Give Me a Crash Course in . . . Pluto, as seen by ‘New Horizons’

What's the big deal about Pluto at the moment, and why all the fuss about a little black-and-white photograph? The picture may look a bit mingy, but don't forget that it was taken by a satellite, New Horizons, and sent back to Earth from five billion kilometres away. And that that image, and many more close-up photographs along with it, were taken as the Nasa satellite shot past Pluto at 14km per second.

I take your point, but it still kind of looks like the moon The stark image might remind you of the moon, but there is one really important thing missing on Pluto: the photograph show no craters. The moon is pockmarked with craters, but they don't seem to be on Pluto.

What difference does that make? Maybe Pluto was lucky and didn't get hit by an incoming meteor or giant rock It makes a difference because Pluto would have to have been extremely lucky not to get hit in the more than four billion years since it formed. Everything else in the solar system got hit. The inner planets were peppered by space debris as they formed and grew from the accumulation of rocky, icy bodies. Earth was likely hit just as often as the moon, but few signs remain to prove it. One exception is the magnificent impact crater in Arizona. The rock that hit us was only 50m across, but it left an enormous hole in the ground, 1.2km across and 170m deep.

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Surprised and excited: Nasa and project staff react as telemetry arrives from the New Horizons spacecraft. Photograph: Brendan Smialowski/AFP/Getty

But some craters would be under water. And rain and wind, and movements of Earth's crust, and volcanoes, would have smoothed out holes on land over time Exactly. But if Pluto was also hit by big rocks, where did the craters go? Somehow they got smoothed out, just like on Earth. But Pluto does not seem to have the liquid water or wind necessary to erode craters. Something else must be happening to get rid of them. That one little photograph from Pluto also shows that the surface of the planet is very young in geological terms. It shows mountains 3,500m high that formed no more than 100 million years ago. That makes them newborns, given that the solar system is 4.6 billion years old. For comparison, the Alps began to lift up 65 million years ago.

So what do no craters and young mountains add up to? They add up to a bunch of surprised and excited planetary scientists. Planets such as Earth are continually renewing their surfaces, caused when big slabs of the surface crust slide about and crash into other big slabs. This is how mountains such as Everest are formed. In other cases one slab slides underneath another, remelting the crust and recycling it back into the earth. The big moons around Jupiter and Saturn can do it, as the enormous gravity of these planets keeps churning the insides of their moons. This keeps their surfaces a bit more active. But Pluto doesn't have a giant planet. The only other way we know of to renew Pluto's surface is if it also remains geologically active like Earth. And Pluto's moon, Charon, also seems to be able to renew its surface. So maybe it remains active as well.

So this really is new stuff coming from Pluto? Absolutely. And there is so much more to be discovered. This image shows only 1 per cent of Pluto's surface, so many more are on the way. And New Horizons had six other instruments on board collecting data during the flyby, so its data will keep scientists busy for years to come.