|Scientists puzzle over images that resemble complex landforms. Click image for larger view. Image Credit: NASA/JPL
When the Cassini spacecraft flew by Titan on October 26, the RADAR instrument peered through the moon's smoggy chemical haze. In an interview with Astrobiology Magazine editor Leslie Mullen, Ralph Lorenz from the University of Arizona's Lunar and Planetary Lab discussed the surprises revealed by Cassini's RADAR results.
Astrobiology Magazine (AM): Could you describe what the recent RADAR measurements uncovered on Titan?
Ralph Lorenz (RL): Titan is a unique moon in the solar system because it has a thick, mostly nitrogen atmosphere. That atmosphere also carries a little bit of methane and photochemical smog. That smog is made up of organic molecules created through the reaction of sunlight on methane.
The smog makes it difficult to see Titan's surface, but RADAR can be used to see through it. When Cassini flies by Titan during the close approaches - and there will be 44 close approaches to Titan over four years - on half of those we'll point the high-gain antenna on Cassini at Titan's surface.
The RADAR operates in a variety of modes. One is to look straight down and transmit RADAR pulses, and then measure the echo. By measuring the echo time, you can measure how high the surface is, creating a topographic profile.
|Radar image of flowing, contrasting geography. Image Credit: NASA/JPL
We can also listen with the RADAR receiver without transmitting, and measure the microwave heat emission from Titan's surface. That tells us about the texture and composition of Titan's surface on a large scale.
But the mode that attracts all the attention is a mode called "synthetic aperture RADAR," or SAR. The beam of the RADAR is quite wide - about a third of a degree by six degrees, so it covers a large area on Titan's surface. But by using SAR we can pick out features much smaller than that, using the echo time and the Doppler shift on the echo to see features as small as 300 by 500 meters.
From that, we get an image of the RADAR reflectivity of the surface. RADAR is not quite the same as an optical image, which just says how a surface reflects light. RADAR reflectivity can depend on how a surface is oriented with respect to the RADAR. If it's tilted towards the RADAR, it looks brighter; if it's tilted away, it can look darker. If the surface is rough, it scatters RADAR imaging much more effectively than a smooth surface, so bright areas tend to be rougher. And it can depend on what the surface is made of. Rock is more reflective than ice, and organic materials are darker than both of those. All these factors come into play, and so you need an educated eye to understand what you're seeing in a RADAR image.
|True color and surface infrared images show features resembling clouds and a continental area about the size of Australia Image Credit: NASA/JPL
During our first close flyby of Titan - a flyby we refer to as "TA" - we got a big long image, a strip about 150 kilometers wide by several thousand kilometers long. That strip represents about half of one percent of the total surface of the moon. By the end of the mission we'll probably map about 20 percent of the surface with RADAR.
Titan looked nothing like we were expecting. It seems to be relatively free of impact craters. We see impact craters all over the solar system, and one big fear was that Titan might look like Jupiter's moon Callisto, which is this old, dead world covered in impact craters. But Titan seems to be almost free of impact craters in this little bit of the surface we've seen. That suggests that the surface has been covered up, maybe by the deposition of organic materials, maybe by weather erosion, and perhaps by volcanism.
A couple of the features we've seen in our RADAR image seem to be indicative of volcanism. Now, volcanism on Titan would be what we call cryovolcanism, or volcanism based on ice. We think Titan is mostly made of ice and rock, roughly in proportions of 50/50. But all the rock would be at Titan's center, forming a core, and water and ice would form a crust and a mantle around that.
|Titan descent by Huygens probe leaving Cassini storage, Christmas 2004. Image Credit: JPL/Space Science Institute
AM: Could you describe how you get a volcano out of ice?
RL: A volcano based on ice needs heat, because the ice crust would have to melt. The heat would come from Titan's interior. The rocks that make up Titan's core have small amounts of radioactive elements in them. All rocks do. This heat cooks Titan from the bottom, creating a layer of liquid water beneath the surface.
If this liquid can reach the surface, perhaps propagating up through cracks, then it would erupt and flow much like molten rock does on the Earth. One big difference is that, on Earth, water gets mixed into the rock, and then the water boils, makes a lot of steam, and that blows out to form big conical-shaped volcanoes like Mount Fuji in Japan.
We think that doesn't happen on Titan because the place of molten rock is taken by water, and the place of water vapor in the lava might be taken by methane. And methane doesn't dissolve very well in water - much less than water dissolves in molten rock - so there wouldn't be this driving, explosive force in volcanoes on Titan. We expect maybe this stuff would just ooze and bubble out of the surface.
The features we've seen with RADAR include one large, pancake-shaped dome. We've seen features like that on the planet Venus. Venus has a very high atmospheric pressure, and that stops the bubbles in magma from expanding. There's another feature on Titan that looks like something has flowed across the surface. It's branched and lobate, like a lava flow.
There could be all sorts of explanations for both of these features, but in this first look at Titan, seeing these is quite interesting.
|Huygens' probe will enter Titan's thick atmosphere and may record alien thunder on its microphone.
It's interesting from an astrobiological point of view in particular. The photochemistry in Titan's atmosphere produces lots of organic compounds, and about 20 compounds have been identified already. But it's an evolutionary dead-end. Titan's atmosphere is so cold, there's very little oxygen or oxygen-bearing compounds in there. All the compounds that have been detected are hydrocarbons - compounds containing carbon and hydrogen - or nitriles - compounds with carbon, hydrogen, and nitrogen. But if this stuff rains out onto the surface, and it interacts with liquid water at any stage, you very quickly form a lot of oxygen-bearing compounds like amino acids. This has been done in the laboratory, and it only takes a few hours.
On Titan, if we have cryovolcanic lava flows of liquid water interacting with this organic stuff that rains out of the atmosphere, it could take thousands of years to freeze solid. As it freezes, it progressively concentrates the stuff that's dissolved in it, and makes an interesting experiment in pre-biotic chemistry. It's an experiment we can't do in the laboratory, because we don't have thousands of years.
So going back to Titan in the future with some sort of vehicle that can sample frozen material, at points where we think there have been these interesting interactions, might tell us how chemical systems increase in complexity and ultimately get to the pre-biotic stages from where life came.
|Cassini image of Saturn rings Image Credit: JPL/NASA
AM: Is the presence of liquid water on Titan still just a hypothesis?
RL: Yes. We know that Titan is made of water and rock, because that's just what the universe is made of. We think Titan is big enough that, as it formed, there was enough kinetic energy released from the in-falling protoplanets to melt the ice, so the rock would have sunk to the interior. So we have this idea of Titan with a rock core and an ice shell.
The heat released from the radio isotopes in the rocks has to get out from Titan's interior. It can do that by just being conducted through material, or by the material moving in convective currents, similar to those that help move the continents around on Earth, or the bubbling that you see in a pan of water on the stove. If that convection is not quick enough, then the material warms to the point where ice melts.
Another factor at play on Titan is its nitrogen atmosphere. That atmosphere came from somewhere, and one of the possibilities is it came from ammonia that was trapped in the ice from which Titan formed. Ammonia acts as antifreeze because it lowers the freezing point of water, and that makes it much easier for Titan to have a liquid layer in its interior, perhaps tens to hundreds of kilometers below the surface.
|The surface of Saturn's moon Titan is hidden beneath a thick hydrocarbon smog. Image Credit: NASA/JPL
The Galileo probe showed that Jupiter's moon Europa has a water layer beneath an ice crust. That wasn't a total surprise, because Europa has a lot of tidal heating as well as radiogenic heating. But Galileo also found that both Ganymede and Callisto seem to have interior water layers. And both Callisto and Ganymede are the same size as Titan.
Determining whether Titan has a liquid water layer is something Cassini will measure directly by measuring the gravity field - the shape of Titan - at different parts in Titan's orbit. Titan's orbit is a little bit eccentric, so it gets flexed, and that distortion depends on how strong Titan is. If Titan is a solid lump of ice, it's quite rigid and it won't change shape much. But if it has a liquid water layer in its interior, then it will flex. So there's a lot of excitement to come in the future measurements of Cassini.
Related Web Pages
Lifting Titan's Veil, by Ralph Lorenz
Saturn Edition, Astrobiology Magaz.
Saturn's Rings in UV
Cassini Closes In on Saturn
Saturn-- JPL Cassini Main Page
Lord of the Rings
Space Science Institute, Imaging Team Boulder, Colorado
Saturn: The Closest Pass
Where is Cassini Now?