2.11 What Huygens found on Titan
John Zarnecki was the lead scientist on the Surface Science Package (SSP) and co-investigator for the Huygens Atmospheric Structure Instrument (HASI) – for the Huygens lander on Titan. In this video he explains the excitement of the first hours after landing and what the team deduced from the data returned to Earth.
Download this video clip.Video player: moons_1_vid064.mp4
Transcript
JOHN ZARNECKI
Well what did we produce? This is what we produced. One floppy disc. This is the entire data take from my instrument, the Surface Science Package, the penetrometer, and eight other sensors.
So there we were, on the evening of January the 14th, with our figures. What did it all mean? We’d foolishly promised that we would appear, all the science teams, would appear in front of the world’s press. We were going to tell them exactly what Titan was all about. So we had to come up with a story.
Well the data that we thought was the most reliable was actually the data from this instrument, the penetrometer. There it is. What is it? A graph - we do like graphs, you might have noticed - of force against depth, as we push into the surface.
We were looking at this, thinking of what we were going to say to the press - what had we done, what was Titan like. And somebody - and we still argue about who it was - piped up from our team, "We’ve landed on crème brûlée!". Look. We’ve hit something hard - there’s a high force, it’s as if we’ve hit a crust. We’ve pushed through that crust. And then there’s something soft, thicker, beneath it.
So half an hour later, I and my colleagues faced the world’s press. And what did I say? "So ähnlich wie crème brûlée," according to the Berliner Zeitung. Similar to crème brûlée, I think it means.
Well maybe it was a quiet day for news, but crème brûlée in Titan was picked up all over the world. There we are, the front page of the Independent. We even made the Sun. It was all over the place.
But of course there were six instruments. Ours was just one of six. What did we learn from the other instruments? Well, there was a camera taking visible images, about a hundred pictures, as we descended through the atmosphere. I’m just going to show you one or two of them.
The first images, from about 150 kilometres -- you can see the height there-- showed absolutely nothing. We were peering into that cloud. All we’re seeing is cloud. And many of the first images were like that.
It wasn’t until we got to about 25 kilometres above the surface, where we started to get a glimpse of that surface. We weren’t sure what it was, but there was something there. A bit further down, 15 kilometres or so - this is in fact a composite -- and we saw a remarkable sight.
We were seeing what look like river channels, with tributaries feeding larger channels. And what was this? Was this a coastline? Were we, in fact, really descending towards a sea or a lake of methane. The scale of this, by the way, is about five kilometres across.
And then finally we landed. And this is what we saw. Absolutely remarkable image. Looks like a desert, but the camera at this point is very low down, just a few inches above the surface, so these pebbles - the largest one there is about six inches across. When I say pebbles, remember Titan is an icy world. So these are pebbles made of ice.
So that’s the picture. But what could we say about this scene? Well let’s take a look then, again, at our data. How do we interpret this? How would we relate this to the surface of Titan. Well it’s obvious, isn’t it? You go to B&Q in Bletchley, and you buy some of this.
Now you probably think that this is material that you buy to make your garden look beautiful. But you’re wrong. This is material which B&Q sells so that planetary scientists can make planetary surfaces.
Either pebbles like these - these are sharp, angular pebbles, maybe like the sort of material that you’d get on a planetary surface if it was bombarded by meteorites, and the surface was broken up. Or material like this. This is much smoother, much rounder pieces.
On Earth, of course, these are produced by the action of water flowing over rock - breaking, chipping off bits of rock, and gradually smoothing them and rounding them off. Maybe the same thing is happening on Titan - not with water and rock, but with liquid methane flowing down those channels that we see, and taking off bits of ice, and rounding and smoothing them off.
So we ran lots of tests, literally hundreds of tests, in the laboratory, with material like this - lots of other materials - to try to understand these graphs. Well to cut a long story short, what we think has happened is that, despite what the picture shows you, we think this little bit of trace here tells us that this whole surface is coated with a thin veneer of something soft. Maybe material which has been raining down out of the atmosphere. Sort of hydrocarbon coating on the surface.
This peak here probably occurred when we hit one of these icy pebbles. We hit it, got a very high resistance, we pushed it aside. And then we pushed down into this material between the pebbles, which is granular material. But it’s icy. So it’s icy chips, probably of this sort of size and consistency. What I can tell you for sure is that we didn’t land on crème brûlée.
What do we learn from all of this? Well one thing that I learn is that pictures are great. They’re wonderful. They tell you a lot. But go for the graphs as well. As scientists, we love graphs, and there’s so much more information in them.
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See also an interview recorded by David Rothery with Sarah Fagents (University of Hawaii) in May 2018 about a proposal to study Titan’s habitability https://www.youtube.com/ watch?v=P7kW8mdSDyg [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)]