Titan, Saturn’s largest moon, is a natural laboratory to study the origins
of life. Like Earth, Titan has a dense atmosphere and seasonal weather
cycles, but the chemical and mineralogical makeup are significantly
different. Now, earthbound researchers have recreated the moon’s conditions
in small glass cylinders, revealing fundamental properties of two organic
molecules that are believed to exist as minerals on Titan.
The researchers will present their results today at the fall meeting of the
American Chemical Society (ACS). ACS Fall 2021 is a hybrid meeting being
held virtually and in-person Aug. 22-26, and on-demand content will be
available Aug. 30-Sept. 30. The meeting features more than 7,000
presentations on a wide range of science topics.
“Simple organic molecules that are liquid on Earth are typically solid icy
mineral crystals on Titan because of its extremely low temperatures, down to
-290 F,” says Tomče Runčevski, Ph.D., the project’s principal investigator.
“We found that two of the molecules likely to be abundant on Titan —
acetonitrile (ACN) and propionitrile (PCN) — occur predominantly in one
crystalline form that creates highly polar nano surfaces, which could serve
as templates for the self-assembly of other molecules of prebiotic
interest.”
Most of what we know now about this icy world is thanks to the 1997-2017
Cassini-Huygens mission to Saturn and its moons. From that mission,
scientists know that Titan is a compelling place to study how life came
about. Like Earth, Titan has a dense atmosphere, but it is mostly made up of
nitrogen, with a touch of methane. It is the only known body in space, other
than Earth, where clear evidence of stable pools of surface liquid has been
found. Fueled by the sun’s energy, Saturn’s magnetic field and cosmic rays,
both nitrogen and methane react on Titan to produce organic molecules of
various sizes and complexities. ACN and PCN are believed to be present in
the moon’s characteristic yellow haze as aerosols, and they rain down on the
surface, settling as solid chunks of minerals.
The properties of these molecules on Earth are well known, but their
characteristics under Titan-like conditions have not been studied until now.
“In the lab, we recreated conditions on Titan in tiny glass cylinders,”
Runčevski says. “Typically, we introduce water, which freezes into ice as we
lower the temperature to simulate the Titan atmosphere. We top that with
ethane, which becomes a liquid, mimicking the hydrocarbon lakes that
Cassini-Huygens found.” Nitrogen is added to the cylinder, and ACN and PCN
are introduced to simulate the atmospheric rainfall. The researchers then
raise and lower the temperatures slightly to imitate the temperature swings
on the surface of the moon.
The crystals that formed were analyzed using synchrotron and neutron
diffraction instrumentation, spectroscopic experiments and calorimetric
measurements. The work, supported by calculations and simulations, involved
Runčevski’s team from Southern Methodist University, as well as scientists
from Argonne National Laboratory, the National Institute of Standards and
Technology, and New York University.
“Our research revealed a lot about the structures of planetary ices that was
previously unknown,” Runčevski says. “For example, we found that one
crystalline form of PCN does not expand uniformly along its three
dimensions. Titan goes through temperature swings, and if the thermal
expansion of the crystals is not uniform in all directions, it may cause the
moon’s surface to crack.” Such detailed knowledge of these minerals could
help the team better understand what the surface of Titan is like.
Runčevski is now preparing crystals of ACN, PCN, and ACN and PCN mixtures to
obtain detailed spectra. “Scientists will then be able to compare these
known spectra to the spectral library collected by Cassini-Huygens and
assign unidentified bands,” he says. The studies will help confirm the
mineral makeup on Titan and will likely provide insights for researchers
working on an upcoming NASA mission to Titan, launching in 2027.
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