The NASA spacecraft Juno has discovered the fifth elusive moon of Jupiter passing through the Great Red Spot of this giant planet, providing astronomers with a rare glimpse of this small but fascinating natural satellite.
Amalthea, seen in two images of Jupiter taken by NASA’s Juno spacecraft on March 7, 2024. (Photo: NASA/JPL-Caltech/SwRI/MSSS).
The most famous moons of Jupiter are its four Galilean satellites: Io, Europa, Ganymede, and Callisto, each with a diameter of several thousand kilometers. The fifth moon of Jupiter, discovered and recognized as the fifth largest among the 95 known moons of the planet, is Amalthea. It was discovered in 1892 by Edward Emerson Barnard, an American astronomer and an exceptional visual observer. He also discovered Barnard’s Star as well as a series of dark nebulae.
Despite being the fifth largest moon of Jupiter, Amalthea is quite modest in size. Its irregular, potato-like shape has a longest axis measuring only 250 km and its narrowest point measuring just 128 km. Gravitational measurements from NASA’s Galileo spacecraft in the early 2000s inferred that Amalthea is more like a pile of rubble loosely held together than a solid rock.
Now, Juno has tracked Amalthea for the first time during the spacecraft’s 59th close flyby of Jupiter, which occurred on March 7 of this year. Juno’s orbit is a long trajectory around the giant gas planet, with a close encounter every 53 Earth days.
Juno detected Amalthea as a small dark spot initially appearing on one of Jupiter’s dark, reddish cloud bands and then moving across the Great Red Spot. The Great Red Spot is a massive anticyclonic storm currently measuring 12,500 km in diameter, while the tiny Amalthea is visualized at a distance of 181,000 km above Jupiter’s cloud tops.
In fact, Amalthea has the third-shortest orbital period among Jupiter’s moons, orbiting the giant planet every 0.5 Earth days on an inner orbit compared to the volcanic moon Io. It shines at an intensity of +14, and its brightness is very close to that of Jupiter.
Amalthea is the reddest body in the Solar System. The identity of this reddish coating has yet to be determined, but one possibility is that it is sulfur expelled by volcanic activity on Io and transported across space to Amalthea.
There is an even deeper mystery surrounding Amalthea, which is that it radiates more heat than it receives from the sun. Where a small moon like Amalthea gets this additional energy remains an open question.
