Hi everyone! Sorry that the posts were thin on the ground last week. The head druid told me that I needed to honor the solstice by taking some time to reflect on the meaning of things [citation needed]. Anyway…since I didn’t blog last week, I failed to post these astonishing pictures of Jupiter’s giant moon Ganymede, which were photographed by NASA’s Juno spacecraft as it slaloms though the Jovian system.

Ganymede as imaged by NASA probe Juno

Although its lack of atmosphere and pockmarked plains of dust make it superficially resemble Earth’s moon, Gannymede is a very strange and unique heavenly object Of the 200 known moons in the solar system, it is the largest. Indeed it is 26% larger than the planet Mercury by volume (although it is only 45% as massive as the metalliferous first planet). Ganymede has a diameter of 5,268 km (3,273 mi), so each pixel in the full size image of the Jovian moon is equal to a kilometer (although you may want to check out the NASA image to really savor that scale–since WordPress has a noteworthy penchant for scrunching up my images in incomprehensible ways).

A photo of the dark side of Ganymede taken by Juno’s incredibly light sensitive navigational camera

Alone among moons in the solar system, Ganymede has a magnetic field, albeit a rather meager one compared to Earth or Jupiter. Scientists surmise that the magnetic field is created by convection within the liquid iron core of the moon–although answers are not forthcoming as to why it has a liquid iron core to begin with (these planetary cores seem to be the real determinant of what planets are like, but I feel like we know precious little about them). Thanks to its size (and maybe thanks also to its magnetosphere), Ganymede has a very thin oxygen atmosphere…but that just creates more question, since elemental Oxygen has a tendency to instantly bond to all sorts of other elements. The 20 percent or so of oxygen in Earth’s atmosphere did not become a mainstay until about 1.5 billion years ago when photosynthesizing bacteria finally became so prevalent that they overcame the constant loss of atmospheric oxygen thanks to oxidation. Hopefully Juno’s survey will help us solve atmospheric mysteries on Ganymede. Ganymede is also believed to have a vast subsurface ocean of icy water tucked away somewehere beneath its surface. Astronomers have reasonably speculated that this Ganymede underworld ocean may contain more water than all of Earth’s oceans combined!

This is the largest version of this interesting cross section which I could find

Ganymede is a Galilean moon–which means it was discovered by the great scientist, and is one of the first objects ever discovered to orbiting another planet (I still sometimes imagine the thrill Galileo must have felt when he realized what he was seeing). I wonder what surprises Juno will send back for us!