Mercury is a funny little world. Made mostly of iron (much of which is solid), with a scuffed, pockmarked surface it might appear to have more in common with a cannonball than a planet. A barren place now, with a silent surface and no atmosphere save for a whisper of oxygen and gaseous sodium atoms, Mercury was once much more active. In its past, it even had oceans.
Oh, except that those oceans were actually made of molten rock.
Over 4 billion years ago, shortly after it had formed, Mercury was a fiery little place. A vast and beautifully deadly ocean of magma may have blanketed the planet. Perhaps created by the violent collisions that happened as the solar system’s planets were forming, or maybe in the face of much higher temperatures than today back when the sun was still contracting, the young Mercury would have stayed partially molten for the first few millions of years of its life.
NASA’s MESSENGER probe has been in orbit around our solar system’s smallest planet for nearly two years now, and it’s been sending back a wealth of data for planetary scientists to pick at. One particular instrument carried by MESSENGER is an X-ray fluorescence spectrometer, able to give us a glimpse at the individual chemical elements locked up in Mercury’s surface rocks.
Interestingly, the surface of the little world is made up of two distinct rock compositions, giving the people analyzing that data a puzzle to solve. What geological processes could lead to two very different types of rock on what appears to be a very continuous planetary surface?
A team at MIT, led by Timothy Grove, decided to answer the question by creating their own version of Mercury — in the lab.
If you know the combination of elements and compounds in any rock, even from another planet, it’s fairly easy to make a fake version from lab chemicals. Grove and company first created fake Mercurian rock, then subjected it to a few different pressures and temperatures, with the aim being to try and figure out what geological processes they might have experienced as they formed.
Rocks are made mostly from crystalline oxides. On Earth, there’s far more oxygen in the planet’s crust than there is in the atmosphere, chemically bound to other elements like silicon, magnesium, and aluminum; rocks behave the same way, chemically, on all four of the inner planets. Using the data from MESSENGER’s X-ray spectrometers, the team analyzed Mercury’s surface composition and determined the ratios of elements present in the planet’s rocky crust, in order to find out precisely what oxides it’s made of. With that knowledge, all they needed to do was make a copy.
As Grove’s team subjected their synthetic Mercury surface to different temperatures. As some rocks started to crystallize out of the synthetic magma, the composition of the remaining molten rock began to change. Very simply, an ancient ocean of magma on a planetary scale was the simplest explanation for the two different types of rock on Mercury’s surface. And I truly do mean ancient. To quote Grove directly: “The crust is probably more than 4 billion years old, so this magma ocean is a really ancient feature.”
While we may never know for certain, what the planets were like earlier in their lives, planetary archaeology like this is the best we can do to get a glimpse of it. The picture may change as we learn more, but as we learn more we’re inevitably going to understand more about the way planets form and evolve.
Image: Mosaic image of Mercury taken by NASA MESSENGER. Credit: NASA