Michael Swanwick, 'Slow Life', reprinted with permission:
The raindrop began forming ninety kilometers above the surface of Titan. It started with an infinitesimal speck of tholin, adrift in the cold nitrogen atmosphere. Dianoacetylene condensed on the seed nucleus, molecule by molecule, until it was one shard of ice in a cloud of billions.
Now the journey could begin.
It took almost a year for the shard of ice in question to precipitate downward twenty-five kilometers, where the temperature dropped low enough that ethane began to condense on it. But when it did, growth was rapid.
Down it drifted.
At fourty kilometers, it was for a time caught up in an ethane cloud. There it continued to grow. Occasionally it collided with another droplet and doubled in size. Finally it was too large to be held effortlessly aloft by the gentle stratospheric winds.
Falling, it swept up methane and quickly grew large enough to achieve a terminal velocity of almost two meters per second.
At twenty-seven kilometers, it passed through a dense layer of methane clouds. It acquired more methane, and continued its downward flight.
As the air thickened, its velocity slowed and it began to lose some of its substance to evaporation. At two and a half kilometers, when it emerged from the last patchy clouds, it was losing mass so rapidly it could not normally be expected to reach the ground.
It was, however, falling towards the equatorial highlands, where mountains of ice rose a towering five hundred meters into the atmosphere. At two meters and a lazy new terminal velocity of one meter, per second, it was only a breath away from hitting the surface.
Two hands swooped an open plastic collecting bag upward, and snared the raindrop.
"Gotcha!" Lizzsie O'Brien cried gleefully.
She zip-locked the bag shut, held it up so her helmet cam could read the bar code in the corner, and said, "One raindrop." Then she popped it into her collecting box.
Sometimes it's the little things that make you happiest. Somebody would spend a year
studying this one little raindrop when Lizzie got it home. And it was just Bag 64 in Collecting Case 5. She was going to be on the surface of Titan long enough to scoop up the raw material of a revolution in planetary science. The thought of it filled her with joy.