Look at NASA's new image of Jupiter's Great Red Spot, and you might catch something unusual. Careful, though: If you don't pay attention, you might miss it. That tiny, dark dot at the top of Jupiter's most famous feature isn't an error from Juno's visible-light camera. It's an itty-bitty moon, made to appear even smaller by the massive gas giant looming in the background.
Amalthea is a fairly small moon in its own right, though, measuring just 83.5 kilometers across (give or take a couple of kilometers since moons aren't perfectly round). For comparison, our own Moon is about 1,737 kilometers across. Even then, Amalthea is the largest of Jupiter's jovian moons or the irregularly-shaped moons that orbit Jupiter most closely. It's also the reddest object in our solar system, thanks to sulfur from fellow Jovian moon Io. According to NASA, Amalthea also appears to radiate more heat than it receives from the Sun.
Snapping a high-quality picture of any jovian moon is a challenge. Not only are jovian moons small and far away, huddling close to Jupiter's surface, but they orbit fairly quickly, making it difficult to avoid motion blur. Jupiter also features intense radiation belts, which tend to limit the lifetime of spacecraft that come within close proximity to the gas giant. Because of this, the below image, captured in January 2000 by the Galileo spacecraft, is the highest-quality image we have of Amalthea.
But NASA's Juno spacecraft is all too happy to grab in-motion pictures of Amalthea. In an X post shared Monday, NASA said Juno—whose primary goal is to understand Jupiter's origin and evolution—got a glimpse of Amalthea above Jupiter's cloud tops. The 13-year-old spacecraft grabbed its photograph during its 59th close flyby in March when it flew roughly 265,541 kilometers over Jupiter's clouds. Once the raw camera data reached Earth, NASA citizen scientist Gerald Eichstädt processed it, making Amalthea a bit more visible.
Though Juno completed its primary mission roughly three years ago, the spacecraft is continuing to gather insights into Jupiter's magnetic field, atmospheric ammonia levels, dust rings, moons, and more. The hope is that Juno's extended mission will help astronomers trace the solar system's history by using Jupiter (which, unlike Earth, has retained its original composition) as a time capsule. Other JunoCam citizen science projects and the camera's raw data are available for public viewing on Nasa's Mission Juno website.