Blank Space no more: New Horizons makes it to Pluto

Dr Robin G. Andrews @squigglyvolcano

New Horizons probe finds Pluto is more than just an icy ball in space: it’s alive.

Well hello there, Pluto. Sorry it took so long. You’re looking great, but I have to be honest with you: we are never ever, ever, getting back together.

Despite humanity’s genuinely epic science project finally reaching Pluto, the last outpost of our solar system in the minds of many, we still cannot bring ourselves to welcome back the prodigal son back into the planetary fold. New Horizons – which has been speeding through the solar system for the last ten years saying hello to various planetary bodies and moons along the way – has finally, after 4.5 billion kilometres – reached Pluto. To quote Douglas Adams, “Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. Imean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.”

There’s a problem though. As beautiful as the little guy is turning out to be – it has a frost feature that looks a lot like a heart, after all – we still can’t accept him back into the planetary relationship. We knew you were trouble, Pluto.

The New Horizons spacecraft was launched back in 2005; back then, Pluto was still a planet. Remarkably, astronomers, planetary scientists and astrophysicists had yet to come up with the definition of a planet, and controversially they settled on three criteria: firstly, it had to orbit the sun and not undergo nucleosynthesis (essentially, it had to definitely not be a star); secondly, it had to be in hydrostatic equilibrium (essentially, it had to be a sphere, with its gravitational forces overcoming any mechanical, rigid forces in the planetary rock); lastly, and sadly for Pluto, it had to have “cleared the neighbourhood” of any other objects orbiting the sun.

This means that it has to be the gravitationally dominant object in the region. Unfortunately, little Pluto shares its orbit with many other similarly sized objects in a distant region of our corner of the Milky Way, the Kuiper Belt. Their paths crisscross often.

Oh Pluto, how could you? You’ve been hanging out with the wrong crowd. You said you would change but you didn’t.

So Pluto definitely still isn’t a planet; it remains a dwarf planet. But oh my, how beautiful you seem to be. Nine years ago, Pluto remained an agglomeration of grey pixels; back in 2005, we saw Pluto as a gleaming light, alongside its moons Charon, Styx, Nix, Kerberos – and later Hydra, all five named after the mythology of the Greek underworld. Just yesterday, our little spacecraft sent back incredible images of Pluto and Charon, showing us the surfaces in incredible detail.

As many other outlets have widely reported, there are enormous mountain ranges on Pluto made almost entirely from water ice approximately the same height as Mount Fuji here on our own pale, blue dot. Smooth plains show frosts of nitrogen and methane ice; the large heart-shaped feature has been named after the world’s discover, Clyde Tombaugh. In 1930, the twentysomething young astronomer officially detected the then-ninth planet in the night sky; today, his ashes are speeding out of the Pluto system, nestled on board New Horizons. Pluto’s largest moon, Charon, has enormous chasms and similarly icy protrusions and plains.

So is Pluto the little iceball we have always presumed it would be, being so far away from the Sun? Even at the speed of light, the data from New Horizons is taking four and a half hours to reach Earth. By comparison, light travelling at the same speed from the Sun to the Earth takes only eight minutes.

Well, no. And that’s the great surprise. Pluto may not be a planet, but it’s certain active and “alive”.

How do we know this? A few telltale signs have given it away.

Firstly, there are no surface craters on Pluto or indeed Charon. Pluto has a thin atmosphere; this could theoretically protect the surface somewhat from larger meteors by causing them to burn up as they enter the planetary body’s upper atmosphere. Charon, however, has no atmosphere, and thus no protection in this regard. Yet, both have no observable pockmarks indicative of meteorite or comet impacts. Pluto shares its orbit with many millions of rocky and icy bodies in the Kuiper Belt, and as such should have some impact craters, but it doesn’t.

Mercury, a small planet much more proximal to the Sun and far away from the Kuiper Belt and the Asteroid Belt between Mars and Jupiter, has plenty of impact craters. Mars has its fair share, as does our own Moon. Even Earth has some, including some enormous ones that have led to infamous extinction events.

But Earth is indeed the odd-one-out here: many of our craters have been eroded over time. This is partly because we have an active atmosphere, with climate and weather patterns slowly chipping away or burying many of our impact craters. However, the main reason a vast number of ancient impact craters on Earth have been removed from view is in fact plate tectonics. Our Earth’s enormous tectonic plates have been moving around for a very, very long time, creating and destroying continents and oceans. Lava has erupted onto the surface of our world, changing the landscape forever. Earth has a fairly young surface when compared to most of the rocky celestial objects in our solar system.

Another planetary body in our solar system with no noticeable impact craters is the innermost Galilean moon of Jupiter, Io. A remarkable world, it is the most volcanically active object we know of. There are volcanoes on this low-gravity world that erupt molten material through an incredibly thin atmosphere up to heights of 400km above the surface. In contrast, Mount Everest is approximately 8km high. These volcanic plumes spread sulphurous material out into space, beyond the moon’s atmosphere, whilst surface lava continuously covers the surface and recycles it. If there is an impact on Io by a meteorite, the evidence will rapidly be removed. This world’s surface is even younger, then, than Earth’s.

So in order for Pluto to have no impact craters, it – and Charon – must have very young surfaces. In order to remove impact craters – and indeed, form mountains – the surface layer of these two distant, icy objects must be moving. And although the mechanisms for volcanism and surface tectonic activity on Earth and Io are generated quite differently, they both rely on the same thing: the generation of heat from within, and its escape to the surface.

Pluto’s not a dead, icy world: it, and its major moon, are active. Heat is escaping the cores of these two spherical strangers and has clearly been active in shaping the enormous icy mountains seen only yesterday by New Horizons.

The question is: where is this heat coming from? Earth is powered by two heat sources: radiogenic heat – that is, heat produced by the natural decay of radioactive elements – and primordial heat kept inside the Earth, producing during its explosive, cataclysmic formation. Io’s heat is powered by something called tidal heating: the powerful gravitational forces of Jupiter and its nearby moons causes the generation of heat through frictional melting within the core of the fiery moon.

Pluto and Charon are far too small for any radiogenic heat to persist, and it is unlikely there are high concentrations of radiogenic elements within their sub-surfaces anyway. Tidal heating seems unlikely for two bodies no larger than Earth’s own moon. There is currently a huge debate as to whether or not there is enough primordial heat to sustain such active surfaces.

Mountain ranges, even water ice ones, require tectonic forces to be occurring, that much is clear. Perhaps, rather excitingly, there are cryovolcanoes on Pluto: instead of erupting molten magma as we think of it on Earth, or indeed on Venus and Io, ammonia-water mixtures could store heat and erupt as ice volcanoes, with molten ice – yep, that’s water – shooting up through volcanic mountains or fissures, covering the surface impact craters in frozen blankets.

With many more announcements to come from NASA, it is likely more secrets are yet to be revealed. It’s an exhilarating time for science, and for humanity in general: earlier this year, we landed a probe on a comet travelling faster than a speeding bullet. Now, we’ve reached the furthest major planetary body in our solar system, and have finally, finally said hello.

But you’re still not a planet, Pluto. You’re looking happy and we’re glad you’ve found so many new friends, but you are going to have to get over this whole dwarf planet classification. Don’t worry; there won’t be any bad blood.

…shake it off, you know?

Click here to see views of Pluto through the years


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