Sky & Telescope - November 2022 - 24

Impact Threats
Fernbank Science Center's scanning electron
microscope. Many contained packets of
zirconium oxide and silicon dioxide, created
when intense heat decomposed grains
of the hardy mineral zircon (zirconium
silicate). This chemical transformation
requires temperatures exceeding 1,670°C
(3,040°F), much hotter than any grassfire.
Some of the zircon particles had been
completely converted, yet they still retained
their original outlines within the glass. This
indicated that the zircons scattered in the soils
were not only exposed to extremely high temperatures
but also cooled very rapidly, thereby preventing mixing
within the molten material.
Then Scott made an even more surprising discovery. Samples
of glass collected all along the 75-km corridor are riddled
with unmelted meteoritic particles. Many of the smaller particles
are composed of exotic minerals (including serpentinite
clasts with troilite, a rare iron sulfide mineral) that were
altered by exposure to water at low temperatures. Laboratory
experiments constrain the conditions of formation and
survival of some of these minerals to the low temperatures
found in primitive, water-rich bodies from the outer solar
system. Samples from Comet Wild 2, returned to Earth by
NASA's Stardust spacecraft, also found these " alien " minerals.
For these and other reasons, we think the Pica bolide was
likely a comet. The French team may have failed to detect the
meteoritic traces because the bulk-chemical-analysis method
GREEN GLASS Thin-section view of folded glass
showing the typical green color, bubbles, and density
variations. The large bubble at top is roughly 200
microns wide.
they employed wasn't sensitive enough to
pick up the tiny contribution from microscopic
particles amid all the glass.
The long corridor containing the glass
fields, combined with the fact that samples
containing the same meteoritic inclusions
were collected at sites tens of kilometers apart,
suggests two possible scenarios. In the first scenario,
the object arrived as a single body that entered
the atmosphere at a low grazing angle and fragmented as it
passed through the atmosphere, producing a series of airbursts.
Alternatively, the object may have broken up prior to
entering the atmosphere and arrived as a swarm at a steeper
angle of 30° to 45° relative to Earth's surface. Comets and
their rubble-pile kin among the asteroids are fragile bodies,
but comets in particular easily split into clusters of large fragments
while traversing the inner solar system.
Radiocarbon dating of organic matter in soils directly
beneath and in contact with the glasses gives ages ranging
from 11,500 to 12,300 years ago. To be clear, these results
only provide the age for the sediments, not for the glasses.
Prior work in this region has demonstrated that there are surfaces
side by side that could have an age difference of 1,000
years, explaining why the French researchers concluded that
there was a range of ages for the glasses.
1 TWO-FACED Some
Pica glasses were still
partially molten when they
were tossed onto grassy
matts. This example shows
both the glossy upper surface
that quenched in air
and the side that landed on
the grass, creating imprints
of grass.
1
2 TWISTED The melting
process was complex. This
example reveals numerous
small, elongated blebs of
glass that fused together.
The entire assemblage was
twisted after slamming into
the surface before cooling
completely.
3 FUSION The fused soils
resulted in glassy bombs
with embedded pieces of
twigs and grass. Because
the grasses have a highsilica
content, they actually
turned into glass as well.
4 KNEADED Many
glasses show evidence
of being folded into much
larger masses before fully
cooling. As a result, the
overlapping glasses look
like kneaded dough.
5 FLOWING This close-up
shows once-molten glass
overlapping and wrapping
around other, less mobile
packets. This pattern reveals
that the molten glass
not only flowed after being
heated but also cooled
suddenly, freezing this
texture in place.
2
3
1 cm
1 cm
24 NOVEMBER 2022 * SKY & TELESCOPE
GREEN GLASS: P. H. SCHULTZ ET AL. / GEOLOGY 2021; IMAGES OF GLASS: P. H. SCHULTZ (4);
FUSION: P. H. SCHULTZ ET AL. / GEOLOGY 2021
Sky & Telescope - November 2022

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