Amalthea (moon)

• 1. Introduction

  ›

  On the 9th of September 1892, astronomer Edward Emerson Barnard spotted a small moon circling Jupiter and named it Amalthea. It was the fifth moon of Jupiter ever found, and it holds the third-closest orbit to the planet of any moon known today. In size, Amalthea ranks fifth as well, smaller only than the four giant moons Galileo Galilei had discovered through his own telescope back in 1610. Barnard's discovery ended that long gap. It also marked an ending of its own: Amalthea was the last moon anyone would ever find by looking directly through an eyepiece. Every satellite discovered after it turned up on a photographic plate or a digital sensor instead. So what does this small, easily overlooked moon actually look like up close? What is it made of, and why does its orbit behave in ways that trace back to another Jovian moon entirely? What might it be like to stand on its surface and watch Jupiter fill the sky? The rest of this program follows the clues that telescopes and spacecraft have gathered about one small, strange moon.

• 2. A Late, Official Name

  ›

  Camille Flammarion first suggested the name Amalthea. It comes from a nymph of Greek mythology who nursed the infant Zeus, the Greek equivalent of Jupiter, with goat's milk. Barnard had made his discovery using a 36 inch, or 91 centimeter, refractor telescope at Lick Observatory. For decades afterward, astronomers used the name informally while the moon's official designation stayed Jupiter V, a Roman numeral standing for the fifth satellite found. The International Astronomical Union did not formally adopt the name Amalthea until 1976, even though the name had already been in informal use for decades. Until that official adoption, the moon was most commonly known simply as Jupiter V. The number V would follow the moon quietly for most of the twentieth century. Even so, scientists had already begun wondering what such a small, oddly shaped body might actually be made of.

• 3. A Puzzle Of Density

  ›

  Amalthea orbits Jupiter at a distance of 181,000 kilometers, about 2.54 Jupiter radii out. Its orbit carries a small eccentricity of 0.003 and a tilt of 0.37 degrees against Jupiter's equator. Both figures are surprisingly large for a moon riding so close to its planet. Scientists trace that tilt and stretch back to Io, the innermost Galilean moon. In the past, Amalthea repeatedly locked into mean-motion resonances with Io. These are orbital relationships where the two moons' periods form a simple ratio, such as m to n. Those past encounters excited Amalthea's orbit into its present, slightly lopsided shape.

  The best ellipsoidal fit to Amalthea's shape measures 250 by 146 by 128 kilometers. That gives it a surface area likely between 88,000 and 170,000 square kilometers, probably close to 130,000. Like Jupiter's other inner moons, Amalthea is tidally locked, keeping the same long axis pointed at the planet at all times.

  Before the 5th of November 2002, Amalthea's irregular shape had convinced scientists that it must be a strong, rigid body. A moon made of ice or other weak material, they reasoned, would have been pulled into a rounder shape by its own gravity. That idea broke down on that date, when the Galileo orbiter flew within 160 kilometers of Amalthea. Scientists used the deflection in its own path to calculate the moon's mass for the first time. Its volume had already been worked out from images, to within about 10 percent. Combined with the new mass figure, that gave Amalthea a density as low as 0.86 grams per cubic centimeter. That is low enough that the moon is now thought to be a relatively icy body, a very porous rubble pile, or something in between.

  Infrared spectra gathered later by the Subaru telescope pointed toward hydrous minerals in Amalthea's surface. That hints the moon could not have formed where it orbits today. The young, hot Jupiter would have melted any ice that close in. That leaves two live possibilities. Either Amalthea formed farther out and migrated inward, or it began somewhere else in the Solar System entirely and was later captured. No camera on Galileo recorded the historic 2002 flyby itself. Radiation had knocked out its imaging system back in January of that year. The other pictures scientists have of Amalthea remain fairly low in resolution.

• 4. The Reddest Surface

  ›

  Of every body in the Solar System, Amalthea carries the reddest surface yet measured. One explanation points to sulfur, possibly arriving from Io, though other non-ice material could produce the same reddish cast. Scattered across Amalthea's major slopes are patches of a less-red, brighter tint, and researchers still do not know what causes them. The moon's surface as a whole runs slightly brighter than those of Jupiter's other inner satellites. It is also not evenly reflective: the hemisphere that leads in Amalthea's orbit shines about 1.3 times brighter than the trailing hemisphere. The likely reason is that the leading side takes impacts more often, and at higher speed. That digs up brighter material, presumably ice, from beneath the reddened surface.

  Amalthea radiates more heat than it receives from the Sun. Researchers trace that surplus to three sources. Jovian heat flux contributes under 9 kelvins, and reflected sunlight off the planet adds under 5 kelvins. A third source, charged-particle bombardment, adds under 2 kelvins. Io shows this same trait of radiating extra heat, though for different underlying reasons.

• 5. Craters And Bright Spots

  ›

  Amalthea's heavily cratered surface carries only four named features: two craters and two bright spots, or faculae. The larger crater, Pan, stretches 100 kilometers across and reaches at least 8 kilometers deep, enormous relative to the moon that holds it. The second crater, Gaea, measures 80 kilometers across and is likely twice as deep as Pan. Both names were approved in 1979. Pan honors the Greek goat-god described as the son of Amalthea and Hermes. Gaea honors the Greek mother-earth goddess said to have brought the infant Zeus to Crete.

  Amalthea's two bright spots sit on the edge of ridges, on the side of the moon that faces away from Jupiter. Ida Facula spans 50 kilometers and is named for Mount Ida on Crete. Lyctos Facula spans 25 kilometers and is named for the Cretan city of Lyctus. Both names were also approved in 1979, alongside the two craters. Where the craters draw their names from mythological characters, the faculae borrow theirs from real places tied to the same myths. The same impacts that carved Pan and Gaea into the surface also kick debris free of Amalthea's weak gravity. That material does not simply fall back down.

• 6. Dust, Rings And Moonlets

  ›

  Amalthea's gravity is weak and its shape irregular. At the points on its surface closest to and farthest from Jupiter, escape velocity is no more than 1 meter per second. Tidal force from Jupiter compounds the effect. Even a micrometeorite impact can knock dust free of the moon entirely, and that escaping dust supplies the Amalthea gossamer ring.

  During its close flyby, the Galileo orbiter's star scanner picked up nine brief flashes near Amalthea's orbit, flashes that looked like small, previously unknown moonlets. Because only one instrument, from one location, ever saw them, their exact distances could not be pinned down. Their sizes could run anywhere from gravel to stadium-sized. Nobody knows for certain where they came from: they might be objects captured into orbit, or debris blasted loose by meteor impacts on Amalthea itself. On Galileo's very last orbit, about an hour before the spacecraft's own destruction, its star scanner caught one further flash. This time Amalthea sat on the far side of Jupiter. That suggests these particles form a ring of their own near Amalthea's orbital path, rather than simply trailing the moon.

• 7. A Sky Ruled By Jupiter

  ›

  Seen from just above Jupiter's cloud tops, Amalthea would blaze at a magnitude of minus 4.7, roughly as bright as Venus appears from Earth. Its disc would only span about 5 arcminutes, barely enough to resolve as more than a point. Because Amalthea's orbital period runs only about 20 percent longer than a Jupiter day, it would crawl across Jupiter's sky rather than race across it. Moonrise to moonset would stretch past 29 hours. Science journalist Willy Ley once proposed Amalthea as a base for observing Jupiter. He cited its closeness to the planet, its near-synchronous orbit, and a small size that would make landing straightforward.

  Flip the view around: Jupiter would dominate Amalthea's sky at roughly 46 degrees across, about 85 times wider than a full moon looks from Earth. Because Amalthea keeps the same face toward Jupiter at all times, the planet would not appear to move at all. It would never rise for an observer standing on the moon's far side. Once each orbit, the Sun would vanish behind Jupiter's bulk for about an hour and a half. Amalthea's short rotation period leaves it just under six hours of daylight. Jupiter would also outshine a full moon by a factor of about 900. That light, though, gets spread across an area roughly 8,500 times larger. So each patch of Jupiter's disc would not actually look as bright as the full moon does from Earth. It would take robotic spacecraft, not the astronaut base Willy Ley imagined, to actually confirm what Amalthea's own surface looks like up close.

• 8. Voyager, Galileo And Fiction

  ›

  In 1979, Voyager 1 and Voyager 2 returned the first images sharp enough to make out features on Amalthea's surface. They also measured its visible and infrared spectra and its surface temperature. The Galileo orbiter later finished mapping Amalthea's surface in more detail. On the 5th of November 2002, at the end of its mission, Galileo made its final satellite flyby. It passed about 244 kilometers from Amalthea's center, at a height of roughly 160-170 kilometers. The encounter bent Galileo's own trajectory enough to send it plunging into Jupiter in September 2003.

  Amalthea has also turned up in fiction, providing the setting for science-fiction stories by Arthur C. Clarke, James Blish, and Arkady and Boris Strugatsky.

  In 2006, scientists refined Amalthea's orbit further using measurements taken by New Horizons during its flyby of Jupiter. NASA's Juno probe, which reached Jupiter in 2016, is scheduled to make a close flyby of Amalthea on the 25th of April 2027.