Geology of the Solar System 4: Mercury

Science!

Because Mercury is tiny, about 1/3 of Earth’s radius, and is rather far away, being the closest planet to the Sun, there is little we can discover about this planet using Earth-based telescopes.  We sent some satellites there instead, the Mariner 10 in the 1970’s and, more recently, the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging – they had to work pretty hard to make that acronym work).

Mercury rotates on its axis once every 58.6 Earth days, and it takes about 88 Earth days to orbit around the sun.  This means that there are exactly two rotations for every 3 orbits around the sun.  Added with Mercury’s highly eliptical orbit, this all makes for an extremely long solar day (the length of time from one sunrise to the next on the surface), at 176 Earth days.  This means that the surface experiences huge temperature fluctuations, with lots of time to reach extremely high temperatures when exposed to the Sun, and also lots of time to cool down to some of the lowest temperatures in the solar system when not exposed.  Here are some cool animations to help you get a sense of what Mercury’s rotation and orbit look like:

From space: http://www.messenger-education.org/Interactives/ANIMATIONS/Orbit_Rotation/orbit_rotation_full.htm

From the surface (the coolest!): http://www.messenger-education.org/Interactives/ANIMATIONS/Day_On_Mercury/day_on_mercury_full.htm

Mercury’s density is 5.44 g/cm3, it has a very thin (practically non-existent) atmosphere of dilute Na with some K, O, and He (salty!)(1), and it has a very weak but distinct magnetic field.

Internal Structure

Having formed close to the Sun, Mercury is made up mostly of high condensate elements like Fe, Si, Ca, Mg, and Ti, and has very low concentrations of volatile elements like K, O, and C that would normally make up an atmosphere or hydrosphere.  It does have some water ice in shaded polar regions, inside of craters, but this was probably introduced to the planet by volatile-rich bolides and would not have originated there.

Almost all of Mercury’s volume is taken up by its core, about 85%. The exact structure is still not fully understood and there is some debate, but the image below is the interpretation that fits my notes the most closely, and is based off of information gathered from MESSENGER.

Mercury’s magnetic field is likely created by convection in the partially liquid middle core, and is probably not pure Fe, as it would have solidified long ago if it was, but iron sulphide, which has a lower melting point.  There is also a lithosphere, made up of Mercury’s mantle and crust.  Unlike the above picture suggests, we cannot actually differentiate between the mantle and crust, and they may not actually exist as separate entities at all.

The crust is mostly of anorthosite composition,  an igneous intrusive rock (formed by the cooling and solidification of molten rocks) that is almost 100% plagioclase feldspar.

<— Like this.

Geologic Evolution

This is all speculative, as we cannot actually obtain rock samples and bring them back for testing.  We can determine a relative sequence of events based on crater densities and styles on the Moon, for whose rock ages are known.

Stage 1: Accretion and differentiation – The formation of the planet would have been closely followed by core formation.  Mercury’s oddly large core and high density suggest the core was formed either from refractory-rich materials or even from a major impact that might have stripped the planet of its silicates and volatiles.  As the surface cooled and solidified, the high thermal state within led to expansion and fracturing of the thin lithosphere. The surface of Mercury is characterized by large, multiring craters and areas of heavily cratered terrain.

Stage 2: Heavy bombardment – This stage created highlands, which are heavily cratered areas mixed with intercrater planes, which might have been formed from lava flows or ejecta blankets from bolide impacts, which obscure earlier features. The core would have still been molten at this point, and the mantle would have acted like an asthenosphere.

Stage 3: Excavation of the Caloris Basin – Towards the end of the heavy bombardment period, a huge multiring basin (1550 km) and impact crater (40 km) was created when something massive hit the planet.  This impact was so powerful, seismic waves made it all the way to the antipode (the exact opposite side of the planet) and created a hilly and lineated terrain about 500 km across.  The crater was then effected by further extensional fracturing, creating spider-like patterns from its center.

This is a false-colour image (Mercury isn’t really bright blue and orange) to show how large the Caloris Basin is (that giant orange cirle in the middle of the photograph). (2)

Towards the end of this period, the planet began to cool enough to allow for global contraction and further fracturing of the crust.  The core was also mostly solid by this point.

Stage 4: Formation of smooth plains – The extrusion of basaltic lavas occurs, obscuring older features like impact craters and creating lowlands.  They remain more or less smooth because of the reduced rate of impact by this time. As contraction continued throughout this period, the lithosphere thickened.

Stage 5: Inactivity and light cratering – The cooling and contraction have now been completed, so there is no longer any tectonic activity (never plate tectonics, just extension and contraction fracturing) or volcanic activity. The asthenosphere would have been thin at the beginning of this period, and now no longer exists.

Footnotes

1. For the chemistry-challenged: Na=Sodium, K=Potassium, O=Oxygen, He=Helium, Fe=Iron, Si=Silicon, Ca=Calcium, Mg=Magnesium, Ti=Titanium, C=Carbon

2. To look at a boat load of awesome pictures of Mercury from the MESSENGER mission, check out this website: http://thebigfoto.com/mercury-from-space

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