Amalthea (am'-ul-thee'-a, Greek Αμάλθεια) is the third of Jupiter's known moons. It was discovered on September 9 1892 by Edward Emerson Barnard using the 36 inch (91 cm) refractor telescope at Lick Observatory. Amalthea was the last moon to be discovered by direct visual observation (as opposed to photographically), and was the first new moon of Jupiter to be discovered since Galileo Galilei discovered the Galilean moons in 1610. It is named after the nymph of Greek legend who nursed the infant Zeus (Jupiter) with goat's milk.
It is the largest of the inner group of jovian moons that bears its name.
The name "Amalthea" was not formally adopted by the IAU until 1975, although it had been in informal use for many decades earlier after its suggestion by Camille Flammarion. Before 1975 it was most commonly known simply by its Roman numeral designation as Jupiter V.
Not to be confused with the asteroid 113 Amalthea.
Physical characteristics
Amalthea is the reddest object in the solar system, even redder than the planet Mars. The reddish color is apparently due to sulfur originating from Io. Bright patches of green appear on the major slopes of Amalthea, but the nature of this color is currently unknown.
Galileo images showing Amalthea's irregular shape
Amalthea is irregularly shaped, with dimensions of 270 × 168 × 150 km; the long axis is oriented toward Jupiter. It is also 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.
Amalthea's irregular shape and large size led to conclusion that it is a fairly strong, rigid body; if it were composed of ices or other weak materials its own gravity would have pulled it into a more spherical shape. However, near the end of its mission the Galileo orbiter made a close flyby of the moon. The exact mass of Amalthea was measured and its density was found to be as low as 1.8 g/cm³—it must be either a relatively icy body or a very porous "rubble pile". Recent measurements from the Subaru telescope suggest that the moon is indeed icy, indicating that it cannot have formed in its current position, since the hot primordial Jupiter would have melted it. Therefore it is likely a captured asteroid.
Like all the inner moons of Jupiter it is tidally locked with the planet, its long axis pointing towards Jupiter at all times. Like Io, Amalthea radiates more heat than it receives from the Sun. This is probably due to the electrical currents induced within it by its orbit through Jupiter's magnetic field.
During the Amalthea encounter, the Galileo orbiter's star scanner detected nine flashes which appear to be small rocks near the orbit of the moon. Since they were sighted only from one location, their true distances could not be measured. The rocks may be anywhere from gravel to stadium-size. Their origins are unknown, but they may be gravitationally captured into current orbit or they may be ejecta from meteor impacts on the moon.
Amalthea in fiction
- Arthur C. Clarke's short story Jupiter V (1951) is set on Amalthea; its plot depends on the moon's weak gravity, and explores what might happen if an astronaut were thrown from its surface.
- The Way to Amalthea (in Russian, Путь на Амальтею) is an early scifi story by Boris and Arkady Strugatsky written in 1959. A solar sail-propelled cargo spaceship visits the Jupiter system to deliver a cargo to a manned science station on Amalthea. Damaged by meteorites, the spaceship falls into Jupiter, but survives the pressure and floats in the dense atmosphere; the crew manage to work around the damaged sail and start the main engine to escape Jupiter and finally reach Amalthea.
References
E. E. Barnard, Discovery and Observation of a Fifth Satellite to Jupiter, Astronomical Journal 12 (1892), 81–85
External links