Amalthea (moon) - Physical Characteristics

Physical Characteristics

The surface of Amalthea is very red (that is, its reflectivity increases with the wavelength from the green to near-infrared). 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.

Amalthea is irregularly shaped, with the best ellipsoidal approximation being 250 × 146 × 128 km. From this, Amalthea's surface area is likely between 88,000 and 170,000 square kilometers, or somewhere near 130,000. Like all other inner moons of Jupiter it is tidally locked with the planet, the long axis pointing towards Jupiter at all times. Its surface is heavily scarred by craters, some of which are extremely large relative to the size of the moon: Pan, the largest crater, measures 100 km across and is at least 8 km deep. Another crater, Gaea, measures 80 km across and is likely twice as deep as Pan. Amalthea has two prominent and named mountains, Mons Lyctas and Mons Ida, with local relief reaching up to 20 km.

Amalthea's irregular shape and large size led in the past to a conclusion that it is a fairly strong, rigid body, where 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 (its volume had been calculated previously—to within 10% or so—from a careful analysis of all extant images). 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, more likely, 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. Unfortunately, NASA didn't publish any picture of Amalthea during this famous flyby on November 5, 2002, and the resolution (number of pixels) of the released Jovian satellite pictures is generally low.

Amalthea radiates slightly more heat than it receives from the Sun, which is probably due to the influence of Jovian heat flux (<9 kelvins), sunlight reflected from the planet (<5 K), and charged particle bombardment (<2 K). This is a trait shared with Io, although for very different reasons.