YSM Issue 97.1

FOCUS
Astrochemistry
A
little beyond Earth, an unassuming
lump of rock quietly orbits around
the Sun. This is the asteroid
(162173) Ryugu, and although it may
not seem impressive at first glance, it has
borne witness to billions of years of cosmic
history. Ryugu was forged in the furnace of
the early Solar System when the Sun was
still a young protostar and the planets were
nothing more than knots of gas, dust, and
rock in a churning disk. Devoid of active
geological processes or atmosphere, Ryugu’s
composition has remained unchanged since
its birth, making it a perfect chemical
time capsule.
So, when the Japanese spacecraft
Hayabusa2 returned with samples from
Ryugu’s surface, it held the promise of
unlocking new insights into the chemical
history of our solar system, and, perhaps,
shedding light on the origins of life
on Earth. The international team that
analyzed the Ryugu samples consisted
of experts from various academic fields
including astrophysicists, statisticians,
biologists, geologists, chemists, and
more. In just four years since Hayabusa2’s
return, this team has already made
remarkable discoveries about the
samples’ composition, such as identifying
the presence of nucleobases (the building
blocks of genetic material) and amino
acids (the building blocks of proteins).
Two of the key scientists on the project
were Sarah S. Zeichner, a postdoctoral
researcher in geochemistry at the California
Institute of Technology, and José C. Aponte,
an astrochemist at NASA.
In a recent study, their team used the
Ryugu samples to uncover clues about
the cosmic origins of a special class of
organic molecules: polycyclic aromatic
hydrocarbons (PAHs).
Carbonic Clues
Until recently, the study of the
chemical history of the Solar System has
been limited to studying meteorites—
meteoroids that have fallen to Earth.
But this can be problematic: only certain
kinds of meteoroids can survive the
perilous journey through the atmosphere,
and their chemical composition might
be altered once they hit the ground. It’s
important to look at returned samples
rather than meteorites because especially
for organic molecules, it’s very easy for
meteorites to become contaminated
[once they fall onto the surface of
Earth],” Zeichner said. “In addition,
the atmosphere is very discriminating
in terms of what meteorites can make
it to Earth, which we think creates a
preservation bias.” That’s why sending
a spacecraft directly to Ryugu for
samples—rather than waiting for it to
come to us—was so appealing.
Zeichner and her team focused their
study on PAHs within the Ryugu samples.
PAHs are rings of carbon and hydrogen
that range from the humble six-carbon
benzene to sixty-carbon behemoths.
“PAHs are produced through many
natural processes here on Earth,” said
Allison Karp, a post-doctoral researcher
at Yale and co-author of the study. “They
are found in petroleum products and [are]
considered EPA-regulated pollutants. They
are also produced through biomass burning.”
PAHs are interesting for several reasons.
First, they are similar to refractory
carbon, which is a type of long-lasting
organic compound. Refractory carbon is
the oldest kind of organic matter present
in Earth’s rock record and is thus key to
understanding the development of life.
Second, PAHs are ubiquitous throughout
the galaxy and represent a significant
portion of the galactic carbon budget.
Radio surveys have shown that PAHs
make up around twenty percent of all
carbon in the Milky Way, making them
important tracers for carbon chemistry
on the largest scales. Terrestrial and
extraterrestrial carbon compounds can
be distinguished by the ratios of carbon
isotopes they have. Carbon isotopes are
different versions of carbon atoms, where
the number of neutrons in the nucleus
varies. On Earth, the isotope carbon-12
( 12 C), which contains six protons and six
neutrons, is much more common than
carbon-13 ( 13 C), which has an additional
neutron. Both isotopes are stable and nonradioactive.
Aponte explained that the
prevalence of 12 C is due to biology favoring
it. “Biological processes require [using]
the least possible amount of energy,”
Aponte said. This preference arises
because breaking bonds between two 12 C
atoms requires less energy than breaking
bonds between two 13 C atoms. In the Solar
System, however, the ratio of 12 C to 13 C is
much lower. Checking the approximate
ratio between the two isotopes helps verify
whether the detected PAHs are actually
from space.
Examining the ratio of isotopes with
more refinement can also help discern
which pathways for PAH formation are
most likely. Although the precise origins
of PAHs remain unknown, several
hypotheses have been proposed to explain
their formation. The most widely accepted
hypothesis is that PAHs were formed in
a “hot” process, forged in the scorching,
energetic environments around dying stars.
However, there is a flaw in this hypothesis.
“Once PAHs are expelled into interstellar
space, they are quickly broken down by UV
and shockwave radiation, about as fast as
they can be created in stellar environments.
But how can these timescales be similar if
PAHs make up twenty percent of the carbon
in the galaxy?” Zeichner said. In other
words, if PAHs can be easily broken down
with common processes, there shouldn’t be
so many of them.
A New Origin Story
IMAGE COURTESY OF WIKIMEDIA COMMONS
The asteroid Ryugu imaged by the Hayabusa2 lander.
The fact that PAHs have accumulated
to such an enormous extent indicates that
another formation mechanism must be at
play. Astrochemists have proposed that
PAHs could also be formed in the interstellar
medium, which exists in the space between
stars within a galaxy. Specifically, they
believe that PAHs could be formed in
molecular clouds—dense regions of gas
and dust that serve as nurseries for young
stars—within the interstellar medium. But
molecular clouds are cold—about ten
Kelvin, or 260 degrees Celsius below
the freezing point—meaning that only
20 Yale Scientific Magazine March 2024 www.yalescientific.org