From Impact Melt Formation By Low-Altitude Airburst Processes, Evidence From Small Terrestrial Craters and Numerical Modeling. By H. E. Newsom, and M. B. E. Boslough we read:
“Airbursts in the lower atmosphere from hypervelocity impacts have been called upon to explain the nature of the Tunguska event and the existence of unusual impact-related silicate melts such as the Muong-Nong tektites and Libyan Desert Glass of western Egypt. Impact melts associated with impact craters, however, have been traditionally attributed to shock melting of the target material that experiences strong shock compression and heating. The characteristics of impact melts from small terrestrial craters (less than km diameter) leads to the possibility that the airburst phenomena may have been responsible for these melts. This conclusion is supported by numerical modeling of the airburst phenomena using super computer class facilities at Sandia National Laboratories.”
And, in a poster by Dr Boslough titled, The Nature of Airbursts and their Contribution to the Impact Threat. we read:
“Ongoing simulations of low-altitude airbursts from hypervelocity asteroid impacts have led to a
re-evaluation of the impact hazard that accounts for the enhanced damage potential relative to the standard point-source approximations. Computational models demonstrate that the altitude of maximum energy deposition is not a good estimate of the equivalent height of a point explosion, because the center of mass of an exploding projectile maintains a significant fraction of its initial momentum and is transported downward in the form of
a high-temperature jet of expanding gas. This “fireball” descends to a depth well beneath the burst altitude before its velocity becomes subsonic. The time scale of this descent is similar to the time scale of the explosion itself, so the jet simultaneously couples both its translational and its radial kinetic energy to the atmosphere. Because of this downward flow, larger blast waves and stronger thermal radiation pulses are experienced at the
surface than would be predicted for a nuclear explosion of the same yield at the same burst height. For impacts with a kinetic energy below some threshold value, the hot jet of vaporized projectile loses its momentum before it can make contact with the Earth’s surface. The 1908 Tunguska explosion is the largest observed example of this first type of airburst. For impacts above the threshold, the fireball descends all the way to the ground, where it expands radially, driving supersonic winds and radiating thermal energy at temperatures that can melt silicate surface materials. The Libyan Desert Glass event, 29 million years ago, may be an example of this second, larger, and more destructive type of airburst. The kinetic energy threshold that demarcates these two airburst types depends on asteroid velocity, density,
strength, and impact angle.
Airburst models, combined with a re-examination of the surface conditions at Tunguska in 1908, have revealed that several assumptions from the earlier analyses led to erroneous conclusions, resulting in an overestimate of the size of the Tunguska event. Because there is no evidence that the Tunguska fireball descended to the surface, the yield must have been about 5 megatons or lower. Better understanding of airbursts, combined with the diminishing number of undiscovered large asteroids,
leads to the conclusion that airbursts represent a large and growing fraction of the total impact threat.”
Ok, that’s pretty interesting stuff. But what if we don’t need to go all the way to Libya to find locations where a geo-ablative airburst has produced melt formations?
About 75 miles southeast of Albuquerque, New Mexico, there is a place that looks extremely promising. Beyond apparent patterns of material movements I see in the image data, I don’t know anything about the place. And field work remains to be done there. But, unless I miss my guess, at 34.482019, –105.573173 there is a four mile wide, multiple fragment, oblique impact, geo-ablative, airburst scar that may warrant a closer look.
I did a few impact experiments of my own, using various rifles, velocities, and target surfaces. Nothing formal, heck, I didn’t even take notes. But I can tell you that it is easy to make something like we see here happen in an experiment.
From 15 kilometers up, the material movements we see there seem to describe a four mile wide cluster of fragments hitting at low trajectory, from the west. And they must have shed a lot of velocity in the atmosphere.
If I’ve got it right, the target surface would’ve been a shallow lake. And the angle of impact was about 30 degrees.
We can use Google Earth’s ‘Historical Image’ function to look at different images of the same place. It’s a nice feature. Because it lets you see the same place at different times of the year, and in different lighting conditions. The black, and white, version of the place gives us a little better contrast to discern how the ejecta, was blown to the east from the main body of impacts.
And when we look in the outlying areas we start to find places that look like they just might be a meteorite hunters dream.
If you’re interested in helping, and you live close enough to go get a closer look at these features, and maybe grab a few rock samples, I’d sure like to hear from you.