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Pluto revealed raises questions about its origins

New Horizons has revealed some surprising features on Pluto and its moon Charon. It's currently thought that a collision created this enigmatic duo, but planetary scientist Dr Andrew Prentice believes another process may explain some of their features.

Last week, Alan Stern, the leader of the NASA New Horizons mission to Pluto said, "I'm a little bit biased, but I think the solar system saved the best for last."

Nothing could have prepared us to see that extraordinary heart-shape pattern on Pluto's surface or the 3500-metre high mountains set in vast plains of ice. Nor did anyone expect cliffs of ice and a 9000-metre deep canyon on the surface of Pluto's largest moon Charon.

Another surprising discovery was the startling lack of impact craters, especially on the surface of Pluto.

In the space of just 24 hours, from just before New Horizons arrived at Pluto and the historic post-flyby press conference of July 15, our perception of Pluto and Charon changed from that of being very ancient worlds with surfaces heavily cratered like those of Moon and Mercury, to apparently young worlds with surprisingly few craters, just pock-marked here and there.

This led Jeff Moore, leader of the New Horizons geology, geophysics and imaging team, to declare the surfaces of Charon and Pluto may be barely 100 million years old. This is barely a blink in the 4560-million year age of the solar system.

Some unknown physical process must be operating to help keep the surfaces of Pluto and Charon young and geologically active. But what could that heat source be?

As Pluto and Charon are now locked in a fixed orbit about one another, we can rule out heat coming from the frictional dissipation of tidal movements in their crusts. On Earth, tides are raised in the crust and ocean by the Moon and the Sun and the energy from these can harvested in wave farms. Not so for Pluto, however, as the tides have all long gone.

The feeble heat coming from the decay of the long-lived radioactive isotopes of potassium, uranium and thorium inside Pluto's rocky core can also be ruled out. So what is going on?

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Collision vs fission

The currently accepted model for how Pluto and its moons formed is known as the 'collision theory' or the 'giant impact model'.

This theory, which has also been used to explain the formation of our Moon, suggests the planets formed from a flattened disc of material consisting of trillions of small solid bodies called 'planetesimals' that surrounded the early Sun. Many of these planetesimals, or building blocks, still remained after the bulk of the growth of each planet had been completed some 4.5 billion years ago.

Then some 600 million years later, during a period known as the Late Heavy Bombardment, it is proposed that the giant planets Jupiter and Saturn started to migrate outwards and stirred up the disc of left-over building blocks. It is proposed that one of these bodies struck the early Earth to form the Moon and another struck Pluto to produce a disc of debris from which it is thought that Charon and the smaller moons were formed.

But I believe another process drove the creation of Pluto, Charon and the other moons -- and may explain some of the features we see in the New Horizons images.

In a paper published in the Australian Journal of Astronomy in 1993, I proposed that Pluto and Charon were once part of a single body, called proto-Pluto. This body had formed by the gravitational accumulation of a swarm of smaller bodies of ice and rock that had existed in a gas ring cast off from the equator of the primitive Sun, some 4.56-billion years ago.

In those days the Sun was once a huge spinning cloud of gas and dust and had a size greater than Neptune's orbit. I suggested that heat produced by the accumulation of the small bodies may have caused the ices inside proto-Pluto to melt. As the ices began to melt, the lighter liquids would rise to the surface to create a deep ocean.

I also suggested that because of the huge amount of angular momentum that exists at the edge of the solar system, proto-Pluto may have been spinning very rapidly with a period of just two or three hours. The rising of the light liquids to the surface was the last straw for proto-Pluto. It caused it to spin up further to the point that it split part into two bodies.

This process of splitting is known as 'rotational fission'. It had been studied in the early 1900s by many distinguished mathematicians (including Henri Poincaré, George Darwin, Sir James Jeans and Raymond Lyttleton) as a possible mechanism for understanding how our Moon may have formed from the early Earth.

The possibility that fission may also provide a dynamical explanation for the origin of the Pluto-Charon binary system was first put forward separately by F. Mignard and D. N. C. Lin, soon after Charon's discovery in 1978.

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Pluto

Can the fission hypothesis explain the mysterious heart-shaped region, now named Tombaugh Regio in honour of Pluto's discoverer Clyde Tombaugh? What about mountains of ice that and the icy plains of Sputnik Planum within the Heart that are made up of polygonal segments, and separated by dark lines and hillocks?

It is my belief that the huge bright white region that makes up Tombaugh Regio is the launch site of the material that made Charon and the small moonlets of Pluto. When fission of proto-Pluto took place, the crust of Pluto would have ruptured at its weakest point.

The huge mass of ejected liquid that formed Pluto's moons then shot off into space, leaving behind a gaping gash. Liquids would have continued to spill out onto the surface for many hours after that cataclysmic event occurred and then later froze over to form the heart-shaped patch that we see today.

The flat plains of Sputnik Planum may be a thin layer of water ice sitting atop an ancient bed of methane ice that was once warm and giving off heat through convection, forming a patterned crust that remained after everything had cooled.

The polygon structures are tell-tale signs of convection, as Moore from the New Horizons team alluded to last week.

Perhaps, the dark lines and hillocks in Sputnik Planum reveal a bed of methane ice that sits below. The bright spikey peaks in Norgay Montes have got me stumped! They seem to sit at the edge of an ancient terrain that escaped being splashed by fission liquids.

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Charon

A feature that is common to both the fission and the collision theories is that all of the moons started off in highly eccentric orbits. In the fission theory the moons were ejected from the interior of a rapidly-spinning liquid proto-Pluto.

In the collision theory, a large object struck a solid Pluto and tore off part of its mantle to form a debris disk of rocks and ices. The moons of Pluto accreted from the solids within the debris disc, and thus it was anticipated that their surfaces would be heavily battered by impacts craters.

But the surfaces of Pluto and Charon do seem to have far too few craters than expected according to the collision theory.

In the fission theory, no collisions are involved so the surfaces are expected to be crater-free. In a paper I presented to the Lunar and Planetary Science conference in Houston earlier this year, I predicted that the surfaces of Charon and the other moons should be 'very flat and smooth and consist solely of water ice'.

In my model, the impact craters that are on Charon were created long after it had formed. The fact that very few impact craters exist today suggests that it may have had none to start with!

Possible tectonic activity on Charon was first proposed by US geophysicists Amy Barr and Geoff Collins based on the collision theory, but their work can be applied equally to moons created by the fission model.

The very strong tidal forces that converted Charon's orbit to the circular one that exists today would have also induced enormous tectonic forces in the interior of Charon, especially so near the equator where gravity is the weakest. Not only would these tidal forces have changed the initial shape of Charon from something resembling a football to that of a sphere but they would also, most likely, have created the remarkable rifts, chasms and cliffs that New Horizons probe has observed.

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Four moons

If the fission theory is correct, all of the smaller moons of Pluto should have smooth surfaces, be almost crater-free and have perfect football-like shapes. Mathematically, we call such shapes Jacobi ellipsoids.

The low resolution photographs of Hydra released today suggests, however, that it may be an irregular body.

One very good reason that Hydra may not have a regular shape today is that it shares orbital resonance with the two smallest moons Styx and Kerberos. The three bodies Hydra, Styx and Kerberos could be the shattered remnants of two former ellipsoidal moons of Pluto that collided soon after the Pluto system formed, when the whole family of moons had highly chaotic orbits.

Meanwhile, the new images show Nix does have a jelly bean-shape very closely resembling an ellipsoid. It has only one possible crater. That is a massive boost for the fission theory if this holds up, but to be certain we must wait until October when the high resolution images of Nix are downloaded from the spacecraft.

If Nix does later prove to have an irregular shape like Hydra, and a surface battered by impact craters, then we must conclude that the collision model provides a better description for the formation of the smaller moons of Pluto.

The discovery of rings around Pluto would reinforce the collision theory as these are not expected in the fission model. If that turns out to be the case, then I shall be disappointed but not unhappy. It is only by putting up new ideas and testing them can we hope to unravel the mysteries of nature. That is how science works.

While the origin of Pluto, Charon and the smaller moons are still very much open for debate, one thing is very certain -- this is the absolutely staggering contribution which the New Horizons Mission is about to make to our understanding of the physical, chemical and geological processes that were at work 4.5 billion years ago, when our solar system was formed.

The discoveries to be made by New Horizons mission will soon prove how very little we know about the workings of nature, even in our own very small corner of the Cosmos. As Michelangelo said: Ancora imparo!

Related: New images reveal ice mountains on Pluto

Related: Pluto's weird moons tango with gravity

About the author: Dr Andrew Prentice is an astronomer and planetary scientist at Monash University and Adjunct Professor of the University of Southern Queensland.



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