Amalthea (moon)
Edward Emerson Barnard stood at the eyepiece of a 36 inch refractor telescope on the 9th of September 1892. He was working at Lick Observatory when he spotted a faint point of light near Jupiter. This object became the fifth moon ever found orbiting that planet. It marked the final time astronomers would find a planetary satellite using only their eyes and glass lenses. Every discovery after this moment relied on photographic plates or digital sensors instead. The moon remained unnamed for decades until the International Astronomical Union formalized its title in 1976. Camille Flammarion had suggested calling it Amalthea long before that official adoption took place. Before 1976, scientists simply referred to it as Jupiter V in informal discussions.
Amalthea orbits Jupiter at a distance of 181,000 kilometers from the planet's center. Its path tilts 0.37 degrees relative to Jupiter's equator while maintaining an eccentricity of 0.003. These values remain unusually high for such a close inner satellite. Gravitational resonance with Io explains why the orbit behaves this way. Past interactions caused Amalthea to pass through several mean-motion resonances with the larger moon. These encounters excited both the inclination and the eccentricity over time. The ratio of orbital periods between the two bodies forms a rational number like m:n. This gravitational dance keeps Amalthea within the outer edge of the Amalthea Gossamer Ring. Dust ejected from its surface contributes to the ring structure surrounding the planet.
A targeted flyby by the Galileo orbiter on the 5th of November 2002 revealed the true nature of Amalthea. The spacecraft passed within 160 kilometers of the moon to measure its mass directly. Scientists calculated the volume previously using images accurate to within 10 percent. The resulting density proved as low as 0.86 grams per cubic centimeter. Such a value suggests the body consists of porous water ice or a very loose rubble pile. Recent infrared spectra from the Subaru telescope indicate the presence of hydrous minerals. This composition implies the moon formed farther from Jupiter before migrating inward. A hot primordial Jupiter would have melted any ice in its current position if it had started there. The irregular shape measures approximately 250 by 146 by 128 kilometers. Its surface area spans somewhere near 130,000 square kilometers.
Pan stands as the largest crater on Amalthea with a diameter of 100 kilometers and a depth of at least 8 kilometers. Another feature named Gaea measures 80 kilometers across and likely reaches twice that depth. Two bright spots known as Lyctos Facula and Ida Facula appear along ridge edges. These faculae reach widths up to 25 kilometers each. They sit on the anti-Jupiter side of the moon's surface. The leading hemisphere appears 1.3 times brighter than the trailing one. Scientists believe impacts excavate bright material like ice from the interior. This process creates substantial asymmetry between the two sides. The surface remains heavily scarred by craters despite the small size of the body. Some features exceed expectations relative to the overall dimensions of the satellite.
Micrometeorite impacts eject dust particles from the surface of Amalthea into space. Escape velocity at points closest to Jupiter never exceeds 1 meter per second. This low threshold allows debris to drift away easily after collisions. The ejected material forms the Amalthea Gossamer Ring around the planet. During its final flyby, Galileo detected nine flashes suggesting nearby moonlets exist near the orbit. Their true distances remained unmeasured because observations came from only one location. These objects may range in size from gravel to stadium-sized chunks. One additional moonlet appeared during the last orbit just before destruction. It likely formed a ring structure near Amalthea's path due to gravitational capture or impact ejecta. No images captured these potential fragments directly.
Voyager 1 and Voyager 2 obtained the first resolved images of Amalthea in 1979. They measured visible and infrared spectra alongside surface temperature data. The Galileo orbiter completed detailed imaging throughout the 1990s. Its final satellite flyby occurred on the 5th of November 2002 at roughly 160 kilometers altitude. Cameras had been deactivated earlier that year due to radiation damage but mass calculations succeeded anyway. New Horizons refined orbital measurements during its 2006 flyby of Jupiter. NASA's Juno spacecraft continued distant observations after arriving in 2016. A simulated view shows Jupiter spanning 46 degrees across the sky from Amalthea. This perspective makes the planet appear 85 times wider than Earth's full moon. Sunlight disappears behind Jupiter for an hour and half each revolution while daylight lasts under six hours.
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Common questions
When was the moon Amalthea discovered by Edward Emerson Barnard?
Edward Emerson Barnard discovered the moon Amalthea on the 9th of September 1892 while working at Lick Observatory. This event marked the final time astronomers found a planetary satellite using only eyes and glass lenses.
What is the orbital distance and shape of the moon Amalthea around Jupiter?
The moon Amalthea orbits Jupiter at a distance of 181,000 kilometers from the planet's center with an irregular shape measuring approximately 250 by 146 by 128 kilometers. Its orbit tilts 0.37 degrees relative to Jupiter's equator while maintaining an eccentricity of 0.003.
How did scientists determine the density and composition of the moon Amalthea in 2002?
Scientists determined the density of the moon Amalthea as 0.86 grams per cubic centimeter during a Galileo orbiter flyby on the 5th of November 2002. This low value suggests the body consists of porous water ice or a very loose rubble pile containing hydrous minerals.
What are the largest surface features named on the moon Amalthea?
Pan stands as the largest crater on the moon Amalthea with a diameter of 100 kilometers and a depth of at least 8 kilometers. Another feature named Gaea measures 80 kilometers across and likely reaches twice that depth along with bright spots known as Lyctos Facula and Ida Facula.
Why does the moon Amalthea contribute dust to the Amalthea Gossamer Ring around Jupiter?
Micrometeorite impacts eject dust particles from the surface of the moon Amalthea into space because escape velocity never exceeds 1 meter per second near Jupiter. This debris drifts away easily after collisions to form the Amalthea Gossamer Ring surrounding the planet.