Monday, 3 March 2014

The Curious Core of Mercury

After a three decade long gap without a visiting spacecraft, the Messenger spacecraft closes in on Mercury. This first image revealed a whole new side to the closest planet to the Sun (Literally: no one had ever seen this side of the planet before).
A Random Tour of the Solar System, Part One.

Hello! After realising that this blog was beginning to turn into "exciting space news of the week", I've decided to mix it up a bit and embark on an irregular tour of the Solar System. My vague aim is to get through all eight planets by the end of my PhD, but we'll see...

Basic facts and figures about the planets are available across the internet, so instead of a general overview I'm going to concentrate on one or two things I find interesting about each planet. First up is the both the smallest planet and the closet to the Sun, Mercury!

Space can be a dangerous place.

On Friday two astronomers from the University of Bristol came to talk to us about their work in trying to figure out how solar systems are formed. This is a very active area, spurred on by the huge numbers of exoplanets, planets in orbit around stars other than the Sun, that have been discovered in the past twenty years.

Although we have the rough series of events that cause planets to form figured out, the details are proving tricky. The models and simulations are very inefficient, only producing planets in a few cases. This is a problem, because planets are everywhere. Just last week, the team working on the Kepler space telescope, a dedicated planet hunter, announced the discovery of 715 new planets (although opinion amongst the exoplanet PhD students at Warwick is divided as to how much we can believe that result).

Teams around the world are working on how to explain how all of these planets came to be. The group from Bristol is focusing on how the newly forming planets interact with each other, and in particular what happens when they collide.
The evidence for those collisions is all around us. The leading theory as to how Earth gained a Moon so similar in size to it involves a massive collision between the young Earth and another object, roughly as big as Mars. From the huge cloud of debris left after that cataclysmic event came the Earth and Moon we know today.

Further out in the Solar System, the ice giant Uranus orbits on its side, again most likely the result of a massive collision. And a collision may have paid an even bigger part in the history of Mercury.

In 1974 the Mariner 10 spacecraft became the first man-made object to visit Mercury, swinging past in a brief flyby. As Mariner 10 curved around the innermost planet, a rocky ball with a diameter only a third of the size of the Earth's, the tiny spacecraft's orbit was subtly shaped by Mercury's gravity. Because of this, astronomers were able to accurately measure the mass of the Solar System's smallest planet for the first time.

What they found was surprising: Mercury appeared to be far too heavy, a much denser object than they had expected. The density of a planet should increase as its mass goes up, with the material squeezed tighter and tighter by the increased gravity. The results from Mariner 10 showed that Mercury has a much higher density than Mars, the second smallest planet, despite having only half of the mass.

This means that Mercury's iron core, which only makes up a small part of the volume of the other rocky planets, must be huge. The core takes up 85% of Mercury's volume, most of the planet. In contrast, Earth's core makes up only 15% of its volume.

Comparison of the interior structure of Earth and Mercury. The core, which makes up only a small part of the Earth, takes up the bulk of Mercury's interior.

According to our current knowledge of how to make a planet, Mercury could never have formed with a core that big. The structure of a planet is determined firstly by how large they are, but also what they're made of.

Once they get big enough that their interior melts, the heaver elements such as iron sink to the middle and form the core, leaving behind the lighter rocks that make up the mantle and crust above it. This process, known as differentiation, has occurred on all of the rocky planets, as well as some of the larger asteroids.

If all of the planets formed out of roughly the same mix of stuff, which we've no reason to believe that they didn't, then a planet the size of Mercury simply wouldn't have enough iron to form such a large core. A core that big should have formed a much larger planet around it. Something strange happened to Mercury, leaving it with a lot less material that it should have had.

There are a number of ideas as to what that strange thing was. When the solar system was first formed the Sun was a lot more variable than it was now, going through massive temperature swings as it settled down into being a star. If Mercury formed close in to the Sun when it was relativity cool, the next time the Sun heated up it could have melted the new planet's outer layers, blowing them away in a intense solar wind.

Another suggestion also points to Mercury's closeness to the new star. Mercury is only just over a third of the distance away from the Sun than the Earth is. That close, the protoplanetery disk of gas and dust, out of which the planets formed, was very dense. Many of the small clumps of material know as planetesimals, the first stages in building a world, would have been caught by the friction of the disk, spiralling down into the Sun before they could begin to form planets. This would have affected the less dense material more, leaving only the heavier stuff to be built into Mercury.

The third explanation is a lot more dramatic. There is one very easy way to get rid of a large chunk of a planet: hit it with something big. Very big.

Mercury's huge core may have been the result of a massive collision, a glancing blow blasting away its outer layers.
The collision hypothesis suggests that another roughly Mercury-sized planet, long since gone, smashed into it soon after it formed. Mercury was dealt a glancing blow, blasting away its outer layers.

The debris would have been quickly scattered away, pushed by the gravity  of nearby Venus and accelerated away by the effects of the Sun's heat, before it had a chance to collapse back onto Mercury. The molten ball of iron and rock left by the collision would then have slowly cooled into the Mercury we see today.

Mariner 10 flew past Mercury twice more, flying too fast to go into orbit, before heading off into space, never to encounter the planet again. Without enough information, which of the three ideas about how Mercury gained its massive core was correct would remain a mystery for over thirty years.

Shielded from the intense heat from the Sun, Messenger closes in on Mercury in this artist's impression
On the 3rd of August 2004 the Mercury Surface, Space Environment, Geochemistry and Ranging, or Messenger, spacecraft blasted off from Cape Canaveral in Florida. Four years later it flew past Mercury.

Like Mariner 10 it would fly past twice more. Unlike Mariner 10, Messenger would be back again, and this time it would be there to stay. In March 2011 Messenger became the first man-made object to enter into orbit around Mercury.

The environment Messenger found itself in is, to say the least, harsh. To protect it from the heat of the Sun, one side of the spacecraft is hidden under a thick shield. But the surface of Mercury can reach temperatures of over 400 degrees Celsius during the day, heating up space around it and threatening Messenger's unprotected underside. Messenger therefore was put into a very elliptical orbit, spending most of its time away from the planet before quickly swooping down to study the baking hot surface below.

There Messenger used its array of instruments to try and shed light on the mystery of Mercury's massive core. Far more advanced than Mariner 10, Messenger was able to study the chemistry of the rocks in Mercury's crust in detail.

In particular, Messenger was looking for potassium, along with uranium and thorium. Potassium is part of a group of elements known as volatiles, with a much lower boiling temperature than the other two. If Messenger found that Mercury had a lot less potassium than uranium of thorium, it would suggest that the surface had once been molten, supporting either the collision hypothesis, or the idea that Mercury had been melted by a hot young Sun.

However, Messenger found just the opposite. There was more potassium than uranium or thorium, showing that the surface couldn't have melted. It seems like that dramatic collision never happened.

That puts us back to square one in many respects, as the third theory, the one which didn't require melting, is far lest developed than those that did. The leading theory now suggest that Mercury formed out of iron-rich meteorites and comets, but planetary scientists are still a long way from a complete explanation for how this odd little planet came to exist.

Messenger is still in orbit, continuing to study the solar system's smallest planet. In 2023 it will be joined by the massive (and over budget) Bepi Columbo spacecraft. Maybe then Mercury will finally give up its secrets.

I seem to have got a bit carried away taking about the core, so I'll have to leave my second Mercury mystery for another post. For now, I'll just tell you what it is. Mercury, which as I mentioned has daytime temperatures of upwards of 400 degrees, appears to have water ice at its south pole. Ice! There's a lot more to Mercury that just a big core.

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The areas of Mercury that receive little or no sunlight, shown in orange . Protected from the roasting heat, Messenger has detected water ice in the deep craters at Mercury's south pole

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