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in the atmosphere of Titan, large patches of two trace gases glow near
the north pole, on the dusk side of the moon, and near the south pole,
on the dawn side. Brighter colors indicate stronger signals from the two
gases, HNC (left) and HC3N (right); red hues indicate less pronounced
signals.Image Credit: NRAO/AUI/NSF
maps of Saturn’s moon Titan reveal large patches of trace gases shining
brightly near the 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.
The pair of patches was spotted by a NASA-led international team of
researchers investigating the chemical make-up of Titan’s atmosphere.
“This is an unexpected and potentially groundbreaking discovery,”
said Martin Cordiner, an astrochemist working at NASA’s Goddard Space
Flight Center in Greenbelt, Maryland, and the lead author of the study.
“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.”
The mapping comes from observations made by the Atacama Large
Millimeter/submillimeter Array (ALMA), a network of high-precision
antennas in Chile. At the wavelengths used by these antennas, the
gas-rich areas in Titan’s atmosphere glowed brightly. And because of
ALMA’s sensitivity, the researchers were able to obtain spatial maps of
chemicals in Titan’s atmosphere from a “snapshot” observation that
lasted less than three minutes.
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, or carbon-based, 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.
In this study, the researchers focused on two organic molecules, hydrogen isocyanide (HNC) and cyanoacetylene (HC3N),
that are formed in Titan’s atmosphere. At lower altitudes, the two
molecules appear concentrated above Titan’s north and south poles. These
findings are consistent with observations made by NASA’s Cassini
spacecraft, which has found a cloud cap and high concentrations of some
gases over whichever pole is experiencing winter on Titan.
The surprise came when the researchers compared the gas
concentrations at different levels in the atmosphere. At the highest
altitudes, the gas pockets appeared to be shifted away from the poles.
These off-pole locations are unexpected because the fast-moving winds in
Titan’s middle atmosphere move in an east–west direction, forming zones
similar to Jupiter’s bands, though much less pronounced. Within each
zone, the atmospheric gases should, for the most part, be thoroughly
The researchers do not have an obvious explanation for these findings yet.
“It seems incredible that chemical mechanisms could be operating on
rapid enough timescales to cause enhanced ‘pocket’' in the observed
molecules,” said Conor Nixon, a planetary scientist at Goddard and a
coauthor of the paper, published online today in the Astrophysical
Journal Letters. “We would expect the molecules to be quickly mixed
around the globe by Titan’s winds.”
At the moment, the scientists 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.
Further observations are expected to improve the understanding of the
atmosphere and ongoing processes on Titan and other objects throughout
the solar system.
NASA’s Astrobiology Program supported this work through a grant to
the Goddard Center for Astrobiology, a part of the NASA Astrobiology
Institute. Additional funding came from NASA’s Planetary Atmospheres and
Planetary Astronomy programs. ALMA, an international astronomy
facility, is funded in Europe by the European Southern Observatory, in
North America by the U.S. National Science Foundation in cooperation
with the National Research Council of Canada and the National Science
Council of Taiwan, and in East Asia by the National Institutes of
Natural Sciences of Japan in cooperation with the Academia Sinica in