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"Nothing in Physics Prohibits Impact-Produced Rings"
From my email (no site available)

Thursday, June 27, 2002
By Dr. Mark Boslough
Sandia National Laboratories

Tom,

Thanks for your detailed explanation of 2-body celestial mechanics. If that had been the mechanism we had suggested for generating a planetary ring system, I can assure you that the editors of JGR would have been smart enough not to accept our paper! I realize that you do not have a copy of our paper and could not have known what we suggested, but it is based on the experimental work of Schultz and Gault (GSA Special Paper 247, 1990).

Quoting from our paper:

"At low impact angles (30 degrees from the horizontal), the original impactor disrupts and ricochets downrange at a significant fraction of its incoming velocity. The ricochet component becomes embedded in and accelerated by an expanding vapor cloud. Continued interaction between the solid debris and the turbulent expanding vapor cloud can potentially provide the non-ballistic force that allows some fraction of the debris to be inserted into orbit." For a simulation of such an impact on a planetary scale, see Dave Crawford's movies:
http://sherpa.sandia.gov/planet-impact/asteroid/

According to Schultz & Gault (1990): "In general, conditions favoring injection of significant quantities of projectile and target into orbit appear to be a 10 to 20 degree impact between 15 and 20 km/s into an ocean or into carbonate sediments... such an event for 10-km-diameter body would be likely over a time interval of 300 m.y." In my opinion, our planet has had an impact-produced ring system many times in its history. One does not need to run a fancy GCM to recognize that such a ring would have had climatological consequences, but the GCM lets us get a handle on second-order effects (first-order being the obvious cooling from reduced insolation).

It is noteworthy also that the "giant impact" hypothesis for the formation of the moon involves the formation of a ring system by impact, albeit at a much greater scale. This is dynamically possible because, again, it is not a simple 2-body celestial mechanics problem; there are hydrodynamic forces as well as distributed body forces.

There are similar assumptions built into your second objection. Melosh (Icarus 59, 234, 1984) showed how material can be ejected from a planet to high velocity by shock-wave interference and non-uniaxial loading effects without strong shock effects (let alone melting or vaporization). Likewise, O'Keefe and Ahrens (Science 234, 346, 1986) modeled vapor-plume entrainment as a way of getting weakly shocked rocks off a planet, similar to what Schultz and Gault observed experimentally and what we invoked. The Gladman et al. article you site does not question the possibility of getting SNCs off Mars, as you implied.

Just in case you think these are contrived, egg-headed theoretical tricks that could not ever really work, have a look at the proceedings of the 1992 Hypervelocity Impact Symposium. My colleague, Lalit Chhabildas, cleverly applied similar ideas to enable the launching of an unmelted projectile at speeds exceeding 12 km/s (Chhabildas et al., Int. J. Impact Eng., 14, 1993). See Boslough et al. (Int. J. Impact Eng., 14, 1993) for a flash x-ray radiograph of an unmelted, intact plate being launched to 10 km/sec by a single impulse!

The lesson is that when you do real-world physics, the idealizations you were taught as an undergraduate do not always apply. Real life is messier, and it is the messiness that is the most interesting!

Regards,
Mark Boslough

Dan

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