Monday, July 06, 2009
ENCELADUS was a familiar name in ancient Greece. Gaia, the Earth to you and me, was fertilised by the blood of Uranus and gave birth to children known as the Gigantes, one of whom was Enceladus.
He’s forgotten nowadays, but, thanks to the activities of a space-probe, this giant may regain his former celebrity; a moon of Saturn, named after him, has become hot property in the search for extra-terrestrial life.
Giant radio telescopes scan the sky, 24 hours a day, listening for faint sounds of civilisations on the planets of distant stars. Alas, no strains of angelic music, or of ET phoning home, have been heard. But are we looking, or listening, in the right place? According to papers in the June issue of Nature, Enceladus might be a better bet for our search. This strange moon has an ocean under its surface, with salts similar to our earthly ones, raising the tantalising possibility that it supports primitive forms of life.
Saturn, with its famous rings of orbiting flotsam and jetsam, is a gigantic ball of liquid and gas, one and a half billion kilometres from Earth. It has eight substantial moons and many smaller ones. NASA’s Cassini space-probe went into orbit around Saturn on June 30, 2004 and began a series of close encounters with the planet’s moons. The most interesting was expected to be the largest, Titan, which is believed to resemble Earth before life evolved.
When Cassini approached Enceladus, it made its most extraordinary discoveries. Sprays of icy particles are being ejected, in what look like giant geysers, from the moon’s south pole, generating a vapour-plume three times as high as the moon is wide. Saturn’s E-ring is thought to consist of material spewed from Enceladus. Cassini examined the composition of the E-ring, and Frank Postberg, and colleagues from leading German Universities and institutes, found that its ice grains contain between 0.5% and 2% sodium salts. Table salt, the most familiar, is a combination of sodium and chlorine. Such compounds wash out of rocks on Earth, making the seas salty. The ones on Enceladus must also have come from rocks immersed in water, so there has to be an ocean under the moon’s surface.
Saturn and its satellites are so far away that our radio signals take 50 minutes to get to them. At such a distance, the Sun’s radiation is so weak that water on Enceladus should be frozen, so how can there be a liquid sea there? The scientists have come up with an intriguing explanation. Enceladus, which is only 500km in diameter, is constantly being squeezed and stretched by the gravitational fields of its sibling moons as they pass close by. These give the little moon a kind of planetary indigestion; constant tummy rumbles.
The internal heaving generates heat and causes the belching of vapour clouds. Even the external appearance of Enceladus has deteriorated. It won’t win any planetary beauty competitions; cracks and fissures disfigure its surface. It’s not even spherical any more, but the tides generated by the fierce gravitational forces generate enough heat to keep its underground sea from freezing.
If there’s a sea, there could be life, but what sort of life? Our Earth formed as a red-hot ball of liquid and gas 4.7bn years ago. A billion years later, the surface had cooled sufficiently for life to get going; our oldest microbe-like fossils are 3.5m years old. It would seem that life gets going readily, provided the essential ingredients, water energy and salts, are present. The forms life might take in the dark, cavernous ocean of Enceladus would be very different to those on Earth. Photosynthesis, the harnessing of the Sun’s energy to produce sugars, is the basis of most of the life-forms we know about, but is not an option on Enceladus. But life can exist in the most unlikely places. Bacteria live in the vents of undersea volcanoes on Earth. Perhaps the inhabitants of Enceladus resemble them.
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