410 years ago today, Galileo discovered the moon Ganymede orbiting Jupiter. This giant moon is larger than Mercury, and has a rocky looking surface and probably a nickel iron core that is responsible for its strong magnetic field. Between this surface and that core is where things get interesting. Based on a variety of different kinds of observations, it appears that Ganymede has a subsurface ocean that is heated by the constant flexing and stretching the world experiences as it interacts gravitationally with the other Galilean moons. These oceans may be the largest in our solar system, with a possible depth of 800km. If there is life in the oceans, we don’t have a way to detect it, at least not today.

This is a reminder that life may exist in all sorts of places where we can’t remotely detect it, and all these discussions of goldilocks zones only apply to surface life, like us, and we have no way of knowing what kind of life, if any, is common out there among the stars.

Read more about Ganymede at:

• Decades-Old Data Shows Auroras on Jupiter’s Moon Ganymede in a New Light (Space.com)
• Ganymede (Wikipedia)