A Triple Conjunction

0105-4x5color.aiOn 24 January, the Hubble Space Telescope captured the rare occurrence of three of Jupiter’s largest moons moving across the face of the gas-giant planet: Europa, Callisto, and Io.

They are three of the four Galilean moons, named after the 17th-century scientist Galileo Galilei who discovered them with a small telescope. Their orbital periods around Jupiter range from 2 days to 17 days. The Galilean moons can often be seen transiting the face of Jupiter and casting shadows onto its cloud tops., but seeing three of them transiting the face of Jupiter at the same time is rare, occurring only once every five or ten years.

On the left, the moons Callisto and Io are above Jupiter’s cloud tops. The shadows from Europa, Callisto, and Io are strung out from left to right. Europa is not visible in this image. Approximately 42 minutes later (right-side image), Europa has entered the frame at lower left. Slower-moving Callisto is above and to the right of Europa. Fastest-moving Io is approaching the eastern limb of the planet; its shadow is no longer visible on Jupiter. Europa’s shadow is toward the left side of the image, and Callisto’s shadow to the right. The moons’ orbital velocities are proportionally slower with increasing distance from the planet. Ganymede, the other Galilean moon, was outside Hubble‘s field of view and too far from Jupiter to be part of this conjunction.

Each of these moons has a distinctive color. The cratered surface of Callisto is brown; the smooth icy surface of Europa is yellow-white; and the volcanic, sulfur-dioxide surface of Io is orange. The fuzziness of each depends on each moon’s distance from Jupiter. The farther away the moon, the softer the shadow because it is more spread out across the disk.

Image Credit: NASA

Why is the Giant Red Spot Red?

jupiterganymede_hstThe color of Jupiter’s Great Red Spot is probably the result of simple chemicals being broken apart by sunlight in the planet’s upper atmosphere. At least that’s what’s suggested by analysis of data from lab experiments and from NASA’s Cassini mission. (“Hold it,” I hear you cry. “Cassini is a Saturn mission.” Yes, it is, but it did a Jupiter flyby on the way.) Those results contradict the other leading theory for the origin of the spot’s striking color—that the reddish chemicals are stirred up from beneath Jupiter’s clouds.

Jupiter possesses three main cloud layers which occupy specific altitudes in its skies; from highest to lowest they are ammonia, ammonium hydrosulfide and water clouds. In lab experiments researchers zapped ammonia and acetylene gases, known to be present in that upper layer, with ultraviolet light, simulating the Sun’s effects on those materials. This produced a reddish material, which the team compared to the Great Red Spot as observed by Cassini’s Visible and Infrared Mapping Spectrometer (VIMS). They found that the light-scattering properties of their red concoction nicely matched a model of the Great Red Spot in which the red-colored material is confined to the uppermost reaches of the giant cyclone-like feature. If red material were being transported from below, it should be present at other altitudes as well, which would make the red spot redder still.

When the same sort of test were performed on ammonium hydrosulfide which makes up a lower layer, the researchers found that, instead of a red color, the products their experiment produced were a shade of bright green.

The Great Red Spot is a long-lived feature in Jupiter’s atmosphere that is as wide as two earths.

Image Credi: NASA

The Eye of Jupiter

jupitereye_0Not really. The trick is that the planet seems to be looking back at us because Hubble happened to catch the shadow of Ganymede, Jupiter’s largest moon, as it moved across the Giant Red Spot. Hubble was monitoring changes in that huge storm last April when the moon’s shadow moved across the center of the storm. For a moment, Jupiter became Cyclops.

Image Credit: NASA


IDL TIFF fileThis isn’t a picture of the Earth. It’s Jupiter as seen in ultraviolet light by the Hubble Space Telescope. Just as on Earth, Jupiter’s aurorae are curtains of light resulting from high energy electrons following the planet’s magnetic field into the upper atmosphere. Collisions with atmospheric atoms and molecules produce the observed light.

Image Credit: NASA