Investigating Titan’s Surface
|Jonathan Lunine of the Lunar and Planetary Laboratory at the University of Arizona|
Image Credit: space.com
Jonathan Lunine, a professor of planetary science and physics at the University of Arizona’s Lunar and Planetary Laboratory in Tucson, Arizona, is also an interdisciplinary scientist on the Cassini/Huygens mission. Lunine presented a lecture entitled “Titan: A Personal View after Cassini’s first six months in Saturn orbit” at a NASA Director’s Seminar on January 24, 2005.
This edited transcript of the Director’s Seminar is Part 1 of a 4-part series.
“Much of the evidence for the nature of geology on Titan comes from the radar data from Cassini. The imaging system has also provided excellent information, as has the VIMS, the visible and infrared mapping spectrometer. But they are limited by the atmosphere, where haze scatters light and methane absorbs it.
The first radar images came from the Titan flyby in October. About 1 percent of the surface was mapped, and some very interesting features were seen that tell us something about the potential for internal activity and cryovolcanism on Titan.
|Titan’s changing face as dark and light patches rotate in circulation. Image Credit: JPL/Space Science Institute|
The radar found one feature on Titan that looks like radar data of basaltic flows on the Earth or on Venus. On those planets, lobate, rough terrains are the result of lava that’s poured out and then hardened on the surface. But Titan doesn’t have basaltic flows. Titan presumably has a rock core, but we assume it’s differentiated, because if it were undifferentiated and the rock were to melt and have volcanism, it would melt the ice and Titan would differentiate anyway. So the flow feature on Titan is not rock.
Furthermore, the heat flow on Titan, due to accretional, radiogenic and also some tidal heating, is quite a bit less than 10 percent the Earth’s mean heat flow. So the lava-like feature on Titan must be something else – presumably cryovolcanism, the melting of water ice.
The other striking thing about the radar and imaging data is that they show there’s between a handful and no impact craters on Titan. We expect that the thick atmosphere of Titan would screen out impactors that are smaller than a kilometer, so craters less than 10 kilometers in size should not be present. But we ought to see bigger craters. When we look at the other Saturnian satellites, like Tethys, they’re covered with craters both large and small.
|Titan seen in different filters tuned for atmospheric depths. Click image for larger view. |
Image Credit: Keck
Titan’s lack of craters indicates a very young surface. Craters are either being obliterated by resurfacing, or they’re being buried by organics – tens or hundreds of meters of organic deposits. It could be both.
The DISR camera on Huygens – the descent imager spectral radiometer – had a side-looking, a downward-looking, and an upward-looking imager. The upward-looking was designed to look for the solar aureole, to see optical effects that will allow for the shape and size of the haze particles to be determined. But the probe was swinging so wildly at high altitudes that the side-looking imager saw the solar aureole.
The camera saw dendritic features flowing into a dark flat area, and we thought, maybe the darkest regions are liquid. The probe landed in either a dark area or the boundary between a darker and brighter terrain, and that landing area was solid.
|Icy pebbles on Titan. Click image for larger view. Credit: ESA|
The Huygens probe returned 80 minutes of data while it was on the surface of Titan, and it was still transmitting when the Cassini orbiter got low enough on the horizon that the communications lock was lost. Ground-based radio telescopes on Earth were still listening to the probe for at least another hour more. It was the Energizer bunny in terms of battery performance.
The probe was designed only for an atmospheric descent – the European Space Agency did not want to design a lander – but there was a possibility of up to 30 minutes of transmission on the surface. Just in case the probe survived, the gas chromatograph mass spectrometer (GCMS) had a heated inlet. That was a brilliant decision, because it’s how we found out that liquid methane is at the landing site on Titan. Three minutes after landing, the GCMS detected an increase in methane, which lasted for an hour, and maybe longer – the instrument was probably getting cold by that point because everything was tailing off. We don’t know how much the heated inlet and the lamp on the DISR were heating the surface, because we don’t know how far into the surface the Huygens probe actually went.
The inlet itself is heated to 90 degrees Celsius, well over the critical point of methane. The methane probably didn’t see that temperature below the surface. But there was clearly a condensed phase of methane that was being heated and vaporized and found its way into this GCMS inlet and continued in a stable manner for over an hour. So this was not a little patch of methane, this was a lot of methane. That is the smoking gun, or vaporizing gun, that these dendritic features were carved by methane.
|Shorelines may be dry but intermittently defined by drainage channels of methane rain. Click image for larger view. Credit: ESA|
But what about the ethane? If there’s ethane/methane there, the ethane is a product of the methane photolysis. Shouldn’t we see ethane in the GCMS? The answer is, yes we should, but it wasn’t seen. Now, maybe the ethane’s not there. It drained away, or the photochemistry is not occurring in any significant level as we understand it, or ethane is not the primary product of that chemistry. Or perhaps the temperature to which the subsurface was being subjected by the heated inlet was low enough that ethane, which is three orders of magnitude lower vapor pressure at 95 degrees Kelvin than methane, simply wasn’t being vaporized to the same extent. That’s possible, and still to be determined.
The impact was detected and measured by the surface science package. The surface is soft – it has the consistency of wet sand. But there is either a hard crust right at the top of that, or the probe hit a pebble first, and then bounced into the soft stuff.
Wet sand is consistent with liquid methane in a water-ice matrix, or a solid organic matrix of some kind. The pebbles we see in the surface image are either ice, or they’re some organic crud. We just don’t know.”
Listen to sounds from the microphone onboard the Huygens during its descent (wav file format, approx. 600 kB each):