Moon gases: life starters?

Titan, Saturn’s largest moon

To simulate sunlight hitting Titan's atmosphere to see what chemicals were produced, researchers zapped nitrogen and methane in a laboratory with high-energy UV light. Shown here are some of Saturn's rings, the tiny moon Epimetheus, and the much larger Titan in the background.

  • Exposing nitrogen and methane to UV light
  • Resulted in nitrogen-containing “brown gunk”
  • Such chemicals might be found on Saturn’s moon Titan

The first experimental evidence showing how atmospheric nitrogen can be incorporated into organic macromolecules has been reported by a University of Arizona (UA) team.

The finding suggests the type of organic molecules that might be found on Titan, the moon of Saturn that scientists think is a model for the chemistry of pre-life Earth.

Earth and Titan are the only known planetary-sized bodies that have thick, predominantly nitrogen atmospheres, said Hiroshi Imanaka, who conducted the research while a member of UA’s chemistry and biochemistry department.

How complex organic molecules become “nitrogenated” in settings like early Earth or Titan’s atmosphere is a big mystery, Imanaka said.

“Titan is so interesting because its nitrogen-dominated atmosphere and organic chemistry might give us a clue to the origin of life on our Earth,” said Imanaka, now an assistant research scientist in the UA’s Lunar and Planetary Laboratory. “Nitrogen is an essential element of life.”

Nitrogen fluorescing blue under UV light

Nitrogen fluorescing blue under UV light

Smog world could hide life

However, not just any nitrogen will do. Nitrogen gas must be converted to a more chemically active form that can drive the reactions that form the basis of biological systems.

Imanaka and Mark Smith converted a nitrogen-methane gas mixture similar to Titan’s atmosphere into a collection of nitrogen-containing organic molecules by irradiating the gas with high-energy UV rays. The laboratory set-up was designed to mimic how solar radiation affects Titan’s atmosphere.

Most of the nitrogen moved directly into solid compounds, rather than gaseous ones, said Smith, a UA professor and head of chemistry and biochemistry. Previous models predicted the nitrogen would move from gaseous compounds to solid ones in a lengthier stepwise process.

Titan looks orange in colour because a smog of organic molecules envelops the planet. The particles in the smog will eventually settle down to the surface and may be exposed to conditions that could form life, said Imanaka, who is also a principal investigator at the SETI Institute.

However, scientists don’t know whether Titan’s smog particles contain nitrogen. If some of the particles are the same nitrogen-containing organic molecules the UA team created in the laboratory, conditions conducive to life are more likely, Smith said.

Laboratory observations such as these indicate what the next space missions should look for and what instruments should be developed to help in the search, Smith said.

Brown gunk held the key

The UA researchers wanted to simulate conditions in Titan’s thin upper atmosphere because results from the Cassini Mission indicated “extreme UV” radiation hitting the atmosphere created complex organic molecules.

Therefore, Imanaka and Smith used the Advanced Light Source at Lawrence Berkeley National Laboratory’s synchrotron in Berkeley, California, to shoot high-energy UV light into a stainless steel cylinder containing nitrogen-and-methane gas held at very low pressure.

Hiroshi Imanaka inside the Advanced Light Source

Hiroshi Imanaka stands next to the experiment inside the Advanced Light Source

The researchers used a mass spectrometer to analyse the chemicals that resulted from the radiation.

Simple though it sounds, setting up the experimental equipment is complicated. In addition, many researchers want to use the Advanced Light Source, so competition for time on the instrument is fierce. Imanaka and Smith were allocated one or two time slots per year, each of which was for eight hours a day for only five to 10 days.

Completing all the necessary experiments took years.

At the beginning, they analysed only the gases from the cylinder. But he didn’t detect any nitrogen-containing organic compounds.

Imanaka and Smith thought there was something wrong in the experimental set-up, so they tweaked the system. But still no nitrogen.

“It was quite a mystery,” said Imanaka. “Where did the nitrogen go?”

Finally, the two researchers collected the bits of brown gunk that gathered on the cylinder wall and analysed it with what Imanaka called “the most sophisticated mass spectrometer technique.”

Imanaka said, “Then I finally found the nitrogen!”

Imanaka and Smith suspect that such compounds are formed in Titan’s upper atmosphere and eventually fall to Titan’s surface. Once on the surface, they contribute to an environment that is conducive to the evolution of life.

Adapted from information issued by University of Arizona / Hiroshi Imanaka / Doug Archer / NASA / JPL / Space Science Institute.

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