Amalthea [am-al-THEE-uh] is one of Jupiter's smaller moons. It was named after the nymph who nursed the infant Jupiter with goats milk. It was discovered in 1892 by the American astronomer Edward Emerson Barnard while making observations from the Lick Observatory with a 36 inch (91 centimeter) refractory telescope. Amalthea was the last moon in the solar system to be discovered through direct visual observation. It was also the first moon of Jupiter to be discovered since Galileo's discovery of the four Galilean Moons in 1610.

Amalthea is extremely irregular, having dimensions of about 270x165x150 kilometers in diameter. It is heavily scarred by craters, some of which are extremely large relative to the size of the moon. Pan, the largest crater, measures 100 kilometers across and is at least 8 kilometers deep. Another crater, Gaea, measures 80 kilometers across and is probably twice as deep as Pan. Amalthea has two known mountains, Mons Lyctas and Mons Ida with local relief reaching up to 20 kilometers. The surface is dark and reddish in color apparently caused by a dusting of sulfur originating from Io's volcanoes. Bright patches of green appear on the major slopes of Amalthea. The nature of this color is currently unknown.

Amalthea rotates synchronously with its long, blunt axis pointed towards Jupiter. Because of Amalthea's close proximity to Jupiter, it is exposed to the intense Jovian radiation field. It continuously receives high doses of energetic ions, protons, and electrons produced by the Jovian magnetosphere. In addition it is bombarded with micrometeorites, and heavy sulfur, oxygen, and sodium ions that have been striped away from Io.

Animation of Amalthea

• Animation of Amalthea.

Views of Amalthea

Amalthea
This image of Amalthea was acquired by the Voyager 1 spacecraft on March 5, 1979. (Credit: Calvin J. Hamilton)

Four Galileo Views Of Amalthea
These four images of Jupiter's moon, Amalthea, were taken by Galileo's solid state imaging system at various times between February and June 1997. North is approximately up in all cases. Amalthea, whose longest dimension is approximately 247 kilometers (154 miles) across, is tidally locked so that the same side of the satellite always points towards Jupiter, similar to how the near side of our own Moon always points toward Earth. In such a tidally locked state, one side of Amalthea always points in the direction in which Amalthea moves as it orbits about Jupiter. This is called the "leading side" of the moon and is shown in the top two images. The opposite side of Amalthea, the "trailing side," is shown in the bottom pair of images. The Sun illuminates the surface from the left in the top left image and from the right in the bottom left image. Such lighting geometries, similar to taking a picture from a high altitude at sunrise or sunset, are excellent for viewing the topography of the satellite's surface such as impact craters and hills. In the two images on the right, however, the Sun is almost directly behind the spacecraft. This latter geometry, similar to taking a picture from a high altitude at noon, washes out topographic features and emphasizes Amalthea's albedo (light/dark) patterns. It emphasizes the presence of surface materials that are intrinsically brighter or darker than their surroundings. The bright albedo spot that dominates the top right image is located inside a large south polar crater named Gaea. (Courtesy NASA/JPL)

Topographic Map of Amalthea
This is a topographic map of Amalthea. It is based upon the shape model of Phil Stooke. As with all maps, it is the cartographer's interpretation; not all features are necessarily certain given the limited data available. This interpretation stretches the data as far as possible. (Courtesy A. Tayfun Oner)

Shaded Relief Map of Amalthea
This is a shaded relief map of Amalthea, a small satellite of Jupiter. As with all maps, it is the cartographer's interpretation; not all features are necessarily certain given the limited data available. This interpretation stretches the data as far as possible. (Courtesy Phil Stooke)

Family Portrait of the Small Inner Satellites of Jupiter
These images, taken by Galileo's solid state imaging system between November 1996 and June 1997, provide the first ever "family portrait" of the four small, irregularly shaped moons that orbit Jupiter in the zone between the planet's ring and the larger Galilean satellites. The moons are shown in their correct relative sizes, with north approximately up in all cases. From left to right, arranged in order of increasing distance from Jupiter, are Metis (longest dimension is approximately 60 kilometers or 37 miles across), Adrastea (20 kilometers or 12 miles across), Amalthea (247 kilometers or 154 miles across), and Thebe (116 kilometers or 72 miles across). While Amalthea, the largest of these four tiny moons, was imaged by NASA's two Voyager spacecraft in 1979 with a resolution comparable to what is shown here, the new Galileo observations represent the first time that Metis, Adrastea, and Thebe have been seen as more than points of light. (Courtesy of NASA/JPL)

Jupiter Adrastea Thebe

Copyright © 1997 by Calvin J. Hamilton. All rights reserved.