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Icy Titan Spawns Tropical Cyclones

Object: Mini-hurricanes of methane rain
Location: North Pole of Saturn's moon Titan

With a maximum surface temperature of -180 °C, Saturn's icy moon Titan is no tropical paradise, at least by earthly standards. But it may still have tropical cyclones, and at what sounds like the unlikeliest of places – near its north pole.

These mini-hurricanes have never been observed anywhere but Earth. If they exist on Titan, that would add to a growing list of features that the distant moon shares with our planet, from lakes, hills, caves and sand dunes to fog, mist, smoggy haze and rain clouds.

Though cyclones - a large family of storms in which winds spiral inward to a low-pressure zone, such as the eye of a hurricane or tornado – have been glimpsed on Mars and Saturn, a tropical cyclone is a special case that is driven by the heat of evaporation from a warm sea. These storms involve a lot of rain as well as gale-force winds, and have not been glimpsed anywhere but Earth.

As Titan is the only body in the solar system apart from Earth known to have liquid on its surface and, therefore, rain (Titan is so cold that its rain is in the form of liquid methane, not water), Tetsuya Tokano of the University of Cologne in Germany decided to calculate what it would take for Titan to have its own mini-hurricanes.

Methane seas

The first thing that would be required, he says, is the right blend of hydrocarbons in the moon's lakes and seas. "We know ethane is present, and methane probably is," he says. The methane is crucial because it evaporates much more readily, and could deliver the heat needed to drive the storm.

Assuming the methane fraction is large enough, Tokano calculated the heat it would carry and how that would be converted into kinetic energy to power a storm. He reckons that the resulting storm would not be as powerful as hurricanes or typhoons on Earth, but that they could produce surface winds of up to 20 metres per second (72 kilometres per hour). That's 10 times the average wind velocity on Titan: on Earth, it's equivalent to the wind speeds of a midsize tropical storm – and two-thirds those needed for a full-scale hurricane.

Tokano also looked at where these could storms could form – and discovered that the 1200-kilometer-long Kraken Mare, and the smaller Ligeia and Punga mares, are the only seas on Titan large enough to support the growth of a tropical cyclone. All three are situated near Titan's North Pole, making a contrast to the tropical cyclones on Earth.

As on Earth, however, any mini-hurricanes on Titan would be seasonal. Tokano says the storms could form in Titan's northern summer, lasting up to 10 days and reaching hundreds of kilometres in diameter, limited by the size of the lakes.

Spectacular storm

It's now spring on northern Titan, and solar warming of the north pole should make the storms possible from 2015 to 2021. (Because Titan is so much further away from the sun than Earth, its year – and therefore its seasons - are much, much longer.)

That means that when mini-hurricane season next returns to Titan, the Cassini spacecraft, which started orbiting it in 2004, will still be watching. The craft's orbit gives it a better view of Titan's poles than terrestrial telescopes, and its mission is scheduled to continue until 2017.

"It would be spectacular to see this kind of storm over Kraken Mare," says Elizabeth Turtle of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. "This gives us a specific type of storm to look for."

Failure to spot a storm during this period would not tell us much, however, says Tokano, as any of a number of factors might cause Cassini to miss it, or it might just be a slow season.

Because of its similarities to Earth, Titan looks like a good place to hunt for extra-terrestrial life. Though Tokano wouldn't be drawn on how tropical cyclones might feed into this picture, one thing is clear: the frigid moon is certainly living up to its reputation as one of the most intriguing places in our solar system.



Titan's disappearing craters

A relatively "fresh" crater called Sinlap (left) and an extremely degraded crater called Soi (right). Image credit: NASA/JPL-Caltech/ASI/GSFC

New reserarch using observations from NASA's Cassini spacecraft suggest that Saturn's largest moon Titan may look much younger than it really is because its craters are getting erased as dunes of exotic, hydrocarbon sand are slowly but steadily filling in the craters.

"Most of the Saturnian satellites, Titan's siblings, have thousands and thousands of craters on their surface. So far on Titan, of the 50 percent of the surface that we've seen in high resolution, we've only found about 60 craters," said Catherine Neish, a Cassini radar team associate based at NASA's Goddard Space Flight Center. "It's possible that there are many more craters on Titan, but they are not visible from space because they are so eroded. We typically estimate the age of a planet's surface by counting the number of craters on it (more craters means an older surface). But if processes like stream erosion or drifting sand dunes are filling them in, it's possible that the surface is much older that it appears."

Neish and her team compared craters on Titan to craters on Jupiter's moon Ganymede. Ganymede is a giant moon believed to have a water ice crust, similar to Titan, so craters on the two moons should have similar shapes. However, Ganymede has almost no atmosphere and thus no wind or rain to erode its surface.

This research is the first quantitative estimate of how much the weather on Titan has modified its surface

Titan on the other hand is the only moon in the solar system with a thick atmosphere, and the only world besides Earth known to have lakes and seas on its surface. However, with surface temperatures of around minus 290 degrees Fahrenheit (94 kelvins), the rain that falls on Titan is not water but liquid methane and ethane, compounds that are normally gases on Earth.

"This research is the first quantitative estimate of how much the weather on Titan has modified its surface," said Neish.

The source of Titan's methane remains a mystery as methane in the atmosphere is broken down over relatively short timescales by sunlight. Fragments of methane molecules then recombine into more complex hydrocarbons in the upper atmosphere, forming a thick, orange smog that hides the surface from view. Some of the larger particles eventually rain out on to the surface, where they appear to get bound together to form the sand.

"Since the sand appears to be produced from the atmospheric methane, Titan must have had methane in its atmosphere for at least several hundred million years in order to fill craters to the levels we are seeing," says Neish. However, researchers estimate Titan's current supply of methane should be broken down by sunlight within tens of millions of years, so Titan either had a lot more methane in the past, or it is being replenished somehow.

The difference in depth between craters on Titan and Ganymede is best explained by filling from windblown sand, although erosion from liquids and viscous flow might contribute to the modification of Titan's craters. The team thinks these considerations need further investigations.  A paper about this research was published online in the journal Icarus December 3, 2012.



Titan's Alien Seasons --New Discoveries

Detailed observations of Saturn's moon Titan have now spanned 30 years, covering an entire solar orbit for this distant world. Dr Athena Coustenis from the Paris-Meudon Observatory in France has analysed data gathered over this time and has found that the changing seasons of Titan affect it more than previously thought. Dr Coustenis presented these results at the European Planetary Science Congress in Madrid on Friday 28th September.

"As with Earth, conditions on Titan change with its seasons," says Coustenis. "We can see differences in atmospheric temperatures, chemical composition and circulation patterns, especially at the poles. For example, hydrocarbon lakes form around the north polar region during winter due to colder temperatures and condensation. Also, a haze layer surrounding Titan at the northern pole is significantly reduced during the equinox because of the atmospheric circulation patterns. This is all very surprising because we didn't expect to find any such rapid changes, especially in the deeper layers of the atmosphere."

The main cause of these cycles is solar radiation. This is the dominant energy source for Titan's atmosphere, breaking up the nitrogen and methane present to create more complex molecules, such as ethane, and acting as the driving force for chemical changes. Titan is inclined at around 27 degrees, similar to the Earth, meaning that the cause of seasons – sunlight reaching different areas with varying intensity due to the tilt – is the same for both worlds.

"It's amazing to think that the Sun still dominates over other energy sources even as far out as Titan, over 1.5 billion kilometres from us," explains Coustenis. To draw these conclusions data was analysed from several different missions, including Voyager 1 (1980), the Infrared Space Observatory (1997), and Cassini (2004 onwards), complemented by ground-based observations. Each season on Titan spans around 7.5 years, while it takes 29.5 years for Saturn to orbit the Sun, so data has now been gathered for an entire Titan year, encapsulating all seasons.

 "Titan is the best opportunity we have to study conditions very similar to our own planet in terms of climate, meteorology and astrobiology and at the same time a unique world on its own, a paradise for exploring new geological, atmospheric and internal processes," concludes Coustenis.

Evidence of changing weather patterns in the skies over Titan's southern region are revealed in thefalse color images at the top of the page obtained by the Cassini spacecraft's visual infrared mapping spectrometer over flybys of this largest of Saturn's satellites. In the first image (left), obtained on the Oct, 26, 2004 Titan flyby, from a distance of some 200,000 kilometers (124,300 miles), Titan's skies are cloud-free, except for a patch of clouds observed over the south pole near the bottom of the image.

In contrast, the image on the right shows a recent view of this same area of Titan obtained seven weeks later on the second close Titan flyby on Dec. 13, 2004, from a distance of 225,000 kilometers (139,800 miles). This image clearly shows that several extensive patches of clouds have formed over temperate latitudes. The appearance of these clouds reveals the existence of weather.

Tracking these features is currently underway by scientists, who hope to gain a better understanding of global circulation, regional weather patterns, and localized meteorology in Titan's skies.

The colors red, green, and blue represent near-infrared images obtained at 2.01 micron, 2.83 micron and 2.13 micron, respectively. These colors explore the surface and atmosphere of Titan with varying effectiveness. The red color images the surface at a wavelength (2.01 micron) where the surface is relatively bright, making the surface appear reddish in these color images. The green color (2.83 micron) images the surface as well, but due to enhanced absorption of sunlight by the surface and lower atmosphere, the surface is relatively dark here compared to the red. The blue color (2.13 micron) is at a wavelength where sunlight cannot reach the surface at all due to strong absorption by the atmospheric gas methane.

In contrast to the reddish surface, bright clouds at a relatively high altitude (here, about 30 kilometers (19 miles) above the ground) residing above most of the atmospheric absorption appear whitish in these representations, as they reflect sunlight effectively in all three near-infrared colors.



Cassini Faz Imagem Angulada Espetacular de Saturno

A sonda Cassini fez a imagem angulada acima de Saturno, mostrando a região sul do planeta sendo atingida com os anéis de forma diagonal e espetacular.

O norte em Saturno está para cima e rotacionado em 16 graus para a esquerda. Essa imagem foi feita com a sonda olhando em direção ao lado sul não iluminado dos anéis a aproximadamente 14 graus abaixo do plano dos anéis. Os anéis geram sombras largas no hemisfério sul do planeta.

A lua Encelado, com seus 504 quilômetros, aparece como um pequeno e brilhante facho de luz na parte inferior esquerda da imagem.

A imagem acima foi feita com a câmera de grande angular da sonda Cassini no dia 15 de Junho de 2012, usando um filtro espectral sensível ao comprimento de onda do infravermelho próximo centrado em 752 nanômetros. A imagem acima foi obtida  a uma distância de aproximadamente 2.9 milhões de quilômetros de Saturno com o conjunto Sol-Saturno-Cassini em fase com ângulo de 72 graus. A escala da imagem é de 17 quil6ometros por pixel.

A missão Cassini-Huygens é um projeto corporativo da NASA e da Agência Espacial Europeia e a Agência Espacial Italiana. O Laboratório de Propulsão a Jato, uma divisão do California Institute of Technology em Psadaena, gerencia a missão para o Science Mission Directorate da NASA  em Washington, D.C. O módulo orbital Cassini e suas duas câmeras a bordo foram desenhadas, desenvolvidas e montadas no JPL. O centro de operações de imagens está baseado no Space Science Institute em Boulder no Colorado.