A multinational team of astronomers studying the chemical make-up of the atmosphere on Saturn’s moon Titan has revealed large zones of two organic molecules – hydrogen isocyanide (HNC) and cyanoacetylene (HC3N) – near the moon’s north and south poles. These regions are curiously shifted off the poles, to the east or west, so that dawn is breaking over the southern region while dusk is falling over the northern one.
This image is a composite of several images taken during two separate Titan flybys in 2006. Image credit: NASA/JPL/University of Arizona.
Titan’s atmosphere has long been of interest because it acts as a chemical factory, using energy from the Sun and Saturn’s magnetic field to produce a wide range of organic molecules.
Studying this complex chemistry may provide insights into the properties of Earth’s very early atmosphere, which may have shared many chemical characteristics with present-day Titan.
The scientists, led by Dr Martin Cordiner of NASA Goddard Space Flight Center and Catholic University of America in Washington, DC, used the extreme sensitivity and resolution of the Atacama Large Millimeter/submillimeter Array (ALMA) to track the atmospheric distributions of HNC and HC3N, which initially appeared to be concentrated evenly over Titan’s north and south poles.
The surprise came when the team compared the gas concentrations at different levels in the atmosphere.
At the highest altitudes, the zones of HNC and HC3N were shifted away from the poles.
These off-pole concentrations are unexpected because the fast-moving, east-west winds in Titan’s middle atmosphere should thoroughly mix the molecules formed there.
Large patches of HNC (left) and HC3N (right) glow near the north pole of Titan, on the dusk side, and near its south pole, on the dawn side; brighter colors indicate stronger signals from the two gases; red hues indicate less pronounced signals. Image credit: NRAO / AUI / NSF.
The team does not have an obvious explanation for these findings yet.
“This is an unexpected and potentially groundbreaking discovery. These kinds of east-to-west variations have never been seen before in Titan’s atmospheric gases. Explaining their origin presents us with a fascinating new problem,” said Dr Cordiner, who is the first author of a paper published on the Astrophysical Journal Letters (arXiv.org preprint).
At the moment, the researchers are considering a number of potential explanations, including thermal effects, previously unknown patterns of atmospheric circulation, or the influence of Saturn’s powerful magnetic field, which extends far enough to engulf Titan.
“Alternatively, I don’t think we could rule out some kind of peculiar atmospheric circulation pattern,” Dr Cordiner added.
“These ALMA observations give us new insights into how organic molecules, the building blocks of life, form and evolve in a planet-like environment,” said co-author Dr Anthony Remijan of National Radio Astronomy Observatory in Charlottesville.
“It is exciting to imagine the new discoveries ALMA will enable as we look more deeply at other interesting objects in our Solar System.”
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M. A. Cordiner et al. 2014. ALMA Measurements of the HNC and HC3N Distributions in Titan’s Atmosphere. ApJ 795, L30; doi: 10.1088/2041-8205/795/2/L30
