A huge arrow-shaped storm blows across the equatorial region of Titan in this image from NASA's Cassini spacecraft, chronicling the seasonal weather changes on Saturn's largest moon.
Credit: NASA/JPL/SSI
BRISTOL: Spring storms shower the surface of Titan with freezing cold methane seasonally, announced an international team of astronomers who analysed data from the NASA's Cassini spacecraft.
Until now, scientists had no definitive explanation for dried up river channels running through the vast expanse of sand dunes found around the equator, but the latest research shows that methane rivers flow seasonally during April storms and then disappear as the methane rapidly evaporates.
“The majority of dry channels can only be explained by rainfall,” said Tetsuya Tokano, who published an accompanying Perspective article in the journal Science.
Dry riverbeds mystery
Titan may be a moon spinning around Saturn 750 million miles from Earth, but with a dense atmosphere, dunes, mountains, volcanoes and lakes it is in many ways the most similar body in the solar system to our own planet.
Methane lakes have been found near the poles but equatorial regions of Titan are almost exclusively arid. Although riverbed channels can be seen around the equator it had been thought that they were formed in the past when the equator was wetter.
This new research by Elizabeth Turtle and colleagues published in Science is the first observational evidence of substantial rainfall happening now near Titan’s equator.
Probing Titan from above
Bright methane clouds are monitored from telescopes on Earth, but rainfall is much more difficult to observe. Fortunately, rainfall can change the surface in ways that can be seen from the NASA probe Cassini which has been exploring Saturn and its moons since 2004.
Cassini collects data using high-resolution cameras to survey Titan’s surface. Turtle and colleagues analysed these data from which they “detected a large cloud, which was soon followed by darkening of a huge area in the vicinity of this cloud,” said Tokano.
Whilst wind storms and volcanic activity could explain these findings, the scientists concluded that rainfall from a huge methane storm, 500,000 square kilometres across, is the most likely reason for the darkening they saw.
Encouragingly, computer simulations predicted substantial equatorial rainfall beforehand which agrees with these latest observations.
Earth’s frozen alter-ego
Titan has a thick atmosphere mostly made of nitrogen, like on Earth. Earth and Titan also share many geological features, such as erosion caused by wind and rain. But the extreme cold (-180 degrees Celsius) caused by being so far from the Sun means that these features are built from very different materials.
“The mountains are made of frozen water ice, the dunes are made of particles of organic matter which snow down from the sky, and the lakes are full of liquid methane, which evaporates, forms clouds and falls as rain that carves the channels,” said David Grinspoon, curator of astrobiology at the Denver Museum of Nature and Science, who was not involved in the study.
Titan has a hydrological cycle with rivers, vapour, clouds and rainfall, similar to that on Earth. According to Grinspoon, the findings “support the view that Titan is very much a frozen alter-ego of our home planet where a methane cycle is, in many respects, playing the role that a water cycle plays on Earth.”
“They support the emerging picture of Titan as the second most dynamic place in the modern solar system,” he said.
