nearly 8000 miles in a circle around Europa's equator there is only ice, unrelieved. Any hills or ridges slope very gradually to a few hundred feet at most. Europa is smoother and rounder than a billiard ball and stiller than an arctic lake. I flexed my fingers in my gloves. This is winter. Beyond the mania of its 3½ day orbit, Europa's only activity is the creation of more ice. When its surface buckles and cracks, more water and rock fill the fissure and freeze. No one knows exactly how this happens. You might hear a crack like thunder explode the silence, and the ice might jerk and split at sonic speeds beneath your feet, and pressurized water flood suddenly into the crack. Or more likely, the ice warps as it does on the pond, with creaks and twists and throatless barks. Water then seeps in slowly from underneath and freezes in the -270 F cold. Craters are erased by this process in short periods of time, say 30 million years. Otherwise there are no shapes or points of reference on Europa, no figures at all. Underneath the ice enormous lines criss-cross like Martian canals, but to someone standing on the surface they'd be invisible and wouldn't exist. There would be nothing to see or hear for millennia at a time, only the disk of Jupiter, 420,000 miles away, filling a sixth of the sky, illuminating flat, unending, polished nothing.

Survival here would involve the generation of motion in utter stillness. The excitation of molecules somewhere, at some innermost area which can hold out the cold. The real activity of winter is whatever supersedes the absence of warmth. On Europa the problem of shelter would concern not only the heat of your physical body but, the same as in Maine, your mind, which would be cooped in by the cold itself, not just the bad or absent roads. On Europa you would invent shapes and figures merely to survive. Your mind would seize upon a gloss of planetlit ice and thaw it into someplace else. To be clear: heat is motion. Temperature is a measure

of molecular activity. Absolute zero is the total absenceof heat energy, the temperature at which all molecular motion stops. This condition is abstractly represented on the Kelvin scale by the expression 0 K. On the Celsius scale it is -273. Fahrenheit, -459. It's difficult to imagine that kind of cold even on Europa, which is relatively warm at -270 F, or in Maine, which is relatively warmer, though not much. The cold is not the problem because you can always give in to it. You can always sleep, drift away toward death, when your energy subsides. You can extinguish much sooner than absolute zero; human beings seem to succumb to the stillness at levels of inactivity much more active than even Europa's. In arctic Canada people do not go out alone when it's colder than -50 F. The core of what is human can solidify long before hell freezes over. Even more difficult to imagine in deep cold is the survival of consciousness. On the surface of Europa, as on the frozen pond, consciousness would be simply the search for warmth, which, again, is activity. The moon-blankness is startling in a way, like the blankness

of the snow-encrusted pond. The mind shivers at the thought of Europa. It's so cold, it's bright: it reflects Sunlight and Jupiter-light with an albedo (or reflectivity) of .62, in astronomy-speak. The Earth, blue and white in space, has for comparison an albedo of .39. The Moon's is .11. Europa's surface temperature is on average lower than its neighbors' because it actually reflects heat away. Of the four Galilean moons of Jupiter, Europa is (with Io) in a warmer orbital zone than Ganymede and Callisto. In the strain and tug of Jupiter's gravitation, Europa is geologically active deep inside, pushing ice and rock slowly upward in a way that Ganymede and Callisto don't. Io is a different story. It is roughly as cold as Europa at the surface, but a place of volcanoes and sulfur compounds, a burning upper hell in contrast to Europa-Cocytus, lower hell. Triton, circling Neptune billions of miles further on, is the coldest place in the solar system, with a mean temperature of -391 F. But even there, seasons lasting decades alter the surface ice, and eruptions of nitrogen crystals blow through the tenuous atmosphere. By contrast, Callisto, 750,000 miles beyond Europa, is completely motionless. Its surface temperature is about -253 F, but nothing internal disturbs that surface, at any interval of time. Rocks inhabit its ice, and so it's dark, albedo .17. Callisto's cold, unlike Europa's, takes form. Instead of an absence of shapes and figures, Next page Previous page Fires of the Sun home The Mind Errant

Winter Moons