Search for life

into the unknown

During the early seventies, our knowledge about outer planets, Jupiter, Saturn, Uranus and Neptune, was derived from observations with the naked eye and telescopes. In fact, we knew very little about the outer planets then. In 1610, Galileo discovered Jupiter’s four largest moons, Io, Europa, Ganymede and Callisto, using a telescope. The first telescopic observation of Saturn was made by him in the same year. However, Galileo was not able to distinguish Saturn’s rings. Instead, he thought the planet may have two large moons on either side of it or there were large ear-like structures around the planet.

The Dutch astronomer Christiaan Huygens observed Saturn half a century later, in 1659. As his telescope was better, he realised that the ‘arms’ around Saturn were actually a system of rings. He was also the first to observe Saturn’s moon, Titan. Better telescopes have since helped reveal that the rings are really a system of particles. The Italian astronomer Giovanni Domenico Cassini discovered four other major moons of Saturn: Iapetus and Rhea, in 1671 and Tethys and Dione, in 1684. The first planet found with the help of a telescope was Uranus, discovered by Sir William Hershel in 1781. Neptune was first mathematically predicted and then it was observed in 1846. This discovery confirmed, in a very dramatic fashion, Isaac Newton’s theory of gravitation.

Exploring the beyond
The exploration of outer planets by space probes began in March 1972 with National Aeronautics and Space Administration (NASA), the US government’s space agency, launching Pioneer 10. Before this, no man- made vehicle had passed through the asteroid belt into the Outer Solar System. When Pioneer 10, became the first probe to traverse this region between Mars and Jupiter, scientists were relieved to find that the journey was not as hazardous as thought to be.

They then launched Pioneer 11 in April 1973 which reached Jupiter a year later. Thereafter, it headed towards Saturn and in 1979 became the first craft to explore the ringed planet. The two Voyager probes, which were launched in 1977, took advantage of the unique alignment of the Outer Planets to embark on a Grand Tour of the Solar System. Voyager 1 reached Jupiter in 1979, before flying to Saturn in 1980. Voyager 2 following behind was able to ‘slingshot’ around Saturn in 1981, hurtling on to Uranus in 1986 and Neptune in 1989. Later visits to Jupiter and Saturn were made by NASA’s Galileo Orbiter in 1989 and more recently by the Cassini/Huygens spacecraft, respectively.

The more scientists studied the data, of visiting probes and the orbiting Hubble Space Telescope returned from some of these far-off worlds, the more it became apparent that the conditions for life could indeed exist on some of them. But it was not the giant planets that had the greater possibility of harbouring life, but their moons. Previously scientists thought that with very little to no warmth from the Sun, the satellites of the Outer Planets, would be just frozen balls of ice and rock.

There are a handful of satellites in the outer Solar System that are very interesting from the perspective of astrobiology: Europa, Enceladus and Ganymede. Spacecraft mission results from Galileo and more recently, Cassini have provided significant evidence of a liquid ocean under an icy crust for each of these satellites. Europa, a satellite of Saturn, has a rocky mantle and a metallic outer core like Earth. But, its crust is icy and below the crust might lie an ocean larger than the Earth’s ocean. This ocean is heated by tidal forces of Jupiter: which is orders of magnitude higher than the tidal force in the Earth-Moon System. Enceladus, another satellite of Saturn, is believed to have an ocean at least 10 km deep: the lateral extent of the ocean is not clear. Significantly, Cassini detected huge water-ice geysers on Enceladus. The plumes contain ice particles, organic molecules and water molecule. Jupiter’s huge moon Ganymede, which is the biggest moon of the Solar System, also has a very large salty ocean, beneath its frozen surface. Some of the other satellites of the outer planets which may also have hidden oceans include Saturn’s moon Titan and Neptune’s moon Triton.

Subterranean ocean’s presence
A common thread that links some of the satellites of the Outer Planets is the presence of a subterranean ocean. On Earth, water is invariably associated with life. The burning question of course, is whether these oceans represent habitable environments where life exists today or could have existed in the past. Life in these oceans cannot be photoautotrophic because the oceans are subterranean. The lifeform has to be able to withstand extreme cold temperatures, radiation and extreme pH values.

The possibility of habitable environments in the Jovian and Saturnian satellites has prompted ESA (European Space Agency) to start planning a mission called JUICE (Jupiter icy moons explorer) to Jupiter and its moons. JUICE is expected to be launched in 2022 and will arrive at Jupiter in 2030. Further, a few weeks back NASA has funded development of the Europa Clipper Mission that can potentially be launched sometime in the 2020s.

Our quest for life is taking us to the satellites of Jupiter and Saturn. This gigantic new frontier now beckons us. However, we should remember that our Solar System is huge, extending much farther than where the planets end. Our search for habitability may even take us further into the vast empty spaces beyond the orbit of Neptune, among the trillions of small very remote icy worlds that move in long slow paths in the Kuiper belt and in the Oort Cloud, extending to an amazing distance of 1.87 light-years away from the Sun!

(The author leads the Rover Operations on Opportunity Rover as Chair of the Science Operations Working Group of the NASA Mars Exploration Rover Mission.)