2.5 Making plans for Titan
In the video Titan is referred to as the only moon with an atmosphere. In fact, the truth is that Titan is the only moon with a dense atmosphere. Neptune’s largest moon, Triton, also has an appreciable atmosphere. Triton is smaller than the Galilean moons of Jupiter, but with a surface temperature of only −235 °C it has been able to retain a thin atmosphere of 99% nitrogen plus traces of methane and carbon monoxide, with a surface pressure about one-fifty-thousandth of Earth’s (0.02 millibars or about 20 microbars). This may be miniscule, but it is enough to form clouds of tiny nitrogen-ice crystals at heights of a few kilometres, analogous to Earth’s cirrus clouds, which are made of tiny crystals of water-ice.
The Titan Mare Explorer (TiME) referred to near the end of the video was short-listed for development by NASA, but was dropped in favour of a Mars mission in 2012. However in 2019 a different mission concept called Dragonfly was approved. To find out about that, visit the link at the end of this section.
Use the downloadable table to compare some of the characteristics of Titan and Triton.
Download this video clip.Video player: moons_1_vid056.mp4
Transcript
NARRATOR
There is only one moon in the whole Solar System with an atmosphere - Titan, largest moon of Saturn. Shrouded in thick haze, until recently, the surface of this mysterious world remained a mystery.
JOHN ZARNECKI
Every time I look through the telescope and I see Titan, I find it incredible to think that something we designed and built is sitting there on the surface. It landed in 2005, it's there now, it will always be there. Incredible.
ATHENA COUSTENIS
Titan is a very intriguing object. It has been so for centuries now that we've been studying it, 350 years. We had never been able to glimpse through its thick atmosphere before. And so with Cassini-Huygens, this was our chance to go there and explore this exciting new world.
ANNOUNCER
And lift off of the Cassini spacecraft on a billion-mile trek to Saturn.
NARRATOR
Launched in 1997, the Cassini-Huygens spacecraft was the most ambitious mission ever sent to the outer Solar System. The massive Cassini orbiter carried with it a tiny lander called Huygens.
ATHENA COUSTENIS
It took seven and a half years to bring the orbiter and the probe it was carrying out to the Saturnian system. The Huygens probe was launched into Titan in December, 2004. And made a fantastic descent January, 2005, through Titan's atmosphere, landed on the surface. I think, it has been the most thrilling moment in my whole career.
JOHN ZARNECKI
The probe carried six instruments. And one of those was ours, led by the Open University. And it measured a whole range of parameters on the surface. And ours was the first instrument to touch the surface when we landed.
MARK LEESE
When we were initially designing the instrument, we were told that we would get a maximum of three minutes on the surface before the probe died. Perhaps the hardest thing was not actually knowing what we were going to land on and what we have to measure. So the landing was a very, very tense moment.
ANNOUNCER
We think we detected surface.
[APPLAUSE]
JOHN ZARNECKI
It's still hard for me to describe the emotions of that day. You know, it was 15 years of work. It was the culmination of that.
ATHENA COUSTENIS
We are screwed in front of our screens, you know, looking to see the data coming down. We don't know what to expect at all.
In the beginning, you can't see anything. It's the haze. It's the clouds. And all of a sudden, you begin to see these dark lines and this huge dark area in the middle. And you're thinking, what is this? And you don't know. But it's going further down, further down. Before you know it, you're looking at channels.
You see this shoreline, it looks very much like Cote d'Azur. And then you see all these rivers and streaks coming down into the lake. Some of them are bright and some of them are dark. And we still don't know what makes them bright or dark. But we do know that there was a point in time where liquid was flowing through these streaks and these rivers.
I actually saw this image hanging there on the wall that had come just through the computer. And I said, 'What's that Mars image doing right there? Please take it away. We're waiting for Titan.' And my colleague from the team turned around and said, 'That's Titan.' And I felt like - I started shaking. This is Titan? Really? It's got pebbles. It's got colour. I had never imagined this. It's fantastic.
DOMINIC FORTES
The Huygens probe landed on a flat area of fine grained sediments, rounded pebbles, and small channels. Not dissimilar to the area where I'm standing now. I'm on a mudflat by the side of a tidal river estuary. You have lots of fine grained sedimentary material being exposed, lots of little river channels, and small rounded pebbles, rather like this one which I've just picked up.
The liquid here, obviously, is water. But on Titan, it's far too cold for water. It's 180 degrees below zero. And the kind of liquid that we think is present on Titan is a mixture of methane and ethane.
ATHENA COUSTENIS
The Huygens probe actually showed us that there was indication of liquid methane on Titan's surface. We saw it through evaporation. We saw it through the channels. And probably the dry lake.
But since then, we've got real evidence that there's liquid methane on Titan's surface. And that came from the mother spacecraft, from Cassini orbiter, who took those beautiful images in the north pole of Titan showing the extended lakes. That they're there, and also that these lakes are filled with real liquid.
DOMINIC FORTES
We know that the lakes exist both at the north pole of Titan, and at the south pole of Titan. And in Titan's current season, the areal distribution is much greater in the north than in the south.
ATHENA COUSTENIS
On Earth, we have the water vapour cycle that brings us the clouds, the rain, and sun. On Titan, the same thing we have, but with methane.
DOMINIC FORTES
The current thinking is that you get these very high-energy rainfall events-- similar to monsoons on the Earth-- which produce flash flooding and deposit very large amounts of material, both fine grained and coarser material, in a very short period of time. And then the area dries up.
MARK LEESE
We knew we were on some kind of dried up riverbed or dried up lakebed. So my first thought was actually, we hit the right place but the wrong season. The liquid's all gone. So we need to come back here.
NARRATOR
Following the great success of Cassini-Huygens there are now plans to return with a mission called the Titan Mare Explorer.
JOHN ZARNECKI
With Huygens, my hope had always been that we would splash down on Titan. But of course, we didn't know where the seas were. If they even existed. But now with the TiME project, we're going to aim for the centre of one of the largest seas.
MARK LEESE
The plan is that the probe will have a direct entry into Titan, descend under a heat shielded parachute rather like Huygens, and then splash down on the surface. The lifetime for the nominal mission is six Titan days, which is actually 96 Earth days.
JOHN ZARNECKI
Once we're there, we're going to study everything we can about that sea. And then hopefully as we drift in the wind, and we'll end up near one of the shores, we can see how a hydrocarbon sea sculpts a shore on an icy body like Titan.
ATHENA COUSTENIS
For someone like me who has followed the Cassini-Huygens mission from the beginning, there's only one thing we really want to do - we want to go back.
JOHN ZARNECKI
Titan is, of course, unique in our Solar System. The only planetary satellite with an atmosphere. Not just any old atmosphere-- it's a very complex, rich atmosphere. It's got an exotic surface, hydrocarbon seas and lakes, what a mix. And I'm biased, but I think it's the most exciting place in the Solar System.
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See also: The dragonfly mission to Titan. Description and a brief animation about a “rotorcraft” (helicopter) for 2028 launch and 2036 arrival.