The Outer Solar System Holds A Beautiful Surprise
Saturn’s moon Titan has more in common with the Earth than any other known world
The Solar System contains a diverse range of worlds, from the four rocky inner planets to the outer four gas giants which have between them over two hundred known moons. In addition, there are countless millions of other objects including dwarf planets (of which Pluto is one), asteroids and comets. Many of them have been the subject of intense study whether by telescope or visiting spacecraft.
But most of these worlds are very different from the Earth. Mercury and almost all the moons and asteroids have no atmosphere to speak of and most have ancient, geologically inactive surfaces pockmarked with craters. Comets have long elliptical orbits that take them close to the Sun and out to the far reaches of the Solar System. Venus has an overwhelmingly thick, choking atmosphere with the highest surface temperature and pressure of any rocky world in the Solar System. Mars is a cold, dusty, desert world with a thin atmosphere. Both Mars and Venus are thought to have had vast reservoirs of water on their surfaces, but no longer do so.
And yet there is one world in the Solar System that stands out from the rest as being more like the Earth than any other: Titan.
Titan is the largest moon of Saturn, and the second largest in the Solar System (after Ganymede). It was first discovered in 1655 by the great Dutch polymath Christiaan Huygens and his brother Constantijn, who were inspired by Galileo’s discovery of the four largest moons of Jupiter (Io, Europa, Ganymede and Callisto — all of which are visible in binoculars) in 1610, to discover their own moon.
In 1944 the Dutch American astronomer Gerard Kuiper discovered using spectroscopy that Titan has methane in its atmosphere and further observations showed that the atmosphere was hazy and extended.
On Nov 12th 1979 the Voyager I spacecraft few by Titan and discovered that its atmosphere is primarily (~95%) made of nitrogen. And thus the first unique similarity with Earth, for Titan and the Earth are the only two bodies in the Solar System whose atmospheres are dominated by nitrogen.
Voyager I was also able to measure the surface pressure exerted by Titan’s atmosphere and found it to be 1.45 atm (i.e. 1.45 x Earth’s surface pressure) — easily the closest to that of the Earth than any other body in the Solar System.
But these two rather mundane sounding similarities were only the beginning of the parallels to be drawn between Titan and the Earth.
Unfortunately, both Voyagers’ visible-light cameras were unable to penetrate the haze of Titan’s atmosphere to see the surface, and that what was required was to both image Titan in the infrared from close quarters to see though the haze and ideally to send a lander to the surface where in-situ measurements and images could be taken.
Enter the Cassini-Huygens mission.
Cassini-Huygens
Shortly after the Voyager I and Voyager II (which flew by Titan in Aug 1981) spacecraft visited the Saturn system, NASA (National Aeronautics and Space Administration) and ESA (European Space Agency) combined forces to build the Cassini-Huygens spacecraft.
The spacecraft consisted of an orbiter called Cassini (built by NASA), attached to which was a lander called Huygens (built by ESA). And the lander was destined for Titan.
The Cassini-Huygens spacecraft was launched on Oct 15th 1997 and arrived at Saturn on July 1st 2004, whereupon a propulsion burn placed it into orbit around the ringed planet. Cassini was designed to take scientific measurements and images of Saturn, its rings and its moons with Titan being of particular interest.
On Christmas Day 2004 Cassini deployed Huygens towards Titan. Huygens took nearly three weeks to arrive at Titan, during which Cassini, Huygens and Titan encircled Saturn together like a carefully choreographed dance, with Cassini ‘passing’ Huygens to Titan.
Huygens’ journey to Titan was in complete free fall and without any ability to command it from the Cassini orbiter, there was nothing ground engineers could do to alter its course should its trajectory be incorrect. For those involved it must have been the most nail-biting three weeks of their scientific careers.
Accelerating continuously as it approached Titan, Huygens hit Titan‘s upper atmosphere at 25,000 km/h on Jan 14th 2005. Its heat-shield, followed by two parachutes, slowed Huygens down until it landed gently on the surface after a descent through the atmosphere of just under two and half hours.
And what it found was astonishing.
The findings of Huygens
During its descent Titan took images of bright ice-covered hills and darker valleys covered in a mixture of complex organic molecules call thiolins. It also saw a pair of long parallel lines of dunes disappearing off into the distance.
The probe sampled the aerosols in the atmosphere as it descended and found them to consist of nitrogen containing organic molecules.
Intriguingly it saw what appeared to be very Earth-like systems of complex drainage channels cut into hillsides which strongly suggested the presence of liquid runoff either now or in the past.
Closer to the surface Huygens imaged its landing site, which had the appearance of a shoreline into which river channels flowed, although the site itself did not have liquid on the surface. However, upon landing, Huygens detected a sudden release of methane gas from the surface caused by the heat of its landing, indicating that the surface had liquid methane absorbed within it.
Once on the surface, Huygens imaged a sector towards the south. The panorama was remarkably familiar: a dry riverbed scattered with water-ice rocks and pebbles, with hills in the distance. The pebbles were rounded, indicating that they had been shaped by erosion: almost certainly by liquids, not winds, since the surface winds were measured by Huygens to be very slight (1–2 m/s).
Taken together Huygens’ instruments indicated that Titan has a ‘methanological’ cycle, analogous to the hydrological (water) cycle on Earth, in which methane exists in liquid droplets near or on the surface which evaporates into the atmosphere where it is rained down to complete the cycle.
But firm evidence of the presence of liquids on the surface was to come from the Cassini orbiter.
The Cassini orbiter and Titan
Cassini, like Huygens, was fairly bristling with instruments to take images, perform spectroscopy, measure magnetic fields strengths and particle fluxes etc. Crucially it had the RADAR instrument, which was used, among other things, to determine with considerable accuracy the topography of Titan, i.e. how its surface height varies with location. The designers of Cassini knew that standing bodies of liquid have a much lower radar reflectivity than does a solid surface and were hopeful that such ‘radar-dark’ patches would be detected. They were not to be disappointed.
In Jan 2007 it was announced that the Cassini RADAR instrument had detected 75 circular to irregularly shaped radar-dark patches north of +70⁰ latitude. The presence of nearby channels and that the patches were in surface depressions added credence to the conclusion that they were indeed the long sought after lakes.
On July 8th 2009 Cassini observed the glint of reflected sunlight from lake Jingpo Lacus in the northern hemisphere, as shown in the spectacular and beautiful image below. By this point the evidence for substantial bodies of liquid hydrocarbons on Titan was incontrovertible.
Numerous other lakes and three ‘seas’ were subsequently discovered by Cassini. Most are filled with liquid methane and at least one, the footprint-shaped Ontario Lacus, also contains liquid ethane and liquid propane. Several of them are substantial: Kraken Mare, the largest of the three seas, has a surface area of 400,000km² and a maximum depth of 160m. Indeed, the total inventory of hydrocarbons on Titan is thought to be hundreds of times greater than Earth’s.
Ontario Lacus is the largest lake in the southern hemisphere and is notable for its remarkable shallowness: with a surface area of ~15,000km² it has average depth of between 40cm and 3.2m and a maximum depth of between 2.4m and 7.9m.
So here is another similarity between the Earth and Titan: these are the only known worlds in the Solar System to have substantial bodies of liquids on their surface.
Cassini also discovered that the dunes imaged by Huygens during its descent encircle the moon’s equatorial regions. Images of dunes taken by Cassini show them as swathes of multiple parallel lines like finely drawn contours on a map that flow around around any obstacles in their path.
The dunes are though vast compared to dunes on Earth: they are estimated to be about 100m tall, 1–2 km wide and hundred of kilometres long. They are thought to be composed of sand grains made of complex organic molecules that settled out from Titan’s atmosphere, rather than silicates (sand) as they are on Earth.
Titan’s atmosphere: A chemist’s dream
At the top of Titan’s atmosphere its dominant gases, methane and nitrogen, are bombarded with photons from the Sun and charged particles from Saturn’s magnetosphere. These break the atomic bonds in both gases and allow their atoms of both to recombine in numerous ways to form other molecules, many of which have chains and / or rings of carbon atoms.
Titan’s atmosphere has therefore a rich organic chemistry, thought to have much in common with Earth’s early atmosphere. And although Titan’s atmosphere is largely devoid of oxygen, this was also fortunately true in Earth’s early atmosphere as oxygen would’ve been toxic to the earliest organisms.
These conditions produce an extraordinary range of molecules, from the simplest hydrocarbons, (e.g. methane, ethane, propane, acetylene, hydrogen cyanide) to more complex molecules containing rings of carbon atoms, e.g. benzene and PAHs (Polycyclic Aromatic Hydrocarbons), but also molecules that are thousands of times the mass of hydrogen whose structures are unknown called thiolins, which form the haze that obscures the surface in visible light and which also settle onto the surface and give it its characteristic orange hue.
Looking in the mirror
And so, against all the odds, there is a world in the outer Solar System whose features we recognize in so many ways: a substantial nitrogen-rich atmosphere, seas and lakes, river channels and deltas, dunes, clouds and a rich organic chemistry.
Human beings love being near water. Seas, lakes and rivers are among the most beautiful features that Earth has to offer. It’s easy to imagine standing by the shoreline of, say, Kraken Mare, and looking across its mirror-like surface so still and unperturbed, softly reflecting the light of the distant Sun. And in the sky, Saturn, the giant, majestic ringed planet hangs permanently in the sky, completing the familiar, and yet otherworldly, beautiful scene.
All this, and yet Titan isn’t even a planet, but a moon.
How Nature fascinates and up-ends our expectations.