WHAT was it like, this new blue planet that swam into the ken of Johann Galle 150 years ago in September 1846? For most of the intervening years, astronomers could give only very superficial answers to this question, but when Voyager 2 swept past Neptune in August 1989, its sensors provided a wealth of data that were to resolve not a few of the enigmas.
Neptune is four times larger than the Earth. Its diameter is some 30,000 miles, and it rotates upon its axis very rapidly, completing a revolution every 16 hours or thereabouts. It moves in an almost circular orbit 3,000 million miles distant from the sun, and it takes 164 years to complete a single journey.
The giant planet has no solid surface in the way we understand the term. It consists as far as we know, of a rocky core at a temperature of about 7,000 C; the outer portions of this core mix with the lower regions of a vast, liquid ocean to form a sea of mud, and the ocean itself peters out gradually into an atmosphere of hydrogen and helium with a tiny trace of methane. It is the methane which gives Neptune its characteristic bluish tinge, absorbing the red and other long wavelengths of the incident sunlight, and reflecting the remaining blue light back to space.
Being 30 times more distant from the sun that we are, Neptune receives very little in the way of solar heat. Temperatures in the outer reaches of its atmosphere, however, range from -150C to -200C, which makes it less cold than one might expect; it is assumed that the extra heat must rise from the planet's molten interior. Neptune's light blue atmosphere is punctuated by several much darker areas, one of which, the "Great Dark Spot", is large enough to contain our Earth. This giant storm system, for such it is, rotates anti clockwise like the Great Red Spot on Jupiter, and its periphery is decorated by a ring of high cirrus clouds of frozen methane.
But perhaps the strangest feature of Neptune's hyperactive weather system is the wind. Gales often approaching 1,000 mph sweep in continuous motion round and round the planet, making it the windiest spot in the solar system. Unlike our winds, which are driven by the energy streaming towards us from the sun, Neptunian winds are largely powered by the heat emanating from the planet's core. But it is one of Neptune's many remaining mysteries that if we add together the energy it receives from the distant sun, and that originating from its internal source, the total is well short of that required to drive such a large mass of gas around the planet at these supersonic speeds.