Ice and Fire

It's pretty much a golden age for exobiologists. Once the preserve of cranks, science fiction writers and supermarket tabloids, the search for evidence of life on other planets is may still be based on speculation and extrapolation rather than actual hard evidence, but it's now a sober and respectable area of legimate scientific research, covered soberly and respectfully in popular science magazines and broadsheet newspapers.

Latest headlines concern findings by the Cassini spacecraft: it seems that not only is hydrogen in Titan's atmosphere flowing down to the moon's surface and disappearing, but there's a distinct lack of acetylene at the surface, too; acetylene should be formed from methane and ethane and snow out onto the surface, but so far it hasn't been detected. Most likely, the disappearance of hydrogen and low levels of acetylene are due to some kind of catalytic chemical process, but exobiologists have pointed out that it could be the signature of some form of biological activity. The surface of Titan is far too cold to support any form of life that uses water as a solvent, but there's abundant liquid methane and ethane, which rains out of the atmosphere, carves rivers into the surface, and forms lakes and seas. Acetylene would be the best energy source for life based on liquid methane, which would consume hydrogen much as we consume oxygen. Right now, on Titan, some frigid, curious slime mould could be edging its way with infinite patience across a pebbly beach towards the Huygens lander, wondering if it's evidence for life above the eternal clouds.

Jupiter's moon Io is one the least likely venues for life in the Solar System. Although it's only a little larger than Earth's moon, it exhibits extremes of geological activity. Tidal effects stretch and flex its crust, and the resulting friction generates enough heat to power volcanoes that spew molten sulphur at temperatures of over 1600 degrees Centigrade. Areas between the volcanoes are covered with sulphur dioxide snow chilled to -130 degrees C, almost all water has long ago been driven off, and besides all that, it's drenched in Jupiter's radiation fields. But astrobiologist Dirk Schulze-Makuch has pointed out that Io would have possessed plenty of water when it first formed, and if life arose before the water was blasted away by Jupiter's radiation, it could have retreated underground, into rocks or even lava tubes, which would provide protection from radiation and conserve and moisture and nutrients (and would be ideal homes for any intrepid human colonists).

It's unlikely, but not impossible. Extremophile bacteria, the poster children of exobiology, exploit all kinds of forbidding niches on Earth, from hydrothermal vents to Antarctic dry valleys, and lithotrophic extremophiles extend the biosphere deep into the Earth's crust, obtaining energy from all kinds of organic and inorganic sources. The oddest example is Desulfurodis audaxviator, discovered in 2008 deep in a South African gold mine. It survives without light or oxygen, using energy generated by the radioactive decay of uranium and other elements in the surrounding rock to drive its thrifty metabolism. When you contemplate this microscopic, one-species, completely self-sufficient ecosystem, life on Io or Titan doesn't seem so unlikely after all.

O Brave New World

Surfacing from working on the ongoing (about halfway through the second draft, killing my darlings like there's no tomorrow), here's the cover for the UK mass-market paperback of Gardens of the Sun. It isn't published until August, but you can preorder it right now. The artwork is by the terrific space artist Don Dixon - how great is that? You can see the original here, and do check out everything else too, while you're there. It shows Uranus from the surface of Miranda, its strangest moon - and one of the strangest moons in the Solar System (which is saying a lot). From the novel:

Most of Uranus’s thirty-plus moons were small chunks of ice or carbonaceous material. One group orbited just outside the outer edge of the ring system; another occupied distant and irregular orbits, wanderers captured by Uranus’s gravitational field. And between these two shoals of tiny moons were five massive enough to have achieved hydrostatic equilibrium, contracting into spheres under the force of their own gravitational fields. Four were much alike, balls of dirty ice wrapped around silicate cores, spattered with impact craters, dusted by dark materials flung outward by the chains of collisions that had created the ring system, fractured by varying degrees of ancient geological activity. But the smallest of the larger moons, Miranda, was not only the strangest of Uranus’s family of satellites, it was one of the strangest moons in the Solar System: a patchwork of cratered, banded and ridged terrains broken by mountainous ridges and monstrous fault canyons up to twenty kilometres deep, as if hammered together from pieces of half a dozen different bodies by some inept god who’d afterwards slashed and hacked at his botched creation in a fit of rage. An early theory about its formation suggested that it had been shattered several times by massive impacts and the larger fragments had randomly clumped together, exposing sections of the core in some places and sections of the original surface in others, but later research showed that its haphazard topography was the result of intense geological activity driven by tidal heating at a time in the deep past when it had possessed a far more eccentric orbit.

Stretched and kneaded every time it swung close around Uranus, Miranda had bubbled and blistered and cracked like a snowball wrapped around a hot coal. Eruptions of icy magma had flooded older terrain and created smooth plains. Coronae, huge domes edged with concentric patterns of ridges and grooves, had grown at the top of upwellings of warm ice that penetrated and deformed overlying strata. And after it had settled into its present orbit, it had cooled and frozen through and through. Its surface had contracted and tectonic activity had scored it with deep grabens formed by extensional faulting, while compressional strain had raised systems of ridges and valleys and thrown up escarpments several kilometres high.

This violent geological history had created a varied and chaotic moonscape that, patched with varied terrain, cut by the rifts and grooves of transition zones and gigantic scarps and grabens, provided a wealth of hiding places. The refugees elected to settle in the deep groove of a narrow chasm in the high northern latitudes, and put to work the two crews of construction robots they’d brought with them.