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ssdfmay be the remnants of a score of Galilean-mass satellites that formed
early in Jupiter's history.[8][10]
Simulations suggest that, while the disk had a relatively high mass at any
given moment, over time a substantial fraction (several tens of a percent) of
the mass of Jupiter captured from the solar nebula was passed through it.
However, only 2% of the proto-disk mass of Jupiter is required to explain
the existing satellites.[8] Thus, several generations of Galilean-mass
satellites may have been in Jupiter's early history. Each generation of
moons might have spiraled into Jupiter, because of drag from the disk, with
new moons then forming from the new debris captured from the solar
nebula.[8] By the time the present (possibly fifth) generation formed, the disk
had thinned so that it no longer greatly interfered with the moons' orbits.[10]
The current Galilean moons were still affected, falling into and being
partially protected by an orbital resonance with each other, which still exists
for Io, Europa, and Ganymede: they are in a 1:2:4 resonance. Ganymede's
larger mass means that it would have migrated inward at a faster rate than
Europa or Io.[8] Tidal dissipation in the Jovian system is still ongoing and
Callisto will likely be captured into the resonance in about 1.5 billion years,
creating a 1:2:4:8 chain.[11]
The outer, irregular moons are thought to have originated from captured
asteroids, whereas the protolunar disk was still massive enough to absorb
much of their momentum and thus capture them into orbit. Many are
believed to have been broken up by mechanical stresses during capture, or
afterward by collisions with other small bodies, producing the moons we
see today.[12]
History and discovery[edit]
See also: Timeline of discovery of Solar System planets and their moons
Visual observations[edit]
Jupiter and the Galilean moons as seen through a 25 cm (10 in) Meade
LX200 telescope
Chinese historian Xi Zezong claimed that the earliest record of a Jovian
moon (Ganymede or Callisto) was a note by Chinese astronomer Gan De
of an observation around 364 BC regarding a "reddish star".[13] However,
the first certain observations of Jupiter's satellites were those of Galileo
Galilei in 1609.[14] By January 1610, he had sighted the four massive
Galilean moons with his 20× magnification telescope, and he published his
results in March 1610.[15]
Simon Marius had independently discovered the moons one day after
Galileo, although he did not publish his book on the subject until 1614.
Even so, the names Marius assigned are used today: Ganymede, Callisto,
Io, and Europa.[16] No additional satellites were discovered until E. E.
Barnard observed Amalthea in 1892.[17]
Photographic and spacecraft observations[edit]
Voyager 1 discovery image of the inner moon Metis on 4 March 1979,
showing the moon's tiny silhouette against the backdrop of Jupiter's
clouds
With the aid of telescopic photography with photographic plates, further
discoveries followed quickly over the course of the 20th century. Himalia
was discovered in 1904,[18] Elara in 1905,[19] Pasiphae in 1908,[20] Sinope in
1914,[21] Lysithea and Carme in 1938,[22] Ananke in 1951,[23] and Leda in
1974.[24]
By the time that the Voyager space probes reached Jupiter, around 1979,
thirteen moons had been discovered, not including Themisto, which had
been observed in 1975,[25] but was lost until 2000 due to insufficient initial
observation data. The Voyager spacecraft discovered an additional three
inner moons in 1979: Metis, Adrastea, and Thebe.[26]
Digital telescopic observations[edit]
No additional moons were discovered until two decades later, with the
fortuitous discovery of Callirrhoe by the Spacewatch survey in October
1999.[27] During the 1990s, photographic plates phased out as digital
charge-coupled device (CCD) cameras began emerging in telescopes on
Earth, allowing for wide-field surveys of the sky at unprecedented
sensitivities and ushering in a wave of new moon discoveries.[28] Scott
Sheppard, then a graduate student of David Jewitt, demonstrated this
extended capability of CCD cameras in a survey conducted with the Mauna
Kea Observatory's 2.2-meter (88 in) UH88 telescope in November 2000,
discovering eleven new irregular moons of Jupiter including the previously
lost Themisto with the aid of automated computer algorithms.[29]
From 2001 onward, Sheppard and Jewitt alongside other collaborators
continued surveying for Jovian irregular moons with the 3.6-meter (12 ft)
Canada-France-Hawaii Telescope (CFHT), discovering an additional
eleven in December 2001, one in October 2002, and nineteen in February
2003.[29][1] At the same time, another independent team led by Brett J.
Gladman also used the CFHT in 2003 to search for Jovian irregular moons,
discovering four and co-discovering two with Sheppard.[1][30][31] From the
start to end of these CCD-based surveys in 2000–2004, Jupiter's known
moon count had grown from 17 to 63.[27][30] All of these moons discovered
after 2000 are faint and tiny, with apparent magnitudes between 22–23 and
diameters less than 10 km (6.2 mi).[29] As a result, many could not be
reliably tracked and ended up becoming lost.[32]
