A giant cloud of ethane has been found near Titan's north pole. The finding suggests that ethane rain or snow could accumulate around the moon's poles, partially accounting for Titan's missing ethane oceans.
Ethane is produced in Titan's atmosphere by the breakdown of methane, which is abundant there. Before NASA's Cassini spacecraft reached Titan, scientists expected this ethane to have accumulated in huge amounts, perhaps forming oceans of liquid ethane mixed with liquid methane. The amount expected would have covered the entire moon in a layer of ethane 300 metres deep.
But Cassini observations have revealed only a few suspected lakes (see Titan may be a land of lakes after all).
Their composition is uncertain, but even if they contain a lot of ethane, they would not be able to account for the expected accumulation.
Now, scientists have found a huge cloud of ethane hanging over the moon's north pole. Caitlin Griffith of the University of Arizona in Tucson, US, and her team detected the cloud in images made with Cassini's Visual and Infrared Mapping Spectrometer (VIMS).
Ethane snow
The instrument detected a layer of reflective particles about 40 kilometres above the surface, around the altitude where the atmosphere is at its coldest. The researchers believe the particles are made of frozen ethane.
Methane is also known to condense in Titan's atmosphere, perhaps raining down on the surface from time to time (see Methane rivers and rain shape Titan's surface).
But at 1 to 3 microns across, the particles seen in the newly discovered cloud are smaller than expected for condensed methane. That suggests they are made of ethane instead, the researchers say.
There are downdrafts in the atmosphere above Titan's polar regions that should carry these ethane particles down to the surface.
"We think that ethane is raining or, if temperatures are cool enough, snowing on the north pole right now," Griffith says. "When the seasons switch, we expect ethane to condense at the south pole during its winter."
Volcanic burps
If ethane gets funnelled towards the poles in this way, it might offer one reason why so little ethane has been found closer to the equator, she says.
Because ethane is produced from the breakdown of methane, measuring the amount that has accumulated can tell scientists how long methane has been present in Titan's atmosphere, Griffith says.
If the process described by her team has been at work since Titan formed, a layer of ethane 2 kilometres thick should have formed at the poles, she says. "But if we look at the south pole we don't see evidence for the amount of ethane that we would need to have if methane was in the atmosphere over the lifetime of Titan," she told New Scientist.
The methane could instead be a relatively recent addition to the moon's atmosphere, Griffith says. "You have to speculate that maybe Titan is volcanically active and still burping out methane."
Polar migration
But she cautions that it is still not clear how much ice is at the south pole, because the images of the region were taken from far away. And Cassini has not been able to image the north pole because it has been in winter darkness since the spacecraft arrived.
Owen Toon of the University of Colorado in Boulder, US, who is not a member of Griffith's team, says the idea that ethane migrates towards the poles is plausible. "Of course it's not confirmed yet," he told New Scientist. "We'd have to wait for observations to see if we really observe a polar cap with ethane or not."
Michael Flasar of NASA's Goddard Space Flight Center in Greenbelt, Maryland, US, who was not involved in Griffith's study, says some of the ethane could be hidden underground. But he agrees that it might also be accumulating near the poles.
"It may be that as we get more observations some things will click into place about what's going on at the surface," he told New Scientist.
Journal reference: Science (vol 313, p 1620)
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