In the badlands, ranchers used to tell us how hot it was in the summer. We'd get back to camp after a day of walking around parched staring at the ground looking for bones ready to make any promise for a cold drink. In the middles of continents, temperatures always swing to the extremes. Northeastern Montana is about as middle as it gets. They'd say things like "It was 104 above in the shade." Which is to say, above zero. The need to specify 'above' with a number like that always struck me as very ominous.
It only really ever hits about fifty below out there. But 150 degrees Fahrenheit still sounds like a pretty hefty swing. Driving along today in Chattanooga with the open window like a blowdryer, swearing off the air conditioner in the truck, I realized that Earth has it pretty good anyway. Magically good, in fact. Thinking of that narrow ambient energy range where our puzzle of cellular chemical gymnastics can occur, the surface of the Earth began to look like a membranous oasis wrapped around a smoldering rock slowly orbiting a radioactive bomb.
Between the hottest equatorial temperatures and the coldest polar ones, there's a range of only about 200ºF. In Tennessee tonight it will dip to the 70s after a full day above 100º. In the winter we might get an ice storm, but 0ºF is almost unheard of. Today on Mercury though, it got close to 700ºF during the day and could fall to negative 100ºF tonight. That's a hundred below, in the shade. Here on Earth, we enjoy thermal dampening from water, atmosphere, biomass, lucky distance from the sun, rotational speed, and a zillion other things that allow our lifeforms to take the shapes they do.
That's not to say life couldn't happen in a different way. I tried to imagine a cell with dozens of different metabolic pathways capable of operating anywhere along an 800º range of temperatures with reactions optimized for every heat wave or cold snap. But that lifeform would probably not look like a crow. Or a valiant sprig of dry sage. Or a person on a motorbike. That planet might not have forests of photosynthesizing trees, dry and smoking, tipped over the edge to quick oxidation. The sun carries on shining through a haze of particles, shaped by life, released by heat and fire, splashing down on skins of lichen stretched over globs of rock melted by inner fire and congealed by distance on that same membranous shell of this same old planet. These terrestrial aliens.
