Last Friday I attended a bonfire party. There were four things in large quantities present at this gathering -- adults, children, dogs and alcohol. This does not necessarily bode well for the evening's outcome, but that is another story. After a few hours there were only three things to be found in large quantities - sleeping dogs, sleeping children and drunk adults.
The bonfire was very large and subsequently very hot. It was a cool night so I was wearing a t-shirt, sweater and a leather coat. As the fire grew larger and hotter I shed the coat and sweater and took to standing at a given distance from the fire. On a cool night with a warm fire there is a definite comfort boundary. A few feet too far and you are cold, a few feet too near and you are sweltering. It made me think of other such boundaries in nature. I could think of two. The first is the event horizon of a black hole and the second was the Moho. The Moho is the boundary that separates the Earth's crust and mantle. And I knew that it had a longer name but I couldn't remember what it was. So I was standing next to the fire as it burned down to a large pile of glowing embers contemplating the longer name and considering my chances of getting the answer out of one of the other people around the fire when I was approached by one of those inebriated others.
This guy put his arm around my shoulder, to steady himself I'm sure, and said, "look at that," as he pointed to the glowing embers, "It's like we are on some hot red planet like Mars." I responded by telling him that Mars, while hot in the sunlight, doesn't actually burn. He sad, "Yeah, cool, maybe it's like that planet right next to the sun." And I named Mercury as the closest planet to the sun and he said, "Yeah, that's the one. So we're on Mercury and it's burning." And then I had to explain that oxidation does not occur on Mercury due to the lack of atmosphere and subsequently oxygen. He muttered something about me being smart and stumbled into the darkness beyond the lit ring of the dwindling fire as another guy approached me. "I was thinking that it's like that planet that is turned inside out." That one took me a few seconds. I was trying to think about an inside out planet when I realized, "You mean Io, the moon of Jupiter with all the volcanoes and no visible crater impacts?" To which he responded, "Dude, that's it! You are way smart! You deserve a beer." I tried to explain that I don't drink alcohol but he was already meandering to the coolers. At this point I decided that it was best to keep my query about the Moho to myself.
Halfway home I remembered that it was the Mohorovic Discontinuity. And on Dirty Dancing they called it "my dance space."