Voyager, Cassini-Huygens and the Lakes of Titan

Titan, the largest of Saturn’s 53 named moons, has always been one of the most intriguing objects in the Solar System.  It was first discovered by Christian Huygens in 1655, and just using an Earth based telescope it has an appearance of a barely resolved reddish disk. However through spectroscopic observations astronomers quickly realised that this reddish colourisation was caused by something quite special – an atmosphere.

It was not until the Voyager missions that astronomers could get a closer look at Titans atmosphere. Voyager 1 was launched September 5th 1977, with an aim to explore the outer Solar System, but such was the eagerness to discover more about Titans atmosphere that the planners programmed Voyager 1 to pass very close to Titan to take photographs and readings, even though it meant that the spacecraft could not use Saturn’s gravity to propel itself further out of the Solar System to Uranus and Neptune and complete its mission. Instead, Voyager 1 is now flying some 12 billion miles further away from Earth in a direction away from the ecliptic plane -the plane in which all the planets orbit the Sun – and is now being used to study the outer heliosphere at the very edge of the solar system

Voyager 1's image of Titans atmosphere
Voyager 1’s image of Titan’s atmosphere

However despite the spacecraft’s close pass with Titan, its surface remained a mystery, but Voyager 1 was able to provide much more detail about the atmosphere. Radio and Infrared observations showed that the atmosphere was actually made up of mostly Nitrogen (90%) and Argon (just under 10% at most) and a small percentage of methane with traces of hydrogen, hydrocarbons ethane and propane, and carbon monoxide. Due to Titans weaker gravitational pull the atmosphere also extends out some ten times farther into space than our own.

In 1982, a couple of years after the Voyager 1’s primary mission had ended, the European Science Foundation and the American National Academy of Sciences formed a working group which laid the foundations for a Saturn Orbiter and a Titan Probe, which culminated in the launch of the Cassini-Huygens mission on 15th October 1997. Cassini would be the Saturn orbiter and Huygens the Titan probe, and the mission took seven years to reach Saturn, arriving on 1st July 2004. Before Cassini-Huygens arrived though radio astronomers using the Arecibo telescope reported the first evidence for liquid hydrocarbon lakes on Titan, which was something the mission would now try to prove.

Cassini-Huygens on the launch pad
Cassini-Huygens on the launch pad

On 27th October 2004, Cassini made the first of 45 planned fly bys of Titan when it flew just 1200km above the surface and through the atmosphere. It was able to take the first radar images of the moon’s surface, which revealed it to be relatively level. On 25th December, the Huygens probe was released, and it entered Titans atmosphere on the 14th January. As it fell towards Titan it recorded its first image, which was speculated to be drainage channels flowing to a possible shoreline, shown below.  After Huygens landed on the surface it managed to send 90 minutes worth of data back to Earth via Cassini before its batteries ran out. It produced photographs of a dry landscape, but which may have had liquid acting on its surface recently, though hydrocarbon lakes might not have currently existed at the Huygens landing site at the time.

Huygens landing site on Titan
Huygens landing site on Titan
Image from Huygens as it decended 16km above Titans surface, showing the
Image from Huygens as it decended 16km above Titans surface, showing drainage channels to a possible shoreline
Titans surface imaged by Cassini in 2004
Titans surface imaged by Cassini in 2004

On further fly bys between 2006 and 2007, the Cassini probe was able to make radar and camera observations which revealed several large features in the North Polar Region interpreted as large expanses of liquid methane and/or ethane. During a close Cassini flyby in December 2007 the visual and mapping instrument observed a lake in Titan’s South Polar Region. This instrument identified chemically different materials based on the way they absorb and reflect infrared light. Based on this instrument’s observations, scientists concluded that at least one of the large lakes observed on Saturn’s moon Titan does in fact contain liquid, that liquid being hydrocarbons, and have positively identified the presence of ethane. This makes Titan the only object in the Solar system other than Earth known to have stable ambient-temperature liquid on its surface.

Cassini radar images revealing lakes in North Polar Region of Titan
Cassini radar images revealing lakes in North Polar Region of Titan

More recently, in September 2014, the NASA Cassini mission published a report in which they believed that these lakes were replenished by rainfall, though how exactly the liquids move and cycle through Titans crust and atmosphere is still unknown. And within the last month Cassini finally got the opportunity to produce the photograph that astronomers were first expecting when the Cassini probe first arrived at Titan 10 years ago. It caught sunlight glinting off of Titan’s northern lakes in one amazing image, ending all doubt that there is hydrocarbon liquid on Titan.

Near infrared sunlight reflecte of of Titans hydrocarbon lakes
Near infrared sunlight reflected of of Titans hydrocarbon lakes

But what next for Titan? Well already in design is a submarine to someday explore the lakes and seas of Titan, and is part of twelve other proposals being considered by NASA’s Innovative Advanced Concepts (NIAC) programme. Should the proposal get the necessary funding it is hoped that the submarine would be able to use the hydrocarbon ‘sea water’ as a fuel source as it explored the depths of Titan. Its sounds a mad idea, but having just landed a probe on a comet 300 million miles from Earth who knows what’s next in store for our space exploration?

An artists impression of the lakes of Titan
An artists impression of the lakes of Titan

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