For decades I underestimated you.
Recently in this space, I callously lumped you in with the Moon, writing that you both “…are dead.” in an unflattering comparison with our planetary system’s more active and beautiful orbs.
In lectures, articles, books and street corner rants I confidently described you as an extreme case, an “end-member” planet, interesting mostly in comparison with other less dense, rocky and senescent orbs. Your story has served as a cautionary tale – “don’t let this happen to your planet”: furious impact pummeling during formation at the sun-seared hot extreme of condensed matter, leaving only a metallic core with a flimsy mantle of battered, desiccated rocky material from which all of the “volatiles”, (the good stuff, the active, beautiful and biogenic materials, water, sulfur, carbon etc.), were exiled, driven off to regroup elsewhere in the solar system where the forces of heat and destruction were less concentrated and light molecules could gather peacefully in clouds, ices, oceans or bound inside rocks.
You were a topic of forensic planetary investigation – the time and manner of your death of interest for planetary formation theories – but not a place where very much, save the knocking about of surface atoms by angry solar particles, would be happening today.
As of this writing parts of this story are still true, but much of it as been discredited in the best possible way. Mercury is still the outlier at the dense, scorched inner end of our planetary system. Vast areas are indeed ancient and cratered, but NASA’s sharp-eyed MESSENGER spacecraft in giving Mercury its first complete going over, has found things that should not be there. Turns out the hot little rock/metal dynamo has been ignoring our theories all along and harboring areas of dynamic change and renewal, excavation and evaporation and who knows what. Many of its craters are dotted with clusters of small rimless hollows, not old enough to be dented with later craters. Something’s happening there. What it is ain’t exactly clear but, by analogy with similar pits on Mars, it seems that surface material is evaporating. Perhaps these are widespread deposits of sulfur-rich rocks. If so, what they hell are they doing there? We teach in cosmochemistry class that sulfur is among the most volatile elements, and has no place on such a world. Mercury would have flunked Cosmochemistry.
But (in science at least) reality wins. So it’s our textbooks that flunk Mercury’s lessons.
I was taught the rules of planetary chemistry by the geniuses who figured them out. My PhD advisor John Lewis derived the theory of “equilibrium condensation” by which we still basically understand which type of planetary matter forms at which distance from the sun – rock and metal in close where its hot, ice and gas far from the sun. (His advisor, Nobel laureate Harold Urey was the first person to apply modern chemical theory to the formation of the Earth and planets.)
Our minds want to impose order, find the simple, underlying patterns beneath the surface of the unruly universe. Chemistry pointed toward a well-ordered solar system but physics came along and jumbled everything up. When proto-planets grew almost to planet size they were tossed hither and thither in a mosh pit of mutual gravitation. The clean chemical seating chart was trampled on and some metal and rock were tossed far from the sun and some ice and water-rich rock was sprinkled throughout the crowd (and, perhaps, somehow sulfur ended up plentiful on Mercury where we all know it does not belong).
We planetary profs will recover from the indignity, and our newer texts will be, if not exactly right, surely less wrong. That the Mercurian surface is also mercurial – and surprisingly lovely – reminds us that we have only begun to explore our own solar system and that there are many delightfully embarrassing and instructive surprises in store. Someday when our descendants have thoroughly explored dozens of planetary systems then they’ll know which, if any, of our precious ideas endured the relentless bombardment of real data.