Amalthea (moon)

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Amalthea

Galileo image of Almathea
Discovery
Discovered by	E. Barnard
Discovered in	1892
Orbital characteristics
Mean radius	181,400 km
Eccentricity	0.0031
Revolution period	11h 57.4m
Inclination	0.388°
Is a satellite of	Jupiter
Physical characteristics
Mean diameter	166.9 km
Surface area	km2
Mass	7.43×1018 kg
Mean density	1.8 g/cm3
Surface gravity	0.066 m/s2
Surface Gravity (Earth = 1)	0.00673
Rotation period	11h 57.4m
Axial tilt	°
Albedo	0.05
Surface temp.	min mean max K K K
Atmospheric pressure	0 kPa

Amalthea (pronounced "am al THEE a") 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 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.

The combination of Amalthea's irregular shape and large size implies that Amalthea 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. Like all of Jupiter's moons it is tidally locked with the planet, its long axis pointing towards Jupiter at all times. Its composition is probably more similar to an asteroid's than any of the Galilean moons, and it may be captured. 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.

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 was thrown from its surface.

References

E. E. Barnard, Discovery and Observation of a Fifth Satellite to Jupiter, Astronomical Journal 12 (1892), 81–85

External links

• Animation of Amalthea

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