During its 2006 flyby of Titan, the Cassini Space Probe captured some of the most detailed images of Saturn’s largest moon. Amongst them was one showing the lofty cloud formations over Titan’s north pole (shown above). Interestingly enough, these cloud formations bear a strong resemblance to those that are seen in Earth’s own polar stratosphere.
However, unlike Earth’s, these clouds are composed entirely of liquid methane and ethane. Given Titan’s incredibly low temperatures – minus 185 °C (-300 °F) – it’s not surprising that such a dense atmosphere of liquid hydrocarbons exists, or that seas of methane cover the planet.
What is surprising, however, is the fact that methane crystals also exist in this atmosphere. Eight years after the photos of Titan’s north pole were taken, astronomers have concluded that this region also contains trace amounts of methane ice.
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“The idea that methane clouds could form this high on Titan is completely new,” said Carrie Anderson, a Cassini participating scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the study. “Nobody considered that possible before.”
Other stratospheric clouds had been identified on Titan already, including clouds of ethane – a chemical formed after methane breaks down. Delicate clouds of cyanoacetylene and hydrogen cyanide, which form from reactions of methane byproducts with nitrogen molecules, have also been found there.
But clouds of frozen methane were thought unlikely in Titan’s stratosphere. Because the troposphere traps most of the moisture, stratospheric clouds require extreme cold. Even the stratosphere temperature of minus 203 °C (-333 °F), observed by Cassini just south of the equator, was not cold enough to allow the scant methane in this region of the atmosphere to condense into ice.
What Anderson and her Goddard co-author, Robert Samuelson, noted is that temperatures in Titan’s lower stratosphere are not the same at all latitudes. This was based on data taken from Cassini’s Composite Infrared Spectrometer and the spacecraft’s radio science instrument, which showed that the high-altitude temperature near the north pole was much colder than that just south of the equator.
It turns out that this temperature difference – as much as 6 °C (11 °F) – is more than enough to yield methane ice.
Other observations made of Titan’s cloud system support this conclusion, such as how certain regions appear denser than others, and the larger particles detected are the right size for methane ice. They also confirmed that the expected amount of methane – 1.5%, which is enough to form ice particles – is present in the lower polar stratosphere.
What’s more, the observation confirms certain models of how Titan’s atmosphere is thought to work.
According to this model, Titan has a global circulation pattern in which warm air in the summer hemisphere wells up from the surface and enters the stratosphere, slowly making its way to the winter pole. There, the air mass sinks back down, cooling as it descends, which allows the stratospheric methane clouds to form.
“Cassini has been steadily gathering evidence of this global circulation pattern, and the identification of this new methane cloud is another strong indicator that the process works the way we think it does,” said Michael Flasar, Goddard scientist and principal investigator for Cassini’s Composite Infrared Spectrometer (CIRS).
Like Earth’s stratospheric clouds, Titan’s methane cloud was located near the winter pole, above 65 degrees north latitude. Anderson and Samuelson estimate that this type of cloud system – which they call subsidence-induced methane clouds (or SIMCs for short) – could develop between 30,000 to 50,000 meters (98,000 to 164,000 feet) in altitude above Titan’s surface.
“Titan continues to amaze with natural processes similar to those on the Earth, yet involving materials different from our familiar water,” said Scott Edgington, Cassini deputy project scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. “As we approach southern winter solstice on Titan, we will further explore how these cloud formation processes might vary with season.”
The results of this study are available online in the November issue of Icarus.
Further Reading: NASA/GSC