... Europa, the fourth largest moon of Jupiter, was recently discovered to have a thin oxygen atmosphere by scientists using the Hubble Space Telescope. ... Jupiter's moon Io has active volcanoes. ... It is, in fact, a Seyfert type 2 spiral galaxy. The "S" is actually a lane of stars, gas and dust circling the core. Jupiter's Colorful Clouds . ... What makes the colors in Jupiter's clouds? ... The Hydra Cluster of Galaxies . ... The view looks down on the Moon.. Tonight: A Blue Moon . ...