6th Largest Moon of Jupiter - Amalthea (3rd Moon outwards from Jupiter)

Greyscale Galileo images of Amalthea

Amalthea is the largest of the inner satellites of Jupiter, the sixth largest overall in size. Only the four Galilean moons and Himalia are larger. Amalthea is the 34th largest moon in the Solar System currently known.

Discovery

Amalthea 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 planetary satellite to be discovered by direct visual observation (as opposed to photographically) and was the first new satellite of Jupiter since Galileo Galilei's discovery of the Galilean satellites in 1610.

Naming

The satellite is named after the nymph Amalthea from Greek mythology who nursed the infant Zeus (the Greek equivalent of Jupiter) with goat's milk.

Stats

Diameter (mean): 167 km

Semi-major axis: 181,366 km

Orbital Period: 0.498 days

Rotation Period: Synchronous

Orbit

Amalthea is in a close orbit around Jupiter and is within the outer edge of the Amalthea Gossamer Ring, which is formed from dust ejected from its surface.

Amalthea is tidally locked with the planet, the long axis pointing towards Jupiter at all times.

From Amalthea's surface, Jupiter would be an astonishing sight in its sky, roughly 92 times larger than the full moon. Because Amalthea is in synchronous rotation, Jupiter would not appear to move, and would be invisible from one side of Amalthea.

The Sun would disappear behind the planet's bulk for an hour and a half each revolution. Amalthea's short rotation period gives it just under six hours of daylight.

Physical characteristics

The surface of Amalthea is very red. The reddish color may be due to sulfur originating from Io or some other non-ice material.

Bright patches of green appear on the major slopes of Amalthea, but the nature of this color is currently unknown.

The surface of Amalthea is slightly brighter than surfaces of other inner satellites of Jupiter. There is also a substantial asymmetry between leading and trailing hemispheres: the leading hemisphere is 1.3 times brighter than the trailing one. The asymmetry is probably caused by the higher velocity and frequency of impacts on the leading hemisphere, which excavate a bright material — presumably ice — from the interior of the moon.

Formation

Amalthea's irregular shape and large size led in the past to a conclusion that it is a fairly strong, rigid body. It was argued that a body composed of ices or other weak materials would have been pulled into a more spherical shape by its own gravity.

However, on November 5, 2002, the Galileo orbiter made a targeted flyby that came within 160 km of Amalthea and the deflection of its orbit was used to compute the moon's mass. In the end, Amalthea's density was found to be as low as 0.86 g/cm³, so it must be either a relatively icy body or very porous "rubble pile" or, something in between.

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. It is therefore likely to have formed farther from the planet or to be a captured Solar System body.