File:The icy moons of Jupiter (potw2241a).jpg

English: This Picture of the Week shows two of Jupiter’s moons, the icy Ganymede and Europa, which have been imaged in the infrared using the SPHERE instrument on ESO’s Very Large Telescope. Whilst Europa is quite similar in size to our own Moon, Ganymede is the largest moon in the whole Solar System – it’s even bigger than the planet Mercury! Their orbits around Jupiter are slightly elliptical, so they get closer and further away from the planet as they orbit it. This results in the moons being stretched and squeezed by the gravitational pull from Jupiter at periodical intervals. This creates frictional heat, warming the insides of the moons, which has made them geologically active. Europa in particular is likely to have active plumes and geysers erupting from the oceans of liquid water beneath the thick ice cover that makes up the surface. Estimates on the abundances of chemical species on the marble-like surfaces of these moons could be made thanks to these new images and also spectra, which have been published in two studies by Oliver King and Leigh N. Fletcher at the University of Leicester in the United Kingdom. They found that the bright regions of Ganymede consist mainly of water in the form of ice with hints of various salts, and that they have formed more recently than the older darker patches, whose composition still remains a mystery to astronomers.Observing these moons with ground-based telescopes is challenging, because they look as small as a 1 Euro coin seen from 3-5 km away. Earth’s atmosphere would completely blur these images, but SPHERE’s adaptive optics system corrects these distortions, delivering very sharp images with details as small as 150 km.