How much I’ve missed writing these posts since Cassini passed away. Unsurprisingly, it’s after the probe’s demise that we’ve really begun to realise how much of Cassini’s images and data we were consuming on a daily basis, all of which is gone. There’s no more the steady stream of visuals of Saturn’s rings, bands, storms and panoply of moons – in fact all of which have been replaced by Jupiter’s rings, bands, storms and panoply of moons thanks to Juno. Nonetheless, one entire area of the Solar System has been darkened in my imagination. Until the next full mission to the Saturnian system (although nothing of the kind is in the works), we’ll have to make do with what Cassini data trickles down through NASA’s and ESA’s data-processing sieves.
One such is a new study about the temperature of the air high above Titan’s poles. Before Cassini’s death-dive into Saturn, the probe spent some time studying the moon’s polar atmosphere. Researchers from the University of Bristol who obtained this data noticed something odd: the part of the atmosphere over Titan’s poles began to develop a warm spot over late 2009 but that by 2012, it had become a ‘cold spot’. By 2015, the temperature at about 550 km above had dropped to 120 K (that’s a little below the temperature at which supercooled water turns into a glass).
On Earth, a warm spot forms over the poles because of two principle reasons: how Earth’s wind circulates around the planet and because of the presence of carbon dioxide. During winter, air over the corresponding hemispheric pole sinks down, becomes compressed and heats up. Moreover, the carbon dioxide present in the air also emits the heat it has trapped in its chemical bonds.
In 2012, astronomers using Cassini data had found that Titan also exhibits a wind circulation process that is moon-wide. It can be understood as Titan having two atmospheres, or layers, one on top of the other. In the lower atmosphere, there are three Hadley cells; each cell represents a distinct air circulation system wherein air rises up for 10 km or so from near the equator, moves up/down towards subtropical regions, sinks back down and returns to the equator along the surface. In the second, upper atmosphere, air moves between the two poles directly in a unified, global Hadley cell.
Now, remember that Titan’s distance from the Sun means that one Titan-year is 29.5 Earth-years, that each Titanic season lasts over seven Earth-years and that seasonal shifts are much slower on the moon as a result. However, in 2012, scientists studying Cassini data found that the rate at which the air over one of Titan’s poles was sinking into the pole – like the air does on Earth – was happening really quickly: according to Nick Teanby, a researcher at the University of Bristol and also the lead author of the latest study, the rate of subsidence increased from 0.5 mm/s in January 2010 to 1.5 mm/s in June 2010. In other words, it was a shift that, unlike the moon’s seasons, happened rapidly (in just 12 Titanic days).
The same study concluded that Titan’s atmosphere was thicker than previously thought because trace gases like ethane, hydrogen cyanide, acetylene and cyanoacetylene were found to be produced at an altitude of over 500 km over the poles thanks to photochemical reactions induced by ultraviolet radiation and high-energy electrons streaming in from the Sun. These gases would then subside into the lower atmosphere over the polar region – which brings us to the latest study. It says that, unlike what carbon dioxide warming Earth’s atmosphere, the (once) trace gases actually cool the atmosphere, resulting in the dreadfully cold spot over Titan’s poles. They also participate in the upper Hadley cell circulation.
This is similar to a unique phenomenon observed over Saturn’s south pole in 2005.
What a beauty you are, Titan. And I miss you, Cassini, more than I miss many other things in life.
Links to previous editions:
- Why Titan is awesome #1
- Why Titan is awesome #2
- Why Titan is awesome #3
- Why Titan is awesome #4
- Why Titan is awesome #5
- Why Titan is awesome #6
- Why Titan is awesome #7
- Why Titan is awesome #8
- Why Titan is awesome #9
Featured image: Cassini’s last shot of Titan, taken with the probe’s narrow-angle camera on September 13, 2017. Credit: NASA.