/ Some of the features of Titan‘s atmosphere, like its polar vortex, can be seen with visible light. Share this story
Weather prediction is a tricky business. And it‘s even more difficult when you only have a single deceased orbiter to get your data from. This week, via a , the Cassini probe tells us of on Titan.
Cassini had a couple of imaging spectrometers on board. These created pictures of Titan, with each pixel in the image telling us something about the materials in the atmosphere and on the ground within the region covered by the pixel. In 2009 and 2010, during the equinox, Cassini observed a sudden and short-lived brightening on Titan.
The brightening was not visible to the naked eye. Instead, it was mostly visible in the infrared (a wavelength longer than we can see). What could have caused it?
Is it a bird, is it a plane?
There were numerous possibilities: maybe a volcano had erupted? Maybe a storm had changed the lay of the land, making it more reflective? Was it methane clouds?
All of these possibilities were examined. It turns out that lava and volcanic hot spots would cool too slowly, so they were eliminated. The ground pre– and post-brightening seemed the same, so that couldn’t have been the issue, either.
That left atmospheric effects. The nice thing about spectroscopy is that once you know what you‘re looking at (e.g., Titan’s atmosphere), you can sometimes tell how deep the changes are. It’s like this: the atmosphere is quite transparent for some wavelength ranges (called windows). Right at the edge of the window, there is some absorption, but not too much. If something changes at low altitudes, the atmosphere above it will wash out those changes at the edges of the window.
The washed-out nature of the changes told the scientists that whatever happened, it was in the lower atmosphere. Not only that, but the remaining spectra definitely indicated that an organic compound was involved. Was it really a heavier organic solid in the atmosphere, or were the scientists being fooled by liquid methane, which looks similar spectrally?
By taking the light reflected from the ground on a clear day and manually adding in the changes due to methane clouds, the researchers could almost fit the data. But the clouds had to be at very low altitude and consist of very small drops. This seemed unlikely to the researchers, but maybe it was the answer?
Not much wind at the equator
To investigate, the researchers turned to climate and weather modeling. Their models showed that while methane clouds could form at the equator (where the brightening was observed), they would all be too high in the atmosphere. Not only that, the required droplet size was much too small. Here, the researchers use the word unphysical, which means such a small droplet would evaporate immediately or grow rapidly, such that the population of small droplets is always zero. In other words: we were not looking at clouds of methane.
In the end, they concluded that the event had to be a dust storm. Not like the sand we have here on Earth, but sands of frozen organics—think solid gasoline blowing in the wind.
But that raised yet another question. How did the particles get into the air? When , it threw up a bit of dust. But it was also found that in the equatorial regions, there isn’t much wind. A set of experiments and more models show that winds between 9km/h and 40km/h would be required to kick up dust into the atmosphere. Atmospheric models, combined with observations, predict average wind speeds of just 1km/hr.
This, the researchers say, probably accounts for the rarity of these observations. Dust storms are only kicked off when a powerful short-lived gust lifts some dust into the air. That dust can kick up more dust at much-lower wind speeds. This triggers a short-lived uprising of dust that brightness the Titan sky momentarily.
It is possible that there is more evidence of dust storms hidden in Cassini’s archive of data. But we will have to wait until the researchers get their next turn with the ouija board to see if the data can be found.
Nature Geoscience, 2018, DOI: ()