Life in our Solar System

After exactly two decades in space, on the 15th of September Cassini transmitted its final images to Earth as it prepared for its final mission: a death dive into the atmosphere of Saturn. Throughout its 20-year history, Cassini had transmitted important information back to its home on Earth, much of which rewrote our knowledge of the solar system. However, it’s most invaluable transmissions is undoubtedly those which suggested the possibility for alien life. Through Cassini’s endeavour, it was found that one of Saturn’s moon could be hosting life as we speak. That moon is ‘Titan’.

Titan: Earth’s ugly step-sister

Titan revealed: Picture of Titan with its dense nitrogen atmosphere removed
Image credit: NASA/JPL/University of Arizona/University of Idaho

Prior to Cassini’s approach, very little was known of Titan. Although scientists postulated that Titan had a dense nitrogen atmosphere, that was all that they could assume from the information that was then available. However, this was all about to change due to in no small part – a mission that soon become known as the first ever successful landing of a space probe in the outer solar system.

In 2005, Cassini’s on board probe (the ‘Huygen probe’) detached itself from its sister satellite and began a 2-and-a-half-hour descent into Titan. Throughout its descent, the Huygen collected important data about the atmosphere of Titan. Upon landing, the Huygen was also able to take pictures of Titan’s surface. Data collected by the probe was complemented by those gathered on Cassini’s own sensors, and together, this stream of information were to transform our perceptions of our solar system, entirely.

The results obtained by Huygen and Cassini was a turning point in our search for another Earth like planet. Underneath that dense nitrogen atmosphere, Titan unveiled a rich and diverse landscape just like on Earth. From dunes and deserts to lakes and seas, Titan appeared to mimic the features of our planet in its own surreal way.

Pictures taken of Titan’s surface during the descent and landing of the Huygen probe
Image Credit: ESA/NASA/JPL/University of Arizona

However, there is a catch. Unlike on Earth, Titan’s lakes are not constituted of water. Instead its lakes are made primarily of liquefied methane thanks to its extremely cold climate. Methane is a natural gas that is typically present in areas of decay such as landfills, marshes and septic systems. Whenever you smell a disturbing odour that gives off the smell of “rotten eggs”, it is likely due to the presence of methane. Because of these lakes, the clouds on Titan are also made up of this compound. So, instead of raining water, it rains methane.

Furthermore, rather than the silicate-based sand that is present on Earth, ‘sand’ on Titan is made up of ice that has been coated with other compounds.

So how on earth could Titan contain life?

Well, although Titan appears to be inhospitable for species that evolved on Earth, it is still a possibility that Titan hosts its own unique and equally bizarre lifeforms. To prove this, in 2015, scientists back on Earth hypothesised the existence of the “azotosome”.

All living cells (the basic building block of life) on Earth have a plasma membrane which controls what comes in and goes out of the cell. This plasma membrane is constituted of a lipid bilayer, which is essentially two layers of fat stacked together. When a lipid bilayer encircles some liquid, the entire structure is referred to as a liposome. Unfortunately, on Titan the climate is too cold to support the existence of lipid bilayers made of the same fatty compounds as those found on Earth.

Enter azotosomes. Chemical engineers at Cornwell University hypothesise that instead of cell membranes made of fatty lipids, the residents of Titan would have membranes that are primarily composed of methane-based compounds. This membrane, dubbed the “azotosome”, would make use of the natural elements that are abundant in Titans large methane seas such as nitrogen, carbon and hydrogen. Via experimentation, the researchers found that these candidate compounds were indeed able to form primitive cell-like structures which are just as stable as the liposomes back on Earth.

Although hypothetical for now, the vision of these scientists could be very well proved as reality with another mission to discover more of Titan’s hidden secrets in the works. In the near future, NASA plans to send an unmanned submarine to Titan to explore its northernmost sea – a dark, deep and insidious body of liquefied methane aptly named the “Kraken mare”.

Exploring the Kraken mare
Image credit: NASA