Two Groups of New Moons Nine of the new moons are part of a distant outer swarm of moons that orbit the planet in the opposite direction of Jupiter’s spin rotation (retrograde). These distant retrograde moons are grouped into at least three distinct orbital groupings and are thought to be the remnants of three once-larger parent bodies that broke apart during collisions with other bodies. The newly discovered moons take about two years to orbit Jupiter. Two of the new discoveries are part of a closer, inner group of moons that orbit in the same direction as the planet’s rotation (prograde). These moons all have similar orbital distances and angles of inclinations around Jupiter and so are thought to also be fragments of a larger moon that broke apart. These two newly discovered moons take a little less than a year to orbit Jupiter. The Oddball “Our other discovery is a real oddball and has an orbit like no other known Jovian moon,” Sheppard explained. “It’s also likely Jupiter’s smallest known moon, being less than one kilometer in diameter.” This new oddball moon is more distant and more inclined than the inner group of moons, and it takes about one and a half years to orbit Jupiter. So, unlike the closer-in group of moons, this new oddball prograde moon has an orbit that crosses the outer moons orbiting in the opposite direction. As a result, head-on collisions are much more likely to occur between the oddball and the outer moons, orbiting in the opposite direction. “This is an unstable situation,” said Sheppard. “Head-on collisions would quickly break apart and grind the objects down to dust. It’s possible the various orbital moon groupings we see today were formed in the distant past through this same mechanism. The team thinks this small oddball moon could be the last-remaining remnant of a once-larger inner group of moons that formed some of the outer moons during past head-on collisions. The name Valetudo has been proposed for it, after the Roman god Jupiter’s great-granddaughter, the goddess of health and hygiene. Early Solar System Dynamics Understanding the complex influences that shaped a moon’s orbital history can teach scientists about our Solar System’s early years. For example, the discovery that these new moons are still abundant suggests the collisions that created them occurred after the era of planet formation, when the Sun was still surrounded by a rotating disk of gas and dust that formed the planets. Because of their sizes—one to three kilometers—these moons are more influenced by surrounding gas and dust. If these raw materials had still been present when Jupiter’s first generation of moons collided to form its current clustered groupings of moons, the drag exerted by any remaining gas and dust on the smaller moons would have been sufficient to cause them to spiral inwards toward Jupiter. Their existence shows that they were likely formed after this gas and dust dissipated. Confirming the Moons The initial discovery of most of the new moons was made on the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory in Chile, operated by the National Optical Astronomical Observatory of the United States. The telescope recently was upgraded with the Dark Energy Camera, making it a powerful tool for surveying the night sky for faint objects. Several telescopes were used to confirm the finds, including the 6.5-meter Magellan telescope at Carnegie’s Las Campanas Observatory in Chile; the 4-meter Discovery Channel telescope at Lowell Observatory in Arizona; and the 8-meter Subaru telescope, the University of Hawaii’s 2.2-meter telescope, and the 8-meter Gemini North telescope, all in Hawaii. NASA’s Jet Propulsion Laboratory confirmed the calculated orbit of the unusual oddball moon in 2017 to double-check its location prediction during the 2018 recovery observations to make sure the new interesting moon was not lost.  SUPPORT: A NASA Planetary Astronomy grant partially funded this research, including data gathered with the 6.5-meter Magellan telescope. This project used data obtained with the Dark Energy Camera, which was constructed by the Dark Energy Survey-collaborating institutions. Observations were partly obtained at Cerro Tololo Inter-American Observatory, operated by the National Optical Astronomical Observatory, operated by the Association of Universities for Research in Astronomy under contract with the National Science Foundation. These are images of the oddball moon Valetudo from the Magellan telescope in May 2018. The moon can be seen moving relative to the steady-state background of distant stars. Jupiter is not in the field but is off to the upper left. Image courtesy Scott Sheppard Carnegie’s Scott Sheppard Image courtesy Roberto Molar Candanosa Carnegie Science | Fall 2018 11