Weather on Titan, moon of Saturn

The weather where I have been recently has been rather unhelpful, oscillating between torrential downpour and sizzling humidity when the sky is clear. It got me thinking about weather elsewhere, in the moons and planets of the solar system and beyond. I am particularly fond of the planet Saturn and its entourage of moons, so today I will be zooming on one moon in particular- Titan!

In visible light, Titan appears almost featureless. When observed in the infrared wavelength however, a whole new story is revealed, shedding light on lakes, rivers, valleys, dunes and a complex weather system which shares many similarities to our weather cycle on Earth. A telescope that has been particularly instrumental in exploring this is the powerful and multipurpose James Webb Space Telescope.

Above: the planet Saturn and one of its moon Titan, as captured by the Cassini spacecraft. Image credit: NASA/JPL-Caltech/SSI

This fascinating moon, one of several around Saturn, is a cloudy body with hydrocarbon lakes and even occasional rain in the form of methane! It should be noted that Titan is a chilly -180 Celsius, so surface water is rock solid ice, potentially with liquid water oceans deep below. Covered in a cloak of hazy smog, it is the only other body in the solar system to have its own weather cycle.

Similar to Earth, Titan’s atmosphere is mostly nitrogen, and has its own weather cycle, clouds and rain. Unlike on Earth however, Titan’s weather cycle runs on methane, rather than water, but acts in a similar way to ours, evaporating from the surface, rising into the atmosphere, and condensing to form clouds. The rain on Titan is cold and oily, but would make for an interesting setting in a science fiction film (side note, two separate friends, one an astrophysics PhD student and the other a former government minister, have been chatting to me rather a lot about this genre recently). While an interesting genre for many, I prefer the wonder of the real objects out in space, the weird and the wonderful, which keep people like me busy with scientific puzzles.

Above: These images of Titan were taken by the NASA/ESA/CSA James Webb Space Telescope on 11July 2023 (top row) and the ground-based W.M. Keck Observatories on 14 July 2023 (bottom row). They show methane clouds (denoted by the white arrows) appearing at different altitudes in Titan’s northern hemisphere. Credit: NASA, ESA, CSA, STScI, Keck Observatory

These images above were taken with the Webb and Keck observatories, both of which are used by one of my favourite lecturers from Exeter. These telescopes observed clouds in the mid to northern latitudes of Titan, where this hemisphere is currently experiencing summer. This is the first time such clouds have been observed in the northern hemisphere, while cloud convection has previously been seen at southern latitudes. This is interesting because most of Titan’s lakes and seas are in its northern half, and evaporation from lakes is a major potential methane source, starting the methane weather cycle, evaporating from rivers and lakes on the surface to create clouds.

Titan has weaker gravity than Earth owing to its smaller mass. Due to this, Titan’s atmospheric layers can extend to 45km in altitude above the surface, almost four times as far as on Earth where our atmosphere extends to around 12 kilometres. Atmospheres can be highly influential in whether an object can host life, indeed life on Earth as we know it would not be possible without our atmosphere, which provides protection from harmful UV radiation as well as a means to cycle gas and liquids. Titan is of great interest to astrobiologists owing to its interesting complex organic chemistry, a fancy way of saying it involves carbon, which is thought to be a crucial element for habitability. Organic molecules form the basis for all life on Earth, and studying such molecules on other worlds such as Titan may help scientists understand the pathways that led to the origins of life on Earth; indeed, my masters in planetary science at Cambridge last year covered this in some detail and it is a ripe and growing field.

Titan’s chemistry is driven by a fairly simple carbon molecule called methane, symbol C2H4, meaning 2 carbon and 4 hydrogen atoms bonded together. Atmospheric methane can be split either by sunlight or energetic electrons from Saturn’s magnetosphere, and can subsequently recombine to make other carbon bearing molecules. The Webb telescope has been revolutionary in allowing scientists to see some of this chemistry in action throughout the various chemical stages for the first time, rather than just observing the start and end products. Through the cycling of methane in the atmosphere, and subsequent breaking and recombination in other carbon molecules means that new molecules can end up in Titan’s surface. Additionally, some hydrogen escapes the atmosphere during the process. This depletes the methane supples over time, unless it is somehow replaced. Possible replenishing of methane supplies could come from cracks in the moon’s crust and interior over long timescales, however if this does not occur, Titan will slowly lose its atmosphere and become a barren, atmosphere depleted world. Atmospheric depletion is thought to have occurred on Mars, where water molecules were broken up and the resulting hydrogen was lost to space, leaving behind a desert planet as we see it today. As hydrogen is the lightest of all elements, this is also the first one to escape from atmospheres as it can do so the easiest.

Atmospheric escape is a key issue in planetary habitability studies, as if atmospheres are depleted too quickly, there is insufficient time for life to emerge even if conditions are temporarily favourable. As such, this provides a new frontier for planetary science and habitability studies, an exciting and growing field.