an autodidact meets a dilettante…

a cool place – and note the tiger stripe

The Curiosity landing has been fabulously successful, and it’ll certainly be worth keeping tabs on the rover’s findings. I posted recently on the possibility of life on Mars, not a couple of billion years ago, as many Mars experts think probable, but right now. The Curiosity rover, as we know, will be investigating this possibility further, but meanwhile there are other possibilities of finding extra-terrestrial life in this solar system, and one of the best places to look, I’m reliably informed, is Enceladus, a tiny moon of Saturn.

Enceladus is only about 500 kilometres in diameter, but its surface has intrigued astronomers ever since Voyager 2revealed detailed features in the early eighties, indicating a wide range of terrains of varying ages. Data from the Cassini spacecraft that performed fly-bys in 2005 showed a geologically active surface, with the most spectacular feature being a large volume of material, mostly water vapour, issuing from the southern polar region. This indicated the existence of ice volcanoes, or cryovolcanoes, which have also been observed elsewhere, and were in fact first observed by Voyager 2 on Triton, Neptune’s largest moon. However, on Enceladus what we have are more like geysers spewing out material from an area known by observers as ‘the tiger stripes’, a series of prominent, geologically active ridges. This material is now known to account for much of the outermost E ring of Saturn, within which Enceladus has its orbit, though a certain amount falls back onto the moon as snow.

Finding water on any object in the solar system obviously excites the souls of astrobiologists. A report from a May 2011 conference on Enceladus stated that this moon “is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it”. However, there are plenty of sceptics, or I should say cautious questioners. First, the existence of water vapour spumes doesn’t necessarily entail liquid water below the surface – for, in spite of the thrill of detecting snow in large quantities on the surface, liquid water is generally regarded as essential to finding life. And even if we assume liquid water…

Some analysts argue that the spumes may be a result of sublimation – a change from a solid, icy state to a vapour, missing out on the liquid phase – or of the decomposition of clathrate deposits. A clathrate is a type of ice lattice that traps gas [methane clathrates are found at the polar regions of Earth]. However, the recent discovery of salt in these plumes has made these possibilities less plausible. Salt is more likely to be associated with liquid water, but hydrogen cyanide, also recently found, would have been expected to react with liquid water to form other compounds, not found as yet. In short, the jury is still out on the presence of liquid water.

And assuming there is liquid water, how could we test for life within it? With great difficulty, obviously. Analysts would be searching for biomarkers, ‘chemicals that appear to have biological rather than geophysical origins’ [Cosmos 44, p78]. Photosynthetic production wouldn’t be an option, so other systems are being hypothesised, including a methanogenic system in which methane is synthesised from carbon dioxide, or a system of metabolizing acetylene, which occurs on Earth. Traces of acetylene have been found on Enceladus. Other biomarkers include amino acids with the right chirality – that’s to say a strong chiral preference, one way [as found on Earth] or its opposite. Amino acids with no chiral preference are likely to be abiotic.

To test for such biomarkers would require new instrumentation and another visit to this intriguing moon. Something else to look forward to. What would we do without anticipation?