Jupiter’s tiny moon Amalthea has been spotted by NASA’s Juno spacecraft while it was passing through the giant planet’s Great Red Spot. This provided astronomers a rare glimpse of this small but fascinating natural satellite.
What is Amalthea?
Jupiter’s most famous four Galilean satellites or moons are Io, Europa, Ganymede and Callisto. Each of these moons is several thousand kilometers wide. Amalthea is the fifth moon of Jupiter, which is also the fifth largest of the planet’s 95 known moons. Edward Emerson Barnard, an American astronomer who was a noted visual observer, discovered it back in 1892. He also found Barnard’s star and a variety of dark nebulae.
Although Amalthea is Jupiter’s fifth largest moon, its size is quite modest. The irregular shape resembles a potato: its long axis spans just 250 kilometers (155 miles) and its narrowest point measures just 128 kilometers (79 miles). Gravity measurements taken by NASA’s Galileo space probe in the early 2000s revealed that Amalthea is little more than a pile of loose debris rather than solid rock.
When was Jupiter’s tiny moon Amalthea discovered?
Juno first discovered Amalthea during the spacecraft’s 59th close flyby of Jupiter on March 7 of this year. Juno’s orbit is looping and long surrounding the gas giant, with a close encounter (called a “perijove”) every 53 Earth days; It was supposed to move to a shorter orbit, but an engine misfire caused by faulty valves forced Juno to stay put for the duration.
Juno discovered Amalthea as a small black spot that first appeared against one of Jupiter’s dark, ruddy cloud belts and then passed through the Great Red Spot. The scale is incredible; The Great Red Spot is a massive anticyclonic storm with a current diameter of 12,500 km (7,767 miles), while the smaller Amalthea is imaged 181,000 km (112,500 miles) above Jupiter’s cloud surfaces.
Amalthea, in fact, has the third-shortest orbit of all of Jupiter’s moons, orbiting the giant planet every 0.5 Earth days in the inner orbit relative to the orbit of the volcanic Io. It shines at magnitude +14, and because it’s so close to Jupiter’s glare, Barnard did an incredible job in discovering it.
Amalthea: Reddest object in the solar system
NASA’s Voyager 1 and Voyager 2 probes and the Galileo spacecraft took close-up images of Amalthea, revealing several bright spots and craters on the small moon, as well as the mysteriously red colour of its surface. In fact, Amalthea is the reddest body in the solar system. The identity of this red layer remains unknown, but it is possible that it is sulfur emitted by the volcanoes of Io and transported via space to neighboring Amalthea.
Amalthea mystery
What is even a deeper mystery about Amalthea is it emits a little more heat than what it gets from the sun. Where does a moon as small as Amalthea get this extra energy? Various explanations have been proposed and the truth could be any of them or a combination of them.
For example, Amalthea gets all the heat that Jupiter radiates and reflects, while the influence of Jupiter’s gravity could create tidal stresses on Amalthea, producing heat. Then Jupiter’s enormous magnetic field creates a magnetic bubble that is the second-largest structure in the Solar System after the Sun’s magnetic bubble, the heliosphere. Amalthea’s short orbit is anchored deep in Jupiter’s magnetosphere, a region where there are radiation belts of charged particles that can bombard Amalthea’s surface and transfer energy. Finally, the magnetosphere might even be able to induce electrical currents in Amalthea’s core, thus generating additional heat.
Whatever the answer, it adds to the charm of this fifth moon, often forgotten along with its famous larger siblings, whose story can be just as tantalizing.