Pluto is a small, rocky object, smaller than the Moon, orbiting the Sun in a wildly elliptical path ranging from 30 to 50 times the Earth-Sun distance, taking it from closer than Neptune to far beyond the outermost planet.
It takes about 248 years to complete each trip. During its "year,” the heat and light it receives from the Sun ranges from 0.1% and 0.04% of what we receive on Earth. This means Pluto is cold, with temperatures ranging from -220 C in summer to -240 C in winter.
Like Earth, Pluto probably started as a lump of molten rock. Because of its small size it cooled relatively quickly and was soon solid all the way through. If there were ever tectonic plate motions, they would have ceased at that point.
From then on the only forces shaping the surface would have been impacts by other orbiting objects. With no weather or plate motions to erase them, by now we would have expected Pluto's surface to be a pulverized mess of overlapping craters.
If so, since the Solar System contains lots of small bodies covered with craters, why would we bother to send a spacecraft to have a close look at Pluto?
The reason is that Pluto is one of a unique class of objects orbiting the Sun far beyond the outermost known planets.
These are known as Kuiper Belt Objects. They are not planets. They are lumps of frozen raw material left over from the construction of the Solar System. Out there in the deep freeze, they should be pristine. We have extensively examined the planets in the Solar System, however we had never had a close look at Pluto and other objects in the Kuiper Belt. This was the rationale behind the New Horizons Space Mission.
There was one other issue that aroused curiosity. We would expect Pluto to be a dull, grey object, yet many observers reported that Pluto looks pink. This suggested concentrations of organic (carbon-based) compounds are present on the surface. These could be a preserved sample of the mixture of chemicals received by the Earth when it formed, which formed basis for the development of life.
The spacecraft was launched on Jan. 19, 2006, and arrived at Pluto on July 14, 2015. The images it sent back demolished our preconceptions about what that world would be like.
Instead of a dead, icy rockball smothered with impact craters, the images showed a complex, exotic and active world, with terrain resembling what we see in the high Arctic here on Earth.
They showed rocky highlands, cratered, but not heavily, leading down into something resembling a frozen sea. There are large patches of reddish organic chemicals. On the shore are large, angular lumps of ice, similar to the shattered ice we see where ocean ice meets the shore.
The "sea" ice is broken into huge polygonal plates, divided by narrow lanes of whiter ice. In this case the ice is frozen nitrogen. Those plates indicate the frozen nitrogen must have been subject to occasional melting, forming a temporary liquid nitrogen sea.
These images raise big questions. Firstly, even on the highlands there is not a lot of cratering suggesting something has recycled the surface. Secondly, if the "sea ice" is occasionally melted, where is the heat coming from?
With a melting point of -210 C, nitrogen should remain well frozen throughout the Plutonian year. It looks as though Pluto gets an occasional warming for which we have no explanation. The core of that little world should now be cold and solid.
Close encounters with large planets could produce tidal heat, which is what warms Jupiter's moons Io and Europa, but there seems no indication there has been any such encounter.
Pluto is a puzzle, and puzzles are what make science interesting.
• Mercury and Mars lie very low in the sunrise glow.
• Jupiter and Saturn are visible in the southern sky and Venus may be seen low in the sunset glow.
• The Moon will reach first quarter on the Nov. 11.
This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.