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What happens when a planet melts?
For Mercury, the closest planet to our sun, this is no idle question.
At the risk of it being understated, Mercury is a very hot planet.
With daytime temperatures reaching an incredible 430°C, the temperature of some wood-burning
fires, the rocks and dust on Mercury’s surface bake beneath a blistering heat that pushes
them towards their limits.
It’s not the hottest planet in the solar system – that honour goes to Venus, thanks
to its thick atmosphere – but it’s certainly up there.
The Mercury we know today has actually cooled considerably over the years.
There is ice at its polar caps, and in our last video, we discussed the signs on its
surface that show it has contracted over time as its interior became colder.
So, what was it like back then?
I’m Alex McColgan, and you’re watching Astrum.
Join with me today on a trip back through time, to an age billions of years ago when
the rocks on Mercury were not just approaching a melting point but were frequently tipped
past it.
When meteors rained from the sky, and when lava flowed through canyons like rivers.
When the soil burst and bubbled in volcanic eruptions that carved out massive craters
in the planet’s crust.
To an age when the planet’s surface melted.
When Mercury was a lava planet.
When rock is sufficiently heated, its solid structure breaks down and it turns into the
gloopy, viscous liquid known as magma, with a viscosity – or “runniness” – 10,000
to 100,000 times more viscous than water.
For a point of reference, this is a similar viscosity to tomato ketchup, although I would
not recommend putting this on your food.
Depending on the rock type, magma forms at temperatures of at least 600°C, but potentially
as high as 1300°C. So, for Mercury to have begun to melt, we know that it must have reached
at least these temperatures.
In spite of being much less runny than water, lava can still travel for great distances
before stopping.
This is because once the surface of lava hardens, it forms an insulating layer that keeps the
rest of the lava within protected so it can flow freely.
How do we know this happened on Mercury?
The clues can be found in craters like Raditladi.
Scientists estimate that Raditladi is a relatively young crater – likely under a billion years
old - with well-preserved walls and a floor relatively clear of other, later impacts.
It’s large – over 25km in diameter.
Notice how rough the hills are around the crater, and yet inside is a smooth plain?
This is no coincidence.
Originally, the terrain inside Raditladi was likely about as rugged as the hills around
it.
So why is it so smooth now?
The answer is lava.
When lava is left on its own, it will try to form the flattest surface possible, just
like water does if you put it in a bowl, as it is dragged down under the effects of gravity.
The same happened here: an asteroid crashed into the planet’s surface, and the crater
quickly filled with lava.
Once the lava inside cooled, it formed the smooth plain you see here.
But where did this lava come from?
There are two theories.
The first is that the impact of the meteor triggered a creeping volcanic eruption, as
magma from beneath the surface rose up through cracks to fill the basin.
The second explanation is that the surface within the crater got so hot due to the impact
of the meteor, that it pushed the already hot rock crust over the tipping point into
melting.
This kind of lava is known as impact melt.
The true explanation is likely a combination of both.
Now that we know that smoothness is a sign of lava flow, we suddenly realise that there
are numerous other craters on Mercury that similarly must have been filled with lava.
Just take a look at Rustaveli, where crags of mountain can be seen poking up through
the smooth lava layer: Or Copland.
Polygnotus.
Or Rachmaninov.
Rachmaninov is particularly interesting, as here you can see the stronger indicators of
lava bubbling up through from beneath the surface to the centre of the crater.
Take a look at the strange crinkled cracks forming a rough circle inside the central
crater.
Such cracks are a signal that a slower outpouring of magma pushed up from beneath the surface,
breaking the plain, then cooling, then pushing up and then cooling again under the effects
of Mercury’s fluctuating temperature.
Here, and in many of these impact craters, the collisions from space triggered deep volcanic
activity from within Mercury’s shell.
But lava didn’t just flow within the craters.
Take a look at the valley known as Angkor Vallis.
Here you can see clear signs of smooth lava flow, but this time moving like a river.
The lava travelled from high to low ground, until it eventually poured into the basin
next to it.
Flows like these ended up filling massive seas, taking up vast swathes of the planet
and turning them the more orangey colour we see today.
Scientists have begun to recognise this tell-tale orange colour as a sure sign of volcanic activity,
and from it a more detailed picture has begun to emerge of conditions on early Mercury,
that make it even less hospitable.
Areas like this one to the north east of Rachmaninov are likely formed by volcanic activity.
When MESSENGER flew over this area in 2015, it took detailed photographs of it, and found
the surface to be covered in a fine dust: Upon review, it was obvious what this dust
was – volcanic ash, that must have fired out of vents and covered the terrain around
it.
NASA scientists likened it to snow – “fiery, hot, angry snow”.
So, it wasn’t just lava flowing beneath your feet you’d have to contend with on
Mercury, but burning ash falling from the sky.
And that was just the calmer volcanoes.
The final indicator of volcanic activity on Mercury hints at eruptions so destructive
that whole chunks were scooped out of the planet.
Take a look at this crater Navoi.
This is no impact crater.
When a crater is formed onto a hard surface, one that’s not sufficiently hot to melt
into lava, a central peak is usually formed.
This is because when the crater walls suddenly find themselves exposed, gravity suddenly
exerts itself on all that loose particulate, which rushes down the walls of the newly scooped
out basin towards the centre.
Once there, having built up momentum, it comes crashing into all the rocks and landslide
that is sliding down from the other side of the crater.
The two sides meet, and all that momentum and energy forces them to keep moving in the
only direction they can – up.
You see this same effect more clearly when you throw a large rock into water.
The water of the newly formed basin rushes in to fill the gap, but then crashes into
water from the other side and all of it shoots upward in a powerful secondary splash.
But unlike water, the sand and loose rock of a crater does not level out, but forms
a central peak.
Depending on what angle the meteor impacted, this peak is either perfectly rounded, or
possibly tear-drop-shaped.
However, the raised central formation of Navoi is neither of these things.
As scientists looked at this, they came to the conclusion left, that this crater was
not formed by an impact at all.
Instead, it had been carved out through the force of an erupting volcano.
At 66km in diameter, the amount of force exploding upwards that would have been necessary to
carve out this crater and scatter its remnants for kilometres all around it must have been
truly massive.
So there you have it.
Meteors raining from the sky, tipping the rocks they landed on over the melting point.
Volcanoes bursting forth, either filling the landscape slowly with bubbling magma in lakes
and fiery rivers, or choking the air with burning ash – not that there was any air
to begin with, beyond thick toxic gases emitted with the eruptions.
And even the ground you could stand on might at any moment explode beneath your feet.
This is what it was like when a planet was melting.
Mercury is quiet now.
As near as we can tell, there are no longer any active volcanoes on the planet.
Although the sun still bakes down on it, the unbridled fury that raged beneath its surface
is now calm and soothed.
Yet, for all those who know how to look, the evidence of what once was is still there,
locked in the geological record.
It’s the scars that tell the story of a violent past.
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