Wreathed in its trademark orange haze of cloud, morning mist and naturally produced photochemical smog, Titan, the biggest of Saturn's huge retinue of moons, is one of the solar system's most intriguing worlds.
With its organic-rich chemistry and Earth-like processes sculpting the surface, Titan resembles a frozen version of our home planet, several billion years ago, before primordial life started pumping oxygen into the atmosphere.
Recent observations reveal that the gas giant's super-sized natural satellite – at 5150 kilometres in diameter, larger than the planet Mercury – has a river stretching towards a darkish delta for about 400 kilometres, following an almost straight course not unlike Egypt's Nile.
Much of what has been gleaned about this enigmatic alien realm has mystified scientists. But not for much longer: NASA's Saturn-orbiting spacecraft, Cassini, will today (February 17, US time) swoop low across Titan's surface, making crucial measurements of what scientists believe is a subsurface sea containing as much liquid as all the Earth's oceans combined.
"Cassini's current flyby is primarily dedicated to measuring Titan's gravity," says Elizabeth Turtle of Johns Hopkins University's Applied Physics Laboratory in the US. A better understanding of the moon's gravity will reveal more about the size and composition of its subsurface sea.
Cassini, named after the Italian astronomer who made several discoveries about the ringed planet and its multiple moons, will fly within 1978 kilometres of the surface during the orbiter's closest approach, Dr Turtle explains.
"The analysis of gravity data is quite involved and so it would be some months before the implications for Titan's interior structure are understood."
A team of scientists, led by Luciano Iess of Italy's Sapienza University, previously studied how Titan's gravity and shape alter as the giant moon orbits its parent planet. The scientists calculated the liquid's viscosity and found it to be consistent with that of water. If true, this would offer the chance of finding extraterrestrial life.
It is not yet possible to peer into inside Titan, says CSIRO astrophysicist Kurt Liffman. So the presence of a liquid ocean is based on models of the interior. "Measurements of Titan's shape as the moon travels around Saturn can be compared with predictions from different models," Dr Liffman notes.
Gravitational forces on Titan are inversely proportional to the distance between Titan and Saturn. "So when Titan is closer to Saturn, it is stretched more into an Aussie-rules football-like shape – with the long axis pointing towards Saturn – relative to when it is further way," Dr Liffman says. "In Titan's case, the stretching is too large for a solid body of rock and ice."
Testing the waters
Some scientists believe that, in addition to vast quantities of water, Titan's ocean might also contain ammonia and perhaps methane.
"Based on the estimated abundances of nitrogen, hydrogen and carbon, plus the expected temperatures and pressures in the Saturnian region of the disc of gas and dust that surrounded the early sun, it is reasonable to expect the presence of ammonia and methane within Titan," Dr Liffman says.
In the view of Monash University astrophysicist Andrew Prentice, the subsurface ocean most likely consists of pure methane and beneath that a layer of ammonia ice that is quite soft.
"My model predicts a fairly thin layer of liquid methane about six kilometres deep and located 50 kilometres below Titan's outer solid crust of water-ice," Dr Prentice explains. "Beneath the liquid layer there is probably a 14-kilometre-thick layer of soft ammonia ice."
It would be very tough for any life-form to survive in a subsurface ocean since there is so little sunlight that reaches the surface, Dr Prentice says. "Titan's source of internal heat through the decay of short lived radio nuclides is also very tiny."
By contrast, Jupiter's large Galilean moon Europa has a greater chance of harbouring life-forms.
Moon in profile
Discovered in 1655 by Christiaan Huygens, Titan has been scrutinised by telescopes and, more recently, by spacecraft.
In 2004, a small NASA probe called Huygens separated from its mother craft, Cassini. After several weeks, the probe parachuted down to Titan's surface and, for several hours, relayed close-up images to Earth.
"The Huygens probe imaged channels, but its descent through the atmosphere was at low latitudes and so it didn't detect any lakes," Dr Turtle says.
Since then, scientists have deduced that beneath the moon's clouds lurk large lakes, some spanning thousands of square kilometres. That would make them roughly the size of the Black Sea or Caspian Sea; one is thought to be bigger than Lake Superior, the world's largest freshwater lake by surface area.
"The lakes and seas are found at high latitudes and predominantly in the northern hemisphere, although there are some near the southern pole," Dr Turtle adds.
"There appear to be no permanent bodies of liquid at equatorial latitudes. However, there was once a large rainstorm over low latitudes and there may still be some areas of ponded runoff – although they've been receding."
Titan has seasons, says Candy Hansen, Cassini scientist at NASA's Jet Propulsion Laboratory in Pasadena, California.
"That's partly why there are more lakes right now in the northern hemisphere, and why some lakes are evaporating," Dr Hansen explains. "These seasonal changes occurred as the sun has moved from the southern to the northern hemisphere in the last eight years that Cassini has been exploring Titan."
The differences between the northern and southern summers are also caused by Saturn's orbital eccentricity and possibly longer-term orbital variations, she adds.
Titan is in a state of deep freeze, with surface temperatures averaging about minus 180 degrees. If the lakes contained water they would be frozen solid. But these lakes hold liquid hydrocarbons, mainly ethane and some methane, that freeze at much lower temperatures than water.
The remote moon has something resembling Earth's water cycle, with surface liquids evaporating and returning as rain. But on Titan, it rains ethane and methane – not water – the only moon in our solar system to have such a cycle.
At Titan's South Pole, the Cassini orbiter once detected fog, caused by surface methane evaporating and then condensing into humid air.
Other likely surface features include wandering streams and riverbeds, probably carved by liquid ethane, beaches – that get more exposed as the lakes they border slowly evaporate – plains, ancient craters and volcanoes, though probably not active ones.
A question of life
Could life exist in Titan's liquid ethane lake? Weird life maybe, suggests Dr Prentice. But even the hardiest of microbes would struggle to tough it out, he says.
Titan appeals to life scientists, in part, because its planet-like atmosphere is denser than that of Mercury, Earth, Mars or Pluto. It is, in fact, the solar system's only moon with a thick atmosphere and a pressure about 50 per cent higher than that at sea level on Earth.
Its atmosphere consists mainly of nitrogen, the Earth's main gas – as well as hydrogen. (Some pundits say it might be similar to what the atmosphere was like here many aeons ago.)
Titan is also the only place in the solar system, apart from Earth, to have liquid both on and beneath its surface.
Cassini's next flyby of Titan will be April 5, at 1400 kilometres, followed by another on May 23 at 970 kilometres.
The Titan Saturn System Mission was a study performed a few years ago of a flagship mission to the Saturnian system, including an exploration of Titan via a montgolfiere (a hot-air balloon), and a lake lander, explains Dr Turtle. "Unfortunately there are no plans to fly it at this point."
Another proposed mission, known as the Titan Mare Explorer (TiME), was proposed to NASA's Discovery Program and was one of three missions selected for further study.
NASA announced last year its selection of one of these missions to fly. "Unfortunately for Titan, TiME was not funded," Dr Turtle says.
"The chosen Discovery mission is InSight, a long-awaited Mars geophysical lander, which will perform seismic and temperature measurements – by means of a subsurface probe – as well as precise measurements of orbital motions. This will help answer important questions about Mars's deep interior and the formation of the terrestrial planets."
Check out details of previous flybys at:
Read about Cassini's latest activities at:
Explore the Planetary Society's account of all Cassini encounters at: www.planetary.org/explore/space-topics/space-missions/cassinis-tour.html
Learn more about gravity, and how it works, in the schools textbook Oxford Big Ideas (Australian curriculum) Science 7 (Oxford University Press, 2011)
Please send bright ideas for new topics to email@example.com