NASA has released a pair of images of Jupiter's moon Amalthea, taken by the Juno Spacecraft during a close flyby on 7 March 2024. At the time of the flyby the spacecraft was only 265 000 km from the Jovian moon (about 69% of the distance between the Earth and its Moon), though Amalthea still appears as a tiny speck, since it is only about 84 km in diameter.
Amalthea was discovered on 9 September 1892 by American astronomer Edward Emerson Barnard, then working at the Lick Observatory on Mount Hamilton in California. Other than the four moons discovered by Galileo Galilei in 1610, Amalthea is the only Jovian moon to have been discovered by direct observation. The remaining 90 know moons of Jupiter have been discovered either by photographic surveys or spacecraft flybys.
Amalthea is irregularly shaped, being roughly 250 km long, 146 km wide, and 128 km deep, and tidally locked to Jupiter, so that it always has the same side facing towards the planet and the same end forward, with its long axis aligned with its orbit.
Images of Amalthea captured by the Galileo Spacecraft; (left) from a distance of 446 000 km on 12 August 1999, (right) from a distance of 374 000 km on 26 November 1999. NASA/JPL/Wikimedia Commons. Amalthea orbits Jupiter at a distance of 181 000 km, making it the third closest known moon to the planet; since the Roche Limit of Jupiter (distance below which a satellite will be torn apart by tidal forces) is about 80 000 km, it is unlikely that there are many undiscovered moons closer to the planet than Amalthea. The current orbit of the moon is extremely circular and has a very low inclination, but it has been calculated that it goes through periodic episodes of resonance with Io, the innermost of the Galilean Moons, during which its orbit becomes more eccentric and tilted.
The surface of Amalthea is extremely red, and brighter than the other inner satellites of Jupiter, with a leading hemisphere 1.3 times as bright as the trailing hemisphere. This has been theorized to be because of an accumulation of sulphur ejected from Io onto the surface of Amalthea, although without visiting the moon it is unlikely that this can ever be established.
Amalthea as imaged by the Voyager 1 Spacecraft on 5 March 1979. Calvin Hamilton/NASA/Wikimedia Commons. A number of large craters have been observed on the surface of Amalthea. The largest of these has been named Pan, and is 100 km wide and at least 8 km deep. Another crater, Gaea, is only about 80 km wide, but thought to be at least twice as deep as Pan.
Largely inside the orbit of Amalthea lies the Amalthea Gossamer Ring, a faint dust ring which extends from about 129 000 km above Jupiter to about 182 000 km, which reaches its maximum thickness, about 2300, close to the orbit of Amalthea. The Galileo Spacecraft passed through this ring in 2002-2003, finding that it is made of particles between 0.2 μm and 5.0 μm in diameter, with some larger bodies close to the moon. It is thought that the ring is made up of material ejected from the surface of Amalthea, and that the larger objects may represent debris from a collision.
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