21 December 2010

Strange new world

Cosmos Magazine
Saturn's enigmatic moon Titan has turned out to be an unexpected treasure trove of Earth-like landscapes and bizarre weather systems – and there are even tantalising hints of a vast and warm underground sea sloshing inside.

The first light of dawn is reflected off a lake on Titan. This image, taken by NASA's Cassini spacecraft on 17 December 2009, confirmed the presence of liquid in the moon's northern hemisphere. Credit: NASA

FOR THE PAST 25 years, John Zarnecki has been haunted by a double nightmare. The physicist – head of the space sciences department at Britain’s Open University in Milton Keynes – led the team that designed, tested and manufactured key instruments for Europe’s spaceprobe, Huygens. The craft took 10 years to construct and required a further eight years to reach its target: Titan, the giant moon of Saturn.

“All that time, I lived with the prospect of the project being cancelled and then, once Huygens was on the launch pad, that it would blow up or that there would be a major breakdown when it reached deep space,” he recalls.

Hundreds of other engineers, physicists, chemists and administrators who had helped build and launch Huygens were in a similar nervous situation, of course. Hence the renewed tension when, on 14 January 2005, at the European Space Operations Centre in Darmstadt, Germany, the probe, which had been carried to Saturn on NASA’s spacecraft Cassini, swept into Titan’s thick orange atmosphere.

“If Huygens had failed, it would have been a disaster. We knew we wouldn’t get another shot at Titan for 20 years,” says Zarnecki.

That was bad enough. But Zarnecki was afflicted by another unpleasant dream. He and his colleagues had spent years persuading the European Space Agency (ESA) to fund Huygens by talking up Titan as if it was the Solar System’s hottest piece of scientific real estate.

“To convince the administrators who handled the dosh, we told them this was a great place to visit. Yet we only had a few hunches that Titan might be interesting.”

In 1980, the Voyager 1 spacecraft had flown past Titan and returned images of a world covered by an orange photochemical haze of nitrogen, methane and a few other hydrocarbon gases. The surface temperature was pinpointed at -179˚C. Not much to get excited about, you would have thought.

However, calculations suggested that, at this temperature, methane can behave like a gas, a liquid or a solid – much as water behaves on Earth: as ice, as vapour or as water. There could be seas of methane on Titan, it was argued. It was also possible that complex hydrocarbons were being made in the upper atmosphere where the Sun’s ultraviolet radiation was interacting with methane. And on the ground, channels could be bursting with torrents of liquid methane.

“That’s the picture we sold to the agency,” says Zarnecki. “But I was never fully convinced. At the back of my mind, I couldn’t help worrying that we would end up with egg on our faces, that Titan would turn out to be really dull and boring – just a lump of ice, with a smooth, bland icy surface with nothing going on there.”

Then Huygens began its two-and-a-half hour descent by parachute through Titan’s atmosphere, beaming back data from its sensors and cameras to Cassini, which was sweeping through space near the moon. Eventually, the probe – named after the 17th century Dutch astronomer Christiaan Huygens, who discovered Titan – settled gently on the moon’s surface, still transmitting data.

Its signals were then relayed by Cassini across the void to ESA scientists and engineers. Even at the speed of light, they took a further 67 minutes to reach Earth.

This was, to put it mildly, a cliffhanger of a mission. I was in the Darmstadt control room that day, and I can vividly remember the atmosphere: it crackled with tension and nervous anticipation. Then, the first results came in. They showed the probe had worked perfectly and, more to the point, they revealed Titan to be one of the most unusual places ever visited by a spacecraft. Zarnecki’s twin nightmares were unfounded.

“We could see river channels, lakes and seas, dunes, and possibly low volcanism, some sort of movement of the crust,” he says. “There was weather. There was meteorology in the lower atmosphere. Everything that we had promised was there, and a whole lot more. But how it all fitted together – well, we are only just beginning to understand that now.”

IN FACT, THE DATA sent back by Huygens was limited: the equivalent of a single flash memory card. While this may not be much in the grand scheme of things, it is enough to keep scientists happy.

It’s the first time any such data has been obtained from the surface. “It was not the quantity that mattered in this case, but the quality, and this really was fantastic data. And until we go back to Titan in 15 or 20 years, we ain’t going to do much better,” adds Zarnecki.

Certainly, the past five years of analyses of Huygens’ results has revealed a startling world, one that is even stranger than anticipated: its chemistry and meteorology is more complex than Mars’s; it is bigger than Mercury; and its atmosphere is thicker and denser than Earth’s.

“This is a moon that would be a planet,” wrote Ralph Lorenz and Christophe Sotin, two U.S.-based scientists who worked on the Cassini-Huygens mission, in an article in Scientific American in early 2010.

Just consider those lakes of methane. They make Titan the only object in the Solar System, apart from Earth, that has large bodies of liquid on its surface. Radar images, returned by Cassini as it has swept past Titan on its orbit round Saturn, have revealed several great seas – Ligeia Mare, Kraken Mare and Punga Mare – which are made up of a dark liquid that is almost certainly methane.

“They look like seas, they behave like seas, and as far as I am concerned, they are seas,” says Zarnecki. “Some radar images even show rivers flowing into them, while other pictures – taken years apart – show these mare regions contracting and expanding. In other words, there are ‘wet’ and ‘dry’ seasons on Titan.”

And just as water evaporates from seas and lakes on Earth to create clouds and rain, so the methane seas of Titan generate their own meteorology, albeit a very unusual one. Just consider how the rain falls on Titan. The moon’s gravity is one seventh that of Earth, so raindrops (of methane, of course) will be quite large, because gravity plays a key role in limiting the size of raindrops on Earth.

Titan’s atmosphere is also much thicker than that of Earth, so those drops will fall slowly and gently. In addition, Huygens confirmed speculation that complex hydrocarbons are indeed being created in Titan’s upper atmosphere. Thus the rain on Titan falls in gently descending, large blobs of oily hydrocarbons – a truly surreal prospect.

Given the presence of this organic raw material, it is perhaps surprising that Titan is not bursting with life.

The trouble is that the place is so cold. Think of it more as a chilled leftover from the formation of the early Solar System, suggests Alphonso Diaz, science associate administrator at NASA, which collaborated with Europe on Huygens.

“Titan is a time machine that gives us a chance to look at conditions that existed on early Earth,” he said after results from the probe were received at Darmstadt.

And then there are the dunes. Some are over 100 m high, matching Earth’s largest – such as the dunes of the Namib and Saharan deserts – and stretch for hundreds of kilometres, vast ripples on the surface of Titan created by the powerful winds that sweep across this strange world. However, these dunes are not made of sand, as on Earth, but consist of mounds of solid hydrocarbons – “rather like heaps of coffee grounds,” say Lorenz and Sotin.

On top of these wonders, Huygens also discovered ice on Titan. As the probe settled on the moon’s surface, cameras photographed a landscape of pebbles, which are made, not of stone, but of ice – evidence that water exists in at least one form on this distant world.

If it wasn’t for the tremendous cold, life could be a beach on Titan. And you never know, says Zarnecki, conditions could one day get a lot sunnier.

“What is going to happen in a couple of billion years when the Sun turns into a red giant star and expands? The oceans on Earth will boil away and we will be enveloped. Earth won’t be a good place to be. But what about Titan? The temperature there will go up a couple of hundred degrees. The ice will start melting. This will be the new Eden.”

In fact, Titan may not have to wait that long to flourish. Life could have already evolved there, some scientists argue. A meteorite impact on the moon could release sufficient energy to melt those ice rocks that pepper its surface and create a nice warm lake filled with hydrocarbons. Perfect for the spontaneous generation of simple lifeforms.

“The key question is: would that meteorite-impact lake last long enough to create life?” adds Zarnecki. “I don’t know, but it would be a nice experiment to try.”

In any case, there is another, even more exciting route that could have led to the evolution of life on Titan, say scientists. Detailed observations of the moon’s surface features, picked out by Cassini’s radar imaging cameras, have revealed an odd feature: the moon’s rotation is not constant.

There are noticeable changes in the length of the day on Titan. On its own, this is not particularly significant. Scientists have also measured tiny discrepancies in the spins of Earth and Mars, a phenomenon that produces minute variations in the length of a day here and on the red planet.

However, the spin discrepancies observed on Titan are an order of magnitude greater. “So how can you explain that?” asks Zarnecki. “Well, the only currently plausible explanation goes as follows. Deep below the surface there is a layer of liquid that is tens of kilometres thick and which covers the entire interior of Titan.

“The surface is therefore decoupled from the core of Titan. In effect, the liquid layer acts as the fluid in a giant ball-bearing which allows Titan’s crust and core to spin at different rates, hence those irregularities in its spin and the length of day there.”

As to the make-up of that liquid layer, most evidence suggests that it is made up of water. “Electrical measurements made by Huygens [have] hinted at an electrically conductive layer of material about 45 km below the surface, and water is the prime candidate,” state Lorenz and Sotin.

This only leaves one key question to answer: is this layer made of liquid water or solid ice? Zarnecki believes all the signs suggest the former. “We know from density measurements of Titan that it almost certainly has a rocky core, as Earth has,” says Zarnecki.

“And if that core contains radioactive isotopes, it will be kept hot by the radiation released – just as it does on Earth. And thanks to Huygens, which carried a mass spectrometer to Titan, a radioactive isotope of argon that is known as argon-40 has already been detected. The crucial point is that argon-40 is produced by the decay of a radioactive isotope of potassium called potassium-40, which is found in rocks on Earth.

“In other words, we can now assume that radioactive decay is also occurring in the rocks of Titan, which strongly supports the idea radioactive decay is occurring in its rocky core, which will be, therefore, hot – thus keeping that vast underground layer of water in a liquid state,” he explains. “It is another fabulous result.”

Thus, scientists have discovered there is a rich soup of hydrocarbons on the surface of Titan, while the moon also seems to have a subterranean ocean of water – a combination which suddenly makes this remote moon of Saturn a very interesting destination indeed.

The question is: can that soup and that underground ocean mix together? In other words, have complex hydrocarbons been filtering down to a warm underground ocean where they could have evolved into primitive lifeforms?

Could there be thriving colonies of bugs living happily below Titan’s surface? It’s a possibility that has Zarnecki and others excited.

Not surprisingly, this makes planetary scientists very keen to return to Titan soon – like yesterday. And although the moon is very remote – Huygens travelled almost 3.2 billion km to get there – it does have advantages over other worlds in our Solar System when it comes to exploration. In particular, Titan has a very thick atmosphere and a very low gravity: ideal conditions for flying, either by plane or balloon.

“From what we know about the winds on Titan, we reckon we could do a balloon trip round it in a couple of weeks,” says Zarnecki. “We could do several trips and survey the surface, pick out the really interesting areas and then get lower to dangle some instruments over the side. We could let a rope down and dip it into the lakes. Or we could sail robot ships on its methane seas and become the first extraterrestrial mariners.”

It sounds fantastic. Nevertheless just such a mission is now being contemplated by NASA and ESA. This would be a joint uncrewed mission – dubbed TANDEM, for ‘Titan AND Enceladus Mission’ – which would journey to Saturn’s two most interesting moons, Titan and tiny Enceladus (from which Cassini observed geyser-like plumes of liquid water and organic material being sprayed from surface fractures).

Two separate spacecraft would be carried to Saturn and would then make their individual ways to the two moons to study them. The Titan element would carry some form of aircraft, and possibly a boat.

Such a mission would be enormously expensive, however, costing several billion dollars. But there is an alternative. NASA’s Discovery Program (a series of highly focussed, cost-capped scientific missions, dubbed “faster, better, cheaper”) envisions a mission called ‘TiME’ – for Titan Mare Explorer – that would cost around US$350 million.

It involves using an Atlas rocket to fire a probe directly at Titan in 2015 so that the probe would splashdown on the Ligeia Mare without any complicated manoeuvring around the moons of Saturn. “We are pretty confident we can land that way and carry a suite of instruments dedicated to liquid measurements,” says Zarnecki.

Thus Titan – a fuzzy orange pinprick of light when observed by telescope from Earth – has revealed itself to be one of the most fantastic destinations in the Solar System and the subject of a range of missions that wouldn’t have seemed credible a decade ago.

“Titan used to give me nightmares, but now I cannot wait to be involved in another mission that will take us back there,” says Zarnecki. “It has proven to be everything we dreamt about and a great deal more. One day we are going to fly round it in an airship or sail on its methane seas or burrow below its surface for signs of simple biology on Titan. It’s an astronomer’s dream come true.”

Robin McKie, the London-based contributing editor of Cosmos, is science editor of Britain’s The Observer.

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