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Posted at 11:15 AM ET, 05/12/2011

On Saturn’s moon Titan, spring rains but no spring flowers


Bright clouds building up near the equator of Titan (October 2010) soaking the surface beneath with methane rains. (NASA JPL)

At last, spring has sprung. Heavy, persistent rains have arrived sometimes overflowing riverbeds and gullies. River and lake ice have thawed. Tropical weather systems are migrating north. And at northern latitudes, average temperatures have finally risen above the freezing point of -297 degrees F.

297 degrees F below zero? Did you catch that? There is no error. I’m not talking about freezing/melting point of water, but rather methane. And the coming of spring is occurring not on Earth but on Titan, the largest moon of the planet Saturn.

It’s currently the equivalent of April in Titan’s 27 earth year-long seasonal cycle. Titan is unique in the solar system in that it is the only moon with an atmosphere and “weather”. Its atmosphere is primarily composed of nitrogen (like Earth) but rich in hydrocarbons, primarily methane. Titan is also the only object in the solar system except for Earth known to have stable bodies of surface liquid and ice, but free flowing or frozen methane rather than water.

In a qualitatively reasonably close parallel, methane is the equivalent of water in the super-chilled “water” cycle, climate, and weather on Titan. Just don’t expect the equivalent of April showers on Titan to bring May flowers.

What we know about Titan’s opaque atmosphere comes from the data provided by the Cassini-Huygens spacecraft, which achieved orbit around Saturn on July 1, 2004 after being launched nearly eight years earlier. As the Cassini component of the spacecraft continued to circle Saturn, the second component, the Huygens probe, separated from the orbiter, ultimately reaching and then descending through Titan’s atmosphere to the surface (Jan 14, 2005) while radioing scientific data to Earth (what a ride that must have been!). Observations of Titan’s surface features were obtained by Cassini’s mapping radar.


Radar-image data from the Cassini spacecraft showing Titan lakes of methane on its surface, indicated by the darker coloring in the image above. (NASA JPL)
On frigid Titan, spring showers are torrents of methane rain flooding northern lakes and rivers. Many of the geological features on Titan are formed by erosion by this liquid methane, not dissimilar to the role of water in shaping features on Earth.

In a recent National Geographic article, Ralph Lorenz, a planetary scientist from Johns Hopkins University, noted that erosion is another example of many Earthlike processes found to be operating on Titan. He adds, “The physical conditions and working materials are different, but the processes themselves and the phenomena that result—whether sand dunes, river channels, or rain clouds—are remarkably similar”.

One other specific example mentioned is the formation of fog, clouds, and precipitation from evaporation of methane from methane lakes into colder “air” passing over them – “exactly what happens on Earth”.

(Note: I’ve found nothing that refers to methane snow, but I suppose at least partially frozen methane is possible or as a component of mixed precipitation – and likely equally as difficult to forecast as hereabouts.)

Until recently Titan’s equatorial regions were believed to be arid with the appearance of vast sand dunes, gullies, and dry stream beds of Earth’s deserts, but on Titan features are carved into the surface of perpetually frozen water ice.

It had been assumed that the surface reflected the remnants of a past (methane) wetter climate. However, late last year near the beginning of Titan’s spring, Cassini observed a recently formed massive cloud near Titan’s equator- something never seen before, according to planetary scientist Elizabeth Turtle in a recent article in Science.

Over the next few months, Cassini captured short-lived surface changes over a 193,000 square mile area of Titan’s surface. Scientists concluded the changes were most likely from surface wetting by methane rain.

A statement from Elizabeth Turtle is one which I instinctively relate to:

It’s amazing to be watching such familiar activity as rainstorms and seasonal changes in weather patterns on a distant, icy satellite. These observations are helping us to understand how Titan works as a system, as well as similar processes on our own planet.

It is believed now that tropical clouds and precipitation encircle Titan’s equatorial regions in response to seasonal changes in the limited solar radiation reaching it (about 1% of the amount reaching Earth), much like the Intertropical Convergence Zone (ITCZ) on Earth.

On Earth, the oceans confine the ITCZ to tropical zones as it migrates north and south across the equator as seasons change. Its latitude largely governs the locations of wet and dry seasons within the tropics. On Titan, however, the ITCZ equivalent moves to polar latitudes in its seasonal cycle, such that the tropics are largely bereft of clouds and precipitation near the summer and winter equinoxes.

While much has been learned about Titan since arrival of Cassini-Huygens, there remain many more questions than answers on the nature of physical processes responsible for the observed (and yet to be observed) weather and climate on Titan. The same can probably be said of the weather and climate on planet where we all live.

One of the most, if not the most, intriguing aspects of Titan is not the weather but that Titan is viewed as analogous to the early Earth billions of years ago. Given the abundance of organic chemicals, there is reason to believe that some form of microbial extraterrestrial life might have developed in Titan’s dark cold lakes or in possible oceans of methane beneath the moon’s surface. In fact, there have been suggestions that anomalies in composition of Titan’s atmosphere are consistent with such life forms, but which also could be explained by exotic non-living chemistry.

Postscript:

Coincidentally, NASA just announced that it is considering sending a boat for splashdown in one of Titan’s methane lakes to probe for complex organic molecules and signs of alien life – and, not incidentally, it could be the first probe to experience rain on another world. If the project gets the OK, lift off would be in 2016 with arrival and splashdown (via parachute) by 2023. Any sailors out there willing to go along for the ride?

By  |  11:15 AM ET, 05/12/2011

Categories:  Tracton, Science, Latest, Space

 
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