Amalthea (moon)

Amalthea

Missing image Amalthea_PIA02532.png Galileo image of Amalthea
Discovery
Discovered by	E. Barnard
Discovered on	September 9, 1892
Orbital characteristics
Mean radius	181,995 km (0.001217 AU)
Eccentricity	0.0046637841
Periastron	181,150 km (0.00121 AU)
Apastron	182,840 km (0.00122 AU)
Revolution period	0.49817905 d (11 h 57 min 23 s)
Orbital circumference	1,144,000 km (0.008 AU)
Orbital velocity	max: 26.691 km/s mean: 26.567 km/s min: 26.443 km/s
Inclination	25.61° (to the ecliptic) 0.36° (to Jupiter's equator)
Is a satellite of	Jupiter
Physical characteristics
Mean diameter	172 km (262×146×134 km)
Surface area	397,600 km2
Volume	~2,430,000 km3
Mass	2.1×1018 kg
Mean density	0.862 g/cm3
Surface gravity	0.0018 m/s2 (0.002 g)
Escape velocity	0.0058 km/s
Rotation period	synchronous
Equatorial rotation velocity	69 km/h
Axial tilt	zero
Albedo	0.09
Surface temp.	min mean max K ~122 K K
Atmospheric pressure	0 kPa

Amalthea (am'-ul-thee'-a, 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 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 as Jupiter V.

Not to be confused with the asteroid 113 Amalthea.

Contents

1 Physical characteristics 2 Amalthea in fiction 3 References 4 External links

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.

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.

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] (http://galileo.ftecs.com/Amalthea/html/AGU_A34_122303_1313.htm). 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] (http://ww.space.com/scienceastronomy/almathea_update_021209.html) 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.

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.

References

E. E. Barnard, Discovery and Observation of a Fifth Satellite to Jupiter, Astronomical Journal 12 (1892), 81–85 (http://adsabs.harvard.edu//full/seri/AJ.../0012//0000081.000.html)

External links

• Animation of Amalthea (http://www.star.ucl.ac.uk/~idh/solar/cap/jup/vamalthe.htm)
• Images of Amalthea (http://pages.preferred.com/%7Etedstryk/amalthea.html)
• Another find for Galileo (http://galileo.jpl.nasa.gov/news/display.cfm?News_ID=4939) – Galileo Millennium Mission status report (2003-04-09)
• Jupiter's Amalthea Surprisingly Jumbled (http://galileo.jpl.nasa.gov/news/display.cfm?News_ID=3919) – JPL press release (2002-12-09)
• Jupiter's Moon Had A Far-Flung Past (http://www.naoj.org/Pressrelease/2004/12/23/index.html)   Subaru news release (2004-12-23)

bg:Амалтея (спътник)

de:Amalthea (Mond) fr:Amalthée (lune) gl:Amaltea, lúa it:Amaltea (astronomia) nl:Amalthea (maan) ja:アマルテア (衛星) sk:Amalthea (mesiac) sr:Амалтеа sv:Amalthea (måne) zh:木卫五