The ‘suicide’ plunge of NASA’s Cassini spacecraft into Saturn later today marks the end of an amazing 13-year-long mission during which the redoubtable spacecraft made many important discoveries about Saturn and two of its icy moons. These include more accurate measurements of Saturn’s magnetic field and a better understanding of its famed ring system, which is an excellent model for the gas and dust that girdled the sun billions of years ago from which the planets formed. But Cassini’s death dive also spells the beginning of a longer mission for scientists to resolve one of the greatest mysteries of the solar system: does Titan, Saturn’s giant moon, harbour life as the available evidence suggests?
Launched in 1997, Cassini was one of the biggest interplanetary exploration missions ever, and had the European lander Huygens piggybacking on it on a seven-year voyage to the Saturn system. In January 2005, Huygens parachuted on to Titan’s surface and sent back the first detailed images of the alien moon—images which suggested liquid pools of a strange substance that Cassini would later identify as methane.
One of the important reasons behind NASA’s decision to send Cassini nosediving on Saturn was to prevent the spacecraft from accidentally wandering too close to Titan and Saturn’s other major moon, Enceladus. For there was a risk of contaminating them with possible terrestrial germs that might have stowed away on board the spacecraft. Titan is home to seas, rivers and lakes of methane while Enceladus has a subsurface ocean hidden under a thick cover—both potential havens for micro-organisms.
Cassini science team members hug as the final loss of signal from the Cassini spacecraft is confirmed | AFP
For a world barely 500 km across, Enceladus has amazed scientists no end. Once Cassini confirmed the existence of a mysterious watery underworld beneath the moon’s frozen shell, scientists wondered how water could exist in liquid form so far from the Sun. One theory is that the gravitational tugs of the mother planet (Saturn) and other moons have kept the ocean water sloshing around without freezing. In which case, it is possible hydrothermal springs abound in the ocean and they provide energy for any micro-organisms that may exist in this dark domain. We will only know for sure when NASA sends its Enceladus Life Finder mission to look for organic molecules on the alien moon in a couple of years.
More than Enceladus, however, Titan is the real focus for planetary scientists who believe the giant moon could be a prebiotic laboratory of sorts, with its atmosphere rich in nitrogen and other carbon-based compounds like methane. Larger than Mercury and Pluto, Titan is the second biggest moon in the solar system (after Jupiter’s Ganymede) and the only one in the solar system known to have an atmosphere. The atmosphere cloaks it with an opaque orange haze, much denser than Earth's own atmosphere and, unlike other moons of the solar system, this had earlier prevented astronomers from peering at Titan through terrestrial telescopes. But planetary scientists use radio telescopes and spacecraft-borne radar to listen rather than look, as radio waves can easily penetrate the haze. Cassini’s radar waves bouncing back from Titan’s surface—it takes an hour and a quarter for signals traveling at the speed of light to reach Earth—allowed scientists to hear the whispers of a mysterious moon that spoke of its complex organic chemistry which was so amazingly similar to that of Earth in its infancy.
Although Titan is a very cold place, where the warmest temperatures never rise above –180° C, Cassini’s data reveal an atmosphere of yellow clouds and lakes, rivers and oceans of liquid hydro-carbons on the frozen surface, which is criss-crossed by ridges and great wind-blown streaks. This is a lot like what Earth probably looked like 4.6 billion years ago when simple life forms started emerging here. In fact, even today, a class of terrestrial bacteria called methanogens breathes out methane as a waste product. So it is quite possible that something could have triggered the organic molecules on Titan to engender life three billion years ago, as it had happened on early Earth. This notion was strengthened in 2006 when Cassini made the dramatic discovery of enormous amounts of vinyl cyanide in Titan’s atmosphere. It is not implausible that molecules of vinyl cyanide could have reached Titan’s surface during rains and gathered into compounds with biological membranes of the kind that sheathe living cells on Earth today.
The more scientists discover about the outer planets like Saturn, the more they get to learn how Earth and the rest of the inner solar system were formed. Cassini has enormously helped us in this endeavour. And perhaps even more importantly, the data sent back by this doughty little mechanical explorer has changed the very way we look at habitable worlds beyond our own. This is the lasting legacy of Cassini.
