Earthlike chemistry gives rise to impossible clouds on Titan
Tuesday, September 20, 2016, 7:20 PM -
Earth and Titan are very different from one another, but one specific similarity between the two could be producing "impossible" clouds on Saturn's icy, hydrocarbon moon.
Solid-state chemistry gives rise to Titan's "impossible" clouds
Nearly 36 years ago, in November of 1980, NASA's Voyager 1 probe flew past Saturn, snapping pictures of the ringed planet and its collection of strange moons. In at least one image, taken by the spacecraft's infrared camera, it caught an unusual cloud high up in the atmosphere of Titan, Saturn's largest moon.
Much more recently, NASA's Cassini spacecraft also spied one of these unusual clouds, using the same technology, specifically its composite infrared spectrometer (CIRS).
What's so strange about these particular clouds, given that Titan's atmosphere is almost completely covered in a thick hydrocarbon haze? According to how scientists believe Titan's atmosphere behaves, these clouds shouldn't exist!
Clouds on Titan are expected to form similar to how they do on Earth, just with different materials, given the extreme temperatures that far out from our Sun. So, just as you need an abundant supply of water vapour in Earth's atmosphere, at any specific location, to form a cloud here, there needs to be enough dicyanoacetylene (C4N2) - the chemical compound these Titan clouds are made of - in vapour form in the moon's atmosphere for clouds of it to form there. Then you just need the temperature and pressure to be just right so that the vapour condenses out of the atmosphere into liquid, which can then even freeze into ice crystals.
The problem is, in both of these instances - back when Voyager 1 spotted its cloud and when Cassini spied the more recent one - the spacecraft's instrument didn't detect enough of dicyanoacetylene vapour present to account for the cloud's formation.
According to NASA, both spacecrafts detected only around 1 per cent of the amount of dicyanoacetylene vapour needed to produce a cloud. It's the equivalent of producing clouds in a desert!
To solve this mystery, NASA scientists had to look much closer to home. Since these dicyanoacetylene clouds were forming in Titan's stratosphere, close to the poles, they looked at how polar stratospheric clouds, also known as nacreous clouds, form on Earth.
When chlorofluorocarbon gases make their way into the stratosphere, they don't combine with water vapour, but instead they stick to tiny ice crystals. It's the reactions with these tiny solid pieces of water - thus "solid-state" chemical reactions - that break off chlorine atoms, which go on to split apart ozone and produce the yearly ozone hole over the Antarctic.
The Titan clouds apparently also use solid-state chemical reactions to form.
The possible "solid-state" chemical reactions going on inside layered ice particles in Titan's upper atmosphere, that could make cloud formation there possible. Credit: NASA's Goddard Space Flight Center
According to a NASA press release:
The first step in the proposed process is the formation of ice particles made from the related chemical cyanoacetylene (HC3N). As these tiny bits of ice move downward through Titan's stratosphere, they get coated by hydrogen cyanide (HCN). At this stage, the ice particle has a core and a shell comprised of two different chemicals. Occasionally, a photon of ultraviolet light tunnels into the frozen shell and triggers a series of chemical reactions in the ice. These reactions could begin either in the core or within the shell. Both pathways can yield dicyanoacteylene ice and hydrogen as products.
The researchers suggest that, on Titan, the reactions occur inside the ice particles, sequestered from the atmosphere. In that case, dicyanoacetylene ice wouldn't make direct contact with the atmosphere, which would explain why the ice and the vapor forms are not in the expected equilibrium.
"It's very exciting to think that we may have found examples of similar solid-state chemical processes on both Titan and Earth," lead author of the study, Carrie Anderson, who is a CIRS co-investigator at NASA's Goddard Space Flight Center, said in the press release.
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