Cosmic Time Capsules

Art Courtesy of CC Chong.

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 that have fallen to Earth. But this can be problematic: only certain kinds of meteorites 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 (12C), which contains six protons and six neutrons, is much more common than carbon-13 (13C), which has an additional neutron. Both isotopes are stable and non-radioactive. Aponte explained that the prevalence of 12C 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 12C atoms requires less energy than breaking bonds between two 13C atoms. In the Solar System, however, the ratio of 12C to 13C 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

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, which are 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 very low-energy molecules could form there. In such environments, PAHs are more likely to be formed with higher-mass isotopes of carbon, such as 13C, due to the lower energy required to form bonds. In the case where a PAH has two 13C atoms, which is called a double-13C substitution, the energy is lowered even more. “At very cold temperatures, having this isotopic substitution really matters for the stability of the molecule. So by measuring the amount of double-13C substitutions in the molecules within extraterrestrial samples, we can see if they carry the fingerprint of cold, interstellar chemistry,” Zeichner said. 

Zeichner and her team set out to do just that, using the 13C concentrations in Ryugu’s PAHs to probe how they might have formed. But the team faced a problem: they had almost no PAHs to analyze. The team had only been allocated a few drops of dissolved rock samples to work with. “The PAHs in the sample […] were three orders of magnitude lower than the concentration you would need to do traditional isotope measurements,” Zeichner said. In the end, Zeichner found a way around this problem, using new techniques to eliminate the noise from her mass spectrometer, which measures the mass of molecules and can be used to measure the content of different isotopes. Using this method, her team was able to make precise measurements of the 13C isotopes in the rock samples, recording their abundance in five different types of PAHs.

Zeichner also used a similar spectroscopic process on the carbon-based Murchison meteorite, which is approximately seven billion years old. Although it may have experienced some alteration in its chemical composition during its journey through the Earth’s atmosphere, the scientists believed that data gathered from its samples could help with gaining a better understanding of the results from Ryugu. After further spectroscopic measurements and rigorous statistical analysis, the results were in. And they were surprising.

Cold beginnings for life?

Both the Ryugu and Murchison samples had elevated values of 13C. “Within three of the five PAHs we measured, we saw an enrichment in the double-13C content within PAHs relative to what we would expect if they were distributed randomly,” said Zeichner of the Ryugu sample. These low-energy molecules likely formed in an abnormally cold, energy-depleted environment, providing some of the first solid evidence that PAHs are formed in molecular clouds. Additionally, the highest quantities of 13C were recorded from Murchison for fluoranthene—a specific PAH. The exact value matched what the team had expected to observe if the PAHs were indeed synthesized in the cold interstellar medium. 

Thus, for Ryugu and Murchison, it appeared that both the carbon bond formation and the linking of aromatic carbon rings happened at low temperatures. Although “hot” processes would have partially contributed to PAH formation, the chemical analysis pointed to cold formation being the dominant process. 

This result has broad implications for the synthesis of all kinds of organic compounds, which are key for the development of life. “Knowing that a small bit of you could have originated from processes in interstellar clouds, far out in space, is a profound thing to think about,” Zeichner said.

But Zeichner and her team are just getting started. “I think that the analytical advancements are really promising—we’re just scratching the surface of what this particular methodology can do,”  Zeichner added. Recently the team has had their eye on OSIRIS REX, a NASA mission that returned samples of the asteroid Bennu in September 2023. Asteroids are not homogenous, meaning the team’s findings from Ryugu will not necessarily apply to Bennu. When the catalog of data from Bennu is released, the team will be able to determine if their findings align with other asteroids.  Over the next several years, subsequent missions are set to probe far-flung asteroids, planets, and moons looking for organic molecules. With more sample analysis, we will learn fascinating details about the formation of organic compounds from before the Solar System. Whether coalesced among the cold shrouds of molecular clouds or in the vast interstellar medium, what we find on these desolate time capsules will allow us to peer into the history of both the Solar System and life itself.