Sky & Telescope - November 2022 - 26

JET SIMULATION: MARK BOSLOUGH / SANDIA NATIONAL
LABORATORIES; MARTIAN CRATER: NASA / JPL-CALTECH /
ARIZONA STATE UNIVERSITY (2)
Impact Threats
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AIRBURST BLAST An incoming object that vaporizes above the surface in an airburst will continue downward as a churning jet of hot gas. When
this jet hits the surface, it blasts and melts surface soils, creating complex glass deposits.
expanding jet of meteoritic vapor and melt. When this fireball
collides with the surface, it has far more profound effects
than just the shock wave or thermal radiation from an object
exploding at high altitude - one of which is the creation of
surface glasses.
We've actually seen effects of hypersonic fireballs reaching
the surface - not on Earth, but on Venus. During the early
1990s, NASA's Magellan spacecraft used radar to see through
the planet's opaque canopy of clouds and map its surface.
These observations revealed that the dense atmosphere has
shielded the surface from impacts that would gouge craters
smaller than about 5 km wide. Instead of small craters, there
are hundreds of diffuse, radar-bright " splotches " of various
shapes, overprinted by central, radar-dark areas. Scientists
inferred these features are the scars of airburst shock waves,
created by objects that didn't make it to the surface.
The splotches have a peculiar appearance. At that time,
researchers thought that an entering body would flatten
like a pancake before suddenly exploding, but such behavior
wouldn't make marks like these. While trying to understand
these scars, I attempted to replicate airbursts using the hypervelocity
Vertical Gun Range at NASA's Ames Research Center
in California. The results revealed that rather than flattening
into a pancake, the disrupted projectile morphs into a needle
shape, enabling the swarm of fragments to penetrate deep
into the atmosphere and reach the surface. Small fragments
of the projectile literally surf the trailing shock wave.
When these hot clouds of gas and trailing debris collided
with the surface of Venus, they scoured it with intense winds,
sweeping away radar-dark particles that were both delivered
to and dislodged near " ground zero. " They were carried long
distances, until they were blocked by ridges and collected in
crevasses. I could see this effect when I studied Magellan's
radar images closely.
In Chile, we think that similar jets of incandescent gas
and supersonic winds combined to make masses of glass
much larger than a short-lived pulse of thermal radiation
alone could generate. Rather than simply forming a thin,
uniform layer of glass, the violent winds and shock-driven
turbulence created a chaotic mess. Loose soil and silt were
lofted, flash-melted, and fused into larger masses, giving the
chunks and slabs their telltale contorted shapes. Large pebbles
and rocks that couldn't be lofted were only singed by brief
exposure to the intense heat. Meteoritic dust shed during
entry arrived in trailing vortices to salt the mixture, thereby
explaining the presence of certain thermally unstable minerals.
The entire cataclysm probably lasted only tens of seconds.
Preliminary calculations based on previous work suggest
that each of the six glass fields might have required fragments
50 to 100 meters in diameter that vaporized only 1 to 2 km
above the surface, with energies exceeding 3 megatons. The
parent body might have been as large as 1 km in diameter.
Although scientists estimate a similar size and larger energy
for the Tunguska object than for each of the Pica fragments,
COMET BLAST The 20-km-wide crater Santa Fe in Mars's Chryse Planitia looks normal enough in visible light (left). But a nighttime infrared image
from the Mars Odyssey Orbiter's THEMIS instrument (right) reveals twin streaks extending from old crater rims (upper and lower left). These streaks
are interpreted as coarse surface materials exposed when Santa Fe's impact-generated vapor plume interacted with pre-existing obstacles.
26 NOVEMBER 2022 * SKY & TELESCOPE

Sky & Telescope - November 2022

Table of Contents for the Digital Edition of Sky & Telescope - November 2022