Mercury A to Z: Graphite

Hello, friends!  For this year’s A to Z Challenge, I’m giving you a guided tour of the planet Mercury, perhaps the Solar System’s most under-appreciated planet.  In today’s post, G is for:

GRAPHITE

I did not know this prior to doing my research for this A to Z series, but apparently Mercury is the least reflective, darkest colored planet in the Solar System.  For a long time, scientists assumed Mercury’s dark color must have something to do with iron.  Thanks to the planet’s unusually high density, we know that Mercury is an iron-rich planet, after all.  However, NASA’s MESSENGER spacecraft was unable to detect significant amounts of iron on the planet’s surface.

After rethinking their assumptions and reanalyzing MESSENGER’s data, scientists now believe that Mercury’s crust might be covered in carbon, specifically carbon in the form of graphite.  The same material used in pencils.  So how did this happen?  How did Mercury get covered in graphite?

Let’s go back in time.  Billions of years ago, when the Solar System was still forming, Mercury would have been just a giant ball of liquid magma.  During that time, heavier elements, like iron, would have sunk down toward the center of the planet; meanwhile, lighter elements, like carbon, would have floated up toward the planet’s surface.  As a result, when the planet started to cool off and solidify, a significant amount of carbon would have been incorporated into the planet’s crust.

Something similar must have happened on Venus, Earth, and Mars; however, Venus, Earth, and Mars continued to be geologically active for a long time after they formed (fun fact: Earth is still geologically active today!).  Mercury didn’t.  So while volcanic eruptions, plate tectonics, and the like allowed Venus, Earth, and Mars to transform their carbon rich surfaces into more mineralogically mixed planetary crusts, Mercury’s crust stayed more or less the same.

I don’t want to make it sound like Mercury didn’t try.  Some amount of volcanic activity did happen on Mercury, long ago.  And in some cases, asteroid impacts punched through the planet’s crust, causing lava to spill out onto the planet’s surface (remember my post on Caloris Basin?).  But even in regions where Mercury’s original crust has been resurfaced by lava, small asteroid impacts have re-exposed the graphite layer underneath.  Those asteroid impacts have also scattered graphite dust over the surfaces around craters.

Based on what I read, it seems that NASA’s MESSENGER spacecraft was not well equipped to study the graphite on Mercury.  And why would it be?  When MESSENGER was launched, no one knew the graphite was there, and given how expensive space exploration is, you don’t want to load up a spacecraft with equipment that you don’t think you’ll need.  And honestly, who would have expected to go to another planet and find the place is covered in pencil lead?

Anyway, hopefully ESA/JAXA’s BepiColombo Mission will be able to follow up on this when it arrives in Mercury orbit in 2025.

WANT TO LEARN MORE?

Here’s an article from The Conversation about the discovery of graphite on Mercury.

And here’s the original research paper reporting on the discovery.

Also, if it’s true that Mercury is covered in graphite, then the force of asteroid impacts might have turned some of that graphite into diamonds.  Click here to learn more about that.