Forecast for Jupiter's moon Ganymede: Extremely cold and… humid?

Using a combination of old and new data from Hubble Space Telescope, astronomers have found water vapor in the atmosphere of Jupiter’s planet-sized moon Ganymede!

This is very cool news, literally: On most of the moon it’s too cold for water vapor, but in some spots where it’s warmest they found the highest abundance of it, clearly indicating it’s coming from sunlight heating the surface. Plus, bizarrely, a contribution from Jupiter’s ridiculously fierce magnetic field.

Ganymede is the largest...