Using the images captured by the Cassini spacecraft, scientists have found evidence of methane rainfall at the North Pole of Titan, the biggest moon of Saturn. Scientists say this rainfall is the first indication of the beginning of a summer season in the northern hemisphere of Titan.
Cassini spacecraft first arrived in Saturn's orbit in 2004 after completing a seven-year-long journey through deep space. The spacecraft was the fourth probe to visit Saturn and the first to enter its orbit. When Cassini entered the orbit of Saturn, it was summer season in the southern hemisphere of Titan, and Cassini could see clouds, storms, and precipitation in that part of Titan.
Based on the available data, scientists created climate models for Titan, which predicted summer season in the northern hemisphere in the years leading up to the northern summer solstice in 2017.
The spring equinox on Titan fell in 2009 and in 2011, resulting in some atmospheric changes that were interpreted as the start of the southern winter. However, rains predicted by climate models in the north remained undetected.
"Despite what the climate models had predicted, we weren't even seeing any clouds," said Rajani Dhingra, a doctoral student in physics at the University of Idaho, and lead author of the study.
"People called it the curious case of missing clouds."
Dhingra and her colleagues investigated the images captured by Cassini's Visual and Infrared Mapping Spectrometer, and noticed - in one image - a reflective feature covering about 120,000 square kilometres near Titan's North Pole.
Further investigation suggested that the reflective feature likely resulted from the reflection of sunlight off a wet surface. Scientists attributed the reflective feature to a methane rainfall, which was also likely followed by a period of evaporation.
"It's like looking at a sunlit wet sidewalk," Dhingra said.
This is the first observation of summer rainfall on Titan's northern hemisphere.
While people on Earth experience four different seasons in a single year, a season on Titan lasts for about 7.5 Earth years (although there is some variation due to Saturn's orbital eccentricity).
"Summer is happening. It was delayed, but it's happening," Dhingra added.
The team now plans to investigate the reasons behind the delay in the onset of summer on Titan's northern hemisphere.
The findings of the study are published in the journal Geophysical Research Letters.
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