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tiny rock and ice aggregates, but some with indications of geological
activity driven by past tidal heating. Circling Neptune is the largest
of these, Triton, which has a thick icy crust with cryovolcanoes and
evidence of a complex geological history, on which geysers of liquid
nitrogen have been glimpsed. Possibly, beneath that thick crust, a
water ocean lurks there too.
Oceans Unborn
Far out on the edges of our solar system lie the trans-Neptunian
objects, the first of these being the icy bodies in the Kuiper Belt. Out
here signals from the Earth's radio transmitters, travelling at the
speed of light, take some eight hours to arrive. The Kuiper Belt is a
giant doughnut-shaped structure made up of many small planetesi-
mals, the largest being the ex-planet Pluto (which has three moons
of its own), and these are probably made of methane, ammonia, and
water ice. Also in the Kuiper Belt lies a rock/ice body (considered by
some a dwarf planet) named 50000 Quaoar, after the creator god of
the native Tongva people of western North America (a god who is also
called Chingichngish, so in this instance the astronomers were merci-
ful). Quaoar is a little over 600 kilometres across, and recently sur-
prised scientists by giving the spectral signal of crystalline (rather than
amorphous) ice at its surface, which suggests an apparent interior
temperature of at least −170 degrees Celsius. This is terribly cold, but
warmer than had been predicted and, if not enough for a deep-lying
ocean, may well allow cryovolcanism of soft, ammonia-laced ice.
150
Beyond the Kuiper Belt is the 'Scattered Disc', sparsely populated by
yet more icy planetesimals. Farther out still, and now just reached by
Earth's most distantly travelled spacecraft,
Voyager 1
, the edge of the
solar system may lie in the hypothetical Oort Cloud. Nearly one light
year from the Sun, this cloud of (hypothesized) comets is thought
to extend to a quarter of the way towards our nearest star, Proxima
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