Three components are evident in the VIMS map, and they are shown in different colours according to the different characteristics of their light. These are the surface of Titan (shown in orange), the atmospheric haze along the limb (green) and the polar vortex (blue).
When observed with VIMS, the southern polar vortex shows a remarkable difference with respect to other portions of Titan’s atmosphere: a signature of frozen hydrogen cyanide molecules (HCN). This discovery suggests that the atmosphere of Titan’s southern hemisphere is cooling much faster than expected.
The VIMS infrared image was processed by Remco de Kok of Leiden Observatory and SRON Netherlands Institute for Space Research. Copyright: NASA/JPL-Caltech/ASI/University of Arizona/SSI/Leiden Observatory & SRON
The international Cassini mission has revealed that a giant, toxic cloud is hovering over the south pole of Saturn’s largest moon, Titan, after the atmosphere has cooled in a dramatic fashion.
Scientists analysing data from the mission found that this giant polar
vortex contains frozen particles of the toxic compound hydrogen cyanide.
“The discovery suggests that the atmosphere of Titan’s southern
hemisphere is cooling much faster than we expected,” says Remco de Kok
of Leiden Observatory and SRON Netherlands Institute for Space Research,
lead author of the study published in the journal Nature.
Unlike any other moon in the Solar System, Titan is shrouded by a dense
atmosphere dominated by nitrogen, with small amounts of methane and
other trace gases. Almost 10 times further from the Sun than Earth,
Titan is very cold, allowing methane and other hydrocarbons to rain onto
its surface to form rivers and lakes.
Like Earth, Titan experiences seasons as it makes its 29-year orbit
around the Sun along with Saturn. Each of the four seasons lasts about
seven Earth years and the most recent seasonal switch occurred in 2009,
when summer transitioned to autumn in the southern hemisphere.
In May 2012, images from Cassini revealed a huge swirling cloud, several
hundred kilometres across, taking shape at the south pole.
During the first years of Cassini’s exploration of the Saturnian system, Titan sported a ‘hood’ of dense gaseous haze (white) in a vortex above its north pole, along with a high-altitude ‘hot spot’ (red). During this time the north pole was pointed away from the Sun.
At equinox both hemispheres received equal heating from the Sun. Afterwards, the north pole tilted towards the Sun, signalling the arrival of spring, while the southern hemisphere tilted away from the Sun and moved into autumn.
After equinox and until 2011 there was still a significant build up of trace gases over the north pole, but the vortex and hot spot had almost disappeared. Instead, similar features began developing at the south pole, which are still present today.
These observations are interpreted as a large-scale reversal in the single pole-to-pole atmospheric circulation cell of Titan immediately after equinox, with an upwelling of gases in the summer hemisphere and a corresponding downwelling in the winter hemisphere. Copyright: ESA/AOES
This polar vortex appears to be an effect of the change of season, with large amounts of air being heated by sunlight during the northern spring and flowing towards the southern hemisphere.
A puzzling detail about this swirling cloud is its altitude, some 300 km
above Titan's surface, where scientists thought it was too warm for
clouds to form.
“We really didn’t expect to see such a massive cloud so high in the atmosphere,” says Dr de Kok.
Keen to understand what could give rise to this mysterious cloud, the
scientists turned to the rich data from Cassini. After careful scrutiny,
they found an important clue in the spectrum of sunlight reflected by
Titan’s atmosphere.
A spectrum splits the light from a celestial body into its constituent
colours, revealing signatures of the elements and molecules that are
present. The Visual and Infrared Mapping Spectrometer on Cassini takes
spectra at many different points on Titan, mapping the distribution of
the chemical compounds in its atmosphere and on its surface.
“The light coming from the polar vortex showed a remarkable difference
with respect to other portions of Titan’s atmosphere,” says Dr de Kok.
“We could clearly see a signature of frozen hydrogen cyanide molecules –
HCN.”
Vortex on Titan close up
A true-colour image of the south pole vortex observed in Titan’s
atmosphere at about 200–300 km altitude, as seen during a Cassini flyby
of Saturn’s largest moon on 27 June 2012. Since equinox in August 2009,
the seasons have been changing, becoming spring in the northern
hemisphere and autumn in the southern hemisphere. The formation of the
vortex over the south pole indicates the effect of the changing seasons
on the circulation pattern in Titan’s atmosphere, specifically with
cooler air sinking down from warmer, high altitudes. The images were obtained with the Cassini spacecraft narrow-angle camera at a distance of approximately 484,000 kilometres from Titan. Copyright: NASA/JPL–Caltech/Space Science Institute
As a gas, HCN is one of the molecules present in small amounts in the nitrogen-rich atmosphere of Titan. However, finding these molecules in the form of ice was very surprising, as HCN can condense to form frozen particles only if the atmosphere is as cold as –148ºC.
“This is about 100ºC colder than predictions from current theoretical
models of Titan’s upper atmosphere,” explains co-author Nick Teanby from
the University of Bristol, UK.
“To check whether such low temperatures were actually possible, we
investigated a second set of observations from Cassini’s Composite
Infrared Spectrometer, which allows us to measure atmospheric
temperature at different altitudes.”
Unfortunately, no such readings were taken in 2012 at this cloud’s
altitude, but the scientists looked at data from other dates, probing
the atmosphere above and below the vortex.
These data showed that the southern hemisphere has been cooling rapidly,
making it possible to reach the low temperature needed to form the
giant toxic cloud seen on the south pole.
This fast cooling of the southern atmosphere may be a consequence of the
atmospheric circulation, which has been drawing large masses of gas
towards the south ever since the change of season in 2009. As the HCN
gas becomes more concentrated, its molecules shine brightly at infrared
wavelengths, cooling the surrounding air in the process.
Another factor contributing to this cooling is the reduced exposure to sunlight on Titan’s southern hemisphere.
“This surprising result shows how much we are still learning about
Titan’s weather and the complex dynamics of its atmosphere,” says
Nicolas Altobelli, Cassini–Huygens Project Scientist at ESA. “We can
look forward to more fascinating discoveries from Cassini in the next
few years, as it continues to monitor the seasonal changes on Saturn and
its moons.”
Notes for Editors
“HCN ice in Titan’s high-altitude southern polar cloud,” by R. J. de Kok
et al. is published in the journal Nature on 2 October 2014; doi:
10.1038/nature13789
The results are reported by R.J. de Kok, Leiden Observatory and SRON
Netherlands Institute for Space Research, The Netherlands; N.A. Teanby,
University of Bristol, UK; L. Maltagliati and S. Vinatier,
LESIA-Observatoire de Paris, CNRS, UPMC Université Paris 06, Université
Paris-Diderot, France; and P.G.J. Irwin, University of Oxford, UK.
The Cassini–Huygens mission is a cooperative project of NASA, ESA and
Italy’s ASI space agency. Launched in 1997, Cassini arrived in the
Saturn system in 2004 and is studying the ringed planet and its moons.
The Huygens probe was released from the main spacecraft and, in 2005,
parachuted through the atmosphere to the surface of Saturn’s largest
moon, Titan.
Cassini’s initial four-year mission to explore the Saturn System covered
the period July 2004 to June 2008, when Saturn and its moons were
experiencing northern winter and southern summer. The first extended
mission, called the ‘Cassini Equinox Mission’, was completed in
September 2010. This included the spring equinox, on 11 August 2009,
when winter was followed by spring in the northern hemisphere and summer
was followed by autumn in the southern hemisphere.
A second extended mission, the ‘Cassini Solstice Mission’, will continue
until September 2017. This will allow scientists to study the Saturnian
system until after the next seasonal change, the summer solstice in May
2017, which will mean the arrival of northern summer and southern
winter.
NASA’s Jet Propulsion Laboratory, a division of the California Institute
of Technology in Pasadena, manages the mission for NASA’s Science
Mission Directorate, Washington, D.C., USA.
For further information, please contact:
Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Phone: +31 71 565 6799
Mobile: +31 61 594 3 954
Email: markus.bauer@esa.int
Nicolas Altobelli
ESA Cassini–Huygens Project Scientist
Directorate of Science and Robotic Exploration
European Space Agency
Phone: +34 91 813 1201
Email: nicolas.altobelli@sciops.esa.int
Remco J. de Kok
Leiden Observatory
Leiden, The Netherlands
and SRON Netherlands Institute for Space Research
Utrecht, The Netherlands
Phone: +31 88 7775725
Email: R.J.de.Kok@sron.nl
Nick Teanby
University of Bristol, UK
Phone: +44 117 3315006
Email: N.Teanby@bristol.ac.uk
Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Phone: +31 71 565 6799
Mobile: +31 61 594 3 954
Email: markus.bauer@esa.int
Nicolas Altobelli
ESA Cassini–Huygens Project Scientist
Directorate of Science and Robotic Exploration
European Space Agency
Phone: +34 91 813 1201
Email: nicolas.altobelli@sciops.esa.int
Remco J. de Kok
Leiden Observatory
Leiden, The Netherlands
and SRON Netherlands Institute for Space Research
Utrecht, The Netherlands
Phone: +31 88 7775725
Email: R.J.de.Kok@sron.nl
Nick Teanby
University of Bristol, UK
Phone: +44 117 3315006
Email: N.Teanby@bristol.ac.uk
