Facts about Titan, the Solar System’s Most Unique Moon

Titan is my favourite place in the Solar System, and for good reason too. Saturn’s largest and most famous moon has been known for centuries but, until the arrival of the Cassini-Huygens probe in 2004, its true nature had been hidden behind its thick clouds. It is precisely this dense atmosphere that makes Titan so unique, but that’s certainly not where the mysteries and peculiarities end. Today we’ll be exploring facts about Titan, the world that’s like Earth in so many ways, albeit with a completely different chemistry leading to some incredibly bizarre results.

Exploring beneath the Clouds of Titan

Kees Veenenbos

An artist’s impression of the Titanian sunset with Saturn in the background accompanied by 3 other moons.

Like the other gas giant planets, Saturn has an extensive system of natural satellites, also known as moons just like our own. While Earth only has one moon, Saturn has at least 62, in addition to its beautiful rings consisting of millions of icy particles and orbital debris. Most of the moons are very small, being only a few miles in diameter. Irregularly shaped, these moons are basically captured asteroids, taken in by the powerful gravitational forces that Saturn exerts on its cosmic surroundings. However, it’s that tiny point of light, visible through any reasonable telescope, that I’m interested in. This world is Titan.

Discovered by the Dutch astronomer, Christiaan Huygens in 1655, Titan is one of the biggest moons in the solar system. Its diameter is almost half again as big as our own moon, and the world is slightly bigger than Mercury. However, the characteristics that make Titan unique are far more impressive, making it one of the most interesting entities in the solar system. Titan is the only world in the solar system which has liquid on the surface and the only moon known to have anything more than just a trace atmosphere.

Rather like Venus, Titan’s impenetrable atmosphere made it impossible to confirm what lay on the surface beneath. Despite decades of close observation, its true nature remained hidden from the world beneath a thick orange blanket some 370 miles (600 km) thick. The Cassini-Huygens mission that arrived at Saturn in 2004 was on a mission to uncover the mysteries of this peculiar alien world. After mapping Titan’s surface by radar, the Cassini-Huygens mission deployed the first (and so far, the only) landing mission in the outer solar system. What they found was fascinating but, while it answered a lot of questions, it posed many more.

Stepping onto the Titanian Surface

NASA

An artist’s rendering of the Huygens landing site, inspired by photos of the surface taken by the lander before it stopped transmitting 90 minutes after touching down.

The first thing that you would notice if you were to walk on the surface of Titan is that it is a very dark world, receiving only one percent of the amount of sunlight that Earth does. This is, in part, due to its far greater distance from the Sun, but it’s also down to the extremely dense atmosphere. It is also frigidly cold, with surface temperatures averaging at around -180°C. When the Huygens probe landed, it wobbled on an unstable surface, sinking several inches into the ground. The surface had the consistency of wet sand or snow. Rocks and pebbles which appear to be made mostly from water ice were scattered across this swampy terrain.

The extremely cold temperatures and surface pressure (about 1.5 times higher than the pressure on Earth at sea level) on Titan allow for methane and ethane to exist in liquid form. This allows Titan to have small lakes and rivers, composed primarily of these chemicals, flowing across the surface just like water does on Earth, only they only cover a few percent of the moon’s surface. In fact, in December 2012, a river remarkably like the Nile was discovered on Titan, flowing with liquid ethane. It meanders into one of Titan’s largest lakes, Ligeia Mare. Adding to the many oddities of this orange moon is also the possibility that it might contain a liquid ocean beneath its soft and crusty surface.

Titan is much smaller and less massive than Earth and, because of this, it has only a fraction of the amount of gravity that Earth has. This is one of the reasons why the atmosphere extends far above the surface. In fact, the entire mass of the atmosphere is about 20% greater than that of Earth. The atmosphere is composed almost entirely of nitrogen with around 1.4% methane and trace amounts of hydrogen and certain other elements.

Storms occur quite regularly over the Titanian surface, particularly in the south pole region where there is a permanent vortex ravaging the land. It also rains a concoction of liquid methane and ethane, albeit only rarely. The Cassini-Huygens mission discovered puddles of the alien liquid on the surface following a storm. Titan even has seasonal weather patterns like Earth although, due to it being a moon, the seasons work in quite a different way to what we’re used to.

The Cassini-Huygens mission also discovered mountains on Titan, indicating that there has been plenty of geological activity in the moon’s more recent history. These mountains are composed of icy materials and are covered by a layer of methane snow. Titan is a geologically young moon, and it has been hypothesised that there may be cryovolcanoes on the surface, which are found on various other moons belonging to Jupiter and Saturn. Instead of throwing molten rock into the atmosphere, they erupt with volatile materials such as methane and ammonia.

After six years of exploration, the orbiting Cassini probe has put together an almost complete map of the Titanian surface, and the results show some geographical features remarkably like those found on Earth. One of the most prominent areas is the Xanadu, a raised region around the equator about the size of Australia. The area is covered in dunes, valleys, rivers and hills. Beside it is the sunken land of Shangri-La, the darkest region of the Titanian surface. These were originally thought to be seas, although recent reports suggest that they are currently dry.

At the time of writing, the Cassini-Huygens mission had just entered its final phase which, after a highly successful run, will eventually end with the orbiting module crashing into the Saturnian clouds early next year.

Extraterrestrial Life on Titan

Just like Earth, Titan has lakes, rivers and seas, albeit with liquid hydrocarbons rather than water. In this artist’s rendering, a small submarine explores one of the moon’s many lakes.

The presence of a dense atmosphere, liquid lakes and rivers and many other characteristics have made Titan one of the top destinations in the search for alien life. On one hand, many things about Titan are instantly familiar to us, but its different chemistry absolutely precludes the existence of any organism that’s remotely Earth-like.

Despite the completely different chemistry of Titan, this does not necessarily mean that life of some sort does not or cannot exist there. After all, on Titan, methane is analogous to water here on Earth. It exists as a solid, liquid and gas and, for this reason, it has been hypothesised that Titan could have a methane-based ecosystem instead. Scientists have recently determined that Titanian conditions, at least those higher up in the atmosphere, are indeed able to give rise to the chemical reactions required to build complex biogenic molecules.

In some ways, Titan has a similar chemistry to that of the prebiotic Earth, albeit an incredibly cold version. In other words, it appears to have everything necessary for life of some kind to evolve, provided that that life consumes hydrogen instead of oxygen and has an ecosystem based on methane instead of water. Science has found little evidence to suggest why this shouldn’t be possible. After all, the earliest life on Earth lived in a world where there was no oxygen in the atmosphere for a start. The only possible hindrance is that the extremely low temperatures may prevent the chemical reactions required for life to evolve but, in the likely event that life has yet to take hold on Titan’s frigid surface, that’s not to say that this will always be the case.

In six-billion years from now, the Sun will have long since swallowed Mercury and Venus, and every trace of life on Earth will be long gone. As the Sun reaches its later years as a red giant star, Titan will become much warmer. There will be a window of opportunity of up to half a billion years in which Titan will develop even more Earth-like characteristics, perhaps even giving basic forms of life a chance to exist long after humanity and almost every trace of its existence has disappeared.

Conclusion

Although we’ve learned much about Titan over the last 12 years, we’ve probably uncovered more questions than revealed answers. Much remains unknown about the surface of this bizarre moon, and scientists continue to mull over the data collected by the enormously successful Cassini-Huygens mission chiefly to make sense of the most unusual chemistry of this alien world.

Unfortunately, it will be some time before we visit the Saturnian moon again, although last year, NASA did award further funding towards a proposal to send a submarine to Titan to explore its methane seas.