Encounter Jupiter's elusive 5th moon

NASA's Juno spacecraft has discovered Jupiter's elusive fifth moon passing through the giant planet's Great Red Spot, giving astronomers a rare glimpse of the natural satellite This small but attractive.

Amalthea, seen in two images of Jupiter taken by NASA's Juno spacecraft on March 7, 2024. (Photo: NASA/JPL-Caltech/SwRI/MSSS).

Jupiter's most famous moons are its four Galilean moons: Io, Europa, Ganymede and Callisto, each several thousand kilometers wide. The fifth moon of Jupiter discovered, and the fifth largest of the planet's 95 known moons, is Amalthea. It was found in 1892 by Edward Emerson Barnard, an American astronomer and an excellent visual observer. He also discovered Barnard's Star as well as a series of dark nebulae.

Although it is Jupiter's fifth largest moon, Amalthea is quite modest in size. Irregularly shaped like a potato, its long axis stretches just 250km and its narrowest point is just 128km long. Gravity measurements by NASA's Galileo spacecraft in the early 2000s deduced that Amalthea was little more than a pile of rubble loosely held together rather than solid rock.

Now, Juno has tracked Amalthea for the first time, during the spacecraft's 59th flyby of Jupiter, which took place on March 7 this year. Juno's orbit is a long-haul one, orbiting the gas giant, with a close encounter every 53 Earth days.

Juno discovered Amalthea as a small dark spot that initially appeared above one of Jupiter's dark, rosy cloud belts and then moved across the Great Red Spot. The Great Red Spot is a vast anticyclone currently 12,500km in diameter, while tiny Amalthea is pictured 181,000km above Jupiter's cloud tops.

In fact, Amalthea has the third shortest orbit of any of Jupiter's moons, orbiting the giant planet every 0.5 Earth days in an inner orbit compared to that of the volcano Io. It shines at magnitude +14, and in its brilliance is very close to Jupiter.

Amalthea is the reddest celestial body in the Solar System. The identity of this red coating is unknown, but one possibility is that sulfur was spewed by volcanoes on Io and traveled through space near Amalthea.

There is an even deeper mystery with Amalthea, which is that it radiates more heat than it receives from the sun. Where a small moon like Amalthea gets this extra energy from is an open question.