Cassini Probe Spots Methane Ice Crystals In Titan’s Atmosphere

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.

“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.

A composite image of Titan's atmosphere, created using blue, green and red spectral filters to create an enhanced-color view.  Image Credit: NASA/JPL/Space Science Institute
A composite image of Titan’s atmosphere, created using blue, green and red spectral filters to create an enhanced-color view. Image Credit: NASA/JPL/Space Science Institute

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

Observing Alert: Bright Spot On Uranus Reported

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There’s nothing like a dynamic solar system… and right now another planet is being heard from. According to various sources, a bright spot – possibly a developing storm – has been spotted on Uranus.

“Professional observers this morning (October 27) reported a very bright cloud on Uranus, using the Gemini telescope, and need amateur confirmation if possible, to obtain a rotation period.” says John H. Rogers, Jupiter Section Director, British Astronomical Association. “Near-infrared filters may have the best chance of detecting it. It was recorded in the 1.6 micron band, which is further into the IR than amateurs can reach, but your usual near-IR filters might be successful. I think that methane filters are not especially promising, as these clouds on Uranus are overlaid by a methane-rich layer of atmosphere, but would be worth trying anyway. Anyone who has a 1-micron filter should have a go too.”

At this point in time, information is limited, but professional images taken using the 8.1-metre Gemini Telescope North on Hawaii have recorded a region said to be ten times brighter than the planetary background. The bright spot is believed to be attributed to methane ice. ““This is an H-band image, centered at 1.6 microns, close to the wavelength of maximum contrast for such features. Its contrast will decrease with decreasing wavelength, and will likely not be detectable by amateur astronomers, except possibly at the longer CCD wavelengths where the Rayleigh scattering background can be suppressed.” says Larry Sromovsky, of the University of Wisconsin-Madison. “Looking with a methane band filters at 890 nm might be productive, especially if the feature continues to brighten.”

“The feature is not very large; instead its prominence is due to its high altitude, placing it above the intense absorption of methane in the deeper atmosphere. This is much higher than the 1.2-bar methane condensation level and thus it is expected to be predominantly composed of methane ice particles.”

Dr Sromovsky added: “The latitude of the feature is approximately 22.5° north planetocentric, which is a latitude nearly at rest with respect to the interior. So it should rotate around Uranus’ axis with nearly a 17.24-hour period. At the time of the image, the feature’s longitude was 351° West. That could change slowly in either direction.

“The low latitude is unusual. Previous exceptionally bright cloud features on Uranus were at close to 30° North, both in 1998 (Sromovsky et al. 2000, Icarus 146, 307-311) and in 2005 (Sromovsky et al. 2007, Icarus 192, 558-575). The 2005 feature oscillated ±1° about its mean latitude. The new feature might also oscillate in latitude, in which case its longitudinal drift rate might also vary with time.”

Hang in there, UT readers! Right now we have two of our best astrophotographers doing their best to give us an exclusive look! This page will be updated as more information becomes available.

Partial Quote Source: Skymania News Release.