Mercury's Gold

One of the aims of classical alchemists was figuring out a way to turn base metals into precious metals, often using the liquid metal mercury in their processes.  Ultimately, it was a fool's quest (although they did discover a lot about chemistry in the process).  This is my clever way to introduce a post about fool's gold and Mercury.

A week or so ago, March 19-23. the Lunar and Planetary Science Conference (LPSC) was held in Houston, Texas.  A few of the papers presented caught my eye since I'm teaching a course on Solar System Astronomy this semester.

A group of researchers presented a paper titled Mercury's Internal Structure as Constrained by Messenger Observations where they discussed some of the most recent findings from NASA's MESSENGER probe which is currently orbiting and studying the planet Mercury.

Since we know how large Mercury is (mean radius = 2.4397 x 10⁸ cm) and we can accurately determine its mass from its orbital characteristics (3.3022×10²⁶ g), we can calculate its bulk density (r) which is mass per unit volume.

[The radius and mass values I used are from Wikipedia.  The radius was multiplied by 100,000 to convert kilometers to centimeters and mass was multiplied by 1,000 to convert kilograms to grams.]

   r = 3.3022×10²⁶ g / [(4/3) p (2.4397 x 10⁸ cm)³]

   r = 3.3022×10²⁶ g / (6.0827 x 10²⁵ cm³)

   r = 5.43 g/cm³

OK, so what?  Well it turns out that this is an anomalously high density for such a small planet.  The average density of crustal rocks is around 2.7 g/cm³.  This means that the interior of Mercury has to be much more dense.  Since, for a variety of reasons, we know that terrestrial planets have internal cores composed primarily of iron, the size of this iron core can then be calculated.

The Earth, for example, has an internal structure composed of an iron core, a mantle, and the crust.  The crust is, of course, solid.  Many people mistakenly believe the mantle of the Earth is molten magma but it's also solid.  It's properties change with depth, due to pressure and temperature changes, but it's primarily plastic.  This means it's soft and flows while remaining solid (I illustrate this in my class with Silly Putty).

The core is segregated into two parts - an inner solid core surrounded by an outer liquid core.  As the Earth cools over geologic time spans, the core is gradually solidifying.  The movements of liquid iron in the Earth's outer core, by the way, generates the Earth's magnetic field.

Mercury also has a core, mantle, and crust but the core is much larger proportionally than we would expect.  Mercury may once been much larger but lost much of its outer core and mantle from a large collision (there is a lot of indirect evidence on other planets and moons for the occurrence large collisions early in the history of our solar system).

By analyzing the rotation of Mercury on its axis, planetary scientists are able to determine that the solid outer crust and mantle of the planet is decoupled from a solid core by a liquid layer.  This is interpreted to mean that the outer part of Mercury's iron core is still liquid.  Mercury does have a magnetic field as well.

The MESSENGER spacecraft has been making some detailed measurements of Mercury's gravity field and this data, along with geochemical data obtained from analysis of surface rocks on the planet, allows planetary scientists to develop models of the interior of the planet (see the paper for all the messy mathematical details).

One model leads to the possibility that there is a layer of solid iron sulfide (FeS₂) at the core-mantle boundary.  This is the mineral pyrite, also known as "fool's gold" (you would be a real fool to confuse it with gold because it has very different physical properties), and relatively common here on Earth.

Image from Sky & Telescope

Pyrite's a beautiful mineral, often forming golden cubic crystals.  It's neat to imagine the interior of a planet has a layer of this stuff.  It's also neat to see how astronomers can figure out what's in the interior of a planet without ever stepping foot on it.  Science rocks!