Shores of Titan's Lakes May Be Made of Strange Crystals

Saturn's largest moon, Titan, is a large, icy, planet-like world that's dotted with lakes filled with liquid hydrocarbons, such as methane and ethane.

Data collected by previous missions to the moon has shown that the shores of some of these lakes in the drier equatorial regions contain evidence of a residue from a material that has evaporated, much like the rings of dirt which form on a bathtub.

Now, a team of scientists think they know what this material is, indicating that these alien shorelines are covered in a strange kind of crystal which is not found naturally on Earth, according to research which was presented on Monday at the 2019 Astrobiology Science Conference in Seattle, Washington.

To make their findings, the team—led by Morgan Cable from the California Institute of Technology—essentially recreated some of the conditions that exist on Titan in their laboratory.

They used a cryostat—a device made up of chamber which can maintain very low temperatures—to which they initially added liquid nitrogen. This lowered the temperature inside the chamber.

Next, the scientists heated the chamber just enough so that the liquid nitrogen turned into gas, mimicking the composition of Titan's atmosphere. Finally, they added the substances found in Titan's lakes—ethane and methane—in addition to other hydrocarbons.

After creating these Titan-like conditions, the team then waited to see what kind of materials would form. Upon initial investigation of the cryostat chamber, the scientists identified crystals of the chemical benzene—a toxic, volatile and highly flammable liquid hydrocarbon which is found in crude oils and its byproducts, such as gasoline.

However, these were no ordinary benzene crystals. Usually the molecules in benzene are shaped like a snowflake consisting of hexagonal rings of carbon atoms. But the benzene crystals that the scientists produced were different, incorporating ethane molecules—in what's known as a "co-crystal."

Next, the researchers identified another co-crystal made from solid acetylene and butane—two compounds which are commonly used for fuel. On Earth, these substances are only found naturally as gases, but the research suggests that they may be common as a solid in Titan's freezing temperatures in the form of this co-crystal.

Furthermore, the "bathtub rings" that are thought to exist around Titan's lakes may originate from acetylene and butane evaporating, a process which leaves behind the crystal mineral as a residue, the scientists say.

"We have discovered a molecular mineral that is stable in the same conditions present on the surface of Titan, a moon of Saturn," the authors wrote in the study. "This molecular mineral is made up of acetylene and butane, two organic molecules that are produced in Titan's atmosphere and fall down onto the surface. We call these 'molecular minerals' because they behave just like minerals do here on Earth, but instead of being made up of things like carbonates or silicates, they are made up of organic molecules."

"We think the 'bathtub rings' around Titan's lakes might be made up of this material, because acetylene and butane both dissolve well in liquid methane and ethane compared to other molecules," they said.

Despite their findings, the scientists say that the presence of these co-crystals on the surface of Titan needs to be confirmed by future observations of the moon.

A false-color, near infrared view of Titan's northern hemisphere collected by NASA's Cassini spacecraft shows the moon's seas and lakes. Orange areas near some of them may be deposits of organic evaporite minerals left behind by receding liquid hydrocarbon. NASA / JPL-Caltech / Space Science Institute