Jupiter's cloud belts are always filled with spots, streaks, and swirls, but this image shows five spots of another kind. Three are the black shadows of Jupiter's moons Io, Ganymede, and Callisto. Two lighter dots are Io and Ganymede themselves. (Callisto is outside the frame, to the right.)
Triple shadow transits like this are rare. This one the first and last this decade took place on March 28th from 8:00 to 8:19 Universal Time, when Jupiter was in good view from the Americas and well-prepared amateur observers made a point of getting their scopes out. Hubble scientists chose this time to make an infrared movie of the planet, from which this frame is taken.
The imaging was done with Hubble's NICMOS infrared camera. In the rendering here, red, green, and blue represent near-infrared wavelengths of 1,900, 1,660, and 1,080 nanometers, respectively.
"In the near infrared, methane gas in Jupiter's atmosphere limits the penetration of sunlight," explains a NASA press release, "which causes clouds to appear in different colors depending on their altitudes. Studying clouds in near-infrared light is very useful for scientists studying the layers that make up Jupiter's atmosphere." Yellow clouds are high, red ones are lower, and blue ones are lower still. The green near the polar limbs is thin haze very high in the atmosphere. The blue tint of Ganymede arises from the absorption of longer infrared wavelengths by water ice on its surface. Io's white comes from its bright sulfur compounds.
The diagram below shows the positions of Jupiter's moons in their orbits at the time.