Amalthea (moon)

Amalthea (am'-əl-thee'-ə, Template:IPA2, Greek Αμάλθεια) is the third moon of Jupiter (in order of distance from the planet), and the fifth in order of discovery, hence its Roman numeral designation of Jupiter V. 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 satellites of Jupiter.

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 as Jupiter V despite Barnard's desire to call it "Columbia" in honor of the 400th anniversary of Columbus' discovery of the Americas.

Not to be confused with the asteroid 113 Amalthea.

1 Physical characteristics
2 The view from the surface
3 Amalthea in fiction
4 References
5 External links

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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.

Image:Amalthea PIA01074.jpg

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 0.8 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.

On November 5, 2002, a little less than a year before it was crashed into Jupiter (on September 21, 2003), the Galileo orbiter made its last moon flyby when it came within 160 km of Amalthea [1]. 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 (note: the value listed in the infobox is the simple ellipsoid volume, not the NASA value). The result [2] was surprising: Amalthea's overall density is close to the density of water ice, although the moon is almost certainly not a solid hunk of ice. One possibility is that the moon consists of many pieces that cling together from the pull of each other's gravity, mixed with empty spaces, where the pieces don't fit tightly together. Even then, the solid parts of Amalthea are apparently less dense than Io. This finding extends an emerging pattern of irregularly shaped moons and asteroids as porous rubble piles.

During the Amalthea encounter, the Galileo orbiter's star scanner detected nine flashes which appear to be small moonlets near the orbit of Amalthea. Since they were sighted only from one location, their true distances could not be measured. The moonlets may be anywhere from gravel to stadium-sized. Their origins are unknown, but they may be gravitationally captured into current orbit or they may be ejecta from meteor impacts on the moon. On the next and final orbit, Galileo detected more of these moonlets. However, this time Amalthea was on the other side of the planet, so it is probable that the particles form a ring around the planet near Amalthea's orbit.[3] No spacecraft is planned to study this region, so the objects will remain mysterious for a long time.

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The view from the surface

Jupiter would be an astonishing sight in Amalthea's night sky: 46 degrees across, it would appear roughly 92 times larger than the full Moon. In spite of its swift orbit, the Sun would disappear behind the planet's bulk for an hour and a half each revolution. Though Jupiter would also appear 900 times brighter than the full Moon, because its light would be spread over an area some 8500 times greater, it wouldn't look as bright per square unit.

From Jupiter's surface--or rather, from just above its cloudtops--Amalthea would be very bright, shining at magnitude −4.7; however, at only 5 arcminutes across its disc would be barely discernable. Like the Martian moon Deimos, Amalthea's orbital period is only slightly longer than its parent planet's day (~20% in this case), which means it would cross Jupiter's sky very slowly; the time between moonrise and moonset would be over 29 hours.

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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 is a scifi story by Boris and Arkady Strugatsky written in 1959.
In one early version of 2001: A Space Odyssey novel the giant monolith is located on the surface of Amalthea, as told by Arthur C. Clarke in the book The Lost Worlds of 2001 (1972).

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References

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

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External links

Animation of Amalthea
Satellite Physical Parameters (JPL)
Images of Amalthea
Another find for Galileo – Galileo Millennium Mission status report (2003-04-09)
Jupiter's Amalthea Surprisingly Jumbled – JPL press release (2002-12-09)
Jupiter's Moon Had A Far-Flung Past Subaru news release (2004-12-23)

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