Metis (moon)
Metis is the innermost known natural satellite of Jupiter. This small, irregular body orbits within the planet's main ring. Together with Adrastea, Metis supplies the dust from which that ring is formed.[5][6]
Size and physical characteristics
Metis has a volumetric mean radius of approximately 21.5 km. A sphere of equal volume would be about 43 km in diameter, but the moon itself is noticeably elongated. Its dimensions are comparable to the span of a large terrestrial city; the Moon, by comparison, is more than 3,400 km across.[1][2]
The mass is estimated at approximately 3.7 × 1016 kg, with an uncertainty of about 2.4 × 1016 kg. The mean density is also poorly constrained: JPL gives a reference value of roughly 0.9 ± 0.8 g/cm³. These measurements do not yet determine the proportions of rock, ice and internal void space.[1]
Gravity on Metis is extremely weak. The mean mass and radius give an acceleration of about 0.005 m/s² in a spherical approximation. Material ejected by impacts can escape the moon and enter independent orbits around Jupiter.[1][3]
Orbit and rotation
Metis orbits approximately 128,000 km from the centre of Jupiter. One revolution takes only 7 hours 4.5 minutes, allowing the moon to circle the planet almost three and a half times during one Earth day. Its orbit is nearly circular and lies almost exactly in Jupiter's equatorial plane.[2]
The orbit is prograde, in the same direction as Jupiter's rotation. Metis rotates synchronously: its rotation period equals its orbital period, and the same side always faces the planet.[2]
Metis circles Jupiter faster than the planet rotates. Together with Adrastea, it lies inside Jupiter's synchronous-orbit radius. This distinguishes the two small inner moons from the Galilean satellites, whose orbital periods are much longer.[7]
Surface
Metis has a dark, reddish surface. Its geometric albedo is approximately 0.06, meaning that it is much darker than an ideal diffuse disc of the same size when viewed near full illumination. The visible surface differs strongly from the bright ice-rich terrain of Jupiter's large moons.[2][3]
Galileo images show the moon's irregular outline and unevenly illuminated topography. The image acquired on 4 January 2000 resolves features approximately 3 km across. The large-scale form is therefore known much better than its small craters, boulders and fine surface texture.[8]
Connection with Jupiter's main ring
Jupiter's main ring consists predominantly of fine dust. Its principal sources are thought to be Metis and Adrastea. Interplanetary particles strike these moons and eject surface material fast enough to overcome their weak gravity.[3]
The released particles continue around Jupiter on orbits similar to those of their source moons. Because Metis and Adrastea travel almost exactly in the planet's equatorial plane, the main part of the associated ring has only a small vertical thickness.[6]
More distant Amalthea and Thebe supply other components of the system—the gossamer rings. Different small moons are therefore associated with distinct parts of Jupiter's dusty surroundings.[6]
The impact-ejecta origin of the rings was established through Galileo observations. In 1996–1997, the spacecraft acquired a series of images of the rings and small moons that connected the structure of the dust components with the locations of their likely sources.[3]
Discovery and name
Stephen Synnott discovered Metis while analysing Voyager 1 images obtained during the spacecraft's Jupiter encounter in 1979. The moon received the provisional designation S/1979 J 3 and the permanent Roman-numeral designation Jupiter XVI.[4][9]
The moon is named after Metis of Greek mythology, the first wife of Zeus and mother of Athena. Like many Jovian moons, its name comes from the mythological circle surrounding the god.[7]
Spacecraft exploration
Voyager data led to the moon's discovery, while Galileo later returned more detailed images. Observations of the inner moons were made during close approaches to Jupiter and Io in late 1999 and early 2000.[9][8]
On 4 January 2000, Galileo photographed Metis from approximately 293,000 km. NASA's published series shows it at the same scale as Amalthea and Thebe, illustrating the size differences even among Jupiter's small inner moons.[8]
Metis is important not only as an individual body. Observations of the moon and its surrounding dust reveal how impacts erode small satellites and transfer material from their surfaces into planetary rings.[3]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Planetary Satellite Physical Parameters // NASA Jet Propulsion Laboratory, Solar System Dynamics.
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 Jovian Satellite Fact Sheet // NASA Space Science Data Coordinated Archive.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Galileo Finds Jupiter's Rings Formed by Dust Blasted Off Small Moons // NASA Jet Propulsion Laboratory. 15 September 1998.
- ↑ 4.0 4.1 Planetary Satellite Discovery Circumstances // NASA Jet Propulsion Laboratory, Solar System Dynamics.
- ↑ Inner Satellites of Jupiter: Metis // Institut de mécanique céleste et de calcul des éphémérides, Paris Observatory.
- ↑ 6.0 6.1 6.2 Jupiter's Main and Gossamer Ring Structures // NASA Jet Propulsion Laboratory. 15 September 1998.
- ↑ 7.0 7.1 Metis // NASA Science.
- ↑ 8.0 8.1 8.2 Best Images Yet of Thebe, Amalthea and Metis, PIA02531 // NASA Science; NASA/JPL/Cornell University. 24 April 2000.