1. Thera Station

The thing nobody tells you about Europa is how loud the ice is.

It groans. It cracks. Jupiter pulls on it every three and a half days and it flexes like a living thing, and the sound comes up through the floor of Thera Station in long, low notes that you feel in your back teeth.

Ana Ruiz had stopped noticing it by her second month. By her fourth she slept through quakes that would have sent her mother running for a doorway in Monterrey. Night shift was four people: Ana on the drill, Okonkwo on reactor, Delacroix on comms, and the station AI, which everybody called Pickles for reasons that predated all of them.

"Hydrophone three's doing something weird," Pickles said at 0214 station time.

"Define weird," Ana said, not looking up from her coffee.

"Rhythmic. Not geological."

She looked up.

The waveform on the main display was a series of sharp spikes separated by silence. Two. Then three. Then five. Then seven.

Delacroix put her mug down very carefully, the way you put down something you don't want to drop.

"Pickles," Ana said. "Tell me that's an echo."

"It is not an echo," Pickles said. "Eleven."