VOICE OVER: Cold, distant and mysterious - for decades after it’s discovery in 1930,
little was known about the geology of Pluto.
STÈPHANE LABROSSE: First seen from the Earth Pluto is just a very tiny dot. But it was
approached by a mission in 2015, called New Horizons.
VOICE OVER: NASA’s New Horizons probe was the first spacecraft to explore Pluto up close
and it sent back some fascinating images.
STÈPHANE: One of the things that the mission could see was some very intriguing polygonal patterns.
And I was like wow, that’s very neat! What could explain these patterns?
VOICE OVER: The patterns can be seen across the smooth surface of a basin filled with
nitrogen ice. Now, a team of researchers has a new explanation for Pluto’s mysterious polygons.
TITLE: Pluto’s mysterious polygons
VOICE OVER: Looking at New Horizons’s high resolution images of Pluto,
several researchers were intrigued by markings in the region known as Sputnik
Planitia - where dark lines formed geometric shapes several tens of kilometres wide.
STÈPHANE: When it was first observed, the people that worked on the mission - they
very rapidly thought about convection. Because
thermal convection, in particular convection in solids, is known to make this type of patterns.
VOICE OVER : Even though Sputnik Planitia is made of solid ice,
slow moving currents are still the most likely explanation.
STÈPHANE: Solids, if you look at them on long time-scales, behave like liquids.
We can see that with glaciers in mountains - we know that glaciers flow down the mountain
so are like rivers, but very viscous rivers. The ice of Sputnik Planitia is also solid
but it convects on timescales of hundreds of thousand years.
VOICE OVER: The convection currents that make these kinds of patterns
are usually driven by heating from below.
STÈPHANE: That’s what happens when you heat up a pan, you can see patterns, you can see,
you know, motion in the pan, and that’s caused by convection. Because the fluid that is below gets
less dense because it gets warmer and so because it’s below and less dense it wants to move up.
On the other side the fluid that is on top, it’s cooler so it’s more dense and wants to go down.
VOICE OVER: But there was something about the patterns on Pluto’s surface
that didn’t quite fit with the idea of convection currents being driven by Pluto’s internal heat.
STÈPHANE: Heating from below would lead to actually a reverse pattern compared to what we observe,
that is flow going up on the polygon’s sides, instead of going down as we observe.
VOICE OVER: The visible lines delineating the polygons are troughs,
showing that the convection currents must be rising up in the centre of each shape
and sinking at the edges - the opposite to what you’d get if you were heating from underneath.
STÈPHANE: So we still think that convection is at the origin of these polygons,
but instead of heating from below we think it is better explained by cooling the top.
VOICE OVER: To explain this cooling, Stéphane and his team turned to sublimation.
This is the process, similar to evaporation,
by which the nitrogen ice of Sputnik Planitia turns from a solid straight into a gas.
STÈPHANE: This cools down the surface in a very similar way to the situation we encounter in every
day life when we get out of water and we feel cold, because water is evaporating which requires
latent heat to do so and it cools down the skin. And so we think that’s exactly what’s happening.
VOICE OVER: To test this theory, the team developed a computer simulation of the surface.
STÈPHANE: And we found that with reasonable parameters for Sputnik Planitia we obtained
the same type of patterns as what we observe.
VOICE OVER: Their model shows for the first time that sublimation
can drive planetary-scale patterns. The team are confident in their findings,
but it might be a while before all their hypotheses can be fully tested.
STÈPHANE: It would be good to go back there and test some of the ideas that we propose,
but of course this is very far away and it is very difficult to send missions there…
but I’m confident yes.