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.

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

Bright Streak on Amalthea
These two images of Jupiter's small, irregularly shaped moon Amalthea form a 'stereo pair' that helps scientists determine this moon's shape and the topography of its surface features. Features as small as 3.8 kilometers (2.4 miles) across can be resolved in these images, making them among the highest-resolution images ever taken of Amalthea.

The large impact crater visible in both images, near the right-hand edge of Amalthea's disk, is about 40 kilometers (about 29 miles) across; two ridges, tall enough to cast shadows, extend from the top of the crater in a V-shape reminiscent of a 'rabbit ears' television antenna. To the left of these ridges, in the top center portion of Amalthea's disk, is a second large impact crater similar in size to the first crater. To the left of this second crater is a linear 'streak' of relatively bright material about 50 kilometers (31 miles) long. In previous spacecraft images of Amalthea taken from other viewing directions, this bright feature was thought to be a small, round, bright 'spot' and was given the name Ida. These new images reveal for the first time that Ida is actually a long, linear 'streak.' This bright streak may represent material ejected during the formation of the adjacent impact crater, or it may just mark the crest of a local ridge. Other patches of relatively bright material can be seen elsewhere on Amalthea's disk, although none of these other bright spots has Ida's linear shape. (Courtesy NASA/Cornell University)

Best images yet of Thebe, Amalthea and Metis
These images of the inner Jovian moons Thebe, Amalthea, and Metis (left to right) are the highest-resolution images ever obtained of these small, irregularly shaped satellites. The images resolve surface features as small as 2 kilometers (about 1.2 miles) across for Thebe; 2.4 kilometers (about 1.5 miles) across for Amalthea, and 3 kilometers (about 1.9 miles) across for Metis.

The moons are shown in their correct relative sizes, with sunlight coming from the right. We are viewing the side of each moon that faces permanently away from Jupiter, and north is approximately up in all cases. The large white region near the south pole of Amalthea marks the location of the brightest patch of surface material seen anywhere on these three moons. This unusual material, which sits inside a large crater named Gaea, has been greatly overexposed; accordingly, the white area on this image is somewhat larger than the actual bright area on Amalthea. Note also the 'scalloped' or 'sawtooth' shape of Amalthea's terminator (the line between day and night, at the left-hand edge of Amalthea's disk), which indicates that parts of this satellite's surface are very rough, with many small hills and valleys. (Courtesy NASA/Cornell University)

Jupiter Small Satellite Montage
This image is A montage of images of the small inner moons of Jupiter from the camera onboard NASA's Galileo spacecraft shows the best views obtained of these moons. The top two images show the moon Thebe.

The next two images show the moon Amalthea; they were taken with the Sun directly behind the observer, an alignment that emphasizes patterns of intrinsically bright or dark surface material. The third image from the top is a view of Amalthea's leading side, the side of the moon that 'leads' as Amalthea moves in its orbit around Jupiter. This image looks 'noisy' because it was obtained serendipitously during an observation of the Jovian satellite Io (Amalthea and Io shared the same camera frame but the image was exposed for bright Io rather than for the much darker Amalthea). The fourth image from the top emphasizes prominent 'spots' of relatively bright material that are located near the point on Amalthea that faces permanently away from Jupiter. The bottom image is a view of the tiny moon Metis. (Courtesy NASA/Cornell University)

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/Cornell University)

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 Cornell University)