The New Horizons spacecraft has passed by Pluto and is continuing its journey out into space.
Its data is trickling to us over the nearly six billion kilometres distance back to Earth. Those first images were nothing like what we expected. The Solar System comprises two main zones. The Earth and other planets lie in the Inner Solar System. Pluto lies at the boundary with the Outer Solar System. This region consists of two parts. The inner part is known as the Kuiper Belt, and consists of possibly millions of icy, rocky objects up to around 2500 km in diameter. The outer part is known as the Oort Cloud, and is filled with an even larger number of icy, dusty objects with diameters up to a few kilometres.
In the Outer Solar System, far from the Sun, temperatures are low and everything – even gases – are deeply frozen. Until now we believed these objects have changed little since the Solar System formed, about 4.5 billion years ago, making them Dead Sea Scrolls telling us about our earliest history. Pluto is now categorized as one of the largest and nearest members of the Kuiper Belt, making it the primary target of the New Horizons space mission.
At about 39 times the Earth’s distance from the Sun, we expected Pluto to have been frozen solid since it formed. It would resemble a deep frozen version of the Moon, whose surface has not been changed by geological processes for billions of years, and today we see a record of all the objects that have hit it. It is covered with impact craters. We expected Pluto to be covered with craters too. The Earth has been similarly hit, but continuous recycling of its surface by plate tectonics has removed almost all of them. Those first images from Pluto tell a completely different story. Its surface has great flat plains and jagged mountain ranges and no craters. This means its surface must be young, no older than 100 million years.
There are objects in the outer Solar System with very young surfaces, but these are all moons orbiting close to giant planets. The continuous flexing due to tidal forces inflicted on them by the planets they orbit generate tremendous heat, enough to melt their cores and drive volcanic activity. Io, Jupiter’s closest moon is the most volcanic object in the Solar System. Europa, the next moon out, is warmed by volcanism to the point where we believe that under a thick icy outer layer lies a deep ocean of water. However today Pluto is not orbiting close to anything big enough to produce any tidal heating. It should be frozen solid all the way through.
It has been suggested that Pluto once orbited Neptune, one of the giant planets, where it could have undergone tidal heating. Then, about 100 million years ago it was diverted into the orbit it has today, and the tidal heating stopped. Maybe something collided with Pluto 100 million years ago, completely melting its surface and erasing all the craters, mountains and other features. Yet another suggestion is that Pluto is being heated by the decay of radioactive elements in its core. We know that this process contributes to the heating of our planetís core. The amount of heating is likely to be small in a body like Pluto, which is much smaller than the Earth. However, if Pluto is made mainly of ice and frozen gases rather than rock, it would require much less heat to melt it. So we might find ice volcanoes erupting gas and liquid water instead of lava. Since the mean surface temperature is around 230 C this would rapidly freeze, forming both mountains and plains.
Now beyond Pluto, New Horizons is heading for other Kuiper Belt Objects. Will they be just like Pluto or will they be different? My guess is that some will be like, some less like, and some utterly different. Thatís what makes all this so exciting.
Venus and Jupiter still lie close together in the sunset glow, but as they drop deeper into the glow Jupiter is getting harder to see. Saturn lies in the south. The Moon will be Full on the 31st.
Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory, Penticton.