. Just piles of rubble held together by various forces, and this macroporosity varying greatly from one celestial object to another. It is a percentage of void space between cracks, separate pieces held together by their own gravity and not perfectly matching with their joining sides, rubble and dust on their surface, etc. I.e. a measure of how solid/hollow an asteroid is, or its bulk, large-scale porosity — macroporosity.

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  • $\begingroup$ That argument about them being strength-less bodies deformed by tidal forces seems awfully strange. If they were truly without strength, this deformation would only exist along the gradient of tidal forces, but in reality they are spinning and not consistently aligned that way. I may have also used the term "bulk density" wrong... $\endgroup$ – AlanSE Sep 20 '13 at 13:18
  • $\begingroup$ I like to imagine some would also be formed as a molten rock ejecta by impacts to low gravity celestials. I.e. the shock droplet, and then slowly solidify forming all kinds of shapes by twisting and turning, and possibly hollow core as the material cools down and compresses. Further impacts with micrometeorites would then design its surface and crack its core into gravel. It would be in a sense a bit like filled hard candy, with the filling going all dry during the millennia. :D $\endgroup$ – TildalWave Sep 20 '13 at 13:50

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