Observations from NASA’s Cassini spacecraft provide evidence of rainfall on the north pole of Saturn’s moon Titan. The rainfall would be the first indication of the start of a summer season in the northern hemisphere of the hazy moon.
Cassini arrived in the Saturnian system in the southern summers of 2004.
As expected, the Cassini team observed cloud cover, storms and precipitation on the south pole of Titan.
Like Earth, the moon has an axial tilt (27 degrees) and its seasons vary over its year (30 Earth years).
Ever since this shift in season began, the researchers eagerly waited for observations indicating cloud-cover and precipitation that went missing from the northern latitudes.
“The whole Titan community has been looking forward to seeing clouds and rains on Titan’s north pole, indicating the start of the northern summer, but despite what the climate models had predicted, we weren’t even seeing any clouds,” said Rajani Dhingra, a doctoral student at the University of Idaho.
“People called it the curious case of missing clouds.”
Dhingra and co-authors identified a reflective feature near Titan’s north pole on an image taken June 7, 2016, by Cassini’s Visual and Infrared Mapping Spectrometer.
The feature covered approximately 46,332 square miles (120,000 km2) and did not appear on images from previous and subsequent Cassini passes.
Analyses of the short-term reflective feature suggested it likely resulted from sunlight reflecting off a wet surface.
The study attributes the reflection to a methane rainfall event, followed by a probable period of evaporation.
“It’s like looking at a sunlit wet sidewalk,” Dhingra said.
This reflective surface represents the first observations of summer rainfall on the moon’s northern hemisphere.
“Summer is happening. It was delayed, but it’s happening. We will have to figure out what caused the delay, though,” Dhingra said.
“Additional analyses suggest the methane rain fell across a relatively pebble-like surface.”
“A rougher surface generates an amorphous pattern as the liquid settles in crevasses and gullies, while liquid falling on a smooth surface would puddle in a relatively circular pattern.”
The findings were published in the journal Geophysical Research Letters.
Rajani D. Dhingra et al. Observational evidence for summer rainfall at Titan’s north pole. Geophysical Research Letters, published online January 16, 2019; doi: 10.1029/2018GL080943