ADVANCES
18 Scientific American, October 2020DEVESH RANJAN, STAM LAB AND GEORGIA INSTITUTE OF TECHNOLOGYPAVISHAGetty Imagesbations along the line where the two gases
meet are amplified by the push-and-pull
nature of the blast waves, forming mush-
room-shaped disturbances that quickly
grow larger. These curls of gas create tur-
bulence, eventually forming low-density
bubbles and long, high-density spikes. If
a spike travels fast enough, it can break off
and accelerate like a bullet to pierce sever-
al layers of gas.
Researchers have puzzled over outflows
of heavy elements that come from deep
within supernova explosions and behave
similarly to those spikes. “It isn’t clear if
we’re seeing something due to the intrinsic,
asymmetric nature of the explosion or if it’s
actually due to the turbulence happening,”
says Anthony Piro, a supernova theorist at
Carnegie Observatories in Pasadena, Calif.,
who was not involved in the research. The
new study illustrates how strong an effect
turbulence might have, Piro says, although
further checks are necessary to understand
the model’s limitations.
Most supernova models incorporate
basic assumptions about what happens at
the smaller scales to avoid weeks of extra
computational time, Piro says. The new
research helps to evaluate such assump-
tions. Scientists now “can see different,
smaller-scale structures evolve,” says Caro-
lyn Kuranz, a physicist at University of
Michigan, who was also not involved in the
work. The structures created by the experi-
ment resemble what she has seen while
investigating how plasmas mix: “Theory
predicted that they should be similar, and
[the researchers] found them to be.”
According to Piro, the experiment pro-
vides “an amazing confirmation of a lot of the
physics” involved in supernovae. He says it
will help calibrate the models he works with
while giving scientists a better understand-
ing of supernova and remnant observations.
Building a supernova-in-a-box came
with its own challenges. Earth’s gravity is
much weaker than a dying star’s, and tiny
commercial blasters produce far less ener-
gy. But “even though the explosive pop is
smaller, the other things we are dealing
with are also smaller, so the ratios match,”
says Benjamin Musci, a Georgia Tech grad-
uate student and the study’s lead author.Preventing gases from bouncing off the
experiment’s sidewalls, which obviously do
not exist in space, “was a long and arduous
battle,” Musci says. It took him nearly a year
to figure out a surprisingly simple solution:
lining the walls with packing foam from a
new computer’s box. The material absorbs
the gases, stopping them from reflecting.
“This foam gets blown to bits every so often
by the explosive, so it slows down our run
time,” he adds. “But without it the physics
would be completely different.”
Another concern is dimensionality. Piro
notes that gases expanding in two dimen-
sions tend to act differently than they would
in three, creating larger eddies and taking
longer to break apart. This is something the
researchers may work on in the future.
Previous supernova experiments have
been performed at larger scales, says the
study’s principal investigator, Georgia Tech
astrophysicist Devesh Ranjan. Sites such asGEOPHYSICSMetal
Detecting
New research reveals geology
behind ore deposit hotspotsCopper, lead and zinc are essential for
modern technology’s electronics and
batteries. Demand has skyrocketed, and
mining companies are depleting known
deposits faster than prospectors can find
more. Now an international team of scien-
tists has discovered a relationship between
deposits of these metals and the thickness
of the lithosphere (the earth’s crust and
upper mantle), providing a reliable way to
locate these crucial resources.
The project began by chance, says
Mark Hoggard, first author of the new
study and a geologist at Harvard Univer-
sity and Columbia University’s Lamont-Doherty Earth Observatory. His co-
author Karol Czarnota, a researcher
at Geoscience Australia, was visiting
Harvard and mentioned noticing—and
wondering why—metal deposits in north-
ern Australia seemed to align with areas
where the lithosphere’s thickness varies.
The research team found that this con-
nection applies globally, hinting at more
places to search for the hidden ores. The
study, published in July in Nature Geosci-
ence, comprehensively maps the correla-
tion between known metal deposits and
lithosphere thickness and proposes a
potential mechanism for that correlation.
The lithosphere can reach up to 300
kilometers below the surface, making its
thickness “actually really hard for geophys-
icists to measure,” says Maureen Long, a
Yale University geophysicist, who was not
involved in the study. To calculate the lith-
osphere’s thickness, seismometer readings
are typically used to record how fast earth-
quake vibrations travel through the planet.Long notes, however, that the world’s lim-
ited number of earthquakes and seismom-
eters means “our ability to resolve the
earth’s structure is not perfect.”
To create a high-resolution world map
of lithosphere thickness, Hoggard and his
colleagues combined and calibrated exist-
ing regional and global models, adding
temperature and pressure data from litho-
spheric rocks carried to the surface in vol-
canic eruptions. They found that metal
deposits tend to appear where the litho-Salt Lake City copper mineGases expand outward in the milliseconds after a simulated supernova blast.© 2020 Scientific American