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Places for life

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Ken Tapping, July 01, 2009

In the sky this week...

> Jupiter and Neptune are currently close together in the sky, and rise around midnight; Saturn lies in the Southwest during the evening.

> Mars and Venus rise in the early hours.

> The Moon will be Full on June 7, the Moon of the Walking Thunder.


When we recently looked at the latest work on searching for evidence of life on planets orbiting other stars, the emphasis was on finding planets like ours, warm ones with liquid water and living creatures on their surfaces. Is that the only possibility? Life might take many forms, some so exotic we have not even imagined them yet, and might not recognize them when we see them. However, since the universe seems so well tuned for producing the ingredients for carbon-based life, with our Earth being an example of a planet loaded with that form of life, let's restrict ourselves to that.

For many years the standard candidates for planets bearing carbon-based life were at the right distance from their stars for liquid water to be present on their surfaces. However, now it looks as though we can expect to find life in places where that condition is not even closely met. Moreover, the key lies right under our noses, on the bottom of our oceans.

Along the mid-ocean ridges, where hot magma is not that far below the seabed, seawater finds its way underground, where it gets heated under pressure to hundreds of degrees and dissolves various minerals, many of them sulphur-bearing. The resulting cocktail, loaded with dissolved hydrogen sulphide, is then ejected from vents on the seabed. As it meets the cold seawater, it cools and the minerals start to drop out of solution. The result is that the vent water is loaded with dark particles, and looks like black smoke. This unlikely mixture, although poisonous to us, provides nourishment for rather bizarre creatures, which thrive on the sulphur-laden chemicals in the water, and have no use for oxygen. The essential thing is that these little communities centred on the black smokers have no use for sunlight or air. The Sun could go out and they would survive, in little warm, liquid oases beneath frozen oceans. In other words, here we have a possibility of life with different needs, and which could survive in places we could not. All we need is enough heat to drive these hot, mineral-laden jets. On our world the heat comes from the decay of radioactive elements in its core, and the tidal kneading it gets from the Moon.

That brings us to Europa, one of Jupiter's largest moons, located in the frigid outer reaches of the Solar System. It is covered by a layer of ice with good evidence of liquid water underneath. This is kept from freezing by heat released by Jupiter's gravity gently kneading Europa. If this heat is sufficient to drive a few black smokers, there could be life there too. Actually, now we have another unlikely-seeming candidate. It is Enceladus, one of the moons of Saturn, even further from the Sun. It looks like Europa – an ice-covered ball. However, it is warmer than it should be and experiences eruption of water, methane and other chemicals. There could easily be a dark, warm ocean under that ice. The heat to drive these eruptions would be coming from the gravitational kneading it gets from Saturn. Big eruptions suggest smaller eruptions – black smokers, maybe with exotic life forms that we simply need to go and see.

Ken TappingKen Tapping is an astronomer at the National Research Council Herzberg Institute of Astrophysics (NRC-HIA), and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9 Tel (250) 493-2277, Fax (250) 493-7767,