News From Elsewhere

In December this year it will be forty years since human beings escaped low Earth orbit; forty years since the end of Apollo. But although there are no cities on the Moon, or expeditions to Mars, the asteroids, and the moons of Jupiter, it's a golden age for space science. Just about every day I'm excited by a new piece of research, a new discovery, a new image of some sublime extraterrestrial landscape.

A couple of days ago, for instance, there was advance publicity for an astrophysics paper describing the computer simulation of trajectories of free-floating planets in star clusters. The investigators discovered that if the number of planets equalled the number of stars in the cluster, then over time some 6 - 8 per cent of the planets were captured by stars. Given that ejection of planets appears to pretty common during formation of planetary systems, not only is the Galaxy teeming with rogue planets, but many star systems may contain captured planets, distinguished by distant and eccentric orbits.  Far-fetched? We have an example of a similar mechanism right here in the Solar System.The outermost moons of Saturn are almost certainly captured bodies that were ejected from the Kuiper belt, out beyond Pluto's orbit, and were captured by Saturn as they migrated inwards.  These little moons possess distant, inclined and often irregular orbits.  They cluster in three groups - Inuit, Gallic and Norse - and the members of each group may well be fragments of a parent body that broke up.  The best known and largest irregular moon is Phoebe, a couple of hundred kilometres in diameter, orbiting in the opposite direction to the inner moons, and the source of dark material that forms Saturn's largest ring (which was unknown until a few years ago) and darkens the leading hemisphere of the ying-yang moon, Iapetus.

That dark ring was discovered by examination of data and images collected by the Cassini spacecraft, still in orbit around Saturn, and still working hard after entering orbit around Saturn some eight years ago. One of the Cassini team's latest discoveries is that one of the hydrocarbon lakes in the south polar region of Saturn's largest moon, Titan, appears to behave like a salt pan in Namibia; both are depressions that drain in the dry season and refill from below, fed by groundwater (or ground hydrocarbons, in the case of Titan's lake). Titan and Earth are the only bodies in the Solar System known to possess hydrological cycles. Although Earth's cycle is based on water, and Titan's on methane, ethane and propane, there amazing similarities. Cassini's extended missions have allowed it to observe seasonal changes in Titan's rain patterns and the size of its lakes, something a human-crewed mission would be hard-pressed to do, given that Saturn takes 29.7 years to complete an orbit, and seasons on the gas giant and Titan are correspondingly longer than seasons on Earth. But maybe somewhere on Titan there are dry salt pans, old, large, and very flat, like the Bonneville salt pans in Nevada, but composed of something like asphalt. Imagine the drag-racing possibilities...