YSM Issue 97.1
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
Astrochemistry<br />
FOCUS<br />
very low-energy molecules could form<br />
there. In such environments, PAHs are<br />
more likely to be formed with highermass<br />
isotopes of carbon, such as 13 C,<br />
due to the lower energy required to form<br />
bonds. In the case where a PAH has two<br />
13<br />
C atoms, which is called a double- 13 C<br />
substitution, the energy is lowered even<br />
more. “At very cold temperatures, having<br />
this isotopic substitution really matters<br />
for the stability of the molecule. So by<br />
measuring the amount of double-13C<br />
substitutions in the molecules within<br />
extraterrestrial samples, we can see<br />
if they carry the fingerprint of cold,<br />
interstellar chemistry,” Zeichner said.<br />
Zeichner and her team set out to do<br />
just that, using the 13 C concentrations in<br />
Ryugu’s PAHs to probe how they might<br />
have formed. But the team faced a problem:<br />
they had almost no PAHs to analyze. The<br />
team had only been allocated a few drops of<br />
dissolved rock samples to work with. “The<br />
PAHs in the sample [...] were three orders<br />
of magnitude lower than the concentration<br />
you would need to do traditional isotope<br />
measurements,” Zeichner said. In the end,<br />
Zeichner found a way around this problem<br />
using new techniques to eliminate the<br />
noise from her mass spectrometer, which<br />
measures the mass of molecules and can<br />
be used to measure the content of different<br />
isotopes. Using this tool, her team was able<br />
to make precise measurements of the 13 C<br />
isotopes in the rock samples, recording their<br />
abundance in five different types of PAHs.<br />
Zeichner also used a similar spectroscopic<br />
process on the carbon-based Murchison<br />
meteorite, which is approximately seven<br />
billion years old. Although it may have<br />
experienced some alteration in its chemical<br />
composition during its journey through the<br />
Earth’s atmosphere, the scientists believed<br />
that data gathered from its samples could<br />
help with gaining a better understanding<br />
of the results from Ryugu. After further<br />
spectroscopic measurements and rigorous<br />
statistical analysis, the results were in, and<br />
they were surprising.<br />
Cold Beginnings<br />
Both the Ryugu and Murchison samples had<br />
elevated values of 13 C. “Within three of the five<br />
PAHs we measured, we saw an enrichment<br />
in the double- 13 C content [within PAHs]<br />
relative to what we would expect if they were<br />
distributed randomly,” said Zeichner of the<br />
Ryugu sample. These low-energy molecules<br />
likely formed in an abnormally cold, energydepleted<br />
environment, providing some<br />
of the first solid evidence that PAHs are<br />
formed in molecular clouds. Additionally, the<br />
highest quantities of 13 C were recorded from<br />
Murchison for fluoranthene—a specific PAH.<br />
The exact value matched what the team had<br />
expected to observe if the PAHs were indeed<br />
synthesized in the cold interstellar medium.<br />
Thus, for Ryugu and Murchison, it<br />
appeared that both the carbon bond<br />
formation and the linking of aromatic<br />
carbon rings happened at low temperatures.<br />
Although “hot” processes would have<br />
partially contributed to PAH formation, the<br />
chemical analysis pointed to cold formation<br />
being the dominant process.<br />
This result has broad implications for the<br />
synthesis of all kinds of organic compounds,<br />
which are key for the development of life.<br />
“Knowing that a small bit of you could have<br />
originated from processes in interstellar<br />
clouds, far out in space, is a profound thing<br />
to think about,” Zeichner said.<br />
But Zeichner and her team are just<br />
getting started. “I think that the analytical<br />
advancements are really promising—<br />
we’re just scratching the surface of<br />
what this particular methodology can<br />
do,” Zeichner added. Recently the team<br />
has had their eye on OSIRIS REX, a<br />
NASA mission that returned samples<br />
of the asteroid Bennu in September<br />
2023. Asteroids are not homogenous,<br />
meaning the team’s findings from Ryugu<br />
will not necessarily apply to Bennu.<br />
When the catalog of data from Bennu<br />
is released, the team will be able to<br />
determine if their findings align with<br />
other asteroids. Over the next several<br />
years, subsequent missions are set to<br />
probe far-flung asteroids, planets, and<br />
moons looking for organic molecules.<br />
With more sample analysis, we will learn<br />
fascinating details about the formation<br />
of organic compounds from before the<br />
Solar System. Whether coalesced among<br />
the cold shrouds of molecular clouds or<br />
in the vast interstellar medium, what we<br />
find on these desolate time capsules will<br />
allow us to peer into the history of both<br />
the Solar System and life itself. ■<br />
PHOTO COURTESY OF S.S. ZEICHNER COMMUNICATED BY LYNNA THAI<br />
Zeichner, the first author of the study, prepares vials<br />
of meteorite samples for analysis.<br />
ABOUT THE<br />
AUTHORS<br />
DIYA NAIK<br />
MAX WATZKY<br />
DIYA NAIK is a first-year physics major in Pierson. Apart from <strong>YSM</strong>, she is currently a member of the<br />
Yale Undergraduate Quantum Computing group and an avid enjoyer of bad science puns.<br />
MAX WATZKY is a first-year astrophysics major in Benjamin Franklin. Outside of <strong>YSM</strong>, he conducts<br />
research on the origins of stellar-mass black holes and plays trombone in the Yale Concert Band.<br />
THE AUTHOR WOULD LIKE TO THANK Dr. Sarah Zeichner, Dr. José C. Aponte, and Dr. Allison Karp<br />
for their time and enthusiasm in sharing their work.<br />
FURTHER READING<br />
Zeichner, S. S., Aponte, J. C., Bhattacharjee, S., Dong, G., Hofmann, A. E., Dworkin, J. P., Glavin, D. P.,<br />
Elsila, J. E., Graham, H. V., Naraoka, H., Takano, Y., Tachibana, S., Karp, A. T., Grice, K., Holman, A. I.,<br />
Freeman, K. H., Yurimoto, H., Nakamura, T., Noguchi, T., … Eiler, J. M. (2023). Polycyclic aromatic<br />
hydrocarbons in samples of Ryugu formed in the interstellar medium. Science, 382(6677), 1411-1416.<br />
http://doi.org/10.1126/science.adg6304<br />
www.yalescientific.org<br />
March 2024 Yale Scientific Magazine 21